WO1991006165A1 - Soft decision decoding with channel equalization - Google Patents

Soft decision decoding with channel equalization Download PDF

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Publication number
WO1991006165A1
WO1991006165A1 PCT/US1990/005359 US9005359W WO9106165A1 WO 1991006165 A1 WO1991006165 A1 WO 1991006165A1 US 9005359 W US9005359 W US 9005359W WO 9106165 A1 WO9106165 A1 WO 9106165A1
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information
time
signal
confidence
recovered
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PCT/US1990/005359
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French (fr)
Inventor
Gerald Paul Labedz
David Edward Borth
Phillip David Rasky
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Motorola, Inc.
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Application filed by Motorola, Inc. filed Critical Motorola, Inc.
Priority to EP90916969A priority Critical patent/EP0495006B1/en
Priority to DE69029330T priority patent/DE69029330T2/en
Priority to KR1019920700836A priority patent/KR960008984B1/en
Publication of WO1991006165A1 publication Critical patent/WO1991006165A1/en

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/01—Equalisers
    • 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
    • H04L25/03—Shaping networks in transmitter or receiver, e.g. adaptive shaping networks
    • H04L25/03006—Arrangements for removing intersymbol interference
    • H04L25/03178—Arrangements involving sequence estimation techniques
    • H04L25/03312—Arrangements specific to the provision of output signals
    • H04L25/03318—Provision of soft decisions

Definitions

  • This invention is concerned with soft decision decoding.
  • this invention is concerned with soft decision decoding during equalization.
  • This invention is particularly concerned with deciding at a receiver which of a predetermined set of symbols was transmitted, by using an error correction or detection decoder operating on a recovered signal having a number of possible values greater than the number of symbols in the set - called soft decision decoding. More particularly, in a communications network having time-dispersed signals, this invention is concerned with soft decision decoding using information derived during equalization.
  • convolutional error correction coding In digital transmission over a medium vulnerable to corruption, such as a fading radio channel, convolutional error correction coding is used to distribute transmitted information over time such that it can still be reconstructed, despite some corruption.
  • An error correction decoding technique such as Viterbi algorithm channel decoding, is used to reconstruct the originally-transmitted information from the corrupted signal.
  • ISI intersymbol interference
  • the ISI is especially severe in severely delay-spread channels where the various reflections arrive at the receiver delayed by amounts comparable to a significant fraction of a symbol time.
  • adaptive equalization -- attempting to account for these multipath, time- dispersal effects on the signal and realign the echoes in time -- is employed prior to error correction or detection.
  • equalization attempts to distill the information carried in the composite, multipath signal to a definitive decision about each symbol received -- a "hard decision". For example, in the binary case, where one of only two symbols can be sent, the output of the equalizer can only take on one of two values, these values being referred to as the equalizer's hard decisions.
  • This invention takes as its object to overcome these shortcomings and to realize certain advantages presented below, and exploits the recognition that fading, microfading and intersymbol interference make the composite signal envelope a poor choice for developing soft information.
  • a mechanism for soft decision decoding comprises: radio reception of a time-dispersed signal, at least partly equalizing those time-dispersal effects, recovering information contained in the signal, multiplying with that recovered information the absolute value of the at- least-partly-equalized signal (scaled by a number obtained from signals during which at least part of the information to be recovered is distributed), and error-correcting the multiplied information by a Viterbi algorithm channel decoding scheme of error correction. Accordingly, soft decision information is generated from within the equalization process itself. DESCRIPTION OF THE DRAWINGS
  • the single Figure is a block diagram of the preferred embodiment of the invention.
  • the single Figure is a block diagram of the preferred embodiment of the invention. It illustrates, coupled in series, a radio having a receiver (RF/IF), equalizer (EQ) and channel decoder (DECODER); the receiver has radio frequency (RF), intermediate frequency (IF) and demodulation (DEMOD) stages, and the equalizer (EQ) has a channel matched filter (MF) and a maximum likelihood sequence estimator (MLSE), all of which are well understood by those ordinarily skilled in this field (see G. Ungerboeck, "Adaptive Maximum Likelihood Receiver for Carrier- Modulated Data-Transmission Systems", IEEE Trams. Comm., Vol. COM-22, No. 5, May 1974, pp.
  • a complex synchronization correlator (not unlike US4,829,543 and US4,852,090, both assigned to Motorola) provides the coefficients for the matched filter (MF).
  • a radio frequency digital signal exhibiting delay-spread characteristics is received, converted to an intermediate frequency and demodulated into n samples per symbol.
  • the matched filter (MF) is matched to the multipath channel characteristics and approximately accounts for and realigns the multipath echoes while the MLSE makes "hard" ( ⁇ 1 ) one-symbol-at-a-time decisions about the symbol received.
  • the Viterbi algorithm channel decoder DECODER
  • the confidence information is developed after the effects of fading, microfading, multipath and intersymbol interference have been at least partly accounted for by channel matched filtering.
  • This altered signal is now more reliable than the composite signal envelope in that some of the undesirable characteristics of the composite envelope have been removed through matched filtering.
  • the hard decision output of the MLSE ( ⁇ 1 in the preferred embodiment) is weighted (multiplied) by the absolute value (ABS) of the matched filter (MF) output, suitably delayed to account for the processing time of the MLSE (DELAY) to provide soft information on which better error correction evaluations can be made in the Viterbi algorithm channel decoder (DECODER).
  • the absolute value of the matched filter output is taken so that the MLSE's decisions are multiplied only by the magnitude, and not the sign, of the sample being output by the matched filter (MF). That is, the MLSE's output decisions are weighted by a measure of reliability for each of those decisions, the reliability in this case being taken as the amplitude of the input to the MLSE at a given symbol sample time.
  • Added improvement is obtained by progressively scaling (SCALE) the absolute value for the current transmission by the average of the current transmission strength and the strength of the signal in the recent past.
  • SCALE progressively scaling
  • a digitized speech block of 260 bits is coded, reordered and diagonally interleaved over eight TDMA bursts.
  • the MLSE hard decisions ( ⁇ 1) for each burst are weighted by the soft decision information SD n , which includes a factor for the average of the averages of the absolute values of the matched filter outputs for the current burst and several previous bursts. This is calculated by the following method.
  • the factor Q m has the effect of accentuating samples of especially weak bursts and symbols on a faded channel, making their soft decision values even lower than in the non-scaled case.
  • the length of time the scaling factor Q m represents can be a variable.
  • TDMA Time Division Multiple Access
  • FDMA Frequency Division Multiple Access
  • a mechanism for soft decision decoding comprises: radio reception of a time-dispersed signal, at least partly equalizing those time-dispersal effects, recovering information contained in the signal, multiplying with that recovered information the absolute value of that at-least-partly-equalized signal (scaled by a number obtained from current conditions and past history), and error-correcting the multiplied information by a Viterbi algorithm channel decoding scheme of error correction. Accordingly, soft decision information is generated from within the equalization process itself.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Power Engineering (AREA)
  • Error Detection And Correction (AREA)
  • Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
  • Dc Digital Transmission (AREA)
  • Multi Processors (AREA)
  • Detection And Prevention Of Errors In Transmission (AREA)

Abstract

In a communications network having time-dispersed signals, there is provided a mechanism for soft decision decoding. It comprises: radio reception of a time-dispersed signal, at least partly equalizing those time-dispersal effects, recovering information contained in the signal, multiplying with that recovered information the absolute value of that at-least-partly-equalized signal (scaled by a number derived from channel conditions over a time during which at least part of the information to be recovered is distributed), and error-correcting the multiplied information by a Viterbi algorithm channel decoding scheme of error correction. Accordingly, soft decision information is generated from whithin the equalization process itself.

Description

SOFT DECISION DECODING WITH CHANNEL EQUALIZATION
THE FlEUP OF INVENTION
This invention is concerned with soft decision decoding.
More particularly, in a communications network having time-dispersed signals, this invention is concerned with soft decision decoding during equalization.
This invention is particularly concerned with deciding at a receiver which of a predetermined set of symbols was transmitted, by using an error correction or detection decoder operating on a recovered signal having a number of possible values greater than the number of symbols in the set - called soft decision decoding. More particularly, in a communications network having time-dispersed signals, this invention is concerned with soft decision decoding using information derived during equalization.
BACKGROUND OF THE INVENTION
In digital transmission over a medium vulnerable to corruption, such as a fading radio channel, convolutional error correction coding is used to distribute transmitted information over time such that it can still be reconstructed, despite some corruption. An error correction decoding technique, such as Viterbi algorithm channel decoding, is used to reconstruct the originally-transmitted information from the corrupted signal.
in a digital radiotelephone system, high data rate digital radio transmissions encounter serious intersymbol interference (ISI) due to multiple reflections and echoes off buildings and other objects in the transmission path. The ISI is especially severe in severely delay-spread channels where the various reflections arrive at the receiver delayed by amounts comparable to a significant fraction of a symbol time. Thus, adaptive equalization -- attempting to account for these multipath, time- dispersal effects on the signal and realign the echoes in time -- is employed prior to error correction or detection. The problem is that equalization attempts to distill the information carried in the composite, multipath signal to a definitive decision about each symbol received -- a "hard decision". For example, in the binary case, where one of only two symbols can be sent, the output of the equalizer can only take on one of two values, these values being referred to as the equalizer's hard decisions.
It is well recognized that better decisions can be made about the value of any individually-transmitted symbol with the availability of "soft information". That is, if during error correction, it were known to the decoder what the quality of the signal was when the symbol was received, a better decision could ultimately be made about what the true value of the transmitted symbol really was. This soft information represents a measure of the confidence held in any given symbol detection. But any such soft information has ordinarily been removed by equalization.
Conventional wisdom dictates that soft information be developed at or ahead of the radio demodulator (at the RF, IF or discriminator stage), at a point that is rich in signal quality information (signal strength, noise and distortion). See, for example, Figure 6 of Tadashi Matsumoto, "Soft Decision Decoding of Block Codes Using Received Signal Envelope in Digital Mobile Radio", IEEE Journal on Selected Areas in Comm., Vol. 7, No.1 , January 1989, or "Modulation and Channel Coding in Digital Mobile Radio Telephony", Proceedings of the Nordic Seminar on Digital Land Mobile Radiocommunication, Espoo, Finland, 5-7 February, 1985, pp. 219-227. Yet it is not widely recognized that the composite signal envelope yields erroneous information in the severely delay-spread channel. This may possibly be due to the phenomenon of individual signal paths causing rapid signal envelope changes (hereinafter coined "microfading"), changes taking place much more rapidly than is known for conventional Rayleigh fading at a given receiver speed.
This invention takes as its object to overcome these shortcomings and to realize certain advantages presented below, and exploits the recognition that fading, microfading and intersymbol interference make the composite signal envelope a poor choice for developing soft information.
SUMMARY OF THE INVENTION
In a communications network having time-dispersed signals, there is provided a mechanism for soft decision decoding. It comprises: radio reception of a time-dispersed signal, at least partly equalizing those time-dispersal effects, recovering information contained in the signal, multiplying with that recovered information the absolute value of the at- least-partly-equalized signal (scaled by a number obtained from signals during which at least part of the information to be recovered is distributed), and error-correcting the multiplied information by a Viterbi algorithm channel decoding scheme of error correction. Accordingly, soft decision information is generated from within the equalization process itself. DESCRIPTION OF THE DRAWINGS
Additional objects, features and advantages of the invention will be more clearly understood and the best mode contemplated for practicing it in its preferred embodiment will be appreciated (by way of unrestricted example) from the following detailed description, taken together with the accompanying drawing in which:
The single Figure is a block diagram of the preferred embodiment of the invention.
DETAILED DESCRIPTION
The single Figure is a block diagram of the preferred embodiment of the invention. It illustrates, coupled in series, a radio having a receiver (RF/IF), equalizer (EQ) and channel decoder (DECODER); the receiver has radio frequency (RF), intermediate frequency (IF) and demodulation (DEMOD) stages, and the equalizer (EQ) has a channel matched filter (MF) and a maximum likelihood sequence estimator (MLSE), all of which are well understood by those ordinarily skilled in this field (see G. Ungerboeck, "Adaptive Maximum Likelihood Receiver for Carrier- Modulated Data-Transmission Systems", IEEE Trams. Comm., Vol. COM-22, No. 5, May 1974, pp. 624-636, and Figure 2 of D'Avella, Moreno & SanfAgostino, IEEE Journal on Selected Areas in Comm. Vol. 7, No.1, January, 1989, pp. 122-129). A complex synchronization correlator (not unlike US4,829,543 and US4,852,090, both assigned to Motorola) provides the coefficients for the matched filter (MF).
Conventionally, a radio frequency digital signal exhibiting delay-spread characteristics is received, converted to an intermediate frequency and demodulated into n samples per symbol. The matched filter (MF) is matched to the multipath channel characteristics and approximately accounts for and realigns the multipath echoes while the MLSE makes "hard" (±1 ) one-symbol-at-a-time decisions about the symbol received. Ordinarily, the Viterbi algorithm channel decoder (DECODER) would attempt to perform error correction on the hard decisions and decode the transmitted information.
Rather than following conventional wisdom and developing soft decision information from the composite signal envelope in the radio demodulator, the confidence information is developed after the effects of fading, microfading, multipath and intersymbol interference have been at least partly accounted for by channel matched filtering. This altered signal is now more reliable than the composite signal envelope in that some of the undesirable characteristics of the composite envelope have been removed through matched filtering. Accordingly, the hard decision output of the MLSE (±1 in the preferred embodiment) is weighted (multiplied) by the absolute value (ABS) of the matched filter (MF) output, suitably delayed to account for the processing time of the MLSE (DELAY) to provide soft information on which better error correction evaluations can be made in the Viterbi algorithm channel decoder (DECODER). The absolute value of the matched filter output is taken so that the MLSE's decisions are multiplied only by the magnitude, and not the sign, of the sample being output by the matched filter (MF). That is, the MLSE's output decisions are weighted by a measure of reliability for each of those decisions, the reliability in this case being taken as the amplitude of the input to the MLSE at a given symbol sample time.
Added improvement is obtained by progressively scaling (SCALE) the absolute value for the current transmission by the average of the current transmission strength and the strength of the signal in the recent past. In the next generation digital cellular system for Europe, a digitized speech block of 260 bits is coded, reordered and diagonally interleaved over eight TDMA bursts. The MLSE hard decisions (±1) for each burst are weighted by the soft decision information SDn, which includes a factor for the average of the averages of the absolute values of the matched filter outputs for the current burst and several previous bursts. This is calculated by the following method. If for a given burst m there are N symbols to be decided upon by the MLSE (N = 114 in the preferred embodiment), then decoded, at the rate of one sample per symbol at the output of the matched filter, a particular sample can be designated as Xmn where n = 1 , . . ., N. At the end of processing each burst through the matched filter, an average Am is formed
N
Am= X | Xmn | /N n = 1
A number M of these averages are can then be averaged, giving Qm according to
Figure imgf000008_0001
for the burst m. If the MLSE hard decisions for a burst m are given by dn, n = 1 , . . ., N then the soft decision information passed to the convolutional decoder for each of the samples would be
SDn = dn X x ιm π /Q -,m
The factor Qm has the effect of accentuating samples of especially weak bursts and symbols on a faded channel, making their soft decision values even lower than in the non-scaled case. The length of time the scaling factor Qm represents can be a variable. The preferred embodiment is for a burst-oriented processing in a Time Division Multiple Access (TDMA) system, and it has been found that M = 8 can produce an improvement over the case where no scaling at all is used. However, using no scaling at all, Qm = 1 , still produces an improvement over merely using the MLSE's hard decisions dn in the Viterbi algorithm convolutional decoder. It would not be necessary, however, to restrict the present invention to burst-oriented TDMA systems. In a continuous- time Frequency Division Multiple Access (FDMA) radio transmission system, training and synchronizing could take place at some interval, whether a specific pattern of symbols is used or not. As this training adjusts the transfer function of the matched filter, the output could still be used as in the equations above as soft decision information, multiplying the hard decision of the MLSE by the corresponding absolute value of the matched filter output. If scaling is to be performed, it could be done as a running average, over some appropriate time, of the absolute value of the matched filter outputs.
The attendant advantages of this invention include computational simplicity while developing reliable soft decision information from within the equalization process itself.
Better performance yet might be achieved by using the MLSE branch metrics (scaled as above) instead of the MF output to develop the soft information. This might involve taking the metric computation at each state transition (say equation 31 or 32 of Ungerboeck, cited earlier) and multiplying it by the output hard decision for which symbol was transmitted at that time. This is still, in effect, attaching a confidence factor to each of the MLSE's hard decisions.
Thus, in a communications network having time-dispersed signals, there has been provided a mechanism for soft decision decoding. It comprises: radio reception of a time-dispersed signal, at least partly equalizing those time-dispersal effects, recovering information contained in the signal, multiplying with that recovered information the absolute value of that at-least-partly-equalized signal (scaled by a number obtained from current conditions and past history), and error-correcting the multiplied information by a Viterbi algorithm channel decoding scheme of error correction. Accordingly, soft decision information is generated from within the equalization process itself.
While the preferred embodiment of the invention has been described and shown, it will be appreciated by those skilled in this field that other variations and modifications of this invention may be implemented. For example, the instant invention is not limited to these particular (matched) filters, (maximum likelihood) sequence estimators or (Viterbi algorithm channel decoder) error correctors; it may be advantageous with decision feedback equalizers, for instance.
These and all other variations and adaptations are expected to fall within the ambit of the appended claims.

Claims

CLAIMSWhat we claim and desire to secure by Letters Patent is:
1. A method of soft decision decoding comprising:
receiving a time-dispersed signal,
at least partly accounting for those time-dispersal effects,
recovering information contained in the signal,
associating with that recovered information, after some time- dispersal effects have been accounted for, information indicative of the confidence held in its recovery,
and evaluating the recovered information considering the confidence held therein,
whereby the confidence information is "soft decision" information for soft decision decoding.
2. A method as claimed in Claim 1 above, wherein accounting for those time-dispersal effects and recovering information comprises equalization.
3. A method as claimed in Claim 1 above, wherein accounting for those time-dispersal effects comprises at least partial equalization of multipath time-dispersal effects.
4. A method as claimed in Claim 1 above, wherein accounting for those time-dispersal effects and recovering information comprises adaptive equalization with channel matched filtering and sequence estimation.
5. A method as claimed in Claim 1 above, wherein accounting for those time-dispersal effects and recovering information comprises adaptive equalization with channel matched filtering and maximum likelihood sequence estimation.
6. A method as claimed in Claim 1 above, wherein accounting for those time-dispersal effects and recovering information comprises decision feedback equalization.
7. A method as claimed in Claim 1 above, wherein the recovered information is weighted by the confidence information comprising the absolute value of the at-least-partly-equalized information.
8. A method as claimed in Claim 1 above, wherein the recovered information is weighted by the confidence information comprising a scaling of the absolute value of the at-least-partly-equalized information.
9. A method as claimed in Claim 1 above, wherein confidence information is obtained from partly equalized information.
10. A method as claimed in Claim 1 above, wherein confidence information is obtained from the absolute value of matched filtered output information.
11. A method as claimed in Claim 1 above, wherein confidence information is obtained from progressively scaling the confidence information over an interval of interest.
12. A method as claimed in Claim 8 above, wherein confidence information is obtained from progressively scaling the absolute value over the interval over which at least some of the information to be recovered is distributed.
13. A method as claimed in Claim 2 above, wherein confidence information is obtained from a point after at least partial equalization.
14. A method as claimed in Claim 1 above, wherein receiving comprises radio frequency reception.
15. A method as claimed in Claim 1 above, wherein evaluation comprises error correction.
16. A method as claimed in Claim 1 above, wherein evaluation comprises error correction/detection.
17. A method as claimed in Claim 1 above, wherein evaluation comprises channel decoding.
18. A method as claimed in Claim 1 above, wherein evaluation comprises Viterbi algorithm channel decoding.
19. A method as claimed in Claim 1 above, wherein association comprises multiplication of the recovered information with the confidence information.
20. A method of soft decision making comprising:
receiving a time-dispersed signal,
accounting for some of those time-dispersal effects,
recovering information contained in the signal,
associating with that recovered information, after some of the time-dispersal effects have been accounted for, information indicative of the confidence held in its recovery.
21. An apparatus for soft decision making comprising, coupled in series:
means for receiving a time-dispersed signal,
means for accounting for some of those time-dispersal effects,
means for recovering information contained in the signal,
means for associating with that recovered information, after some of the time-dispersal effects have been accounted for, information indicative of the confidence held in its recovery.
22. A method of soft decision making comprising:
receiving a time-dispersed signal,
at least partly equalizing those time-dispersal effects,
recovering information contained in the signal,
multiplying with that recovered information, after at least partial equalization, information indicative of the confidence held in its recovery.
23. A method of soft decision making comprising:
radio reception of a time-dispersed signal,
at least partly equalizing those time-dispersal effects,
recovering information contained in the signal,
multiplying with that recovered information the absolute value of that at-least-partly-equalized signal, scaled over the interval over which at least part of the information to be recovered is distributed,
and error-correcting the multiplied information by Viterbi algorithm channel decoding.
24. A method of soft decision making comprising:
radio reception of a time-dispersed signal,
at least partly equalizing those time-dispersal effects,
recovering information contained in the signal,
multiplying with that recovered information the absolute value of that at-least-partly-equalized signal,
and error-correcting the multiplied information by Viterbi algorithm channel decoding.
25. An apparatus for soft decision making comprising, coupled in series:
means for radio reception of a time-dispersed signal,
means for at least partly equalizing those time-dispersal effects,
means for recovering information contained in the signal,
means for multiplying with that recovered information the absolute value of that at-least-partly-equalized signal,
and means for error-correcting the multiplied information by Viterbi algorithm channel decoding.
PCT/US1990/005359 1989-10-13 1990-09-24 Soft decision decoding with channel equalization WO1991006165A1 (en)

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EP90916969A EP0495006B1 (en) 1989-10-13 1990-09-24 Soft decision decoding with channel equalization
DE69029330T DE69029330T2 (en) 1989-10-13 1990-09-24 SOFT DECISIVE DECODING WITH CHANNEL EQUALIZATION
KR1019920700836A KR960008984B1 (en) 1989-10-13 1990-09-24 Soft decision decoding with channel equalization

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PT95577A (en) 1992-08-31
CN1050958A (en) 1991-04-24
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KR920704483A (en) 1992-12-19
EP0495006A1 (en) 1992-07-22
CA2066270C (en) 1995-03-28
PT95577B (en) 1998-07-31
DE69029330T2 (en) 1997-05-28
IE80438B1 (en) 1998-07-15
ES2095261T3 (en) 1997-02-16
AU6729490A (en) 1991-05-16
JP2591343B2 (en) 1997-03-19
MX171792B (en) 1993-11-15
US5271042A (en) 1993-12-14
DE69029330D1 (en) 1997-01-16
JPH05504872A (en) 1993-07-22
CA2066270A1 (en) 1991-04-14
CN1017294B (en) 1992-07-01
TR26468A (en) 1995-03-15
AU637166B2 (en) 1993-05-20
EP0495006B1 (en) 1996-12-04
EP0495006A4 (en) 1992-10-07

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