U.S. patent application number 09/725351 was filed with the patent office on 2001-04-12 for method and apparatus for detecting zero rate frames in a communications system.
Invention is credited to Chen, Tao, Patel, Shimman.
Application Number | 20010000221 09/725351 |
Document ID | / |
Family ID | 23532338 |
Filed Date | 2001-04-12 |
United States Patent
Application |
20010000221 |
Kind Code |
A1 |
Chen, Tao ; et al. |
April 12, 2001 |
Method and apparatus for detecting zero rate frames in a
communications system
Abstract
Techniques for detecting zero rate frames in a received data
transmission. A modulated signal is received and demodulated in
accordance with a particular demodulation format to generate
demodulated symbols. The demodulated symbols are partitioned into a
number of received frames. For each received frame, a quality
metric is computed and compared against a threshold value. The
threshold value is selected based, in part, on the quality metrics
of received frames. Based on the comparison result, the received
frame is indicated as being either transmitted and received in
error (i.e., erased or bad) or not transmitted at all (i.e., zero
rate or empty). The quality metric can relate to an energy of a
received frame, a distance between a received frame and a codeword
corresponding to the received frame, or other metrics. The
threshold value can be selected based on the quality metrics
computed for decoded frames or received frames identified as good,
and can be dynamically adjusted based on current information
available at the receiver. The method is advantageously used in a
CDMA communications system.
Inventors: |
Chen, Tao; (San Diego,
CA) ; Patel, Shimman; (San Diego, CA) |
Correspondence
Address: |
QUALCOMM Incorporated
5775 Morehouse Drive
San Diego
CA
92121-1714
US
|
Family ID: |
23532338 |
Appl. No.: |
09/725351 |
Filed: |
November 29, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09725351 |
Nov 29, 2000 |
|
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|
09388029 |
Sep 1, 1999 |
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Current U.S.
Class: |
375/340 |
Current CPC
Class: |
H04L 1/20 20130101; H04B
2201/70705 20130101; H04L 1/208 20130101; H04W 52/26 20130101; H03M
13/3738 20130101 |
Class at
Publication: |
375/340 |
International
Class: |
H03D 001/00 |
Claims
What is claimed is:
1. In a wireless communication system, a method comprising:
receiving a modulated signal; demodulating the modulated signal to
generate demodulated symbols; partitioning the demodulated symbols
into a plurality of received frames; computing a quality metric for
a fraction of at least one of the plurality of received frames;
performing a comparison of the quality metric for the fraction of a
particular received frame against a threshold value, wherein the
threshold value is selected based, in part, on the quality metrics
of received frames; and determining if the particular received
frame a zero rate frame based on the comparison.
2. The method of claim 1, wherein the quality metric relates to an
energy of a received frame.
3. The method of claim 2, wherein the quality metric relates to a
distance between the particular received frame and a codeword
corresponding to the at least one frame.
4. The method of claim 3, further comprising: decoding the
particular received frame to form a decoded frame, the decoded
frame comprising decoded data bits.
5. The method of claim 3, wherein computing the quality metric
further comprises: correlating the decoded data bits of the
particular received frame with a data portion of the modulated
signal.
6. A wireless communication device performing the method of claim
1.
7. A method for detecting zero rate frames in a received data
transmission, the method comprising: receiving a modulated signal;
demodulating the modulated signal to generate demodulated symbols;
partitioning the demodulated symbols into a plurality of received
frames; decoding the plurality of received frames to form decoded
frames, each of the decoded frames comprising decoded data bits;
computing a quality metric for at least one of the plurality of
received frames by correlating decoded data bits to encoded data;
performing a comparison of the quality metric for a particular
received frame against a threshold value, wherein the threshold
value is selected based, in part, on the quality metrics of
received frames; and determining if the particular received frame a
zero rate frame based on the comparison.
8. The method of claim 7, wherein the threshold is based on a
quality metric of a good frame.
9. The method of claim 7, wherein the threshold is based on
statistics of quality metrics of a plurality of received
frames.
10. The method of claim 7, wherein computing the quality metric
further comprises: computing the quality metric for at least a
portion of a received frame.
11. A wireless apparatus, comprising: a demodulator adapted to
demodulate received modulated signals and generate demodulated
symbols; a decoder coupled to the demodulator and adapted to decode
demodulated symbols and generate decoded data bits, wherein the
demodulated symbols are partitioned into frames; and a memory
storage device adapted for storing: a first set of computer
readable instructions adapted to calculate a quality metric of at
least one frame by correlating decoded data bits to a data portion
of the received modulated signals; a second set of computer
readable instructions adapted to compare the quality metric to a
threshold value; and a third set of computer readable instructions
adapted to determine if the at least one frame is an empty frame
based on the comparison.
Description
CROSS REFERENCE
1. This application is a continuation of application Ser. No.
09/388,029, filed on Sep. 1, 1999, and entitled "Method and
Apparatus for Detecting Zero Rate Frames in a Communications
System," now allowed.
BACKGROUND OF THE INVENTION
2. I. Field of the Invention
3. The present invention relates to data communications. More
particularly, the present invention relates to novel and improved
method and apparatus for detecting zero rate frames in a data
transmission.
4. II. Description of the Related Art
5. Many modern day communications systems currently exist for
transmitting data from a source device to a destination device.
Among these systems, code division multiple access (CDMA)
communications systems are efficient data transmission systems that
employ spread spectrum techniques to utilize an entire available
signal bandwidth. CDMA systems use other techniques to further
enhance system capacity while providing the required level of
performance. Such techniques include dynamic adjustment of the
transmit power level and data transmission at a variable rate.
6. In CDMA systems, communication between users is conducted via
one or more base stations. A first user on one mobile station
communicates to a second user on a second mobile station by
transmitting data on a reverse link to a base station. The base
station receives the data and can route the data to another base
station. The data is then transmitted on the forward link of the
same base station, or a second base station, to the second mobile
station. The forward link refers to transmission from the base
station to the mobile station, and the reverse link refers to
transmission from the mobile station to the base station.
7. Data transmissions for CDMA systems occur in frames of data. To
enhance system capacity, the rate of each frame can be selected
from one of a number of possible rates (e.g., full, half, quarter,
and eight rates), depending on the amount of data to be
transmitted. For some CDMA systems, transmission occurs in
specified (e.g., 20 msec) time intervals, with each interval
comprising a single larger (20 msec) frame or a number of smaller
(5 msec) frames. Each frame can include a data transmission or no
data transmission. A frame with no transmission is commonly
referred to as a zero rate (or empty) frame.
8. The variable and zero rate frames allow the CDMA system to
increase capacity by decreasing the transmit power level, and thus
reducing interference, when smaller amounts or no data is present
for transmission. At the receiving device, a detection scheme is
necessary to detect whether a frame was received correctly (i.e., a
good frame) or received in error (i.e., an erased or bad frame), or
whether no transmission occurred (i.e., a zero rate or empty
frame). This information may be required, for example, to adjust
the transmit power level at the transmitting source to maintain a
specified level of performance.
9. As can be seen, techniques that can accurately identify zero
rate frames are highly desirable.
SUMMARY OF THE INVENTION
10. The present invention provides novel and improved techniques
for detecting zero rate frames in a received data transmission.
Zero rate detection can be achieved using various methods.
Typically, a quality metric is computed for a received frame that
cannot be reliably decoded and compared against a threshold value.
Based on the comparison result, the received frame is indicated as
being either transmitted and received in error (i.e., erased or
bad) or not transmitted at all (i.e., zero rate or empty). In
accordance with different aspects of the invention, the threshold
value can be 1) selected based on the quality metrics computed for
decoded frames, 2) selected based on the quality metrics computed
for received frames identified as good, and 3) dynamically adjusted
based on current information available at the receiver. These
features increase accuracy in identifying zero rate frames by
taking into account the operating conditions of the receiver.
11. An embodiment of the invention provides a method for
identifying zero rate frames in a received data transmission. In
accordance with the method, a modulated signal is received and
demodulated in accordance with a particular demodulation format to
generate demodulated symbols. The demodulated symbols are
partitioned into a number of received frames. For each received
frame, the symbols are decoded and certain decoding metrics (e.g.,
symbol error rate, CRC, and so on) are checked to determine the
success of decoding. If decoding fails, or if a rate determination
algorithm (RDA) needs to distinguish between zero rate and erased
frames, a quality metric is computed and compared against a
threshold value. The threshold value is selected based, in part, on
the quality metrics of received frames. A particular received frame
is identified as being a zero rate frame or not a zero rate frame
based on the comparison. The method is advantageously used in a
CDMA communications system.
12. The quality metric can relate to an energy of a received frame,
a distance between a received frame and a codeword corresponding to
the received frame, or other metrics. The energy can be computed as
a sum of square symbols for the received frame. The distance can be
computed by decoding a received frame, re-encoding the decoded data
(if a non-systematic code is used at the transmitting device), and
performing a dot product of the received frame with the decoded or
re-encoded frame. The threshold value can be selected based, in
part, on the computed quality metrics of decoded frames identified
as good frames, and can also be dynamically adjusted.
13. Another embodiment of the invention provides a receiver
subsystem in a communications system. The receiver subsystem
includes a demodulator coupled to a data processor. The demodulator
receives and demodulates a modulated signal in accordance with a
particular demodulation format to generate demodulated symbols. The
data processor is configured to: 1) partition the demodulated
symbols into a number of received frames, 2) compute a quality
metric for each received frame, 3) compare the quality metric for a
particular received frame against a threshold value, and 4)
identify the particular received frame as being a zero rate frame
or not a zero rate frame based on the comparison. The threshold
value is selected based, in part, on the quality metrics of
received frames.
14. The data processor can include: 1) a decoder that receives and
decodes the received frames to generate decoded frames, 2) a CRC
circuit that receives and checks the decoded frames to identify
good frames among the decoded frames, 3) an encoder that receives
and re-encodes the decoded frames, or a combination thereof. The
quality metric can relate to an energy, a distance, or other
metrics of the received frame.
15. Yet another embodiment of the invention provides a receiver
subsystem used in a CDMA communications system and operable to
identify zero rate frames in a received data transmission. The
receiver subsystem includes a demodulator, a decoder, a CRC
circuit, and a metric calculation unit. The demodulator receives
and demodulates a modulated signal in accordance with a particular
demodulation format to generate demodulated symbols. The decoder
receives the demodulated symbols as a plurality of received frames,
and decodes the received frames into decoded frames. The CRC
circuit receives and checks the decoded frames to identify good
frames among the decoded frames. The metric calculation unit
computes a quality metric for each of the plurality of received
frames, compares the quality metric for a particular received frame
against a threshold value, and identifies the particular received
frame as being a zero rate frame or not a zero rate frame based on
the comparison. The threshold value is selected based, in part, on
the quality metrics of received frames.
BRIEF DESCRIPTION OF THE DRAWINGS
16. 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:
17. FIG. 1 shows a diagram of an embodiment of a communications
system that comprises multiple cells;
18. FIG. 2 shows a block diagram of an embodiment of a portion of a
base station for generating a fundamental channel and a control
channel for the forward link transmission;
19. FIG. 3 shows a block diagram of an embodiment of a portion of a
mobile station for processing the fundamental and control channels
received on the forward link transmission;
20. FIG. 4 shows a block diagram of an embodiment of a decoding
unit within the mobile station; and
21. FIG. 5 is a plot showing two probability density functions
(PDFs) for two hypotheses (H.sub.0 and H.sub.1) of a received data
frame.
DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS
22. FIG. 1 shows a diagram of an embodiment of a communications
system 100 that comprises multiple cells 110A-110G. Each cell 110
is serviced by a corresponding base station 120. Various mobile
stations 130 are dispersed throughout the communications system. In
an embodiment, each mobile station 130 communicates with one or
more base stations 120 on the forward and reverse links, depending
on whether the mobile station is in soft handoff. In FIG. 1, the
solid line with the arrow indicates a data transmission from a base
station to a mobile station. A broken line with the arrow indicates
that a mobile station is receiving the pilot signal, but no data
transmission, from the base station. The reverse link communication
is not shown in FIG. 1 for simplicity.
23. As shown by FIG. 1, each base station can transmit data to one
or more mobile stations at any given moment. The mobile stations,
especially those located near a cell boundary, can receive data
transmission and pilot signals from multiple base stations. If the
pilot signal of a particular base station is above a particular
threshold, the mobile station can request that base station to be
added to the active set of the mobile station. In an embodiment,
each mobile station can receive data transmission from zero or more
members of the active set.
24. The present invention can be applied to code division multiple
access (CDMA) systems, time division multiple access (TDMA)
systems, frequency division multiple access (FDMA) systems, and
other communications systems. The International Telecommunications
Union (ITU) recently requested the submission of proposed methods
for providing high rate data and high-quality speech services over
wireless communication channels. The majority of the proposals
operate within a code division multiple access environment. For
clarity, the present invention is described in terms of the
submission by the Telecommunications Industry Association (TIA)
entitled "The cdma2000 ITU-R RTT Candidate Submission," herein
after referred to as IS-2000. However, the teachings of the present
invention are equally well suited to application to other CDMA
standards proposed to the ITU. One of the proposals was issued by
the European Telecommunications Standards Institute (ETSI),
entitled "The ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT
Candidate Submission," hereafter referred to as WCDMA. The contents
of these submissions are public record and are well known in the
art.
25. FIG. 2 shows a block diagram of an embodiment of a portion of
the base station for generating a fundamental channel and a control
channel for the forward link transmission. The fundamental channel
can be used to send primary data from the base station to the
mobile station. In the case of speech transmissions, the
fundamental channel carries speech data. The control channel
carries control data such as status and signaling information to
the mobile station. For clarity, the invention is described for
forward link transmissions from the base station to the mobile
station, but is equally applicable for reverse link transmissions
from the mobile station to the base station.
26. As shown in FIG. 2, a message generator 212 generates and
provides control messages to a cyclic redundancy check (CRC) and
tail bit generator 214. Generator 214 appends a set of CRC bits
used to check the accuracy of the decoding at the mobile station.
The CRC bits are parity bits generated based on the contents of the
particular control message. Generator 214 further appends a set of
tail bits to the control message to clear the memory of the decoder
at the mobile station. The formatted control message is then
provided to an encoder 216 that encodes the message with a
particular encoding format. Encoder 216 provides forward error
correction (FEC) coding of the control message. In a specific
embodiment, encoder 216 is a rate one-half or a rate one-quarter
convolutional encoder, as defined by the IS-2000 submission. The
encoded symbols from encoder 216 are provided to a symbol puncturer
220 that punctures, or removes, some of the symbols in accordance
with a particular puncturing pattern. The unpunctured symbols are
provided to an interleaver 222 that reorders the symbols in
accordance with a particular interleaving format. The interleaved
symbols are provided to a modulator 230.
27. A variable rate data source 232 generates variable rate data.
The data can comprise speech, video, facsimile, multimedia,
electronic mail messages, and other forms of digital data. An
example of a method for transmitting data in code channel frames of
fixed duration is described in U.S. Pat. No. 5,504,773, entitled
"METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION,"
assigned to the assignee of the present invention, and incorporated
herein by reference. Generally, variable rate data source 232 can
support any number of rates, and can also support zero rate for no
data transmission.
28. In a specific embodiment, variable rate data source 232 is a
variable rate speech encoder such as the one described in U.S. Pat.
No. 5,414,796, entitled "VARIABLE RATE VOCODER," assigned to the
assignee of the present invention, and incorporated herein by
reference. Variable rate speech encoders are popular in wireless
communications because their use increases the battery life of
wireless communication devices and enhances system capacity with
minimal impact on perceived speech quality. The Telecommunications
Industry Association has codified some popular variable rate speech
encoders in such standards as Interim Standard IS-96 and Interim
Standard IS-733. These variable rate speech encoders encode the
speech signal at four possible rates based on the level of voice
activity. These rates are referred to as full rate, half rate,
quarter rate, and eighth rate. Each rate is associated with a
particular number of bits used to encode a frame of speech, with
the full, half, quarter, and eight rates respectively using one,
one-half, one-quarter, and one-eight a specified maximum number of
bits to encode the frame. The rate can vary on a frame-by-frame
basis.
29. Variable rate date source 232 provides the data, in frames, to
a CRC and tail bit generator 234. Generator 234 appends a set of
CRC bits used to check the accuracy of the decoding at the mobile
station. Again, the CRC bits are parity bits generated based on the
contents of the particular data frame. Generator 234 also appends a
set of tail bits to the data frame to clear the memory of the
decoder at the mobile station. The formatted frame is then provided
to an encoder 236 that encodes the frame with a particular encoding
format. Encoder 236 provides forward error correction coding of the
data. In a specific embodiment, encoder 236 is a convolutional or a
turbo encoder operated at either rate one-half or rate one-quarter,
as defined by the IS-2000 submission. The encoded symbols from
encoder 236 are provided to a symbol repetition generator 238 that
repeats the encoded symbols of lower rate frames. The symbols are
then provided to a puncturing element 240 that punctures some of
the symbols in accordance with a particular puncturing pattern to
provide a particular number of symbols for each frame. The
unpunctured symbols are provided to an interleaver 242 that
reorders the symbols in accordance with a particular interleaving
format. The interleaved symbols are provided to modulator 230.
30. In an embodiment, modulator 230 modulates the fundamental and
control channels in accordance with a particular CDMA modulation
format and provides a modulated signal to a transmitter (TMTR) 252.
For example, modulator 230 can scramble the data with a long PN
sequence, spectrally spread the data with short PN sequences, cover
the data with Walsh codes, and quadrature modulates the data with
an inphase and a quadrature carrier signal. Transmitter 252
amplifies, filters, and upconverts the signal. The forward link
signal is then provided through a duplexer 254 and transmitted from
an antenna 256. The elements shown in FIG. 2 are described in
further detail in the IS-2000.
31. FIG. 2 shows a simplified block diagram of the fundamental and
control channels. Other channels are also available for data
transmission on the forward link but not shown in FIG. 2 for
simplicity.
32. FIG. 3 shows a block diagram of an embodiment of a portion of
the mobile station for processing the fundamental and control
channels received on the forward link. The forward link signal from
the base station is received by an antenna 312, routed through a
duplexer 314, and provided to a receiver (RCVR) 316. Receiver 316
downconverts the received signal to a baseband frequency in
accordance with a demodulation format that is complementary to the
modulation format (e.g., QPSK) used at the base station. The
baseband signal is then provided to, and demodulated by, a
demodulator (DEMOD) 318 to provide demodulated symbols. Demodulator
318 performs functions complementary to those performed at the base
station (e.g., decovering, despreading, and descrambling). The
demodulated symbols are provided to a de-interleaver (DEINT) 320
that reorders the symbols in accordance with a de-interleaving
format that is complementary to the interleaving format used at the
base station. The reordered symbols are provided to a decoding unit
322 that decodes the symbols to provide an estimate of the
transmitted frame. Using the CRC bits, if any, included in the
transmitted frame, the estimate of the transmitted frame is then
checked to determine the accuracy of the frame estimate. The
decoded data is provided to a processor 330.
33. In an embodiment, the mobile station performs a blind decoding
on the forward link signal. Blind decoding describes a method of
decoding variable rate data in which the receiver does not know a
priori the rate of the data transmission. In an embodiment, the
mobile station deinterleaves, accumulates, and decodes the data in
accordance with each possible rate hypothesis (e.g., full, half,
quarter, eight, and zero rates and erasure). One of the decoded
frames is selected as the best estimate based on one or more
quality metrics such as the symbol error rate, the CRC check, the
Yamamoto metric, the frame energy, and other metrics.
34. FIG. 3 also shows some of the circuit elements used to transmit
erasure indicator bits (EIBs) to the base station for forward link
power control. In an embodiment, the EIBs are multiplexed with the
reverse traffic data and provided to a modulator (MOD) 332 that
combines the EIBs with the traffic data at particular locations
defined by the IS-2000 submission. The combined EIBs and traffic
data are modulated by modulator 332 using a particular modulation
format. The modulated data is provided to a transmitter (TMTR) 334
that upconverts, amplifies, and filters the signal prior to
transmission to the base station via antenna 312. In an embodiment,
the reverse link signal is a CDMA signal that is modulated in
accordance with the IS-2000 submission.
35. To enhance system capacity, the CDMA system is designed to
transmit data using frames of various frame formats and rates. Each
frame format can be defined by a particular frame length, a
particular coding format, and (possibly) some other attributes. For
example, in accordance with the IS-2000 submission, data is
transmitted in 5 msec or 20.cndot.L msec frames, where L is 1, 2,
or 4. The rate of each 20.cndot.L msec frame can also be selected
from one of a number of possible rates (e.g., eight or more rates),
depending on the amount of data to be transmitted and other
considerations. For an IS-2000 compliant system, transmission
occurs in 20 msec intervals, with each interval comprising one 20
msec frame, four 5 msec frames, or a portion of a longer frame.
Each frame can include a data transmission or no transmission. The
5 msec frame has less processing delay, and is particularly useful
for transmitting control messages that need to be acted on quickly.
As currently specified by the IS-2000 submission, a zero rate frame
can be transmitted on a 5 msec frame or a 20 msec frame on the
dedicated control channel (e.g., when there are no control messages
to send), and a zero rate frame may be transmitted on the
fundamental channel when the transmitter is out of power. A zero
rate frame may also be transmitted on a particular (e.g.,
supplemental) channel if there is no information (e.g., no voice
data) to send.
36. FIG. 4 shows a block diagram of an embodiment of decoding unit
322. The demodulated data from de-interleaver 320 is provided to a
number of frame decoders 410A through 410N. Each frame decoder 410
can be used to decode a data frame based on a particular decoding
hypothesis (i.e., a particular frame format and rate). A data
processor can be designed to include all elements, additional
elements, and/or a subset of the elements in frame decoder 410.
37. On the forward link, data transmission at lower rates is
achieved by repeating each code symbol N times (where N is 1, 2, 4,
or 8) to achieve a particular symbol rate. Each transmitted symbol
is also scaled by 1/N to provide approximately the same amount of
energy per code symbol. At the receiver, each set of N repeated
symbols are accumulated and scaled to provide a combined soft
decision symbol that is representative of the original code
symbol.
38. Within each frame decoder 410, the demodulated data is provided
to a symbol accumulator 412 that accumulates sets of N received
symbols based on a hypothesized rate of 1/N. For example, if the
frame decoder is configured to decode an eight rate frame, symbol
accumulator 412 accumulates sets of eight received symbols to
generate a soft decision symbol for each set. Each soft decision
symbol is representative of the original symbol at the transmitting
device. The soft decision symbols are provided to a decoder 414
that decodes the symbols to provide decoded data. Decoder 414 is
designed based on the encoder used at the transmitting source. For
example, a Viterbi decoder is preferably used to decode
convolutionally encoded data. Decoder 414 or other external
circuits may further be designed to provide a frame quality metric
such as a symbol error rate, a CRC check, a Yamamoto quality
metric, or a combination thereof, that can be used to determine the
quality of the decoded frame. The Yamamoto quality metric is
particularly useful for lower rates when CRC bits are not
available.
39. An efficient decoding scheme for data is disclosed in U.S. Pat.
No. 5,933,462, entitled "SOFT DECISION OUTPUT DECODER FOR DECODING
CONVOLUTIONALLY ENCODED CODEWORDS," and U.S. Pat. No. 5,710,784,
entitled "MULTIRATE SERIAL VITERBI DECODER FOR CODE DIVISION
MULTIPLE ACCESS SYSTEMS APPLICATIONS," both assigned to the
assignee of the present invention, and incorporated herein by
reference.
40. For some rate hypotheses that include CRC bits, the decoded
data is provided to a CRC circuit 416 that checks the CRC bits
appended with each decoded frame. CRC check is known in the art and
further defined by the particular CDMA standard being implemented
(e.g., IS-95-A or IS-2000). In an embodiment, CRC circuit 416
provides a one-bit result for each checked frame. In a specific
implementation, the CRC result is a logic zero ("0") if the CRC
check indicates a good frame and a logic one ("1") if the CRC check
indicates a frame that is not good (i.e., erased or empty).
41. Decoding unit 322 can be designed in various configurations.
For example, for an IS-2000 compliant system, decoding unit 322 can
include a number of frame decoders 410 operated in parallel, with
each frame decoder 410 configured to decode a particular decoding
hypothesis. The rate determination can be performed based on the
symbol error rate, the CRC result, the Yamamoto quality metrics,
other metrics, or a combination thereof. One such decoder design is
disclosed in U.S. Pat. No. 5,774,496, entitled "METHOD AND
APPARATUS FOR DETERMINING DATA RATE OF TRANSMITTED VARIABLE RATE
DATA IN A COMMUNICATIONS RECEIVER," assigned to assigned to the
assignee of the present invention, and incorporated herein by
reference.
42. In FIG. 4, for ease of understanding, decoding unit 322 is
shown as having multiple parallel paths for processing the
demodulated symbols. However, a single decoding path using shared
circuit elements is preferred in some implementations to reduce the
amount of required circuitry. In the shared decoder
implementations, the demodulated symbols are stored in a buffer
(not shown in FIG. 4) as they are received and repeatedly provided
to a frame decoder for decoding. The frame decoder is reconfigured
for a different decoding hypothesis for each pass of the data.
Other implementations of decoding unit 322 can be contemplated and
are within the scope of the invention.
43. Detection of a zero rate frame may be required for many
applications. In an IS-2000 system, a power control mechanism is
provided to adjust the transmit power of the forward link signal
based on the decoded forward link frames at the mobile station. The
mobile station decodes the forward link frames and determines
whether the frames are good, erased, or not transmitted. The base
station is instructed to adjust the forward link transmit power
level based on the decoded frames. For example, the base station
can be instructed to decrease its transmit power to the mobile
station if a decoded frame is good, increase the transmit power if
the decoded frame is bad (or erased), and do nothing if no
transmission (or zero rate) is detected. The quality of the
communication and the capacity of the system are dependent, in
part, on the ability to accurately detect erased and zero rate
frames.
44. IS-2000 defines a power control mechanism for the forward link.
Specifically, when operating in certain specified modes, the mobile
station is required to set all power control bits on a Reverse
Power Control Subchannel during a 20 msec period to an EIB, which
is defined by the following:
45. 1) The EIB bit is set to "0" in the second transmitted frame
following the detection of a good 20 msec frame on the Forward
Fundamental Channel or the Forward Dedicated Control Channel.
46. 2) The EIB is set to "0" in the second transmitted frame
following the detection of at least one good 5 msec frame without
detection of any bad (i.e., erased) 5 msec frame.
47. 3) The EIB is set to "1" in the second transmitted frame for
all other cases.
48. The IS-2000 specification is tabulated in Table 1 for various
decoding scenarios.
1 TABLE 1 1.sup.ST 2.sup.ND 3.sup.RD 4.sup.TH Scenario 5 msec 5
msec 5 msec 5 msec 20 msec EIB 1 Bad Bad Bad Bad Good 0 2 Good
Empty Empty Empty Bad/Empty 0 3 Empty Good Empty Empty Bad/Empty 0
4 Empty Empty Good Empty Bad/Empty 0 5 Empty Empty Empty Good
Bad/Empty 0 6 Good Good Empty Empty Bad/Empty 0 7 Good Empty Good
Empty Bad/Empty 0 8 Good Empty Empty Good Bad/Empty 0 9 Empty Good
Good Empty Bad/Fmpty 0 10 Empty Good Empty Good Bad/Empty 0 11
Empty Empty Good Good Bad/Empty 0 12 Good Good Good Empty Bad/Empty
0 13 Good Good Empty Good Bad/Empty 0 14 Good Empty Good Good
Bad/Empty 0 15 Empty Good Good Good Bad/Empty 0 16 Good Good Good
Good Bad/Empty 0 17 Bad 5 msec frame(s) anywhere Bad/Empty 1 18
Empty Empty Empty Empty Empty 1
49. As shown in Table 1, the EIB is set to logic low if: 1) the
received 20 msec frame is decoded as a good frame, or 2) at least
one received 5 msec frame within a 20 msec time interval is decoded
as a good frame AND a bad (i.e., erased) frame is not detected. A
frame can be identified as being good by performing a CRC check on
the decoded frame. For the second case in which at least one 5 msec
frame is detected as being good, the remaining 5 msec frames in the
20 msec time interval need to be identified as being either bad or
empty. Zero rate detection is thus needed for this case.
50. Referring to Table 1, when the EIB is set to zero, the decoder
has information from at least one good frame. In accordance with an
aspect of the invention, for improved detection accuracy, the
information from good frames can be used to assist in determining
whether a decoded frame is bad or empty.
51. Zero rate detection can be achieved using various methods.
Generally, a quality metric is computed for a received frame and
compared against a threshold value. Based on the comparison result,
the received frame is indicated as being either transmitted and
received in error (i.e., erased or bad) or not transmitted at all
(i.e., zero rate or empty). In accordance with an aspect of the
invention, the threshold value can be selected based on the quality
metrics computed for received (and possibly decoded) frames. In
accordance with another aspect of the invention, the threshold
value can be selected based on the quality metrics computed for
received frames identified as good. In accordance with yet another
aspect of the invention, the threshold value can be dynamically
adjusted based on current information (or future information, if
the current decision is delayed) available at the receiver. These
features increase accuracy in identifying zero rate frames by
taking into account the operating conditions of the receiver.
52. In one zero rate detection method, the sum of the squared
symbols is computed and compared against a threshold value. The sum
of the squared symbols is indicative of the energy of the received
frame. A data transmission is indicated if the computed energy is
greater than an energy threshold value, and no transmission is
indicated if the computed energy is less than the threshold
value.
53. FIG. 4 shows a block diagram of the circuitry used to detect
zero rate using the sum of the squared symbols. The soft decision
symbols from symbol accumulator 412 are provided to a sum of
squared symbols element 422. Element 422 squares each received soft
decision symbol in a particular frame and sums the squared symbols
within the frame. The sum result represents the computed energy for
the frame and is provided to processor 330. In an embodiment,
processor 330 considers two hypotheses for the computed energy
value, which are:
54. H.sub.0--the computed energy contains only noise, and
55. H.sub.1--the computed energy contains signal plus noise.
56. Specifically, processor 330 determines whether the computed
energy is likely to contain only noise (i.e., hypothesis H.sub.0)
or signal plus noise (i.e., hypothesis H.sub.1). Based on the
result of this determination, a received frame is indicated as
being erased or zero rate. The zero rate determination is described
in more detail below.
57. The computation of the noise and the signal plus noise of a
particular communications channel is described in further detail in
U.S. Pat. No. 5,903,554, entitled "METHOD AND APPARATUS FOR
MEASURING LINK QUALITY IN A SPREAD SPECTRUM COMMUNICATION SYSTEM,"
assigned to the assignee of the invention, and incorporated herein
by reference.
58. In a second zero rate detection method, the decoded symbols are
re-encoded and correlated with the soft decision symbols. For a
particular frame, a dot product is performed between the (encoded)
soft decision symbols and the re-encoded symbols. The dot product
is indicative of a distance between the received vector (i.e., the
received frame) and its nearest codeword (i.e., the re-encoded
frame). The computed distance is compared against a distance
threshold. A data transmission is indicated if the computed
distance is less than a distance threshold value, and no
transmission is indicated if the computed distance is greater than
the threshold value.
59. FIG. 4 also shows a block diagram of the circuitry used to
detect zero rate using the computed distance. The decoded bits from
decoder 414 are provided to an encoder 424 that encodes the bits
using the same encoding format used at the transmitting source for
the particular decoding hypothesis. For example, encoder 424 can be
a convolutional or turbo encoder and can be a rate one-half or rate
one-quarter encoder, as defined by the IS-2000 submission. The code
symbols from encoder 424 are provided to a dot product element 426
that also receives the soft decision symbols from symbol
accumulator 412. Dot product element 426 performs a dot product of
the soft decision symbols with the re-encoded symbols in a manner
known in the art, and provides the result to processor 330. The dot
product result is indicative of the distance between the received
and re-encoded frame. Processor 330 then considers the two
hypotheses (described above) for the computed distance.
60. In a third method for zero rate detection, which is a variation
of the second method, the decoded data bits are correlated with the
soft decision symbols. For a systematic code, the encoded data
includes the original data and coded (or parity) data. This
property allows the decoded data bits to be correlated with the
data portion of the encoded data (i.e., the soft decision symbols).
This method eliminates the need for re-encoding, which simplifies
the decoding circuitry and shortens the processing time to detect
zero rate frame. This method is especially applicable to an IS-2000
compliant system, which employs a systematic turbo code for the
supplemental channel on the forward link.
61. In the description above, the quality metrics are computed for
each received frame. However, the quality metric can be computed
for a fraction of a frame or for multiple frames, and this is
within the scope of the invention.
62. For many CDMA systems, a pilot signal is transmitted on the
forward or reverse link to allow the receiving station to perform
various functions. As part of the signal processing, the pilot
signal is recovered and used to coherently demodulate the forward
link signal. Thus, the demodulated symbol includes a factor that is
related to the pilot energy.
63. FIG. 4 includes circuitry used to compute several quality
metrics (e.g., energy and distance) for identifying zero rate
frames. Typically, only one quality metric is computed, and the
frame decoder in FIG. 4 can be simplified. The quality metrics can
also be computed in various manners such as by hardware
specifically designed to perform the functions described herein, by
software programmed to perform the described functions, or a
combination of both. For example, sum or squared symbols element
422 can be implemented by software executed on processor 330.
Processor 330 can be implemented in a microcontroller, a
microprocessor, a digital signal processing (DSP) chip, or an
application specific integrated circuit (ASIC) programmed to
perform the function as described herein.
64. As noted above, the computed quality metric for a particular
frame is compared against a threshold value to determine whether a
zero rate or erased frame was received. The quality metric can be
the energy computed using the first method, the distance computed
using the second and third methods, or other metrics. The energy
and distance metrics have an inverse relationship. Specifically, a
zero rate frame is more likely if the computed energy is low or if
the computed distance is large. For simplicity, the following
description is directed toward the computed energy, but may be
modified to cover the computed distance or other metrics.
65. FIG. 5 is a plot showing two probability density functions
(PDFs) for the two hypotheses (H.sub.0 and H.sub.1) of a received
frame, which are identified above. A PDF 510 corresponds to
hypothesis H.sub.0 in which the computed energy contains
predominantly noise, and a PDF 512 corresponds to hypothesis
H.sub.1 in which the computed energy contains signal plus noise.
PDF 510 has a mean of x.sub.0 and a standard deviation of
.sigma..sub.0, and PDF 512 has a mean of x.sub.1 and a standard
deviation of .sigma.1. The mean of PDF 510 is less than the mean of
PDF 512, as expected. The distance between x.sub.1 and x.sub.0
corresponds to the mean signal energy of the received frames.
66. If PDFs 510 and 512 are known, a threshold 514 can be set at a
value x.sub.TH such that a desired outcome is achieved. For
example, if the desired outcome is to obtain the same likelihood of
detection error for either hypothesis, then a threshold value
x.sub.TH1 can be selected such that an area 520 to the right of
x.sub.TH1 and under PDF 510 is equal to an area 522 to the left of
x.sub.TH1 and under PDF 512. If the threshold value is set lower
than x.sub.TH1, toward x.sub.0, the probability of missing
hypothesis H.sub.1 is decreased but the probability of false
detection for hypothesis H.sub.0 is increased. That is, if the
threshold value is set lower than x.sub.TH1, a computed value
belonging to hypothesis H.sub.1 is more likely to be correctly
identified but a computed value belonging to hypothesis H.sub.0 is
more likely to be incorrectly identified.
67. The desired outcome may be dependent on various considerations.
For example, if the zero rate determination is used for controlling
the transmit power of a transmitting source, it may be more
desirable to err toward transmitting more power than necessary
(which may reduce system capacity) than to transmit less power than
required (which may degrade performance).
68. In an embodiment, PDF 510 is estimated from the computed
metrics from empty frames and PDF 512 is estimated from the
computed metrics from good and bad frames. As noted above, a frame
can be identified as being correctly decoded (i.e., good) based on
the result of the CRC check. Frames that do not pass the CRC check
are identified as either bad or empty. In an embodiment, statistics
such as the mean and standard deviation are computed or evaluated
using nominal operation conditions. Subsequently, the statistics
are computed using information from both good and bad frames as
they occur in the actual operation. The mean and standard deviation
can also be computed for the metrics associated with empty frames.
Initially, statistics for empty frames can be estimated from the
total noise power Nt on a known channel (e.g., the pilot channel).
The estimation of Nt is disclosed in the aforementioned U.S. Pat.
No. 5,903,554.
69. In an embodiment, the PDFs are assumed to be Gaussian. The
shape of a Gaussian PDF is uniquely defined for a given mean and
standard deviation. PDF 512 can be determined from the mean and
standard deviation for good and bad frames, and PDF 510 can be
determined from the mean and standard deviation for empty frames.
Based on these PDFs, the threshold value x.sub.TH can be selected
such that the desired outcome is achieved, as described above.
70. For zero rate detection, the computed metric for a particular
frame is compared against the threshold value x.sub.TH. If the
computed metric is less than the threshold value x.sub.TH, the
frame is identified as a zero rate or empty frame. Otherwise, if
the computed metric is greater than the threshold value x.sub.TH,
the frame is identified as a bad or erased frame. It should be
noted that a different selection criterion is used if the quality
metric is the computed distance between received and decoded
frames.
71. The threshold value x.sub.TH can be adjusted to account for
additional information available to the receiver. For example,
since PDF 512 for hypothesis H.sub.1 is initially determined from
frames known to be good, and that good frames tend to contain more
energy than erased frames, the average x.sub.1 will be slightly
higher than the true mean value for hypothesis H.sub.1. Thus, the
threshold value x.sub.TH can be slightly skewed or offset to the
left of x.sub.1. Viewed differently, since the computed metric is
known to be from an empty frame (i.e., no transmission) or a bad
frame (having low received energy, which is possibly the reason the
frame is decoded as an erasure), it is likely to be a smaller value
and the threshold value should be offset to the left accordingly.
The amount of offset can be determined based on system simulation,
empirical measurements, or by other means. For example, lab
measurement can be made on a number of transmitted frames. The
(mean) difference in the computed metrics between good and bad
frames can then be determined and stored as a parameter in the
receiving device. Subsequently, the computed mean for good frames
can be offset by this mean difference to derive an estimate of the
mean for bad frames. The threshold value can then be set based on
the estimated mean for bad frames and the estimated mean for empty
frames.
72. The threshold value can also be adjusted based on other
available information, such as power control information. The
transmit power of the transmitting source may be adjusted by a
power control loop to provide a particular level of performance
(e.g., a particular frame-error-rate FER) at the receiving device.
In one implementation, the power control loop measures the quality
(e.g., the Eb/Nt) of the received signal, compares the measured
signal quality against a set point, and adjusts the transmit power
of the transmitting source such that the signal quality is
maintained at the set point. The set point is adjusted such that
the desire performance is achieved. In this implementation, the
threshold value can be adjusted, for example, by the difference
between the set points for good and bad frames. For example, if the
set point is 5 dB when a frame is correctly decoded and 4 dB when a
frame is incorrectly decoded, the threshold value can be adjusted
downward by 1 dB.
73. The threshold value can also be adjusted based on a decision
feedback decoding scheme. Initially, the threshold value can be set
to a particular value based on information then available, such as
the initial estimated statistics for empty frames and bad frames.
Thereafter, decoding is performed iteratively whereby information
from incorrectly decoded frames is used to update the statistics
for PDFs 510 and 512. For example, a frame incorrectly decoded can
be estimated as either an empty frame or a bad frame, and the
compute metric for this frame is used to update the statistics for
PDF 510 or 512, respectively. In this manner, the decoded data is
used in decoding future data (e.g., via the adjustment of the
threshold value).
74. By setting the threshold value x.sub.TH based on measurements
computed at the receiver, the operating conditions of the
particular receiver are taken into consideration in making the zero
rate determination. For example, if the receiver requires more
power to maintain a particular level of performance, this fact will
be taken into account in setting the threshold value.
75. The threshold value can also be dynamically adjusted as
operating conditions change. The computed statistics (e.g., mean
and standard deviation) can be determined based on a weighted
average of the computed metrics. Numerous weighting schemes can be
implemented. For example, the metrics can be weighted equally,
weighted more heavily toward more recent measurements (i.e., a
"leaky" average), or weighted using other schemes (e.g., frames
located near good frames may be weighted more heavily).
76. Various other factors can also be used in setting the threshold
value. For example, the set points for frames correctly and
incorrectly decoded can also be used to adjust the threshold value.
The set points can be averaged in the manners described above. The
amount of adjustment in the threshold value can also be dependent
on, for example, the number of fingers in a rake receiver used to
demodulate the signal.
77. For clarity, the invention has been described for detecting
zero rate frames on the forward link. For some CDMA systems (e.g.,
IS-95-B), on the reverse link, the code symbols at lower rates are
transmitted at full power but pseudo-randomly transmitted in one of
N possible symbol locations. For example, for eighth rate
transmission, each code symbol is transmitted in one of eight
possible symbol locations, with the location being selected by a
long PN sequence. At the base station receiver, a selector unit
selects the code symbols at the proper locations based on the
hypothesized rate. Thus, the base station decoder for the lower
rates includes a selector in place of the symbol accumulator. The
decoder for the reverse link signal in an IS-2000 CDMA system is
further described in the aforementioned IS-2000 submission.
78. For clarity, many aspects of the invention are described for a
specific implementation in a CDMA system that complies with
IS-2000. However, the invention can be adopted for used with other
CDMA systems. One specific CDMA system is disclosed in U.S. Pat.
No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS
COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS" and
U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING
WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM." Another specific
CDMA system is described in U.S. patent application Ser. No.
08/963,386, entitled "METHOD AND APPARATUS FOR HIGH RATE PACKET
DATA TRANSMISSION," filed Nov. 3, 1997. These patents and patent
applications are assigned to the assignee of the present invention
and incorporated herein by reference.
79. CDMA systems can be designed to conform to a number of
currently defined CDMA standards, and current or future proposed
standards. For example, the CDMA system can be designed to conform
to "TIA/EIA/IS-95-A Mobile Station-Base Station Compatibility
Standard for Dual-Mode Wideband Spread Spectrum Cellular System" or
TIA/EIA/IS-98-A, -B, and -C entitled "Recommended Minimum
Performance Standard for Dual-Mode Spread Spectrum Cellular and PCS
Mobile Stations," hereinafter referred to as the IS- 95-A and IS-98
standards, respectively. CDMA systems can also be designed to
conform to the IS-2000 or the WCDMA standards being proposed by the
standards bodies ETSI and ARIB. These various CDMA standards are
incorporated herein by reference.
80. The invention can also be adopted for use with other types of
communications systems such as time division multiple access
(TDMA), frequency division multiple access (FDMA), and amplitude
modulation (AM) schemes such as amplitude companded single sideband
(ACSSB).
81. 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.
* * * * *