U.S. patent application number 11/544234 was filed with the patent office on 2007-02-01 for method and apparatus for transmitting messages in a wireless communication system.
Invention is credited to Stein Lundby, Rajesh Pankaj, Byron Yafuso.
Application Number | 20070025284 11/544234 |
Document ID | / |
Family ID | 25121387 |
Filed Date | 2007-02-01 |
United States Patent
Application |
20070025284 |
Kind Code |
A1 |
Pankaj; Rajesh ; et
al. |
February 1, 2007 |
Method and apparatus for transmitting messages in a wireless
communication system
Abstract
Techniques to reduce transmit power required for transmission of
messages from an access terminal to reduce interference to
transmissions from other access terminals. In one aspect, messages
to be transmitted are defined and/or coded such that they may be
detected at different received signal qualities. The codewords may
be defined having different distances to their nearest codewords.
In another aspect, messages to be transmitted are assigned to
different points in a signal constellation, with the points being
located such that they may be received at different signal
qualities. Codewords that may be received at a lower signal quality
may be assigned to messages more likely to be transmitted at higher
transmit power levels (e.g., when the access terminal is located
further away) or to more frequently transmitted messages.
Inventors: |
Pankaj; Rajesh; (San Diego,
CA) ; Lundby; Stein; (Solana Beach, CA) ;
Yafuso; Byron; (San Diego, CA) |
Correspondence
Address: |
QUALCOMM INCORPORATED
5775 MOREHOUSE DR.
SAN DIEGO
CA
92121
US
|
Family ID: |
25121387 |
Appl. No.: |
11/544234 |
Filed: |
October 5, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09781012 |
Feb 10, 2001 |
7126930 |
|
|
11544234 |
Oct 5, 2006 |
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Current U.S.
Class: |
370/320 ;
375/E1.024 |
Current CPC
Class: |
H04B 2201/709709
20130101; H04W 52/54 20130101; H04B 1/7103 20130101 |
Class at
Publication: |
370/320 |
International
Class: |
H04B 7/216 20060101
H04B007/216 |
Claims
1. In a wireless communication system, a method for transmitting a
control message from a first entity to a second entity, comprising:
at the first entity, measuring at least one characteristic of a
channel through which a signal is received from the second entity
to produce channel state information; forming the control message
indicative of the channel state information; and transmitting the
control message from the first entity to the second entity at a
particular power level determined based at least in part on the
control message.
2. The method of claim 1, wherein the control message comprises a
particular codeword selected from among a plurality of possible
codewords.
3. The method of claim 2, wherein the power level is determined
based on a minimum distance of the selected codeword.
4. The method of claim 2, wherein the power level is determined
based on an expected frequency of the selected codeword being
transmitted.
5. The method of claim 2, wherein the power level is determined
based on a particular number of times the selected codeword is
repeated for a transmission.
6. The method of claim 1, wherein the control message is a data
rate control message indicative of a rate for a data transmission
requested from the second entity.
7. The method of claim 1, wherein the at least one characteristic
comprises a carrier-to-noise-plus interference ratio (C/I).
8. The method of claim 1, wherein the control message is selected
from among a plurality of data rate control messages.
9. The method of claim 2, wherein the selected codeword has a
minimum distance based on quality of the channel.
10. The method of claim 2, wherein the selected codeword has a
minimum distance based on frequency in which the control message is
transmitted.
11. In a wireless communication system, a method for transmitting a
message from a first entity to a second entity, comprising:
identifying a codeword associated with the message, wherein the
identified codeword is one of a plurality of codewords defined for
an alphabet, and wherein at least two codewords in the alphabet
have unequal distances to their nearest codewords; and transmitting
the identified codeword from the first entity to the second
entity.
12. The method of claim 11, further comprising: determining a
transmit power level for the identified codeword, and wherein the
identified codeword is transmitted at the determined transmit power
level.
13. The method of claim 12, wherein the transmit power level for
the identified codeword is based at least in part on the distance
of the identified codeword to its nearest codeword.
14. The method of claim 12, wherein the transmit power level for
the identified codeword is determined to achieve a particular level
of performance.
15. The method of claim 14, wherein the particular level of
performance is approximately one percent frame error rate or
better.
16. The method of claim 11, wherein the message to be transmitted
is one of a plurality of possible messages, and wherein the
plurality of codewords in the alphabet are assigned to the
plurality of possible messages in accordance with a particular
assignment scheme.
17. The method of claim 16, wherein the plurality of codewords in
the alphabet are assigned to the plurality of possible messages
such that messages more likely to be transmitted at higher transmit
power levels are assigned with codewords having larger distances to
their nearest codewords.
18. The method of claim 16, wherein the plurality of codewords in
the alphabet are assigned to the plurality of possible messages
such that messages more likely to be transmitted are assigned with
codewords having larger distances to their nearest codewords.
19. The method of claim 11, wherein the alphabet includes N
codewords having minimum distances of d.sub.1 through d.sub.N, and
wherein the minimum distances conform to the following:
d.sub.1.gtoreq.d.sub.2.gtoreq. . . .
.gtoreq.d.sub.N-1.gtoreq.d.sub.N, and d.sub.1>d.sub.N.
20. The method of claim 11, wherein the message identifies a
particular data rate for a data transmission requested by the first
entity from the second entity.
21. The method of claim 11, wherein the first entity is an access
terminal in the wireless communication system.
22. The method of claim 11, wherein the wireless communication
system is a CDMA system.
23. In a wireless communication system, a method for transmitting a
message from a first entity to a second entity, comprising:
identifying a codeword associated with the message, wherein the
identified codeword is one of a plurality of codewords defined for
an alphabet, and wherein at least two codewords in the alphabet may
be transmitted with different amounts of energy for a particular
level of performance under similar link condition; determining a
transmit power level for the identified codeword; and transmitting
the identified codeword at the determined transmit power level.
24. The method of claim 23, wherein at least two codewords in the
alphabet have unequal distances to their nearest codewords.
25. The method of claim 23, wherein the plurality of codewords in
the alphabet are associate with a plurality of points in a signal
constellation, and wherein at least two points in the signal
constellation have unequal distances to their nearest
codewords.
26. The method of claim 25, wherein the plurality of points in the
signal constellation are selected from points in signal
constellations for quadrature phase shift keying (QPSK), M-ary
phase shift keying (M-PSK), M-ary quadrature amplitude modulation
(M-QAM), or a combination thereof.
27. The method of claim 23, wherein at least two codewords in the
alphabet have unequal lengths.
28. The method of claim 27, further comprising: encoding the
identified codeword in accordance with a particular coding
scheme.
29. The method of claim 23, wherein the message to be transmitted
is one of a plurality of possible messages, and wherein the
plurality of codewords in the alphabet are assigned to the
plurality of possible messages such that messages more likely to be
transmitted at higher transmit power level are assigned with
codewords requiring lower transmit power to achieve the particular
level of performance.
30. The method of claim 23, wherein the message to be transmitted
is one of a plurality of possible messages, and wherein the
plurality of codewords in the alphabet are assigned to the
plurality of possible messages such that messages more likely to be
transmitted are assigned with codewords requiring less transmit
power to achieve the particular level of performance.
31. An access terminal in a wireless communication system,
comprising: a receiver for receiving a signal from an access
network and determining at least one characteristic of a forward
link channel through which the signal is received; a data processor
configured to form a control message indicative of a state of the
forward link channel; and a transmitter unit configured to transmit
the control message at a particular transmit power determined based
at least in part on the control message.
32. An access terminal in a wireless communication system,
comprising: a data processor configured to receive and process a
codeword for a message, wherein the codeword is one of a plurality
of codewords defined for an alphabet, and wherein at least two
codewords in the alphabet may be transmitted with different amounts
of energy for a particular level of performance under similar link
condition; and a transmitter unit operatively coupled to the data
processor and configured to transmit the processed codeword.
33. The access point of claim 32, further comprising: a controller
operatively coupled to the data processor and configured to provide
a signal indicative of transmit power level to be used for the
processed codeword.
34. The access point of claim 32, further comprising: a signal
quality measurement unit configured to receive samples for a
received signal and to determine a received signal quality of
signals transmitted from one or more transmitting sources, and
wherein the processed codeword is transmitted at a power level
based in part on the received signal quality of a transmitting
source to which the processed codeword is transmitted.
35. A communication unit in a wireless communication system,
comprising: a receiver configured to receive a signal from a
transmitting source and determine at least one characteristic of a
communication link through which the signal is received; a data
processor configured to form a message indicative of a state of the
communication link; and a transmitter unit configured to transmit
the message at a particular transmit power determined based at
least in part on the message.
36. An access point in a CDMA system comprising the communication
unit of claim 35.
37. An apparatus in a wireless communication system, comprising:
means for receiving a signal from a transmitting source and
determining at least one characteristic of a communication link
through which the signal is received; means for forming a control
message indicative of a state of the communication link; and means
for transmitting the control message at a particular transmit power
determined based at least in part on the control message.
38. The apparatus of claim 37, wherein the control message
comprises a codeword selected from among a plurality of codewords
defined for an alphabet, and wherein at least two codewords in the
alphabet may be transmitted with different transmit power for a
particular level of performance under similar link condition.
Description
CLAIM OF PRIORITY UNDER 35 U.S.C. .sctn.120
[0001] The present Application for Patent is a Continuation and
claims priority to patent application Ser. No. 09/781,012, entitled
"Method And Apparatus For Transmitting Messages In A Wireless
Communication System," filed Feb. 10, 2001, now allowed, and
assigned to the assignee hereof and hereby expressly incorporated
by reference herein.
BACKGROUND
[0002] 1. Field
[0003] The present invention relates to data communication. More
particularly, the present invention relates to a novel and improved
method and apparatus for transmitting messages in a wireless
communication system.
[0004] 2. Description of the Related Art
[0005] Wireless communication systems are widely deployed to
provide various types of communication such as voice, data, and so
on. These systems may be based on code division multiple access
(CDMA), time division multiple access (TDMA), or some other
modulation techniques. A CDMA system provides certain advantages
over other types of system, including increased system
capacity.
[0006] A CDMA system may be designed to support one or more CDMA
standards such as (1) the "TIA/EIA-95-B Mobile Station-Base Station
Compatibility Standard for Dual-Mode Wideband Spread Spectrum
Cellular System" (the IS-95 standard), (2) the "TIA/EIA-98-C
Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum
Cellular Mobile Station" (the IS-98 standard), (3) the standard
offered by a consortium named "3rd Generation Partnership Project"
(3GPP) and embodied in a set of documents including Document Nos.
3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the
W-CDMA standard), (4) the standard offered by a consortium named
"3rd Generation Partnership Project 2"(3GPP2) and embodied in a set
of documents including "TR-45.5 Physical Layer Standard for
cdma2000 Spread Spectrum Systems," the "C.S0005-A Upper Layer
(Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,"
and the "C.S0024 cdma2000 High Rate Packet Data Air standards are
incorporated herein by reference. A system that implements the High
Rate Packet Data specification of the cdma2000 standard is referred
to herein as a high data rate (HDR) system. Proposed wireless
systems also provide a combination of HDR and low data rate
services (such as voice and fax services) using a single air
interface.
[0007] In a wireless communication system, the transmit power
required for a transmission is dependent on the propagation (or
path) loss between a transmitting entity (e.g., an access terminal)
and a receiving entity (e.g., an access point). As an access
terminal moves further away from the access point, the path loss
typically increases. Consequently, more transmit power is required
so that the transmission can be received at the required signal
quality for the desired level of performance (e.g., one percent
frame error rate). However, the higher transmit power for this
transmission causes more interference to the transmissions from
other access terminals. The higher transmit power also causes
faster depletion of battery power on mobile wireless devices.
[0008] There, is therefore, a need in the art for a way to provide
HDR services that minimizes interference and depletion of battery
power.
SUMMARY
[0009] The disclosed embodiments provide techniques to reduce the
amount of transmit power required for transmission of selected
messages from an access terminal. In a first aspect, the reduction
in transmit power is based on the expected path loss associated
with the reverse link, thus tending to extend the operating range
of an HDR access terminal, and at the same time decreasing reverse
link interference in adjacent cells. In another aspect, the
reduction in transmit power is based on the relative frequency with
which an HDR access terminal is expected to send each type of
message, thus tending to minimize reverse link interference in a
serving cell. Both of these aspects also have the benefit of
tending to extend battery life of a mobile wireless device such as
a mobile HDR access terminal. The techniques described herein can
also be applied to forward link transmissions from an access point.
Various other aspects of the invention are also presented.
[0010] The invention provides methods and system elements that
implement various aspects, embodiments, and features of the
invention, as described in further detail below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The features, nature, and advantages of the present
invention will become more apparent from the detailed description
set forth below when taken in conjunction with the drawings in
which like reference characters identify correspondingly throughout
and wherein:
[0012] FIG. 1 is a diagram of a wireless communication system that
supports a number of users, and which can implement various aspects
of the invention;
[0013] FIG. 2 is a diagram of a packet transmission scheme used in
the HDR system;
[0014] FIG. 3 is a block diagram of a reverse link architecture
employed in the HDR system and capable of transmitting data rate
control (DRC) messages and other information;
[0015] FIGS. 4A and 4B are diagrams graphically illustrating an
alphabet of codewords having equal distance and unequal distances,
respectively, to the nearest codewords;
[0016] FIGS. 5A and 5B are diagrams of two signal constellations
having points selected from different modulation formats;
[0017] FIG. 6A is a block diagram of an embodiment of an access
terminal, in accordance with various aspects of the invention;
and
[0018] FIG. 6B is a block diagram of an embodiment of a portion of
a transmit (TX) data processor, which may be used to process DRC
messages for various schemes described herein.
DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0019] FIG. 1 is a diagram of a wireless communication system 100
that supports a number of users, and which can implement various
aspects of the invention. System 100 may be designed to support one
or more CDMA standards and/or designs (e.g., the cdma2000 standard,
the HDR specification). For simplicity, system 100 is shown to
include three access points 104 (which may also be referred to as
base stations) in communication with two access terminals 106
(which may also be referred to as remote terminals or mobile
stations). The access point and its coverage area are often
collectively referred to as a "cell".
[0020] Depending on the CDMA system being implemented, each access
terminal 106 may communicate with one (or possibly more) access
points 104 on the forward link at any given moment, and may
communicate with one or more access points on the reverse link
depending on whether or not the access terminal is in soft handoff.
The forward link (i.e., downlink) refers to transmission from the
access point to the access terminal, and the reverse link (i.e.,
uplink) refers to transmission from the access terminal to the
access point.
[0021] In a CDMA system, the cells may be operated on the same
frequency band (i.e., with a frequency reuse of one, or K=1) to
achieve better utilization of the available system resources. In
this case, the transmission from each transmitting entity (e.g.,
access terminal) acts as interference to the transmissions from
other transmitting entities. To minimize interference and increase
system capacity on the reverse link, the transmit power of each
transmitting access terminal is controlled such that a desired
level of performance (e.g., one percent frame error rate, or 1%
FER) is achieved while minimizing the amount of interference to
other transmitting access terminals. This transmit power adjustment
is achieved by a power control loop maintained for each
transmitting access terminal. The power control loop adjusts the
transmit power level of the access terminal such that a
transmission is received by the access point at a target signal
quality (i.e., a particular signal-to-noise-plus-interference, C/I)
needed for the desired level of performance.
[0022] In the example shown in FIG. 1, access terminal 106a is
located near access point 104a, and access terminal 106b is located
near the cell boundaries of access points 104a, 104b, and 104c. For
this example, both access terminals are using the same coding and
modulation to transmit. Since access terminal 106a is located
(relatively) close to access point 104a, its transmit power may be
adjusted to a (relatively) low level to achieve the desired level
of performance at access point 104a. This low transmit power is
possible since the path loss is approximately proportional to the
4th law of the distance between the transmitting and receiving
entities (i.e., path loss .varies. (distance).sup.4). Because of
the low transmit power level and further because of the greater
distances between access terminal 106a and access points 104b and
104c, the transmission from access terminal 106a causes little
interference to other transmissions received at access points 104b
and 104c.
[0023] In contrast, access terminal 106b is located further away
from access points 104a, 104b, and 104c. Due to the longer
distances to these access points, the transmit power of access
terminal 106b is likely to be adjusted to a high level to achieve
the same level of performance. Because of the high transmit power
level and the shorter distances between access terminal 106b and
access points 104b and 104c, the transmission from access terminal
106b is likely to cause more interference to other transmissions
received at access points 104b and 104c.
[0024] As seen by the above example, when an access terminal is
located near an access point, less transmit power is required for a
transmission, and the transmission causes little interference at
other access points. In contrast, when an access terminal is
located further away from an access point, more transmit power is
required for a transmission, and the transmission causes more
interference at other access points.
[0025] Various techniques are provided herein to reduce the amount
of transmit power required for transmission of messages from an
access terminal, which then results in less interference to the
transmissions from other access terminals. Some of these techniques
are briefly described below.
[0026] In one aspect, messages to be transmitted from an access
terminal are defined and/or coded such that they may be received by
an access point at different received signal qualities. In one
implementation, an alphabet of codewords is defined whereby at
least some of the codewords have different "distances" to their
nearest codewords (i.e., different minimum distances, d.sub.min).
As used herein, an "alphabet" is a collection of individual
codewords, each of which (1) is represented by a specific value or
sequence of bits, (2) may be associated with a particular meaning
by a system (e.g., a particular data rate), and (3) is selectable
for transmission as all or a part of a message. For digital codes,
minimum distance, d.sub.min, relates to the minimum number of bit
errors in a received codeword necessary to cause an equal or
greater correlation with an incorrect codeword. Minimum distance,
d.sub.min, may also refer to the distance between points in a
(typically multi-dimensional) signal constellation. A codeword with
a larger minimum distance may be correctly detected at a lower
signal quality, and a codeword with a smaller minimum distance
typically requires higher signal quality for proper reception.
[0027] In certain embodiments, codewords with larger minimum
distances may be advantageously assigned to messages more likely to
be transmitted by access terminals located further away from the
access point, which would normally need to transmit their messages
at higher transmit power levels due to greater path loss. This
alphabet and codeword assignment scheme allow access terminals to
transmit their messages using less power when located further away
from the access point, which then reduces the amount of
interference to transmissions from other access terminals in
adjacent cells and may further extend the range of the access
terminal.
[0028] In some other embodiments, codewords with larger minimum
distances may be advantageously assigned to more frequently
transmitted messages. Since these messages may be received at a
lower signal quality, they cause less interference to messages from
other transmitting access terminals. The reduced interference may
increase the capacity of the reverse link.
[0029] In another aspect, messages to be transmitted are assigned
to different points in a signal constellation. In such an aspect,
d.sub.min refers to the distance between a point on the signal
constellation and the nearest other point in the same signal
constellation. The points in the signal constellation may be viewed
as codewords in an alphabet, and may be selected from various
modulation formats such as quadrature phase shift keying (QPSK),
M-ary phase shift keying (e.g., 8-PSK), quadrature amplitude
modulation (e.g., 16-QAM, 64-QAM), and others. A custom signal
constellation may also be generated having points at various
defined locations. The location of the points in the signal
constellation may be defined such that the points may be received
at different signal qualities (i.e., the points have different
distances to their nearest neighbor points). In certain
embodiments, messages expected to be transmitted at higher transmit
power level (e.g., from an access terminal located further away
from an access point and having greater path loss) are assigned to
points that may be received at lower signal qualities, and thus may
be transmitted at lower transmit power. And in some other
embodiments, more frequently transmitted messages are assigned to
points that may be received at lower signal qualities, which may
result in less interference and increased link capacity.
[0030] In yet another aspect, the transmit power used to transmit a
message is adjusted by varying the length of the codeword used, and
hence the length of the transmit duration. For example, messages to
be transmitted from an access terminal are defined with different
lengths. In one implementation, an alphabet of codewords is defined
whereby at least some of the codewords have different lengths. For
a given link condition, a shorter length codeword may be
transmitted at the same transmit power level but over a shorter
time interval relative to a longer length codeword, or at a lower
transmit power level over the same time interval. Shorter length
codewords may be assigned to messages more likely to be transmitted
by an access terminal located further away from an access point,
which would tend to reduce the amount of interference in the
system. Alternatively or additionally, shorter length codewords may
be assigned to more commonly transmitted messages, which would also
tend to extend the battery life of the access terminal.
[0031] The message transmission schemes described herein may be
used for any set of defined messages to be transmitted on any
channel on the forward or reverse link.
[0032] These message transmission schemes may also be used for
other wireless communication systems and for other CDMA systems
that may support one or more other CDMA standards and/or
designs.
[0033] For clarity, various aspects, embodiments, and features of
the invention are now described for a specific implementation in
conjunction with a data rate control (DRC) channel defined for the
reverse link in a high data rate (HDR) system. The disclosed
aspects and embodiments may be equally applied to other types of
system, such as a hybrid system that supports high rate packet data
services and voice services concurrently or other types of systems
mentioned above.
[0034] In the HDR system, each access point transmits packet data
to access terminals within its coverage, one at a time, in a
time-division multiplexed manner. An access point transmits packet
data to an access terminal at or near the peak transmit power
level, if at all. Whenever an access terminal desires a data
transmission, it sends a packet data request in the form of a DRC
message to a selected access point. The access terminal measures
the signal quality of the forward link signals (e.g., the pilot
references) received from a number of access points, determines the
access point having the best received signal quality, identifies
the highest data rate supported by the best received link, and
sends a DRC message indicative of the identified data rate. This
DRC message is transmitted on the DRC channel and directed to the
selected access point associated with the best received signal
quality. The selected access point receives the DRC message and
schedules a data transmission for the access terminal at the
identified data rate.
[0035] As shown in FIG. 1, access terminal 106a is located
(relatively) close to access point 104a and likely to experience
smaller path loss. To maintain the desired level of performance
while minimizing interference to other transmitting access
terminals, the transmission from each access terminal is power
controlled such that it is received at the target signal quality
needed for the desired level of performance. Because of the smaller
path loss, access terminal 106a is able to transmit the DRC message
for the identified data rate at a lower transmit power level and
still be received by the access point at the target signal quality.
In contrast, access terminal 106b is located further away from
access point 104a and likely to experience greater path loss.
Because of the greater path loss, access terminal 106b is required
to transmit the DRC message at a higher transmit power level to
achieve the target signal quality
[0036] FIG. 2 is a diagram of a packet transmission scheme used in
the HDR system. Initially, a request for a data transmission is
received from an access terminal. In response, one or more Physical
Layer packets are generated by an access point and transmitted to
the access terminal starting at time slot n. Each packet includes a
particular number of data bits (e.g., 1024 bits in the HDR system)
and may be transmitted as one or more "slots". The number of slots
for each packet is dependent on the data rate, and four slots are
included in the example packet shown in FIG. 2. For each slot, the
access terminal receives and processes (e.g., decovers,
demodulates, deinterleaves, and decodes) the slot, and further
determines whether the packet has been received correctly. The
access terminal is able to recover the transmitted packet based on
a partial transmission because the data modulation symbols
generated for the packet are repeated a number of times for lower
data rates and transmitted.
[0037] In an HDR system, each access terminal desiring a data
transmission continually measures the received quality of forward
link signals received from one or more access points. The access
terminal then directs DRC messages to the access point having the
best measured forward link signal quality. The DRC message
transmission continues until the requested data transmission is
successfully received by the access terminal. A portion of the
reverse link capacity is utilized for this continual transmission
of DRC messages by access terminals requesting data
transmissions.
[0038] In an exemplary embodiment, a DRC message identifies the
particular access point from which data is being requested, and
also indicates the data rate at which that data should be
transmitted, if at all. An access point receives DRC requests from
multiple access terminals during each time slot, but transmits to
only one access terminal per time slot. Because the access terminal
might not receive a forward link transmission in response to each
DRC message, the access terminal continuously sends DRC messages in
every time slot. If the access terminal fails to send a DRC message
in a reverse link time slot, it will generally not receive any
forward link data in the corresponding forward link time slot.
[0039] FIG. 3 is a block diagram of a reverse link architecture 300
employed in the HDR system and capable of transmitting DRC messages
and other information (e.g., pilot, reverse rate indicator (RRI),
acknowledgment (ACK), and packet data).
[0040] Examples of such signal structures are described in detail
in the aforementioned cdma2000 standard. For simplicity, only the
processing for the DRC message is described herein. The HDR system
supports a number of different data rates for data transmission on
the forward link. Each of the supported forward link data rates is
associated with a respective DRC value. In the cdma2000 standard,
each of 16 possible DRC values is represented by a 4-bit value. A
DRC processor 330 receives the DRC value for the identified data
rate, which represents a message to be transmitted, and provides a
code sequence for the message.
[0041] Within DRC processor 330, the DRC value is mapped to an
assigned 8-bit bi-orthogonal codeword (or DRC codeword) by a
bi-orthogonal encoder 332. The 8-bit DRC codeword is then repeated
twice in block 334 to generate 16 binary symbols to be transmitted
per active slot. The binary symbols are then mapped (e.g.,
"0".fwdarw.+1, and "1".fwdarw.-1) by a signal mapping element 336.
Each mapped binary symbol is further covered by a coverer 338 with
a particular 8-ary Walsh function, W.sub.i.sup.8, provided by a
Walsh cover generator 340. This Walsh function, W.sub.i.sup.8, is
the one assigned to the selected access point having the best link
to the access terminal.
[0042] The 16 binary symbols in the two repeated DRC codewords are
used to generate 128 Walsh chips by coverer 338. Each Walsh chip is
further covered by a coverer 342 with a 16-bit Walsh function,
W.sub.8.sup.16 (i.e., a sequence of "1111111100000000"). The 128
Walsh chips from coverer 338 for each active slot are thus covered
to generate 2048 chips. The sequence of 2048 chips for the DRC
message is then combined with other data within a combiner and
modulator 350, modulated, and transmitted over one time slot, which
is defined as 1.667 msec in the cdma2000 standard.
[0043] Table 1 lists the 16 DRC values and their corresponding DRC
codewords, as defined in the cdma2000 standard. As noted above, the
DRC values are representative of the forward link data rates, with
the mapping between the data rates and DRC values being defined in
the cdma2000 standard. TABLE-US-00001 TABLE 1 DRC Value DRC
Codeword 0 0000 0000 1 1111 1111 2 0101 0101 3 1010 1010 4 0011
0011 5 1100 1100 6 0110 0110 7 1001 1001 8 0000 1111 9 1111 0000 10
0101 1010 11 1010 0101 12 0011 1100 13 1100 0011 14 0110 1001 15
1001 0110
[0044] Table 2 lists the 8-ary Walsh functions, W.sub.i.sup.8, that
may be assigned to the access points. By covering the DRC codeword
for the identified data rate with the specific Walsh function,
W.sub.i.sup.8, assigned to the selected access point, the selected
and neighbor access points are able to easily determine whether or
not the DRC message has been sent to them. Only the access point
assigned with that Walsh function, W.sub.i.sup.8, processes the DRC
message for scheduling data to the access terminal. TABLE-US-00002
TABLE 2 Walsh Function Walsh Sequence W.sub.0.sup.8 0000 0000
W.sub.1.sup.8 0101 0101 W.sub.2.sup.8 0011 0011 W.sub.3.sup.8 0110
0110 W.sub.4.sup.8 0000 1111 W.sub.5.sup.8 0101 1010 W.sub.6.sup.8
0011 1100 W.sub.7.sup.8 0110 1001
[0045] Referring back to Table 1, DRC codewords are selected such
that each codeword (e.g., "0000 0000") differs from its compliment
(e.g., "1111 1111") by eight bit positions, and further differs
from all other codewords by four bit positions. For this "alphabet"
DRC codewords, the minimum distance, d.sub.min, between the
codewords is equal to four. For a transmitted DRC codeword, an
access point is able to correctly detect the codeword if fewer than
d.sub.min/2 bits in the codeword are received in error. Otherwise ,
if d.sub.min/2 or more bits are received in error, the codeword may
be erroneously detected.
[0046] In accordance with an aspect of the invention, an alphabet
of codewords is defined such that at least some of the codewords
have a variety of different minimum distances. For this alphabet,
the minimum distances for some codewords are smaller than average
while the minimum distances for some other codewords are larger
than average. A codeword with a smaller minimum distance must be
received at a higher power level to achieve a higher C/I needed for
the desired level of performance (e.g., 1% FER). Correspondingly, a
codeword with a larger minimum distance may be received at a lower
power level since a lower C/I is required for the same level of
performance.
[0047] In certain embodiments, codewords with larger minimum
distances are assigned to messages more likely to be sent by access
terminals which would have required higher transmit power levels
(e.g., by access terminals located further away from the access
point and experiencing greater path loss). In other embodiments,
codewords with smaller minimum distances are assigned to more
frequently transmitted messages.
[0048] FIG. 4A is a diagram graphically illustrating an alphabet of
codewords having equal minimum distance to the nearest codewords.
In this example, the codewords are represented as points 412
equally spaced on a circle 410 in a 2-D plane. Because of the equal
spacing, the distance between any pair of adjacent codewords is
d.sub.A. The distance from the center of circle 410 and any
particular point 412 can be representative of the transmit power
(P.sub.S) for the point, and the distance from this point outward
(i.e., toward the edge of a circle 414) can be representative of
noise (P.sub.N). In this example, any codeword may be correctly
received if the noise is less than d.sub.A/2 (i.e.,
P.sub.N<d.sub.A/2). If the noise is greater than or equal to
d.sub.A/2, the codeword may be erroneously detected as another
codeword (i.e., an adjacent codeword). Because of the equal
codeword spacing, the codewords in this alphabet are equally
susceptible to noise. Thus, the same received signal quality (C/I)
is required for each codeword for a particular desired level of
performance.
[0049] FIG. 4B is a diagram graphically illustrating an alphabet of
codewords having unequal distances to the nearest codewords. In
this example, the codewords are represented as points 422 unequally
spaced on a circle 420 in the 2-D plane. The eight codewords are
spaced such that the distance between each pair of adjacent
codewords ranges from d.sub.B1 to d.sub.B4, where
d.sub.B1<d.sub.B2<d.sub.B3<d.sub.B4. Codeword A has the
smallest distance, d.sub.B1, to the nearest codewords B and H, and
is more susceptible to noise. This codeword may be correctly
received if the noise is less than d.sub.B1/2 (i.e.,
P.sub.NA<d.sub.B1/2). Consequently, a higher received signal
quality (C/I) is needed for the desired level of performance.
[0050] In contrast, codeword E has the largest distance, d.sub.B4,
to the nearest codewords D and F, and is less susceptible to noise.
This codeword may be correctly received if the noise is less than
d.sub.B4/2 (i.e., P.sub.NE<d.sub.B4/2). Thus, a lower received
signal quality is needed for the same level of performance, which
allows this codeword to be transmitted at a lower transmit power
level.
[0051] The examples of FIGS. 4A and 4B were chosen because they are
easy to graphically illustrate on a flat sheet of paper. A person
skilled in the art will appreciate that the same principles also
apply to coding over any other single or multi-dimensional spaces
where a distance metric can be defined.
[0052] Referring back to FIG. 1, access terminal 106a is located
(relatively) close to access point 104a. Because of the smaller
path loss, access terminal 106a is likely to request transmission
at a high data rate (e.g., 614.4 kbps or higher) from this access
point. In contrast, access terminal 106b is located further away
from access point 104a. Because of the greater path loss, access
terminal 106a is likely to request transmission from this access
point at a lower data rate (e.g., 76.8 kbps or lower).
[0053] If, as is the case in the cdma2000 standard, the minimum
distance between the DRC codewords is relatively uniform, then all
codewords must be transmitted by the access terminals such that
they are received by the access point at the target signal quality.
This is achieved by controlling the transmit power such that
codewords from access terminals with greater path loss are
transmitted at higher transmit power levels, and codewords from
access terminals with smaller path loss are transmitted at lower
transmit power levels. For the example shown in FIG. 1, if both
access terminals 106a and 106b concurrently request data
transmission from access point 104a, access terminal 106b would
transmit its DRC message at a higher transmit power level than
would access terminal 106a to achieve the target received signal
quality at access point 104a.
[0054] The path loss versus distance is approximately equal for the
forward and reverse links. Consequently, a DRC message for a
progressively lower data rate is (disadvantageously but
necessarily) transmitted at a progressively higher transmit power
level. This could cause more interference to reverse link signals
of cells adjacent to access point 104a. The higher transmit power
for a longer time period may further shorten the access terminal's
operating life if it is a mobile unit operating on battery
power.
[0055] Table 3 lists an alphabet whereby at least some of the
codewords have unequal minimum distances, and which may be used for
the DRC messages. In this example, the alphabet includes 16
codewords {A, B, . . . P} assigned to the 16 DRC values {0, 1, . .
. 15}. These 16 codewords may be used for up to 16 data rates
{R.sub.0, R.sub.1, . . . R.sub.15}. Each codeword in the alphabet
has a particular distance d.sub.x to the nearest codeword (i.e., a
particular minimum distance), which is listed in columns 4 and 8 of
Table 3. TABLE-US-00003 TABLE 4 DRC Data Minimum DRC Code- Data
Minimum Value Rate Codeword Distance Value word Rate Distance 0
R.sub.0 A d.sub.0 8 I R.sub.8 d.sub.8 1 R.sub.1 B d.sub.1 9 J
R.sub.9 d.sub.9 2 R.sub.2 C d.sub.2 10 K R.sub.10 d.sub.10 3
R.sub.3 D d.sub.3 11 L R.sub.11 d.sub.11 4 R.sub.4 E d.sub.4 12 M
R.sub.12 d.sub.12 5 R.sub.5 F d.sub.5 13 N R.sub.13 d.sub.13 6
R.sub.6 G d.sub.6 14 O R.sub.14 d.sub.14 7 R.sub.7 H d.sub.7 15 P
R.sub.15 d.sub.15
[0056] In an embodiment, the codewords for the alphabet are defined
such that the minimum distances for the codewords maintain the
following relationships:
d.sub.0.gtoreq.d.sub.i.gtoreq.d.sub.2.gtoreq.. . .
.gtoreq.d.sub.l3.gtoreq.d.sub.l424 d.sub.15, and
d.sub.0>d.sub.15. As shown by the above relationships, at least
some (and not necessarily all) of the codewords in the alphabet
have different minimum distances.
[0057] In certain embodiments, the codewords in the alphabet are
assigned such that messages more likely to be transmitted at higher
transmit power levels are assigned to codewords having larger
minimum distances. As noted above, for the DRC messages,
progressively higher transmit power levels are typically needed for
progressively lower data rates. Thus, in an embodiment, the
codewords are assigned to the data rates such that codewords with
progressively larger minimum distances are assigned to
progressively lower data rates. For the codeword assignment shown
in Table 3, the data rates may be defined to maintain the following
relationship: R.sub.0.ltoreq.R.sub.1.ltoreq.R.sub.2.ltoreq.. . .
.ltoreq.R.sub.13.ltoreq.R.sub.14.ltoreq.R.sub.15.
[0058] Based on the above alphabet and codeword assignment, an
access terminal located further away from an access point is likely
to request data transmission at a lower data rate, which would be
assigned with a codeword having a larger minimum distance. This
codeword may then be transmitted at a lower relative transmit power
level than would otherwise be required for a codeword with an
average minimum distance.
[0059] The above embodiment can be extended to any type of
transmission on the forward link where different codewords
correspond to transmissions requiring different C/I. Codeword
assignment based on data rates is applicable for the HDR system
because, to be received with equal quality, low data rates require
lower C/I than high data rates. Thus, the HDR system assigns lower
data rates to users located far from the access point. The codeword
assignment can be based on the required C/I in some other manner.
For example, a particular system may assign all users the same data
rate, but different spreading codes. If the spreading codes are not
the same, the users close to the access point can be assigned (not
quite as good) spreading codes that are more susceptible to be
interfered than the ones assigned to users located far away. The
same concept can be applied to an FDMA system, where some frequency
bands (e.g., unlicensed frequency bands) have more interference
than others.
[0060] In some other embodiments, codewords in the alphabet are
assigned such that messages more frequently transmitted are
assigned to codewords having larger minimum distances. This allows
commonly transmitted messages to be transmitted at lower power
levels, which may reduce interference and increase link
capacity.
[0061] In the above-described HDR system, the 8-bit DRC code word
is repeated and covered twice to generate 2048 chips for each
active time slot. For an alphabet having codewords with different
minimum distances, the codewords can be defined to have lengths of
8, 16, 32, 64, and so on, up to 2048 bits. Longer codeword length
generally provides more flexibility in selecting a set of codewords
having varying minimum distances. Codewords of any length may be
used and are within the scope of the invention. Table 5 shows an
example of a simple alphabet with four codewords having different
distances to the nearest codewords. In this example alphabet,
codeword A has distances of 4, 3, and 3 to codewords B, C, and D,
respectively. Codeword B has distances of 4, 1, and 1 to codewords
A, C, and D, respectively. Because of the larger distance to other
codewords in the alphabet, codeword A may be correctly received at
a lower C/I. This allows codeword A to be transmitted at a lower
transmit power level. Codeword A may thus be advantageously
assigned to the lowest supported data rate (e.g., 38.4 kbps). The
remaining codewords may be assigned to the other supported data
rates in a similar manner based on their minimum distances.
TABLE-US-00004 TABLE 5 DRC Value Data Rate Codeword Sequence 0 38.4
kbps A 0000 1 76.8 kbps B 1111 2 153.6 kbps C 1110 3 307.2 kbps D
0111
[0062] In accordance with another aspect of the invention, messages
to be transmitted are assigned to different points in a signal
constellation. The signal constellation may include points from
various modulation formats such as, for example, QPSK, 8-PSK,
16-QAM, 32-QAM, 64-QAM, and others. The location of the points in
the signal constellation and the assignment of the points to the
messages may be dependent on various factors such as, for example,
the expected transmit power level for the messages, the frequency
of the messages, and so on.
[0063] FIG. 5A is a diagram of a signal constellation having seven
points selected from two different modulation formats. In this
diagram, each point in the signal constellation is associated with
a respective message that may be transmitted. In quadrants 1, 2,
and 3, QPSK is employed and three different messages are assigned
to points 512a, 512b, and 512c. And in quadrant 4, 16-QAM is
employed and four different messages are assigned to points 514a,
514b, 514c, and 514d.
[0064] As seen in FIG. 5A, the points are closer to one another as
the modulation order increases from QPSK to 16-QAM. The larger
distance between points 512a, 512b, and 512c for QSPK results in
these points being more immune to erroneous detection due to noise.
Note that in the example shown in FIG. 5A, the minimum distance for
point 512b is greater than the minimum distance for points 512a and
512c. The points in the constellation need not be arranged in
rectangular fashion as shown, but may be arranged in any way that
produces the desired relative transmit levels. For example, a
double-log scale (i.e., log in the x and y coordinates) may be used
to define the points in the constellation to produce approximately
even reduction in the minimum distance.
[0065] Some QPSK points may be transmitted at a lower transmit
power level than others. These QPSK points may be assigned to
messages likely to be transmitted at higher transmit power level
(e.g., from an access terminal located further away from an access
point). Alternatively, the QPSK points may be assigned to more
frequently transmitted messages, which would result in less
interference at the access point since these messages may be
transmitted with less power due to the larger minimum distance.
Conversely, the smaller distance between points 514a, 514b, 514c,
and 514d for 16-QAM results in these points being more susceptible
to erroneous detection due to noise (relative to QPSK). As a
result, these 16-QAM points may be transmitted at a higher transmit
power level than for the QPSK points.
[0066] FIG. 5B is a diagram of a signal constellation having 23
points selected from four different modulation formats. As seen in
FIG. 5B, the points are closer to one another as the modulation
order increases from QPSK to 8-PSK, to 16-QAM, and to 64-QAM.
Again, points with larger distances to neighbor points may be
transmitted at lower transmit power level, and may be assigned to
messages more likely to be transmitted at higher transmit power
(e.g., messages likely to be sent by remote terminals located
further away from the access point). Conversely, points with
smaller distance to neighbor points are transmitted at higher
transmit power level, and may be assigned to messages more likely
to be transmitted at lower transmit power (e.g., messages likely to
be sent by remote terminals located closer to the access
point).
[0067] Other signal constellations may also be defined for any set
of messages. The points in the signal constellation may be defined
such that the distance between any particular point and its nearest
neighbor point is based on the transmit power level expected to be
used for that message. Messages expected to be transmitted at
higher transmit power level are associated with points having
larger distances to the nearest neighbor.
[0068] In accordance with yet another aspect of the invention,
messages to be sent from an access terminal are associated with
codewords having varying lengths. For a particular link condition,
the shorter length codewords may be transmitted at the same
transmit power level but over shorter time intervals relative to
the longer length codewords. The shorter length codewords may also
be transmitted at the same transmit power level, but could be
repeated and then punctured similar to that performed for the
reverse link in the IS-95 system. Alternatively, these shorter
length codewords may be transmitted over the same time interval as
that of longer length codewords, but at reduced transmit power
levels. Shorter length codewords may be assigned to more commonly
transmitted messages, which would tend to reduce the amount of
interference in the system. Alternatively or additionally, shorter
length codewords may be assigned to messages more likely to be
transmitted at higher transmit power by access terminals located
further away from the access point, which would also tend to reduce
the amount of interference. The codewords may be encoded prior to
transmission.
[0069] The codewords may be defined based on the probability of
occurrence of the associated messages. A message with a higher
probability of occurrence may be associated with a shorter length
codeword, and a message with a lower probability of occurrence may
be associated with a longer length codeword. The generation of
these codewords may be achieved in a manner similar to that used to
generate a Huffman code, which is known in the art and not
described herein.
[0070] Referring back to FIG. 1, the areas further away from the
access points comprise a longer portion of the system's coverage
area than the areas near the access points. If the access terminals
are equally likely to be located anywhere throughout the coverage
area of the system even (and even if this is not true), more of the
terminals are likely to be located further from the cell. These
access terminals are also likely to request data transmissions at
lower data rates.
[0071] Table 6 example of an alphabet of codewords having varying
lengths and assigned to the DRC values. In this example, the DRC
values 0 through 15 are assumed to be decreasingly likely to be
sent. Thus, the most likely DRC value of 0 is assigned with a
codeword having the shortest length of 2, the next most likely DRC
value of 1 is assigned with a codeword having the next shortest
length of 3, and so on, and the least likely DRC value of 15 is
assigned with a codeword having the longest length of 7.
TABLE-US-00005 TABLE 6 DRC Values Codeword 0 00 1 010 2 0110 3 0111
4 1000 5 1001 6 10100 7 10101 8 10110 9 10111 10 110000 11 110001
12 110010 13 110011 14 1101000 15 1101001
[0072] In one embodiment, the shorter length codewords are
transmitted within a shorter time period corresponding to their
lengths. In another embodiment, the shorter length codewords are
transmitted within the same time interval as the longer length
codewords (e.g., over an entire time slot), but at reduced transmit
power levels. In this case, a codeword may be repeated as many
times as necessary to fill the available number of chips in the
time slot. The longer transmission period allows the shorter length
codeword to be transmitted at a lower power level.
[0073] Various processing, coding, and/or transmission schemes may
be used in conjunction with the variable-length codewords. These
schemes may be employed to increase the likelihood of correctly
detecting the codewords or to achieve a particular level of
performance.
[0074] In one scheme, a variable-length codeword is encoded prior
to transmission. The encoding may be achieved based on a
convolutional code or some other code known in the art. For a given
number of coded bits, a shorter length codeword may be encoded with
a stronger code than for a longer length codeword. The stronger
code allows the encoded codeword to be correctly received at a
lower received signal quality, which may allow the codeword to be
transmitted at a lower transmit power level.
[0075] FIG. 6A is a block diagram of an embodiment of access
terminal 106, which is capable of implementing various aspects of
the invention. On the forward link, signals from the access points
are received by an antenna 612, routed through a duplexer 614, and
provided to an RF receiver unit 622. RF receiver unit 622
conditions (e.g., filters, amplifies, and downconverts) and
digitizes the received signal to provide samples. A demodulator 624
receives and processes (e.g., despreads, decovers, and pilot
demodulates) the samples to provide recovered symbols. Demodulator
624 may implement a rake receiver that processes multiple instances
of the received signal and generates combined recovered symbols. A
receive data processor 626 then decodes the recovered symbols,
checks the received frames, and provides the output data.
[0076] The samples from RF receiver unit 622 may also be provided
to an RX signal quality measurement unit 628 that measures the
quality of the received signals from the access points (e.g., based
on the received pilots). The signal quality measurement can be
achieved using various techniques, including those described in
U.S. Pat. Nos. 5,056,109 and 5,265,119.
[0077] Controller 630 receives the signal quality measurements for
the access points, determines the best received link based on the
signal quality measurements, determines the data rate supported by
the best received link, and determines the codeword associated with
the data rate. The codeword is then provided to a transmit data
processor 642 for processing and transmission back to the selected
access point.
[0078] On the reverse link, the message (i.e., codeword) is
processed by a transmit (TX) data processor 642, further processed
(e.g., spread, modulated) by a modulator (MOD) 644, and conditioned
(e.g., converted to analog signals, amplified, filtered, quadrature
modulated, and so on) by an RF TX unit 646 to generate a reverse
link signal. The reverse link signal is then routed through
duplexer 614 and transmitted via antenna 612 to the access
points.
[0079] FIG. 6B is a block diagram of an embodiment of a portion of
TX data processor 642, which may be used to process DRC messages
for various schemes described herein. Within a DRC process 660, a
DRC value for a DRC message (or DRC symbol) is mapped to an
assigned codeword by a codeword look-up element 662. The mapped
codeword may be one of a number of codewords with different minimum
distances or different lengths. Alternatively, the mapped codeword
may be representative of a particular point in a signal
constellation. Depending on the particular implementation, the
mapped codeword may be repeated and/or punctured by a
repetition/puncture element 664. For some implementations,
repetition/puncture element 664 is not used and may be omitted from
DRC processor 660.
[0080] The codeword is then mapped by a signal point mapping
element 666. For the scheme whereby DRC messages are mapped to
different points in the signal constellation, signal point mapping
element 666 maps the received codeword to the corresponding point.
For other schemes, the codeword may be mapped as described above
(e.g., bits in the codeword may be mapped such that "0".fwdarw.+1,
and "1".fwdarw.-1). The mapped codeword may then be scaled by a
gain element 667. As noted above, a codeword with a larger minimum
distance may be transmitted with less transmit power, and this
codeword would be scaled smaller by gain element 667. Conversely, a
codeword with a smaller minimum distance may be scaled larger by
gain. element 667. Thus, the codeword is scaled by a factor related
to the signal quality at which the codeword may be received.
[0081] The scaled codeword is then covered by a coverer 668 with a
particular 8-ary Walsh function, W.sub.i.sup.8, provided by a Walsh
cover generator 670. This Walsh function, W.sub.i.sup.8, is the one
assigned to the selected access point having the best link to the
access terminal. Each Walsh chip from coverer 668 is further
covered by a coverer 672 with a 16-bit Walsh function,
W.sub.8.sup.16 (i.e., a sequence of "1111111100000000") to generate
the required number of chips. The sequence of (e.g., 2048) chips
for the DRC message is then combined with other data within a
combiner, and the combined data is provided to the next processing
element (e.g., modulator 644). The processing of the message
transmission from the access terminal may be achieved using an
architecture similar to that shown in FIG. 6A. Depending on the
particular scheme used for the message, the detection of the
message may be performed within the demodulator (e.g., demodulator
624) or the receive data processor (e.g., processor 626). If the
messages are associated with different points on a signal
constellation, the demodulator can compare the received point
versus the possible points in the signal constellation and declare
the most likely transmitted message based on the comparison of the
received and possible points. And if the messages are associated
with different codewords (e.g., of different minimum distances or
different lengths), the receive data processor can process the
received codeword and declare the most likely transmitted message
based on the comparison of the received and possible codewords.
[0082] For clarity, various aspects, embodiments, and features of
the message transmission schemes of the invention have been
specifically described for the DRC messages in the HDR system. The
message transmission schemes described herein may be used for any
set of defined messages to be transmitted on any channel on the
forward or reverse link. The message transmission schemes of the
invention may also be used for other wireless communication systems
and for other CDMA systems that may support one or more other CDMA
standards and/or designs.
[0083] The foregoing description of the preferred embodiments is
provided to enable any person skilled in the art to make or use the
present invention. Various modifications to these embodiments will
be readily apparent to those skilled in the art, and the generic
principles defined herein may be applied to other embodiments
without the use of the inventive faculty. Thus, the present
invention is not intended to be limited to the embodiments shown
herein but is to be accorded the widest scope consistent with the
principles and novel features disclosed herein.
* * * * *