Maximum Likelihood Sequence Estimation Equalizer Research Articles

Joint prefiltering and MLSE equalization of space-time-coded transmissions over frequency-selective channels

The problem of designing a front-end prefilter to improve the performance and/or reduce the complexity of maximum likelihood sequence estimation equalization of space-time-coded signals is addressed in this paper. The front-end prefilter performs channel shortening without excessive noise enhancement and is constrained to be a finite impulse response filter for practical implementation. Transmission scenarios emphasized assume two transmit antennas (with delay diversity or space-time trellis coding) and either one or two receive antennas. Extensions to more antennas are straightforward. Various design parameters (such as number of prefilter taps, number of equalizer states, and decision delay) are optimized using Monte Carlo simulations in a typical urban EDGE environment.

100-Mbit/s single-chip Q-VLMS MLSE equalizer LSI for TDMA mobile radio communications

A single-chip 100-Mbit/s burst-operation two-tap maximum likelihood sequence estimation (MLSE) equalizer LSI for QPSK signals is introduced. It also supports two-branch diversity combining. Three new techniques are used to realize this fast equalizer LSI: the quantized variable-gain least mean squares (VLMS) algorithm, which has small processing delay with fast convergence characteristics; a simple complex-valued multiplication scheme based on inverting the sign and switching the in-phase and quadrature-phase components; and a parallel structure to minimize the processing delay of path memory. The chip, containing 75 kgates, is manufactured using the 0.45-/spl mu/m-CMOS gate array process. The supply voltage is 3.3 V. This LSI offers higher processing speed than any other conventional equalizer chip for mobile radio communications.

Design and performance of 32-kbit/s ?/4-QPSK VLMS MLSE equalizer

This article considers an adaptive maximum-likelihood sequence estimation (MLSE) equalizer that can process signals at high speed and discusses its implementation in hardware, together with its performance. The variable-gain least-mean-square (VLMS) algorithm, for which circuit construction is relatively easy, is used for channel estimation. The estimation process is applied to each state to improve tracking performance. The subinterval sampling structure is used to avoid performance deterioration due to the sampling phase. As the first step, pipeline processing for efficient channel estimation and sequence estimation, as well as reduction of computational complexity by avoiding matrix manipulation in the fractional-interval VLMS algorithm, are discussed. The experimentally constructed VLMS-MLSE equalizer can process the air interface for a 384-kbit/s π/4-QPSK signal in real time at 32 kbit/s. It is shown that the floor error in the two-wave Rayleigh fading channel with a delay spread of 500 ns can be reduced to 10–5 or less and that a diversity gain of 5 dB is realized for BER = 10–3. When the experimentally constructed equalizer is applied to a selective diversity receiver system, the same performance is experimentally demonstrated as the theoretical value for delayed detection when there is no delayed wave in the same channel. © 2000 Scripta Technica, Electron Comm Jpn Pt 1, 83(5): 47–56, 2000

High‐speed QVLMS–MLSE equalizer for mobile communications

This paper considers the adaptive maximum-likelihood sequence estimation (MLSE) equalizer as a means of delay distortion compensation that is indispensable in high-speed signal transmission of megabit order in mobile communications. Improvement of the processing speed and reduction of the circuit scale are discussed. As a method for channel estimation we propose the quantized variable-gain least-mean-squares (QVLMS) algorithm, in which multiplication processes are eliminated by approximating the gain function in the VLMS algorithm by a power of 2. A method for reducing the circuit scale is also proposed, utilizing symmetry of mapping rotation in the PSK/QAM modulation. Based on the proposed method, a two-tap equalizer for QPSK modulation was constructed in the laboratory, and it is shown that the circuit can be realized at a scale of approximately 40 kgates even at the FPGA level. An equalizer was also constructed for a two-wave independent multipath Rayleigh fading channel with a transmission rate of 1.5 Mbits/s and a delay spread of 650 ns, and excellent performance was realized, reducing the floor error to less than 10–5 and completing initial convergence in eight symbols. © 1999 Scripta Technica, Electron Comm Jpn Pt 1, 82(10): 40–49, 1999

Orthogonal-transformed variable-gain least mean squares (OVLMS) algorithm for fractional tap-spaced adaptive MLSE equalizers

A fast channel-estimation scheme for adaptive maximum-likelihood sequence-estimation (MLSE) equalizers called the orthogonal-transformed variable-gain least mean squares (OVLMS) algorithm is proposed. This algorithm requires only as many operations as the least mean squares algorithm in spite of its excellent performance. Furthermore, an operational complexity reduction method is proposed in which the orthogonal matrix is reconfigured as eigenvectors with valid eigenvalues. The OVLMS algorithm is theoretically analyzed and is shown to have both a fast acquisition and a good tracking performance. An equalizer using OVLMS (OVLMS-MLSE) experimentally attains a 5-dB improvement in bit-error rate (BER) performance at BER of 1.0/spl times/10/sup -4/ over coherent detection. The OVLMS-MLSE is found to be free of the degradation caused by sampling phase error. Finally, the OVLMS-MLSE equalizer is experimentally verified to synchronize within five symbols.

MLSE and soft-output equalization for trellis-coded continuous phase modulation

This paper presents two equalizer structures for trellis-coded continuous phase modulation (TC-CPM) on multipath fading intersymbol interference (ISI) channels. An equivalent discrete-time (DT) model is developed by combining the tapped-delay-line (TDL) model of the frequency-selective channel and by oversampling at the receiver. The (noninterleaved) fractionally spaced maximum-likelihood sequence estimation (MLSE) equalizer performs continuous phase modulation (CPM) demodulation, trellis-coded modulation (TCM) decoding, and channel equalization by exploiting the finite state nature of the ISI-corrupted TC-CPM signal. Both simulation and analytical results show diversity-like improvement when performing joint MLSE decoding and equalization. For the interleaved soft-output equalizer, the soft symbol metric is delivered to the TCM decoder by using a forward and backward recursion algorithm. Three variants of the soft-output equalizer are examined. We conclude that the backward recursion is essential to partial response CPM schemes, and with moderate complexity, the soft-output equalizer can have a substantial advantage over a noninterleaved MLSE equalizer.

An adaptive truncated MLSE receiver for Japanese personal digital cellular

This paper describes a dual-mode Japanese personal digital cellular receiver that uses an adaptive truncated symbol-spaced maximum-likelihood sequence-estimation (MLSE) equalizer in one mode and a tangent type differential detector in the other. The receiver employs a channel estimation and symbol synchronization procedure that uses the known phase shifts between successive symbols in the synchronization word. Per-survivor processing is used to track the channel variations and carrier frequency offset. Simulation results are presented for multipath Rayleigh fading channels having various delay profiles. Comparisons between the regular symbol-spaced truncated MLSE equalizer and a fractionally spaced truncated MLSE equalizer are also furnished.

Fast channel impulse response estimation scheme for adaptive maximum likelihood sequence estimation equalizer—proposal of variable‐gain least mean square algorithm

AbstractThis paper discusses the channel estimation algorithm in the adaptive maximum likelihood sequence estimation (MLSE) and proposes the variable‐gain least square (VLMS) algorithm that can realize a fast acquisition by a simple configuration. When the sent code is a random sequence, VLMS is represented by a deterministic canonical equation based on the fact that a deterministic autocorrelation matrix for VLMS can be diagonalized.It is shown that VLMS can be realized with nearly the same computational complexity as least‐mean square (LMS) algorithm. It is shown theoretically by analysis that VLMS can realize the same fast acquisition and tracking characteristics as those of the recursive least square (RLS) algorithm. An algorithm also is derived which is an extension of the basic idea of VLMS and can be used for the fractional MLSE equalizer. It is shown experimentally that the fractional MLSE equalizer with VLSE algorithm can compensate the performance deterioration due to the sampling phase error. In the two‐wave Rayleigh fading environment with faT = 4.0 × 10−4, the floor error can be suppressed below 10−4. It is shown also experimentally that the proposed equalizer has an excellent synchronization performance in which the initial acquisition can be completed in approximately six symbols.

Analysis of LMS-adaptive MLSE equalization on multipath fading channels

We consider a practical maximum-likelihood sequence estimation (MLSE) equalizer on multipath fading channels in conjunction with an adaptive channel estimator consisting of a least mean square (LMS) estimator and a linear channel predictor, instead of assuming perfect channel estimates. A new LMS estimator model is proposed which can accurately characterize the statistical behavior of the LMS estimator over multipath fading channels. Based on this model, a new upper-bound on block error rate is derived under the consideration of imperfect channel estimates. Computer simulations verify that our analytical results can correctly predict the real system performance and are applicable over a wide range of the step size parameter of the LMS estimator.

MLSE equalization and decoding for multipath-fading channels

The performance of a receiver using a combined MLSE (maximum likelihood sequence estimation) equalizer/decoder and D-diversity reception is analyzed for multipath Rayleigh fading channels. An upper bound on the (decoded) bit error probability is derived. Comparisons to simulation results show that this upper bound is quite tight when the system has a high signal-to-noise ratio or when diversity reception is used. The upper bound involves an infinite series that must be truncated at a point where the remainder can be safely assumed to be small. An algorithm based on a one-directional stack algorithm is proposed for this calculation because it makes efficient use of computer memory.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Reduced-state sequence estimation for coded modulation of intersymbol interference channels

The authors investigated the detection of trellis codes designed for channels that are intersymbol interference free when they operate in the presence of intersymbol interference. A well-structured reduced-state sequence estimation (RSSE) algorithm is described which can achieve the performance of maximum-likelihood sequence estimation (MLSE) with drastically reduced complexity. Well-defined reduced-state trellises are first constructed by merging the states of the ML supertrellis using set partitioning principles. Then the Viterbi algorithm is used to search these trellises. A special case of RSSE, called parallel decision-feedback decoding, uses the encoder trellis, yet on channels with large attenuation distortion it can provide a significantly better performance than linear equalization. The performance of RSSE is examined analytically and through simulation, and then compared to that of MLSE and ideal decision-feedback equalization. It is noted that the performance advantage of RSSE can be obtained without significantly increasing the decoding delay or complicating an adaptive implementation. >