Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
1999-04-30
2002-10-01
Le, Amanda T. (Department: 2634)
Pulse or digital communications
Equalizers
Automatic
C375S232000, C375S340000, C375S350000
Reexamination Certificate
active
06459728
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority of European Patent Application No. 98303418.2, which was filed on April 30, 1998.
FIELD OF THE INVENTION
This invention relates to channel and data estimation methods and apparatus in mobile radio communications and in particular to an equalizer that compensates for channel distortion by iterating data and channel estimation procedures on a block-by-block basis.
BACKGROUND OF THE INVENTION
In digital mobile radio communications, transmission channels suffer from severe distortion due to frequency selective fading. In addition, channel characteristics are normally time-varying due to the relative motion of fixed and mobile stations. Therefore, in order to allow for reliable transmission, the receiver must be able to estimate and compensate for channel distortion on a block-by-block basis. Various channel estimation and channel equalization methods have been proposed in literature and are commonly used in practical systems such as mobile cellular communication systems employing the European wireless digital cellular standard “GSM′. In most cases the receiver performs channel equalization on the received signal using Maximum Likelihood (ML) or Maximum A Posteriori (MAP) probability data estimation, based on the knowledge of the Channel Impulse Response (CIR). Most practical systems employ training sequences to enable the CIR to be estimated before the equalizer start-up. Fast time varying, fading channels require the changing channel response to be tracked and adjusted dynamically by the receiver for the duration of the received signal. Tracking of the CIR may be performed by means of decision directed algorithms, where tentative decisions from the equalizer are used to update the initial CIR estimate. Examples of receiver systems which perform channel estimation and channel equalization may be found in the following articles: “Bit Synchronization and Timing Sensitivity in Adaptive Viterbi Equalizers for Narrowband TDMA Digital Mobile Radio 25 Systems”, A. Baler, G. Heinrich and U. Wellens, Proc. IEEE Vehicular Technology Conference, June 1988, pp 377-384; “Correlative and Iterative Channel Estimation in Adaptive Viterbi Equalizers for TDMA Mobile Radio”, ITG-Fachbericht No. 107, VDE Verlag, April 1989, pp 363-368; “Simulation and Hardware Implementation of a Viterbi Equalizer for the GSM TDMA Digital Mobile Radio System”, A. Baler, G. Heinrich, P. Shoeffel and W. Stahl, Proc. 3'd Nordic Seminar on Digital Land Mobile Radio Communications, September 1988 pp. 13.7.1. 13.7.5.
The effectiveness of the channel estimation strategy, and thus the overall equalization performance, depends heavily on the reliability of the initial CIR estimate. There is a requirement for an improved estimation strategy that can function with or without training sequences.
SUMMARY OF THE INVENTION
According to a first aspect of the invention there is provided a method of estimating channel impulse response and data in a signal transmitted over a channel in a communication system. The method estimates the channel impulse response using correlative channel sounding. The estimated channel impulse response is used to estimate the data in the signal, and an output is provided. The channel impulse response estimating step is repeated, at least once, using the previous output and an improved channel impulse response estimate is provided, for use in a repeated data estimating step.
According to a second aspect of the invention there is provided apparatus for estimating channel impulse response and data in a signal transmitted over a channel in a communication system. The apparatus has a channel impulse response estimator for providing an initial channel impulse response estimate using correlative channel sounding, a data estimator for providing an initial estimate of data in the transmitted signal, and an output. The channel impulse response estimator has a first input for receiving the transmitted signal, a second input for receiving a feedback signal from the apparatus output, and an output. The data estimator has a first input for receiving the channel impulse response estimate, a second input for receiving the transmitted signal, and an output.
The invention provides a reduced-noise CIR estimate, which is needed for the equalization of the received signal, in the case of multipath propagation environment, and thus provides improved receiver performance.
The equalizer performance is improved considerably by iterating the data and channel estimation procedure on a block-by-block basis. In particular, after a first pass in which the initial channel estimate is obtained by resorting to the known training bits, one or more iteration can be performed, using data symbol decisions from the equalizer output derived in the previous iteration, together with the original training sequence, to obtain a new CIR estimate.
In systems in which the information bits are encoded (and possibly interleaved) prior to modulation, the above strategy can be further improved by using the more reliable decisions obtained by a) re-encoding (and possibly re-interleaving) the channel decoder output, or b) simply taking (and possibly re-interleaving) the most significant bit of the A Posteriori values for the coded bits provided by a soft-in/soft-out channel decoder. Computer simulations carried out for the particular case of the GSM TCWFS transmission scheme show that, as compared with the conventional channel estimation approach (that is, correlative channel sounding by training sequence), the invention provides a significant performance improvement even with just one iteration.
The invention may also be used in those cases where no training sequence is available and the data estimation is performed by starting with an arbitrary channel estimate.
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Mignone, V. et al “A New Channel Estimation Method To Improve The Spectrum Efficiency In Digital Terrestrial Television Systems” “International Broadcasting Convention 1994 No. 413 Sep. 14, 1995” pp. 122-128.
WU, L. et al “A New Short-Block Digital Transmission Scheme With Adaptive MLSE For Mobile Radio Channels” “Proceedings Of The Vehicular Technology Conference, Stockholm Jun. 8-10, 1994” pp. 243-247.
Bair, A. et al “Bit Synchronization And Timing Sensitivity In Adaptive Viterbi Equalizers for Narrowband-TDMA Digital Mobile Radio Systems” “38thIEEE Vehicular Technology Conference: Telecommunications Freedom—Technology On The Move Philadelphia, PA Jun. 15-17, 1988” pp. 377-384.
D'Avella, R. et al “An Adaptive MLSE Receiver For TDMA Digital Mobile Radio” “IEEE Journal On Selected Areas In Communications Jan. 1989, vol. 7 No. 1” pp. 122-129.
Chang, K. et al “Iterative Joint Sequence And Channel Estimation For Fast Time-Varying Intersymbol Interference Channels” “Communications—Gateway To Globalization Proceedings Of The International Conference On Communications, Seattle, WA Jun. 18-22, 1995 vol. 1” pp. 357-361.
Fukawa, K. et al “Adaptive Equalization With RLS-MLSE For Frequency Selective Fast Fading Mobile Radio Channels” “Countdown To The New Milennium, Phoenix, AZ Dec. 2-5, 1991 vol. 1” pp. 548-552.
Baier, A. “Correlative And Iterative Channel Estimation In Adaptive Viterbi Equalizers For TDMA Mobile Radio Systems” “Stochastic Models And Methods In Information Technology Nurenberg, W. Germany Apr. 12-14, 1989 vol. 107” pp. 363-368.
Baler, A. et al “Simulation And Hardware Implementation Of A Viterbi Equalizer For The GSM TDMA Digital Mobile Radio System” “Proc. 3'd Nordic Seminar On Digital Land Mobile Radio Communications, Sep. 1988” pp. 13.7.1-13.7.5.
Bauch, G. et al “Iterative Equalization And Decoding In Mobile Communications Systems” “Proc. EPMCC '97, ITG-Fachbericht No. 145, VDE Verlag, Oct. 1997” pp. 307-312.
European Search Report, Dated Sep. 18, 1998.
Bar-David Israel
Luschi Carlo
Yan Ran-Hong
Agere Systems Guardian Corp.
Botos Richard J.
Le Amanda T.
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