U.S. patent application number 11/799155 was filed with the patent office on 2007-09-06 for coded reverse link messages for closed-loop power control of forward link control messages.
This patent application is currently assigned to InterDigital Technology Corporation. Invention is credited to James A. JR. Proctor.
Application Number | 20070206542 11/799155 |
Document ID | / |
Family ID | 38015786 |
Filed Date | 2007-09-06 |
United States Patent
Application |
20070206542 |
Kind Code |
A1 |
Proctor; James A. JR. |
September 6, 2007 |
Coded reverse link messages for closed-loop power control of
forward link control messages
Abstract
Multiple field units in a CDMA system are synchronized for
communication with a base station using shared forward and reverse
link channels. In an illustrative embodiment, each field unit is
assigned a time slot in a forward link channel to receive messages
from the base station. Likewise, each field unit is assigned a time
slot on a common reverse link channel for transmitting messages to
the base station. Timing alignment and power level control among
each of many field units and the base station is achieved by
analyzing coded transmissions received at the base station in a
corresponding time slot as transmitted by each field unit. The
codes may be orthogonal, pseudonoise (PN), or other codes. The
power level of forward control channel messages can thus be
individually controlled. In this way, minimal resources are
deployed to maintain communication and precise synchronization
between a base station and each of multiple users, minimizing
collisions between field units transmitting in adjacent time slots
on the reverse link.
Inventors: |
Proctor; James A. JR.;
(Melbourne Beach, FL) |
Correspondence
Address: |
VOLPE AND KOENIG, P.C.;DEPT. ICC
UNITED PLAZA, SUITE 1600
30 SOUTH 17TH STREET
PHILADELPHIA
PA
19103
US
|
Assignee: |
InterDigital Technology
Corporation
Wilmington
DE
|
Family ID: |
38015786 |
Appl. No.: |
11/799155 |
Filed: |
May 1, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10137116 |
May 1, 2002 |
7218623 |
|
|
11799155 |
May 1, 2007 |
|
|
|
60288927 |
May 4, 2001 |
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Current U.S.
Class: |
370/335 |
Current CPC
Class: |
H04W 72/0473 20130101;
H04L 5/006 20130101; H04W 24/08 20130101; H04W 52/0206 20130101;
H04W 84/042 20130101; H04L 5/0007 20130101; H04W 56/001 20130101;
H04W 72/087 20130101; H04W 28/04 20130101; H04W 72/04 20130101;
H04W 24/10 20130101; H04B 7/2618 20130101; Y02D 70/449 20180101;
H04B 7/2637 20130101; H04W 72/085 20130101; Y02D 30/70
20200801 |
Class at
Publication: |
370/335 |
International
Class: |
H04B 7/216 20060101
H04B007/216 |
Claims
1. A method for supporting wireless communications, the method
comprising: assigning a field unit a first time slot in a reverse
link channel; receiving and analyzing messages from said field unit
at a base station in the first time slot; and maintaining
synchronization between said field unit and said base station when
said field unit is inactive or standby mode.
2. The method of claim 1 further comprising: assigning said field
unit a second time slot in a forward link channel.
3. The method of 2, wherein said field unit transmits a timing
reference signal in said first time slot when said first unit is in
standby mode.
4. The method of claim 2 further comprising: said field unit
indicating to said base station to remain in the standby mode.
5. The method of claim 2, wherein said reverse link channel is a
heartbeat-type channel.
6. The method of claim 5, wherein said heartbeat-type channel
comprises an active channel and a standby channel.
7. The method of claim 6 further comprising: transmitting in said
standby channel and indication to remain in the standby mode.
8. The method of claim 6 further comprising: transmitting in said
active channel a request for allocation of reverse link traffic
channels for transmitting data.
9. The method of claim 6 further comprising: transmitting power
control information including whether messages over said forward
link channel should be increased in power or decreased in
power.
10. The method of claim 9 wherein said standby channel and said
active channel include said power control information.
11. A system for supporting wireless communication comprising: a
base station for assigning a field unit a first time slot in a
reverse link channel, and receiving and analyzing messages from
said field unit in said first time slot; and said base station and
said field unit maintaining synchronization when said field unit is
in an active mode or standby mode.
12. The system of claim 11 further comprising said base station
for: assigning said field unit a second time slot in a forward link
channel.
13. The system of claim 12, wherein said field unit transmits a
timing reference signal in said first time slot when said field
unit is in standby mode.
14. The system of claim 12 further comprising: said field unit
indicating to said base station to remain in the standby mode.
15. The system of claim 12, wherein said reverse link channel is a
heartbeat-type channel.
16. The system of claim 15, wherein said heartbeat-type channel
comprises an active channel and a standby channel.
17. The system of claim 16 further comprising: transmitting in said
standby channel and indication to remain in the standby mode.
18. The system of claim 16 further comprising: transmitting in said
active channel a request for allocation of reverse link traffic
channels for transmitting data.
19. The system of claim 16 further comprising: transmitting power
control information including whether messages over said forward
link channel should be increased in power or decreased in
power.
20. The system of claim 19 wherein said standby channel and said
active channel include said power control information.
Description
CROSS REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a continuation of U.S. application Ser.
No. 10/137,116 filed on May 1, 2002 which in turn claims the
benefit of U.S. Provisional Application No. 60/288,927 filed on May
4, 2001 which are incorporated by reference as if fully set
forth.
BACKGROUND
[0002] Increasing use of wireless telephones and personal computers
has led to a corresponding increase in demand for advanced
telecommunication services that were once thought practical only
for specialized applications. In the 1980s, wireless voice
communication became widely available through cellular telephone
networks. Such services were thought at first to be for the
exclusive province of businessmen because of expected high
subscriber costs. The same was also true for access to remotely
distributed computer networks, whereby until very recently, only
business people and large institutions could afford the necessary
computers and wireline access equipment.
[0003] As a result of the widespread availability of affordable new
technologies, the general population now increasingly desires to
have not only wireline access to networks such as the Internet and
private intranets, but also wireless access as well. Wireless
technology is particularly useful to users of portable computers,
laptop computers, hand-held personal digital assistants and the
like who prefer access to such networks without being tethered to a
telephone line.
[0004] There still is no widely available satisfactory solution for
providing low cost, high speed access to the Internet, private
intranets, and other networks using the existing wireless
infrastructure. This is most likely an artifact of several
unfortunate circumstances. First, the typical manner of providing
high speed data service in the business environment over a wireline
network is not readily adaptable to the voice grade service
available in most homes or offices. For example, such standard high
speed data services do not necessarily lend themselves to efficient
transmission over standard cellular wireless handsets because
wireless networks were originally designed only to provide voice
services. As a result, present day digital wireless communication
systems are optimized for voice transmissions, although certain
schemes such as CDMA do provide some measure of asymmetrical
behavior for the accommodation of data transmissions. For example,
the data rate specified by the Telecommunication Industry
Association (TIA) for IS-95 on the forward traffic channel is
adjustable in increments from 1.2 kbps up to 9.6 kbps for so-called
Rate Set 1, and increments from 1.8 kbps up to 14.4 kbps for Rate
Set 2. On the reverse link traffic channel, however, the data rate
is fixed at 4.8 kbps.
[0005] At best, existing wireless systems therefore typically
provide a radio channel that can accommodate maximum data rate
transfers of 14.4 kilobits per second (kbps) over a forward link
direction. Such a low data rate channel does not lend itself
directly to transmitting data at rates of 28.8 or even 56.6 kbps
that are now commonly available using inexpensive wireline modems,
not to mention even higher rates such as the 128 kbps that are
available with Integrated Services Digital Network (ISDN) type
equipment. Data rates at these levels are rapidly becoming the
minimum acceptable rates for activities such as browsing web
pages.
[0006] Although wireline networks were known at the time when
cellular systems were initially developed, for the most part, there
was no provision made for such wireless systems to provide higher
speed ISDN- or xDSL-grade data services over cellular network
topologies.
[0007] In most wireless systems, there are many more potential
users than radio channel resources. Some type of demand-based
multiple access system is therefore required.
[0008] Whether the multiple access is provided by the traditional
Frequency Division Multiple Access (FDMA) using analog modulation
on a group of radio frequency carrier signals, or by schemes that
permit sharing of a radio carrier frequency using Time Division
Multiple Access (TDMA), or Code Division Multiple Access (CDMA),
the nature of the radio spectrum is such that it is expected to be
shared. This is quite dissimilar to the traditional environment
supporting data transmissions in which the wireline medium is
relatively inexpensive and is not typically intended to be
shared.
[0009] Other factors to consider in the design of a wireless system
are the characteristics of the data itself. For example, consider
that access to web pages generally is burst-oriented, with
asymmetrical data rate transmission requirements in a reverse and
forward direction. In a common application, a user of a remote
client computer first specifies the address of a web page to a
browser program. The browser program then sends the web page
address data, which is usually 100 bytes or less in length, over
the network to a server computer. The server computer then responds
with the content of the requested web page, which may include
anywhere from 10 kilobytes to several megabytes of text, image,
audio, or even video data. The user thereafter may spend several
seconds or even several minutes reading the content of the page
before downloading another web page.
[0010] In an office environment, the nature of most employees'
computer work habits is typically to check a few web pages and then
to do something else for an extended period of time, such as
accessing locally stored data or even terminating use of the
computer altogether. Therefore, even though such users may remain
connected to the Internet or private intranet continuously during
an entire day, actual use of the high speed data link is usually
quite sporadic.
[0011] If wireless data transfer services supporting Internet
connectivity are to coexist with wireless voice communication, it
is becoming increasingly important to optimize the use of available
resources in wireless CDMA systems. Frequency re-use and dynamic
traffic channel allocation address some aspects of increasing the
efficiency of high performance wireless CDMA communication systems,
but there is still a need for more efficient utilization of
available resources.
SUMMARY
[0012] A significant limitation of forward link capacity involves
the amount of carrier power that can be allocated to dedicated
traffic payload channels. Overhead channels such as pilot and
paging consume power that may be otherwise utilized for
transmitting data to users. A major limitation of so-called common
channels in CDMA systems is the lack of power control associated
with the messages to the individual users. Power control allows an
increase in capacity as only the power on a per-user basis is
allocated, allowing the residual power to be used for traffic
payload. Common channels such as the paging channel have no method
of closed-loop power control feedback to the base station. Because
of this, enough power must be allocated to all messages to meet the
minimum performance for all users in the network. This causes
significant waste as much of this power may be reallocated to
traffic.
[0013] Users can require on-demand and sporadic high speed
throughput of data on a wireless communication link. For example,
remote users can be connected to the Internet over a wireless link
that supports on-demand high speed throughput capability for
downloading an object file such as a web page. Such users can
remain in a standby mode when no data payloads are transmitted in a
reverse link direction. To support such users, it is advantageous
to maintain synchronization with a base station even while the link
is not actively being used to transmit or receive data. This can be
achieved by maintaining a minimal connection with the base station
even when no data is being actively transferred between the base
station and a specific field unit. A shared channel in the reverse
link called a heartbeat channel can be used to maintain the minimal
connection by transmitting a minimal indication from a field unit
to keep it synchronized with the base station.
[0014] This invention is a method of providing closed-loop feedback
for messages on the forward link meant to control users in a
standby mode utilizing a reverse link time slotted heartbeat
channel. In the heartbeat channel, during the user's time slot, one
of several codes may be transmitted. One code ("heartbeat") is used
to notify the base station that the field unit desires to remain in
a standby mode. Another code ("heartbeat with request to go
active") is used to notify the base station that the field unit is
ready to begin transmitting a data payload to the base station.
Since the duration of these time slots is adequate to support more
than one code, additional signaling may be provided by utilizing
additional codes. For instance, codes to indicate a power up/power
down power control scheme may be sent simultaneously with the
heartbeat and heartbeat with request messages. This is provided by
arranging the messages in a code matrix where one axis indicates
heartbeat or heartbeat with request, and the other axis indicates
power up/power down to the base station.
[0015] Accordingly, a method for supporting wireless communications
includes allocating a common control channel to support
synchronized communications from a transmitter to multiple
receivers and assigning a time segment in which the transmitter
communicates an indication to a target receiver by generating a
signal at an adjusted power level over the common control channel
for each of the respective receivers. Information indicating
whether to increase or decrease power level transmissions for the
control channel communications at the transmitter is transmitted as
an encoded signal for each receiver.
[0016] The invention allows feedback from a field unit to the base
station to provide closed loop power control of the individual
transmitted messages on a per-user basis. Additionally, other
information may be conveyed in this manner.
[0017] For example, a coding matrix may communicate a request to
remain in heartbeat or to make a request to go active. In addition,
the field unit may communicate power control information indicating
whether powers levels for a forward link control message should be
increased or should be decreased in power. These four conditions
can therefore be handled by encoding the transmission with four
different message codes.
[0018] The message codes may be orthogonal codes, such as Walsh
codes, other types of codes such as quasi-orthogonal pseudonoise
(PN) codes, or may be PN codes.
[0019] The point is, both the desired heartbeat state, as well as
the power control feedback messages for the control channel, can be
handled by encoding the transmissions in the appropriately assigned
time slot of a reverse link control channel, such as heartbeat
standby or heartbeat request active channel.
[0020] This scheme provides a method for providing closed loop
feedback so that power control on even a commonly shared forward
control channel may be implemented. Therefore, one major limitation
of managing forward link capacity is eliminated, since only enough
power needs to be allocated to individual messages to meet the
minimum performance for individual users in the system, rather than
the minimum performance for all users collectively.
[0021] According to another aspect of the invention, feedback from
a field unit to the base station provides closed loop power control
of a dedicated control channel on a forward link using encoded
transmissions on a shared control channel of a reverse link.
BRIEF DESCRIPTION OF THE DRAWING(S)
[0022] The foregoing and other objects, features and advantages of
the invention will be apparent from the following more particular
description of preferred embodiments of the invention, as
illustrated in the accompanying drawings in which like reference
characters refer to the same parts throughout the different views.
The drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention.
[0023] FIG. 1 is a general diagram illustrating a wireless
communication system according to the principles of the present
invention.
[0024] FIG. 2A is a timing diagram illustrating heartbeat and Link
Quality Management (LQM) slot timing according to the principles of
the present invention.
[0025] FIG. 2B is a table illustrating an assignment of codes to
heartbeat and heartbeat request active channel messages.
[0026] FIG. 3 is a diagram illustrating an exemplary bit definition
of an LQM slot according to the principles of the present
invention.
[0027] FIG. 4 is a graph illustrating a field unit requesting to go
active and the allocation of traffic channels to transmit a data
payload in a reverse link direction according to the principles of
the present invention.
[0028] FIG. 5 is a block diagram supporting channel synchronization
according to the principles of the present invention.
[0029] FIGS. 6A and 6B are flow charts illustrating how forward and
reverse channels are synchronized according to the principles of
the present invention.
[0030] FIG. 7 is a graph illustrating pulse sampling techniques for
identifying a timing mark for synchronizing forward and reverse
channels according to the principles of the present invention.
[0031] FIG. 8 is a table showing attributes of an active, standby
and idle mode for synchronizing a field unit to a base station
according to the principles of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0032] FIG. 1 is a diagram of a wireless communication system 100
according to the principles of the present invention. A base
station 25 maintains wireless communication links with each of a
plurality of field units 42A, 42B, 42C (collectively, field units
42) as shown. Such wireless links are established based upon
assignment of resources on a forward link 70 and a reverse link 65
between the base station 25 and field units 42. Each link 65 or 70
is typically made up of several logical reverse link channels 55
and several logical forward link channels 60.
[0033] As shown, communication system 100 supports wireless
communication between an interface 50 and network 20. Typically,
network 20 is a Public Switched Telephone Network (PSTN) or
computer network such as the Internet, internet or intranet.
Interface 50 is preferably coupled to a digital processing device
such as a portable computer 12, to provide wireless access to the
network 20. Consequently, portable computer device 12 has access to
network 20 based on communications over a combination of both
hard-wired and wireless data links.
[0034] In a preferred embodiment, the forward link channels 60 and
reverse link channels 55 are defined in wireless communication
system 100 as Code Division Multiple Access (CDMA) channels. That
is, each CDMA channel is preferably defined by encoding and
transmitting data over the channel with an augmented pseudo random
noise (PN) code sequence. The PN coded data is then modulated onto
a radio frequency carrier. This enables a receiver to decipher one
CDMA channel from another knowing only the particular augmented PN
code assigned for a given channel. In accordance with the preferred
embodiment, each channel preferably occupies a 1.25 MHz band
consistent with the IS-95 CDMA standard and is capable of
transmitting at 38.4 kbps.
[0035] Forward link channels 70 include at least three logical
channels. As shown, this includes a forward control message
channel, such as a Link Quality Management (LQM) channel 60L, a
paging channel 60P, and multiple traffic channels 60T.
[0036] Reverse link 65 includes a heartbeat standby channel 55HS,
heartbeat request active channel 55HRA, access channel 55A and
multiple traffic channels 55T. Generally, the reverse link channels
55 are similar to the forward link channels 60 except that each
reverse link traffic channel 55T can support variable data rates
from 2.4 kbps to a maximum of 160 kbps.
[0037] Data transmitted between base station 25 and field unit 42A
typically consists of encoded digital information, such as
hypertext transfer protocol (HTTP) encoded Web page data. Based on
the allocation of multiple traffic channels in the reverse link 65
or forward link 70, higher data transfer rates can be achieved in a
particular link between the base station 25 and field unit 42A.
However, since multiple field units 42 compete for bandwidth
allocation, a field unit 42A may have to wait until resources are
free to be assigned traffic channels to transmit a data
payload.
[0038] As shown in FIG. 2A, the forward link LQM channel 60L is
partitioned into a predetermined number of periodically repeating
time slots 310 for the transmission of messages to each of multiple
field units 42. Each field unit 42 identifies messages directed to
itself based upon messages received in its assigned time slot 310.
In other words, field units 42 monitor messages received in their
respectively assigned time slots 310 to receive information from
base station 25.
[0039] The reverse link heartbeat standby channel 55HS and
heartbeat request active channel 55HRA are also shared among
multiple users. These channels are both partitioned into
periodically repeating time slots 310 so that the time slots 310 in
each heartbeat channel align with each other and also the time
slots of the LQM channel 60L. A time slot 310 of the reverse link
heartbeat channels 55HS or 55HRA is assigned to one of many field
units 42 for transmitting heartbeat-type messages to the base
station 25 over either the heartbeat standby channel 55HS or
heartbeat request active channel 55HRA. Accordingly, the base
station 25 identifies from which field unit 42A a message is
transmitted based upon the receipt of a message in a particular
time slot.
[0040] The pair of shared channels in the reverse link are called
heartbeat channels because one aspect of the present invention
involves transmitting a minimal indication from a field unit 42A to
keep it synchronized with the base station 25. The heartbeat
channels 55HS, 55HRA and LQM channel 60L are described in more
detail below.
[0041] As previously mentioned, another aspect of the present
invention involves maintaining a minimal maintenance link between
each of multiple field units and a base station 25 even when they
are not presently transmitting a data payload in a reverse link
direction. This scheme maintaining synchronization is particularly
advantageous in applications where each of the multiple field units
sporadically request to go active and transmit data in a reverse
link. Since each of the field units 42 is already synchronized with
the base station 25 via the minimal link, a field unit 42A can be
assigned reverse link traffic channels 55 and, when assigned,
almost immediately transmit a data payload in a reverse link
direction without interfering with other channels. That is, a field
unit 42A does not need to go through a lengthy process of
re-synchronizing itself with the base station 25 when traffic
channels are eventually assigned for its use.
[0042] In the following description, reference is again generally
made to FIG. 1, but more specific details of LQM channel 60 and
heartbeat channel 55H are referenced to FIG. 2A.
[0043] To establish a synchronized link with base station 25, field
units 42 transmit messages on access channel 55A to base station
receiver 35 via field unit transmitter 40. These messages are then
acknowledged and processed at base station 25. If available,
resources are allocated by base station 25 to establish a
bi-directional communication link with the requesting field unit
42A.
[0044] Within the forward link 70, the paging channel 60P is used
by the base station transmitter 30 to send overhead and paging
messages or commands to the field unit receiver 45. Overhead
information includes data such as system configuration parameters
for establishing wireless links with field units 42.
[0045] As mentioned previously, wireless communication system 100
includes a heartbeat channel 55HS and heartbeat request active
channel 55HRA in the reverse link 65 and link quality management
channel (LQM) 60L in the forward link 70. These channels are shared
between the base station 25 and multiple field units 42. That is,
the base station 25 transmits messages to multiple field units 42
using the same forward link LQM channel 60L, where a message to a
particular field unit 42A is transmitted in an assigned time slot
310. In this way, time slot assignments serve as a way of
addressing messages to a particular field unit and corresponding
communication link.
[0046] The principles of the present invention are advantageously
deployed to support users that require on-demand and sporadic high
speed throughput of data on a wireless communication link. For
example, remote users at PC device 12 can be connected to the
Internet over a wireless link that supports on demand high speed
throughput capability for downloading an object file such as a web
page. Users can remain in a standby mode when no data payloads are
transmitted in a reverse link direction. For example, use of a link
can be minimal for a period of time so that the user can review a
web page. To support such users, it is advantageous to maintain
synchronization with the base station 25 even while the link is not
actively being used to transmit or receive data. This is achieved
in wireless communication system 100 by maintaining a minimal
connection with the base station 25 even when no data is being
actively transferred between the base station 25 and a specific
field unit 42A.
[0047] One aspect of the minimal connection between a field unit
42A and base station 25 involves adjusting timing of the field unit
42A so that its timing is properly aligned with base station 25.
Another aspect of the minimal connection involves adjusting the
power level output of the field unit 42A so that it transmits at a
low but detectable power level.
[0048] As mentioned, repeatedly creating or reviving connections
for users who sporadically need a link can be time consuming and
result in the inefficient use of resources. It is also inefficient
to reserve resources such as traffic channels 55T on a continuous
basis for subscribers who are not presently transmitting data.
Accordingly, traffic channels 55T are allocated on an as-needed
basis to support data transfers, optimizing the use of available
resources in wireless communication system 100.
[0049] FIG. 2A is a timing diagram more particularly illustrating
the heartbeat standby channel 55HS, heartbeat request active
channel 55HRA and LQM channel 60L. Preferably, there are two LQM
channels 60L combined with a total of four heartbeat-type channels
including two heartbeat standby channels 55HS and two heartbeat
request active channels 55HRA since coded channels are typically
allocated in pairs. However, only one of each channel type is shown
in FIG. 2A for illustrative purposes. Of course, the paired sets of
channels can be used to support twice the number of users.
[0050] As shown, 64 time slots (in each direction) are defined per
EPOCH period in each of the heartbeat standby 55HS, heartbeat
request active channel 55HRA and LQM 60L channels. Up to 48 field
units 42 in the standby mode can be supported along with up to 16
users in the active mode. The EPOCH period in the illustrated
embodiment is 13.3 mS, so that each time slot is 208 mS or 256 PN
code chips. Because time slots repeat on a periodic basis, the base
station 25 can exchange information with a particular field unit 42
every EPOCH or 13.3 mS.
[0051] Data transmissions on the LQM channel 60L are maintained by
base station 25, which is preferably used as a master timing
reference. Field units 42, therefore, must synchronize themselves
to base station 25, and specifically to the LQM channel 60L, in
order to communicate with the base station 25 and transmit within
an assigned time slot.
[0052] Generally, a link between base station 25 and a field unit
42A is maintained in one of three modes: active, standby or idle.
Precise synchronization between the base station 25 and a
particular field unit 42A is maintained only for field units 42 in
the active and standby mode. FIG. 7 provides more details about
mode types maintained for a particular link between the base
station 25 and a field unit 42A. This aspect of the present
invention will be discussed later in the specification.
[0053] Each field unit 42A in the standby or active mode is
assigned one time slot in the forward link LQM channel 60L and one
time segment in the reverse link heartbeat-type channels.
Accordingly, information is targeted to a field unit 42A based upon
the transmission of a message in a particular time slot. For
example, a field unit 42A assigned to time slot #1 demodulates and
decodes information received in time slot #1 on the forward link
LQM channel 60L, while data transmitted back to base station 25 is
transmitted by field unit 42A in time slot #1 of the reverse link
heartbeat standby channel 55HS or heartbeat request active channel
55HRA. Both base station 25 and field unit 42A identify to which
link a message is directed based on receipt of a message in a
particular time slot 310.
[0054] Preferably, there is a timing offset between time slots in
each respective channel, allowing the base station 25 time to
process a message received in an assigned time slot and then
respond accordingly over the LQM channel 60L in a following portion
of a cycle. Thus, messages transmitted over the LQM channel 60L
include feedback messages that are used to adjust transmitting
characteristics of a field unit 42A.
[0055] It should be noted that although the LQM channel 60L is used
as a timing reference as described above, the principles of the
present invention equally apply where the heartbeat-type channels
55HS and 55HRA are used in a forward link and LQM-type channel is
used in a reverse link. In other words, base station 25 is
optionally synchronized with respect to a field unit 42A.
[0056] In the standby mode, synchronization is maintained between
the forward link LQM channel 60L and reverse link heartbeat standby
channel 55HS based upon messages sent in the appropriate time slot
on the LQM channel 60L indicating to a particular field unit 42
whether messages transmitted to the base station 25 from that field
unit 42 are received in the appropriate time slot. For example,
message transmissions from the field unit transmitter 40 to base
station 25 are analyzed at base station receiver 35 to achieve fine
tuning alignment between the base station 25 and each of multiple
field units 42.
[0057] As shown in FIG. 2A, time slots A.sub.1 through A.sub.16 of
the LQM channel 60L are reserved for field units 42 in the active
mode, indicating that traffic channels are assigned to a field unit
42A in a reverse link direction and data is being transferred from
the field unit 42 to the base station 25. Contrariwise, time slots
1-48 of the LQM channels 60L are reserved for field units 42
operating in the standby mode that are not presently transmitting a
data payload over a reverse link of communication system 100.
[0058] At any given time, there are preferably no more than 48 of
the 64 time slots of the heartbeat channel 55H or LQM channel 60L
assigned to respective field units 42. This ensures that on
completion of a data transfer between a field unit 42A and base
station 25, a field unit 42A in the active mode assigned an active
time slot can revert back to the standby mode and consequently be
assigned an unused standby mode time slot 310 again.
[0059] Preferably, field units 42 in the standby mode are assigned
an unused active time slot 310 as close to the EPOCH mark M1 as
possible when they are placed in the active mode. For example, if
48 field units are assigned standby mode LQM slots S.sub.1,
S.sub.2, . . . S.sub.48, a field unit 42A set to the active mode
would be assigned active mode time slot A.sub.1 in the LQM channel.
The next active time slot 310 to be assigned to a field unit 42A
would be the lowest numbered and unused time slot such as A.sub.2,
assuming A.sub.1 is then in use.
[0060] It should be noted that heartbeat standby channel 55HS also
includes additional time slots for transmitting messages from an
active field unit 42A, i.e., a field unit 42A transmitting data in
a reverse link over assigned traffic channels, to base station 20.
Preferably, reverse LQM time slots 250 are allocated for
transmitting link quality information from a corresponding active
field unit 42A to base station 20. In this way, base station 20 can
be notified of a corresponding link quality of transmissions on
forward channels between the base station 20 and field unit 42.
[0061] In a specific application utilizing the reverse LQM time
slots 250, a field unit 42A can monitor the quality of a forward
link signal from the base station 20 and transmit a modulated
message including forward error correction information to the base
station 20 in an assigned LQM time slot 250. Based on these
feedback messages transmitted in an LQM time slot 250, properties
of the transmitted signal from base station 20 can be adjusted so
that subsequent messages on the forward link channels to the field
unit 42A can be properly detected. For instance, field unit 42A can
monitor whether a signal transmitted by the base station 20 on a
forward link traffic channel is transmitted at an appropriate power
level so that the power level of the received signal is within a
desired range, e.g., a selected signal-to-noise ratio. In this
instance, the message sent in the reverse LQM time slot 250 can
indicate whether base station 20 should increase or decrease its
power level output on the forward channel.
[0062] The heartbeat standby channel 55HS therefore supports at
least two types of communications between multiple field units 42
and base station 20. A first field unit 42A in the standby mode
transmits a timing reference signal that is monitored at base
station 20 for adjusting timing alignment of the corresponding
filed unit 42A. As recently discussed, a second field unit 42B in
the active mode is assigned a reverse LQM time slot 250 for
transmitting a message to base station 20. Preferably, the message
transmitted in a reverse LQM time slot 250 includes a data message
that is demodulated and decoded at base station 25 to determine the
contents of the message.
[0063] The mere RF (Radio Frequency) transmission in a time slot on
the heartbeat standby channel 55HS by a field unit 42A in the
standby mode itself is an indication to the base station 20 that
the field unit 42A desires to remain in the standby mode. As
mentioned, the latter transmission by a field unit 42A in the
standby mode preferably does not include an encoded and modulated
message including forward error correction information.
[0064] FIG. 3 is a timing diagram illustrating an exemplary mapping
of bits in a forward link LQM time slot 310 according to the
principles of the present invention. As shown, there are 16 bits
transmitted in each time slot 310, although this can vary depending
on the application. One bit of the LQM time slot 310 is LQM timing
bit 311 that indicates whether a field unit message transmission
received at the base station 25 on a last message cycle is
accurately received within an assigned time slot 310. This ensures
that other field units 42 transmitting messages in adjacent time
slots of the same reverse link channel 65 do not interfere with
each other.
[0065] In a preferred embodiment, the LQM timing bit 311 indicates
whether a field unit 42A is to advance or retard its timing on the
reverse link 65. A logic one indicates that timing should be
advanced 1/8 of a chip while a logic zero indicates that timing
should be retarded 1/8 of a chip. In this way, the base station 25
individually synchronizes communication links between the base
station 25 and each of a plurality of field units 42. Said
differently, timing of message transmissions from corresponding
field units 42 are frequently adjusted, so that corresponding
messages are received in the assigned time slots at the base
station 25. Consequently, a field unit 42 can synchronize itself
with the base station 25 even though it is moving very fast
relative to the base station 25.
[0066] In a preferred embodiment, the base station 25 transmits
information on the LQM channel 60L based on BCH coding. This
enables a receiving field unit 42 to detect and correct errors. For
example, the use of a 15,7 code allows up to 2 errors to be
corrected and up to 3 errors to be detected. As shown in FIG. 3,
there are 8 parity bits 313 for error correction and detection.
[0067] Referring again to FIG. 2A, a timing diagram illustrates the
heartbeat standby channel 55HS and heartbeat request active channel
55HRA. As shown, time slot numbering is selected for both channels
so that they line up with each other. For example, time slot #1 for
each heartbeat channel are aligned with each other in a given time
segment, T.sub.SLOT.
[0068] The heartbeat standby channel 55HS and heartbeat request
active channel 55HRA serve different functions. For example, a
field unit 42A assigned use of a particular time slot 310 transmits
over the heartbeat standby channel 55HS in order to provide an
indication to base station 25 that the field unit 42 desires to
remain in the standby mode. On the other hand, a field unit 42A
alternatively transmits over the appropriate time slot 310 of the
heartbeat request active channel 55HRA to provide an indication to
the base station 25 that the field unit 12 desires the allocation
of reverse link traffic channels for transmitting a data payload
from the field unit 42A to base station 25.
[0069] FIG. 2B is a table illustrating in more detail how the
communication on the heartbeat standby channel 55HS and heartbeat
request active channel 55HRA may be encoded to provide different
indications to the base station 25 of certain conditions in the
field unit 12. For example, a coding matrix as shown may be used to
communicate four different messages. One axis of the matrix
represents communicating a request to remain in heartbeat mode or a
request to go into an active mode. The other axis indicates power
control information for forward link control messages, such as
whether control messages transmitted on a shared forward link
control channel, such as the LQM channel 60L, should be increased
in power or decreased in power. The base station can respond to the
power up/power down indications by adjusting transmissions on the
forward link by a set amount, e.g. .+-.1 db.
[0070] These four conditions can therefore be handled by encoding
the heartbeat standby channel 55HS or heartbeat request active
channel 55HRA slots with one of four different message codes. The
message codes may be orthogonal codes, such as Walsh codes, or may
be other types of codes such as quasi-orthogonal PN codes, or PN
codes.
[0071] The point is, both the desired heartbeat state, as well as
the power control feedback messages for the LQM channel can be
handled by encoding the transmissions in the appropriately assigned
time slot 310 of the heartbeat standby channel 55HS or heartbeat
request active channel 55HRA.
[0072] This provides a method for providing closed loop feedback so
that power control on a commonly shared control channel, such as
the LQM channel, may be implemented. Control is implemented on as
fine a granularity as a per-user basis. Therefore, one major
limitation of managing forward link capacity is eliminated, since
only enough power needs to be allocated to each forward link
message to meet the minimum performance for individual users in the
network, rather than the minimum performance for all users
collectively in the network.
[0073] The principles of the present approach apply also to systems
that use dedicated control channels. For example, in IS-2000 Rev.
C, a fundamental channel is defined within an assigned traffic
channel to provide dedicated control. The encoded transmissions
sent over the heart beat channels of the present system can be used
to provide closed loop power control of the dedicated control
channels. That is, the invention does not require the forward link
to be a shared control channel.
[0074] In a preferred application, the heartbeat standby channel
55HS, heartbeat request active channel 55HRA and LQM channel 60L
are all defined by unique code such as long PN (Pseudo-Random
Noise) codes. Accordingly, base station 25 detects a message from a
field unit 42A in an assigned time slot by detecting whether or not
a field unit 42A transmits an RF (Radio Frequency) signal over the
corresponding uniquely coded channel. A transmission within an
assigned time slot of either heartbeat channel need not include a
meaningful data payload that must be demodulated because the mere
coded RF transmission by a field unit 42A within a time of a
channel itself indicates to base station 25 whether the
corresponding field unit 42A desires to remain in the standby mode
or go active.
[0075] In one application, field unit 42A transmits unmodulated
data including a short PN code, a long PN code, and an orthogonal
code such as a Walsh code in an assigned time slot 310 of the
heartbeat-type channel, i.e., the heartbeat standby channel 55HS of
the heartbeat request active channel 55HRA. Thus, the message as
received in a time slot 310 is easily identified without having to
decode a corresponding data payload message. A field unit 42A can
then transmit at a lower power level than would otherwise be
necessary if the field unit transmitted an indication including a
coded message or data payload.
[0076] Since field units 42 transmit during an assigned time
segment over only one of the pair of heartbeat channels including
heartbeat standby channel 55HS and heartbeat request active channel
55HRA, the combination of transmitted RF power on these channels is
effectively that of a single channel.
[0077] A marker is preferably included within a time slotted
message of either heartbeat channel so that base station 25 can
analyze whether a corresponding field unit 42 is properly
synchronized. More specifically, field unit 42 transmits a marker
at a predetermined position in a time slot 310 and base station 25
then sends a message in the appropriate time slot 310 of the
forward link LQM channel 60L to indicate whether the field unit
should advance or retreat its timing of future message
transmissions.
[0078] Another aspect of the present invention involves maintaining
a power feedback loop between each of multiple field units 42 and
base station 25. The indication transmitted in a time slot of the
heartbeat standby channel 55HS or heartbeat request active channel
55HRA is analyzed at base station 25 to determine the strength of
the received RF signal as transmitted by a corresponding field unit
42A. For example, the power output of a field unit 42A can be
adjusted based on a signal-to-noise ratio of the signal received at
base station 25. If the signal strength is lower than a desired
level or outside a specified range as detected by base station 25,
a feedback message generated by base station 25 is communicated to
the field unit 42A in the appropriate forward link for adjusting
its power level for subsequent transmissions on the heartbeat type
channels. In this way, the power level of a field unit 42A can be
adjusted to reduce co-channel interference based on power
adjustment messages transmitted to a field unit 42A in successive
LQM time slots 310. The power level of a transmitting device can be
gradually increased or decreased so that it has minimal impact on
other channels.
[0079] The aforementioned method of adjusting the power output
level of a field unit 42A is similar to the method as previously
described for synchronizing a field unit 42A to base station 25 via
feedback messages. However, in the power feedback control loop, the
power level output of the field unit 42A is adjusted via feedback
messages instead of timing. Thus, the power level of a field unit
42A can be adjusted while in a standby mode so that, in the event
that the transmitter goes active transmitting a data payload to the
receiver, the power level of the transmitter is optimized to reduce
co-channel interference.
[0080] The power feedback loop provides a reference for
transmitting an RF signal at a specified power level so that the
field unit 42A can determine at what level the field unit 42A
should transmit an FEC (Forward Error Correction) coded message
over other channels such as reverse link traffic channels. More
specifically, a field unit 42A recently assigned to the active mode
can determine at what level to transmit a data payload to base
station 25 depending on the modulation-type and FEC code to be used
for transmitting the data payload using the power level
transmission on the heartbeat channel as a reference.
[0081] Both power and timing feedback loops can be implemented
simultaneously so that the power output level and timing of a field
unit 42A is optimized for potentially sporadic data transmissions.
Thus, power and timing of a field unit 42A is optimally adjusted
even in a dynamic environment where the signal to noise ratio and
signal path transmission delay of the field unit 42A changes almost
instantaneously. A link is maintained even during changing
environmental conditions.
[0082] In the standby mode, power level optimization is achieved
based upon messages sent in the appropriate time slot 310 on the
LQM channel 60L indicating to a particular field unit 42A whether
RF transmissions form the field unit 42A to the base station 25 are
received at an appropriate power level. For example, signal
transmissions from the field unit transmitter 40 to base station 25
are analyzed at base station receiver 35 to achieve fine tuning
power level adjustments for each of multiple field units 42.
[0083] FIG. 3 is a timing diagram illustrating an exemplary mapping
of bits in a forward link LQM time slot 310 according to the
principles of the present invention. As shown, there are at least
16 bits transmitted in each time slot 310, although this can vary
depending on the application.
[0084] One bit of the LQM time slot 310 is LQM power level control
bit 312 that indicates whether a field unit transmission on the
heartbeat standby channel 55HS or heartbeat request active channel
55HRA received at base station 25 in a previous Epoch cycle is
detected to be within a desired power level range. This feedback
message in the LQM time slot 310 is monitored at the field unit 42A
to adjust the power output level of the field unit 42A so that
power output of the field unit 42A is minimal but detectable at
base station 25. Of course, the power output level of the field
unit 42A is adjusted above the minimal detectable level so that
transmissions from the field unit 42A are still detectable even if
there is a slight change in environmental conditions.
[0085] Notably, if the transmission by a field unit 42A is so low
that it is not detectable at base station 25, a feedback message in
the LQM time slot 310 will be generated indicating that the field
unit 42A shall increase its power output level a predetermined
amount so that the base station 25 can hopefully detect a
transmission by the field unit 42A in a following Epoch. Power
feedback messages transmitted over multiple Epochs to the field
unit 42A can be used to gradually adjust its power output level.
This gradual change in power output by the field unit 42A minimally
impacts the quality of other channels. In other words, the field
unit 42A preferably does not transmit at such a high power level
that it causes undue interference with other field units 42
transmitting on other coded channels.
[0086] In a specific application, the LQM power level control bit
312 indicates whether a field unit 42A is to increase or decrease
its power level output for transmissions on the reverse link 65. A
logic one indicates that timing should be increased by, for
example, 1/2 dB while a logic zero indicates that power level
output of the field unit 42A should be decreased by 1/2 dB so that
the received signal at base station 25 falls within a desired
signal-to-noise ratio range. In this way, base station 25
individually adjusts the power level of communication links between
the base station 25 and each of a plurality of field units 42. Said
differently, power output levels of corresponding field units 42
are frequently adjusted, so that corresponding indications are
received at a desired power level at base station 25. Consequently,
the power output level of a field unit 42 can be continuously
adjusted so that it is optimally set even though the field unit 42A
may be moving very fast relative to the base station 25, i.e., the
reverse link path loss may be changing and the power output level
of the field unit 42A will be adjusted accordingly for supporting
continued communications with base station 25.
[0087] As mentioned, power adjustments are made at the field unit
42A based upon the state of the LQM timing bit 312. Initially,
timing is adjusted by a first predetermined amount such as 1/2 dB
in the appropriate direction depending on the state of this bit.
However, if the field unit 42A receives 8 "increase" power bits in
a row or 8 "decrease" power bits in a row over as many EPOCHs,
power adjustments of the field unit 42A are based on 1 db instead
of 1/2 dB for the following LQM power control bits 312 of the same
state. In this way, the optimal power output level of the field
unit 42A can be achieved more quickly when the power level for a
link is grossly out of adjustment.
[0088] Once the field unit 42A determines that the power output
level is overcorrected, i.e., the polarity of the LQM timing bit
312 changes state from one epoch to the next, power output
adjustments at the field unit 42A revert back to 1/2 db for each
subsequently received LQM power control bit 312. When power
synchronization is achieved between a field unit 42 and base
station 25, the LQM power control bit 312 will typically be set to
alternating logic ones and zeros for several successive EPOCH
cycles. In other words, power control output at the field unit will
jitter 1/2 dB when synchronization is practically achieved between
the base station 25 and field unit 42A. This amount of jitter is
tolerable for maintaining such synchronization links. Of course, a
filter can be implemented at the field unit 42A so that the power
output does not jitter from one Epoch to the next of the filed unit
42A.
[0089] Rather than transmit a single LQM power control bit 312, the
LQM timing slot 310 can also include a multi-bit message indicating
an amount that the corresponding field unit 42A is to increase or
decrease it power output level.
[0090] FIG. 4 is a timing diagram illustrating a field unit 42A
requesting to be assigned reverse link traffic channels according
to the principles of the present invention. As shown, a field unit
42A in the standby mode is assigned a particular time slot 310 in
Epoch E.sub.1. As previously discussed, the field unit 42A
transmits over the assigned time slot 310 of the heartbeat standby
channel 55HS to remain in the standby mode. In response to this
reverse link indication from the field unit 42A, base station 25
transmits a feedback message in the appropriate time slot 310 of
the LQM channel 60L in Epoch E.sub.1 for maintaining
synchronization of the link. As discussed, this feedback message
can include both power and timing control adjustment
information.
[0091] Epoch E.sub.2 illustrates a similar circumstance where the
field unit 42A continues requesting to remain in the standby mode.
Consequently, the repetitive function of monitoring a timing marker
within a time slot 310 and providing corresponding feedback in the
reverse link over the LQM channel 60L ensures that the
corresponding link is synchronized in the event that the field unit
42 desires to transmit a data payload in a reverse link
direction.
[0092] In following Epoch E.sub.3, field unit 42A indicates to base
station 25 a request to go active so that it will be assigned
reverse link traffic channels to transmit a data payload. As
mentioned, this is achieved by generating an RF signal in the
appropriate time slot 310 of the heartbeat request active channel
55HRA. Depending on a number of available reverse link traffic
channels, there can be a delay between the time a field unit
requests to go active and the time traffic channels are actually
assigned for use by the field unit 42A. Thus, it is desirable to
repeat a request to go active by transmitting in an assigned time
slot 310 of the heartbeat request active channel 55HRA at the base
station 25. Since timing adjustment feedback messages are also
transmitted to the field unit 42A based on messages received on the
heartbeat request active channel 55HRA, precise synchronization and
power control of the corresponding link between base station 25 and
field unit 42 is maintained for subsequent Epochs E.sub.4 and
E.sub.5.
[0093] Prior to or during Epoch E.sub.5, field unit 42A is notified
which traffic channels are allocated for transmitting its data
payload in a reverse link direction.
[0094] Epoch E.sub.6 and E.sub.7 illustrate that field unit 42A has
been assigned use of reverse link traffic channels 55T for
transmitting a data payload. Notably, the field unit 42A no longer
transmits an indication to the base station 25 over either the
heartbeat standby channel 55HS or the heartbeat request active
channel 55HRA. However, a link quality message is still transmitted
in a forward link direction from the base station 25 to adjust
timing of the field unit 42. The timing adjustment feedback
messages are based on markers transmitted over the reverse link
traffic channels 55T. As shown, in Epoch E.sub.6 and E.sub.7, an
LQM message is transmitted to the field unit 42A in a newly
assigned active time slot between A.sub.1 and A.sub.16. Thus,
transmissions from the base station 25 to field unit 42A have
shifted to a new time slot 310. Of course, prior to Epoch E.sub.6,
field unit 42A must be notified of the traffic channels 55T on
which it is to transmit a data payload and the newly assigned
active time slot 310 in which the field unit 42A is to receive a
time-slotted LQM message.
[0095] As mentioned, markers are included with the data payload
transmissions over the reverse link traffic channel 55T to base
station 25 where they are analyzed. In this instance, the minimal
feedback timing adjustment messages are generated based on the
markers received within the reverse link traffic channels 55T. The
timing adjustment messages are transmitted in the newly assigned
active time slot A.sub.1 of the forward link LQM channel 60L.
[0096] After a data payload is transmitted over the reverse link
traffic channel 55T, the field unit 42A is placed in the standby
mode as shown in Epoch E.sub.8 and E.sub.9. Accordingly,
synchronization is again maintained based on a feedback loop
between a field unit 42A and base station 25. More specifically,
messages transmitted in a time slot 310 of the heartbeat standby
channel 55HS are again analyzed at base station 25 and timing
adjustment feedback information is transmitted in the standby time
slot S.sub.1 of the forward link LQM channel 60L to precisely
synchronize the field unit 42A with the base station 25.
[0097] FIG. 5 more particularly shows hardware components at base
station 25 that are used to achieve synchronization and power
control of the reverse link 65 and forward link 70. Information
transmitted in a time slot 310 as assigned for use by a field unit
42A is analyzed by a corresponding heartbeat correlation filter
such as heartbeat standby correlation filter 440 or heartbeat
request active correlation filter 445. Generally, the unique codes
of each heartbeat channel are monitored in different time slots 310
to detect a request by a corresponding field unit 42A to be placed
in the active mode or remain in the standby mode. Thereafter, the
base station 25 will set field unit 42A to the active mode by
assigning it the appropriate resources if a request to go active is
detected. Note that heartbeat standby correlation filter 440 is
used to identify a long PN code corresponding to a request by the
field unit 42A to remain in the standby mode, while heartbeat
request active correlation filter 445 at base station 25 identifies
a long PN code corresponding with a request to be placed in the
active mode.
[0098] Regardless on which heartbeat-type channel a field unit 42A
transmits in an assigned time slot 310, the marker from the field
unit 42A is monitored by a pulse timing analyzer 422. It is then
determined whether the message transmission by a corresponding
field unit 42A is received early or late within a time slot 310 at
base station 25. Preferably, the strongest received diversity
string in a time slot 310 will be designated as the time alignment
string for analyzing timing of the message received over heartbeat
standby channel 55HS or heartbeat request active channel 55HRA.
[0099] Time slot alignment is preferably based on the correlation
profile of the pilot in a particular string, which is analyzed
using correlation filters as mentioned. The output of the
correlation filters 440, 445 include 256 samples, which represent
64 lags at 4 samples per lag. The 256 sample output-window
represents the total correlation time span of the base station 25.
Preferably, the time alignment point or marker in a time slot 310
is sample number 80, which allows 20 lags for precursor and 44 lags
for post cursor channel information.
[0100] Generally, the computation of the time alignment error is
based on a determination of where the centroid or peak lies in a
given sample string. For example, each field unit 42A transmitting
in its assigned time slot 310 over either the heartbeat standby
channel 55HS or heartbeat request active channel 55HRA includes a
marker, i.e., the peak signal, located at a predetermined position
within a time slot. The strongest pilot path for the channel and 2
samples on either side of the main path, i.e., 1 and 1/4 chips, is
statistically analyzed to determine the centroid or peak of a
marker within a time slot. The centroid of the samples in FIG. 7
are calculated based on the following equation: L = [ t .times. Q
.function. ( t ) ] Q .function. ( t ) ##EQU1## where L is a
position of the centroid in a time slot, t is the sample time along
the X-axis, and Q(t) is the magnitude of a sample at a given sample
time. For example, L is calculated based on the results as shown in
FIG. 7: L = ( .25 * 76 ) + ( 5 * 77 ) + ( 1.0 * 78 ) + ( .8 * 79 )
+ ( .6 * 80 ) .25 + .5 + 1.0 + .8 + .6 ##EQU2## L = 78.317
##EQU2.2##
[0101] Again, the timing alignment error is determined by comparing
the timing of the computed centroid to the desired time set point
of 80, which is chosen as the reference point for timing alignment
within a time slot 310. Since the centroid in the example above is
estimated to be 78.317, timing is early and, therefore, the
corresponding LQM timing bit 311 will be set to a logic "one"
indicating that the corresponding field unit should advance its
timing reference by 1/8 of a chip so that subsequent messages are
transmitted 1/8 of a chip later in time slot 310. This overall
feedback technique in the present invention ensures continuous
fine-tuning the time alignment between base unit 25 and each of
multiple field units 42.
[0102] Preferably, the time error is calculated by taking the
integer of twice the difference between the desired set point
sample 80 and L. For example, time_error=integer [(L-80)*2]
[0103] If the time_error result is negative, the LQM timing bit 311
is set to a logic "one." Conversely, the LQM timing bit 311 is set
to a logic "zero" when time_error is positive.
[0104] Referring again to FIG. 5, processor 426 analyzes timing
data and generates time_error for synchronizing the reverse link
heartbeat channels 55H and forward link LQM channel 60L. LQM time
slotted messages are then transmitted by LQM signal generator 450
on LQM channel #1 60L to provide timing adjustments for the
corresponding field unit 42A as mentioned.
[0105] If a field unit 42A in the standby mode transmits a request
to go active by transmitting in an assigned time slot of the
heartbeat request active channel 55HRA, such a request is detected
at heartbeat request active correlation filter 445. As previously
discussed, the timing characteristics of an active mode request
detected at heartbeat request active correlation filter 445 is also
analyzed to determine timing errors as described above for
maintaining alignment on a particular link between the base station
25 and each field unit 42A.
[0106] If resources are available for allocating traffic channels
55T, the requesting field unit 42A is placed in the active mode by
base station 25, where configuration details for setting up the
data transfer are handled by processor 426. For example,
information regarding new LQM time slot assignments, i.e.,
assignment of an active mode time slot A.sub.1 . . . A.sub.16, is
sent to a corresponding field unit 42A over, for example, the
paging channel 60P. Reverse link traffic channels 55T are then
allocated for transferring a data payload from field unit 42A to
base station 25.
[0107] While in the active mode, synchronization of the forward and
reverse link is maintained based on messages transmitted over the
LQM channel 60L and traffic channels 55T since the heartbeat
channel time slot is no longer dedicated on the reverse link 65 for
use by the transmitting field unit 42A. More specifically, a timing
marker is included in the reverse link traffic channel
transmissions so that base station 25 can monitor whether data
payload field unit 42A is early or late in its timing.
[0108] Messages transmitted by a field unit 42A in the active mode
are transmitted to base station 25 over traffic channels 55T and
the corresponding traffic channel signal is fed into the traffic
channel correlation filter 430 at base station 25 for detection of
pilot symbol timing markers. Preferably, a field unit 42A transmits
a sequence of 32 pilot symbols in an assigned time slot 310 as a
timing marker. The traffic channel 55T is then analyzed by pulse
timing analyzer 420 to determine whether such messages are early or
late with respect to a desired synchronization of the field unit 42
with base station 25.
[0109] The process of analyzing a pulse or marker for estimating
the centroid is similar to that described earlier in the
specification for messages and corresponding markers such as long
PN codes on either heartbeat channel 55HS or 55HRA. However, when
field unit 42A is in the active mode, pilot symbols in the traffic
channels 55T are used as a timing reference mark rather than long
PN codes. Again, see FIG. 7 and related discussion above for
details regarding how a timing marker is analyzed to identify
whether a field unit 42A should advance or retard its timing.
[0110] FIG. 8 is a table illustrating different operational modes
according to the principles of the present invention and how
synchronization is maintained between a field unit and base station
for each of the modes.
[0111] Preferably, timing alignment of the base station 25 and
field units 42 is based upon the LQM timing bit 311 as transmitted
in an assigned active time slot A.sub.1 . . . A.sub.16 on the
forward link 70. When the receipt of data messages transmitted by
the active field unit 42 are received early or late with respect to
an assigned time slot, the LQM timing bit 311 is set accordingly to
advance or retard timing of future message transmissions on the
traffic channels 55T.
[0112] Although a single traffic channel correlation filter 430 is
shown for detecting a marker in a single traffic channel 55T,
multiple traffic channels 55T are optionally analyzed to coordinate
timing alignment between the reverse link 65 and forward link
70.
[0113] As mentioned, access channel 55A is used by the field units
42 to transmit requests for establishing a synchronization link
with the base station 25. Typically, messages on the access channel
55A are transmitted on a random basis. Hence, a message collision
may occur if two or more link requesting field units 42 happen to
transmit a link request message on the access channel 55A at the
same time.
[0114] If a collision is detected on the access channel 55A, the
collision is made known to the field units 42 based upon a message
generated by paging channel signal generator 455 over paging
channel 60P. Each field unit 42 will then retransmit their request
to establish a synchronization link on the access channel 55A based
on a random back off time, making it less likely that a collision
will occur on a second or other subsequent attempt.
[0115] Access channel 55A, also shown in FIG. 5, is fed into access
channel correlation filter 435. Preferably, a field unit 42
transmits a sequence of 32 pilot symbols including information
identifying the field unit 42A requesting a synchronization link. A
received sequence of pilot symbols is analyzed by pulse timing
analyzer 422 to determine initial timing information of the field
unit 42A with respect to the base station 25. Since the field units
42 randomly transmit requests on the access channel 55A, it is
necessary to determine an initial timing error between the field
unit 42 and base station 25 for achieving a coarse synchronization
of the forward and reverse link channels.
[0116] If it is determined by the base station 25 that a
synchronization link will be established between the base station
25 and requesting field unit 42A, an appropriate acknowledgment
message is transmitted over the forward paging channel 60P to the
base station 25 to the corresponding field unit 42A. Among other
information transmitted over the forward paging channel 60P to the
field unit 42, a heartbeat time slot assignment, an LQM time slot
assignment, and synchronization information such as coarse timing
adjustment information is also transmitted to the field unit 42.
Thus, a field unit 42A newly assigned to the standby mode can
transmit an indication over one of the heartbeat-type channels for
maintaining more precise synchronization with base station 25.
[0117] As mentioned, coarse timing adjustment information is
transmitted on the forward paging channel 60P to roughly
synchronize the link requesting field unit 42A with respect to base
station 25. Preferably, a 10-bit signed number is transmitted to
the field unit 42A indicating an amount to advance or retard its
timing with respect to the link request message of the field unit
42 as previously transmitted on the access channel 55A. Each least
significant bit (LSB) in the 10-bit signed number is appropriately
weighted. For example, an LSB can represent 16 chips. Based on this
timing correction information, the corresponding field unit 42A
adjusts its coarse timing relative to the base station 25.
Thereafter, messages are then transmitted in the appropriate
reverse link time slot of the heartbeat channel 55HS, 55HRA or
traffic channel 55T. Fine-tuning is thereafter achieved by
analyzing transmissions by field unit 42A at base station 25 and
providing synchronization information over the LQM channel 60L
feedback path.
[0118] In addition to transmitting in the appropriate time slot,
coarse and fine synchronization with the base station 25 renders it
possible for a field unit 42 to receive information in its assigned
time slot in the forward link.
[0119] FIGS. 6A and 6B are a flow charts providing details of how a
wireless communication link is established between field unit 42A
and base station 25. There are typically multiple field units 42
requesting communication links in a particular service area, where
each mobile or field unit 42A is located at a different distance
with respect to base station 25. For example, some field units 42
can be located very close to base station 25 while others are
located very far away. Hence, the time it takes for a signal to
travel from a particular field unit 42A to base station 25 is
different for each field unit 42. Precise timing alignment of a
specific field unit 42 and the base station 25 is therefore
important to avoid or minimize collisions between field units 42
transmitting in adjacent time slots.
[0120] If all field units 42 transmitted in real time without
taking into account the distance to base station 25 and
corresponding delay, message transmissions in an assigned time slot
from a particular field unit would be skewed, i.e., messages at the
base station would be received slightly out of an assigned time
slot. Therefore, message transmissions from each field unit 42A are
precisely adjusted as previously discussed to prevent this skewing
phenomenon.
[0121] Not only does distance from a field unit 42A to base station
25 effect timing alignment, so does the environment in which a
field unit 42A transmits a message. For example, building
structures, atmospheric conditions and other geographical terrain
will effect the path of a signal transmitted from a field unit 42A
to base station 25. Therefore a field unit 42 changing position
merely a few feet in several seconds can have a substantial impact
on timing of a signal path, thus, effecting timing alignment
between a reverse link 65 and forward link 70. Based on the
principles of the present invention, the previously described
method of continuously adjusting timing transmissions in the shared
reverse channel 65 minimizes collisions among multiple field units
42 transmitting to base station 25 in adjacent time slots.
[0122] Step 510 in FIG. 6A shows an entry point of the flow chart
for establishing a wireless communication link. In step 515, access
channel 55A is monitored by base station 25 to detect requests by
field units 42 to establish wireless synchronization links with
base station 25. A link request message received at base station 25
includes a sequence of pilot symbols followed by data identifying
the link requesting field unit 42A. Based on the data information
received over access channel 55A, base station 25 is able to access
characteristics of the corresponding field unit 42.
[0123] If no standby time slots are available for establishing a
new synchronization link, the connection request by a field unit
42A is denied as shown in step 525. A message is then transmitted
to the corresponding field unit 42A on the forward link paging
channel 60P to indicate that no time slots are available and the
field unit 42A must try again at a later time to establish a
standby synchronization link.
[0124] If resources are available to establish a new link in step
520, base station 25 analyzes the timing of the request message as
received from a field unit 42A on access channel 55A in step 530.
As mentioned, the sequence of 32 pilot symbols are analyzed to
determine the location of the peak pulse or marker in the reverse
link 65. Based on the time when this random message is received
with respect to the base station's master time reference EPOCH
mark, M1, and the distance that the field unit 42A is located from
base station 25, a coarse time adjustment message is generated by
the base station 25 to synchronize timing between the link
requesting field unit 42A and base station 25. This coarse timing
information, preferably a 10-bit signed number indicating how a
field unit 42 should adjust its timing to align the field unit with
the base station EPOCH mark, is sent to the field unit 42A over the
forward link paging channel 60P in step 535. The field unit 42A
then adjusts its timing reference accordingly so that subsequent
messages are transmitted in an assigned time slot on the reverse
link 65. Timing alignment also ensures that the field unit 42A can
receive messages from base station 25 in the appropriate time slot
of the forward link LQM channel 60L.
[0125] Following in step 540, base station 25 assigns two time
slots to the link requesting field unit 42A over paging channel
60P. One time-slot assignment indicates the time slot in which the
field unit 42A is to receive LQM messages from the base station 25
over the LQM channel 60L. Another time-slot assignment indicates in
which time slot 310 of the reverse link field unit 42 is to
transmit over a heartbeat-type channel to base station 25. Based
upon these time slot assignments, the base station 25 and field
units 42 can determine to which link a message pertains as the time
slot itself indicates to which target a message is directed.
[0126] While in the standby mode, base station 25 monitors periodic
messages in an assigned time slot for a transmission on either the
heartbeat standby channel 55HS or heartbeat request active channel
55HRA by a corresponding field unit 42A. For example, a marker
received in a time slot of either channel is analyzed at base
station 25 to correct timing alignment as mentioned between base
station 25 and field unit 42A. If the message in a time slot is
received early or late at base station 25, timing of future
transmissions by the field unit 42 in an assigned time slot 310 on
a reverse link heartbeat channel is appropriately retarded or
advanced based upon the LQM timing bit 311 for a particular field
unit 42A in step 542.
[0127] Timing adjustments are made at the field unit 42A based upon
the state of the LQM timing bit 311. Initially, timing is adjusted
by 1/8 of a chip in the appropriate direction depending on the
state of this bit. However, if the field unit 42A receives 8 retard
bits in a row or 8 advance bits in a row over as many EPOCHs,
timing adjustments of the reference at the field unit 42A at are
based on 1/2 of a chip instead of 1/8 of a chip for the following
LQM bits 311 of the same state. In this way, synchronization
between the base station 25 and field unit 42 is achieved more
quickly when timing for a link is grossly out of adjustment.
[0128] Once the field unit 42A determines that timing is
overcorrected, i.e., the polarity of the LQM timing bit 311 changes
state from one epoch to the next, timing adjustments at the field
unit 42 revert back to 1/8 of a chip for each subsequently received
LQM timing bit 311. When synchronization is achieved between a
field unit 42 and base station 25, the LQM timing bit 311 will
typically be set to alternating logic ones and zeros for several
successive EPOCH cycles. In other words, timing at the field unit
will jitter 1/8 of a chip when synchronization is practically
achieved between the base station 25 and field unit 42A. This
amount of jitter is tolerable for maintaining such synchronization
links.
[0129] If field unit 42A receives another 8 cycles of timing
adjustment corrections in the same direction such that 16
successive LQM bits 311 are the same state, the time adjust
correction is set to 1 chip per received LQM timing bit 311. As
stated earlier, when over-correction is detected, timing
adjustments at the field unit are again based on 1/8 of a chip for
each received LQM timing bit 311 again.
[0130] In addition to monitoring timing pulses for aligning message
transmissions of each field unit 42, base station 25 also
determines on which heartbeat channel a field unit 42A transmits
during its assigned time segment T.sub.SLOT. It is then determined
in step 547 whether a field unit 42A requests to be set to the
active mode based on whether the field unit 42A transmits over the
heartbeat request active channel 55HRA. If so, the base station
allocates appropriate resources such as traffic channels 55T in the
reverse link 65 to support the data transfer in step 550.
Additionally, base station 25 is assigned an active time slot for
use by a field unit 42, i.e., one available time slot between
A.sub.1-A.sub.16, in the forward link LQM channel 60L to maintain a
synchronization loop. While in the active mode, as mentioned, the
field unit 42A maintains synchronization with base station 25 based
on a sequence of well-placed pilot symbol markers in the traffic
channels 55T, upon which the base station 25 issues timing
adjustments in the appropriate time slot 310 using the forward link
LQM timing bit 311. Additionally, the field unit 42A transmits data
over the reverse link traffic channels 55T in step 555 before
returning to the main loop again at step 560. At this re-entry
point into the main loop again, the field unit 42 is then
reassigned a standby mode time slot 310.
[0131] If a field unit 42A has not been in the standby mode too
long in step 560, base station 25 determines whether the field unit
42A has made a request to terminate a wireless link between base
station 25 and corresponding field unit 42A in step 572. Without
the request to tear down a link, processing loops back to step
542.
[0132] If field unit 42 generates a request to tear down a
corresponding link in step 572, base station 25 acknowledges such a
request in step 575 by sending a message to the field unit 42 and
tears down the communication link. This is one way of terminating
the flow chart as shown in step 580.
[0133] Referring again to step 560, if it is determined that the
field unit 42A is inactive too long, i.e., in standby mode not
transmitting data, the base station revokes the assigned LQM and
heartbeat channel slots for use by other users and maintains an
idle connection with the field unit 42A in step 565.
[0134] When it is determined that field unit 42A requests to go
active again in step 582, process flow continues at the beginning
of the flow chart to reestablish a synchronized link in step 570.
In such a case, connectivity is reestablished based in part on the
prior connection. For example, it is not necessary to go through
the entire configuration process since data maintained with respect
to the corresponding recently active link is advantageously used to
minimize the overhead associated with reviving the previous
connection.
[0135] Flow continues at step 585 if base station 25 fails to
detect a request by the field unit 42 to go active again in step
582. If base station 25 fails to receive a response from an idle
field unit 42A in a specified time out period in step 585, base
station 25 pings the field unit 42 on forward page channel 60P to
elicit a response by the field unit 42 in step 587. If the field
unit 42A does not respond in step 590, it is assumed that the field
unit 42A is shut down and an idle connection is therefore no longer
maintained for that particular field unit 42. If the field unit 42A
responds to the ping in step 590, process flow continues in step
595 at START (step 510) of the flowchart to reestablish the link as
a standby connection.
[0136] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
scope of the invention encompassed by the appended claims.
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