Sequential sequence estimation for trellis-coded modulation onmultipath fading ISI channels

A new receiver structure is proposed for trellis-coded modulation on multipath fading intersymbol interference (ISI) channels that permits the use of large-state trellis codes. The receiver uses a sequential sequence estimator with the Fano algorithm, and a channel estimator consisting of a fast nonrecursive start-up algorithm for training and the LMS algorithm for tracking. During the tracking mode, the channel estimates are updated dynamically by using recent tentative decisions produced by the sequential sequence estimator. This approach results in good tracking even on rapidly varying channels, and reduces the degradation in performance of the sequential sequence estimator due to channel estimation error. The effect of fading is mitigated using both implicit time diversity in the form of interleaved trellis-coded modulation and explicit antenna diversity     

Performance of trellis codes for a class of equalized ISI channels

The performance of trellis codes is examined for a class of intersymbol interference (ISI) channels that occur in high-frequency radio systems. The channels considered are characterized by in-band spectral nulls and by a rapid time variation. The baseline modulation technique is 4QAM (four-point quadrature amplitude modulation). When spectral nulls are absent, performance of fractionally spaced linear equalizers and trellis decoders is found to be near ideal and to be better than using symbol-spacing in the equalizer. However, error propagation in the feedback path, resulting from equalizer-based decisions, ruins the performance of the combination of decision-feedback equalizers and trellis decoders when spectral nulls are present. Their performance can be improved by using fractionally spaced feedforward equalizer sections and by designing the decoder to compensate for ISI. Rate 2/3 codes are found to outperform rate 1/2 codes in error performance     

Performance analysis of asymptotically optimal noncoherentdetection of trellis-coded multi-amplitude/-phase modulation signals inGaussian noise and ISI channels

Performance upper bounds for noncoherent receivers employed in conjunction with single and multi-amplitude/-phase signals, transmitted over time dispersive and Gaussian noise channels are derived. Based upon a metric which has been previously derived by the authors, we present analytical expressions and computer generated results for the performance of asymptotically optimal noncoherent detection over such channels. As a typical application of the developed theoretical analysis, we consider wideband telecommunication systems. Where time dispersion resulting in intersymbol interference (ISI) is one of the significant sources of system performance degradation. Numerical evaluation of the optimal noncoherent decoding algorithms, shows the proposed bounds to be an effective and efficient means of evaluating the performance of the noncoherent receivers under investigation. Using the derived bounds, performance evaluation results for modulation schemes such as π/4-shift DQPSK (differential quadrature phase shift keying), 8- and 16-DQAM (differential quadrature amplitude modulation), at very low bit-error rates (BER), which would otherwise pose impractically high computational loads when using Monte-Carlo error counting techniques, are readily obtained. At BER>10^-4 evaluation results generated via computer simulation have verified the tightness of the bounds     

Optimized signal constellations for trellis-coded modulation onAWGN channels

Previously, performance gains over Ungerboeck type trellis-coded modulation schemes were obtained by optimizing (by hand) the signal constellation. Using genetic algorithms and simulated annealing, we have found additional cases with performance gains over the Ungerboeck type schemes     

Performance bounds for trellis-coded modulation on time-dispersivechannels

The performance of trellis-coded modulation (TCM) on additive white Gaussian noise channels is well understood, and tight analytical bounds exist on the probability of the Viterbi decoder making a decision error. When a channel is also time-dispersive, the performance of TCM systems has been studied mainly by simulation. However, simulation is limited to symbol error probabilities greater than 10^-6 and is not a particularly useful tool for designing codes. Tight analytical bounds on the error probability of TCM on time-dispersive channels are required for a more thorough study of performance. Moreover, the design of good codes and optimum metrics for time-dispersive channels requires tight analytical bounds. In this paper we derive analytical upper bounds, which, although requiring numerical techniques for tractable evaluation, are tight for a wide range of time-dispersive channel conditions. The bounds are based on a union bound of error events that leads to a summation of pairwise error probabilities, which are themselves upper bounded     

Optimum asymmetric constellation design for trellis-coded modulation over gaussian channels

This letter studies the optimization of asymmetric constellations for trellis-coded modulation (TCM). Unlike the conventional criterion of maximizing the free Euclidean distance, the criterion here is to minimize the bit error rate (BER) of TCM systems. The letter presents an easy way to derive the BER upper bound for any TCM codes, and proposes the method for designing optimum asymmetric constellations by minimizing the bound. The given method can be applied to TCM systems with arbitrary state numbers. Numerical results verify that all the codes designed by the proposed method achieve better BER performance than symmetric ones.     

The effects of constellation density on trellis-coded modulation infading channels

Dense constellations such as 16-QAM (quadrature amplitude modulation) have not seen much use in mobile communication because of their greater peak-to-average power ratio and their seemingly greater sensitivity to noise and channel interference. It is demonstrated that dense constellations can actually improve performance. Using a completely analytical method, three constellations are compared with the same net throughput of 2 bits/symbol: uncoded QPSK (quadrature phase-shift keying), rate 2/3 TCM (trellis-coded modulation) 8-PSK and rate 1/2 TCM 16-QAM. Comparison on the basis of average power puts TCM 16-QAM 5-dB ahead of TCM 8-PSK (phase-shift keying). Even comparison on the basis of peak power gives TCM 16-QAM a 2.44-dB advantage over TCM 8-PSK. QPSK is much poorer than either     

Error performance of MPSK trellis-coded modulation overnonindependent Rician fading channels

This paper presents new upper bounds on the pairwise error probability (PEP) of trellis-coded modulation (TCM) schemes over nonindependent Rician fading channels. Cases considered are coherent and pilot-tone-aided detection and differential detection of trellis-coded multilevel phase-shift keying (TC-MPSK) systems. The average bit-error probability P_b can be approximated by truncating the union bound. This method does not necessarily lead to an upper bound on P_b, and, hence, the approximation must be used with simulation results. In addition, for Rayleigh fading channels with an exponential autocovariance function, bounds resembling those for memoryless channels have been derived. The bounds are substantially more accurate than Chernoff bounds and hence allow for accurate estimation of system performance when the assumption of ideal interleaving is relaxed     

A performance analysis of trellis-coded modulation schemes overRician fading channels

This paper presents a saddle point approximation (SAP) method to compute the pairwise error probability (PEP) of trellis-coded modulation (TCM) schemes over Rician fading channels. The approximation is applicable under several conditions, such as finite and ideal interleaving, ideal coherent and pilot-tone aided detection, and differential detection. The accuracy of this approximation is demonstrated by comparison to the results of numerical integration. When ideal interleaving is assumed, an asymptotic approximation for the PEP of ideal coherent, pilot-tone aided or differentially detected TCM is derived. This asymptotic approximation of the PEP is in a product form and much tighter than the ordinary Chernoff bound on the PEP. Also, based on the SAP, the effect of finite interleaving depth on the error performance of TCM schemes over Rician and shadowed Rician channels is studied     

Blind equalization for an application of unitary space-time modulation in ISI channels

Transmit diversity schemes have gained attention due to the promise of increased capacity and improved performance. Among these schemes, unitary space-time modulation and differentially encoded unitary space-time modulation allow for simple noncoherent decoding for flat-fading channels. In this paper, a new blind equalization algorithm for these transmission schemes in intersymbol interference (ISI) channels is proposed. A matrix-type constant modulus algorithm that exploits the unitary structure of the space-time codes is developed. The equalizer is paired with a noncoherent decoder, resulting in a completely blind, low-complexity method for decoding in the presence of ISI. A noiseless convergence analysis is conducted and verified via simulation in both noiseless and noisy cases. The performance of the overall system is evaluated via simulation and semi-analytically, and the achieved performance is between that of the ideal zero-forcing and the minimum-mean squared-error equalizers.     

Improved codes for space-time trellis-coded modulation

Space-time coded modulation has been shown to efficiently use transmit diversity to increase spectral efficiency. We propose new trellis codes found through systematic code search. These codes achieve the theoretically maximal diversity gain and improved coding gain compared to known codes     

EXIT charts for turbo trellis-coded modulation

In this letter, we extend the previously proposed extrinsic information transfer charts (EXIT) method to the analysis of the convergence of turbo codes to turbo trellis-coded modulation (TTCM) schemes. The effectiveness of the proposed method is demonstrated through examples. The proposed method provides a convenient way to systematically compare between schemes and thus can be used as a tool in the design of TTCM.     

Efficient performance computations for trellis-coded modulation

The algorithm given by Rouanne and Costello (1989) for the computation of the distance spectrum is improved for trellis-coded modulation schemes having uncoded bits, i.e., for trellis diagrams having parallel paths. It is shown that, when through a trellis corresponding to such kind of codes, all parallel transitions (labeled by signal selectors) between states are considered as a single branch labeled by a subset, then defining subset selector distance polynomials makes the computational complexity of the distance spectrum dependent on the number of states as compared to the complexity of Rouanne and Costello algorithm which depends on the number of paths to be extended     

Performance of interleaved trellis-coded differential 8-PSKmodulation over fading channels

The performance of trellis-coded differential octal phase-shift keying (coded 8-DPSK) with differentially coherent detection and soft-decision Viterbi decoding is investigated. A suitable receiver is presented whose signal processing is based on Nyquist signaling, requiring only one complex sample per modulation interval. Symbol synchronization and automatic frequency control are performed in a decision-directed way. Bit-error-rate (BER) performance over Gaussian, Rayleigh, and Rician channels is determined by means of computer simulations. The performance of coded 8-DPSK on the Gaussian channel is shown for a four-state convolutional trellis code. The unquantized outputs of up to three symbol detectors with delays of 1, 2, and 3 symbol periods are used for metric computation. The coding gain which includes losses due to timing and frequency synchronization errors is found to be 2.5 dB at BER=10^-5 with respect to uncoded 4-DPSK. Much larger gains are achieved for fading channels if interleaving is applied. Using an eight-state trellis code the performance is determined on Rayleigh and Rician channels for various Doppler spreads and interleaver sizes     

Symbol-interleaver design for turbo trellis-coded modulation

A design method for a code-matched symbol-interleaver for turbo trellis-coded modulation is proposed. The method constructs an interleaver by code matched criteria to eliminate some input error events of low symbol Hamming weight (SHW) sequences of the code which can produce low SHW codewords. Numerical results confirm that the interleaver can lower the error floor at moderate to high signal-to-noise ratio.     

A maximum likelihood approach for estimating DS-CDMA multipathfading channels

We propose a maximum likelihood approach for near-far robust synchronization of asynchronous direct sequence code division multiple access (DS-CDMA) systems operating over multipath channels. The algorithm is suitable for use in, for instance, a slotted system where each user transmits a short data burst with an embedded training sequence. The algorithm is shown to outperform the standard sliding correlator estimator. The Cramer-Rao bound is derived and is used to indicate the best performance that can be achieved by an unbiased estimator     

Generalized APP detection of continuous phase modulation over unknown ISI channels

Techniques for computing soft information in the presence of unknown intersymbol interference are presented, with a particular focus on iterative detection of serially concatenated continuous phase modulation. The techniques are centered around the recursive least-squares algorithm, thus enabling unsupervised detection. In particular, we employ bidirectional estimation.     

Adaptive trellis-coded multiple-phase-shift keying for Rayleighfading channels

An adaptive scheme for trellis-coded modulation of MPSK signals, called adaptive trellis-coded multiple-phase-shift keying (ATCMPSK), is proposed for slowly Rayleigh fading channels. The adaptive scheme employs a slightly modified rate 1/2 convolutional encoder and the corresponding Viterbi decoder to realize a family of codes of different rates which are employed according to channel conditions. During poor channel conditions, trellis-coded QPSK (TCQPSK) together with repetition schemes are employed. As channel conditions improve, higher rate schemes such as trellis-coded 16 PSK are used. An interleaving/deinterleaving method suitable for the adaptive scheme is proposed. Theoretical bounds for the error performance and an exact expression for the throughput of the proposed adaptive scheme are derived, and are compared against simulation results. Simulations have been performed to measure the performance of the scheme for different parameters and some nonideal conditions. It is shown that ATCMPSK results in considerable improvement in bit-error-rate (BER) performance of MPSK signals. Under ideal conditions, gains in the range of 3-20 dB are achieved over conventional fixed rate pragmatic trellis-coded schemes     

Adaptive trellis-coded modulation for bandlimited meteor burstchannels

The throughput performances of three adaptive information rate techniques on the bandlimited meteor burst channel are investigated. Closed-form expressions for throughput are derived based on the channel model commonly used in the literature. The throughput performance is compared to the conventional fixed information rate modem and upper bounds on throughput improvement over the fixed rate modem are derived. It is shown that an adaptive technique that uses trellis-coded modulation (TCM) with three phase-shift keyed (PSK) signal sets can increase throughput over the conventional fixed rate modem by more than a factor of 3. Data from the US Air Force High Latitude Meteor-Scatter Test Bed confirm the superiority of the adaptive TCM technique. A practical implementation is suggested that uses a single rate 1/2 convolutional code for all three PSK signal sets. The use of this single code, versus the three best Ungerboeck codes, results in a throughput loss of less than 2%. An expression for the theoretical information capacity of the bandlimited meteor burst channel is derived