Showing papers on "Precoding published in 1995"

MMSE decision-feedback equalizers and coding. I. Equalization results

Abstract: The minimum mean-squared-error decision-feedback equalizer (MMSE-DFE) has properties that suggest that it is a canonical equalization structure for systems that combine equalization with coded modulation. The structure and performance of the MMSE-DFE are succinctly derived using linear-estimation-theoretic principles in this first part of this two-part paper. The front-end of the MMSE-DFE, called the "mean-square whitened matched filter" (MS-WMF), is preferable in some ways to a matched filter or a whitened matched filter as a canonical receiver front end. In a coded system, the feedback filter of the MMSE-DFE may be implemented in the transmitter using precoding. The MMSE-DFE can perform significantly better than a zero-forcing decision-feedback equalizer, particularly at moderate-to-low SNR's and on severe-ISI channels. The MMSE-DFE is biased. The optimum unbiased MMSE-DFE is the MMSE-DFE with the bias removed. Removing bias improves error probability, but reduces the SNR to SNR/sub MMSE-DFE,U/=SNR/sub MMSE-DFE/-1. It is shown that this SNR relationship is a particular case of a very general result and that SNR/sub MMSE-DFE,U/ gives a more realistic estimate of SNR. The results are extended to partial response equalization and to equalization with correlated inputs in an appendix. >

Achievable rates for Tomlinson-Harashima precoding

Abstract: This article examines Tomlinson-Harashima precoding (1971, 1972) on discrete-time channels having intersymbol interference and additive white Gaussian noise. An exact expression for the maximum achievable information rate of zero-forcing (ZF) THP is derived as a function of the channel impulse response, the input power constraint, and the additive white Gaussian noise variance. Information rate bounds are provided for the minimum mean-square error (MMSE) THP. The performance of ZF-THP and MMSE-THP relative to each other and to channel capacity is explored in general and for some example channels. The importance of symbol rate to ZF-THP performance is demonstrated.

Spread-signature CDMA: efficient multiuser communication in the presence of fading

Abstract: A new class of orthogonal code-division multiple-access (CDMA) systems is developed for efficient multiuser communication in environments subject to multipath fading phenomena. The key characteristic of these new systems, which we refer to as "spread-signature CDMA" systems, is that the associated signature sequences are significantly longer than the interval between symbols. Using this approach, the transmission of each symbol of each user is, in effect, spread over a wide temporal and spectral extent, which is efficiently exploited to combat the effects of fading. These systems generalize and improve on the spread-response precoding systems. Both efficient signature sets and efficient receiver structures for such systems are developed. Several aspects of the performance of the resulting spread-signature CDMA systems are presented, including both the achievable bit-error rate characteristics and the effective capacity of such systems. The results suggest that spread-signature CDMA may be an attractive alternative to conventional CDMA in a variety of application scenarios. >

A hybrid equalizer arrangement for use in data communications equipment

Abstract: A hybrid type of decision feedback equalizer (DFE) structure that approaches optimal DFE performance is described. The hybrid DFE includes both an intersymbol interference DFE (ISI-DFE) and a noise predictive DFE (NP-DFE). The hybrid DFE structure is designed so that one form of tap values dominates over the other. This allows only the dominate tap values to be used by the transmitter during precoding and reduces the transmitter complexity. In particular, a hybrid DFE is designed so that the I(z) coefficient values dominate over the N(z) coefficient values, where the notation I(z) and N(z), as known in the art, represent the resulting coefficient values after adaptation of the ISI-DFE and the NP-DFE, respectively. A respective Tomlinson precoding scheme in the transmitter uses only the I(z) values. Since only the I(z) values are used in the precoder of the transmitter, the NP-DFE of the receiver is kept active during the communications phase. Conveniently, the continued use of the NP-DFE in the receiver allows the receiver to track small changes in the channel.

Method for synchronization of transmitter and receiver at mobile radio system

Abstract: The present invention relates to a method at mobile radio system for synchronization of transmitter and receiver. The system relates to MC/DS-CDMA-system. In the system information is transmitted digitally in frames. For the purpose a data channel, a pilot channel and a synchronization channel are created. In the data channel the data blocks (D0, D1, D2, etc.) are separated by a guard space Δ. In the pilot channel, respective the synchronization channel, the information is arranged in blocks which are synchronous with each other. The repeating distance, (A), of the pilot channel, corresponds to the length of one of the data blocks (D0, D1, D2, etc.). The repeating distance for the synchronization channel is S, corresponding to a number of blocks in the data channel. In the synchronization channel information is introduced into the blocks which indicate their relation to the data channel. The method in this way allows an identification of the position of the pilot channel, which indicates the position of the synchronization channel, at which a decoding of the information of the synchronization channel appoints the position of the data channel.

Analysis of the Impact of Channel Estimation Errors on the Performance of a Decision-Feedback Equalizer in Fading

Abstract: A coherent receiver with a decision-feedback equalizer (DFE) operating on a Rayleigh fading channel under a suitable adaptive algorithm is considered. In the analysis of a DFE, a common assumption is that the receiver has perfect knowledge of the channel impulse response. However, this is not the case in practice, and for a rapidly fading channel, errors in channel tracking can become significant. We analyze theoretically the impact of these errors on the performance of a multichannel DFE. The expressions obtained for the achievable average MPSK bit error probabilities depend on the estimation error covariance. In order to specify this matrix, we focus on a special case when a Kalman filter is used as an optimal channel estimator. In this case, the probability of bit error can be assessed directly in terms of channel fading model parameters, the most interesting of which is the fading rate. Our results show the penalty imposed by imperfect channel estimation, as well as the fading-induced irreducible error rates. >

Partial-response equalization in magneto-optical disk readout: a theoretical investigation

Abstract: We analyze the application of partial-response equalization and maximum-likelihood sequence estimation in magneto-optical readout. Two filters are proposed, and several aspects of their performance are examined. Filter I has 8 states in its state-transition diagram and is therefore easier to implement than filter II, which has 32 states. We discuss the required signal-to-noise ratio as function of the recorded bit density for these filters. The effects of jitter and bloom on the eye patterns of the output signals are also examined by computer simulation. This analysis indicates that filter II is somewhat superior to filter I, presumably because the output of filter II is more similar to the actual readout signal. We determine the distribution of Euclidean distance between pairs of output sequences and compute upper bounds on the probability of sequence error for both filters. Using two different methods of precoding (i.e., mapping of the user data to the magnetic pattern on the disk), we also compute the probability of bit error for the user data and show that one precoding scheme is slightly better than the other.

Using flexible precoding for channels with spectral nulls

Abstract: Flexible preceding has been proposed for channel pre-equalisation at the transmitter side. In the Letter a slight modification of the receiver is presented which enables the use of flexible preceding for channels with spectral nulls at DC and/or the Nyquist frequency. Simulation results covering the performance are given. >

Blind equalization for digital cable transmission with Tomlinson-Harashima precoding and shaping

Abstract: A new system concept for high-rate digital transmission over twisted pair lines with precoding is presented. To avoid retransmission of channel data, filters at the transmitter side are fixed and designed for a reference application. Intersymbol interference caused by mismatch of designed filters and actual situation, is cancelled by an adaptive FIR equalizer. Implementation is thereby simplified considerably compared to the case of optimum tuned filters, whereas the resulting SNR-loss is negligible. Because Tomlinson-Harashima precoding is not suited for many blind equalization algorithms, a modification of the precoder is proposed, resulting in a structure called dynamics shaping, which enables blind equalization by the Sato algorithm.

Method and apparatus for reducing precoding loss when using a post-comb filtering approach to reduce co-channel interference in high definition television transmission

Abstract: A system for transmitting and receiving digital data which utilizes pre-coding at a transmitter, and selects post-coding at a receiver when co-channel interference not present or post-comb processing at the receiver when co-channel interference is present. The system includes a symbol interleaver at the transmitter and a symbol deinterleaver at the receiver which minimizes the byte errors by organizing the data input at the pre-coder at the transmitter such that the symbol errors and their respective propagated errors are re-organized so as to be disposed adjacent to each other, thereby forming adjacent symbol errors. The effect of this re-organization is to increase the probability that each pair of adjacent symbol errors correspond to bits which lie in the same byte.

Dynamics limited shaping for fast digital transmission

Abstract: A new combined precoding/shaping technique for fast digital transmission is proposed. Major advantages of this "dynamics shaping" are: dynamics of the signal at the input of the decision device are reduced by a great amount. Thereby, A/D-conversion, adaptive equalization, and symbol timing are rather facilitated. A trade-off between signal dynamics at the transmitter output, decision device input and the SNR-gain by noise whitening is offered. For dynamics limitation relevant in practice, gains up to 6 dB are achieved. Nevertheless, system complexity is lower than in other precoding/shaping techniques. Numerical results are presented for a HDSL application in the German Telekom subscriber network.

Source coding and transmission of signals over time-varying channels with side information

Abstract: We look at the problem of transmitting information over time-varying channels with side information, where for time-varying channels the statistics of the channel change with time and by channel side information we mean the current state of the channel. We show that when this side information is available at both the transmitter and the receiver, then for the power-constrained channel, the power allocation policy that achieves minimum end-to-end distortion is not necessarily the same as the one required for maximum transmission rate.

Suboptimal detection for intersymbol interference inflicted cable channels

Abstract: This paper presents some features of combining the design of a finite impulse response linear equalizer, which minimizes the noise variance (anchoring the first non zero sample), with detection techniques, which are frequently used in severe intersymbol interference inflicted channels. These include symbol by symbol detection using zero forcing decision feedback equalizer and Tomlinson filtering, and sequence detection invoking maximum likelihood sequence estimation and decision feedback sequence estimation (DFSE). Two trellis coded modulation schemes for this channel are also presented. The first uses Tomlinson precoder, for which we examine a DFSE with an unfolded branch metric, and the second employs DFSE (with states that combine the code states with part of the channel states). It is demonstrated that for the twisted pair and coax cable channels DFSE with 2-4 states achieves a very good performance when compared to other examined detection techniques in the uncoded transmission case. For the coded transmission case, it is shown that the performance of the coding scheme with Tomlinson precoding is close to the performance of the idealized coding scheme that invokes a sample whitened matched filter and an ideal tail canceller, and it improves (/spl sim/2 dB for bit error rate of 10/sup -4/) on the performance of a Viterbi algorithm with modulo folded metric (the commonly used metric in this case). A simple analytical upper bound for the error probability of a DFSE that accounts for the error propagation is also investigated.

Digital signal recording apparatus

Abstract: In digital signal recording apparatus using I-NRZI modulation for recording, the need for intermittently reading or intermittently writing buffer storage is eliminated by using parallel-bit precoding to generate the channel words that are selected between for recording. The precoders (106) perform precoding on an accelerated basis using ripple-through integration of the alternate successive bits used to form each channel word. Two precoders (106) generate (n+1)-parallel-bit channel words at a channel word rate slower by a factor of (n+1) than the rate of a system clock for the I-NRZI modulation. This leaves additional time during each channel word interval to carry out a decision procedure, which determines which of the channel words generated by the two precoders is to be selected for recording. There is also sufficient additional time for completing a subsequent updating procedure, in which precoding information stored in the precoder that did not generate the selected channel word is altered, to conform to precoding information stored in the precoder that did generate the selected channel word. The parallel-bit channel words from the precoders (106) are converted to serial-bit format for recording with a bit rate equal to that of the system clock. The parallel-bit channel words from the precoders are converted to serial-bit format with an effective bit rate that is substantially higher than that of the system clock, to provide a signal for timely implementing the decision and updating procedures.

One and two dimensional parallel partial response for parallel readout optical memories

Abstract: We extend partial response (PR) precoding to two-dimensions and consider it, as well as parallel one-dimensional (1D) PR, for use in parallel readout optical memory systems. We also develop expressions for optically implementable two-dimensional (2D) zero-forcing equalizers to be used in conjunction with these forms of PR precoding.

Adaptive quantization for fading channels with feedback

Abstract: The effect of the presence of a feedback channel on the transmission of information was first considered by Shannon, who showed that the capacity of a memoryless channel is not increased by the existence of a feedback link even if the feedback link is noiseless. Later it was shown that the information on a feedback channel can be used to improve considerably the performance of channel coding. In this work we study the transmission of an information source through a fading channel with feedback, modeled by a finite-state channel in the Gilbert-Elliot sense. We show that by employing the feedback information in the quantizer design for this finite-state channel, one can achieve lower overall distortion compared to the case where feedback is not available. The feedback channel is used to estimate the channel state using a hidden Markov model, and a quantizer matched to the channel state is chosen based on this information.

Source correlation in a K-out-of-M multiple-access channel

Abstract: A K-out-of-M multiple-access channel is a mathematical model for transmission of information between a large number of potential transmitters and one receiver, where at any transmission interval only a (usually small) number of transmitters are actively transmitting their messages. We study the problem of matching between correlated information sources and a K-out-of-M multiple-access channel. We show that by employing the correlation between the information sources and using a two-step coding scheme better results can be achieved compared to separate source and channel coding. A coding and decoding scheme is described and sufficient conditions for reliable transmission of the correlated information sources via the K-out-of-M multiple-access channel, based on this scheme are obtained. >

The effect of an randomly time-varying channel on mutual information

Abstract: We examine the effect of a randomly time-varying channel on mutual information between the receiver and sender when the channel is m/sup th/ order Markov. Such channels often occur in mobile communications and can affect the achievable rate. If the channel is perfectly known, then the mutual information between a receiver and an arbitrary number of senders may be found, even if the channel is time-varying.

Phase precoding with constant amplitude for slowly fading channels

Abstract: The paper presents a novel phase precoding (pre-equalization) method for slowly fading channels, which achieves intersymbol interference (ISI) cancellation with only transmitted signal phase being precoded. The method improves the transmission performance without increasing the complexity of portable units. Theoretical analysis and computer simulation results demonstrate that, over frequency-selective Rayleigh fading and Rician fading channels, differential quadrature phase-shift-keying (QPSK) using the channel precoder has significantly lower bit error rates than those using a conventional decision-feedback equalizer (DFE), because the precoder achieves a higher efficiency of the transmitted signal power and is not subject to error propagation. The study is of interest to indoor radio communications.

A Technique to Reduce Error Propagation in M-ary eedback Equalization

Abstract: A technique is described for reducing the error prop- agation resulting from the use of decision feedback equalization in an M-ary, varying system. The technique is applicable when the actual system response varies moderately about a nominal response that can be approximated by a partial response. The decision feedback equalizer (DFE) is supplemented with partial response precoding and detection such that the resulting error propagation is a function of only the difference between the actual system response and the partial response. Simulation results for a system with a nominal response equal to the duobinary partial response are given to illustrate the performance improvement. I. INTRODUCTION REQUIREMENT often faced in digital communications system design is spectrally efficient transmission over noisy intersymbol interference (ISI) channels. A method of reducing the degrading effects of the channel that has found many practical applications is decision feedback equalization. A DFE is shown in Fig. 1. The feedforward filter (FFF) acts primarily as a phase equalizer which suppresses the precursive ISI, while the feedback filter (FBF) uses decisions on the transmitted data in an attempt to remove the postcursive ISI.

Spread-response precoding system for wireless transmission

Abstract: Method and apparatus for preparing a message stream of symbols for wireless transmission by pre-coding it through convolution with a characteristic "signature" of predetermined symbols identifying a particular user of the message system. The time duration of the signature should exceed the inter-symbol interval of the message stream in order to reduce the likelihood of error caused by fading in the transmission channel.