U.S. patent application number 09/795005 was filed with the patent office on 2002-01-24 for communication system with fast control traffic.
This patent application is currently assigned to Omnipoint Corporation. Invention is credited to Anderson, Gary B., Jensen, Ryan N., Lindsay, Charles L..
Application Number | 20020009070 09/795005 |
Document ID | / |
Family ID | 22403453 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020009070 |
Kind Code |
A1 |
Lindsay, Charles L. ; et
al. |
January 24, 2002 |
Communication system with fast control traffic
Abstract
A method and system for conducting rapid control traffic in a
time division multiple access (TDMA) communication system comprises
a base station communicating with a plurality of user stations in
assigned time slots of a time frame. For bearer traffic, time slots
are assigned to particular user stations for an extended duration.
In unassigned time slots, the base station transmits a general
polling message indicating availability of the time slot. A user
station desiring to hand off communication from one base station to
another uses multiple available time slots at the target base
station for exchanging control traffic messages with the target
base station. The next available time slot is indicated by a slot
pointer in the header of each general polling message to facilitate
rapid exchange of control traffic messages. During handover, the
user station may establish a new link with the target base station
before relinquishing the existing communication link with the old
base station.
Inventors: |
Lindsay, Charles L.;
(Monument, CO) ; Jensen, Ryan N.; (Monument,
CO) ; Anderson, Gary B.; (Carnelian Bay, CA) |
Correspondence
Address: |
LYON & LYON LLP
633 WEST FIFTH STREET
SUITE 4700
LOS ANGELES
CA
90071
US
|
Assignee: |
Omnipoint Corporation
3 Bethesda Metro Centre, Suite 400,
Bethesda
MD
20814
|
Family ID: |
22403453 |
Appl. No.: |
09/795005 |
Filed: |
February 26, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09795005 |
Feb 26, 2001 |
|
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09122565 |
Jul 24, 1998 |
|
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6301242 |
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Current U.S.
Class: |
370/347 ;
370/278 |
Current CPC
Class: |
H04W 36/12 20130101;
H04W 36/0069 20180801; H04W 52/24 20130101; H04W 74/06 20130101;
H04W 72/0446 20130101; H04W 52/04 20130101; H04W 52/08 20130101;
H04L 1/0002 20130101; H04B 7/0805 20130101; H04W 52/36 20130101;
H04W 48/08 20130101; H04B 7/10 20130101; H04W 72/04 20130101; H04W
52/367 20130101; H04B 7/2618 20130101; H04B 7/2628 20130101; H04L
1/20 20130101; H04W 52/40 20130101; H04W 52/362 20130101; H04L
1/0025 20130101; H04W 72/042 20130101; G10L 19/012 20130101 |
Class at
Publication: |
370/347 ;
370/278 |
International
Class: |
H04B 007/212 |
Claims
What is claimed is:
1. In a time division multiple access communication system having a
base station and a plurality of user stations, a method of
communicating between the base station and a user station,
comprising the steps of: establishing a series of time frames, each
time frame comprising a plurality of time slots; and transmitting a
plurality of messages from the base station to the user station,
each of said messages comprising an information element indicating
a location of a subsequent time slot available for
communication.
2. The method of claim 1, wherein said plurality of messages
comprise control traffic messages, and wherein said step of
transmitting a plurality of messages from the base station to the
user station comprises the step of completing a control traffic
transaction.
3. The method of claim 1, further comprising the step of completing
a handshake transaction between said base station and said user
station so as to establish a communication channel, and thereafter
exchanging bearer traffic messages between said user station and
said base station over said established communication channel.
4. The method of claim 1, wherein said step of transmitting said
plurality of messages comprises the step of spread spectrum
encoding said messages.
5. The method of claim 1, further comprising the step of
transmitting a plurality of messages from the user station to the
base station, each of said plurality of messages transmitted in the
time slot indicated by the information element of the preceding
message from the base station.
6. In a communication system comprising at least one base station
capable of communicating with a plurality of user stations, a
method of communication comprising the steps of: transmitting a
first message from the base station, said first message comprising
an information element indicating a relative time position for a
subsequent communication; receiving said first message at a user
station; transmitting, according to said relative time position, a
second message from said user station to said base station; and
receiving said second message at said base station.
7. The method of claim 6 wherein said step of transmitting said
first message comprises the step of spread spectrum encoding said
first message, and wherein said step of transmitting said second
message comprises the step of spread spectrum encoding said second
message.
8. The method of claim 6 wherein said base station and said user
station communicate according to a time division multiple access
protocol in which a series of time frames are each divided into a
plurality of time slots, and wherein said information element
indicates said relative time position by reference to a designated
one of said time slots.
9. A message structure for use in a time division multiple access
communication system, said time division multiple access
communication system operating according to a communication
protocol by which a series of time frames are each divided into a
plurality of time slots, said message structure comprising: a data
segment; and a header segment, said header segment comprising a
next slot pointer.
10. The message structure of claim 9 wherein said next slot pointer
identifies a subsequent time slot available for communication.
11. The message structure of claim 10, wherein said slot pointer
comprises a numerical value indicating a relative number of time
slots until said subsequent time slot available for
communication.
12. The message structure of claim 10, wherein said slot pointer
comprises a numerical value indicating a position of said
subsequent time slot available for communication relative to a
starting point of a time frame.
13. In a time division multiple access communication system wherein
a base station is capable of communicating with a plurality of user
stations, a method of communicating comprising the steps of:
transmitting a first plurality of control traffic messages from a
user station to a base station in a first plurality of time slots,
at least two of said first plurality of time slots within a single
time frame; receiving said first plurality of control traffic
messages at said base station; transmitting a second plurality of
control traffic messages from said base station to said user
station in a second plurality of time slots, at least two of said
second plurality of time slots within a single time frame; and
receiving said second plurality of control traffic messages at said
user station.
14. The method of claim 13 wherein said step of transmitting said
first plurality of control traffic messages comprises the step of
spread spectrum encoding said first plurality of control traffic
messages, and wherein said step of transmitting said second
plurality of control traffic messages comprises the step of spread
spectrum encoding said second plurality of control traffic
messages.
15. The method of claim 13 wherein at least one of the control
traffic messages in said second plurality of control traffic
messages comprises a next slot pointer.
16. The method of claim 15 wherein said next slot pointer indicates
a relative position of a subsequent time slot for transmitting a
control traffic message of said first plurality of control traffic
messages.
17. The method of claim 13 wherein said first plurality of control
traffic messages and said second plurality of control traffic
messages are transmitted over the same frequency band.
18. In a time division multiple access communication system having
a series of time frames each divided into a plurality of time
slots, said time slots collectively comprising a plurality of user
transmission intervals and a plurality of base transmission
intervals, a method of communication comprising the steps of:
transmitting, over a designated frequency band and in a first base
transmission interval, a general poll message from a base station;
receiving said general poll message at a user station; transmitting
in response to said general poll message, over said designated
frequency band and in a first user transmission interval, a general
response message from said user station to said base station;
receiving said general response message at said base station;
transmitting in response to said general response message, over
said designated frequency band and in a second base transmission
interval, a specific poll message from said base station to said
user station; and receiving said specific poll message at said user
station; wherein said general poll message, said general response
message, and said specific poll message are all transmitted within
the time span of a single time frame.
19. The method of claim 18, wherein each time slot is duplex and
comprises a user transmission interval followed by a base
transmission interval.
20. The method of claim 18 wherein said first user transmission
interval and said second base transmission interval are located
within the same time slot.
21. The method of claim 18 further comprising the steps of:
transmitting, over said designated frequency band and in a second
user transmission interval, a user control traffic message from
said user station to said base station; receiving said user control
traffic message at said base station; transmitting, over said
designated frequency band and in a third base transmission
interval, a base control traffic message from said base station to
said user station; and receiving said base control traffic message
at said user station.
22. The method of claim 18, wherein: said general poll message is
transmitted in a first time slot; said general response message is
transmitted in a second time slot; and said general poll message
comprises a next slot pointer identifying to the user station a
slot position of said second time slot.
23. The method of claim 18 wherein said specific poll message
comprises a next slot pointer identifying to the user station a
slot position of a subsequent time slot for communication between
the base station and the user station.
24. The method of claim 18 wherein said steps of transmitting said
general poll message, transmitting said general response message,
and transmitting said specific poll message each comprise the step
of transmitting a spread spectrum signal.
25. The method of claim 18 further comprising the steps of:
connecting a call from a network to said base station; completing
said call from said base station to said user station; and
thereafter exchanging bearer traffic messages between said user
station and said base station during said call.
26. A multiple-user communication system, comprising: a base
station, said base station generating a series of time frames, each
of said time frames comprising a plurality of time slots; and a
plurality of user stations; wherein said base station transmits
control traffic messages during selected ones of said time slots,
each control traffic message comprising a next slot pointer
identifying a subsequent time slot available for communication; and
wherein a user station responding to one of said control traffic
messages does so in the time slot identified by the next slot
pointer of that control traffic message.
27. The multiple-user communication system of claim 26, wherein
each of said time slots comprises a first interval during which a
user message may be transmitted by a user station to which the time
slot is assigned, and a second interval during which the base
station may transmit a base message.
28. The multiple-user communication system of claim 27 wherein said
user message and said base message are each transmitted in a spread
spectrum format.
29. The multiple-user communication system of claim 27 wherein said
base message comprises a base header segment, and wherein said next
slot pointer is contained within said base header segment.
30. In a time division multiple access communication system in
which a time frame is divided into a plurality of time slots, a
method of communication comprising the steps of: communicating
between a mobile station and a first base station; and handing off
communication from said first base station to a second base
station, said step of handing off communication comprising the step
of exchanging a plurality of control traffic messages between said
mobile station and said second base station during multiple time
slots of a single time frame.
31. The method of claim 30, further comprising the step of
establishing a duplex communication link between said mobile
station and said second base station as a result of said step of
exchanging said plurality of control traffic messages.
32. The method of claim 31, further comprising the step of
assigning a time slot for bearer communication to said mobile
station as a result of said step of exchanging said plurality of
control traffic messages.
33. The method of claim 32, further comprising the step of
exchanging bearer traffic messages between said mobile station and
said second base station during said time slot assigned for bearer
communication.
34. The method of claim 30, wherein said step of exchanging a
plurality of control traffic messages between said mobile station
and said second base station during multiple time slots of a single
time frame comprises the step of transmitting a next slot pointer
in each control traffic message transmitted from said second base
station to said mobile station.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of U.S.
application Ser. No. 09/122,565, filed on Jul. 24, 1998, hereby
incorporated by reference as if set forth fully herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The field of the present invention relates to wireless
communication and, more particularly, to communication protocols
for control traffic in a wireless communication system.
[0004] 2. Description of Related Art
[0005] A mobile communication system may generally comprise a set
of "user stations", typically mobile and the endpoints of a
communication path, and a set of "base stations", typically
stationary and the intermediaries by which a communication path to
a user station may be established or maintained. A group of base
stations may be connected to a base station controller, or a
cluster controller, which can in turn be connected to a local
public telephone network through, for example, a mobile switching
center.
[0006] It is generally desirable in a mobile communication system
to achieve the greatest possible user traffic capacity at a base
station, so that fewer base stations need to be deployed in order
to serve user demands. One technique used to allow a base station
to communicate with multiple user stations is use of time division
multiple access (TDMA). In a particular TDMA system, for example, a
time frame is divided into a plurality of smaller time units, or
time slots, and transmissions from the base station and from the
user stations are separated in time so as to avoid collisions. In
addition to separating transmissions in time, transmissions may
also be distinguished by using different assigned frequencies,
thereby resulting in a frequency division multiple access (FDMA)
system. Furthermore, transmissions may be encoded using spread
spectrum techniques, and different cells in a mobile communication
system may be assigned different spread spectrum codes, thereby
differentiating transmissions through code division multiple access
(CDMA).
[0007] Generally, in order to carry out communication between a
base station and a user station, a communication link must first be
established. Establishment of the communication link can be
difficult in a spread-spectrum communication system, due to the
length of time typically required to synchronize the transmitter
and the receiver. Establishment of the communication link and/or
handing off can be more difficult in a TDMA system in which spread
spectrum is used, due to the amount of time usually necessary to
synchronize the transmitter and receiver, especially where the
amount of time available for synchronization within a user
station's time slot is relatively brief.
[0008] Within a mobile communication system, a protocol generally
defines how communication is to be initially established between a
base station and a user station. The protocol may further define
when and how a handoff may be conducted as a user station leaves
the service area or "cell" of one base station and enters the
service area of another base station. Messages exchanged between a
base station and user station for the purposes of establishing or
maintaining a connection, or for handing off communication,
generally can be referred to as control traffic or signaling
traffic. Messages carrying data to be conveyed between the
endpoints of a call are generally referred to as bearer traffic
messages.
[0009] Initial communication between a user station and a base
station can be established either when the user station seeks to
initiate communication with a base station (for example, attempting
to initiate a telephone call), or when the base station attempts to
complete a call to the user station (for example, where the user
station is paged). In many conventional mobile communication
systems, a dedicated control channel is used to assist mobile
stations in establishing communication. According to this
technique, the mobile station first communicates over the control
channel when establishing communication. The base station then
assigns to the mobile station a "permanent" communication channel
for exchanging bearer traffic messages.
[0010] In at least one mobile communication system, however, a user
station can establish initial communication using the same channel
used for transmitting bearer traffic. For example, a system in
which a user station can establish communication by exchanging
control traffic messages in a particular communication channel
(e.g., a time slot of a time frame), and thereafter use the same
channel (time slot) for bearer traffic, is described in U.S. patent
application Ser. No. 08/284,053 filed Aug. 1, 1994, which is
assigned to the assignee of the present invention, and hereby
incorporated by reference as if set forth fully herein.
[0011] The exchange of control traffic messages may also occur
during a handoff of a user station from one base station to
another, usually as the user station moves between service areas.
Typically, in the large majority of conventional mobile
communication systems, handoffs are carried out under the direction
of the base station and/or a mobility control center connected to
the base station. When a communication link starts to break down,
the base station requests a transfer of an ongoing call to a nearby
base station, which becomes the target for handoff. The target base
station may be selected according to criteria developed at the base
station, the user station, or both. A control channel (which may be
the same dedicated control channel as used for establishing
communication, where provided) may be used for the purpose of
assisting the mobile station with the handoff.
[0012] In some mobile communication systems, the user station plays
a larger role in handoff. An example of such a system is generally
described in U.S. patent application Ser. No. 08/284,053,
previously incorporated herein by reference. In at least one
embodiment disclosed therein, the user station not only determines
when to hand off, but also takes steps to initiate a hand off from
its current base station to a different base station.
[0013] It is generally desirable in mobile communication systems to
allow the rapid establishment of communication links between mobile
stations and base stations, and rapid handoff between base
stations, without errors and without inadvertently dropping the
call or losing a communication link. This type of capability would
tend to imply the need for devoting potentially significant
resources (i.e., communication channels and processing speed and
power) to handle link establishment and handoff. Because the
communication environment can be unstable and multiple users may
need to be serviced at the same time, a mobile communication system
is preferably capable of handling multiple service requests for
link establishment or handoff, and doing so quickly and without
errors or dropped calls.
[0014] At the same time, resources available for handling control
traffic messages are usually limited, sometimes severely so, in
part because control traffic resources generally must compete
against bearer traffic resources. Thus, resources dedicated to
control traffic reduce the overall resources available for handling
data or bearer traffic, and vice versa. By setting aside resources
(such as a dedicated control channel or multiple such channels) for
servicing control traffic demands, the base station's user capacity
can be adversely impacted. As a result, a greater number of base
stations may need to be deployed to service a given number of
expected users.
[0015] It would therefore be advantageous to provide a
communication system having a rapid and reliable means for
establishing a communication link between a base station and a user
station. It would further be advantageous to provide a
communication protocol enabling rapid handoffs and control traffic
functions, and which is particularly suited to use in a time
division multiple access environment. It would further be
advantageous to provide a communication protocol having a fast
handoff and control traffic capability well suited to the demands
of spread spectrum communication.
SUMMARY OF THE INVENTION
[0016] In one aspect of the present invention, a method and system
for handing off communication between base stations in a mobile
communication system is provided. In a preferred embodiment of the
invention, a mobile station communicates with a base station using
a time division multiple access (TDMA) and/or time division duplex
(TDD) technique. In such an embodiment, a continuous sequence of
time frames is generated, with each time frame comprising a
plurality of time slots. The base station can communicate with a
plurality of user stations (some or all of which may be mobile
stations), one in each time slot. A mobile station desiring to hand
off exchanges a plurality of control traffic messages with a second
base station to establish communication in a different time slot
with the second base station. The mobile station then releases the
communication channel with the first base station and requests,
through the second base station, the transfer of the call to the
second base station.
[0017] In a preferred embodiment of the present invention, a mobile
station transmits and/or receives a plurality of control traffic
messages in multiple time slots of one or more time frames with the
second (target) base station while in the process of handing off
communication to the target base station, or performing other
control traffic signaling. The second base station provides an
indication to the mobile station of the next available time slot
for control traffic, and, if desired, can temporarily assign
additional time slots to the mobile station during handoff, or
other control traffic signaling.
[0018] In another aspect of the present invention, a method and
system for establishing communication and handing off communication
in a TDMA and/or TDD communication is provided. In one embodiment,
the base station transmits a general poll message in each available
time slot to indicate availability of the time slot. To establish
communication in an available time slot, a user station responds to
the general poll message with a general poll response. The base
station then follows with a specific poll message. The user station
responds with a specific poll response. Normal traffic
communication may thereafter be conducted over an established
communication link. During normal traffic communication, in one
embodiment, each user station transmits information to the base
station during an initial portion of an assigned time slot, and
each user station receives information from the base station during
a latter portion of the same assigned time slot.
[0019] Handover between base stations may be carried out by
establishing a new communication link with a new base station,
while maintaining an old communication link with an original base
station until the new communication link is fully established. The
new communication link may be established in the same manner as the
original link--that is, by using the same handshaking technique
involving a general poll, general response, specific poll, and
specific response messages.
[0020] In another aspect of the invention, a slot pointer
information element within a general polling message provides an
indication of the location of the next available time slot for
communication. The slot pointer may be a numerical value relative
to the current time slot. As part of a specific polling message,
the slot pointer information element provides an assignment of the
time slot channel to be used for future communication by the user
station presently in the process of establishing communication. The
slot pointer may be used to perform rapid handover by allowing the
use of multiple time slots within a time frame for control
traffic.
[0021] In another embodiment, virtual time slots are defined as
part of the timing structure. As used herein, a virtual time slot
is generally a time slot assigned to the same user station with two
transmission intervals non-adjacent in time. For example, a virtual
time slot may be a time slot in which a forward link transmission
and a reverse link transmission for a particular user station are
separated by transmissions to or from one or more other user
stations. In a preferred system in which each physical time slot
has a user transmission interval and a base transmission interval,
a user station may therefore transmit a user message to the base
station during a user transmission interval of a first physical
time slot, and receive a base message from the base station during
a base transmission interval of a second, subsequent physical time
slot. In a particular embodiment, a virtual slot field in the
header of the general polling message indicates whether or not
virtual time slots are provided, thereby enabling operation in
either of two modes, one using virtual time slots and the other not
using virtual time slots.
[0022] A method and system for establishing and maintaining spread
spectrum communication is disclosed with respect to a preferred
embodiment wherein data symbols are encoded using an M-ary direct
sequence spread spectrum communication technique. Further
variations and details of the above embodiments are also described
herein and/or depicted in the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a diagrammatic representation of a cellular
communication system.
[0024] FIG. 1A is a diagram of an arrangement of cells in a
wireless communication system showing an exemplary code and
frequency reuse pattern.
[0025] FIG. 2 is a diagram of one embodiment of a communication
system.
[0026] FIG. 2A is a block diagram of another embodiment of a
communication system, using a GSM-based network
interconnection.
[0027] FIG. 3 is a diagram of a time frame divided into time
slots.
[0028] FIG. 4 is a diagram illustrating a protocol for establishing
a communication link between a base station and a user station.
[0029] FIG. 4A is a message flow diagram corresponding to FIG.
4.
[0030] FIG. 5A is a diagram of a preferred time slot structure.
[0031] FIGS. 5B and 5C are diagrams of a base station transmit data
time frame structure and a user station transmit data time frame
structure, respectively.
[0032] FIG. 6 is a diagram of a time frame structure in accordance
with another embodiment of the invention showing a time frame
divided into virtual time slots.
[0033] FIGS. 7A-7C are diagrams of polling message formats.
[0034] FIGS. 8A and 8B are diagrams of message header formats.
[0035] FIG. 9 is a message flow diagram illustrating call
origination from a user station.
[0036] FIG. 10 is a message flow diagram illustrating call
termination at the user station.
[0037] FIGS. 11A-11C are message flow diagrams illustrating a
handover of a mobile call between two base stations within a
cluster.
[0038] FIGS. 12A and 12B are message flow diagrams illustrating a
handover of a mobile call between two base stations located in
different clusters.
[0039] FIGS. 13A and 13B are diagrams of a base station data packet
and a user station data packet, respectively.
[0040] FIGS. 14A and 14B are timing diagrams showing a time frame
and time slot structure in a linear representation and loop
representation, respectively.
[0041] FIG. 15 is a diagram of a series of consecutive time frames
showing utilization of a particular time slot over a sequence of
time frames.
[0042] FIGS. 16A and 16B are timing diagrams of mobile station
transmissions and base station transmissions, respectively, within
a particular polling loop of the type shown in FIG. 14B, wherein
symmetric time slots are used.
[0043] FIGS. 17A and 17B are timing diagrams of mobile station
transmissions and base station transmissions, respectively, within
a particular polling loop of the type shown in FIG. 14B, wherein
asymmetric time slots are used.
[0044] FIGS. 18A and 18B are timing diagrams showing multiple time
slots utilized for carrying out control traffic operations.
[0045] FIG. 19 is a block diagram of a communication system
illustrating inter-cluster and intra-cluster handoffs.
[0046] FIG. 20 is a block diagram of a transmitter and a receiver
in a spread spectrum communication system.
[0047] FIG. 21 is a diagram illustrating a preferred system
protocol architecture.
[0048] FIG. 22 is a call flow diagram of a call release initiated
by a user station.
[0049] FIG. 23 is a call flow diagram of a call release initiated
by the network.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0050] FIG. 1 is a diagram of a pattern of cells for a
multiple-access wireless communication system 101. The wireless
communication system 101 of FIG. 1 includes a plurality of cells
103, each with a base station 104, typically located at the center
of the cell 103. A plurality of user stations 102, some or all of
which may be mobile, communicate with the base stations 104 to
place and receive calls. Each station (both the base stations 104
and the user stations 102) generally comprises a receiver and a
transmitter.
[0051] A control station 105 may also be provided (comprising a
receiver and a transmitter) to manage the resources of the system
101. The control station 105 (which may comprise a "base station
controller" as described later herein) may assign the base station
104 and user stations 102 in each cell 103 a spread-spectrum code
or a set of spread spectrum codes for modulating radio signal
communication in that cell 103. (Alternatively, a spread spectrum
code or set of spread spectrum codes may be pre-assigned to a cell
103.) The resulting spread spectrum signals are generally spread
across a bandwidth exceeding the bandwidth necessary to transmit
the data, hence referred to by the term "spread spectrum."
Accordingly, radio signals used in a cell 103 are preferably spread
across a bandwidth sufficiently wide that both base station 104 and
user stations 102 in an adjacent cell 103 can distinguish
communication which originates in the first cell 103 from
communication which originates in the adjacent cell 106.
[0052] FIG. 2 is a block diagram of a communication system
architecture utilized in a preferred embodiment of the present
invention. The FIG. 2 communication system comprises a plurality of
base stations 104 for communicating with a plurality of user
stations 102. The base stations 104 and user stations 102 may
operate in a personal communications system (PCS), such as may be
authorized under rules prescribed by the Federal Communications
Commission (FCC).
[0053] Each base station 104 may be coupled to a base station
controller 105 by any of a variety of communication paths 109. The
communication paths 109 may each comprise one or more communication
links 118. Each communication link 118 may include a coaxial cable,
a fiber optic cable, a digital radio link, or a telephone line.
[0054] Each base station controller 105 may also be connected to
one or more communication networks 126, such as a public switched
telephone network (PSTN) or personal communication system switching
center (PCSC). Each base station controller 105 is connected to a
communication network 126 by means of one or more communication
paths 108, each of which may include a coaxial cable, a fiber optic
cable, a digital radio link, or a telephone line.
[0055] The FIG. 2 communication system also may include one or more
"intelligent" base stations 107 which connect directly to a
communication network 126 without interfacing through a base
station controller 105. The intelligent base stations 107 may
therefore bypass the base station controller 105 for local handoffs
and switching of user stations 102, and instead perform these
functions directly over the network 126.
[0056] In operation each base station 104 formats and sends digital
information to its respective base station controller 105 (or
directly to the network 126 in the case of an intelligent base
station 107). The base station controllers 105 receive inputs from
multiple base stations 104, assist handoffs between base stations
104, and convert and format channel information and signaling
information for delivery to the network 126. The base station
controllers 105 may also manage a local cache visitor location
register (VLR) database, and may support basic operation,
administration and management functions such as billing, monitoring
and testing. Each base station controller 105, under control of the
network 126, may manage local registration and verification of its
associated base stations 104 and may provide updates to the network
126 regarding the status of the base stations 104.
[0057] The network 126 connects to the base station controllers 105
for call delivery and outgoing calls. Intelligent base stations 107
may use ISDN messaging for registration, call delivery and handoff
over a public telephone switch. The intelligent base station 107
may have all the general capabilities of a base station 104 but
further incorporate a basic rate ISDN (BRI) card, additional
intelligence and local vocoding.
[0058] The communication system may also be based on a GSM network
interconnection. FIG. 2A is a diagram of a communication system
architecture showing such an interconnection. In the communication
system shown in FIG. 2A, the base stations 104 may connect to a GSM
mobile switching center 112 through a GSM "A" interface. The "A"
interface may be incorporated in base station controllers 105 and
in intelligent base stations 107. Features and functionality of GSM
may be passed to and from the base stations 104 over the "A"
interface in a manner that is transparent to the end user (i.e.,
user stations 102). The GSM mobile switching center 112 may connect
to a PSTN or to other networks, as indicated in FIG. 2A.
[0059] The system may also interconnect to cable television
distribution networks. In such a system, the base stations 104 may
be miniaturized so that they can be installed inside standard cable
TV amplifier boxes. Interfacing may be carried out using analog
remote antenna systems and digital transport mechanisms. For
example, T1 and fractional T1 ("FT1") digital multiplexer outputs
from the cable TV network may be used for interfacing, and basic
rate (BRI) ISDN links may be used to transport digital
channels.
[0060] FIG. 1A is a diagram of a preferred cellular environment in
which the invention may operate. According to FIG. 1A, a
geographical region 201 is divided into a plurality of cells 103.
Associated with each cell 103 is an assigned frequency and an
assigned spread spectrum code. Preferably, three different
frequencies (or frequency groups) F1, F2 and F3 are assigned in
such a manner that no two adjacent cells have the same assigned
frequency (or frequency group) F1, F2 or F3, thereby minimizing RF
interference between adjacent cells. The frequencies may be
assigned on a "permanent" basis, or else dynamically through the
network.
[0061] To further reduce the possibility of intercell RF
interference, different near-orthogonal spread spectrum codes C1
through C7 are assigned as shown in a repeating pattern overlapping
the frequency reuse pattern. Although a repeating pattern of seven
spread spectrum codes C1 through C7 is preferred, a pattern
involving other numbers of spread spectrum codes may be suitable
depending upon the particular application. As with frequencies used
in the cells 103, spread spectrum codes may be assigned on a
"permanent" basis or else dynamically through the network. Further
information regarding a suitable cellular environment for operation
of the invention may be found in U.S. Pat. No. 5,402,413, assigned
to the assignee of the present invention, and hereby incorporated
by reference as if fully set forth herein.
[0062] The use of spread spectrum for carrier modulation permits a
frequency reuse factor of N=3 for allocating different carrier
frequencies F1, F2 and F3 to adjacent cells 103. Interference
between cells 103 using the same carrier frequency F1, F2 or F3 is
reduced by the propagation loss due to the distance separating the
cells 103 (i.e., any two cells 103 using the same frequency F1, F2
or F3 are separated by at least one intervening cell 103, as shown
in FIG. 1A), and also by the spread spectrum processing gain
obtained by the use of near-orthogonal spreading codes.
[0063] Further details regarding an exemplary cellular pattern are
described in, e.g., U.S. Pat. No. 5,402,413 referred to above.
[0064] A preferred embodiment of the invention achieves multiple
access communication by using a time frame divided into multiple
time slots, i.e., time division multiple access (TDMA). FIG. 3 is a
diagram showing a timing structure for a particular TDMA system.
According to the timing structure of FIG. 3, communication over
time is broken into a continuous series of time frames 301. A
single complete time frame 301 is shown along a timeline 310 in
FIG. 3; similar time frames are assumed to precede and follow time
frame 301 in a continuous pattern along the timeline 310.
[0065] Time frame 301 is divided into a plurality of time slots 302
numbered consecutively TS1, TS2 . . . TSN, each of which may
support duplex communication with a user station 102. Time frame
301 may be thought of as a "polling loop" or a time loop, as
depicted in FIG. 3, whereby user stations 102 are communicated with
sequentially over the time frame 301 in a manner analogous to
polling, each user station 102 transmitting and receiving messages
in its designated time slot 302. In the FIG. 3 embodiment, each
time slot 302 comprises a user transmission interval 305, wherein a
user station 102 transmits a user-to-base message to the base
station 104, and a base transmission interval 306, wherein the base
station 104 transmits a base-to-user message to the user station
102. Communication in time slots 302 may be interleaved, such that
user stations 102 transmit in one physical time slot 302 but
receive in a different physical time slot 302.
[0066] In an exemplary TDMA communication system, time frames 301
are each in the neighborhood of 20 milliseconds in duration, and
each time frame 301 comprises sixteen time slots 302 or,
alternatively, eight time slots 302 to support extended range
through increased guard times.
[0067] In some embodiments, a user station 102 may communicate in
more than one time slot 302 in each time frame 301, so as to
support an increased data rate. Similarly, in some embodiments, a
user station 102 may periodically skip time frames 301 and
communicate in some subset of all time frames 301 (e.g., every
other time frame 301, or every fourth time frame 301), so as to
support a reduced data rate where a full speed communication link
is not necessary. Further information about an exemplary TDMA
system supporting variable data rates as described above may be
found in copending U.S. patent application Ser. No. 08/284,053
filed Aug. 1, 1994, previously incorporated herein by
reference.
[0068] FIG. 6 is a diagram of a timing structure employing virtual
time slots, each of which generally comprises a duplex pair (i.e.,
one forward link and one reverse link).
[0069] In FIG. 6, similar to FIG. 3, communication over time is
broken into a continuous series of time frames 601. A single
complete time frame 601 is shown along a timeline 610 in FIG. 6;
similar time frames are assumed to precede and follow time frame
601 in a continuous pattern along the timeline 610.
[0070] Time frame 601 is divided into a plurality of physical time
slots 602 numbered consecutively TS1', TS2' . . . TSN'. Each
physical time slot 602 comprises a user transmission interval 605
wherein a user station 102 transmits a user-to-base message to the
base station 104, and a base transmission interval 606 wherein the
base station 104 transmits a base-to-user message to a user station
102, which could be a different user station 102 than transmitted
to the base station 104 in the same physical time slot 602. Using
virtual time slots, communication in physical time slots 602 may be
interleaved, such that a user station 102 transmits in one physical
time slot 602 but receives in a different physical time slot 602.
The user transmission interval 605 and base transmission interval
606 which define the forward link and reverse link transmissions
for a given user station 102 (and which are generally located in
different physical time slots 602, as depicted in FIG. 6) are
collectively referred to as a "virtual time slot."
[0071] An exemplary virtual time slot 618 is shown in FIG. 6,
associated with a particular user station 102 (e.g., user station
MS2). The virtual time slot 618 comprises two message transmission
intervals, one in each of two physical time slots 602a and 602b.
Virtual time slot 618 has a user transmission interval 605a in the
first physical time slot 602a, and a base transmission interval
606b in the second physical time slot 602b. Between the user
transmission interval 605a and the base transmission interval 606b
of the virtual time slot 618, the base station 104 transmits in a
base transmission interval 606a of the first physical time slot
602a (e.g., to a second user station 102, such as user station
MS1), and another user station 102 (e.g., a third user station 102,
such as user station MS3) transmits in a user transmission interval
605b to the base station 104. In this manner, transmissions to and
from the base station 104 are interleaved.
[0072] Time frame 601 may be thought of as a "polling loop" or a
time loop, similar to time frame 301 of the FIG. 3 embodiment,
whereby user stations 102 are communicated with sequentially over
the time frame 601 in a manner analogous to polling, each user
station 102 transmitting and receiving messages in its designated
virtual time slot 618. The virtual time slots 618 of FIG. 6,
however, are not necessarily identical to the physical time slots
602. An advantage of the FIG. 6 timing structure is that it may
allow extended time for the base station 104 to process channel
characterization data as received from the user station 102.
[0073] In an exemplary TDMA communication system, time frames 601
are each 20 milliseconds in duration, and each time frame 601
comprises sixteen time slots 602 or, alternatively, eight time
slots 602 to support extended range through increased guard
times.
[0074] Further details regarding time frame structures (including
virtual time slots) may be found in copending U.S. patent
application Ser. No. 08/668,483 filed Jun. 21, 1996, hereby
incorporated by reference as if set forth fully herein.
[0075] In some embodiments, a user station 102 may communicate in
more than one virtual time slot 618 in each time frame 601, so as
to support an increased data rate. Similarly, in some embodiments,
a user station 102 may periodically skip time frames 601 and
communicate in some subset of all time frames 601 (e.g., every
other time frame 601, or every fourth time frame 601), so as to
support a reduced data rate where a full speed communication link
is not necessary.
[0076] Communication between a user station 102 and a base station
104 is established in one embodiment by a response from a user
station 102 to a general polling message sent from the base station
104 during an available time slot 302. This process is described in
more detail with reference to FIG. 4, which illustrates a protocol
for establishment of a spread spectrum communication link in, e.g.,
the FIG. 3 communication system. A communication link may be
established in an analogous manner for the FIG. 6 embodiment.
[0077] In the FIG. 4 protocol, a general poll message 401 is
transmitted by the base station 104 in some or all of the time
slots 302 which are available for communication. A user station 102
may monitor transmissions from a base station 104 and ascertain
available time slots 302 by receiving general poll messages 401 in
those time slots 302.
[0078] A user station 102 may "acquire" a base station 104 by a
sequence of handshaking steps. At a general poll step 407, the base
station 104 transmits its general poll message 401 during an
unoccupied time slot 302. The user station 102 receives the general
poll message 401 and, if it was received without error, transmits a
general poll response 404 to the base station 104 in the same time
slot 302 of the following time frame 301 (or in a different time
slot, as explained hereafter). The general poll message 401
preferably comprises a field for a base ID 408b, which may be 32
bits long (for example), and which may be stored or otherwise
recorded by the user station 102. Similarly, the general poll
response 404 preferably comprises a field for a user ID 409, which
may be 32 bits long (for example), and which may be stored or
otherwise recorded by the base station 104.
[0079] Upon receiving a general poll response 404, at a specific
poll step 410 the base station 104 transmits a specific poll
message 402 comprising (among other things) the user ID 409 which
had been previously received by the base station 104 as part of the
general poll response 404. The user station 102 receives the
specific poll message 402 and, if it was received without error and
with the same user ID 409, transmits its specific poll response 405
to the base station 104 in the same time slot 302 of the following
time frame 301 (or in a different time slot, as explained further
herein). The specific poll response 405 comprises the same user ID
409 as the general poll response 404.
[0080] In a particular embodiment, the specific poll response 405
may be eliminated as redundant. The user station 102 may, in such a
case, follow the specific poll message 402 with a user traffic
message 406.
[0081] Upon receiving a specific poll response 405 comprising a
user ID 409 which matches that of the general poll response 404, at
a link-established step 411 the base station 104 may transmit a
traffic message 403. At this point, the base station 104 and user
station 102 have established a communication link 412. The base
station 104 may connect a call through the communication channel,
and the user station 102 may begin normal operation on a telephone
network (e.g., the user station 102 may receive a dial tone, dial a
number, make a telephone connection, and perform other telephone
operations). The base station 104 and user station 102 may exchange
traffic messages 403 and 406, until the communication link 412 is
voluntarily terminated, until faulty communication prompts the user
station 102 to re-acquire the base station 104, or until handoff of
the user station 102 to another base station 104.
[0082] FIG. 4A illustrates a similar exchange of messages in a
message flow diagram format, whereby a user station 102 establishes
communication with a base station 104.
[0083] Should more than one user station 102 respond to the same
general poll message 401, the base station 104 may intentionally
fail to respond with a specific poll message 402. The lack of
response from the base station 104 signals the involved user
stations 102 to back off for a calculated time interval before
attempting to acquire the same base station 104 using the general
poll message 401 and general poll response 404 protocol. The
back-off time may be based upon the user ID 409, and therefore each
user station 102 will back off for a different length of time to
prevent future collisions, in a manner similar to that specified by
IEEE Standard 802.3.
[0084] When an incoming telephone call is received at a base
station 104 at an incoming-call step 413, the base station 104
skips the general poll message 401 and general poll response 404
and moves directly to the specific poll step 410. The base station
104 transmits a specific poll message 402 with the user ID 409 of
the indicated recipient user station 102 on an available time slot
302. As further described herein, each user station 102 listens
regularly for the specific poll message 402 so as to receive the
specific poll message 402 within a predetermined time after it is
transmitted. When the specific poll message 402 is received, the
user station 102 compares the user ID 409 in the message with its
own user ID, and if they match, continues with the link-established
step 411. The base station 104 may thereby establish a
communication link 412 with any user station 102 within
communication range.
[0085] Further details regarding means for establishing
communication (particularly spread spectrum communication) in a
TDMA system may be found in copending U.S. Pat. No. 5,455,822 and
in copending U.S. patent application Ser. No. 08/284,053 filed Aug.
1, 1994, both of which are hereby incorporated by reference as if
fully set forth herein.
[0086] In a preferred embodiment, the general poll message 401
comprises a next slot pointer (contained in a next slot pointer
field 810 shown in and described with respect to FIG. 8A) which
indicates the next time slot 302 (or virtual time slot 618) during
which a general poll message 401 will be transmitted by the base
station 104. In such an embodiment, a user station 102 seeking to
establish communication responds to the general poll message 401 in
the user transmission interval 305 (or 605) of the time slot 302
(or 618) indicated by the next slot pointer, and not necessarily in
the same time slot of the next time frame 301 (or 601). Upon
receiving a general response message 404 from the user station 102
in the time slot indicated by the next slot pointer, the base
station 102 responds with a specific poll message 402. Should more
than one user station 102 respond to a general poll message 401,
the appearance of a general poll message 401 (rather than a
specific poll message 402) in the time slot indicated by the next
slot pointer will cause each user station 102 involved to back off
for a variable period of time depending on the user station ID.
[0087] The specific poll message 402 comprises a temporary
shorthand identifier (nickname) specific to the user station 102
and referred to herein as a "correlative ID." The correlative ID
appears in subsequent signaling messages (in both directions) until
the established link is dropped. In response to the specific poll
message 402, the user station 102 responds with a traffic message
in a time slot 302 (or 618) assigned by a next slot pointer in the
header of the specific poll message 402.
[0088] Further details of how the next slot pointer (sometimes
referred to simply as the slot pointer) is used within preferred
embodiments are described below, after a brief description of
various time intervals within a time slot and basic message
structures and formats. The particular time intervals, messages
structures and formats are meant to be illustrative and to
represent various preferred embodiments for demonstrating the
workings of the invention, and are not meant to limit the invention
to any particular type of message structure or format, or any
particular type of time slot structure.
[0089] FIG. 5A is a diagram of a preferred slot structure, and
FIGS. 5B and 5C are diagrams of a base station transmit data frame
structure and a user station transmit date frame structure,
respectively. In FIG. 5A, a time slot 510 comprises a variable
radio delay gap 505, a user station transmit frame 515, a base
processor gap 525, a guard time 535, a base station transmit frame
545, and a radar gap 555. Each user station transmit frame 515
comprises a user preamble 516, a user preamble sounding gap 519,
and a user station transmit data frame 521. Similarly, each base
station transmit frame 545 comprises a base preamble 547, a base
preamble sounding gap 549, and a base transmit data frame 551.
[0090] FIG. 5B illustrates a preferred message structure for the
base station transmit data frame 551. The message structure of FIG.
5B comprises a base header field 553, a base D-channel field 557, a
base data field 559, and a base cyclical redundancy check (CRC)
field 561. In a preferred embodiment, the base header field 553 is
23 bits, the base D-channel field 557 is 8 bits, the base data
field 559 is 192 bits, and the base CRC field 561 is 16 bits.
[0091] FIG. 5C illustrates a preferred message structure for the
user station transmit data frame 521. The message structure of FIG.
5C comprises a user header field 523, a user D-channel field 527, a
user data field 529, and a user CRC field 531. In a preferred
embodiment, the user header field 523 is 17 bits, the user
D-channel field 527 is 8 bits, the user data field 529 is 192 bits,
and the user CRC field 531 is 16 bits.
[0092] FIGS. 7A-7C are diagrams of preferred polling message
formats. FIG. 7A is a diagram of a general poll message format,
such as may be employed, for example, with general poll message 401
of FIG. 4. As shown in FIG. 7A, a general poll message 701
preferably comprises, in the following sequence, a header field
702, a spare field 703, a zone field 704, a base station controller
(BSC) ID field 705, a base ID field 706, a facility field 707, a
system type field 708, a service provider field 709, a slot quality
field 710, a forward error correction (FEC) field 711, and a frame
control word (FCW) field 712. In a preferred embodiment, the header
field 702 is 24 bits long, the spare field 703 is 16 bits long, the
zone field 704 is 40 bits long, the BSC ID field 705 is 16 bits
long, the base ID field 706 is 32 bits long, the facility field 707
is 32 bits long, the system type field 708 is 8 bits long, the
service provider field 709 is 16 bits long, the slot quality field
710 is 8 bits long, the FEC field 711 is 32 bits long, and the
frame control word field 712 is 16 bits long, for a total of 240
bits.
[0093] The header field 702 identifies the message type and is
described more fully with respect to FIG. 8A. The zone field 704
identifies the paging zone of the specific base station 104. A user
station 102 may move from one base station 104 service area to
another in the same zone without requiring immediate
re-registration. The BSC ID field 705 is a sequence uniquely
identifying the base station controller 105. The base ID field 706
is a sequence uniquely identifying the base station 104. The
facility field 707 describes the services offered by the base
station 104 (e.g., internet access, aggregate data capability,
enhanced voice, etc.). The facility field 707 may include a
sub-field indicating what user stations may have access to the
channel (e.g., 911 calls only, or user stations 102 with specific
access codes). The system type field 708 identifies the type of
system associated with the base station 104. The service provider
field 709 identifies the PCS service provider that operates the
base station 104 (or, if more than one service provider is
available at the base station 104, the service provider that
currently operates the particular time slot). The slot quality
field 710 indicates the relative quality of the time slot in terms
of interference. Generally, the lower the number, the better the
slot quality. The FEC field 711 is used for forward error
correction. The FCW field 712 is used for error detection, and in
one embodiment comprises a sequence of bits and/or phase shifts
determined according to following algorithm:
[0094] 1. Calculate remainder R1 of a seed polynomial SDP modulo-2
divided by a generator polynomial GRP;
[0095] 2. Calculate product P of x.sup.16 and content of the
message 701 preceding FCW field 710;
[0096] 3. Calculate remainder R2 of the generator polynomial GNP
modulo-2 divided by the product P derived in Step 2;
[0097] 4. Calculate modulo-2 sum S of remainder R1 and remainder
R2; and
[0098] 5. Calculate the ones-complement of sum S the result of
which is transmitted in the FCW field 710.
[0099] In a preferred embodiment, the seed polynomial SDP is:
x.sup.k(x.sup.15+x.sup.14+x.sup.13+x.sup.12+x.sup.11+x.sup.10+x.sup.9+x.su-
p.8+x.sup.7+x.sup.6+x.sup.5+x.sup.4+x.sup.3+x.sup.2+x.sup.1+1)
[0100] and the generator polynomial GRP is:
x.sup.16+x.sup.12+x.sup.5+1
[0101] FIG. 7B is a diagram of a specific poll message format (such
as may be employed, for example, with specific poll message 402 of
FIG. 4). As shown in FIG. 7B, a specific poll message 720
preferably comprises, in the following sequence, a header field
721, a correlative ID field 722, a cause field 723, a personal
identifier (PID) field 724, an over-the-air (OTA) map type field
725, an OTA map field 726, a spare field 727, a slot quality field
728, a forward error correction field 729, and an FCW field 730. In
a preferred embodiment, the header field 721 is 24 bits long, the
correlative ID field 722 is 8 bits long, the cause field 723 is 8
bits long, the PID field 724 is 72 bits long, the OTA map type
field 725 is 8 bits long, the OTA map field 726 is 32 bits long,
the spare field 727 is 32 bits long, the slot quality field 728 is
8 bits long, the FEC field 729 is 32 bits long, and the FCW field
729 is 16 bits long, for a total of 240 bits.
[0102] The header field 721, slot quality field 728, FEC field 729,
and FCW field 730 are similar to the analogous fields described for
FIG. 7A. The correlative ID field 722 is used to temporarily
identify one or more channels (i.e., time slots) as being allocated
to a specific user station 102. A correlative ID number is assigned
for the duration of a call connection and is released for reuse by
another user station 102 at the termination of a connection; the
correlative ID number may also be changed during a connection. A
specific correlative ID number may be reserved by the base station
104 for broadcast use. The cause field 723 indicates the cause of
an error occurring during execution of a previous signaling traffic
operation for the particular user station 102. Interpretation of
the cause field 723 message may therefore depend upon the type of
signal traffic involved. Possible cause messages include, for
example, those indicating that the user station 102 is unregistered
or will not be accepted for registration, or that the call has not
been connected or cannot be completed. The PID field 724 comprises
a personal identification number which uniquely identifies the
subscriber (e.g., user station 102). The OTA map type field 725
defines the type of map (e.g., superframe, subframe, etc., as
defined later herein) that follows in the OTA map field 726. The
OTA map field 726 describes the mapping of time slots relative to a
particular user station 102. The format of the OTA map field 726
depends on the map type.
[0103] FIG. 7C is a diagram of a poll response message format (such
as may be employed, for example, with general poll response 404 or
specific poll response 405 of FIG. 4). As shown in FIG. 7C, a poll
response message 740 preferably comprises, in the following
sequence, a header field 741, a first spare field 742, a PID field
743, a service provider field 744, a class field 745, a user
capabilities field 746, a second spare field 747, an FEC field 748,
and an FCW field 749. In a preferred embodiment, the header field
741 is 17 bits long, the first spare field 742 is 16 bits long, the
PID field 743 is 72 bits long, the service provider field 744 is 16
bits long, the class field 745 is 16 bits long, the user
capabilities field 746 is 16 bits long, the second spare field 747
is 32 bits long, the FEC field 748 is 32 bits long, and the FCW
field 749 is 16 bits long, for a total of 233 bits.
[0104] The header field 741 identifies the message type and is more
fully described in FIG. 8B. The PID field 743, FEC field 748, and
FCW field 746 are similar to the PID field 724, FEC field 729, and
FCW field 730, respectively, described with respect to FIG. 7E. The
service provider field 744 identifies the PCS service provider that
the user station 102 wishes to use. The class field 745 specifies
some of the operational parameters being used by the particular
user station 102. The class field 745 may comprise a class type
sub-field and a class information sub-field. The class type
sub-field indicates the user station class type (e.g., DCS1900
class type, or IS-41 class type, etc.), and may also provide an
indication of the power level capability of the user station 102.
The class information sub-field provides operational information
including, for example, revision level, available encryption
algorithms, short message capability, ellipsis notation and phase-2
error handling capability, power class, continuous/discontinuous
transmission, bandwidth (e.g., 20 MHz or 25 MHz), and nominal power
levels. The class type sub-field may, for a GSM-oriented system,
indicate the power level capability of the user station 102. The
user capabilities field 746 identifies the features present in the
user station 102 (e.g., whether the user station 102 can receive a
fax or data connection, whether the user station 102 is capable of
ciphering, etc.).
[0105] FIGS. 8A and 8B are diagrams of preferred polling message
header formats. FIG. 8A is a diagram of a polling message header
format for a base polling message (such as general poll message 401
or specific poll message 402 of FIG. 4). The polling message header
801 comprises a base/mobile indicator (B/M) flag 802, an extended
protocol (E) flag 803, a packet type field 804, a power adjustment
(PWR) field 805, a symmetry field 806, a D-channel suppression
(DCS) flag 807, a virtual slot (VS) flag 808, a slot or channel
utilization (CU) field 809, a slot pointer field 810, a error check
and correct (ARQ) field 811, and a header frame control word (HCF)
field 812. In a preferred embodiment, the B/M indicator flag 802, E
flag 803, PWR field 805, DCS flag 807, and the VS flag 808 are each
1 bit long, the packet type field 804 and symmetry field are each 2
bits long, the CU field 809 and ARQ field are each 3 bits long, and
the slot pointer field 810 and header HCF field 812 are each 4 bits
long, for a total of 23 bits. A twenty-fourth bit of the header 801
is used for the purpose of assisting establishment of the RF
link.
[0106] The B/M indicator flag 802 indicates whether the originator
of the message is a user station 102 or the base station 104. The E
flag 803 is used to indicate whether or not an extended protocol is
in use. The packet type field 804 specifies which of four packet
types is being used, according to Table 8-1A below.
1TABLE 8-1A Packet Field Packet Type 00 Normal traffic 01 Specific
poll 10 Control (signaling) traffic 11 General poll, or general
response
[0107] The packet type field 804 also provides an indication of the
usage of the D-field 557, according to Table 8-1B below.
2TABLE 8-1B Packet Field D-Field Usage 00 D-Channel 01 Correlative
ID 10 Correlative ID 11 Reserved
[0108] The PWR field 805 is a serialized bit stream from the base
station 104 to the user station 102 allowing control of the power
level of the user station 102 transmitter. As each base-to-user
message is received at the user station 102, the PWR bit from the
last message is analyzed along with the current PWR bit to
determine if the power level of the user station 102 transmitter
should be raised, lowered or remain unchanged. Power control action
therefore requires that at least two consecutive base-to-user
messages be received by the user station 102 before any action is
taken. The action taken is dictated according to Table 8-2
appearing below.
3 TABLE 8-2 Last Bit Current Bit Action 0 0 Decrease transmitter
power 1 1 Increase transmitter power 0 1 Leave power unchanged 1 0
Leave power unchanged missing any Leave power unchanged any missing
Leave power unchanged
[0109] The amount of power increase or decrease carried out in
response to receiving commands in the PWR field 805 may be a fixed
or preset amount--e.g., 1 dB for each time frame 301 (or more
frequently if the user station 102 is transmitting in multiple time
slots 302 per time frame 301). Using only a single bit for the PWR
field 805 saves space in the header 553 of the base-to-user
message. The quality metrics generally provide sufficient feedback
to allow small power adjustment steps over time, but not sufficient
feedback to have confidence in making substantial power adjustment
steps. However, because user station transmissions are separated by
time within the general geographic region of a particular base
station 104, strict power control of the user stations 102 is not
required to avoid intracell or intercell interference as it
typically is with CDMA systems not employing time division
techniques.
[0110] The symmetry field 806 is used by the base station 104 to
grant bandwidth to the user station 102. The bandwidth grant
applies to the next time slot 302 (or 618) in the channel. The
symmetry field 806 contents may be interpreted according to Table
8-3 below.
4TABLE 8-3 Symmetry Bits Meaning 00 Symmetric bandwidth grant. Each
direction has been granted one half of the bandwidth. 01 The
maximum bandwidth has been granted to the user station 102, and the
minimum bandwidth has been granted to the base station 104. 10 The
maximum bandwidth has been granted to the base station 104, and the
minimum bandwidth has been granted to the user station 102. 11
Broadcast mode. The entire bandwidth has been granted to the base
station 104. There is no user station 102 packet.
[0111] The DCS flag 807 indicates the usage of the D-channel for
the current message. The DCS flag 807 is set to one value to
indicate that the D-channel is disabled to reserve it for use by
the application using the bearer channel (B-channel), and is set to
another value to indicate that the D-channel is enabled for other
usage. The VS flag 808 indicates whether the base station 104 is
using a virtual slot mode. If the virtual slot mode is active
(e.g., the time slot structure of FIG. 6 is used), then all user
station 102 transmissions occur one time slot earlier than if the
VS mode is inactive.
[0112] The CU field 809 indicates the relative slot utilization for
the base station 104. In a preferred embodiment, the CU field
contents are defined according to Table 8-4 below.
5TABLE 8-4 CU Field Contents Utilization 000 No channels available:
Find another base station 001 One channel available: 911 calls only
010 Two channels available: 911 calls or handover only 011 Few
channels available: Class control is in effect for registrations
and originations 100 Nearly full: Access Unrestricted 101
Moderately full: Access Unrestricted 110 Partially full: Access
Unrestricted 111 All slots available: Access Unrestricted
[0113] Where class control is in effect for registrations and call
originations, access leveling and load leveling classes may be
identified in the facility field 707 of the general poll message
(see FIG. 7A).
[0114] The slot pointer field 810 contains an index which
identifies the next time slot to be used in the current base/user
packet exchange. The user station 102 transmits in the time slot
indicated by the slot pointer to continue the exchange. In a
particular embodiment, the contents of the slot pointer field 810
may take on any of sixteen different values (e.g., binary 0 to 15),
with each value indicating a different relative number of time
slots from the present time slot in which the user station 102 is
to transmit. For example, a value of zero means that the user
station 102 is to transmit in the same slot (in the next frame if
at a regular bandwidth rate, or several frames in the future if
using a sub-frame rate). A value of one means that the user station
102 is to transmit in the next time slot of the present time frame.
A value of two means that the user station 102 is to transmit in
the time slot two places ahead in the present time frame, and so
on. Examples of operation using slot pointers are described further
below.
[0115] The ARQ field 811 allows the receiving entity (either base
station 104 or user station 102) to correct a message error. The
ARQ field 811 comprises three subfields of one bit each: (1) an
"ARQ required" sub-field that indicates whether or not ARQ is
required for the message sent; (2) an "ACK" sub-field indicating
whether or not the sender of the message received correctly the
last message sent; and (3) a "message number" sub-field, which
indicates the message number (zero or one) of the current message.
The ACK sub-field and message number sub-field are always used
regardless of whether the ARQ required bit is set.
[0116] If ARQ is required (as determined by the value of the ARQ
required bit), then the receiving entity performs the following
steps:
[0117] (1) Compares the message number sub-field of the received
message with the message-number sub-field of the previously
received message; if they are the same, the new message is
ignored.
[0118] (2) Checks the ACK sub-field of the received message. If the
value is NAK (indicating that the sender of the message did not
receive the last message correctly), then the receiving entity
resends the old data message; otherwise, it sends a new data
message.
[0119] (3) Complements the message number sub-field bit each time a
new data message is sent.
[0120] (4) If a message is received with a FCW error (as explained
with respect to FIG. 7A), or did not receive a message at all, then
the receiving entity sends its data message with the ACK sub-field
set to NAK.
[0121] The header HCF field 812 is used for a cyclic redundancy
check calculated over the preceding bits of the message header.
[0122] FIG. 8B is a diagram of a polling message header format for
a poll response message (such as general poll response 404 or
specific poll response 405 of FIG. 4). The polling response header
820 comprises a base/mobile indicator (B/M) flag 821, an extended
protocol (E) flag 822, a packet type field 823, a PWR field 824, a
symmetry field 825, a DCS flag 826, a spare field 827, an ARQ field
828, and a header frame control word (HCF) field 829. In a
preferred embodiment, the B/M indicator flag 821, E flag 822, and
DCS flag 826 are each 1 bit long, the packet type field 823,
symmetry field 825, and spare field 827 are each 2 bits long, the
ARQ field 828 is 3 bits long, and the HCF field 829 is 4 bits long,
for a total of 17 bits.
[0123] The B/M indicator flag 821, E flag 822, packet type field
823, PWR field 824, DCS flag 826, ARQ field 828 and HCF field 829
are used for the same purposes as their counterpart fields in the
base station header shown in FIG. 8A. The contents of the symmetry
field 825 in the user station 102 header may be interpreted
according to Table 8-5 below.
6TABLE 8-5 Symmetry Field Meaning 00 Symmetric bandwidth is
requested for the next time slot 01 Maximum bandwidth is requested
for the next time slot 10, 11 (Not presently used)
[0124] In one embodiment in accordance with the header formats of
FIGS. 8A and 8B, the message headers shown in Table 8-6 correspond
to the message types shown (where "1" and "0" are bit values, and
"X" is a bit value that is irrelevant or depends upon the
application and/or system status).
7TABLE 8-6 Message Type Header Contents BS General Poll 1X11 XXXX
XXXX XXXX XXXX XXX BS Specific Poll 1X01 XXXX XXXX XXXX XXXX XXX BS
Control Traffic 1X10 XXXX XXXX XXXX XXXX XXX BS Traffic Message
1X00 XXXX XXXX XXXX XXXX XXX MS General Response 0X11 XXXX XXXX
XXXX X MS Specific Response 0X01 XXXX XXXX XXXX X MS Control
Traffic 0X10 XXXX XXXX XXXX X MS Traffic Message 0X00 XXXX XXXX
XXXX X
[0125] FIG. 13A is a diagram of a base station information packet
showing in octet format fields generally depicted in FIGS. 5B and
8A. FIG. 13B is a diagram of a user station information packet
showing in octet format fields generally depicted in FIGS. 5C and
8B.
[0126] Data may be transmitted between the base station 104 and
user stations 102 using an M-ary spread spectrum technique.
Suitable M-ary spread spectrum transmission and reception
techniques are described in, e.g., U.S. Pat. No. 5,022,047 and in
U.S. Pat. No. 5,692,007, both of which are assigned to the assignee
of the present invention, and both of which are hereby incorporated
by reference as if set forth fully herein. In a preferred
embodiment, the base station 104 and user stations 102 each
transmit M-ary direct sequence spread spectrum signals using spread
spectrum codes (called "symbol codes") of 32 chips. Preferably, N
data bits are transmitted per symbol code, with M different symbol
codes are used to represent up to M different data symbols, where
M=log.sub.2 N. In a preferred embodiment, thirty-two different
symbol codes are used to represent thirty-two different data
symbols, each comprising five bits of data, and differential phase
encoding is used to allow transmission of a 6th bit of data for
each symbol code. Techniques of phase encoding for transmission of
an additional bit of information per symbol code are described in,
e.g., U.S. Pat. No. 5,692,007 referred to above.
[0127] Because the base header field 553 is positioned first in the
base transmit data frame 551, it "loses" the first bit from the
first transmitted data symbol (which is transmitted using a
differential encoding technique) because it is used as a phase
reference bit. Thus the base header field 553, which comprises four
data symbols, is 23 bits in length. The first data symbol comprises
five data bits, and the latter three data symbols each comprises
six data bits. Likewise, because the user header field 523 is
positioned first in the user transmit data frame 521, it "loses"
the first bit from the first transmitted data symbol because it is
used as a phase reference bit. Thus the user header field 523,
which comprises three symbols, is 17 bits in length. The first data
symbol comprises five data bits, and the latter two data symbols
each comprises six data bits.
[0128] Signaling messages (i.e., messages used for control traffic)
may be used to assist in acquisition and maintenance of a channel
from the network. Over-the-air signaling messages may commence with
a "message type" data element located in a message type field. The
message type data element defines the format of the rest of the
message, and acts as an operation code to the destination unit
(either user station 102 or base station 104). Exemplary message
types for over-the-air signaling (i.e., control traffic) messages
appear in Table 9-1 below.
8TABLE 9-1 ACK Acknowledge ANS Answer Incoming Call AUT
Authentication Request AUR Authentication Response BAI Base Assist
Information CIP Set Cipher Mode CNC Call Connected CSC Circuit
Switch Complete DRG De-registration Request DRP Drop Incoming
Connection HLD Hold ORH Originating Handover Request ORG Originate
Call RCP Registration Complete RRQ Registration Request SET Set
Services SPR Specific Response SYN Synchronize THR Target handover
Request TRA Transport Message
[0129] The number of bits of the message type data element used to
identify the type of message depends mainly upon the number of
control traffic message supported by the system. In a preferred
embodiment, the message type is 8 bits in length. Additional
information needed to process or act upon the message may be
contained in other fields in the signaling message.
[0130] Messages exchanged between the base station 104 and base
station controller 105 or other network entities can be mapped to a
local or internal format referred to as "Notes". Some of these
Notes may resemble the over-the-air signaling messages exchanged
between the base station 104 and the user station 102, in order to
expedite processing of the control traffic messages. The base
station controller 105 may act as a protocol interface whereby
signaling messages are translated to a form compatible with the
mobile switching center 112 and/or network.
[0131] The general content of certain over-the-air signaling
messages that play a role in handover and related functions are set
forth in the tables appearing below. The message content may be
viewed as an aspect of "layer three" protocol architecture.
9TABLE 10-1 Hold (CT-HLD) Information Element Length in Bits
Message Type 8 Reserved 152
[0132] Hold (CT-HLD)control traffic messages can be transmitted
either by the base station 104 or the user station 102. They are
generally part of a larger signaling traffic exchange. The user
station 102 sends a CT-HLD control traffic message to the base
station 104 when the user station 102 requires more time to process
data and return a result to the base station 104, or when
responding to a CT-HLD control traffic message from the base
station 104.
10TABLE 10-2 Acknowledge (CT-ACK) Information Element Length in
Bits Message Type 8 ACK Response 8 Ack'd Command 8 Ack State 8
Reserved 128
[0133] Acknowledge (CT-ACK) control traffic messages can be
transmitted by either the base station 104 or the user station 102.
It is not necessary the every exchange of control traffic messages
end with a CT-ACK message.
[0134] The Ack Response information element of the CT-ACK message
contains an acknowledgment response indicator. One of two binary
values (i.e., a "0" bit) indicates success, while the other of the
two binary values (i.e., a "1" bit) indicates failure. The Ack'd
Command information element contains the Message Type of the
specific command being acknowledged. The Ack State information
element contains the current state of the system element (i.e., the
base station 104 or user station 102) which is transmitting the
acknowledge.
11TABLE 10-3 Set Cipher Mode (CT-CIP) Information Element Length in
Bits Message Type 8 Cipher Type 8 Cipher Mode 8 Initialization
Vector 64 Cause Type 8 Cause 8 Reserved 56
[0135] A Set Cipher Mode (CT-CIP) control traffic message is
transmitted from the base station 104 to the user station 102 to
pass pertinent ciphering information to the user station 102 and to
instruct the user station 102 to go into or out of ciphering mode.
When the user station 102 receives the CT-CIP message, the user
station 102 uses the cipher mode parameters to set its ciphering
equipment and then switches into or out of ciphering mode. All
traffic after the switch to cipher mode will be ciphered.
[0136] The Cipher Type information element of the CT-CIP message
indicates the type of encryption to be used by the system (e.g.,
either DCS-1900 or Bellcore "C", for example). The Cipher Mode
information element indicates the encryption mode being requested
by the system. The Initialization Vector information element
contains a value to be used in conjunction with other keying
information to initialize the encryption equipment. The Cause
information element consists of eight bits of an encoded parameter
indicative of what the cause is of an action, and is specific to a
particular control traffic message. For the CT-CIP message, the
Cause information field can be set to contain a code indicating
such things as set/change cipher or synchronize cipher. The Cause
Type information element defines the cause code set to be returned
when either the base station 104 or the user station 102 drops a
connection. The Cause Type is stored as an encoded value that
identifies the code set of the supporting infrastructure. For
example, the Cause Type information field can be set to contain a
value indicating the use of DCS 1900 cause codes or a value
indicating the use of Bellcore Generic "C" cause codes.
12TABLE 10-4 Call Origination (CT-ORG) Information Element Length
in Bits Message Type 8 Service Request 32 Key Sequence Number 8
Class 16 CREF 8 Reserved 88
[0137] The user station 102 sends a Call Originate (CT-ORG)control
traffic message to the base station 104 to request the placement of
an outgoing call.
[0138] The Service Request information element of the CT-ORG
message indicates such things as data versus voice service, use of
CRC and ARQ, symmetry or asymmetry of the channel, whether service
resources are being requested, and frame rate, for example. The Key
Sequence Number information element is used to generate a
communication key in both the base station 104 and the user station
102 without having to explicitly pass the key over the air. The
Class information element specifies some of the operational
parameters of the particular type of user station 102. The Class
information element can be broken down into sub-fields of Class
Type and Class Information. The Class Type sub-field may indicate
the general class of the user station 102 (e.g., DCS1900 or IS-41),
while the Class Information sub-field may indicate such things as
protocol or revision level, encryption algorithm, RF power rating,
power class, continuous or discontinuous transmission, and licensed
or unlicensed bandwidth. The Call Reference ("CREF") information
element specifies the circuit to which data in a transport message
belongs. The CREF field corresponds to the ISDN Call Reference
information element. The CREF information element may contain a
value indicating whether the circuit is, for example, ISDN, DCS1900
or DECT.
13TABLE 10-5 Call Connect (CT-CNC) Information Element Length in
Bits Message Type 8 Connection Number 40 Map Type 8 Map 32 Cause
Type 8 Cause 8 CREF 8 Reserved 48
[0139] The Call Connect (CT-CNC) control traffic message may be
sent from the base station 104 to the user station 102 when a call,
either incoming or outgoing, is completed or when an outgoing call
from the user station 102 is rejected.
[0140] The Connection Number information element of the Call
Connect message specifies the specific network connection which was
allocated to carry the bearer channel of the particular user
station 102 from the base station 104 to the network. Unused
nibbles and octets of this information element are filled with "F"
hex. The Map information element describes the mapping of time
slots to a particular user station 102. The format of the Map
element is dependent upon the Map Type information element in the
same frame. The Map Type information element indicates if the frame
is a "superframe" (aggregated time slots) or "subframe" (single
time slot occurring every N time frames). If a superframe map type,
then each bit in the Map information element corresponds to a
channel relative to the current channel. If a subframe map type,
the Map information element indicates such things as the
submultiplex rate (i.e., the number of frames skipped between
transmissions), the frame phase (i.e., the number of frames skipped
before the first transmission), and the channel phase (i.e., the
number of time slots or channels skipped before the first
transmission). The Cause and Cause Type information elements are as
described with respect to the CT-CIP message. However, for the
CT-CNC message, the Cause information element indicates whether or
not the requested connection has been connected. The CREF
information element is the same as described with respect to the
CT-ORG message.
14TABLE 10-6 Target Handover Request (CT-THR) Information Element
Length in Bits Message Type 8 Old Connection Number 40 Service
Request 32 Key Sequence Number 8 Class 16 Old Base Station ID 32
Old Mobility Country Code (MCC) 16 Old Mobility Network Code (MNC)
8
[0141] The Target Handover Request (CT-THR) control traffic message
is sent from the user station 102 to the target base station 104 to
initiate a terminating handover procedure.
[0142] The Old Connection Number information element of the CT-THR
message specifies the specific network connection which was
allocated to carry the bearer channel of the user station 102 from
the old base station 104 to the network. Unused nibbles and octets
of this information element are filled with "F" hex. The Service
Request, Key Sequence Number and Class information elements are as
described with respect to the CT-ORG message. The Old Base Station
ID information element identifies the originating base station 104
in a handover. The Old MCC information element indicates the
mobility country code of the originating base station in a
handover, and the Old MNC information element indicates the
mobility network code of the originating base station in the
handover.
15TABLE 10-7 Originating Handover Request (CT-OHR) Information
Element Length in Bits Message Type 8 Base ID 32 Mobility Country
Code (MCC) 16 Mobility Network Code (MNC) 8 Reserved 56
[0143] The originating Handover Request (CT-OHR) control traffic
message is sent from the user station 102 to the current base
station 104 to initiate an originating handover procedure.
[0144] The Base ID information element uniquely identifies the
target base station 104. The MCC and MNC information elements
indicate the mobility country code and the mobility network code,
respectively, of the target base station 104.
16TABLE 10-8 Circuit Switch Complete (CT-CSC) Information Element
Length in Bits Message Type 8 Handover Reference 48 Map Type 8 Map
32 Reserved 56
[0145] The Circuit Switch Complete (CT-CSC) control traffic message
is sent from the old base station 104 to the user station 102 to
signal that the network connection is available at the target base
station 104. When sent from the old base station 104, the Map
information element will be all zeroes to indicate that there are
no longer any slots on the old base station 104 for the user
station 102 to utilize.
[0146] The Handover Reference information element is used to
identify a specific handover process that has already been
initiated by an originating handover request sequence. In a DCS1900
infrastructure system, the handover reference number is assigned by
the terminated base station controller 105. The Map Type and Map
information elements are as described with respect to the CT-CNC
message.
17TABLE 10-9 Terminating Handover Complete (CT-THC) Information
Element Length in Bits Message Type 8 Service Request 32 Key
Sequence Number 8 Class 16 Handover Reference Number 48 Reserved
48
[0147] A Terminating Handover Complete (CT-THC) control traffic
message is sent by the user station 102 to the target base station
104 to initiate a terminating handover procedure.
[0148] The Service Request, Key Sequence Number, and Class
information elements are as described for the CT-ORG message. The
Handover Reference Number information element is as described for
the CT-CSC message.
18TABLE 10-10 Specific Response (CT-SPR) Information Element Length
in Bits Message Type 8 Cipher Type 8 Cipher Mode 8 Key Info 64
Class 16 Reserved 56
[0149] The Specific Response (CT-SPR) control traffic message is
sent from the user station 102 to the base station 104 when the
user station 102 is listening for a page and receives a Specific
Poll control traffic message which contains the user station's PID
and which is marked as a "paging" Specific Poll message.
[0150] The Cipher Type and Cipher Mode information elements are as
described for the CT-CIP message. The Key Info information element
contains a value to be used in conjunction with other keying
information to initialize the encryption equipment, and the
contents of this field depend upon the specific type of supporting
infrastructure (e.g., DCS1900). The Class information element is as
described for the CT-ORG message.
19TABLE 10-11 Set Service (CT-SET) (user to base) Information
Element Length in Bits Message Type 8 Reserved 80 Map Type 8 Map
32
[0151] Service Request
[0152] The user station 102 sends a Set Service (CT-SET) control
traffic message to the base station 104 when the user station 102
desires to change the characteristics of the over-the-air
service.
[0153] The Map Type and Map information elements are as described
for the CT-CNC message. The Service Request information element is
as described with respect to the CT-ORG message.
20TABLE 10-12 Set Service (CT-SET) (base to user) Information
Element Length in Bits Message Type 8 Cause Type 8 Cause 8 Connect
Number 40 Reserved 24 Map Type 8 Map 32 Service Request 32
[0154] The base station 104 sends a Set Service (CT-SET) control
traffic message to the user station 102 when the base station 104
wishes to changes the characteristics of over-the-air service.
[0155] The Connection Number, Map Type and Map information elements
of the CT-SET message are as described for the CT-CNC message. The
Cause Type and Cause information elements are as described for the
CT-CIP message. However, the Cause information element for the
CT-SET message indicates whether the link was successfully
established or else failed.
21TABLE 10-13 Release (CT-REL) Information Element Length in Bits
Message Type 8 Cause Type 8 Cause 8 Reserved 136
[0156] The Release (CT-REL)control traffic message is sent by the
base station 104 to the user station 102 when the network releases
the connection in progress or during link setup. The Cause Type and
Cause information elements are as described for the CT-CIP message.
However, the Cause information element for the CT-REL message
indicates whether the release was initiated by the network, or
whether an authentication rejection occurred.
22TABLE 10-14 Base Assist (CT-BAM) Information Element Length in
Bits Message Type 8 Base Assist Information 152
[0157] The Base Assist (CT-BAM) control traffic message is sent by
the base station 104 to the user station 102 whenever the base
station 104 desires to pass information to the user station 102
which will help the user station 102 in making well informed
decisions. The contents of the Base Assist information element vary
depending upon the circumstances.
23TABLE 10-15 Transport (CT-TRA) Information Element Length in Bits
Message Type 8 Transport Data 56
[0158] The Transport (CT-TRA) control traffic message is used for
transporting data between the base station 104 and the user station
102 on the circuit specified by the Call Reference Number (CREF).
The contents of the Transport Data information element varies
depending upon the application, and generally constitutes
application level data.
[0159] Transport control traffic messages differ from other control
traffic messages in that the Message Type information element
contains additional information. The format of the Message Type
field for Transport messages is as follows:
24TABLE 10-15A Message Type Header for Transport Header Element Bit
Position(s) Transport Bit 8 ACK/NAK 7 Message Number 6 CREF 1-5
[0160] The Transport Bit indicates whether or not the message is a
Transport message. The ACK/NAK bit indicates whether or not the
sender received the last message without error. The Message Number
bit indicates the message number (0 or 1) of the current message,
and should alternate for each message sent by the same entity. The
Call Reference identifies the call.
[0161] The values passed as part of Message Type information
element allow the receiving entity (base station 104 or user
station 102) to correct a message error. In one embodiment, the
following steps are undertaken to attempt to correct a message:
[0162] 1) The receiving entity compares the Message Number of the
received message with the Message Number of the previously received
message. If they are the same, the receiving entity ignores the new
message.
[0163] 2) The receiving entity checks the ACK/NAK field of the
received message. If the value is NAK, it resends the old packet,
and if the value is ACK, it sends the new packet.
[0164] 3) Each sender complements the message number each time a
new packet is sent.
[0165] 4) If the receiving entity receives a message with a FCW
error, or if it does not receive a message at all, it resends the
old packet with the NAK bit set.
[0166] In addition to the above messages, various signaling
messages may be used between the base station and the network to
convey information at the call control entity level. Exemplary call
control messages include those appearing in Table 9-2 below.
25 TABLE 9-2 Direction Call Establishment Messages CC-SETUP Both
CC-INFOrmation Both CC-CALL-PROCeeding Network -> User
CC-ALERTING Both CC-PROGress Network -> User CC-CONNECT Both
CC-CONNECT-ACKnowledge Both CC-EMERGENCY-SETUP User -> Network
CC-CALL-CONFIRMED User -> Network Call Release Messages
CC-DISConnect Both CC-RELEASE Both CC-RELEASE-COMplete Both Call
Related Supplementary Services HOLD User -> Network
HOLD-ACKnowledge Network -> User HOLD-REJECT Network -> User
RETRIEVE User -> Network RETRIEVE-ACKnowledge Network -> User
RETRIEVE-REJECT Network -> User DTMF Interaction Start-DTMF User
-> Network Stop-DTMF User -> Network Start-DTMF-ACK Network
-> User Stop-DTMF-Ack Network -> User Start-DTMF-Reject
Network -> User
[0167] The interplay among the various entities involved in the
transfer of signaling messages and other information may be better
understood by reference to FIG. 21, which depicts a preferred
system protocol architecture. As illustrated in FIG. 21, a
preferred user station 102 (designated "MS" in FIG. 21) includes a
Communication Management ("CM") entity, a Mobility Management
("MM") entity, and a Radio Resources ("RR") entity, among others.
The CM and MM entities of the user station 102 communicate with
their counterparts at a mobile switching center 112 (designated
"MSC" in FIG. 21), via links connected across a base station 104
(designated "BS" in FIG. 21) and base station controller 105
(designated "BSC" in FIG. 21). The various types of signaling
interfaces of a preferred embodiment are shown in FIG. 21 by the
arrows connecting like entities.
[0168] The "Layer 3" protocol exchange between the mobile switching
center 112 and the base station controller 105 is characterized by
the BSSMAP protocol. The "Layer 3" protocol exchange between the
mobile switching center 112 and the user station 102 is
characterized by the Direct Transfer Application Part (DTAP). DTAP
is further divided into two logical sublayers, defined by the CM
and MM entities described above. The CM includes call control and
supplementary services management, including short message
service.
[0169] Most DTAP messages are not interpreted by the base station
controller 105 or the base station 104. Rather, they are
transferred to the network by the mobile switching center 112 over
a network interface (such as the GSM A-interface). Most radio
resource (RR) messages are mapped to BSSMAP messages at the base
station controller 112. However, some of these messages are
interpreted by the base station 104 (e.g., paging messages). The
control management (CM) part of the protocol is addressed by an
ISDN based CM message set, referred to as IGCC (ISDN Generic Call
Control). Control management messages from the user station 102 are
directly transferred to the network over the interface at the
mobile switching center 112. Interface adapters at the user station
102 and the base station controller 105 segment control management
(i.e., IGCC) messages into packets, which are individually
transported between the user station 102 and the base station 104
via CT-TRA Control Traffic messages and between the base station
104 and base station controller 105 via Transport Notes. Notes are
transmitted over a CCITT ISDN data link (Q.920/Q.921) The interface
adapters at the user station 102 and base station controller 105
are responsible for ensuring that the packets are sequenced
properly and the entire IGCC message is error free.
[0170] Radio resource (RR) messages and mobility management (MM)
messages take the form of internal Notes between the base station
controller 105 and base station 104, and are mapped at the base
station to over-the-air messages when sent to the user station
102.
[0171] Exemplary message flow diagrams for various calling
functions are shown in FIGS. 9, 10, 11A-11C and 12A-12B. While
generally described with respect to features referenced in the FIG.
3 embodiment, they have equal applicability to the FIG. 6
embodiment.
[0172] An exemplary message flow diagram for call origination from
a user station 102 is shown in FIG. 9. In FIG. 9 messages are
designated by arrows (1) between a user station 102 (abbreviated
"MS") and a base station 104 (abbreviated "BS"), (2) between the
base station 104 and a base station controller 105 (abbreviated
"BSC"), and (3) between the base station controller 105 and a
mobile switching center 112 (abbreviated "MSC"). The MSC 112
generally acts a switch controlling access to the network 106 (as
shown, e.g., in FIG. 2). Control traffic messages between the user
station 102 and the base station 104 are typically preceded by the
initials "CT". The steps numbered 1 through 17 associated with the
arrows appearing in FIG. 9 are explained below:
[0173] 1. A user station application sends a call originate request
to the user station 102 over-the-air controller.
[0174] 2. The user station 102 seizes an available time slot (such
as, for example, time slot 302 in FIG. 3 or virtual time slot 618
in FIG. 6) in accordance with the protocol shown in FIG. 4 and/or
4A. If no time slot is acquired, the user station 102 times out and
attempts to register and then acquire a time slot on another base
station 104.
[0175] 3. Upon successful time slot acquisition, the user station
102 responds to the specific poll message 402 with an ORIGINATE
control traffic (CT-ORG) message. The CT-ORG message includes
circuit reference (CREF) information.
[0176] 4. The base station 104 responds to the CT-ORG message by
sending a control traffic acknowledgment (CT-ACK) message back to
the user station 102. (If no CT-ACK message is received from the
base station 104, the link is dropped and the user station 102
attempts to originate a call on another base station 104.) The base
station 104 assigns time slots and terrestrial bearer channels to
support the service request (if possible) and then sends a Setup
Link NOTE containing the terrestrial bearer information to the base
station controller 112. If, however, the base station 104 is unable
to assign time slots or bearer channels, it returns a control
traffic setup (CT-SET) message indicating a failure status to the
user station 102, and no Setup Link NOTE is sent to the base
station controller 105. The user station 102 then attempts to
originate a call using another base station 104.
[0177] 5. When the base station controller 105 receives the Setup
Link NOTE, it builds a signaling connection control part (SCCP)
channel for the user station 102 based on the PID of the user
station 102. The base station controller 105 also retains the Setup
Link NOTE parameters for use in a later step. If construction of
the SCCP channel fails, a Connect Link NOTE is sent to the base
station 104 indicating a failure status. The base station 104 then
responds by sending a control traffic Set Service (CT-SET) message
to the user station 102 indicating the link failure (see Step 7
below). The link failure is communicated to the user station
application via a Connect message (see Step 10 below).
[0178] 6. After the CT-ACK message is received at the user station
102, the user station 102 and the base station 104 enter a HOLD
sequence while waiting for a link to be established between the
base station 104 and the base station controller 105. During this
sequence, the user station 102 and base station 104 periodically
exchange control traffic HOLD (CT-HLD) messages. If the base
station 104 and/or user station 102 unexpectedly stops receiving
CT-HLD messages or the user station 102 does not subsequently
receive a CT-SET message from the base station 104 after the CT-ORG
message has been sent, then the base station 104 disconnects the
link from the base station controller 105 using call clearing
procedures, and the user station 102 and base station 104 attempt
lost link recovery. If the lost link recovery procedure is
successful, then call origination from the user station 102 is
re-initiated.
[0179] 7. When the SCCP channel is constructed, the base station
controller 105 sends a Connect Link NOTE to the base station 104.
The Connect Link NOTE includes status information from the base
station controller 105.
[0180] 8. The base station 104 then sends a control traffic SET
SERVICE (CT-SET) message to the user station 102. This message
defines the slot structure (i.e., mapping) to be used by the user
station 102. The CT-SET message includes the status information
contained in the Connect Link message received from the base
station controller 105.
[0181] 9. The user station 102 acknowledges receipt of the CT-SET
message by responding with a control traffic acknowledgment
(CT-ACK) message. If the base station 104 does not receive the
CT-ACK message, then the base station 104 disconnects the link from
the base station controller 105 and attempts lost link recovery in
a manner similar to that described with respect to Step 6
above.
[0182] 10. The user station 102 responds to the CT-SET message by
sending a Connect message to the user station application. The
Connect message indicates to the user station application whether
or not the control link has been established.
[0183] 11. The user station 102 and base station 104 then enter a
HOLD sequence by exchanging control traffic hold (CT-HLD) messages,
a condition which is sustained as long as no IGCC Setup message
traffic is being transported from or to the user station
application (see Step 12). If the communication link is lost
between the base station 104 and the user station 102 after the
Connect message has been sent to the user station application, then
the base station disconnects the link from the base station
controller 105 using call clearing procedures. The user station 102
sends a Link Lost message to the user station application. Any
message from the user station application that does not initiate a
new operation will cause the user station 102 to respond with
another Link Lost message.
[0184] 12. The user station application sends an ISDN generic call
control ("IGCC") Setup message through the user station 102 and
base station 104 to the base station controller 105 via control
traffic Transport (CT-TRA) messages and Transport NOTES. The user
station 102 and base station 104 return to the hold sequence
whenever no IGCC messages are available for transport.
[0185] 13. The base station controller 105 sends a Service Request
message to the mobile switching center 112 via a Complete L3 Info
DTAP message.
[0186] 14. The mobile switching center 112 responds to the base
station controller 105 with a call management (CM) Service Accept
DTAP message.
[0187] 15. The user station application completes the call setup
via end-to-end IGCC based call procedures.
[0188] 16. Once the IGCC Call Control has the call established, the
user station application sends a Begin Traffic request to the user
station 102.
[0189] 17. The system enters normal traffic mode, and the
conversation (if voice) or other data path is stable.
[0190] An exemplary message flow diagram for processing a call
originating from the network and terminating at a user station 102
is shown in FIG. 10. In FIG. 10 are shown abstractly by arrows,
similar to FIG. 9, messages between a user station 102 (abbreviated
"MS") and a base station 104 (abbreviated "BS"), between the base
station 104 and a base station controller 105 (abbreviated "BSC"),
and between the base station controller 105 and a mobile switching
center 112 (abbreviated "MSC"). Control traffic messages between
the user station 102 and the base station 104 are typically
preceded by the initials "CT" in FIG. 10. The steps numbered 1
through 33 associated with the arrows appearing in FIG. 10 are
explained below:
[0191] 1. The mobile switching center 112 originates a call by
sending a BSSMAP PAGING message to the base station controller 105.
The BSSMAP PAGING message is sent as a "connectionless" message to
the base station controller 105, and includes the personal
identifier (PID) of the user station 102 being paged.
[0192] 2. The base station controller 105 searches its Location
Register (LR) for the entry of an international mobile station
identifier (IMSI) matching the PID sent in the BSSMAP PAGING
message. If the matching user station PID is not found, then the
base station controller 105 does not respond to the mobile
switching center 112 and the call is dropped.
[0193] 3. If the base station controller 105 identifies the
appropriate entry, the base station controller 105 sends a Page
NOTE to the base station 104 associated with the entry. The service
request type in the Page NOTE is set to zero (indicating that a
NULL service is being requested). The Page Note allows the base
station 104 to page the user station 102 without having to set up a
specific call.
[0194] 4. When the base station 104 receives a Page NOTE with a
NULL service request, the base station 104 sends a SPECIFIC POLL
message (e.g., specific poll message 402 in FIG. 4) with service
type set to zero (indicating a NULL request). The base station 104
queues the Page NOTES and sends SPECIFIC POLL messages
corresponding to the Page NOTES on a cyclic basis. If there are
more Page NOTES than there are available unused time slots (time
slots), the base station 104 sends the SPECIFIC POLL messages
sequentially on the available time slots. Consequently, the
SPECIFIC POLL messages may be spread out over many time frames
(polling loops). The base station 104 continues to issue the
SPECIFIC POLL message until either the user station 102 responds,
or until predetermined time period T.sub.page associated with the
Page NOTE expires (as measured by an internal timer).
[0195] 5. The user station 102 alternates between an inactive or
sleep state and an active state with a predetermined duty cycle.
When the user station 102 wakes up, it scans all SPECIFIC POLL
messages from the base station 104 upon which it is registered. In
one embodiment, the user station 102 scans until the same user
station PID is seen twice. If an user station 102 does not see a
SPECIFIC POLL containing its user station PID (or does not see the
user station PID twice, if applicable), then after a predetermined
monitoring time the user station 102 returns to sleep for a time
period dictated by its duty cycle.
[0196] 6. When user station 102 receives a SPECIFIC POLL control
traffic message containing its user station PID, the user station
102 responds with a SPECIFIC POLL RESPONSE control traffic (CT-SPR)
message (e.g., specific poll response 405 in FIG. 4). If no
SPECIFIC POLL is seen by the user station 102 with its user station
PID, then it goes back to sleep for a predetermined time
period.
[0197] 7. When the base station 104 receives the SPECIFIC POLL
RESPONSE control traffic message from user station 102 having the
matching user station PID, the base station 104 returns an
acknowledgment control traffic (CT-ACK) message to the user station
102, and sends a Page Response NOTE to the base station controller
105. If the base station 104 does not receive a SPECIFIC POLL
RESPONSE control traffic message from the user station 102, it does
not send a Page Response NOTE, and the call is dropped.
[0198] 8. The base station 104 and user station 102 then enter a
slot maintenance mode in which they pass HOLD control traffic
(CT-HLD) messages back and forth. If the base station 104 or user
station 102 unexpectedly stops receiving CT-HLD control traffic
messages, then the base station 104 and user station 102 attempt
lost link recovery. If lost link recovery fails, then the call is
dropped.
[0199] 9. Upon receipt of the Page Response NOTE from the base
station 104, the base station controller 105 builds an SCCP circuit
to the mobile switching center 112 and associates the SCCP circuit
with the user station PID. If the base station 104 does not receive
a Page Response NOTE, the call is dropped. Further, if construction
of the SCCP circuit fails, the Setup Link NOTE so indicates. The
base station 104 responds by sending a control traffic Set Service
(CT-SET) message to the user station 102 indicating the
failure.
[0200] 10. Once the base station controller 105 has built an SCCP
circuit to the mobile switching center 112, the base station
controller 105 sends a BSSMAP Paging Response message to the mobile
switching center 112 over the SCCP circuit.
[0201] 11. The mobile switching center 112 then sends a DTAP Setup
message to the base station controller 105, using the SCCP circuit
associated with the user station's PID.
[0202] 12. When the base station controller 105 receives the DTAP
Setup Message from the mobile switching center 112, the base
station controller 105 sends a Setup Link NOTE to the base station
104 communicating with the particular user station 102.
[0203] 13. When the base station 104 receives the Setup Link NOTE
from the base station controller 105, the base station 104 assigns
radio resources (e.g., time slots) to satisfy the service request
data element. The base station 104 then sends the base station
controller 105 a Service information NOTE detailing the bearer
channels assigned to this call. If the base station 104 does not
receive a Setup Link NOTE, then call clearing procedures are
initiated. If the base station 104 cannot supply the resources
requested by the base station controller 105, this fact is
indicated in a "result" field of the Service Information NOTE.
[0204] 14. The base station 104 communicates the service desired
and the air resources necessary to support the service to the user
station 102 using a Set Service (CT-SET) control traffic
message.
[0205] 15. The user station 102 responds to the CT-SET message by
sending an acknowledge control traffic (CT-ACK) message back to the
base station 104 and sending a Setup Link message to the user
station application. If no CT-ACK message is received by the base
station 104, call clearing procedures are initiated.
[0206] 16. The user station application responds to the Setup Link
message with a Connect Link message.
[0207] 17. The base station 104 and user station 102 then enter a
slot maintenance mode in which they pass HOLD control traffic
(CT-HLD) messages back and forth. If the base station 104 or user
station 102 unexpectedly stops receiving CT-HLD control traffic
messages, the base station 104 and user station 102 attempt lost
link recovery. If lost link recovery fails, call clearing
procedures are initiated.
[0208] 18. After the user station 102 configures itself to provide
the requested service and receives the Connect Link message from
the user station application, the user station 102 responds with a
CONNECT LINK (CT-CNL) control traffic message with the "response"
field set to indicate a successful connection. If the user station
102 cannot satisfy the service request, the user station 102
replies with the "response" field set to indicate failure. If the
user station 102 does not receive a CT-ACK message, the user
station 102 disconnects the link according to call clearing
procedures, and the call is dropped.
[0209] 19. When the base station 104 receives the CT-CNL control
traffic message, it returns a control traffic acknowledgment
(CT-ACK) message to the user station 102. Once the base station 104
has allocated all necessary channel resources, it sends a Connect
Link NOTE to the base station controller 105.
[0210] 20. The user station 102 and base station 104 enter a hold
sequence in which they exchange CT-HLD messages to maintain the
over-the-air channel. If the base station 104 or user station 102
unexpectedly stops receiving CT-HLD control traffic messages, the
base station 104 and user station 102 attempt lost link recovery.
If lost link recovery fails, call clearing procedures are
initiated.
[0211] 21. The base station controller 105 responds to the Connect
Link NOTE by returning a Connect Link NOTE back to the base station
104. The Connect Link NOTE from the base station controller 105
contains a connection number for the call.
[0212] 22. Upon receiving the Connect Link NOTE from the base
station controller 105, the base station 104 sends a CONNECT
COMPLETE (CT-CNC) control traffic message to the user station 102.
The CT-CNC message communicates the connection number for the call
to the user station 102. If the user station 102 does not receive
the CT-CNC message, or the base station 104 does not receive a
CT-ACK message in response, lost link recovery is attempted. If
lost link recovery fails, call clearing procedures are
initiated.
[0213] 23. The user station 102, as suggested in step 22,
acknowledges the CT-CNC control traffic message with a control
traffic acknowledgment (CT-ACK) message.
[0214] 24. Upon receiving the CT-ACK control traffic message, the
base station 104 sends an Acknowledge NOTE to the base station
controller 105, with the command argument set to "Connect Link," to
indicate completion of the link.
[0215] 25. The base station 104 and user station 102 then enter a
slot maintenance mode in which the pass HOLD (CT-HLD) control
traffic messages back and forth. This sequence is sustained as long
as no other message traffic is being transported to or from the
user station application. If the base station 104 or user station
102 unexpectedly stops receiving CT-HLD control traffic messages,
the base station 104 and user station 102 attempt lost link
recovery. If lost link recovery fails, call clearing procedures are
initiated.
[0216] 26. When the base station controller 105 receives the
Acknowledge NOTE from the base station 104, the base station
controller 105 initiates ISDN generic call control (IGCC) message
traffic that sets up the link with the user station 102. The base
station controller 105 uses the information from the DTAP Setup
message (see Step 11) during the IGCC setup process.
[0217] 27. Upon completion of the IGCC setup, an IGCC Call
Confirmed message is sent from the user station application to the
base station controller 105.
[0218] 28. Once the call is confirmed between the user station
application and the base station controller 105, the base station
controller 105 sends a DTAP Call Confirmed message to the mobile
switching center 112 on the SCCP circuit associated with the user
station's PID.
[0219] 29. In response to the DTAP Call Confirmed message, the
mobile switching center 112 sends the base station controller 105 a
BSSMAP Assignment Command message.
[0220] 30. When the base station controller 105 receives the BSSMAP
Assignment Command message from the mobile switching center 112,
the base station controller 105 connects the circuit described by
the Circuit ID code to the base-station-to-base-station-controller
circuit described by the map in the Service Information NOTE. Once
the Assignment Command message has been received from the mobile
switching center 112 and the Connect Link NOTE has been received
from the base station 104, the base station controller 105 sends
the mobile switching center 112 a BSSMAP Assignment Complete
message on the SCCP circuit associated with the user station's
PID.
[0221] 31. When the mobile switching center 112 receives the BSSMAP
Assignment Complete message from the base station controller 105,
the mobile switching center 112 initiates IGCC end-to-end call
control traffic.
[0222] 32. When the connection is complete and the user station
application is ready to accept/send data, the user station
application sends a Begin Traffic message to the user station
102.
[0223] 33. The system then enters normal traffic mode, and the
conversation is stable.
[0224] FIGS. 11A-11C and 12A-12B are message flow diagrams for an
intra-cluster handover and an inter-cluster handover, respectively.
These message flow diagrams may be explained with reference to FIG.
19, which illustrates a particular deployment of base stations in
clusters. In FIG. 19, a mobile switching center 112 120 is
connected to a plurality of base station controllers 105 (also
referred to as cluster controllers). Each base station controller
105 is in turn connected to a plurality of base stations 104. The
base stations 104 are organized into logical groups of clusters
121, such that each cluster 121 of base stations 104 is connected
to a single base station controller 105. A cluster 121 of base
stations 104 need not be geographically adjacent; rather, the
cluster 121 comprises a logical group of base stations 104
regardless of their geographical proximity.
[0225] As used herein, an intra-cluster handover is one in which a
user station 102 transfers communication from the current base
station 104 to a new base station 104 in the same cluster 121
(i.e., in a cluster 121 that is serviced by the same base station
controller 105), and an inter-cluster handover is one in which the
user station 102 transfers communication from the current base
station 104 to a new base station 104 in a different cluster 121
(i.e., in a cluster 121 that is serviced by a different base
station controller 105).
[0226] An exemplary message flow diagram for an intra-cluster
handover is shown in FIGS. 11A-11C. As described in more detail
below, FIG. 11B relates to the case in which the link with the old
base station is maintained, while FIG. 11C relates to the case in
which the link with the old base station is lost. In FIGS. 11A-11C,
similar to FIGS. 9 and 10, transmitted messages are designated by
arrows between a user station 102, a current base station 104
(denoted "Old BS" or "BS1"), a target base station 104 (denoted
"New BS" or "BS2"), a base station controller 105, and a mobile
switching center 112. Control traffic messages between the user
station 102 and either base station 104 are typically preceded by
the initials "CT". The steps numbered 1 through 22 associated with
the arrows appearing in FIGS. 11A-11C are explained below:
[0227] 1. The user station 102 starts in normal stable traffic with
the current base station 104 (BS1).
[0228] 2. The user station 102 monitors a received signal strength
indication (RSSI). Eventually, the RSSI for the current link drops
below a first threshold value L.sub.look (i.e., the threshold value
below which the user station 102 begins to search for a new base
station 104).
[0229] 3. During a portion of the time frame 301 that the user
station 102 does not need to maintain communication in its assigned
time slots with the current base station BS1, the user station 102
switches to the frequency (e.g., F1, F2 or F3) and/or code (e.g.,
C1, C2, C3, C4, C5 or C6) of one of the surrounding base stations
104, as specified by a surrounding base station table, and measures
the RSSI of that base station 104 by observing any traffic from the
base station 104. The user station 102 also records the current
utilization field from the header of the base station 104 traffic
messages (e.g., CU field 809 of FIG. 8A). If the message observed
is a GENERAL POLL message, then the user station 102 also records
the slot quality, base ID, base station controller ID (BSC ID),
service provider, zone and facility of the candidate base station
104. The user station 102 uses this information to calculate a
preference value for the candidate base station 104 and sorts the
entry into a table of preferred base stations.
[0230] 4. When the RSSI of the link to the current base station BS1
drops below a second threshold level L.sub.ho (i.e., the threshold
below which handover is appropriate), the user station 102 selects
the highest preference base station 104 as the target base station
104 (BS2). If the observed time slot 302 at the target base station
BS2 had contained a GENERAL POLL message, then the user station 102
examines the BSC ID of the target base station BS2. If the BSC ID
is not the same as that of the current base station BS1 (i.e., the
current and target base stations are connected to different base
station controllers 105), then the user station 102 executes an
inter-cluster handover (see FIGS. 12A-12B). Similarly, the user
station 102 examines the zone of the target base station BS2, and
if the zone is not the same as the zone of the old base station
BS1, then the user station 102 will commence execution of an
inter-cluster handover (see FIGS. 12A-12B). Otherwise, if the BSC
ID is the same for the current and target base stations and the
zone for both is also the same, then the user station 102 continues
with an intra-cluster handover. If the observed time slot did not
contain a GENERAL POLL message, then the user station 102 attempts
to locate a time slot that has a GENERAL POLL message. The user
station 102 can potentially look at all of the time slots in which
it is not presently communicating and, if desired, can even skip a
transmission on its current time slot to check the same location
time slot on the target base station BS2 for a GENERAL POLL
message.
[0231] 5. The user station 102 acquires the observed time slot of
the target base station BS2. The user station 102 does this by
searching for a GENERAL POLL message from the target base station
BS2, and responding with a GENERAL RESPONSE message to the GENERAL
POLL message. If the user station 102 has not already examined the
BSC ID of the target base station BS2, it does so at this point. If
the BSC ID and the zone match those of the old base station BS1,
then the user station can perform an intra-cluster handover
utilizing the target base station BS2. Otherwise, if either the
zone of the BSC ID of the target base station BS2 does not match
that of the old base station BS1, then the user station 102 does
not respond to the SPECIFIC POLL message, but instead executes an
inter-cluster handover (see FIGS. 12A-12B).
[0232] 6. Assuming an intra-cluster handover is to be performed,
the user station 102 and old base station BS1 maintain traffic
communication over the old link if possible. If not possible, the
old link is dropped.
[0233] 7. In response to the SPECIFIC POLL control traffic message
from the target base station BS2, the user station 102 returns a
TERMINATING HANDOFF REQUEST (CT-THR) control traffic message.
[0234] 8. If the target base station BS2 will accept the handover,
the target base station BS2 responds with a BASE ASSIST (CT-BAM)
control traffic message. The CT-BAM message contains a list of
surrounding base stations 104 which the user station 102 can
monitor for future handovers. The user station 102 responds with a
HOLD (CT-HLD) control traffic message and sets an internal user
station handover timer. The handover is at this stage considered to
be committed in the sense that the user station 102 cannot attempt
a new handover until this attempt is completed. If the user station
102 does not receive a CT-BAM message from the target base station
BS2, the user station 102 will attempt to hand off to the next most
preferable base station 104 it found in Step 3 above. If there are
no other suitable base stations 104, the user station 102 will
proceed with call clearing.
[0235] 9. If the target base station BS2 has accepted the handover,
it sets a base station handover timer and sends a Terminating
Handoff Note to the base station controller 105.
[0236] 10. The base station controller 105 switches the user
station 102 from the old base station BS1 to the new base station
BS2. Specifically, the base station controller 105 switches the
circuit represented by the Circuit ID code associated with the user
station 102 as identified in the local registration (LR) of the
base station controller 105 from the old circuit (described by the
connection number) to a new circuit at the target base station BS2
as described by a Bearer Map in the Terminating Handoff Request
NOTE. The base station controller 105 thereafter associates the
user station 102 with its new location. The base station controller
105 updates the contents of the location register (LR) to reflect
the new location of the user station 102.
[0237] 11. In response to the CT-BAM message from the base station
104, the user station sends a control traffic acknowledge (CT-ACK)
message to the base station 104 to acknowledge receipt of the
surrounding base station list.
[0238] If the link between the user station 102 and the old base
station BS1 can be maintained, the following steps are then carried
out, in accordance with the call flow diagram of FIG. 11B:
[0239] 12. After receiving a CT-ACK message from the user station
102, the target base station BS2 starts issuing SPECIFIC POLL
messages targeted for the user station 102 (using the user station
PID), so that the user station 102 can re-acquire the link on the
target base station BS2.
[0240] 13. When the base station controller 105 completes its
circuit switch, the base station controller 105 sends the target
base station BS2 a Circuit Switch Complete NOTE. In one embodiment,
the Circuit Switch Complete NOTE contains no ciphering
information.
[0241] 14. The base station controller 105 also sends the old base
station BS1 a Circuit Switch Complete NOTE. When the old base
station BS1 receives the Circuit Switch Complete NOTE, the old base
station BS1 sends a CIRCUIT SWITCH COMPLETE (CT-CSC) control
traffic message to the user station 102. The old base station BS1
then clears all tables and circuits related to the call.
[0242] 15. The base station controller 105 then sends a BSSMAP
HANDOVER PERFORMED message to the mobile switching center 112.
[0243] 16. When the user station 102 receives the CT-CSC control
traffic message, the user station 102 responds by switching to the
frequency and code of the new base station BS2. The user station
102 then searches for a SPECIFIC POLL message with the PID field
matching the PID of the user station 102. When the user station 102
finds the appropriate SPECIFIC POLL message, it responds with a
HOLD (CT-HLD) control traffic message. If the user station 102
loses the link to the old base station BS1 before receiving the
CT-CSC message, the user station 102 will switch to the target base
station BS2 and respond to the SPECIFIC POLL message. If the user
station 102 is unable to find a SPECIFIC POLL message with the
proper PID on the target base station BS2, then the call is lost,
and the user station proceeds with call clearing
[0244] 17. When the target base station BS2 sees a CT-HLD message
from the user station 102 and has received the Circuit Switch
Complete NOTE from the base station controller 105, the target base
station BS2 sends a CIRCUIT SWITCH COMPLETE (CT-CSC) control
traffic message to the user station 102.
[0245] 18. When the user station 102 receives the CT-CSC message
from the target base station BS2, the user station 102 cancels its
internal user station handover timer, and responds with bearer
traffic messages. If the user station's handover timer expires
before bearer traffic is received, the connection is lost, and the
user station 102 will proceed with call clearing.
[0246] 19. When the target base station BS2 receives the bearer
traffic messages from the user station 102, the target base station
BS2 cancels its base station handover timer and switches into
traffic mode. If the base station handover timer expires before
bearer traffic is receives, then the connection is assumed lost,
and the base station BS2 will proceed with call clearing.
[0247] 20. A stable bearer channel has been established with the
new base station BS2. Handover is complete.
[0248] Steps 12-19 above assume that the link between the user
station 102 and the old base station BS1 is maintained during
handover. If, however, the link between the user station 102 and
the old base station BS1 is lost, then the following steps are
carried out to complete the intra-cluster handover, in accordance
with the call flow diagram of FIG. 11C:
[0249] 21. If the user station 102 loses the link with the old base
station BS1 before it receives the CT-CSC control traffic message,
then the user station 102 switches to the frequency of the target
base station BS2 and searches for a SPECIFIC POLL message having a
PID field matching the PID of the user station 102. When the user
station 102 finds the appropriate SPECIFIC POLL message, the user
station 102 responds with a HOLD (CT-HLD) control traffic message.
If the user station 102 is unable to find a SPECIFIC POLL on the
target base station BS2, then the call is assumed lost, and the
user station 102 proceeds with call clearing.
[0250] 22. If the target base station BS2 receives a response to
its SPECIFIC POLL message from the user station 102 before the
target base station BS2 has received the Circuit Switch Complete
NOTE from the base station controller 105, the target base station
BS2 responds to the CT-HLD messages from the user station 102 with
CT-HLD messages, in an alternating fashion.
[0251] 23. When the base station controller 105 completes its
switch, it sends the target base station BS2 a Circuit Switch
Complete NOTE.
[0252] 24. When the target base station BS2 receives the Circuit
Switch Complete NOTE from the base station controller 105, the
target base station BS2 sends a CIRCUIT SWITCH COMPLETE (CT-CSC)
control traffic message to the user station 102.
[0253] 25. The base station controller then sends the old base
station BS1 a Circuit Switch Complete NOTE. In one embodiment, the
Circuit Switch Complete NOTE contains no ciphering information.
[0254] 26. The base station controller 105 then sends a BSSMAP
HANDOVER PERFORMED message to the mobile switching center 112.
[0255] 27. When the old base station BS1 receives the Circuit
Switch Complete NOTE, the old base station BS1 sends a CT-CSC
control traffic message to the user station 102. (The user station
102 will not see this message because it has lost the link to the
old base station BS1.) The old base station BS1 then clears all
tables and circuits related to the call.
[0256] 28. When the user station 102 receives the CT-CSC message
from the target base station BS2, the user station 102 responds
with bearer traffic, and cancels its internal user station handover
timer.
[0257] 29. When the target base station BS2 receives a bearer
traffic response from the user station 102, the target base station
BS2 cancels its base station handover timer, and switches into a
traffic mode. A stable bearer channel has been established at this
point.
[0258] The foregoing description pertains to intra-cluster
handovers. A system in accordance with a preferred embodiment is
also capable of performing inter-cluster handovers. Exemplary
message flow diagrams for an inter-cluster handover is shown in
FIGS. 12A-12B. In FIGS. 12A-12B, similar to FIGS. 9-11, messages
are designated by arrows between a user station 102, a current base
station 104 (denoted "Old BS" or "BS1"), a target base station 104
(denoted "New BS" or "BS2"), a current base station controller 105
(denoted "Old BSC" or "BSC1"), a target base station controller 105
(denoted "New BSC" or "BSC2"), and a mobile switching center 112.
Control traffic messages between the user station 102 and either
base station 104 are typically preceded by the initials "CT". The
steps numbered 1 through 33 associated with the arrows appearing in
FIGS. 12A-12B are explained below (with steps 1 through 4 being
identical to those for an intra-cluster handover):
[0259] 1. The user station 102 starts in normal stable traffic with
the current base station 104 (BS1).
[0260] 2. The user station 102 monitors a received signal strength
indication (RSSI). Eventually, the RSSI for the current link drops
below a first threshold value L.sub.look (i.e., the threshold value
below which the user station 102 begins to search for a new base
station 104).
[0261] 3. During a portion of the time frame 301 that the user
station 102 does not need to maintain communication in its assigned
time slots with the current base station BS1, the user station 102
switches to the frequency (e.g., F1, F2 or F3, as shown in the
example of FIG. 1A) and/or code (e.g., C1, C2, C3, C4, CS, C6 or
C7, as shown in the example of FIG. 1A) of one of the surrounding
base stations 104, as specified by a surrounding base station
table, and measures the RSSI of that base station 104 by observing
any traffic from the base station 104. The user station 102 also
records the current utilization field from the header of the base
station 104 traffic messages (e.g., CU field 809 of FIG. 8A). If
the message observed is a GENERAL POLL message, then the user
station 102 also records the slot quality, base ID, base station
controller ID (BSC ID), service provider, zone and facility of the
candidate base station 104. The user station 102 uses this
information to calculate a preference value for the candidate base
station 104 and sorts the entry into a preferred base station
table.
[0262] 4. When the RSSI of the link to the current base station BS1
drops below a second threshold level L.sub.ho (i.e., the threshold
below which handover is appropriate), the user station 102 selects
the highest preference base station 104 as the target base station
104 (BS2). If the observed time slot 302 at the target base station
BS2 had contained a GENERAL POLL message, then the user station 102
examines the BSC ID of the target base station BS2. If the BSC ID
is not the same as that of the current base station BS1 (i.e., the
current and target base stations are connected to different base
station controllers 105), then the user station 102 executes an
inter-cluster handover, as described in further detail in the steps
below. Similarly, the user station 102 examines the zone of the
target base station BS2, and if the zone is not the same as the
zone of the old base station BS1, then the user station 102 will
commence execution of an inter-cluster handover as described in
more detail below. Otherwise, if the BSC ID is the same for the
current and target base stations and the zone for both is also the
same, then the user station 102 executes an intra-cluster handover
(see FIGS. 11A-11C). If the observed time slot did not contain a
GENERAL POLL message, then the user station 102 attempts to locate
a time slot that has a GENERAL POLL message. The user station 102
can potentially look at all of the time slots in which it is not
presently communicating and, if desired, can even skip a
transmission on its current time slot to check the same location
time slot on the target base station BS2 for a GENERAL POLL
message.
[0263] 5. If the user station 102 does not yet know the BSC ID of
the target base station BS2, then the user station 102 responds to
the GENERAL POLL message with a GENERAL RESPONSE message and
examines the BSC ID of the GENERAL POLL message. The GENERAL
RESPONSE message sent by the user station 102 includes the user
station's PID.
[0264] 6. After determining that an inter-cluster handover is to be
performed (based upon the BSC ID and/or zone of the old base
station BS1 and that of the target base station BS2), the user
station 102 sends the old base station BS1 an ORIGINATING HANDOVER
REQUEST (CT-OHR) control traffic message. The CT-OHR message
contains the base ID of the preferred new base station BS2, as
determined by the surrounding base station table, as well as its
mobility country code (MCC) and mobility network code (MNC).
[0265] 7. When the old base station BS1 receives the CT-OHR
message, the old base station BS1 sends an ACKNOWLEDGE (CT-ACK)
control traffic message to the user station 102 to acknowledge the
correct receipt of the CT-OHR message. The old base station BS1
sends an Originating Handover Request NOTE to the old base station
controller (BSC1). The Originating Handover Request NOTE contains
the PID of the user station, the old base station ID, and the MCC
and MNC of the target base station BS2. The old base station BS1
knows the PID of the user station 102 since it was supplied during
the initial slot seizure.
[0266] 8. When the user station 102 receives the CT-ACK message,
the user station 102 and base station 104 resume normal traffic
pending the completion of the circuit switch. If the user station
102 does not receive a CT-ACK message, the user station 102 assumes
that its handover request has not been successful and it will
restart the handover attempt (returning back to Step 4). If it did
receive a CT-ACK message, the user station 102 sets an internal
user station handover timer with a predetermined timeout value. The
handover is now committed in the sense that the user station 102
cannot attempt a new handover until this attempt is completed.
[0267] 9. The old base station controller BSC1 sends a BSSMAP
Handover Required message to the mobile switching center 112 on the
SCCP circuit for the user station 102 (i.e., the SCCP circuit
described by the user station's PID). In a preferred embodiment,
the BSSMAP Handover Required message identifies only a single
cell--the cell serviced by the target base station BS2--in a
preferred cell list.
[0268] 10. The mobile switching center 112 interprets the Handover
Required message and sends a BSSMAP Handover Request message to the
SCCP circuit in the terminating base station controller (BSC2) that
will subsequently be used by the user station 102 upon completion
of the handover. The BSSMAP Handover Request message contains all
of the information necessary to maintain the call in progress,
including, e.g., the channel type, encryption information and
priority. In addition, the BSSMAP Handover Request message contains
the base ID of the target base station BS2.
[0269] 11. The terminating base station controller BSC2 generates a
"handover reference number" and stores the received information in
a small association table for use at a later time. The information
stored in the table is associated with a concatenation of the
handover reference number and the target base station's base ID.
The new base station controller BSC2 then sends a BSSMAP Handover
Request ACK message back to the mobile switching center 112. The
BSSMAP Handover Request ACK message contains the generated handover
reference number in its "level three" information.
[0270] 12. Upon receipt of the BSSMAP Handover Request ACK message,
the mobile switching center 112 sends the old base station
controller BSC1 a BSSMAP Handover Command message on the original
SCCP circuit. The BSSMAP Handover Command message contains the
level three information supplied by the terminating base station
controller BSC2, including the handover reference number. The
handover reference number and the implicit knowledge of the user
station PID (from the SCCP circuit) are all the identification
information needed by the old base station controller BSC1 to
complete the handover.
[0271] 13. After receiving the Circuit Switch Complete NOTE, the
old base station controller BSC1 sends a Circuit Switch Complete
NOTE to the old base station BS1. In place of the connection number
field, this Circuit Switch Complete NOTE contains the handover
reference number from the terminating base station controller BSC2.
The Circuit Switch Complete NOTE also contains the user station PID
that was associated with the SCCP circuit.
[0272] 14. Upon receipt of the Circuit Switch Complete NOTE, the
old base station BS1 sends the user station 102 a CIRCUIT SWITCH
COMPLETE (CT-CSC) control traffic message which contains the
handover reference number. Since the user station 102 has retained
the base ID and frequency of the target base station BS2, the user
station 102 now has all of the information required to complete the
handover. If the old base station BS1 does not receive the Circuit
Switch Complete NOTE, an error has occurred, and the call is torn
down.
[0273] 15. Upon receipt of the CT-CSC message, the user station 102
returns an ACKNOWLEDGE (CT-ACK) control traffic message to the old
base station BS1 to acknowledge the correct receipt of the CT-CSC
control traffic message. If the user station 102 does not receive
the CT-CSC message (which contains the handover reference number
needed to complete the handover), the call cannot be continued on
the target base station BS2, and the call is torn down.
[0274] 16. Upon receipt of the CT-ACK message, the old base station
BS1 clears all resources associated with the user station 102 and
makes the channel available for new communication. If the old base
station BS1 does not receive the CT-ACK message, it will
nevertheless clear all resources associated with the user station
102 and make the channel available for new communication.
[0275] 17. After sending the CT-ACK message, the user station 102
switches to the frequency of the target base station BS2 and seizes
a channel (i.e., a time slot 302) using the slot seizure procedure
described with respect to FIG. 4. Once the user station 102 has
captured a time slot 302, the user station 102 sends the target
base station BS2 a TERMINATING HANDOVER COMPLETE (CT-THC) control
traffic message which contains the handover reference number and
the service request of the user station 102. If the user station
102 fails to seize a channel on the target base station BS2, the
call is lost. The user station 102 will, in such a case, send a
Link Lost message to the user station application.
[0276] 18. When the target base station BS2 receives the CT-THC
message, it compares the BSC ID of the CT-THC message with the BSC
ID of the base station controller 105 to which it is connected.
This comparison allows the target base station BS2 to independently
determine that an inter-cluster handover is required. The target
base station BS2 responds to the user station 102 with a BASE
ASSIST (BAM) control traffic message to acknowledge the correct
receipt of the CT-THC message. The target base station BS2 then
uses the service request information element of the CT-THC message
to allocate bearer channels between itself and the terminating base
station controller BSC2, and sends a Terminating Handover Complete
NOTE to the terminating base station controller BSC2. The
Terminating Handover Complete NOTE contains the PID of the user
station 102 along with the handover reference number and a
description of the bearer channels assigned to support the user
station 102.
[0277] 19. Upon receipt of the CT-BAM message, the user station 102
sends an ACKNOWLEDGE (CT-ACK) control traffic message to the target
base station BS2 to signal correct receipt of the CT-BAM
message.
[0278] 20. The terminating base station BS2 and the user station
102 enter a hold pattern in which they exchange HOLD (CT-HLD)
control traffic messages while awaiting an indication that the
circuit has been switched.
[0279] 21. The terminating base station controller BSC2 uses the
handover reference number and the base ID of the target base
station BS2 to find the associated connection information in the
association table located at the mobile switching center 112. If
the terminating base station controller BSC2 cannot find an
association for the handover reference number and the base ID of
the target base station BS2, then there has been an error and the
call is torn down. Assuming that the proper association is found,
the terminating base station controller BSC2 sends a Circuit Switch
Complete NOTE to the target base station BS2.
[0280] 22. The target base station BS2 responds to the Circuit
Switch Complete NOTE by sending an ACK Circuit Switch Complete NOTE
to the terminating base station controller BSC2.
[0281] 23. When the target base station BS2 receives the Circuit
Switch Complete NOTE, it also sends a CIRCUIT SWITCH COMPLETE
(CT-CSC) control traffic message to the user station 102.
[0282] 24. When the user station 102 receives the CT-CSC message,
it sends an ACKNOWLEDGE (CT-ACK) control traffic message to the
target base station BS2.
[0283] 25. The terminating base station controller BSC2 connects
the bearer channels specified by the target base station BS2 with
the links set up by the mobile switching center 112. The
terminating base station controller BSC2 then sends a Set Cipher
Mode NOTE to the target base station BS2 which contains ciphering
information, if applicable.
[0284] 26. The target base station BS2 uses the ciphering
information to set its ciphering equipment and returns an
Acknowledge Cipher Mode NOTE to the terminating base station
controller BSC2. If the target base station BS2 does not receive
the Set Cipher Mode NOTE, then an error has occurred, and the call
is torn down. (If the Set Cipher Mode NOTE is not received, then
the target base station BS2 does not have the information required
to formulate a CT-SET message to the user station 102, as described
in the following step.)
[0285] 27. The terminating base station controller BSC2 sends a
Handover Detect message to the mobile switching center 112.
[0286] 28. The mobile switching center 112 sends a Handover
Complete message to the terminating base station controller
BSC2.
[0287] 29. After receiving the Set Cipher Mode NOTE, the target
base station BS2 sends a SET CIPHER MODE (CT-CIP) control traffic
message to the user station 102.
[0288] 30. When the user station 102 receives the CT-CIP control
traffic message, it sends an ACKNOWLEDGMENT (CT-ACK) control
traffic message to the target base station BS2.
[0289] 31. The mobile switching center 112 sends a Clear Command to
the SCCP circuit of the user station 102 on the old base station
controller BSC1.
[0290] 32. The old base station controller BSC1 clears its
resources that were allocated to the user station 102 and returns a
Clear Complete message to the mobile switching center 112. There is
no need to send any information to the old base station BS1 since
the old base station BS1 cleared all of its resources allotted to
the user station 102 earlier when the old base station controller
BSC1 sent the CT-CSC control traffic message to the user station
102 in step 14.
[0291] 33. The target base station BS2 clears its base station
handover attempt timer, and the user station 102 likewise clears
its user station handover attempt timer. They enter traffic mode,
and the handover is complete.
[0292] Aspects of the invention are directed to facilitating rapid
control traffic within the timing structure of the communication
system. Handover, establishing communication, or time slot
interchange may be carried out in a rapid manner by utilizing
multiple time slots spaced less than one time frame apart. In such
a manner, the control traffic takes advantage of unused time slots
to avoid having to wait an entire time frame for each opportunity
to exchange messages between the base station 104 and the user
station 102 desiring a transaction. Spare resources are thereby
used for the purpose of speeding up control traffic
transactions.
[0293] In the preferred embodiment wherein the user station 102
transmits prior to the base station 104 in a time slot 302 (or
virtual time slot 618), the slot pointer allows the user station
102 to have knowledge of the next available time slot 302.
Otherwise, the user station 102 may not necessarily know until a
general poll message 401 is received whether or not a particular
time slot is available for communication, and then would typically
have to wait an entire polling loop before responding to the
general poll message 401.
[0294] Knowledge of available time slots 302 is also passed to the
user station 102 in a specific poll message 402 by use of the OTA
map field 726. As noted previously, the OTA map field 726 describes
the mapping of time slots relative to a particular user station
102. Thus, for a time frame 301 with sixteen time slots 302, the
OTA map field 726 in one embodiment comprises sixteen bits. Each
bit may be set to a first value (e.g., "1") to indicate that the
time slot 302 associated with that bit is unavailable, and to a
second value (e.g., "0") to indicate that the time slot 302
associated with that bit is available for communication.
Preferably, the time slot usage is indicated from a standpoint
relative to the current time slot 302 of the user station 302--that
is, the first bit is associated with the immediately following time
slot, the second bit with the next time slot thereafter, the third
bit with the next time slot thereafter, and so on. Alternatively,
the time slot usage may be indicated from a standpoint with respect
to a fixed reference, such as the start of the time frame 301, in
which case the user station 102 needs to have available as
information the relative starting point of the time frame 301.
[0295] FIG. 18A is a timing diagram illustrating rapid control
traffic by utilizing multiple time slots within the span of a
single time frame. In FIG. 18A, a timing diagram including a
plurality of time frames 1401 is shown. A first time frame 1401a
precedes a second time frame 1401b. In each time frame 1401 are a
plurality of time slots 1402, numbered consecutively. Each time
frame 1401 has sixteen time slots 1402. Each time slot has a user
transmission interval 1403 and a base transmission interval
1404.
[0296] In the first time frame 1401a, it is assumed that at least
three time slots 1402 (time slots "2", "8", and "15") are
available. In the second time frame 1401b, it is assumed that at
least two time slots 1402 (time slots "5" and "11") are available.
In time slot "2" of the first time frame 1401a, no user station 102
transmission is sent during the user transmission interval 1403;
only a general poll message (e.g., such as general poll message 401
of FIG. 4) is sent by the base station 104 during the base
transmission interval 1404. The general poll message 401 includes a
next slot pointer ("NSP") set to "6", which indicates that the next
available slot is six slot positions ahead relative to the current
slot; in other words, time slot "8".
[0297] Accordingly, in time slot "8" of the first time frame 1401a,
a user station 102 desiring to establish communication (either
initial communication or handover) with the base station 104
transmits a GENERAL RESPONSE message (e.g., such as general
response message 404 of FIG. 4) during the user transmission
interval 1403 of time slot "8". The base station 104 receives the
GENERAL RESPONSE message, and responds during the base transmission
interval 1404 of time slot "8" with a SPECIFIC POLL message (e.g.,
such as specific poll message 402 of FIG. 4). As part of the
SPECIFIC POLL message, the user station is assigned a correlative
ID (in the present example, the correlative ID is "3"). The next
slot pointer in the present example is "7", which means that the
next available slot is seven slot positions ahead relative to the
current slot; in other words, time slot "15".
[0298] Accordingly, in time slot "15" of the first time frame
1401a, the user station 102 in the present example transmits a
control traffic message "THR" (as defined in Table 9-1 aboved),
indicating a "Target Handover Request." In this case, the user
station 102 seeks to handover to the base station 104 from another
base station 104. The base station 104 responds in the base
transmission interval 1404 of time slot "15" with a control traffic
"ACK" or acknowledge message. The correlative ID of the user
station 102 is sent as part of the acknowledge message, as well as
a next slot pointer indicating that the next available slot is six
slot positions ahead relative to the current slot; in other words,
time slot "5" of the next time frame 1401b.
[0299] Accordingly, in time slot "5" of the second time frame
1401b, the user station 102 in the present example transmits a
control traffic acknowledge (CT-ACK) message or, alternatively, a
control traffic HOLD (CT-HLD) message, as shown in the message flow
diagram of FIG. 11. The base station 104 then has several options
in response. In one embodiment, the base station 104 may respond
with a traffic message in the base transmission interval 1404 of
time slot "5", provided that the call has been connected from the
base station controller 105. Alternatively, the user station 102
can monitor each time slot 1402 until it sees its correlative ID,
and then respond thereafter in accordance with the message directed
to it. As another alternative, the base station 104 may respond in
the base transmission interval 1404 with a control traffic message
assigning a new time slot 1402 to the user station 102.
[0300] In a preferred embodiment, the user station 102 continues to
communicate in the assigned time slot 1402 (i.e., time slot "5") of
each time frame until the call is connected and completed, or is
otherwise dropped. Until communication is fully established, the
base station 104 may transmit a GENERAL POLL message in the base
transmission interval 1404 of time slot "5" indicating the next
available time slot 1402 for other user stations 102 desiring to
establish communication.
[0301] In one aspect, FIG. 18A illustrates a method of establishing
communication between a user station 102 and a particular base
station 104 by exchanging control traffic messages separated by a
time duration less than a time frame 1401. In the example shown in
FIG. 18A, messages are exchanged between the user station 102 and
the base station 104 in three time slots 1402 of a first time frame
1401a, and in two time slots 1402 of a second time frame 1401b.
This technique can provide substantial reductions in the amount of
time needed to establish communication between a user station 102
and a particular base station 104, or to handoff communication to a
new base station.
[0302] Besides being useful for establishing communication (either
initial communication or handover), the same method may be used to
rapidly exchange control messages between a user station 102 and a
base station 104, where such rapid exchange is necessary. The
rapidity of conducting the control traffic may be particularly
useful, for example, in the support of "911" emergency calls or
other time-critical situations.
[0303] In a particular embodiment, one or more time slots 1402 are
reserved for "911" emergency calls, and are not used for
non-emergency bearer traffic. For example, four time slots 1402 may
be held in reserve. These reserved time slots 1402 may also be used
to conduct the rapid control traffic operations described in the
FIG. 18A example. Preferably, at least one time slot 1402 is not
used for anything other than receiving a possible "911" emergency
call. When a "911" emergency call is received, it may pre-empt
other control traffic, and the reserved time slots 1402 may be used
to conduct a rapid establishment of communication for the "911"
call.
[0304] The correlative ID assigned to the user station 102 as part
of the SPECIFIC POLL message may be used to recover from situations
in which subsequent messages are received in error due to
interference or correlation errors. FIG. 18B is a diagram
illustrating the rapid control traffic techniques of FIG. 18A, but
wherein one of the messages to the user station is received in
error. In FIG. 18B, similar to FIG. 18A, a timing diagram including
a plurality of time frames 1411 is shown. A first time frame 1411a
precedes a second time frame 1411b. Each time frame 1411 has a
plurality (e.g., sixteen) of time slots 1412, numbered
consecutively. As in FIG. 18A, each time slot has a user
transmission interval 1413 and a base transmission interval
1414.
[0305] In FIG. 18B, the same control traffic transactions are
carried out in time slots "2" and "8" of the first time frame 1411a
as in FIG. 18A. However, in time slot "15" of the first time frame
1411a, the base message sent in the base transmission interval 1414
is received in error. As a result, the user station 102 may not
know when to expect the next communication from the base station
104, as the next slot pointer has been lost. Accordingly, the user
station 102 monitors the base transmission interval 1414 of each
time slot 1412 until it recognizes its correlative ID (which was
assigned to it as part of the specific poll message). In the
present example, the user station 102 recognizes its correlative ID
in time slot "5" of the second time frame 1411b, and therefore
identifies the message as one intended for it. The user station 102
also reads the next slot pointer (in this case having a value of
"6"), and therefore responds six time slots 1412 later with an
appropriate user message. After recovering from the erroneous
reception in this manner, the exchange between the user station 102
and the base station 104 may proceed as described with respect to
the remaining steps shown in FIG. 18B.
[0306] Thus, loss of the slot pointer does not necessarily prevent
the establishment of communication (or the conducting of other fast
control traffic operations). Recovery from errors is possible by
searching for the correlative ID once communication has been
temporarily disrupted by an error in receiving a message from the
base station 104.
[0307] While the principles of rapid traffic control have been
described in certain aspects of FIGS. 18A and 18B with respect to
the FIG. 3 timing structure, the same principles are applicable to
the FIG. 6 timing structure utilizing virtual time slots. The
principles are also applicable to hybrid systems using frequency
duplex techniques (such as FDD or FDMA) in addition to
TDMA/TDD.
[0308] FIG. 20 is a block diagram of an exemplary transmitter and
receiver in a spread spectrum communication system as may be
employed for spreading and despreading signals in a communication
system in accordance with one or more embodiments of the present
invention. In FIG. 20, a spread-spectrum transmitter 2010 a serial
input register 2021, a symbol table 2022, a modulator 2025, a phase
selector 2026 and a transmitting antenna 2027 for transmitting a
spread-spectrum signal. A spread-spectrum receiver 2050 comprises a
receiver antenna 2051, a down converter 2052, a bank of spread
spectrum demodulators 2056, a best-of-M detector 2057, and an
output data signal 2059.
[0309] In operation, a serial data stream 2012 is received by the
transmitter 2010 and clocked by a data clock 2013 into the serial
input register 2021. When N bits have been clocked into the serial
input register 2021, one of M spread spectrum codes (or "symbol
codes") is selected from the symbol table 2022. For example, five
bits of the serial data stream 2012 clocked into the serial input
register 2021 may be used to select one of 32 possible symbol codes
stored in the symbol table 2022. The selected symbol code is output
from the symbol table 2022 and used by the modulator 2025 to
generate a spread spectrum signal. Another data bit (or possibly
multiple data bits, if desired) of the data stream 2012, exclusive
from those used to select the symbol code, is input to the phase
selector 2026, which determines the phase of the symbol code
selected from the symbol table 2022. For example, the phase
selector may use a single bit (called a "phase control bit" of the
data stream 2012 to determine the phase of the symbol code; if this
phase control bit has a first value (e.g., a "0"), then the symbol
code is transmitted with no phase inversion, while if the phase
control bit has a second value (e.g., a "1"), then the symbol code
is transmitted with a phase inversion of 180 degrees. If two phase
control bits are used, then four possible phases could be selected,
and so on for additional phase control bits.
[0310] The modulator 2025 transmits the selected symbol code using
the phase indicated by the phase selector 2026. The modulator 2025
may transmit using continuous phase modulation, or a similar
technique, so as to minimize spectral splatter. In the transmission
process, the modulator 2025 preferably modulates the selected
symbol code with a carrier signal at a predetermined carrier
frequency. Exemplary spread spectrum modulators are described in,
for example, U.S. Pat. Nos. 5,548,253 and 5,659,574, both of which
is assigned to the assignee of the present invention, and both of
which are hereby incorporated by reference as if set forth fully
herein.
[0311] At the spread-spectrum receiver 2050, the transmitted spread
spectrum signal is received at the receiver antenna 2051 and
down-converted to baseband by the down converter 2052. The baseband
signal is then fed to a bank of spread spectrum demodulators 2056,
each of which is configured to recognize one of the M possible
symbol codes, and each of which outputs a correlation signal
indicating a degree of match with its respective symbol code. The
best-of-M detector 2057 receives the correlation signal from each
of the spread spectrum demodulators 2056, and determines which of
the M symbol codes has been received based on the relative
strengths of the correlation signals. The best-of-M detector 2057
generates an output data signal 2059 based upon the received symbol
codes. The phase of the received symbol code can also be detected,
and further information received by differential phase
decoding.
[0312] Exemplary correlators suitable for use with certain
embodiments of the present invention are described in, among other
places, U.S. Pat. Nos. 5,022,047 and 5,016,255, both of which are
assigned to the assignee of the present invention, and both of
which are incorporated by reference as if fully set forth herein. A
preferred method of correlation is described in U.S. Pat. No.
5,659,574 issued Aug. 5, 1997, assigned to the assignee of the
present invention, and hereby incorporated by reference as if set
forth fully herein. In particular, a multi-bit correlation
technique as described in U.S. Pat. No. 5,659,574 represents a
presently preferred manner of correlating a spread spectrum signal.
U.S. Pat. No. 5,659,574 also sets forth a presently preferred
technique of differential phase encoding and decoding usable in
conjunction with the present invention.
[0313] Spread spectrum communication techniques are further
described in, e.g., Robert C. Dixon, Spread Spectrum Systems with
Commercial Applications (John Wiley & Sons, 3d ed. 1994),
hereby incorporated by reference as if set forth fully herein. A
large variety of spread spectrum systems have been proposed in the
industry, and the particular details of the spread spectrum system
set forth above are in no way meant to be limiting to the scope of
the invention. Moreover, while spread spectrum communication
techniques are utilized in a preferred embodiment of the invention,
many embodiments of the invention are operable without using spread
spectrum.
[0314] Several further variations, modifications and enhancements
of the invention will now be described. User stations 102 in one
embodiment may comprise mobile handsets capable of multi-band
and/or multi-mode operation. The user stations 102 may be
multi-mode in that they may be capable of both spread spectrum
(i.e., wideband) communication and also narrowband communication.
The user stations 102 may be multi-band in the sense that they may
be set to operate on a plurality of different frequencies, such as
frequencies in either the licensed or unlicensed PCS bands. The
user stations 102 may operate in one mode (e.g., wideband) over a
first frequency band, and another mode (e.g., narrowband) over a
second frequency band.
[0315] As an example, a user station 102 may be set to operate on a
plurality of frequencies between 1850 and 1990 MHz, with the
frequencies separated in 625 kHz steps. Each user station 102 may
be equipped with a frequency synthesizer that may be programmed to
allow reception and/or transmission on any one of the plurality of
frequencies. If the user station 102 operates solely in a licensed
PCS band (e.g., 1850 MHz to MHz), the programmable frequency steps
may be in 5 MHz increments, in which case the first channel may be
centered at 1852.5 MHz, the next at 1857.5 MHz, and so on. If
operating in the isochronous band between 1920 and 1930 MHz, the
first channel may be centered at 1920.625 MHz, and the channel
spacing may be 1.25 MHz across the remainder of the isochronous
band. The user stations 102 may or may not be configured to operate
in the 1910 to 1920 MHz band, which at present is set apart in the
United States for asynchronous unlicensed devises.
[0316] Further information regarding dual-mode and/or dual-band
communication is set forth in U.S. patent application Ser. No.
08/483,514 filed on Jun. 7, 1995, hereby incorporated by reference
as if set forth fully herein.
[0317] In one embodiment, a communication protocol provides channel
information to a base station to select an antenna for
communication with a user station 102. Further, the protocol
provides for output power adjustment in a user station 102 and a
base station 104. A preferred power adjustment command from the
base station 104 to the user station 102 may be encoded according
to Table 8-2 appearing earlier herein. Although preferred values
are provided in Table 8-2, the number of power control command
steps and the differential in power adjustment between steps may
vary depending upon the particular application and the system
specifications. Further information regarding antenna diversity and
power adjustment technique may be found in copending U.S. patent
application Ser. No. 08/826,773 filed on Apr. 7, 1997, hereby
incorporated by reference as if set forth fully herein.
[0318] The present invention has been set forth in the form of its
preferred embodiments. It is nevertheless understood that
modifications and variations of the disclosed techniques for
carrying out fast control traffic, and for establishing and
maintaining spread spectrum communication, may be apparent to those
skilled in the art without departing from the scope and spirit of
the present invention. Moreover, such modifications are considered
to be within the purview of the appended claims.
* * * * *