U.S. patent application number 09/727712 was filed with the patent office on 2001-10-25 for cellular communication system with common channel soft handoff and associated method.
Invention is credited to Sarkar, Sandip, Tiedemann, Edward G. JR..
Application Number | 20010034233 09/727712 |
Document ID | / |
Family ID | 26782187 |
Filed Date | 2001-10-25 |
United States Patent
Application |
20010034233 |
Kind Code |
A1 |
Tiedemann, Edward G. JR. ;
et al. |
October 25, 2001 |
Cellular communication system with common channel soft handoff and
associated method
Abstract
A cellular communications system comprising; a mobile unit which
designates an active set of base stations; a base station; and a
base station controller which puts the base stations in the active
set in common channel soft handoff.
Inventors: |
Tiedemann, Edward G. JR.;
(San Diego, CA) ; Sarkar, Sandip; (San Diego,
CA) |
Correspondence
Address: |
QUALCOMM Incorporated
5775 Morehouse Drive
San Diego
CA
92121-1714
US
|
Family ID: |
26782187 |
Appl. No.: |
09/727712 |
Filed: |
November 30, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09727712 |
Nov 30, 2000 |
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09115217 |
Jul 13, 1998 |
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6216004 |
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60090357 |
Jun 23, 1998 |
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Current U.S.
Class: |
455/436 ;
455/442 |
Current CPC
Class: |
H04W 76/28 20180201;
H04W 24/00 20130101; H04W 36/30 20130101; H04W 36/08 20130101; H04W
24/10 20130101; H04W 36/18 20130101; H04W 68/025 20130101; H04W
52/0245 20130101; H04W 68/00 20130101; Y02D 30/70 20200801; H04W
52/0216 20130101 |
Class at
Publication: |
455/436 ;
455/442 |
International
Class: |
H04Q 007/20 |
Claims
1. A method comprising: identifying a mobile unit in a coverage
area of a base station of multiple base stations in a spread
spectrum communication system; monitoring by said mobile unit a
power level of control signals received from said multiple base
stations; identifying by the mobile unit a first set of base
stations in said multiple base stations for which the monitored
power level of the received control signal exceeds a prescribed
level; sending a power measurement message from the mobile unit to
at least one base station in the set that identifies the base
stations in the identified first set; sending to the mobile unit by
at least one base station in said multiple base stations a message
that specifies a second set of base stations that are permitted to
transmit to the mobile unit in soft handoff; communicating a
control message to the mobile unit for soft handoff that identifies
members of both the first set and the second set; wherein at least
one member of the second set of base stations is determined based
on independent soft handoff information other than the information
provided by the mobile unit relating to the power level of control
signals.
2. The method as recited in claim 1 further comprising: performing
a soft handoff communication from the base stations that are
members of both the first set and the second set.
3. The method as recited in claim 1 wherein said independent soft
handoff information is based on said identifying said mobile unit.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of application Ser. No.
09/115,217, filed Jul. 13, 1998, entitled "CELLULAR COMMUNICATION
SYSTEM WITH COMMON CHANNEL SOFT HANDOFF AND ASSOCIATED METHOD," now
allowed, which further claims priority to a provisional application
filed Jun. 23, 1998, entitled "PAGING CHANNEL SOFT HANDOFF," and
assigned application Ser. No. 60/090,357.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates in general to digital cellular
communication systems, and more particularly, to soft handoff in
spread spectrum communication systems.
[0004] 2. Description of the Related Art
[0005] In a code division multiple access (CDMA) spread spectrum
communication system, a shared frequency band is used for
communication with all base stations within that system. An example
of such a system is described in the TIA/EIA Standard TIA/EIA-95-B
entitled "Mobile Station-Base Station Compatibility Standard for
Dual Mode Wideband Spread Spectrum Cellular System", incorporated
herein by reference. The generation and receipt of CDMA signals is
disclosed in U.S. Pat. No. 4,401,307 entitled "SPREAD SPECTRUM
MULTIPLE ACCESS COMMUNICATION SYSTEMS USING SATELLITES OR
TERRESTRIAL REPEATERS" and in U.S. Pat. No. 5,103,459 entitled
"SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR
TELEPHONE SYSTEM", both of which are assigned to the assignee of
the present invention and incorporated herein by reference.
[0006] Radio Frequency (RF) signals are exchanged between a
respective mobile unit and one or more base stations. Mobile units
do not communicate directly with one another. Base stations
communicate with a base station cellular or personal communication
system controller, referred to herein as a base station controller
(BSC) using various media such as ground based wires or a microwave
link, for example. The BSC can route calls to a public switching
telephone network (PSTN) or can route packets to a packet switched
network, such as the Internet. The base station also coordinates
the operation of base stations within the system during soft
handoff for example.
[0007] TIA/EIA-95 is one example of a CDMA communication system.
Communication from a mobile unit to one or more base stations in a
TIA/EIA-95 CDMA system takes place over shared frequency channels
each of which occupies approximately 1.25 MHz of radio frequency
bandwidth. More specifically, communication signals occupying a
given frequency band are discriminated at a receiving station
through the spread spectrum CDMA waveform properties based on the
use of a high rate pseudonoise (PN) code. A PN code is used to
modulate signals transmitted from the base stations and mobile
units. Signals from different base stations can be separately
received at a given mobile unit by discrimination of different PN
codes. For the TIA/EIA-95 standard, these codes are constructed
from a single code, but each base station has a unique time offset
of the PN code. The high rate PN spreading also allows a receiving
station to receive a signal from a single transmission station
where the signal has traveled over distinct propagation paths.
Demodulation of multiple signals is disclosed in U.S. Pat. No.
5,490,165 entitled "DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM
CAPABLE OF RECEIVING MULTIPLE SIGNALS" and in U.S. Pat. No.
5,109,390 entitled "DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE
SYSTEM", both of which are assigned to the assignee of the present
invention and incorporated herein by reference.
[0008] The various channels within a given "forward" (base station
to mobile unit) TIA/EIA-95 CDMA channel include data channels, a
synchronization channel, a pilot channel, and a set of paging
channels, all transmitted from the base station to mobile units.
The pilot channel carries a pilot signal that is a regularly
repeated pattern that is differently phase offset for each base
station. The pilot provides for time reference and for amplitude
and phase tracking. The pilot signal allows mobile units to
identify and become synchronized with the various base stations
that are within range of their communication capability. The
synchronization channel carries additional synchronization
information for use by mobile units. The set of data channels carry
the data associated with the various communication sessions
(usually phone calls) and are directed to individual mobile units.
These data channels are called traffic channels in TIA/EIA-95. The
paging channels are used by the base stations to notify mobile
units when a request to communicate has been received.
[0009] The protocol for paging a subscriber unit typically is
defined so as to evenly distribute power transmitted by the base
station over time, and to reduce power consumption in the mobile
unit. In some CDMA systems, the mobile unit power consumption while
monitoring the paging channel is reduced by dividing each base
station's paging channel into a set of paging "slots" or time
windows during which paging signals directed to mobile units may be
generated, and by assigning a particular CDMA frequency channel,
paging channel and paging slot to each mobile unit in an evenly
distributed fashion. This assignment generally may be performed,
for example, via the use of a hashing function applied to the
International Mobile Station Identity (IMSI) assigned to each
mobile unit, although other unique numbers could be utilized. The
hashing function of set of hashing functions yields a value that
corresponds to a particular set of paging parameters including a
CDMA channel, paging channel, and paging slot. Mobile units are
then configured to "listen" for a paging message over their
assigned CDMA frequency channel and paging channel during the
corresponding paging slot. Since listening for a page requires a
certain amount of power, limiting the time a mobile unit performs
the listening function to a particular paging slot reduces the
overall power consumption of that mobile unit and therefore
increases the life of any battery or other power storage system
utilized by that mobile unit.
[0010] During a mode of operation called "soft handoff", the mobile
station simultaneously exchanges identical communications traffic
on a CDMA traffic channel between two or more different base
stations. In the case of (forward link) reception by a mobile
station, the signals from the multiple base stations can be
coherently combined to improve performance, as with multipath
combining. In fact, the second base station signal can be regarded
as a delayed version of the first, generated actively and
purposely, rather than as a delayed reflection of the first caused
by the environment. See, CDMA, Principles of Spread Spectrum
Communication, by Andrew J. Viterbi, Addison-Wesley Pub. Co., 1995,
pp. 181, 183-184, 198-199 and 222-224. Soft handoff is further
disclosed in U.S. Pat. No. 5,101,501 entitled "SOFT HANDOFF IN A
CDMA CELLULAR TELEPHONE SYSTEM" and U.S. Pat. No. 5,267,261
entitled MOBILE STATION ASSISTED SOFT HANDOFF IN A CDMA CELLULAR
COMMUNICATIONS SYSTEM", both assigned to the assignee of the
present invention and incorporated herein by reference. Similarly,
a mobile unit can simultaneously communicate with two sectors of
the same base station, known as softer handoff, as disclosed in
copending U.S. Pat. No. 5,625,876 entitled "METHOD AND APPARATUS
FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION",
assigned to the assignee of the present invention and incorporated
herein by reference. Handoffs are described as soft and softer
because they make the new connection before breaking the existing
one.
[0011] In a typical CDMA communication system, mobile unit assisted
soft handoff operates based on the pilot signal strength of several
sets of base stations as measured by the mobile unit. In the
TIA/EIA-95 CDMA communication system, for example, an Active Set is
the set of base stations through which active communication is
established. A Neighbor Set is a set of base stations surrounding
an active base station comprising base stations that have a high
probability of having a pilot signal strength of sufficient level
to establish communication. A Candidate Set is a set of base
stations having a pilot signal strength of sufficient level to
establish communication.
[0012] In at least one earlier CDMA system, for example, soft
handoff can be set up between a mobile unit and two or more base
stations in the mobile unit's Active Set. For instance, in such an
earlier CDMA system, when communications are initially established,
a mobile unit typically communicates through a first base station,
and the Active Set contains only the first base station. The mobile
unit monitors the pilot signal strength of the base stations of the
Active Set, the Candidate Set, and the Neighbor Set. When a pilot
signal of a base station in the Neighbor Set exceeds a
predetermined threshold level, the base station is added to the
Candidate Set and removed from the Neighbor Set at the mobile unit.
The mobile unit communicates a message to the first base station
identifying the new base station. A base station controller decides
whether to establish communication between the new base station and
the mobile unit. If the base station controller decides to
establish communication, then it sends a message to the new base
station with identifying information about the mobile unit and a
command to establish communications therewith. A message is also
transmitted to the mobile unit through the first base station. The
message identifies a new Active Set that includes the first and the
new base stations. The mobile unit searches for the new base
station transmitted information signal, and communication is
established with the new base station in soft handoff (without
termination of communication through the first base station). This
process can continue with additional base stations.
[0013] When the mobile unit is communicating through multiple base
stations, it continues to monitor the signal strength of the base
stations of the Active Set, the Candidate Set, and the Neighbor
Set. Should the signal strength corresponding to a base station of
the Active Set drop below a predetermined threshold for a
predetermined period of time, the mobile unit generates and
transmits a message to report the event. The base station
controller receives this message through at least one of the base
stations with which the mobile unit is communicating. The cellular
or personal communication system controller may decide to terminate
communications through the base station having a weak pilot signal
strength.
[0014] While soft handoff generally has been successful on
dedicated channels such as traffic channels between a mobile unit
and one or more base stations in an Active Set, soft handoff
generally has not been available on common channels such as the
paging channel, for example. One reason for the absence of soft
handoff on a typical paging channel is that soft handoff ordinarily
requires sending identical messages from every base station
participating in the soft handoff. In prior systems, however, this
could result in a considerable reduction in paging channel
capacity. For example, let the number of base stations involved in
soft handoff on the paging channel be N. Then the paging channel
capacity could be reduced by a factor of N. In even a small city,
there are tens of cells; and the paging channel capacity could be
substantially reduced. Since all base stations in a network cannot
be in soft handoff, then there would probably have to be boundaries
between groups of cells, and the performance between these groups
of cells would be reduced. It is possible to use sets of
overlapping groups of cells which are in soft handoff. However,
this could mean that many messages would have to be transmitted by
multiple sets of these cells, thus resulting in lower paging
channel capacity. Unfortunately, since the paging channel generally
is not operated in soft handoff, fading and shadowing can cause the
forward link of one base station to become stronger than the
forward link of another base station. This can result in
significant problems when the mobile unit is attempting to perform
system access since it generally cannot as readily perform (hard)
handoff in the System Access State. Moreover, calls often are
dropped due to delays in getting the mobile unit into soft handoff
after the traffic channel is set up.
[0015] A similar problem exists when the mobile unit is first
assigned to a traffic channel, and the mobile unit initially
receives the traffic channel from only one base station. This
similar problem was addressed by a technique included in the
TIA/EIA-95-B standard, called "Soft-Channel Assignment." Soft
Channel Assignment is disclosed in U.S. Pat. No. 6,021,122 which
had been issued on Feb. 1, 2000 and is entitled METHOD AND
APPARATUS FOR PERFORMING IDLE HANDOFF IN A MULTIPLE ACCESS
COMMUNICATION SYSTEM, assigned to the assignee of the present
invention and incorporated herein by reference. Soft Channel
Assignment reduces the problem of delays in getting a station into
soft handoff following traffic channel assignment. During such Soft
Channel Assignment, a Channel Assignment Message carries a list of
base stations which should be in the mobile station's Active Set.
By doing Soft Channel Assignment, the mobile unit is placed into
soft handoff as soon as it is assigned to the traffic channel. This
significantly increases the speed in which the mobile unit can be
placed into soft handoff, thus increasing the reliability of the
call set up.
[0016] Various techniques have been proposed in the TIA/EIA-95-B
specification to reduce the incidence of dropped calls resulting
from fading of the paging channel. Two such techniques are called,
"Access Probe Handoff" and "Access Handoff". The basic principles
are disclosed in U.S. Pat. No. 6,021,122 which had been issued on
Feb. 1, 2000.
[0017] Access Probe Handoff permits a mobile unit to switch to
monitoring a new base station between access probes. The mobile
unit would switch to monitoring a new base station whenever the
forward link of the base station that the mobile unit is monitoring
becomes too weak. Thus, for instance, if the mobile unit does not
receive an acknowledgement to an access probe, and the mobile unit
determines that the paging channel is weak, then the mobile unit
may shift to using a new base station. An Extended System
Parameters Message sent by the base station to the mobile unit
informs the mobile unit of whether it is permitted to perform an
Access Probe Handoff and the set of base stations to which the
mobile unit is permitted to perform the Access Probe Handoff. In
one earlier embodiment, for every base station in the mobile
station's neighbor list, the Extended System Parameters Message has
a one-bit flag that indicates whether an Access Probe Handoff is
permitted to that base station.
[0018] Access Handoff permits a mobile unit to switch to monitoring
a new base station while awaiting a Channel Assignment Message.
Thus, for example, if the mobile unit has received an
acknowledgement to its access probe, and the mobile unit determines
that the paging channel is weak, then the mobile unit may shift to
using a new base station. An Extended System Parameter Message sent
by the base station to the mobile unit informs the mobile unit of
whether it is permitted to perform an Access Handoff and the set of
base stations to which it is permitted to perform the Access
Handoff. In one earlier embodiment, for every base station in the
mobile station's neighbor list, the Extended System Parameters
Message has a one-bit flag that indicates whether an Access Handoff
is permitted to that base station.
[0019] Yet another earlier method specified in TIA/EIA-95-B,
referred to as, Access Entry Handoff, permits a mobile unit to
begin monitoring a new base station from the time in which the
mobile unit receives a page until it transmits a Page Response
Message.
[0020] Traffic channel handoff generally requires the mobile
station to receive an explicit message instructing the mobile unit
to perform the handoff. In Access Probe Handoff, Access Handoff,
and Access Entry Handoff, the mobile unit does not receive an
explicit message instructing it to handoff, but performs a handoff
autonomously to a limited set of base stations about which it has
informed the base station and about which have been pre-enabled by
the base station.
[0021] In order to facilitate Soft Channel Assignment, Access Probe
Handoff, and Access Handoff, in accordance with the TIA/EIA-95-B
specification, the Access Channel Messages sent by a mobile unit
identify the set of base stations whose received signal strengths
are above a certain threshold level, called T_ADD in TIA/EIA-95-B.
The base stations having strengths above T_ADD are those base
stations that would be typically placed into the Active Set if the
mobile station were on the traffic channel. In TIA/EIA-95-B, the
mobile station reports two different lists of base stations in the
Access Channel Message. One list is the IDLE_HANDOFF_LIST. This is
the set of base stations for which the received signal strengths
exceed T_ADD and for which the mobile unit is allowed to perform an
Access Probe or Access Handoff. The latter is controlled by one-bit
flags in an Extended System Parameters Message corresponding to
neighboring base stations. A second set of base stations are those
for which the signal strengths exceed T_ADD, and for which Access
Probe or Access Handoff is not allowed. The base station can use
this information when performing soft channel assignment.
[0022] The illustrative drawing of FIG. 1 provides a hypothetical
example of the operation of Access Probe Handoff, Access Handoff
and Access Entry Handoff.
[0023] Nevertheless, there still have been problems with lost calls
due to the absence of soft handoff on common channels. For example,
there have been problems with lost calls due to loss of the paging
channel during call setup. This results in annoyance to customers
of the system. Furthermore, the amount of power required to be
transmitted on the paging channel is quite large, thus reducing the
overall capacity of the system.
[0024] Thus, there has been a need for soft handoff on common
channels, such as the paging channel, in a cellular system, such as
a spread spectrum communication system. The present invention meets
this need.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is an illustration of an exemplary sequence of
messaging and monitoring activities involving a mobile unit and one
or more base stations when the mobile unit is in a System Access
State in accordance with the TIA/EIA-95-B specification;
[0026] FIG. 2 is a schematic overview of an exemplary CDMA cellular
communication system in accordance with a present embodiment of the
invention;
[0027] FIG. 3 is a block diagram of a mobile unit in accordance
with a present embodiment of the invention, configured for
communications in the CDMA cellular system of FIG. 2;
[0028] FIG. 4 is a block diagram of a base station in accordance
with a presently preferred embodiment of the invention, configured
for communication in the CDMA cellular system of FIG. 2;
[0029] FIG. 5 is a block diagram of a base station controller in
accordance with a present embodiment of the invention, configured
for communications in the CDMA cellular system of FIG. 2;
[0030] FIG. 6 is an illustrative message sequence diagram showing
the re-transmission of paging messages on the F-BCCH in accordance
with a present embodiment of the invention;
[0031] FIG. 7 is an illustrative message sequence diagram showing
the interleaving of re-transmitted paging messages on the F-BCCH in
accordance with an alternative embodiment of the invention;
[0032] FIG. 8 is an illustrative F-QPCH and F-CCCH paging channel
timing diagram in accordance with a present embodiment of the
invention;
[0033] FIG. 9 is an illustrative message sequence diagram showing a
call origination message sequence transmitted in paging channel
soft handoff between a mobile unit, multiple base stations and a
base station controller in accordance with a present embodiment of
the invention;
[0034] FIG. 10 is an illustrative message sequence diagram showing
a call termination message sequence transmitted in paging channel
soft handoff between a mobile station, multiple base stations and a
base station controller in accordance with a present embodiment of
the invention;
[0035] FIG. 11 is an illustrative message sequence diagram showing
an alternative call termination message sequence transmitted in
paging channel soft handoff between a mobile station, multiple base
stations and a base station controller in accordance with a present
embodiment of the invention;
[0036] FIG. 12 is a message sequence diagram illustrating two
different sets of messages transmitted in soft handoff to two
different mobile units by two overlapping sets of base stations
operating synchronously in accordance with a present embodiment of
the invention; and
[0037] FIG. 13 is a message sequence diagram illustrating two
different sets of messages transmitted in soft handoff to two
different mobile units by two overlapping sets of base stations
that operate asynchronously in accordance with a present embodiment
of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] The present invention comprises novel soft handoff on a
common communication channel, such as a paging channel, in a
cellular communication system. The following description is
presented to enable any person skilled in the art to make and use
the invention, and is provided in the context of a particular
application and its requirements. Various modifications to the
preferred embodiment will be readily apparent to those skilled in
the art, and the generic principles defined herein may be applied
to other embodiments and applications without departing from the
spirit and scope of the invention. Thus, the present invention is
not intended to be limited to the embodiment shown, but is to be
accorded the widest scope consistent with the principles and
features disclosed herein.
[0039] Overall System Architecture
[0040] A CDMA cellular communication system in accordance with a
presently preferred embodiment of the invention is disclosed in
U.S. Pat. No. 5,640,414, entitled "MOBILE STATION ASSISTED SOFT
HANDOFF IN CDMA CELLULAR COMMUNICATION SYSTEM," which is assigned
to the assignee of the present invention and which is hereby
incorporated herein by this reference. Referring to the
illustrative drawing of FIG. 2, there is shown an exemplary digital
cellular communication system. A base station controller 10 may be
coupled to the base stations by various means such as dedicated
telephone lines, optical fiber links or by microwave communication
links. In FIG. 2, three such exemplary base stations, 12, 14 and 16
and an exemplary mobile unit 18 are illustrated. Arrows 20a-20b
define possible radio communication links between base station 12
and mobile unit 18. Arrows 22a-22b define possible communication
links between base station 14 and mobile unit 18. Similarly, arrows
24a-24b define the possible radio communication links between base
station 16 and mobile unit 18. A mobile station may be in the
coverage area of a single base station or may be in an area where
it can receive signals from multiple base stations as is shown in
FIG. 2.
[0041] In a presently preferred embodiment of a CDMA cellular
communication system, each base station transmits a upon pilot
channel a "pilot carrier" signal or, for short, pilot signal. The
pilot signal is an unmodulated, direct sequence, spread spectrum
signal transmitted at all times by each base station using a common
pseudorandom noise (PN) spreading code. The pilot signal allows the
mobile units to obtain initial system synchronization, i.e. timing,
in addition to providing a phase reference for coherent
demodulation and a reference for signal strength for comparisons
between base stations for handoff determination.
[0042] The pilot signal as transmitted by each base station in the
present embodiment is of the same PN spreading code but with a
different code phase offset. For example, in a present embodiment
of the invention that pilot signal spreading code is of a PN code
length of 2.sup.15. In this example there are 511 different offsets
from the zero offset, where the offsets are in increments of 64 PN
chips. It is this phase offset which allows the pilot signals to be
distinguished from one another by the mobile station, resulting in
a differentiation between base stations from which they originate.
Use of the same pilot signal code allows the mobile station to find
system timing synchronization by a single search through all pilot
signal code phases. The strongest pilot signal, as identified
through a correlation process for each code phase, is readily
identifiable. The identified pilot signal generally corresponds to
the pilot signal transmitted by the base station with the smallest
path delay, which is often, but not always the strongest base
station.
[0043] Each base station in the present embodiment of the invention
also transmits a sync channel signal which is a modulated, encoded,
interleaved, direct sequence, spread spectrum signal used by the
mobile stations to acquire additional synchronization, system time
and, along with these, other overhead control information.
Information such as system identification, network identification,
a pilot PN sequence offset index, a long code state, current system
time along with other time parameters, and paging channel data rate
are transmitted on the sync channel. It should be noted that the
pilot PN sequence offset index identifies an offset value from a
zero offset pilot PN sequence. The sync channel signal is despread
using the same pilot PN sequence offset as the pilot channel.
[0044] Each base station in the current embodiment also transmits
on one or more paging channels corresponding paging channel
signals. In a presently preferred embodiment of the invention, the
paging channel messages are converted modulated signals for radio
transmission. More specifically, in a current embodiment, the
paging channel messages are encoded, interleaved, scrambled,
modulated, direct sequence, spread spectrum signals. Overhead
messages transmitted on the paging channel include a system
parameters message, which contains general system and base station
overhead information; an access parameters message, which contains
information to be used by the mobile unit on an Access Channel when
accessing the system; a neighbors list message which identifies to
the mobile station the pilot signal PN sequence offset of the
neighboring base stations; a CDMA channel list identifying the 1.25
MHz CDMA channels available in this base station, and an Extended
System Parameters Message which has information pertaining to
access probe handoff and access handoff. Like the sync channel
signals, the paging channel signals are spread and despread using
the same pilot PN sequence offset as the pilot channel. In the
present embodiment, the paging channel is implemented as a slotted
multiple channel structure described more fully below.
[0045] Each base station transmits user information to an intended
mobile unit on a selected one of a plurality of traffic channels.
Each mobile unit is thus assigned to a unique traffic channel for
receiving the mobile unit intended information. In a current
embodiment of the invention, the traffic channel signals are
modulated, interleaved, scrambled, direct sequence, spread spectrum
signals transmitted to mobile units on a respective traffic
channel. Information received in the sync channel message is used
by the mobile unit to de-scramble the traffic channel scrambled
signal.
[0046] Further details on the modulation scheme for the various
channels of the base station are described in U.S. Pat. No.
5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING SIGNAL
WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", assigned to the
assignee of the present invention, the disclosure thereof
incorporated by reference.
[0047] A mobile initiated handoff relies on the mobile unit to
detect the presence or absence of pilot signals, and the signal
strength of the pilot signals. The mobile unit identifies and
measures the signal strength of the pilot signals which it
receives. This information is communicated via the base station(s)
to which the mobile unit is communicating through to the base
station controller (BSC). The BSC upon receiving this information
initiates or tears down the soft handoffs. To streamline the
process of searching for pilots, four distinct sets of pilot
offsets are defined: the Active Set, the Candidate Set, the
Neighbor Set, and the Remaining Set. The Active Set identifies the
base station(s) or sector(s) through which the mobile unit is
communicating. The Candidate Set identifies the base station(s) or
sector(s) for which the pilots have been received at the mobile
unit with sufficient signal strength to make them members of the
Active Set, but have not been placed in the Active Set by the base
station(s). The Neighbor Set identifies the base station(s) or
sector(s) which are likely candidates for the establishment of
communication with the mobile unit. The Remaining Set identifies
the base station(s) or sector(s) having all other possible pilot
offsets in the current system, excluding those pilot offsets
currently in the Active, the Candidate and Neighbor sets. Further
details on the use of these sets in the handoff scheme are
discussed later in further detail.
[0048] When a call is set up, a pseudorandom noise (PN) code
address is determined for use during the course of this call.
Generally, this code address is used to mask the PN long sequence
code in a the code unique the code unique to the communication
between the base station and the mobile unit. The code address may
be either assigned by the base station or preferably be determined
by prearrangement based upon the identity of the mobile unit.
[0049] After a call is set up, the mobile unit continues to scan
the pilot signals transmitted by base stations located in
neighboring cells. Pilot signal scanning continues in order to
determine if one or more of the neighboring base station
transmitted pilot signals rises above a predetermined threshold, a
level which is indicative that communications may be supported
between the base station and the mobile unit. When the pilot signal
transmitted by a base station located in a neighboring cell rises
above the threshold, it serves as an indication to the mobile
station that a handoff should be initiated. In response to this
pilot signal strength determination, the mobile unit generates and
transmits a control message to the base station presently servicing
the call. This control message is relayed on to the base station
controller (BSC).
[0050] Mobile Unit
[0051] FIG. 3 illustrates in block diagram form an exemplary mobile
unit cellular telephone in accordance with a presently preferred
embodiment of the invention. The mobile unit includes an antenna 30
which is coupled through diplexer 32 to analog receiver 34 and
transmit power amplifier 36. Antenna 30 and diplexer 32 are
standard design and permit simultaneous transmission and reception
through a single antenna. Antenna 30 collects transmitted signals
and provides them through diplexer 32 to analog receiver 34.
Receiver 34 receives the RF frequency signals from diplexer 32
which in the current embodiment are typically in the 850 MHz
frequency band for U.S. cellular, the 1.9 GHz frequency band for
U.S. PCS. Then the signals are amplified and down converted to an
IF frequency. This frequency translation process is accomplished
using a frequency synthesizer of standard design which permits the
receiver to be tuned to any of the frequencies within the receive
frequency band of the overall cellular telephone frequency
band.
[0052] The IF signal is then passed through a surface acoustic wave
(SAW) bandpass filter which in the preferred embodiment is
approximately 1.25 MHz in bandwidth. The characteristics of the SAW
filter are chosen to match the wave-form of the signal transmitted
by the bas station which has been direct sequence spread spectrum
modulated by a PN sequence clocked at a predetermined rate, which
in the preferred embodiment is 1.2288 MHz.
[0053] Receiver 34 also performs a power control function for
adjusting the transmit power of the mobile station. Receiver 34
generates an analog power control signal that is provided to
transmit power control circuitry 38. The control and operation of
the mobile station power control feature is disclosed in U.S. Pat.
No. 5,056,109 entitled "METHOD AND APPARATUS FOR CONTROLLING
TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM,"
assigned to the assignee of the present invention, to which the
disclosure is also incorporated by reference.
[0054] Receiver 34 is also provided with an analog to digital (A/D)
converter (not shown) for converting the IF signal to a digital
signal. The digitized signal is provided to each of three or more
signal processors or data receivers, one of which is a searcher
receiver with the remainder being data receivers. For purposes of
illustration only one searcher receiver and two data receivers are
shown in FIG. 3.
[0055] In FIG. 3, the digitized signal output from receiver 34 is
provided to digital data receivers 40 and 42 and to searcher
receiver 44. It should be understood that an inexpensive, low
performance mobile station might have only a single data receiver
while higher performance stations may have two or more, preferably
a minimum of three, to allow diversity reception.
[0056] The digitized IF signal may contain the signals of many
on-going calls together with the pilot carriers transmitted by the
current and all neighboring base stations. The function of the
receivers 40 and 42 is to correlate the IF samples with the proper
PN sequence. This correlation process provides a property that is
well-known in the art as "processing gain" which enhances the
signal-to-interference ratio of a signal matching the proper PN
sequence while not enhancing other signals. The correlation output
is then coherently detected using the pilot carrier offset PN
sequence used for the correlation as a carrier phase reference. The
result of this detection process is a sequence of encoded data
symbols.
[0057] A property of the PN sequence as used in the present
invention is that discrimination is provided against multi-path
signals. When the signal arrives at the mobile receiver after
passing through more than one path, there will be a difference in
the reception time of the signal. This reception time difference
corresponds to the difference in distance divided by the speed of
light. If this time difference exceeds one PN chip, 0.8138
microseconds, in the preferred embodiment, then the correlation
process will discriminate against one of the paths. The receiver
can choose whether to track and receive the earlier or later path.
If two receivers are provided, such as receivers 40 and 42, then
two independent paths can be tracked simultaneously.
[0058] Searcher receiver 44, under control of control processor 46
is for continuously scanning the time domain, around the nominal
time of a received pilot signal of the base station, for other
multi-path pilot signals from the same base station and for other
base station transmitted pilot signals. Receiver 44 will measure
the strength of any reception of a desired waveform at times other
than the nominal time. Receiver 44 uses the ratio of the received
pilot energy per chip to total received spectral density, noise and
signals, denoted as E.sub.c/I.sub.o as a measure of the pilot
signal strength. Receiver 44 provides a signal strength measurement
signal to control processor 46 indicative of the pilot signal and
its signal strength.
[0059] Processor 46 provides signals to data receivers 40 and 42
for each to process a different one of the strongest signals.
Receivers 40 and 42 may process a multipath signal from a single
base station or signals from two different base stations.
[0060] The outputs of receivers 40 and 42 are provided to diversity
combiner and decoder circuitry 48. The diversity combiner circuitry
contained within circuitry 48 adjusts the timing of the two streams
of received signals into alignment and adds them together. This
addition process may be preceded by multiplying the two streams by
a number corresponding to the relative signal strengths of the two
streams. This operation can be considered a maximal ratio diversity
combiner. The resulting combined signal stream is then decoded
using a forward stream error detection decoder also contained
within circuitry 48.
[0061] In the exemplary embodiment convolutional encoding is
utilized. The optimum decoder for this type of code is of the soft
decision Viterbi algorithm decoder design. The resulting decoded
information bits are passed to the user digital baseband circuitry
50.
[0062] Baseband circuitry 50 typically includes a digital vocoder
(not shown). Baseband circuitry 50 further serves as an interface
with a handset or any other type of peripheral device. Baseband
circuitry 50 accommodates a variety of different vocoder designs.
Baseband circuitry 50 provides output information signals to the
user in accordance with the information provided thereto from
circuitry 48. It should be understood that various other types of
service, other than voice, may also be offered to which the
disclosure herein is equally applicable.
[0063] User analog voice signals typically provided through a
handset are provided as an input to baseband circuitry 50. Baseband
circuitry 50 includes an analog to digital (A/D) converter (not
shown) which converts the analog signal to digital form. The
digital signal is provided to the digital vocoder where it is
encoded. The vocoder output is provided to a forward error
correction encoding circuit (not shown) for error correction. This
voice digitized encoded signal is output from baseband circuitry 50
to transmit modulator 52.
[0064] During transmission of call traffic after a traffic channel
has been set up, transmit modulator 52 modulates the encoded signal
on a PN carrier signal whose PN sequence is chosen according to the
assigned address function for the call. The PN sequence is
determined by control processor 46 from call setup information that
is transmitted by the base station and decided receivers 40 and 42.
In the alternative, control processor 46 may determine the PN
sequence through prearrangement with the base station. Control
processor 46 provides the PN sequence information to transmit
modulator 52 and to receivers 40 and 42 for call decoding. Transmit
modulator 52 also modulates the data with a common unshifted
version of the PN code that is used by the base station.
[0065] The output of transmit modulator 52 is provided to transmit
power control circuitry 38. Signal transmission power is controlled
by the analog power control signal provided from receiver 34.
Furthermore, control bits are transmitted by the base stations in
the form power adjustment command and are processed by data
receivers 40 and 42. The power adjustment command is used by the
control processor in setting the power level in mobile station
transmission. In response to the power adjustment commands, control
processor 46 generates a digital power control signal that is
provided to circuitry 38.
[0066] Transmit power control circuitry 38 outputs the power
controlled modulated signal to transmit power amplifier circuitry
36. Circuitry 36 amplifies and converts the IF signal to an RF
frequency by mixing with a frequency synthesizer output signal
which tunes the signal to the proper output frequency. Circuitry 36
includes an amplifier which amplifies the power to a final output
level. The intended transmission signal is output from circuitry 36
to diplexer 32. Diplexer 32 couples the signal to antenna 30 for
transmission to the base stations.
[0067] Control processor 46 is also capable of generating control
messages such as cell-diversity mode requests and base station
communication termination commands. These commands are provided to
transmit modulator 52 for transmission. Control processor 46 is
responsive to the data received from data receivers 40, 42 and
search receiver 44 for making decisions relative to handoff and
diversity combining.
[0068] Base Station
[0069] FIG. 4 illustrates in block diagram form an exemplary
embodiment of the base station equipment in accordance with a
presently preferred embodiment of the invention. At the base
station, two receiver systems are utilized with each having a
separate antenna and analog receiver for space diversity reception.
In each of the receiver systems the signals are processed
identically until the signals undergo a diversity combination
process. The elements within the dashed lines correspond to
elements corresponding to the communications between the base
station and one mobile station. The output of the analog receivers
are also provided to other elements used in communications with
other mobile stations.
[0070] In FIG. 4, the first receiver system is comprised of antenna
60, analog receiver 62, searcher receiver 64 and digital data
receiver 66. This receiver system may also include an optional
digital data receiver 68. Although only one optional digital data
receiver 68 is illustrated it should be understood that several
additional ones may be used. The second receiver system includes
antenna 70, analog receiver 72, searcher receiver 74 and digital
data receiver 76. Again additional optional digital data receivers
(not shown) may be utilized for this receiver system. Also utilized
in signal processing and control for handoff and diversity is base
station control processor 78. Both receiver systems are coupled to
diversity combiner and decoder circuitry 80. Digital link 82 is
utilized to communicate signals to and from the base station
controller (FIG. 5) with base station transmit modulator 84 and
circuitry 80 under the control of control processor 78.
[0071] Signals received on antenna 60 are provided to analog
receiver 62. Received signals amplified by an amplifier in receiver
62 are translated to an IF frequency by mixing with a frequency
synthesizer output signal. The IF signals are bandpass filtered and
digitized in a process identical to that described with reference
to the mobile station analog receiver. The digitized IF signals are
provided to digital data receiver 66, optional data receiver 68 and
searcher receiver 64 and are processed respectively in a manner
similar to that as disclosed with reference to the digital data
receivers and searcher receiver of the mobile station in FIG. 3.
However, the processing by the digital data receivers and searcher
receivers are different for the mobile to base station link from
that used in the base unit to mobile link in several respects.
[0072] In the inbound, or mobile unit to base station link, the
mobile unit does not transmit a pilot signal that can be used for
coherent reference purposes in signal processing at the base
station. Thus, in a present embodiment, the mobile unit to base
station link utilizes a non-coherent modulation and demodulation
scheme using 64-ary orthogonal signaling.
[0073] Searcher receiver 64 is again used to scan the time domain
about the receiver signal to ensure that the associated digital
data receiver 66, and data receiver 68 if used, are tracking and
processing the strongest available time domain signals. This
tracking process is identical to that described with reference to
the mobile station. Searcher receiver 64 provides a signal to base
station control processor 78 which provides control signals to
digital data receivers 66 and 68 for selecting the appropriate
received signals for processing.
[0074] The digital data receiver 66 (and 68) produces estimates of
the received signals and provides weighting of the received data
signals. The weighting function is linked to measured signal
strength. The weighted data is then provided as an output to
diversity combiner and decoder circuitry 80.
[0075] The second receiver system processes the received signals in
a manner similar to that discussed with respect to the first
receiver system of FIG. 4. The outputs from receivers 66 and 76 are
provided to diversity combiner and decoder circuitry 80. Circuitry
80 includes adding and other weighting circuitry that combines the
symbols from digital data receiver 66 with the weighted symbols
from receiver 76. The result is used to determine a set of decoder
weights and symbols for use within a Viterbi algorithm decoder
implemented in circuitry 80.
[0076] The Viterbi decoder is utilized to determine the most likely
information bit sequence. For each vocoder data block, nominally 20
msec. of data, a signal quality estimate is obtained and
transmitted as a mobile station power adjustment command along with
data to the mobile station. The quality estimate is the average
signal-to-noise ration over the 20 msec interval.
[0077] In FIG. 4, optional digital data receiver 68 may be included
for improved performance of the system. This additional data
receiver alone or in combination with additional receivers can
track and receive other possible delay paths of mobile station
transmitted signals. The structure and operation in this receiver
is similar to that described with reference to the digital data
receivers 66 and 76. Receiver 68 is utilized to obtain additional
diversity modes.
[0078] Signals from the BSC are coupled to the appropriate transmit
modulator via digital link 82 under the control of control
processor 78. Transmit modulator 84 spread spectrum modulates,
according to a predetermined spreading function (PN code) as
assigned by control processor 78, the data for transmission to the
intended recipient mobile station. The output of transmit modulator
84 is provided to transmit power control circuitry 86 where under
the control of control processor 78, the transmission power may be
controlled. The output of circuitry 86 is provided to transmit
power amplifier circuitry 88.
[0079] In the preferred implementation, each of a traffic channel
(user data communication channel), sync channel, one or more paging
channels, and pilot channel are modulated by a different Walsh
function sequence. Although only the traffic channels are modulated
with each unique PN code, each traffic channel along with the other
channels are modulated by a common PN sequence. In the exemplary
implementation the pilot channel Walsh function sequence is the
"all zero" sequence thus resulting the pilot signal being the
common PN sequence itself. All signals as modulated by the common
PN sequence are provided to transmit power amplifier circuitry
88.
[0080] Circuitry 88 includes a summer for summing the output of
transmit modulator 84 with the output of other transmit modulators
at the base station. Circuitry 88 further includes a summer for
summing the pilot signal/sync channel signal/paging channel signal
output from generator 90 with the summed transmit modulator output
signals. Circuitry 88 also includes a digital to analog converter,
frequency upconversion circuitry and an amplifier for respectively
converting the digital signals to analogs signals, converting the
IF frequency signals as output from the transmit modulators to an
RF frequency and amplifying the RF signal. The output from
circuitry 88 is provided to antenna 92 where it is radiated to
mobile units within the base station service area.
[0081] Base station control processor 78 has the responsibility for
assignment of digital data receivers and modulators to a particular
call. Control processor 78 also monitors the progress of the call,
quality of the signals and initiates teardown on loss of signal.
The base station communicates with the BSC via link 82 where it is
coupled by a standard telephone wire, optical fiber, or microwave
link.
[0082] Base Station Controller
[0083] FIG. 5 illustrates in block diagram form the equipment
utilized in the BSC of a presently preferred embodiment of the
invention. The BSC typically includes a system controller or system
control processor 100, digital switch 102, diversity combiner 104,
digital vocoder 106 and digital switch 108. In a present
embodiment, the switch 102 is a pocket switch. Although not
illustrated, additional diversity combiners and digital vocoders
are coupled between digital switches 102 and 108.
[0084] When the cell-diversity mode is active, or the BSC is in the
handoff process with the call processed by two or more base
stations, signals will arrive at the BSC from more than one base
station with nominally the same information. However, because of
fading and interference on the inbound link from the mobile station
to the base stations, the signal from one base station may be of
better quality than the signal from the other base station.
[0085] Digital switch 102 is used in routing the information stream
corresponding to a given mobile station from one or more base
stations to diversity combiner 104 or the corresponding diversity
combiner as determined by a signal from system control processor
100. When the system is not in the cell-diversity mode, diversity
combiner 104 may be either bypassed or fed the same information on
each input port.
[0086] A multiplicity of serial coupled diversity combiners (or
selectors) and vocoders are provided in parallel, nominally, one
for each call to be processed. Diversity combiner 104 compares the
signal quality indicators accompanying the information bits from
the two or more base station signals. Diversity combiner 104
selects the bits corresponding to the highest quality base stations
signal on a frame-by-frame basis of the information for output to
vocoder 106.
[0087] In a present embodiment, Vocoder 106 converts the format the
digitized voice signal to standard 64 Kbps PCN telephone format,
analog, or any other standard format. The resultant signals are
transmitted from vocoder 106 to digital switch 108. Under the
control of system control processor 100, the call routed to the
PSTN.
[0088] Voice signals coming from the PSTN intended for a mobile
unit are provided to digital switch 108 to an appropriate digital
vocoder such as vocoder 106 under control of system control
processor 100. Vocoder 106 encodes the input digitized voice
signals and provides the resulting information bit stream directly
to digital switch 102. Digital switch 102 under system control
processor control directs the encoded data to the base station or
base stations to which the mobile unit is communicating. If the
mobile unit is in a handoff mode communicating to multiple base
stations or in a cell diversity mode, digital switch 102 routes the
calls to the appropriate base stations for transmission by the
appropriate base station transmitter to the intended recipient
mobile station. However, if the mobile unit is communicating with
only a single base station or not in a cell diversity mode, the
signal is directed only to a single base station.
[0089] System control processor 100 provides control over digital
switches 102 and 108 for routing data to and from the BSC. System
control processor 100 also determines the assignment of calls to
the base stations and to the vocoders at the BSC. Furthermore,
system control processor 100 communicates with each base station
control processor about the assignment of particular calls between
the BSC and base station, and the assignment PN codes for calls. It
should be further understood that as illustrated in FIG. 5 digital
switches 102 and 108 are shown as two separate switches, however,
this function may be performed by a single physical switching
station.
[0090] It should also be understood the embodiment provided herein
with respect to the system architecture is merely and exemplary
embodiment of the system and that other system architecture may be
employed. For example, as described herein the system controller is
located at the BSC for control of many of the base station
functions and handoff function. In an equally preferred mode, many
of the functions of the system controller may be distributed
throughout the base station.
[0091] Overview of System Operation During Common Channel Soft
Handoff
[0092] In operation, the searcher receiver 44 constantly monitors
received pilot signals to determine which pilot signals exceed a
prescribed threshold value referred to as T_ADD in the present
embodiment. The control processor 46 creates PMI messages which
identify the base stations for which the searcher has measured
pilot signal levels above the threshold. The PMI message can be
sent alone or it can be attached to other messages. These are the
base stations that are proposed by the mobile unit to participate
in common channel soft handoff. Alternatively, the base station
could measure a different value such as the total received power
from different base stations to determine which base stations to
communicate with. The control processor also identifies the mobile
unit sending the message, and it can identify the time slot in
which it will monitor the common channel, the F QPCH and F-CCCH in
the present embodiment. In the current embodiment, however, the
time slot need not be sent by the mobile unit since it is
ascertained based upon mobile unit identity (e.g. IMSI) as
described below. In a presently preferred embodiment, a long code
is used on the F-CCCH to identify the mobile unit to which a
message is directed. A long code is a maximal length sequence, and
the phase of the long code can be used to identify individual
mobile units, though other units could be used to identify the
mobile unit. The use of a long code on the F-CCCH can obviate the
need to use addresses on the F-CCCH to identify mobile units. The
control processor 46 also causes the diversity combiner and decoder
48 to monitor those base stations that have been reported in the
most recent PMI message to have signal levels above the threshold
and that also have been identified by one of the base stations that
are permitted to participate in common channel soft handoff. In the
current embodiment, the mobile unit ordinarily receives soft
handoff permission information from the base station that is the
closest or has the strongest pilot signal. The permission
information indicates which base stations are eligible for common
channel soft handoff. The permission informaton is provided in a
Handoff Table which is described in detail below.
[0093] It will be appreciated that the mobile unit can continually
monitor the pilot signal strengths of multiple base stations. It
can send new updated PMI messages whenever there is a change in the
set of pilot signals that exceed the threshold. Moreover, the
mobile unit can continually alter the set of base staions monitored
for common channel soft handoff. As a result, the set of base
stations monitored by the mobile unit is continually updated to
account for changes in pilot signal strengths, and as described
below, the BSC and the base stations are notified of these updates
so that the common channel soft handoff messages can be tailored to
the needs of the mobile unit.
[0094] The base station that receives the mobile unit PMI message
communicates the message to the BSC. The BSC reads the PMI
information. If a Call Origination or a Call Termination is
initiated with the base station, for example, the system control
processor 100 assembles an appropriate message, such as an
Acknowledge Message or a Page Message or a Channel Assignment
Message, for instance. The BSC uses the permission information in
the Handoff Table to determine which base stations are eligible to
participate in soft handoff. The BSC causes some set of these base
stations to transmit the appropriate messages to the mobile unit
within the appropriate time slot. The set of base stations
designated by BSC to transmit to the mobile unit in common channel
soft handoff may depend upon the base stations identified by the
mobile unit in the PMI information. A specific example of Call
Origination is provided below with reference to FIG. 9. Specific
examples of Call Termination are provide below with reference to
FIGS. 10-16.
[0095] Multiple Common Channel Structure
[0096] A common channel is a communication channel that is
continually shared. For example, messages on a common channel may
be directed from a base station to a first mobile unit in a first
time interval, and may be directed to a second mobile unit during a
second time interval immediately following the first and may be
directed to a third mobile unit during a third time interval
immediately following the second.
[0097] A paging channel is a forward common communication channel
used by a base station to communicate to a mobile unit when the
mobile unit is not assigned to a traffic channel. A CDMA spread
spectrum communication system of a presently preferred embodiment
of the invention has multiple common channels. In the current
embodiment, multiple slotted common channels are employed in order
to reduce mobile unit power consumption since a mobile unit need
only be active for prescribed time intervals.
[0098] Slotted channels deliver messages during prescribed time
intervals or slots. An advantage of the use of slotted mode is that
a mobile unit may conserve power by monitoring such channels only
during the prescribed time slots. Slotted mode also has been
referred to as "sleep mode" since the mobile unit can power-down
and sleep when idle and wake up to monitor the slot.
[0099] A Forward Broadcast Channel (F-BCCH) paging channel
broadcasts overhead information, such as system parameters
messages, and broadcast short messages. Broadcast short messages
are a class of short messages that are directed to a large number
of mobile stations. The overhead messages sent on the F-BCCH
generally will involve information that will be different for
different base stations. A Forward Common Control Channel (F-CCCH)
channel carries messages directed to particular mobile units such
as channel assignment messages. The F-CCCH can also carry broadcast
short messages. A Forward Quick Paging Channel (F-QPCH) carries
indications of pages directed to a mobile station. The base station
transmits a signal to a given mobile station on the F-QPCH whenever
it needs to contact a mobile unit operating in slotted mode.
[0100] F-CCCH
[0101] In a current embodiment of the invention, the F-CCCH is
transmitted intermittently. As a result, a preferable arrangement
is for it to be variable rate/on-off. In this arrangement, the
channel is transmitted for a frame if there is a message to send.
As a result, capacity is not wasted in transmitting this channel
when there is no message to send. Furthermore, the channel can be
transmitted during a frame at one of several different rates, with
the rate being chosen based upon the ability of the base station to
supply the amount of power needed to send data at the particular
rate to the mobile station.
[0102] F-BCCH
[0103] The F-BCCH is a separate logical channel conveying overhead
information. The F-BCCH is not transmitted in a soft handoff mode
since much of the information conveyed is specific to a sector. A
mobile unit may continue to monitor these F-BCCH messages even
after it has established a traffic channel. The mobile unit may
monitor the F-BCCH when it is monitoring the F-CCCH in order to
receive broadcast short messages and in order to update its
overhead information. The mobile unit may also monitor the F-BCCH
when it needs to update overhead information and it is receiving
the F-QPCH. The overhead information might include RAND
(pseudorandom number patterns used for authentication), channel
code parameters and Neighbor Sets, just to mention a few possible
examples.
[0104] The F-BCCH also can be operated in an intermittent mode
similar to the F-CCCH. However, the F-BCCH generally conveys only a
relatively few overhead messages that do not change frequently.
Ordinarily, only mobile units that are first powering on or that
are handing off to the sector need to receive these overhead
messages. Thus, it is usually desired that the F-BCCH be
transmitted with as little amount of power as is necessary. In
order to achieve more efficient lower power operation, F-BCCH
overhead messages are transmitted and then repeated. An
illustrative example of repeating F-BCCH messages is shown in FIG.
6. More specifically, the F-BCCH messages are repeated at known
intervals in a way that the transmitted symbols are exactly the
same.
[0105] In the example in FIG. 6, the messages are repeated during
80 ms intervals. It should be noted that this time interval could
be any value which is known by the mobile unit (or told to the
mobile unit). Furthermore, as shown in FIG. 7, message
transmissions can be interleaved with prior message transmissions.
The main requirement being that messages are repeated at some
interval known to the mobile unit so that it can perform diversity
combining.
[0106] In a present embodiment of the invention, messages on the
F-CCCH are directed to individual mobile units. A mobile unit
monitors the F-QPCH slot to determine whether or not paging
messages are to be sent to the mobile unit on the F-CCCH. More
specifically, a mobile unit monitors page notification information
in its prescribed time slot of the F-QPCH in order to determine
whether or not there is a paging message directed to it. The page
notification information monitored in the F-QPCH time slot is very
short so that the mobile station does not have to expend
significant power determining whether or not there exists an actual
message directed to it. If the page notification information
indicates that there is no paging message directed to the mobile
unit, then the mobile unit can go back to a sleep mode until the
next prescribed F-QPCH time slot. If the page notification
information indicates that there is a paging message directed to
the mobile unit, then the mobile unit monitors the F-CCCH for a
predetermined time interval in an attempt to receive a Page Message
described below with reference to FIG. 10. Alternatively, the
mobile unit could immediately respond on the R-CCCH with a Quick
Page Response Message described below with reference to FIG. 11.
The mobile unit then waits for the page message which can be
transmitted in soft handoff. Another alternative embodiment is to
have the mobile unit respond with a Quick Page Response Message and
then wait for the Channel Assignment Message--that is, the Page
Message is not transmitted.
[0107] F-OPCH
[0108] The F-QPCH carries page notification information. Each
mobile unit is assigned a time slot in the F-QPCH. The F-QPCH time
slots assigned to any given mobile unit are prescribed by the
identity of the mobile unit. There are numerous ways in which a
mobile unit can be identified, such as ESN, IMSI or TMSI to name a
few. Thus, a given mobile unit can be assigned the same F-QPCH time
slots for each of multiple different base stations. That is, the
F-QPCH time slots are assigned independently of base station
identity.
[0109] A presently preferred embodiment of the invention implements
a hashed-On-Off Keying (OOK) Forward Quick Paging Channel (F-QPCH)
in which an on-off keyed (OOK) symbol is used to notify a mobile
station that it must listen to the F-CCCH during the next F-CCCH
paging channel slot. The mobile is identified by the position of
the OOK symbol in the F-QPCH slot.
[0110] More specifically, in a current embodiment the quick paging
channel contains single bit messages to direct slotted-mode mobile
stations to monitor their assigned slot on the paging channel. In a
presently preferred embodiment, the data rate is fixed at 9,600
bps, and the modulation is on-off keying (OOK), where a logical "1"
instructs the mobile station to monitor the F-CCCH, and a logical
"0" instructs the mobile station to return to sleep. In the current
embodiment, one OOK bit is transmitted in each 128 PN chip, symbol
period. The transmit Ec/Ior while sending a 1 bit should be three
decibels below the Pilot Channel Ec/Ior when soft handoff is not
used. Each single bit message is transmitted twice per 80
millisecond slot, once in bit R.sub.1, then again in bit R.sub.2,
where a logical "1" indicates the first bit, which starts at the 80
ms timing boundary.
[0111] In a present embodiment, the set of possible bit positions
over which the quick paging channel is defined starts 80 ms before
the interval on the F-CCCH. This starting point can be defined as
time t, where t is in units of frames, such that
(.left brkt-bot.(t-4)/4.right
brkt-bot.-PGSLOT)mod(16.multidot.T)=0,
[0112] where T=2.sup.i is the slot cycle length in units of 1.28
seconds, and i is the slot cycle index, and PGSLOT stands for page
slot.
[0113] The mobile station hashes to one or two bit positions per
slot. The second bit is transmitted for increased reliability of
the decision. Each hashing randomizes each bit over 340
non-overlapping bit positions. The decorrelation values are defined
so that they update each 1.28 seconds (64 frames), in order to
prevent repeat collisions between mobile stations. The
decorrelation cycle repeats every 23.3 hours.
N=340 bit positions.
DECORR.sub.1.multidot..multidot.(t-4)/64.multidot.mod2.sup.16.
DECORR.sub.2.multidot..multidot.(t-4)/64+1.multidot.mod2.sup.16.
[0114]
1 Word L to be bits 0-15 of HASH_KEY (MIN or IMSI_S). Word H to be
bits 16-31 of HASH_KEY (MIN or IMSI_S).
R.sub.1=.multidot.N.multidot.((40503.multidot.(L.multidot.H.multidot.DECO-
RR.sub.1)) mod 2.sup.16)/2.sup.16.multidot.+1.
R.sub.2=.multidot.N.multidot.((40503.multidot.(L.multidot.H.multidot.DECO-
RR.sub.2)) mod 2.sup.16)/2.sup.16.multidot.+341.
[0115] Referring to the illustrative F-QPCH and F-CCCH timing
diagram of FIG. 8, the possible locations for F-QPCH messages in a
present embodiment of the invention are located from 80
milliseconds to 9.17 milliseconds before the F-CCCH slot for that
mobile station is to begin. This leaves at least 9.17 milliseconds
from the second transmission on the quick paging channel for the
mobile station to prepare for receiving the F-CCCH. The quick
paging channel is not to be scrambled by a long PN code.
[0116] Thus, the base station simply sends a logical "1" in the
correct bit positions in a time period defined above ranging from
9.17 milliseconds to 80 milliseconds before the slot of the paging
channel to alert slotted-mode mobile stations that it is sending a
paging message in the immediately following F-CCCH time slot. The
base station is to perform the hashes to bit positions defined
above, based on mobile unit identity and system time, and set those
bits to logical "1". All other bits are to be set to logical
"0".
[0117] A mobile unit in accordance with the current embodiment
employs a 16 by 16 multiplier and bit-wise exclusive-or to
implement the hashing function which is updated prior to each slot.
The mobile unit may go back to sleep after monitoring a bit. A
false alarm in the first bit requires the mobile to monitor the
second bit position. False alarms in both bits require the mobile
to monitor the F-CCCH.
[0118] An alternative embodiment of the invention implements a
short packet to notify a mobile unit that it must listen to
messages in the next F-CCCH time slot. The mobile unit is
identified by a key in the packet. More specifically, in this
alternative embodiment the quick paging channel carries between one
and twelve quick paging channel packets per paging channel slot.
The format of the quick paging channel packet is shown in the
following Paging Channel Packet.
2 Length Field (bits) Description TYPE 2 The packet type. Possible
frame types are 00 - No mobile stations need to wake up, and this
is the last packet in this slot. 01 - All mobiles stations need to
wake up, and this is the last packet in this slot. 10 - All mobile
stations that hash to value HASH need to wake up, and this is the
last packet in this slot. 11 - All mobile stations that hash to
value HASH need to wake up, and there is at least one more packet
in this slot. HASH 10 The hash value. CRC 12 The CRC.
[0119] In this alternative embodiment, each packet is 24 bits. Each
packet is transmitted in two power control groups (2.5
milliseconds). Therefore, the packet is transmitted at 9600 bps. As
a result, during transmission, the quick paging channel requires
approximately the same power as the paging channel.
[0120] If an incorrectly decoded packet goes undetected, then the
mobile unit may decide not to listen to the paging channel when it
should listen to the paging channel. Therefore, the undetected
error probability should be sufficiently below the desired minimum
call failure probability. In this alternative embodiment, using
only a 12 bit CRC, the probability of undetected error is 1/4096.
Since this is 40 times less than an acceptable call failure rate,
it is sufficiently small. Using the measured Es/Nt or re-encoder
symbol error rate, the probability of undetected error may be
reduced further.
[0121] The value of HASH is dependent on the mobile unit
identification (MSID) and system time in frames (t).
3 N=2.sup.10, DECORR .multidot..multidot.t/64.mu- ltidot.mod
2.sup.16, L=bits 0-15 of HASH_KEY (MIN or IMSI_S), H=bits 16-31 of
HASH_KEY (MIN or IMSI_S), and HASH=.left
brkt-bot.N.times.((40503.times.(L.sym.H.sym.DECORR)) mod
2.sup.16)/2.sup.16.right brkt-bot..
[0122] The quick paging channel packet contains only one HASH
value. Therefore, the overhead associated with TYPE and CRC fields
can be relatively high. This overhead can be reduced by including
more HASH values in each packet.
[0123] Each quick paging channel packet is encoded using the
convolutional encoder used on traffic channel. In a current
implementation a constraint length 9, rate 1/4 convolutional
encoder is employed. The initial encoder state is "0". The above
quick paging channel packet does not have any tail bits. Because
the packet is relatively short, the penalty for tail bits can be
relatively high.
[0124] Quick paging channel packets for page messages in paging
slot N are sent in quick paging channel slot N-1.
[0125] In this alternative embodiment, the start of the first
packet is randomized. This is done in order to randomize the chance
the quick paging channel packets sent from adjacent base stations
will be sent at the same time. In the present embodiment, the start
of the first packet is aligned to the start of a power control
group which occupies a 1.25 ms interval on the Forward Traffic
Channel. The power control group is dependent on the base station
identification (BASE_ID) and the system time in frames (t).
4 N=32, DECORR=.left brkt-bot.t/64.right brkt-bot.mod 2.sup.16,
L=BASE_ID, H=0, and POSITION=.left
brkt-bot.N.times.((40503.times.(L.sym.H.sym.DECORR)) mod
2.sup.16)/2.sup.16.right brkt-bot..
[0126] This randomizes the start of the first packet over the first
40 milliseconds of the 80 millisecond slot. Therefore, since
packets are 2.5 milliseconds long, there can be up to 16 packets
per slot.
[0127] OPCH and F-CCCH Soft Handoff
[0128] In soft handoff, identical information, carried in
identically converted, modulated signals is transmitted from
multiple base stations so that the multiple identical transmissions
can be combined by a single mobile unit through diversity
reception.
[0129] In common channel soft handoff in accordance with the
present invention, identical QPCH and F-CCCH messages are carried
in identically converted, modulated signals and are transmitted by
multiple base stations so that they can be combined by a single
mobile unit through diversity reception.
[0130] In operation, each individual base station sends F-QPCH and
F-CCCH channel soft handoff permission information on its
respective F-BCCH paging channel that indicates which other base
stations are permitted to participate in soft handoff on the on the
F-QPCH and F-CCCH. More specifically, each individual base station
may send messages on its respective F-BCCH paging channel that
identify other base stations that are permitted to transmit,
together with such individual base station, to a given mobile unit
in soft handoff on an F-CCCH. Similarly, each individual base
station may send messages on its respective F-BCCH paging channel
that identify other base stations that are permitted to transmit,
together with such individual base station, to a given mobile unit
in soft handoff on a F-QPCH. In a presently preferred embodiment of
the invention, separate single bit flags may be individually set or
reset for each of several other base stations to indicate which of
those other base stations may communicate in soft handoff with the
given mobile unit on the F-CCCH and F-QPCH paging channels.
Separate sets of flags could be used for the F-CCCH and the F-QPCH.
Alternatively, one single bit flag could be set for each of several
other base stations to indicate which of those other base stations
may communicate in soft handoff with the given mobile unit on the
F-CCCH and F-QPCH paging channels.
[0131] It will be appreciated that the availability of paging
channel soft handoff in accordance with the present invention does
not preclude the use of Access Handoff or Access Probe Handoff or
Access Probe Handoff. Therefore, in a current embodiment, each
individual base station may also send hard handoff permission
information on its respective F-BCCH paging channel that indicates
which other base stations are permitted to participate in (hard)
Access Handoffs.
[0132] The exemplary Handoff Table set forth below illustrates the
handoff permission flags sent by sector "A1" of base station "A" in
its F-BCCH overhead messages. These handoff permission flags
identify other base stations in the Neighbor List of base station
"A" for which Access Handoff is allowed, for which F-CCCH soft
handoff is allowed and for which F-QPCH soft handoff is allowed. In
this example, there are four other base stations in the Neighbor
List of base station A. They are base stations B, C, D and E. In
the Handoff Table, there is an individual flag for every other base
station sector for which Access Handoff is permitted. There is also
an individual flag for every other base station sector for which
F-CCCH soft handoff is permitted. Finally, there is a flag for
every other base station sector for which F-QPCH soft handoff is
permitted. In this example, a flag set to "1" indicates that
handoff is permitted, and a flag set to "0" indicates that handoff
is not permitted.
5HANDOFF TABLE IS-95-B F_CCC FQPC ACCESS_HO_- H_-SOFT_- H_- Base
ALLOWED HANDOFF SOFT_HAN Station Flag Flag DOFF Flag Comments A 1 1
1 Access handoff, 2 F-CCCH soft handoff; F-QPCH soft handoff A 1 1
1 Access handoff, 3 F-CCCH soft handoff; F-QPCH soft handoff B 1 1
1 Access handoff, 1 F-CCCH soft handoff; F-QPCH soft handoff B 1 1
1 Access handoff, 2 F-CCCH soft handoff; F-QPCH soft handoff B 1 1
1 Access handoff, 3 F-CCCH soft handoff; F-QPCH soft handoff C 1 0
1 Access handoff, 1 F-QPCH soft handoff C 1 0 1 Access handoff, 2
F-QPCH soft handoff C 1 0 1 Access handoff, 3 F-QPCH soft handoff D
1 0 0 Access handoff 1 D 1 0 0 Access handoff 2 D 1 0 0 Access
handoff 3 E 0 0 0 1 E 0 0 0 2 E 0 0 0 3
[0133] In this example, a mobile unit in the cell area covered by
sector A1 is permitted to perform F-CCCH soft handoff with any
other sector of base station A and with all three sectors of base
station B. However, a mobile unit in the cell area covered by
sector A1 is not permitted to perform F-CCCH soft handoff with any
of base stations C, D or E. Also, in this example, a mobile unit in
the sector A1 cell area may perform F-QPCH soft handoff with any
other sector of base stations A and with any of the sectors of base
stations B or C, but it may not perform F-QPCH soft handoff with
either of base stations D or E. Finally, a mobile unit in sector A1
may perform an Access Handoff with sectors A2 and A3 of base
station A and with any of the sectors of B, C and D, but it may not
perform an Access Handoff with any sector of base station E.
[0134] Thus, the base station covering a cell area in which a
mobile unit is located sends an F-BCCH message which indicates to
the mobile unit other base stations with which the mobile unit is
permitted to perform F-CCCH soft handoff, QPCH soft handoff or
Access Handoff. This handoff permission information may be used by
the base station to accomplish handoffs before a traffic channel
has been established. For example, the F-CCCH soft handoff
permission information can be used during call setup, in the course
of a call origination message sequence or in the course of a call
termination message sequence as described more fully below.
Moreover, the Access Handoff permission information informs the
mobile unit of which base stations are available for a hard handoff
in case the F-CCCH is lost before a traffic channel is set up.
[0135] A determination of the which base stations in a given base
station's Neighbor List are to perform F-QCPH or F-CCCH soft
handoff may be fixed or it may be changed dynamically under control
of a base station controller. For example, a base station
controller may be programmed to monitor the volume of call
originations or terminations in different base station cell areas,
and to send individual messages, determined by the volume of calls
in different cells, to individual base stations which alter the
sets of other base stations that may perform F-QCPH or F-CCCH soft
handoff with such individual base stations. It will also be noted
that, for the above Handoff Table, some base stations are permitted
to participate only in F-QPCH soft handoff; others are permitted to
participate in both F-QCPH and F-CCCH soft handoffs; and others are
not permitted to participate in either F-QPCH or F-CCCH soft
handoffs. In the present embodiment of the invention, it is
possible that a larger number of base stations will be permitted to
participate in F-QPCH soft handoff than in F-CCCH soft handoff. One
reason for this is that F-QCPH paging channel messages usually are
short, only one or two bits, and F-CCCH paging channel messages
typically are significantly longer. As a result, fewer base
stations may be allowed to participate in F-CCCH soft handoff than
in F-QPCH soft handoff, since it is often much easier for a base
station controller to coordinate short message length F-QCPH soft
handoffs than to coordinate longer message length F-CCCH soft
handoffs.
[0136] The determination of which base stations in a given base
station's Neighbor List are to perform paging channel soft handoff
also may depend upon the specific layout and organization of the
overall cellular system. For instance, in the above example, base
station D is not permitted to participate in F-QPCH soft handoff.
This might be because base station D is in a different registration
zone than the other base stations, for example. Neither F-QCPH soft
handoff nor F-CCCH soft handoff nor Access handoff is permitted for
base station E. This might be because base station E is controlled
by another BSC, for example. Moreover, in some circumstances it
might be desirable to restrict paging channel soft handoff on the
F-CCCH to only sectors of the same base station in order to
synchronize and control the soft handoff from a single base station
cell. Under such circumstances in the above example, F-CCCH soft
handoff would only involve sectors A2 and A3 in addition to A1.
[0137] F-CCCH Soft Handoff During Call Origination Example
[0138] Referring to the illustrative drawings of FIG. 9, there is
provided a timing diagram showing a call origination message
sequence between a mobile unit, multiple base stations and a base
station controller during the origination of a call by the mobile
station. During a call origination, a mobile unit sends an
Origination Message to the base station with the strongest pilot
signal to indicate that a call is to be originated by the mobile
unit, and the base station and the base station controller should
coordinate the process of assigning a traffic channel to the mobile
unit. In the example illustrated in FIG. 9, assume that the mobile
unit is located in sector A1 of base station A when it issues the
Origination Message. In a present embodiment of the invention, the
mobile unit sends the pilot measurement information, previously
described in every message on the Access Channel. Assume that the
pilot signals of base station sectors B1 and C1 are above a
prescribed pilot signal strength level. Then the mobile unit sends
to base station A1 an indication that the pilot signal strengths of
B1 and C1 exceed the threshold. In the preferred embodiment, this
indication is the pilot PN phase of base stations B1 and C1, though
any indicator of these base stations can be used. Furthermore, it
is preferred that the mobile unit sends the strength of base
stations B1 and C1 as well as the strength of base station A1 which
the mobile station is directly monitoring. Note that the mobile
unit previously received the exemplary Handoff Table via overhead
messages on the F-BCCH, which for instance may have been
transmitted from base station A1 to the mobile unit upon entry into
the coverage area of base station A1 as a result of powering up,
entering from a neighboring base station or switching from another
frequency. The Handoff Table indicates that F-CCCH soft handoff is
permitted with B1, and that Access Handoff is permitted with
C1.
[0139] The Origination Message is received by base station "A"
which sends the message to its base station controller. An
Acknowledgement Message is generated at the BSC and is transmitted
from the BSC to base stations A1 and B11.
[0140] The base stations with a pilot signal strength above a
prescribed threshold, as measured by the mobile unit, and that have
F-CCCH-SOFT-HANDOFF set to "1" in the Handoff Table are B1. These
two base stations, therefore, transmit the Acknowledgement Message
to the mobile station in soft handoff.
[0141] If Access Probe Handoff were enabled in base station C1,
then the base station controller would send the acknowledgement to
base station C1. Base station C1 could send the message to the
mobile unit in a soft handoff mode or not; since the mobile unit
has not been told that base station C1 is operating in soft handoff
mode, the mobile station will not combine base station C1 in soft
handoff mode. In order to avoid having to determine the set of base
stations in which to actually send the message, the BSC may send
the Acknowledgement Message to a larger set of base stations. These
base stations may directly transmit the message or they may have
their own filters and determine that a certain message does not
need to be transmitted in the particular base station. Once abase
station receives the Acknowledgement Message, and determines that
it is to be sent, it sends it on the F-CCCH. It should be noted
that those base stations that are in soft handoff must send the
Acknowledgement Message in a soft handoff way which entails
synchronizing the transmissions of the messages from the two base
stations and transmitting the message at exactly the same time as
is done with the IS-95-A traffic channel, for instance. It should
be noted that once the mobile unit sends the Origination Message,
the mobile unit continuously monitors the F-CCCH for awhile. If the
mobile unit has not received the Acknowledgement Message within
some short time interval, it retransmits the Origination Message.
This is similar to what is done in IS-95-A, for example.
[0142] Meanwhile, the BSC sets up a traffic channel for use by the
mobile unit and then sends a Channel Assignment Message (or
information used to determine the Channel Assignment Message) to
base stations A, B and C. The BSC sends the Channel Assignment
Message to base station B since B1 was reported (in the pilot
measurement information in the Origination Message to have a pilot
signal strength above a prescribed threshold as measured by the
mobile unit, and F-CCCH-SOFT-HANDOFF was set to "1". The base
station controller sends the Channel Assignment Message to base
station C since C1 was also reported in the pilot measurement
information in the Origination Message to have a pilot signal
strength above a prescribed threshold as measured by the mobile
unit, and F_ACCESS_HO WAS SET TO "1". The Channel Assignment
Message is transmitted on the F-CCCH in soft handoff mode from base
stations A1 and B1. Consistent with TIA/EIA-95-B, the Channel
Assignment Message is also transmitted by base station C1. The
transmission from base station C1 is not in soft handoff since the
mobile unit is not combining the C1 transmission with the A1 and B1
transmissions.
[0143] F-OPCH Soft Handoff During Call Termination Example
[0144] Referring to the illustrative drawings of FIG. 10, there is
provided a timing diagram showing an exemplary call termination
message sequence between a mobile unit, multiple base stations and
a base station controller. During the call termination sequence,
the BSC and multiple base stations coordinate the assignment of a
traffic channel for communication between the mobile unit and one
or more base stations. More specifically, in the example of FIG.
10, while the mobile unit is located in sector A1 of the cell
region of base station A, base station sectors A1, B1 and C1 send
identical F-QPCH messages at a particular time that may be
determined by the International Mobile Subscriber Station Identity
(IMSI) of the mobile unit and the configuration of the base
station. It will be appreciated that the mobile unit may transmit
its IMSI to a base station during call registration, for example.
Also, as explained above, the BSC is aware of the configuration of
the base stations within the overall system. Generally, base
stations in a given paging region are configured so that they all
transmit a F-QPCH at substantially the same time. In accordance
with the invention, F-QPCHs of those base stations that transmit
their quick pages at substantially the same time can be combined by
a mobile station in a soft handoff. The mobile unit receives the
Handoff Table from base station A1 via overhead messages on the
F-BCCH.
[0145] Referring to the exemplary Handoff Table transmitted to the
mobile unit by the base station A, it will be seen that the
F-QPCH-SOFT-HANDOFF flag is logical "1" for base station sectors
A1, A2, B1, B2, B3, C1, C2, and C3 since these base station sectors
are in the same paging area or are otherwise able to send a quick
page at the same time. Base station A1 has set the
F-QPCH-SOFT-HANDOFF flag to logical "0" for base station D since
this base station is not in the same paging area or is otherwise
unable to send a quick page at the same time as the other base
stations.
[0146] In the example illustrated in FIG. 10, the mobile unit
receives the quick page (F-QPCH) in soft handoff from base station
sectors A1, B1 and C1. The mobile unit monitors the F-QPCH slot
during its designated time slot as described above. When the mobile
unit receives the F-QPCH messages from A1, B1 and C1, it combines
them in a soft handoff mode. Then, as described above, mobile unit
determines whether or not a quick page message has been directed to
it. Once the mobile unit determines that it did in fact receive a
quick page set to logical "1" during its designated time slot, the
mobile unit begins to monitor its prescribed time slot in the
F-CCCH. Next, the base station controller instructs base stations
to send a Page Message with the full address of the mobile unit
such as its IMSI, TMSI or electronic serial number (ESN). In
additional to the identity of the mobile station, the Page Message
contains other information such as the proposed service option.
[0147] There are a number of alternative approaches to sending the
Page Message. In the preferred embodiment certain base stations
send this message out using soft handoff just as with the F-QPCH.
In an alternative approach, a different set of base stations could
be used to send the Page Message in soft handoff. This different
set of base stations could be specified by flags in an additional
column of the Handoff Table.
[0148] Another alternative is to not use soft handoff at all for
the Page Message.
[0149] Moreover, in the presently preferred embodiment, the set of
base stations that are transmitting the Page Message could actually
be smaller than the set indicated by the FQPCH_SOFT_HANDOFF Flags
if the BSC has some additional information on the mobile unit's
location, such as a recent communication with the mobile unit. This
would permit the BSC to determine a smaller set of base stations to
transmit the message than would otherwise be used. Returning to the
preferred method, the mobile unit combines the Page Message of A1,
B1, and C1 in soft handoff since the mobile unit is in sector A1
and the F-QPCH-SOFT-HANDOFF flag is set to logical "1" for B1 in
the Handoff Table, and the B1 pilot signal is above a prescribed
threshold. The mobile station responds to the soft handoff Page
Message by sending a Page Response Message to base station A1 since
it has the strongest pilot signal. The Page Response Message lists
the pilot measurement information for neighboring base stations. In
this example shown in FIG. 10, base stations A1, B1, and C1 are
above the threshold and are reported.
[0150] The process from this point is essentially identical to the
Call Origination process previously described. Base station A1
communicates the Page Response Message to the base station
controller. Next, the base station controller instructs base
stations A1 and B1 to send another Acknowledgement Message to the
mobile station on their F-CCCHs in soft handoff. Another
Acknowledgement Message also is sent to the mobile unit by C1 on
its F-CCCH, but this C1 Acknowledgement Message is not sent in soft
handoff. Finally, the base station controller sets up a traffic
channel for use by the mobile station, and then sends a Channel
Assignment Message (or information used to determine the Channel
Assignment Message) to base stations A, B and C. The base station
controller sends the Channel Assignment Message to base station B
because the mobile unit measured the B1 pilot signal strength to be
above the prescribed threshold value, and because
F-CCCH-SOFT-HANDOFF was set to "1" for B1. The base station
controller sends the Channel Assignment Message to base station C
since the mobile station measured the C1 pilot signal to be above
the prescribed threshold value and because F_ACCESS_HO was set to
"1" for C1. The Channel Assignment Message is transmitted in soft
handoff from base stations A1 and B1. Consistent with TIA/EIA-95-B,
the Channel Assignment Message is also transmitted from base
station C1. The transmission from base station C1 is not in soft
handoff since the mobile station is not combining the C1
transmission with the A1 and B1 transmissions.
[0151] Alternatively, the Page Message can be eliminated, and the
mobile station can instead respond directly to the Quick Page
Message with a Page Response Message. However, the false alarm rate
on the quick paging channel may be sufficiently high that so that
it is preferable for the mobile station to wait for the Page
Message before sending a Page Response Message.
[0152] Referring to the illustrative drawings of FIG. 11, there is
shown an example of a call termination message sequence which
includes a Quick Page Response Message sent by the mobile unit to
inform the base station with the strongest measured pilot signal
strength, base station A1 in the example, the pilot measurement
information for the mobile station. This advantageously informs the
base station of the latest Active Set in which to send the Page
Message. This has the advantage in that the Page Message is
required to be transmitted from fewer base stations since the BSC
is informed of the pilot measurement information. This is in
contrast to the previous example in which the BSC was assumed to
not know pilot measurement information about the mobile station and
thus had to transmit the Page Message in many different base
stations.
[0153] Paging Channel Soft Handoff For Synchronized Based
Stations
[0154] In receiving either the Acknowledgement Message, Page
Message, or the Channel Assignment Message on the F-CCCH, the
mobile unit would diversity combine the signals from the base
stations in soft handoff, just as is done on the Traffic Channel
when the mobile unit is in soft handoff. In order for diversity
combining to work correctly, all of the base stations must send out
F-CCCH paging messages at the substantially same time. In the past,
this has been done on the Traffic Channel by having all of the
forward link frames time synchronized in their transmission. This
is relatively straightforward since the Traffic Channel is a
dedicated channel, and no specific control is required.
Nevertheless, mobile units typically are equipped with deskew
buffers to synchronize signals subject to multipath delay.
[0155] The F-CCCH, however, is a common or shared channel, and the
transmission of paging signals substantially simultaneously by
multiple base stations in different cell regions requires
coordination of their respective transmissions. For example, there
may be more than one mobile unit that requires transmission of
F-CCCH messages in soft handoff. Moreover, each such mobile unit
may identify a different Active Set of base stations with pilot
signals above a prescribed threshold value. Referring to FIG. 12
for instance, there is shown an illustrative example of two
different sets of messages that are to be transmitted by two
different, but overlapping sets of base stations. It will be
appreciated that, in soft handoff in accordance with a presently
preferred embodiment of the invention, each instance of message M1,
transmitted by A1, A2 or B1 has identical information which is
identically converted and modulated so that the messages can be
diversity combined. The same is true for each instance of messages
M2-M5 as well. In this example, M1 is to be transmitted in sectors
{A1, A2, B1} and M2 is to be transmitted in sectors {A1, A2,
C1}.
[0156] In a presently preferred embodiment of the invention, the
BSC performs a scheduling algorithm so that a single common F-CCCH
used by each base station is appropriately utilized. The base
stations are those in the Active Set with the appropriate
permission flags set as described in the above example. Each
instance of a message is scheduled to be sent simultaneously with
each other instance of the same message as shown in FIG. 12.
Message scheduling information is sent to the various base stations
by the BSC.
[0157] It should be noted that this invention has been described in
terms of a single F-CCCH. A plurality of F-CCCH's can be used with
the hashing to a particular F-CCCH based upon the mobile station's
IMSI or other identity similar to that which is done with the
Paging Channel in TIA/EIA-95-B. Then the messages which are sent on
the F-CCCH are directed to the appropriate F-CCCH based upon the
IMSI of the mobile station.
[0158] Paging Channel Soft handoff For Asynchronous Based
Stations
[0159] In an alternative embodiment of the invention, the mobile
unit may have to achieve synchronization of F-CCCH or F-QPCH
messages transmitted by different base stations that operate
asynchronously. Asynchronous base stations are those in which the
timing is not necessarily aligned between them. For example, frames
may offset from one base station to another.
[0160] The mobile unit must deskew the timing of messages
transmitted by different asynchronous base stations before the
messages can be combined to achieve diversity. The message timing
diagram of FIG. 13 illustrates the transmission of messages by base
stations A, B and C, where Sectors A1 and A2 are synchronized with
each other, but sectors B1 and C1 are asynchronous. The messages
are the same as in FIG. 12. Note that for asynchronous base
stations, that the relative timing within each channel is
maintained; however, one channel is skewed between channels.
[0161] In order to deskew messages received on the asynchronous
paging channels of different base stations, the mobile unit
implements a deinterleaver buffer with a length that is at least
the maximum amount of skew between the base stations. The mobile
unit is assumed to know the timing offset of the various base
stations, perhaps as obtained by synchronization patterns which are
embedded in the forward links of these base stations. As the mobile
unit receives a symbol from a particular base station, the mobile
unit does the normal receiver processing functions such as
despreading, removing the orthogonal cover, demodulation in each of
its rake receiver fingers, which are 40 and 42 in FIG. 3. However,
deinterleaving of message information may have to be performed
differently for each rake receiver finger in order to synchronize
the asynchronous messages so that they can be diversity combined.
More particularly, the output of every rake receiver finger which
corresponds to a different base station is separately deinterleaved
so that the asynchronous messages can be aligned. Referring to FIG.
3, this deinterleaving is performed in diversity combiner and
decoder 48.
[0162] For instance, assume the example in FIG. 13 that a first
mobile unit is to receive F-QPCH and F-CCCH paging channel messages
from base station sectors A1, A2 and B1. Also, assume that there
are four rake receiver fingers of the form 40 and 42 in FIG. 3 with
two fingers being assigned for different multipaths from base
station sector A1 and the remaining two fingers being assigned to
A2, and B1. The mobile unit combines the signal from the two rake
fingers from sector A1 as it normally does since these messages are
aligned with each other. As a result there are three remaining
streams, one from each of A1 (the combined signals on two fingers),
A2 and B1.
[0163] For example, the mobile unit takes a given symbol from the
first arriving base station message stream (combined from two
multipath streams) and places it in the deinterleaver buffer. When
the corresponding symbol arrives from the second base station
message stream (the one having timing later than the first base
station), the mobile unit takes the symbol from the deinterleaver
buffer, combines it with the newly arriving symbol, and replaces
the symbol into the deinterleaver buffer. When the corresponding
symbol arrives from the third base station message stream (the one
having timing later than the second base station), the mobile unit
takes the symbol from the deinterleaver buffer, combines it with
the newly arriving symbol, and replaces the symbol into the
deinterleaver buffer. When all the symbols have been placed into
the deinterleaver buffer from all of the base stations, the mobile
unit performs the deinterleaving and then the decoding. Thus,
deinterleaving and decoding are delayed until the entire symbol has
arrived form all three base stations. It should be noted that the
mobile unit can attempt the decoding before it has received the
corresponding symbols from all base stations. This could result in
a faster processing rate but with a possibly higher error rate. It
should also be noted that the mobile unit may require a second
deinterleaver buffer to begin buffering a received symbol from the
next interleaver frame before symbols have been completely received
from the previous interleaver frame. It should also be noted that
there are a number of other equivalent ways to implement this which
should be evident to those skilled in the art.
[0164] F-CCCH Soft Handoff With Headers Example
[0165] In yet another alternative, paging messages can have
information added to them that identify a given mobile unit that is
to receive the F-CCCH messages in soft handoff. This alternative
approach can be quite useful, for example, when there are a large
number of base stations under a BSC in soft handoff. This
alternative approach also may be particularly useful when F-CCCH
soft handoff must be coordinated between multiple base stations
under control of different BSCs. This alternative does not require
the base stations to send the same message at the same time.
Rather, each mobile unit is assigned a short identifier. For
example, this identifier could be a twelve-bit number. At the
beginning of each message transmission from a base station, a coded
version of this identifier is transmitted. It is transmitted with
sufficient coding so that the mobile unit can receive the
identifier correctly, even at relatively low signal levels. This
coding, for example, can be a special block coded representation of
the identifier so that the error rate of the identifier is low when
soft handoff combining is not used. Alternatively, increased power
level could be used. Other methods familiar to those skilled in the
art can also be used. When receiving messages on the F-CCCH, the
mobile unit first decodes the identifier and then the mobile unit
attempts to receive the message. If the mobile unit is unable to
demodulate and decode the message without incurring significant
errors then the mobile unit maintains the code symbols in a buffer
and searches for the identifier from another base station. When it
receives the second indicator, it demodulates the message and
combines symbols with those stored in the buffer from the first
attempt. At the end, the mobile unit again determines whether the
message was correctly received.
[0166] Note that the base station identifiers are not sent
initially in soft handoff. Only after more than one base station is
identified using such identifiers is it possible, under this
approach, to combine F-CCCH messages in soft handoff.
[0167] While a particular embodiment of the invention has been
described in detail, various modifications to the preferred
embodiment can be made without departing from the spirit and scope
of the invention. Thus, the invention is limited only by the
appended claims.
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