CA2220965C - Method and apparatus for determining coding rate in a wireless communication system - Google Patents
Method and apparatus for determining coding rate in a wireless communication system Download PDFInfo
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- CA2220965C CA2220965C CA002220965A CA2220965A CA2220965C CA 2220965 C CA2220965 C CA 2220965C CA 002220965 A CA002220965 A CA 002220965A CA 2220965 A CA2220965 A CA 2220965A CA 2220965 C CA2220965 C CA 2220965C
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/20—Arrangements for detecting or preventing errors in the information received using signal quality detector
- H04L1/208—Arrangements for detecting or preventing errors in the information received using signal quality detector involving signal re-encoding
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
- H04L1/0045—Arrangements at the receiver end
- H04L1/0054—Maximum-likelihood or sequential decoding, e.g. Viterbi, Fano, ZJ algorithms
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
- H04L1/0045—Arrangements at the receiver end
- H04L1/0046—Code rate detection or code type detection
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L1/00—Arrangements for detecting or preventing errors in the information received
- H04L1/20—Arrangements for detecting or preventing errors in the information received using signal quality detector
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L25/00—Baseband systems
- H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
- H04L25/0262—Arrangements for detecting the data rate of an incoming signal
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B2201/00—Indexing scheme relating to details of transmission systems not covered by a single group of H04B3/00 - H04B13/00
- H04B2201/69—Orthogonal indexing scheme relating to spread spectrum techniques in general
- H04B2201/707—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation
- H04B2201/70703—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation using multiple or variable rates
- H04B2201/70705—Rate detection
Abstract
A receiver (500) utilizes parameters generated by a Viterbi decoder (530) to determine one of a plurality of coding rates in which user information is transmitted. The receiver (500) combines the parameters in a predetermined manner, the result of which is a detection statistic (dij). By utilizing the detection statistic (dij), the coding rate at which user information is transmitted is accurately determined.
Description
t! ~ CA 0222096~ 1997-11-13 o M~1~OU AND APPARATUS FOR DE~NG CODING RATE
IN A WIRELESS COMMUNIGATION SYSTEM
S FIELD OF THE INVENTION
The present invention relates generally to wireless communication ~ysLems and, more particularly, to coding rate determination in such wireless communication ~y~lell~s.
BACKGROUND OF THE INVENTION
The current Code Division Multiple Access (CDMA) 15 wireless communication system envisioned for, inter alia, cellular applications in the United States is defined by TIA/EIA/IS-95, Mobile Station-~ase Station Compatibili~y Standard for DUQI Mode Wideband Spread Spectrum Cellular System, July 1993. As is well known in the art, limiting the total amount of system interference 2 0 in a spread spectrum system results in an increase in user capacity.
One technique implemented in the IS-95 spread spectrum system to limit the total amount of system interference is variable rate coding.
Variable rate coding in a wireless communication system 2 5 such as IS-95 takes advantage of the inherent silences of a typical voice communication. In IS-95, coding occurs in 20 millisecond (ms) blocks at either full rate, 1/2 rate, 1/4 rate, or 1/8 rate. If, for example, a 20 ms block to be transmitted to a user of the spread spectrum system contained unvoiced speech or silences but was 3 0 coded at full rate, interference would be added to the system llnnec~s~rily. Such a frame can be transmitted as a 1/8 rate frame without effecting the speech quality while reducing the total amount of interference presented to the system. The 1/2 rate and 1/4 rate are transitional rates. As stated above, reducing the total ~ ~ CA 0222096~ 1997-11-13 amount of interference presented to the system results in an increase in user capacity.
While the method for determining which coding rate to use on the transmitter side is well rl~fine~l in document TIA/EIA/IS-5 96, Speech Service Option Standard for Wideband Spread Spectrum Digft~l Cellular System, April 1993 (Official Ballot Version), a receiver receiving such a variable rate coded tr~n~Inission has no knowledge of which rate was actually tr~n~mit~erl Since the receiver must have some knowledge of the 10 coding rate to perform an accurate reception/demodulation of the transmission, accurately determining the coding rate in the receiver is critical to operation of a IS-95/IS-96 compatible receiver.
Thus, a need exists for a method of accurately determining the coding rate in a receiver capable of receiving variable rate coded 1 5 transmissions.
BRIEF DESCRIP rToN OF THE DRAWINGS
2 0 FIG. 1 generally depicts a block diagram of a wireless communication system which may beneficially implement coding rate determination in accordance with the invention.
FIG. 2 generally depicts a block diagram of a transmitter impl~nl.onte~ in a mobile station compatible with a spread spectrum 2 ~ wireless communication system defined by IS-95.
FIG. 3 depicts a Walsh matrix utilized in a spread spectrum wireless communication system compatible with IS-95.
FIG. 4 generally depicts transmissions of various rate data during a CDMA time frame.
3 0 FIG. 5 generally depicts a receiver implemented in a base transceiver station (BTS) which determines coding rate in accordance with the invention.
~ ~ ,CA 0222096~ 1997-11-13 ~ , .
FIG. 6 generally depicts, in flow diagram form, the coding rate de~ tion filnction (for Rate Set 1) in accordance with the invention.
FIG. 7 generally depicts, in flow diagram form, the process S for erasing a frame (for Rate Se~ 1) using detection statistics dij in accordance with the invention.
~ IG. 8 generally depicts, in flow diagram form, the coding rate determin~t on function (for Rate Set 2) in accordance with the invention.
10FIG. 9 generally depicts, in flow diagram form, the process for erasing a frame (for Rate Set 2) using detection statistics dij in accordance with the invention.
Generally stated, a method of determining coding rate in a receiver implemented in a wireless communication system is disclosed. The receiver is responsive to a communication resource 2 0 conPinin~ user information coded at one of a plurality of coding rates. The method generates parameters indicative of the likelihclod of coding at the plurality of coding rates and combines the parameters in a predetermined manner to produce a detection statistic. Using the detection statistic, the coding rate is then 2 5 determine-l-In the preferred embodiment, the parameters indicative ofthe lik~olihood of coding at the plurality of coding rates further comprise cyclic redundancy check (CRC) information, symbol error rate (SER) information, and quality bit (QB) information. Likewise 3 0 in the preferred embodiment, the parameters are combined in a linear discrimin~nt function to produce the detection statistic, wherein the detection statistic represents two possible coding rates.
-- ~ CA 0222096~ 1997-11-13 The above method is executed in a receiver, which comprises a receiver front end which receives and demodulates the communication resource to produce demodulated user information. A decoder, which is coupled to thë receiver front end, then decodes the demodulated user information and generates parameters indicative of the likelihood of coding at the plurality of coding rates. A processor, which is coupled to the decoder, combines the parameters in a predetermined manner to produce a detection statistic and determines the coding rate based on the (letectit~n statistic.
FIG. 1 generally depicts a block diagram of a wireless communication ~yslelll 100 which may beneficially implement coding rate determination in accordance with the invention. In the ~ler~lled embodiment, the wireless communication ~yslem is a code division multiple access (CDMA) cellular radiotelephone syslelll. As one of ordinary skill in the art will appreciate, however, the coding rate determination in accordance with the invention can be implemented in any wireless communication ~y~l~m which implements variable rate coding.
2 0 Referring to FIG. 1, acronyms are used for conv~ni~nce The following is a list of definitions for the acronyms used in l~IG. 1:
BTS Base Transceiver Station CBSC Centralized Base Station Controller 2~5 EC Echo Canceller VLR Visitor Location Register HLR Home Location Register ISDN Integrated Services Digital Network MS Mobile Station 3 0 MSC Mobile Switching Center MM Mobility Manager OMCR Operations and Maintenance Center - Radio OMCS Operations and Maintenance Center - Switch PSTN Public Switched Telephone Network 3 5 TC Transcoder - ~ CA 0222096~ 1997-11-13 As seen in FIG. 1, multiple BTSs 101-103 are coupled to a CBSC 104. Each BTS 101-103 provides radio frequency (RF) communication to an MS 105-106. In the preferred embodiment, the transmitter/receiver (transceiver) hardware impl.ornente~l in the BTSs 101-103 and the MSs 105-106 to support conveyance of the RF communication resource is substantially defined in the document titled by TIA/EIA/IS-95, Mobile St~tion-Base Sfatiorl Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System, July 1993, available from the Telecommunication Industry Association (TIA). The CBSC 104 is responsible for, inter alia, call processing via the TC 110 and mobility management via the MM 109. Other tasks of the CBSC 104 include feature control and tr~n~missiQn/networking interfacing. For more information on the functionality of the CBSC 104, reference is made to United States Patent 5,475,686 to Bach et al., assigned to the assignee of the present application, and incorporated herein by refe~ ce.
Also depicted in FIG. 1 is an OMCR 112 coupled to the MM
109 of the CBSC 104. The OMCR 112 is responsible for the operations and general maintenance of the radio portion (CBSC
104 and BTS 101-103 combination) of the communication system 100. The CBSC 104 is coupled to an MSC 115 which provides switching capability between the PSTN 120/ISDN 122 and the CBSC 104. The OMCS 124 is responsible for the operations and general maintenance of the switching portion (MSC 115) of the 2~ communication ~y~Lell- 100. The HLR 116 and VLR 117 provide the communication system 100 with user information primarily used for billing purposes. ECs 111 and 119 are implemented to improve the quality of speech signal transferred through the communication ~y~lelll 100.
3 0 The functionality of the CBSC 104, MSC 115, HLR 116 and ~ILR 117 is shown in FIG. 1 as distributed, however one of ordinary skill in the art will appreciate that the functionality could likewise be centralized into a single element. Also, for different configurations, the TC 110 could be located at either the MSC 115 or - ' ~ CA 0222096~ l997-ll-l3 a BTS 101-103. The link 126 coupling the MSC 115 with the CBSC 104 is a T1/E1 link which is well known in the art. By placing the TC 110 at the CBSC, a 4:1 improvement in link budget is reAli7e~1 due to compression of the input signal (input from the T1/E1 link 126) by ~he TC 110. The compressed signal is transferred to a particular BTS 101-103 for tr~nsmi~sion to a particular MS 105-106. Important to note is that the compressed signal transferred to a particular BTS 101-103 undergoes further processing at the BTS
101-103 before tr~n~mission occurs. Put differently, the eventual 1 0 signal transmitted to the MS 105-106 is different in form but the sarne in substance as the compressed signal exiting the TC 110.
When the MS 105-106 receives the signal transmitted by a BTS 101-103, the MS 105-106 will essentially "undo" (commonly referred to as "decode") all of the processing performed by the 1 5 SySI~ 100. When the MS 105-106 transmits a signal back to a BTS
101-103, the MS 105-106 likewise implements its own processing.
After a signal having undergone processing is transmitted by the MS 105-106 (the processing of the signal is to change the form, but not the substance, of the signal) to a BTS 101-103, the BTS 101-103 2 0 will "undo" the processing performed on the signal and transfer to the a~ro~iate point within the system 100. Eventually, the signal will be transferred to an end user via the T1/E1 link 126.
A block diagram of an IS-95 compatible transmitter 200 for use in the system depicted in FIG. 1 is generally depicted in FIG. 2.
2 5 In this embodiment, the transmitter 200 resides in any of the BTSs 101-103. A voice (or data) signal 203 is input to a coding/interleaving section 206, resulting in a coded/interleaved signal 209. The coded/interleaved signal 209 is mapped, plerelably, six symbols at a time, to a unique 64-ary symbol by a 64-ary 3 0 orthogonal modulator 212. In the preferred embodiment, the 64-ary orthogonal modulator is a Walsh matrix, as shown in FIG. 3.
The six coded symbols are mapped by the equation Co + 2C1 + 4C2 + 8C3 + 16C4 + 32Cs = i, where C0 5 are the coded symbols and i is the index of the output Walsh symbol. Because the symbols are CA 0222096~ 1997-11-13 "
either 1 or 0, the equation uniquely maps the six symbols into one of ~e 64 Walsh indexes. The output of the 6~ary modulator 212 is a Walsh symbol which is made up of 64 Walsh chips (i.e., a row in the Walsh matrix depicted in FIG. 3).
S (~onnectell to the 64-ary modulator 212 is a modulo 2 adder 215 which sums a long pseudorandom noise (PN) sequence 218 with the Walsh chips. The output of the adder 215 is split into an in-phase (I) channel 221 and a quadrature-phase (Q) channel 224.
The I ( h~nnPl 221 includes an adder 227 which sums the output of 1 0 the adder 215 with an I PN sequence 230. The Q channel 224 includes an adder 233 which sums the output of the adder 215 and a Q PN sequence 236. The adder 233 is connected to a delay Pl~ nt 239. At this point, the I and Q data are mapped to +1 (i.e., a zero is mapped into a +1 and a 1 is mapped into a -1) and 1 5 bandpass filtered 242, 243, mixed 245, 246 and summed 248 to create the radio frequency (RF) signal 251 which is transmitted to a MS
105 or 106 via an antenna 254. This results in offset QPSK
modulation of the input voice (or data) signal 203.
The input voice (or data) signal 203 can be coded by the coder 2 0 206 a~ either full rate (9600 bps) 404, half rate (4800 bps) 406, quarter rate (2400 bps) 408, or eighth rate (1200 bps) 410. FIG. 4 shows examples of trAn~mi~sions of variable rate data within a CDMA
Time Frame 400. The time frame 400 is made up of sixteen power control groups (PCGs) 402. The PCGs 402 are made up of six Walsh symbols 412 and each Walsh symbol 412 is defined by 64 Walsh chips 414. Finally, each Walsh chip 414 has four PN chips 418. The incoming data rate, (full, half . . . ) is determined by the voice activity of the user. Periods containing unvoiced speech or silences are encoded at eighth rate, while continuous voiced 3 0 speech might be encoded at full rate. Which power control groups are active during the CDMA Time Frame 100 is determined by the long code 218 and by the amount of voice activity.
FIG. 5 generally depicts a receiver 500 in a BTS 101-103 which determines coding rate in accordance with the invention. In actual CA 0222096~ 1997-11-13 implementation, the receiver 500 to be used would be a four-path (or four-"finger") RAKE receiver, the general structure of which is well known in the art. As shown in FIG. 5, the receiver 500 depicts only a single finger (RAKE 1) of the above-mentioned four-finger 5 RAKE receiver. Referring to FIG. 5, an antenna 503 receives the signal 251 over a radio frequency (RF) communication resource which contains user information coded at one of a plurality of co-ling rates. In the preferred embo-lirn~n~, the coding rates are full rate, 1/2 rate, 1/4 rate and 1/8 rate, implementation of which is 1 0 well known in the art.
Upon reception by the antenna 503, the signal 251 is input into a RF downconverter/sampler 509 which processes the signal 251 with well known techniques to obtain an oversampled (e.g.
eight times oversampled) baseband representation 512 of the signal 1 5 251. The baseband representation 512 is input into a despreader 515 which reverses the known transmission modulation of offset QPSK using the long code PN sequence and the I and Q PN
sequences. The despread signal 518 is then input into a Fast ~A/l~m~rd Transform (FHT) 521, which correlates each group of 2 0 sixty four despread samples correspon~ling to a Walsh symbol 412 of the PCG 402 against each of the sixty four possible Walsh codewords to generate 64 correlations as is known in the art. Each of the 64 correlations at the output of the FHT 521 is magnitude squared 522 and summed, via a summing node 524, with the corresponding squared correlations at the FHT output from the other RAKE fingers, and the summed squared correlations at the output of the summing node 524 are input into a metric generator 527. The metric generator attempts to estimate which Walsh indexes (based on the I and Q magnitudes) were transmitted for 3 0 each Walsh symbol 412 of the PCG 402 and generates the decoding metrics of each of the index bits. The output from the metric generator 527 is input into a Viterbi decoder 530.
As is well known in the art, the Viterbi decoder 530 reconstructs the data 203 which was transmitted in the signal 251.
CA 0222096~ 1997-11-13 ~, The construction is performed for all four possible rates. The Viterbi decoder 530 also generates parameters which are indicative of the likplillood of coding at the plurality of coding rates, and are thus useful in the coding rate determination performed by the 5 receiver 500. As shown in FIG. 5, the parameters include cyclic redundancy check (CRC) information, symbol error rate (SER) information, and quality bit (QB) information. The subscript i associated with each parameter (i.e., CRCi, SERi and QBi) represents parameters being output for each possible coding rate.
Thus, the Viterbi decoder 530 outputs 12 different parameters (CRCfull rate, SERfull rate, QBfull rate, CRcl/2 rate, SERl/2 rate, QBl/2 rate, andeso on) for every frame transmitted. For a general discussion of the SER associated with a Viterbi decoder, see United States Patent 5,321,705 to Gould et al., assigned to the assignee of the present application. For a general discussion of the QB
associated with a Viterbi decoder, see "Viterbi Decoding Algorithm for Convolutional Codes with Repeat Request" by N. Yamamoto and K. Itoh, IEEE Trans on Information Theory, Vol. IT-26, No. 5, Sept. 1980.
It should be noted here that the spread spectrum wireless communication ~y~ defined by IS-95 is capable of supporting difr~lel~t speech service options (as defined in IS-96). For example, one speech service option is the default 8 kbps speech service option, commonly referred to as Rate Set 1. Another speech 2 5 service option is the 14.4 kbps speech service option, defined for personal communication systems (PCS) and commonly referred to as Rate Set 2. The coding rate determination functions described below in accordance with the invention are rate set dependent.
For example, the coding rate determination function for Rate Set 1 3 0 (default speech) exploits a-priori information concerning the allowable rate transitions at the speech coder 206 to discrirnin~te between 1/4 rate frames and 1/8 rate frames. On the other hand, the coding rate determination function for rate set 2 (14.4 kbps speech) makes use of different constraints placed on the speech CA 0222096~ 1997-11-13 ,, coder 206 and also leverages the enhanced CRC coding presented in Rate Set 2 (for Rate Set 1, only the full and 1/2 rates undergo CRC, while for Rate Set 2, all rates undergo CRC). The techniques described in accordance with the invention can be easily extended to other speech service options (for example, the enhanced variable rate coding, or EVRC, speech service option) as desired.
As stated above, the Rate Set 1 coding rate determination flm~ion is based on the estimated channel symbol error rate (SER), decoding quality bit (QB), and CRC checks obtained from the 1 0 Viterbi decoder 530 operating under each rate hypothesis. The Rate Set 1 coding rate determination function is performed in the detection statistic generator block 533 as shown in FIG. 5. In the ~er~lled embodiment, the detection statistic generator block 533 is implemented using a 56166 digital signal processor (DSP).
1 5 FIG. 6 generally depicts, in flow diagram form, the coding rate determin~tion function (for Rate Set 1) in accordance with the invention. Referring to FIG. 6, the coding rate determination process begins by accepting, at step 603, the parameters CRCi, SERi and QBi from the Viterbi decoder 530. The process then combines, 2 0 at step 606, the parameters in a predetermined manner to produce a detection statistic dij, where i represents one possible rate and j another. At steps 609 and 612, the CRC status of the two highest possible rates (i.e., full rate and 1/2 rate) are checked. If both CRCs check (i.e., the tests at steps 609 and 612 are both [Y]es), the 2 5 parameters are combined in a predetermined manner to produce the detection statistic d12. At step 615, the coding rate is determined based on the detection statistic d12. Stated specifically, if the detection statistic d12 at step 615 is greater than 0, then the coding rate of the transmitted data is determined to have been full rate.
3 0 However, if the ~ietection statistic dl2 at step 615 is less than 0, then the coding rate of the transmitted data is determined to have been 1/2 rate. If, from tests 609 and 612, only the full rate CRC checks, the coding rate is declared to have been full rate coding. If, from CA 0222096~ 1997-11-13 tests 609 and 618, only the 1/2 rate CRC checks, the coding rate is declared to have been 1/2 rate coding.
Lf neither of the CRCs check at steps 609'and 6~8 (i.e., the tests at steps 609 and 618 are both [N]o), the parameters are combined in 5 a predetermined manner to produce the detection statistic d48. At step 621, the coding rate is determined based on the detection statistic d48 Stated specifically, if the detection statistic d48. at step 621 is greater than 0, then the coding rate of the trànsmitted data is determined to have been 1/4 rate. However, if the detection statistic d48 at step 621 is less than 0, then the coding rate of the transmitted data is determined to have been 1/8 rate. By using the ~etecti~ n statistics dij which represents the parameters CRCi, CRCj, SERi, SERj" QBi and QBj combined in a predetermined manner, the receiver 500 can accurately determine the coding rate of the 15 data signal tr~n~nnitted in accordance with the invention.
The derivation of the detection statistic, dij, proceeds as follows. First, a 4-dimensional space, ~ is established which is spanned by SERi, SERj, QB, and QBj where SERi and SERj are the norm~li7et1 (to the full rate) symbol error rates for rate i and rate j.
20 This is defined as the count of received convolutionally coded symbols that do not agree with the symbols resulting from re-encoding the decoded information bits. A linear discriminant function is then used to combine the parameters in a predetermined manner to produce the detection statistic:
2 5 d,j(x)--w'x + wo = w,SER, + w2SERj + w3QBz + w4QBj + wo where Wij = [Wl W2 W3 W4 ]' and x = [SERi SERj QBi QBj]' and where w is the weight vector and w0 is the decision boundary.
The selection of u7 directly affects the performance of the 3 0 discrimin~nt function. Obviously, ~ should be determined according to how much discriminant information each variable in contains as well as the physical magnitude each variable takes. It can be shown that the optimal weight vector that minimi7es the rate ~ieri~ion error is:
~~j.l (~7,--17j ) ¦¦~"1 (~7i--~7j ~11 S The parameters, sij and ~ can be experimentally estimated. The resulting detection statistics are (all SERs are normalized to full rate):
d,2 =--0.06297SERI + 0.038845ER2 + QBI--QB2 + 1.0 dl2 =--0.6297SERI +0.3884SER2 d;4 = -0.4723SER, + 0.2203SER4 d;8 =--0.33625ERl +0.1177SER8 d24 = -0.3012SER2 + 0.1988SER4 d28 =--0.2396SER2 + ~-1097SER8 d48 = -0.01535SER4 + 0.00986~SER8 + QB4 - QB8 + 0-7 +
d48 =--0.15355ER4 + 0.098655ER8 10 where S is depenll~nt on the rate of the previous frame:
~ 5~ if rate(n--1) = 2 ~0.0, otherwise i.e., the ~le~ i~ion boundary is biased towards 1/4 rate if the previous frame rate is 1/2 rate.
Important to note is that the Rate Set 1 coding rate 15 determin~tion function takes advantage of the rate constraints defined in the IS-96-A speech coder. The rate constraints imposed is that the data rate is only permitted to decrease by one rate per frarne. This information is incorporated into the decision boundary of the detection statistics dij. That is, the decision boundary is variable according to the data-rate of the previous frame. The decision rule is:
choose rate i if dij(x) > O
choose rate j if djj (x) < O
where i, j = 1, 2, 4, 8 and i ~j.
In the Rate Set 1 coding rate determination flowchart depicted in FIG. 6, the following constants apply:
~, = -45, ~ = ~0. Yl = ~35 1 0 oc~ =--35, ,l~2 =--20, Y2 =--20 C~4=O.~ 4=3~7 r4=-34~ ~;4=-15 a~8 = 3-0, ~B8 = 0-5' Y8 = ~30, ~i8 = ~30 In the ~rerelred embodiment, the discriminant function used to combine the parameters is linear, thus scaling will not affect - performance. Thelefore, the detection statistics can be rescaled to fit various hardware implementations. The a and ,B coefficients 15 should also be scaled by the same corresponding factors.
Once the coding rate has been determined, the receiver 500 can determine whether the frame received is either to be kept (i.e., a good frarne) or erased (i.e., a bad frame). FIG. 7 generally depicts, in flow diagram form, the process for erasing a frame using 2 0 detection statistics d 'ii in accordance with the invention. For frames declared full rate, the CRC and d'lj (j=2, 4, 8) are used for erasure detection. For half rate frames, the CRC and d'lz, d'z~ (j=4, 8) as well as the quality bit QB, at step 703, are used to check if the frame is good. Since there are no CRCs for the two lowest rates, 25 erasure detection is carried out by checking quality bit QB and corresponding d'ijs at step 706, 709, 710, and 712.
CA 0222096~ 1997-11-13 The coding rate determination function for Rate Set 2 is different to some degree from that of Rate Set 1 since, in Rate Set 1, 1/4 and 1/8 rate frames are not provided with a CRC while in Rate Set 2, all rates are provided with a CRC. ~l~lition~ïly, in Rate Set I, 5 full rate frames are further protected by a BCH codeword in the speech coder 206, which is not the case for Rate Set 2. Finally, Rate Set 1 and 2 place different rate constraints on the speech coder 206.
As noted above, the coding rate constraints imposed in Rate Set 1 is that the data rate is only permitted to decrease by one rate per 10 frame; in Rate Set 2, the data rate is permitted to decrease by no more than two- rates per frame.
FIG. 8 generally depicts, in flow-diagram form, the Rate Set 2 coding rate de~ermination function in accordance with the inv~rltiorl. As in the Rate Set 1 function, the Rate Set 2 coding rate 15 determin~tion begins by accepting, at step 803, the parameters CRCi, SERi and QBi (i=1, 2, 4, 8) from the Viterbi decoder 530. The process then combines, at step 806, the parameters in a predetermined manner to produce a detection statistic dij, where i represents one possible rate and j another. The process then 2 0 checks, at step 809, whether all of the CRCs are bad. If all CRCs are bad, the frame is more than likely a bad frame (regardless of its rate), thus that frame is erased. If one of the CRCs is good, the process proceeds to step 812, where the CRC for full rate is checked.
If at steps 812 and 815 the CRCs for full and 1/2 rate are good, the 2 5 detection statistic dl2 is used to determine the coding rate.
Specifically, at step 818, if the detection statistic dlz is greater than zero, a full rate frame is declared to have been transmitted. If, however, the detection statistic d12 is less than zero, a 1/2 rate frame is declared to have been transmitted.
3 0 Referring back to step 815, if the CRC for 1/2 rate is not good, the process proceeds to step 821 where the CRC for 1/4 is checked.
If that CRC is good, the detection statistic d14 is used to determine the coding rate. Specifically, at step 824, if the detection statistic d14 CA 0222096~ 1997-11-13 is greater than zero, a full rate frame is declared to have been transmitted. If, however, the detection statistic dl~ is less than zero, a 1 /4 rate frame is dedared to have been -transrnitted.
Referring back to step 821, if the CRC for 1 /4 rate is not good, S the process proceeds to step 827 where the CRC for 1/8 is cherke~l-If ~hat CRC is not good, a full rate frame is declared to have been transmitted. If that CRC is good, the detection statistic dl8 is used to determine the coding rate. Specifically, at step 830, if the detection statistic d1g is greater than zero, a full rate frame is 1 0 declared to have been transmitted. If, however, the detection statistic d1g is less than zero, a 1/8 rate frame is declared to have been transmitted.
An entirely different set of detection statistics are impl~n~nte~ if the test at step 812 is negative. In this situation, 1 5 the process proceeds to step 833 where the CRC for 1/2 rate is checked. If that CRC is good, then the CRC for 1/4 rate is checked at step 836 and, if good, the ~letectiQn statistic d24 is used to determine the coding rate. Specifically, at step 839, if the detection statistic d24 is greater than zero, a 1/2 rate frame is declared to have been 20 transmitted. If, however, the detection statistic d24 is less than zero, a 1/4 rate frame is dedared to have been transmitted.
Referring back to step 836, if the CRC for 1 /4 rate is not good, the process proceeds to step 842 where the CRC for 1/8 rate is checked. If that CRC is bad, a 1/2 rate frame is declared to have 25 been transmitted. If, however, that CRC is good, the detection statistic d28 is used to determine the coding rate. Specifically, at step 845, if the detection statistic d28 is greater than zero, a 1/2 rate frame is declared to have been transmitted. If, however, the detection statistic d28 is less than zero, a 1/8 rate frame is declared 3 0 to have been transmitted.
Referring back to step 833, if the CRC for 1 /2 rate is not good, then the process proceeds to step 848 where the detection statistic CA 0222096~ 1997-11-13 d48 is used to determine the coding rate. Specifically, at step 848, if tl~e detection statistic d48 is greater than zero, a 1/4 rate frame is declared to have been transmitted. If, however, the detection statistic d48 is less than zero, a 1/8 rate frame is declared to have 5 been transmitted. By using the detection statistics dij in accordance with the invention, the receiver is able to accurately determine the coding rate of the data signal transmitted in accordance with the invention.
As in the case for Rate Set 1, once the coding rate is 10 determined, the receiver 500 can determine whether the frame received is either to be kept (i.e., a good frame) or erased (i.e., a bad frame). FIG. 9 generally depicts, in flow diagram form, the process for erasing a frame using detection statistics dij in accordance with the invention. Referring to FIG. 8 and FIG. 9, steps 903 and 906 15 check the full and 1/2 rates, and an erasure is declared if either the associated dijs or the quality bit do not check. For the two lowest rates, erasure detection is carried out by checking CRC, QB and associated d~js as shown in steps 909-914.
As stated above, for Rate Set 2, the coding rate is permitted to 2 0 decrease by no more than two rates per frame. Stated differently, for a Rate Set 2 coder, if the previous frame was selected for full rate coding, the current frame will can only be coded at 1/2 or 1/4 rate, but not at 1/8 rate. This constraint is taken into account in the detection statistics dij for Rate Set 2 listed below.
CA 02220965 l997-ll-l3 d,2 = ~.06382SERl + 0.03849SER2 + QBI--QB2 + 2.3 d;2 = ~.6382SERI + 0.3849SER2 d,4 =~.04468SE~ +0.02237SER4 +QBI--~B4 +2.1 d;4 =-0.4468SERI +0.2237SER4 dl8 =~.03285SER, +0.01181SER8 +QBl--QB8 +1.7 d;8 =-0.3285SER, +0.1181SER8 d24 =~.033675ER2 +0.01848SER4 +QB2--QB4 +1-5 d24 =-0.3367SER2 +0.18485ER4 d28 = ~.02512SER2 + 0.0108 lSER8 + QB2--QB8 + 1.3 d28 =--0.2512SER2 + 0.108 lSER8 -~ -d48 = ~.01706SER4 + 0.0091 38SER8 + QB4--QB8 + 0.8 + ~; -d48 =--0.1706SER4 +0.09138SER8 where 8=0.2 if the previous frame rate is full rate and zero otherwise. ~l~litionally, the following constants apply for the Rate Set 2 coding rate determination function described above with 5 reference to FIG. 8:
40, ,Bl =-30, y, =-25 ~2=-1~, ~2=-257 Y2=-20 CX4 = -8, ~4 = 3, y4 = -14, ~4 = 0.5 (X8 =-12, 138 =-8, Y8 --2, ~8 =-0 5 Finally, the scaling rule described above with reference to Rate Set 1 is likewise valid for Rate Set 2.
While the invention has been particularly shown and described with reference to a=particular embodiment, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. For example, coding rate CA 0222096~ 1997-11-13 determination in accordance with the invention has been described as being implemented in a receiver in a BTS, but the coding rate determination can likewise be implemented in a receiver in a MS. Additionally, all parameters CRCi, SERi and QBi 5 have been combined in a predetermined manner in the preferred embodiment to produce the detection statistic dij, but various combinations of the parameters can be beneficially utilized without departing from the spirit and scope of the invention. The co,~e~onding structures, materials, acts and equivalents of all 10 means or step plus function elements in the claims below are in~n~le-l to include any structure, material, or acts for performing the functions in combination with other claimed elements as spe~ific~lly claimed.
What we claim is:
IN A WIRELESS COMMUNIGATION SYSTEM
S FIELD OF THE INVENTION
The present invention relates generally to wireless communication ~ysLems and, more particularly, to coding rate determination in such wireless communication ~y~lell~s.
BACKGROUND OF THE INVENTION
The current Code Division Multiple Access (CDMA) 15 wireless communication system envisioned for, inter alia, cellular applications in the United States is defined by TIA/EIA/IS-95, Mobile Station-~ase Station Compatibili~y Standard for DUQI Mode Wideband Spread Spectrum Cellular System, July 1993. As is well known in the art, limiting the total amount of system interference 2 0 in a spread spectrum system results in an increase in user capacity.
One technique implemented in the IS-95 spread spectrum system to limit the total amount of system interference is variable rate coding.
Variable rate coding in a wireless communication system 2 5 such as IS-95 takes advantage of the inherent silences of a typical voice communication. In IS-95, coding occurs in 20 millisecond (ms) blocks at either full rate, 1/2 rate, 1/4 rate, or 1/8 rate. If, for example, a 20 ms block to be transmitted to a user of the spread spectrum system contained unvoiced speech or silences but was 3 0 coded at full rate, interference would be added to the system llnnec~s~rily. Such a frame can be transmitted as a 1/8 rate frame without effecting the speech quality while reducing the total amount of interference presented to the system. The 1/2 rate and 1/4 rate are transitional rates. As stated above, reducing the total ~ ~ CA 0222096~ 1997-11-13 amount of interference presented to the system results in an increase in user capacity.
While the method for determining which coding rate to use on the transmitter side is well rl~fine~l in document TIA/EIA/IS-5 96, Speech Service Option Standard for Wideband Spread Spectrum Digft~l Cellular System, April 1993 (Official Ballot Version), a receiver receiving such a variable rate coded tr~n~Inission has no knowledge of which rate was actually tr~n~mit~erl Since the receiver must have some knowledge of the 10 coding rate to perform an accurate reception/demodulation of the transmission, accurately determining the coding rate in the receiver is critical to operation of a IS-95/IS-96 compatible receiver.
Thus, a need exists for a method of accurately determining the coding rate in a receiver capable of receiving variable rate coded 1 5 transmissions.
BRIEF DESCRIP rToN OF THE DRAWINGS
2 0 FIG. 1 generally depicts a block diagram of a wireless communication system which may beneficially implement coding rate determination in accordance with the invention.
FIG. 2 generally depicts a block diagram of a transmitter impl~nl.onte~ in a mobile station compatible with a spread spectrum 2 ~ wireless communication system defined by IS-95.
FIG. 3 depicts a Walsh matrix utilized in a spread spectrum wireless communication system compatible with IS-95.
FIG. 4 generally depicts transmissions of various rate data during a CDMA time frame.
3 0 FIG. 5 generally depicts a receiver implemented in a base transceiver station (BTS) which determines coding rate in accordance with the invention.
~ ~ ,CA 0222096~ 1997-11-13 ~ , .
FIG. 6 generally depicts, in flow diagram form, the coding rate de~ tion filnction (for Rate Set 1) in accordance with the invention.
FIG. 7 generally depicts, in flow diagram form, the process S for erasing a frame (for Rate Se~ 1) using detection statistics dij in accordance with the invention.
~ IG. 8 generally depicts, in flow diagram form, the coding rate determin~t on function (for Rate Set 2) in accordance with the invention.
10FIG. 9 generally depicts, in flow diagram form, the process for erasing a frame (for Rate Set 2) using detection statistics dij in accordance with the invention.
Generally stated, a method of determining coding rate in a receiver implemented in a wireless communication system is disclosed. The receiver is responsive to a communication resource 2 0 conPinin~ user information coded at one of a plurality of coding rates. The method generates parameters indicative of the likelihclod of coding at the plurality of coding rates and combines the parameters in a predetermined manner to produce a detection statistic. Using the detection statistic, the coding rate is then 2 5 determine-l-In the preferred embodiment, the parameters indicative ofthe lik~olihood of coding at the plurality of coding rates further comprise cyclic redundancy check (CRC) information, symbol error rate (SER) information, and quality bit (QB) information. Likewise 3 0 in the preferred embodiment, the parameters are combined in a linear discrimin~nt function to produce the detection statistic, wherein the detection statistic represents two possible coding rates.
-- ~ CA 0222096~ 1997-11-13 The above method is executed in a receiver, which comprises a receiver front end which receives and demodulates the communication resource to produce demodulated user information. A decoder, which is coupled to thë receiver front end, then decodes the demodulated user information and generates parameters indicative of the likelihood of coding at the plurality of coding rates. A processor, which is coupled to the decoder, combines the parameters in a predetermined manner to produce a detection statistic and determines the coding rate based on the (letectit~n statistic.
FIG. 1 generally depicts a block diagram of a wireless communication ~yslelll 100 which may beneficially implement coding rate determination in accordance with the invention. In the ~ler~lled embodiment, the wireless communication ~yslem is a code division multiple access (CDMA) cellular radiotelephone syslelll. As one of ordinary skill in the art will appreciate, however, the coding rate determination in accordance with the invention can be implemented in any wireless communication ~y~l~m which implements variable rate coding.
2 0 Referring to FIG. 1, acronyms are used for conv~ni~nce The following is a list of definitions for the acronyms used in l~IG. 1:
BTS Base Transceiver Station CBSC Centralized Base Station Controller 2~5 EC Echo Canceller VLR Visitor Location Register HLR Home Location Register ISDN Integrated Services Digital Network MS Mobile Station 3 0 MSC Mobile Switching Center MM Mobility Manager OMCR Operations and Maintenance Center - Radio OMCS Operations and Maintenance Center - Switch PSTN Public Switched Telephone Network 3 5 TC Transcoder - ~ CA 0222096~ 1997-11-13 As seen in FIG. 1, multiple BTSs 101-103 are coupled to a CBSC 104. Each BTS 101-103 provides radio frequency (RF) communication to an MS 105-106. In the preferred embodiment, the transmitter/receiver (transceiver) hardware impl.ornente~l in the BTSs 101-103 and the MSs 105-106 to support conveyance of the RF communication resource is substantially defined in the document titled by TIA/EIA/IS-95, Mobile St~tion-Base Sfatiorl Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System, July 1993, available from the Telecommunication Industry Association (TIA). The CBSC 104 is responsible for, inter alia, call processing via the TC 110 and mobility management via the MM 109. Other tasks of the CBSC 104 include feature control and tr~n~missiQn/networking interfacing. For more information on the functionality of the CBSC 104, reference is made to United States Patent 5,475,686 to Bach et al., assigned to the assignee of the present application, and incorporated herein by refe~ ce.
Also depicted in FIG. 1 is an OMCR 112 coupled to the MM
109 of the CBSC 104. The OMCR 112 is responsible for the operations and general maintenance of the radio portion (CBSC
104 and BTS 101-103 combination) of the communication system 100. The CBSC 104 is coupled to an MSC 115 which provides switching capability between the PSTN 120/ISDN 122 and the CBSC 104. The OMCS 124 is responsible for the operations and general maintenance of the switching portion (MSC 115) of the 2~ communication ~y~Lell- 100. The HLR 116 and VLR 117 provide the communication system 100 with user information primarily used for billing purposes. ECs 111 and 119 are implemented to improve the quality of speech signal transferred through the communication ~y~lelll 100.
3 0 The functionality of the CBSC 104, MSC 115, HLR 116 and ~ILR 117 is shown in FIG. 1 as distributed, however one of ordinary skill in the art will appreciate that the functionality could likewise be centralized into a single element. Also, for different configurations, the TC 110 could be located at either the MSC 115 or - ' ~ CA 0222096~ l997-ll-l3 a BTS 101-103. The link 126 coupling the MSC 115 with the CBSC 104 is a T1/E1 link which is well known in the art. By placing the TC 110 at the CBSC, a 4:1 improvement in link budget is reAli7e~1 due to compression of the input signal (input from the T1/E1 link 126) by ~he TC 110. The compressed signal is transferred to a particular BTS 101-103 for tr~nsmi~sion to a particular MS 105-106. Important to note is that the compressed signal transferred to a particular BTS 101-103 undergoes further processing at the BTS
101-103 before tr~n~mission occurs. Put differently, the eventual 1 0 signal transmitted to the MS 105-106 is different in form but the sarne in substance as the compressed signal exiting the TC 110.
When the MS 105-106 receives the signal transmitted by a BTS 101-103, the MS 105-106 will essentially "undo" (commonly referred to as "decode") all of the processing performed by the 1 5 SySI~ 100. When the MS 105-106 transmits a signal back to a BTS
101-103, the MS 105-106 likewise implements its own processing.
After a signal having undergone processing is transmitted by the MS 105-106 (the processing of the signal is to change the form, but not the substance, of the signal) to a BTS 101-103, the BTS 101-103 2 0 will "undo" the processing performed on the signal and transfer to the a~ro~iate point within the system 100. Eventually, the signal will be transferred to an end user via the T1/E1 link 126.
A block diagram of an IS-95 compatible transmitter 200 for use in the system depicted in FIG. 1 is generally depicted in FIG. 2.
2 5 In this embodiment, the transmitter 200 resides in any of the BTSs 101-103. A voice (or data) signal 203 is input to a coding/interleaving section 206, resulting in a coded/interleaved signal 209. The coded/interleaved signal 209 is mapped, plerelably, six symbols at a time, to a unique 64-ary symbol by a 64-ary 3 0 orthogonal modulator 212. In the preferred embodiment, the 64-ary orthogonal modulator is a Walsh matrix, as shown in FIG. 3.
The six coded symbols are mapped by the equation Co + 2C1 + 4C2 + 8C3 + 16C4 + 32Cs = i, where C0 5 are the coded symbols and i is the index of the output Walsh symbol. Because the symbols are CA 0222096~ 1997-11-13 "
either 1 or 0, the equation uniquely maps the six symbols into one of ~e 64 Walsh indexes. The output of the 6~ary modulator 212 is a Walsh symbol which is made up of 64 Walsh chips (i.e., a row in the Walsh matrix depicted in FIG. 3).
S (~onnectell to the 64-ary modulator 212 is a modulo 2 adder 215 which sums a long pseudorandom noise (PN) sequence 218 with the Walsh chips. The output of the adder 215 is split into an in-phase (I) channel 221 and a quadrature-phase (Q) channel 224.
The I ( h~nnPl 221 includes an adder 227 which sums the output of 1 0 the adder 215 with an I PN sequence 230. The Q channel 224 includes an adder 233 which sums the output of the adder 215 and a Q PN sequence 236. The adder 233 is connected to a delay Pl~ nt 239. At this point, the I and Q data are mapped to +1 (i.e., a zero is mapped into a +1 and a 1 is mapped into a -1) and 1 5 bandpass filtered 242, 243, mixed 245, 246 and summed 248 to create the radio frequency (RF) signal 251 which is transmitted to a MS
105 or 106 via an antenna 254. This results in offset QPSK
modulation of the input voice (or data) signal 203.
The input voice (or data) signal 203 can be coded by the coder 2 0 206 a~ either full rate (9600 bps) 404, half rate (4800 bps) 406, quarter rate (2400 bps) 408, or eighth rate (1200 bps) 410. FIG. 4 shows examples of trAn~mi~sions of variable rate data within a CDMA
Time Frame 400. The time frame 400 is made up of sixteen power control groups (PCGs) 402. The PCGs 402 are made up of six Walsh symbols 412 and each Walsh symbol 412 is defined by 64 Walsh chips 414. Finally, each Walsh chip 414 has four PN chips 418. The incoming data rate, (full, half . . . ) is determined by the voice activity of the user. Periods containing unvoiced speech or silences are encoded at eighth rate, while continuous voiced 3 0 speech might be encoded at full rate. Which power control groups are active during the CDMA Time Frame 100 is determined by the long code 218 and by the amount of voice activity.
FIG. 5 generally depicts a receiver 500 in a BTS 101-103 which determines coding rate in accordance with the invention. In actual CA 0222096~ 1997-11-13 implementation, the receiver 500 to be used would be a four-path (or four-"finger") RAKE receiver, the general structure of which is well known in the art. As shown in FIG. 5, the receiver 500 depicts only a single finger (RAKE 1) of the above-mentioned four-finger 5 RAKE receiver. Referring to FIG. 5, an antenna 503 receives the signal 251 over a radio frequency (RF) communication resource which contains user information coded at one of a plurality of co-ling rates. In the preferred embo-lirn~n~, the coding rates are full rate, 1/2 rate, 1/4 rate and 1/8 rate, implementation of which is 1 0 well known in the art.
Upon reception by the antenna 503, the signal 251 is input into a RF downconverter/sampler 509 which processes the signal 251 with well known techniques to obtain an oversampled (e.g.
eight times oversampled) baseband representation 512 of the signal 1 5 251. The baseband representation 512 is input into a despreader 515 which reverses the known transmission modulation of offset QPSK using the long code PN sequence and the I and Q PN
sequences. The despread signal 518 is then input into a Fast ~A/l~m~rd Transform (FHT) 521, which correlates each group of 2 0 sixty four despread samples correspon~ling to a Walsh symbol 412 of the PCG 402 against each of the sixty four possible Walsh codewords to generate 64 correlations as is known in the art. Each of the 64 correlations at the output of the FHT 521 is magnitude squared 522 and summed, via a summing node 524, with the corresponding squared correlations at the FHT output from the other RAKE fingers, and the summed squared correlations at the output of the summing node 524 are input into a metric generator 527. The metric generator attempts to estimate which Walsh indexes (based on the I and Q magnitudes) were transmitted for 3 0 each Walsh symbol 412 of the PCG 402 and generates the decoding metrics of each of the index bits. The output from the metric generator 527 is input into a Viterbi decoder 530.
As is well known in the art, the Viterbi decoder 530 reconstructs the data 203 which was transmitted in the signal 251.
CA 0222096~ 1997-11-13 ~, The construction is performed for all four possible rates. The Viterbi decoder 530 also generates parameters which are indicative of the likplillood of coding at the plurality of coding rates, and are thus useful in the coding rate determination performed by the 5 receiver 500. As shown in FIG. 5, the parameters include cyclic redundancy check (CRC) information, symbol error rate (SER) information, and quality bit (QB) information. The subscript i associated with each parameter (i.e., CRCi, SERi and QBi) represents parameters being output for each possible coding rate.
Thus, the Viterbi decoder 530 outputs 12 different parameters (CRCfull rate, SERfull rate, QBfull rate, CRcl/2 rate, SERl/2 rate, QBl/2 rate, andeso on) for every frame transmitted. For a general discussion of the SER associated with a Viterbi decoder, see United States Patent 5,321,705 to Gould et al., assigned to the assignee of the present application. For a general discussion of the QB
associated with a Viterbi decoder, see "Viterbi Decoding Algorithm for Convolutional Codes with Repeat Request" by N. Yamamoto and K. Itoh, IEEE Trans on Information Theory, Vol. IT-26, No. 5, Sept. 1980.
It should be noted here that the spread spectrum wireless communication ~y~ defined by IS-95 is capable of supporting difr~lel~t speech service options (as defined in IS-96). For example, one speech service option is the default 8 kbps speech service option, commonly referred to as Rate Set 1. Another speech 2 5 service option is the 14.4 kbps speech service option, defined for personal communication systems (PCS) and commonly referred to as Rate Set 2. The coding rate determination functions described below in accordance with the invention are rate set dependent.
For example, the coding rate determination function for Rate Set 1 3 0 (default speech) exploits a-priori information concerning the allowable rate transitions at the speech coder 206 to discrirnin~te between 1/4 rate frames and 1/8 rate frames. On the other hand, the coding rate determination function for rate set 2 (14.4 kbps speech) makes use of different constraints placed on the speech CA 0222096~ 1997-11-13 ,, coder 206 and also leverages the enhanced CRC coding presented in Rate Set 2 (for Rate Set 1, only the full and 1/2 rates undergo CRC, while for Rate Set 2, all rates undergo CRC). The techniques described in accordance with the invention can be easily extended to other speech service options (for example, the enhanced variable rate coding, or EVRC, speech service option) as desired.
As stated above, the Rate Set 1 coding rate determination flm~ion is based on the estimated channel symbol error rate (SER), decoding quality bit (QB), and CRC checks obtained from the 1 0 Viterbi decoder 530 operating under each rate hypothesis. The Rate Set 1 coding rate determination function is performed in the detection statistic generator block 533 as shown in FIG. 5. In the ~er~lled embodiment, the detection statistic generator block 533 is implemented using a 56166 digital signal processor (DSP).
1 5 FIG. 6 generally depicts, in flow diagram form, the coding rate determin~tion function (for Rate Set 1) in accordance with the invention. Referring to FIG. 6, the coding rate determination process begins by accepting, at step 603, the parameters CRCi, SERi and QBi from the Viterbi decoder 530. The process then combines, 2 0 at step 606, the parameters in a predetermined manner to produce a detection statistic dij, where i represents one possible rate and j another. At steps 609 and 612, the CRC status of the two highest possible rates (i.e., full rate and 1/2 rate) are checked. If both CRCs check (i.e., the tests at steps 609 and 612 are both [Y]es), the 2 5 parameters are combined in a predetermined manner to produce the detection statistic d12. At step 615, the coding rate is determined based on the detection statistic d12. Stated specifically, if the detection statistic d12 at step 615 is greater than 0, then the coding rate of the transmitted data is determined to have been full rate.
3 0 However, if the ~ietection statistic dl2 at step 615 is less than 0, then the coding rate of the transmitted data is determined to have been 1/2 rate. If, from tests 609 and 612, only the full rate CRC checks, the coding rate is declared to have been full rate coding. If, from CA 0222096~ 1997-11-13 tests 609 and 618, only the 1/2 rate CRC checks, the coding rate is declared to have been 1/2 rate coding.
Lf neither of the CRCs check at steps 609'and 6~8 (i.e., the tests at steps 609 and 618 are both [N]o), the parameters are combined in 5 a predetermined manner to produce the detection statistic d48. At step 621, the coding rate is determined based on the detection statistic d48 Stated specifically, if the detection statistic d48. at step 621 is greater than 0, then the coding rate of the trànsmitted data is determined to have been 1/4 rate. However, if the detection statistic d48 at step 621 is less than 0, then the coding rate of the transmitted data is determined to have been 1/8 rate. By using the ~etecti~ n statistics dij which represents the parameters CRCi, CRCj, SERi, SERj" QBi and QBj combined in a predetermined manner, the receiver 500 can accurately determine the coding rate of the 15 data signal tr~n~nnitted in accordance with the invention.
The derivation of the detection statistic, dij, proceeds as follows. First, a 4-dimensional space, ~ is established which is spanned by SERi, SERj, QB, and QBj where SERi and SERj are the norm~li7et1 (to the full rate) symbol error rates for rate i and rate j.
20 This is defined as the count of received convolutionally coded symbols that do not agree with the symbols resulting from re-encoding the decoded information bits. A linear discriminant function is then used to combine the parameters in a predetermined manner to produce the detection statistic:
2 5 d,j(x)--w'x + wo = w,SER, + w2SERj + w3QBz + w4QBj + wo where Wij = [Wl W2 W3 W4 ]' and x = [SERi SERj QBi QBj]' and where w is the weight vector and w0 is the decision boundary.
The selection of u7 directly affects the performance of the 3 0 discrimin~nt function. Obviously, ~ should be determined according to how much discriminant information each variable in contains as well as the physical magnitude each variable takes. It can be shown that the optimal weight vector that minimi7es the rate ~ieri~ion error is:
~~j.l (~7,--17j ) ¦¦~"1 (~7i--~7j ~11 S The parameters, sij and ~ can be experimentally estimated. The resulting detection statistics are (all SERs are normalized to full rate):
d,2 =--0.06297SERI + 0.038845ER2 + QBI--QB2 + 1.0 dl2 =--0.6297SERI +0.3884SER2 d;4 = -0.4723SER, + 0.2203SER4 d;8 =--0.33625ERl +0.1177SER8 d24 = -0.3012SER2 + 0.1988SER4 d28 =--0.2396SER2 + ~-1097SER8 d48 = -0.01535SER4 + 0.00986~SER8 + QB4 - QB8 + 0-7 +
d48 =--0.15355ER4 + 0.098655ER8 10 where S is depenll~nt on the rate of the previous frame:
~ 5~ if rate(n--1) = 2 ~0.0, otherwise i.e., the ~le~ i~ion boundary is biased towards 1/4 rate if the previous frame rate is 1/2 rate.
Important to note is that the Rate Set 1 coding rate 15 determin~tion function takes advantage of the rate constraints defined in the IS-96-A speech coder. The rate constraints imposed is that the data rate is only permitted to decrease by one rate per frarne. This information is incorporated into the decision boundary of the detection statistics dij. That is, the decision boundary is variable according to the data-rate of the previous frame. The decision rule is:
choose rate i if dij(x) > O
choose rate j if djj (x) < O
where i, j = 1, 2, 4, 8 and i ~j.
In the Rate Set 1 coding rate determination flowchart depicted in FIG. 6, the following constants apply:
~, = -45, ~ = ~0. Yl = ~35 1 0 oc~ =--35, ,l~2 =--20, Y2 =--20 C~4=O.~ 4=3~7 r4=-34~ ~;4=-15 a~8 = 3-0, ~B8 = 0-5' Y8 = ~30, ~i8 = ~30 In the ~rerelred embodiment, the discriminant function used to combine the parameters is linear, thus scaling will not affect - performance. Thelefore, the detection statistics can be rescaled to fit various hardware implementations. The a and ,B coefficients 15 should also be scaled by the same corresponding factors.
Once the coding rate has been determined, the receiver 500 can determine whether the frame received is either to be kept (i.e., a good frarne) or erased (i.e., a bad frame). FIG. 7 generally depicts, in flow diagram form, the process for erasing a frame using 2 0 detection statistics d 'ii in accordance with the invention. For frames declared full rate, the CRC and d'lj (j=2, 4, 8) are used for erasure detection. For half rate frames, the CRC and d'lz, d'z~ (j=4, 8) as well as the quality bit QB, at step 703, are used to check if the frame is good. Since there are no CRCs for the two lowest rates, 25 erasure detection is carried out by checking quality bit QB and corresponding d'ijs at step 706, 709, 710, and 712.
CA 0222096~ 1997-11-13 The coding rate determination function for Rate Set 2 is different to some degree from that of Rate Set 1 since, in Rate Set 1, 1/4 and 1/8 rate frames are not provided with a CRC while in Rate Set 2, all rates are provided with a CRC. ~l~lition~ïly, in Rate Set I, 5 full rate frames are further protected by a BCH codeword in the speech coder 206, which is not the case for Rate Set 2. Finally, Rate Set 1 and 2 place different rate constraints on the speech coder 206.
As noted above, the coding rate constraints imposed in Rate Set 1 is that the data rate is only permitted to decrease by one rate per 10 frame; in Rate Set 2, the data rate is permitted to decrease by no more than two- rates per frame.
FIG. 8 generally depicts, in flow-diagram form, the Rate Set 2 coding rate de~ermination function in accordance with the inv~rltiorl. As in the Rate Set 1 function, the Rate Set 2 coding rate 15 determin~tion begins by accepting, at step 803, the parameters CRCi, SERi and QBi (i=1, 2, 4, 8) from the Viterbi decoder 530. The process then combines, at step 806, the parameters in a predetermined manner to produce a detection statistic dij, where i represents one possible rate and j another. The process then 2 0 checks, at step 809, whether all of the CRCs are bad. If all CRCs are bad, the frame is more than likely a bad frame (regardless of its rate), thus that frame is erased. If one of the CRCs is good, the process proceeds to step 812, where the CRC for full rate is checked.
If at steps 812 and 815 the CRCs for full and 1/2 rate are good, the 2 5 detection statistic dl2 is used to determine the coding rate.
Specifically, at step 818, if the detection statistic dlz is greater than zero, a full rate frame is declared to have been transmitted. If, however, the detection statistic d12 is less than zero, a 1/2 rate frame is declared to have been transmitted.
3 0 Referring back to step 815, if the CRC for 1/2 rate is not good, the process proceeds to step 821 where the CRC for 1/4 is checked.
If that CRC is good, the detection statistic d14 is used to determine the coding rate. Specifically, at step 824, if the detection statistic d14 CA 0222096~ 1997-11-13 is greater than zero, a full rate frame is declared to have been transmitted. If, however, the detection statistic dl~ is less than zero, a 1 /4 rate frame is dedared to have been -transrnitted.
Referring back to step 821, if the CRC for 1 /4 rate is not good, S the process proceeds to step 827 where the CRC for 1/8 is cherke~l-If ~hat CRC is not good, a full rate frame is declared to have been transmitted. If that CRC is good, the detection statistic dl8 is used to determine the coding rate. Specifically, at step 830, if the detection statistic d1g is greater than zero, a full rate frame is 1 0 declared to have been transmitted. If, however, the detection statistic d1g is less than zero, a 1/8 rate frame is declared to have been transmitted.
An entirely different set of detection statistics are impl~n~nte~ if the test at step 812 is negative. In this situation, 1 5 the process proceeds to step 833 where the CRC for 1/2 rate is checked. If that CRC is good, then the CRC for 1/4 rate is checked at step 836 and, if good, the ~letectiQn statistic d24 is used to determine the coding rate. Specifically, at step 839, if the detection statistic d24 is greater than zero, a 1/2 rate frame is declared to have been 20 transmitted. If, however, the detection statistic d24 is less than zero, a 1/4 rate frame is dedared to have been transmitted.
Referring back to step 836, if the CRC for 1 /4 rate is not good, the process proceeds to step 842 where the CRC for 1/8 rate is checked. If that CRC is bad, a 1/2 rate frame is declared to have 25 been transmitted. If, however, that CRC is good, the detection statistic d28 is used to determine the coding rate. Specifically, at step 845, if the detection statistic d28 is greater than zero, a 1/2 rate frame is declared to have been transmitted. If, however, the detection statistic d28 is less than zero, a 1/8 rate frame is declared 3 0 to have been transmitted.
Referring back to step 833, if the CRC for 1 /2 rate is not good, then the process proceeds to step 848 where the detection statistic CA 0222096~ 1997-11-13 d48 is used to determine the coding rate. Specifically, at step 848, if tl~e detection statistic d48 is greater than zero, a 1/4 rate frame is declared to have been transmitted. If, however, the detection statistic d48 is less than zero, a 1/8 rate frame is declared to have 5 been transmitted. By using the detection statistics dij in accordance with the invention, the receiver is able to accurately determine the coding rate of the data signal transmitted in accordance with the invention.
As in the case for Rate Set 1, once the coding rate is 10 determined, the receiver 500 can determine whether the frame received is either to be kept (i.e., a good frame) or erased (i.e., a bad frame). FIG. 9 generally depicts, in flow diagram form, the process for erasing a frame using detection statistics dij in accordance with the invention. Referring to FIG. 8 and FIG. 9, steps 903 and 906 15 check the full and 1/2 rates, and an erasure is declared if either the associated dijs or the quality bit do not check. For the two lowest rates, erasure detection is carried out by checking CRC, QB and associated d~js as shown in steps 909-914.
As stated above, for Rate Set 2, the coding rate is permitted to 2 0 decrease by no more than two rates per frame. Stated differently, for a Rate Set 2 coder, if the previous frame was selected for full rate coding, the current frame will can only be coded at 1/2 or 1/4 rate, but not at 1/8 rate. This constraint is taken into account in the detection statistics dij for Rate Set 2 listed below.
CA 02220965 l997-ll-l3 d,2 = ~.06382SERl + 0.03849SER2 + QBI--QB2 + 2.3 d;2 = ~.6382SERI + 0.3849SER2 d,4 =~.04468SE~ +0.02237SER4 +QBI--~B4 +2.1 d;4 =-0.4468SERI +0.2237SER4 dl8 =~.03285SER, +0.01181SER8 +QBl--QB8 +1.7 d;8 =-0.3285SER, +0.1181SER8 d24 =~.033675ER2 +0.01848SER4 +QB2--QB4 +1-5 d24 =-0.3367SER2 +0.18485ER4 d28 = ~.02512SER2 + 0.0108 lSER8 + QB2--QB8 + 1.3 d28 =--0.2512SER2 + 0.108 lSER8 -~ -d48 = ~.01706SER4 + 0.0091 38SER8 + QB4--QB8 + 0.8 + ~; -d48 =--0.1706SER4 +0.09138SER8 where 8=0.2 if the previous frame rate is full rate and zero otherwise. ~l~litionally, the following constants apply for the Rate Set 2 coding rate determination function described above with 5 reference to FIG. 8:
40, ,Bl =-30, y, =-25 ~2=-1~, ~2=-257 Y2=-20 CX4 = -8, ~4 = 3, y4 = -14, ~4 = 0.5 (X8 =-12, 138 =-8, Y8 --2, ~8 =-0 5 Finally, the scaling rule described above with reference to Rate Set 1 is likewise valid for Rate Set 2.
While the invention has been particularly shown and described with reference to a=particular embodiment, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. For example, coding rate CA 0222096~ 1997-11-13 determination in accordance with the invention has been described as being implemented in a receiver in a BTS, but the coding rate determination can likewise be implemented in a receiver in a MS. Additionally, all parameters CRCi, SERi and QBi 5 have been combined in a predetermined manner in the preferred embodiment to produce the detection statistic dij, but various combinations of the parameters can be beneficially utilized without departing from the spirit and scope of the invention. The co,~e~onding structures, materials, acts and equivalents of all 10 means or step plus function elements in the claims below are in~n~le-l to include any structure, material, or acts for performing the functions in combination with other claimed elements as spe~ific~lly claimed.
What we claim is:
Claims (8)
1. A method of determining coding rate in a receiver implemented in a wireless communication system, the receiver responsive to a communication resource containing user information coded at one of a plurality of coding rates, the method comprising the steps of:
generating parameters indicative of the likelihood of coding at the plurality of coding rates;
combining the parameters in a predetermined manner to produce a detection statistic; and determining the coding rate based on the detection statistic.
generating parameters indicative of the likelihood of coding at the plurality of coding rates;
combining the parameters in a predetermined manner to produce a detection statistic; and determining the coding rate based on the detection statistic.
2. The method of claim 1, wherein the parameters indicative of the likelihood of coding at the plurality of coding rates further comprise cyclic redundancy check (CRC) information, symbol error rate (SER) information, and quality bit (QB) information.
3. The method of claim 1, wherein the step of combining the parameters in a predetermined manner to produce a detection statistic further comprises combining the parameters in a linear discriminant function to produce a detection statistic.
4. The method of claim 1, wherein the detection statistic represents two possible coding rates.
5. The method of claim 4, wherein the linear discriminant function is defined by dij(~)=~2~+w0=w1SERi+w2SERj+w3QBi+w4QBj+w0 where i and j are the two possible coding rates.
6. A receiver for determining the coding rate of user information transmitted in a wireless communication system, the receiver responsive to a communication resource containing the user information, the user information coded at one of a plurality of coding rates, the receiver comprising:
a receiver front end which receives and demodulates the communication resource to produce demodulated user information;
a decoder, coupled to the receiver front end, which decodes the demodulated user information and generates parameters indicative of the likelihood of coding at the plurality of coding rates;
a processor, coupled to the decoder, for combining the parameters in a predetermined manner to produce a detection statistic and determining the coding rate based on the detection statistic.
a receiver front end which receives and demodulates the communication resource to produce demodulated user information;
a decoder, coupled to the receiver front end, which decodes the demodulated user information and generates parameters indicative of the likelihood of coding at the plurality of coding rates;
a processor, coupled to the decoder, for combining the parameters in a predetermined manner to produce a detection statistic and determining the coding rate based on the detection statistic.
7. The receiver of claim 6, wherein the parameters indicative of the likelihood of coding at the plurality of coding rates further comprise cyclic redundancy check (CRC) information, symbol error rate (SER) information, and quality bit (QB) information.
8. The receiver of claim 6, wherein the processor combines the parameters in a linear discriminant function to produce a detection statistic.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US08/624,233 | 1996-03-29 | ||
| US08/624,233 US5671255A (en) | 1996-03-29 | 1996-03-29 | Method and apparatus for determining coding rate in a wireless communication system |
| PCT/US1997/000435 WO1997037471A1 (en) | 1996-03-29 | 1997-01-15 | Method and apparatus for determining coding rate in a wireless communication system |
Publications (2)
| Publication Number | Publication Date |
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| CA2220965A1 CA2220965A1 (en) | 1997-10-09 |
| CA2220965C true CA2220965C (en) | 2001-04-03 |
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| CA002220965A Expired - Lifetime CA2220965C (en) | 1996-03-29 | 1997-01-15 | Method and apparatus for determining coding rate in a wireless communication system |
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| US (1) | US5671255A (en) |
| EP (1) | EP0830770B1 (en) |
| JP (1) | JP3833260B2 (en) |
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| CN (1) | CN1097921C (en) |
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| CA (1) | CA2220965C (en) |
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| US5883923A (en) * | 1995-09-18 | 1999-03-16 | Oki Electric Industry Co., Ltd. | Data receiver with symbol rate discrimination and statistical analysis functions |
| US6678311B2 (en) | 1996-05-28 | 2004-01-13 | Qualcomm Incorporated | High data CDMA wireless communication system using variable sized channel codes |
| US5920552A (en) * | 1996-05-30 | 1999-07-06 | Lucent Technologies, Inc. | Variable rate coding for wireless applications |
| KR19990067611A (en) * | 1996-09-17 | 1999-08-25 | 요트.게.아. 롤페즈 | Transmission system with improved lock detection |
| US6108372A (en) * | 1996-10-30 | 2000-08-22 | Qualcomm Inc. | Method and apparatus for decoding variable rate data using hypothesis testing to determine data rate |
| US6018546A (en) * | 1997-09-16 | 2000-01-25 | Lucent Technologies Inc. | Technique for soft decision metric generation in a wireless communications system |
| US6009553A (en) * | 1997-12-15 | 1999-12-28 | The Whitaker Corporation | Adaptive error correction for a communications link |
| US6175587B1 (en) | 1997-12-30 | 2001-01-16 | Motorola, Inc. | Communication device and method for interference suppression in a DS-CDMA system |
| US6175588B1 (en) | 1997-12-30 | 2001-01-16 | Motorola, Inc. | Communication device and method for interference suppression using adaptive equalization in a spread spectrum communication system |
| US6112325A (en) * | 1998-01-23 | 2000-08-29 | Dspc Technologies, Ltd. | Method and device for detecting rate |
| US6134278A (en) * | 1998-01-30 | 2000-10-17 | Lucent Technologies Inc. | Methods and apparatus for CDMA rate detection |
| FI108825B (en) * | 1998-03-16 | 2002-03-28 | Nokia Corp | A method for estimating a channel bit error rate and a receiver |
| TW376497B (en) * | 1998-05-26 | 1999-12-11 | Koninkl Philips Electronics Nv | Transmission system for transmitting a main signal and an auxiliary signal |
| JPH11340840A (en) * | 1998-05-28 | 1999-12-10 | Fujitsu Ltd | Mobile communication terminal and transmission bit rate discrimination method |
| US6233230B1 (en) * | 1998-10-22 | 2001-05-15 | Sony Corporation | Neural network IS-95 rate determination |
| US6463031B1 (en) * | 1998-12-03 | 2002-10-08 | Nokia Mobile Phones Limited | Rate determination technique that utilizes modified cumulative metrics to orthogonalize the rates |
| US7042963B1 (en) * | 1998-12-11 | 2006-05-09 | Ericsson Inc. | Methods and apparatus for decoding variably-coded signals based on prior communication |
| US6567466B1 (en) * | 1999-02-16 | 2003-05-20 | Agere Systems Inc. | Method and apparatus for determining the data rate of a received signal in a variable data rate orthogonal spread spectrum communication system |
| US6137845A (en) * | 1999-04-16 | 2000-10-24 | Motorola, Inc. | Method of determining an encoding rate in a communication system |
| US6389067B1 (en) * | 1999-06-10 | 2002-05-14 | Qualcomm, Inc. | Method and apparatus for using frame energy metrics to improve rate determination |
| KR100330244B1 (en) * | 1999-07-08 | 2002-03-25 | 윤종용 | Data rate detection device and method for a mobile communication system |
| US6208699B1 (en) * | 1999-09-01 | 2001-03-27 | Qualcomm Incorporated | Method and apparatus for detecting zero rate frames in a communications system |
| JP3482931B2 (en) | 1999-12-08 | 2004-01-06 | 日本電気株式会社 | Radio communication apparatus and searcher control method for DS / CDMA mobile communication system |
| US7746832B2 (en) * | 2001-01-05 | 2010-06-29 | Qualcomm Incorporated | Method and apparatus for supporting adaptive multi-rate (AMR) data in a CDMA communication system |
| US6732321B2 (en) | 2001-03-27 | 2004-05-04 | Motorola, Inc. | Method, apparatus, and article of manufacture for error detection and channel management in a communication system |
| CN100431287C (en) * | 2002-04-29 | 2008-11-05 | 中兴通讯股份有限公司 | Speed detecting method for variable speed communication system |
| EP2381610B8 (en) | 2004-09-25 | 2017-03-22 | TQ Delta, LLC | Crc counter normalization |
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| JPS57155857A (en) * | 1981-03-23 | 1982-09-27 | Kokusai Denshin Denwa Co Ltd <Kdd> | Maximum likelihood method and apparatus for error |
| US5568483A (en) * | 1990-06-25 | 1996-10-22 | Qualcomm Incorporated | Method and apparatus for the formatting of data for transmission |
| US5115429A (en) * | 1990-08-02 | 1992-05-19 | Codex Corporation | Dynamic encoding rate control minimizes traffic congestion in a packet network |
| US5321705A (en) * | 1990-11-21 | 1994-06-14 | Motorola, Inc. | Error detection system |
| CA2165342C (en) * | 1993-06-18 | 2005-11-15 | Brian K. Butler | Method and apparatus for determining the data rate of a received signal |
| ZA947317B (en) * | 1993-09-24 | 1995-05-10 | Qualcomm Inc | Multirate serial viterbi decoder for code division multiple access system applications |
| JP2818534B2 (en) * | 1993-09-28 | 1998-10-30 | 日本電気株式会社 | Coded modulation method |
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- 1996-03-29 US US08/624,233 patent/US5671255A/en not_active Expired - Lifetime
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- 1997-01-15 EP EP97905569A patent/EP0830770B1/en not_active Expired - Lifetime
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- 1997-01-15 WO PCT/US1997/000435 patent/WO1997037471A1/en active IP Right Grant
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| BR9702322B1 (en) | 2008-11-18 |
| EP0830770A4 (en) | 2001-04-04 |
| US5671255A (en) | 1997-09-23 |
| CN1097921C (en) | 2003-01-01 |
| BR9702322A (en) | 1999-03-09 |
| CN1185880A (en) | 1998-06-24 |
| JP3833260B2 (en) | 2006-10-11 |
| DE69733736D1 (en) | 2005-08-25 |
| KR19990022101A (en) | 1999-03-25 |
| DE69733736T2 (en) | 2006-03-30 |
| CA2220965A1 (en) | 1997-10-09 |
| EP0830770B1 (en) | 2005-07-20 |
| WO1997037471A1 (en) | 1997-10-09 |
| JPH11506597A (en) | 1999-06-08 |
| EP0830770A1 (en) | 1998-03-25 |
| KR100291401B1 (en) | 2001-06-01 |
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