Viterbi-type Algorithm Research Articles

Adaptive Maximum Likelihood Sequence Detection for QPSK Coherent Optical Communication System

A novel Viterbi-type adaptive maximum likelihood sequence detection algorithm is developed and investigated in coherent optical communication systems in the presence of both laser phase noise and nonlinear phase noise. Compared with the nonadaptive version, it can self-adjust the effective memory length of the phase estimator and automatically deliver the optimum bit-error rate performance without requiring any prior statistical knowledge of the optical channel. The phase error variance of the estimator is also analytically investigated.

Sequence detection for MPSK/MQAM with adaptive phase tracking

In [P.Y. Kam, P. Sinha, A Viterbi-type algorithm for efficient estimation of MPSK sequences over the Gaussian channel with unknown carrier phase, IEEE Trans. Commun. COM-43 (1995) 2429–2433], we developed a Viterbi algorithm for efficient detection of M -ary phase-shift keyed (MPSK) sequences received over the additive, white, Gaussian noise (AWGN) channel with an unknown carrier phase. By assuming the carrier phase is constant over some L symbol intervals and using the sequence detection metric in [P.Y. Kam, Maximum-likelihood digital data sequence estimation over the Gaussian channel with unknown carrier phase, IEEE Trans. Commun. COM-35 (1987) 764–767] as the decoding metric, it approaches the performance of coherent detection, provided the observation interval L for forming the decoding metric is sufficiently long. However, this observation interval L is fixed, and chosen based on prior knowledge of the carrier phase characteristics. Thus, the metric is nonadaptive, and cannot be optimized when a priori statistical knowledge of the carrier phase is not available. The work here renders it adaptive by developing a recursive metric that is adapted on-line based on the received signals, without prior knowledge of the carrier phase characteristics. We first derive a metric for the maximum likelihood sequence detector for a general M -ary quadrature amplitude modulation (MQAM) sequence received in the presence of AWGN and an unknown, slowly- and randomly-varying carrier phase. To reduce the receiver complexity, a practical, suboptimal metric is obtained. A recursive form of the metric using adaptive reference estimation as in [P.Y. Kam, K.H. Chua, X. Yu, Adaptive symbol-by-symbol reception of MPSK on the Gaussian channel with unknown carrier phase characteristics, IEEE Trans. Commun. COM-46 (1998) 1275–1279], is proposed. Comparison between the proposed adaptive, recursive metric and those in the literature is provided. Simulation results using a random-walk carrier phase model show the superior performance of the adaptive sequence detector over the original nonadaptive sequence detector of [P.Y. Kam, P. Sinha, A Viterbi-type algorithm for efficient estimation of MPSK sequences over the Gaussian channel with unknown carrier phase, IEEE Trans. Commun., COM-43 (1995) 2429–2433].

Viterbi-Type Inter-Carrier Interference Equalization for OFDM over Doubly Selective Channels

Orthogonal Frequency Division Multiplexing (OFDM) transmission is robust to frequency-selective channels but sensitive to time-selective channels. Time variations of channels generate inter-carrier interference (ICI), which degrades system performance. In this paper, we develop frequency-domain Viterbi-type algorithm to effectively suppress the ICI, by exploiting the property of ICI terms in OFDM symbols. Null subcarriers, which are embedded in OFDM symbols for the reduction of interferences from/to adjacent bands, are exploited to reduce the complexity of the algorithm. Then, an approximate BER expression of Viterbi-type equalizer is derived. Simulations are provided to show the Viterbi-type equalizer works well in time- and frequency-selective (doubly selective) channel with affordable complexity.

Non-Coherent and Mismatched-Coherent Receivers for Distributed STBCs with Amplify-and-Forward Relaying

Cooperative diversity is a transmission technique where multiple nodes in a network cooperate to form a virtual antenna array realizing the benefits of spatial diversity in a distributed fashion. The coherent scenario considered in most existing work on cooperative diversity assumes the availability of perfect channel state information at the relay and destination terminals and is highly unrealistic in practical applications. In this paper, we investigate non-coherent and mismatched-coherent receivers for a cooperative diversity scheme assuming both quasi-static and time-varying fading channels for the underlying cooperative links. Specifically, we consider a distributed space- time block coded (STBC) system in a single-relay scenario operating in the amplify-and-forward relaying mode. Exploiting the orthogonal structure of distributed STBC, we first derive a non-coherent decoding rule which can be implemented in practice by a Viterbi-type algorithm. Although this decoding rule has been derived assuming quasi-static channels,- its inherent channel tracking capability allows its deployment over time- varying channels with a promising performance as a sub-optimal solution. As a possible alternative to non-coherent detection, we investigate the performance of mismatched-coherent receiver (i.e., coherent detection with imperfect channel estimation) within the considered relay-assisted transmission scenario. We further compare the performance of non-coherent and mismatched- coherent receivers to reveal their robustness under various mobility scenarios.

An efficient implementation of a maximum-likelihood detector for space - time block coded systems

We investigate maximum-likelihood (ML) sequence estimation for space-time block coded systems without assuming channel knowledge. The quadratic form of the ML receiver in this case does not readily lend itself to efficient implementation. However, under quasi-static channel conditions, the likelihood function reduces to a simple form similar to the classical correlation receiver in matrix notation. It also allows the development of a recursive expression that can be easily implemented by a Viterbi-type algorithm with a reasonable complexity. Although the receiver is suboptimum for the nonstatic case, its performance is close to the optimum for a range of signal-to-noise ratios.

Low Complexity Receiver Structures for Space-Time Coded Multiple-Access Systems

Multiuser detection for space-time coded synchronous multiple-access systems in the presence of independent Rayleigh fading is considered. Under the assumption of quasi-static fading, it is shown that optimal (full diversity achieving) space-time codes designed for single-user channels, can still provide full diversity in the multiuser channel. The joint optimal maximum likelihood multiuser detector, which can be implemented with a Viterbi-type algorithm, is derived for such space-time coded systems. Low complexity, partitioned detector structures that separate the multiuser detection and space-time decoding into two stages are also developed. Both linear and nonlinear multiuser detection schemes are considered for the first stage of these partitioned space-time multiuser receivers. Simulation results show that these latter methods achieve performance competitive with the single-user bound for space-time coded systems.

Data-aided estimation of carrier frequency for burst detection of QAM

To overcome large carrier frequency offsets in burst demodulation of quadrature amplitude modulation, a new Viterbi-type algorithm coupled with a data-aided frequency estimation is developed. This algorithm can quickly acquire carrier offset, then remove it during the process of demodulation. Its performance is confirmed as approaching that of coherent demodulation, with little loss of short pilot data.

Fault detection and isolation in hybrid systems, a petri-net approach

A new Petri-net based approach is proposed for FDI of hybrid systems. It consists of three components: first, one derives a model of the overall discrete state of the system, in which both stochastic failure events and deterministic control events interact; faults are modeled as the hidden discrete state part of the system. Second, the continuous state part is modeled and ones shows how it can be linked to the hidden discrete part, through statistical residual generation. Third, diagnosis is performed by a Viterbi-type algorithm, which estimates the most likely sequence of hidden failed states, given a sequence of residuals.

Simplified trellis decoder

A new simplified trellis decoder (STD) Viterbi-type algorithm is proposed for fast trellis decoding of rate K/K+1 binary convolutional codes. Viterbi algorithm (VA) computation is dominated by add-compare-select (ACS) operations when k/spl ges/2. The STD can substantially reduce the number of ACS operations and allow for a trade-off between the computational load and the performance of the decoder, The STD is analyzed and simulated for a four-dimensional (4-D) rate 4/5 64-state convolutional encoder specified by the ITU-T V.34 modem recommendation.

Adaptive Viterbi estimation of MPSK sequences over a Rayleigh fading channel

A Viterbi type algorithm is presented for adaptive estimation of MPSK sequences over a Rayleigh fading channel with unknown fading spectrum and unknown additive channel noise intensity. It uses a new, weighted least-squares decision metric. Simulations show significant performance gains over both differential detection and adaptive symbol-by-symbol detection proposed in .

A Viterbi-type algorithm for efficient estimation of M-PSK sequences over the Gaussian channel with unknown carrier phase

A Viterbi-type algorithm for the efficient estimation of a convolutionally/trellis coded, or uncoded M-PSK sequence over the Gaussian channel with unknown carrier phase is presented. It performs coherent detection of M-PSK sequences with no explicit recovery of coherent references. The metric computations involve only linear operations. Theoretical and simulated performance results are presented for various carrier phase processes. The algorithm is called the suboptimum ML estimator (SMLE). Its BER in the presence of a time invariant carrier phase is obtained analytically. The simulation results for the BER with time-invariant as well as time-varying carrier phase processes allow a comparison of the performance of the SMLE with other sequence estimation algorithms, and enable the applicability of the SMLE to burst mode TDMA communications to be ascertained. >