Institute for Systems Research

First, several fundamental issues regarding the statistical properties of fading channels are provided. We demonstrate the constraints, that must be satisfied so that the channel can be regarded as impaired by ﲦlat (i.e., non-frequency-selective) fading with a constant fading factor over each symbol duration. Throughout this thesis these constraints are assumed to be satisfied.

We next investigate the channel capacity and cutoff rates for fading channels with DPSK-modulated input signals and perfect symbol interleaving. The impact of the channel state information (CSI), on these information-theoretic limits is also discussed. We introduce several symbol metrics for soft-decision decoding, and their performance is investigated by analytical derivation as well as by simulation. Furthermore, we define a bit metric for DQPSK modulation, and compare this bit metric to dibit (symbol) metrics.

We then consider the problem of error concealment for mobile radio, communications with a maximum-likelihood soft- decision decoder. A normalized codeword reliability is defined as the decision reliability information when CSI is not available. We employ a given, interpolation algorithm on a particular land mobile radio system and design a rule for selecting unreliable codewords. Simulation results show that error concealment can decrease the minimum operational signal-noise ratio (SNR) by 3 dB or more.

Finally, we address the problem of exploiting the residual redundancy in the source to enhance the channel decoder's performance -i.e., maximum a posteriori (MAP) decoding. We use two simple source models to demonstrate that MAP decoding can achieve significant gain over maximum-likelihood decoding. We employ a practical CELP-based land mobile radio system to show that significant residual redundancy does exist in the output of some source encoders. Simulation results show that a 2 - 3 dB gain can be achieved by MAP decoding (over ML) at low SNR.

We first investigated the reference signal based adaptive diversity combining algorithm, which conventionally relies on feedback symbols in the absence of reference signals. Our computer simulation revealed that on a fast time varying fading channel, error propagation in the decision directed tracking mode severely degrades the performance. We developed a simultaneous diversity combining and decoding technique which incorporated QR decomposition-based recursive least-square parallel weights tracking and M-D decoding algorithms. In contrast to the conventional system where only one set of array weights is kept and updated, in our system, we update M sets of candidate weights. Thus we are able to make a more reliable symbol decision based on D symbols without compromising weights tracking speed. The M-D algorithm was first developed for the binary convolutional codes and then extended to Trellis-coded modulation. This technique significantly reduces error propagation. Simulation results showed that about 8 to 10dB improvement in the total interference suppression at low ISR and about 5dB improvement at high ISR can be achieved with a moderate increase in complexity.

In the next part of the dissertation, we proposed and studied the use of the constrained adaptive array algorithm for extracting signals from interferences at separable directions. This algorithm requires direction-of-arrival (DOA) information and does not need reference signals. However, most of the high resolution DOAs estimation methods are only effective for noncoherent signals, while in mobile radio channels, coherent signals are inevitable. We developed a general spatial smoothing (SS) technique and a forward backward spatial smoothing technique for two dimensional arrays to decorrelate coherent signals from arbitrary directions. We found and proved the necessary and sufficient conditions on an array configuration for applying SS. This array must have an orientational invariance structure with an ambiguity free center array, and the number of subarrays must be larger than or equal to the size of the largest group of coherent signals. We also studies the causes of ambiguities and found some ambiguity free array manifolds. We expanded the application of our SS to several high resolution DOA estimation and constrained adaptive beamforming algorithms. All the predicted results were verified by simulations. In the last part of the dissertation, we investigated the applications of adaptive array technique in DS/CDMA systems. We applied reference-signal- based simultaneous diversity combining and decoding to reduce fading and suppress interference caused by poor synchronization and power control.

The proposed codes are also simulated under less ideal assumptions. For instance, results for a 1 bit/sec/Hz IQ-TCM code without CSI show a significant gain over conventional coding. Finally, simulations over channels with correlated fading were undertaken; it is concluded that an interleaver span of 4v yields performance close to what is achieved with ideal interleaving.

Also included is an analysis of maximal ratio combining when the diversity branches are correlated; the cutoff rate and a tight upper bound on the pairwise error probability are derived. It is shown that, with double diversity, a branch correlation coefficient as high as 0.5 results in only slight performance degradation.

We begin by quantifying the amount of ﲲesidual redundancy inherent in the LSP's of Federal Standard 1016 CELP. This is done by modeling the LSP's as first and second-order Markov chains. Two models for LSP generation are proposed; the first model characterizes the intra-frame correlation exhibited by the LSP's, while the second model captures both intra-frame and inter-frame correlation. By comparing the entropy rates of the models thus constructed with the CELP rates, it is shown that as many as one-third of the LSP bits in every frame of speech are redundant.

We next consider methods by which this residual redundancy can be exploited by an appropriately designed channel decoder. Before transmission, the LSP's are encoded with a forward error control (FEC) code; we consider both block (Reed- Solomon) codes and convolutional codes. Soft-decision decoders that exploit the residual redundancy in the LSP's are implemented assuming additive white Gaussian noise (AWGN) and independent Rayleigh fading environments. Simulation results employing binary phaseshift keying (BPSK) indicate coding gains of 2 to 5 dB over soft-decision decoders that do not exploit the residual redundancy.

Supplementing a satellite link with a parallel terrestrial link may allow mitigating some problems of using ARQ in satellite communication systems. ARQ acknowledgments, and possibly retransmissions as well, can be sent terrestrially in such a hybrid network, and so avoid the large satellite propagation delay. The satellite transmission capability of a receiving station which communicates with the transmitter exclusively by terrestrial means can be eliminated and the system cost correspondingly reduced. Further, multicasting with a hybrid network may allow retransmissions to be conducted without interrupting the flow of new information to all destinations, so throughput need not drastically suffer if retransmissions are required.

The degree to which throughput can be improved by adopting a hybrid network is not clear. A hybrid network's effect on the fidelity of information delivered to the destination(s) is also not clear. An experiment is presented for investigating such error control issues of hybrid networking.

In the first part, TCM schemes that provide high coding gains over the flat, slow Rayleigh distributed fading channel are presented. It is shown that the use of two encoders in parallel used to specify the in-phase and quadrature components of the transmitted signal results in large performance improvements in bit error rates when compared with conventional TCM schemes in which a single encoder is used. Using this approach which we label ﲉ-Q TCM codes with bandwidth efficiencies of 1, 2, and 3 bits/sec/Hz are described for various constraint lengths. The performance of these codes is evaluated using tight upper bounds and simulation.

In the second part, the use of space diversity with three different combining schemes is investigated. Expressions for the cutoff rate parameter are shown for the three combining schemes over the fully interleaved Rayleigh-distributed flat fading channel. Also, tight upper bounds on the pairwise error probability are derived for the three combining schemes. Examples of I-Q TCM schemes with diversity combining are shown. The cutoff rate and a tight upper bound on the pairwise error probability are also derived for maximal ration combining with correlated branches.

In the last part the problem of reliably transmitting trellis coded signals over very noisy channels is considered. Sequence maximum a posteriori (MAP) decoding of correlated signals transmitted over very noisy AWGN and Rayleigh channels is presented. A variety of different systems with different sources, modulation schemes, encoder rates and complexities are simulated. Sequence MAP decoding proves to substantially improve the performance at very noisy channel conditions especially for systems with moderate redundancies and encoder rates. A practical example for coding the CELP line spectral parameters (LSPs) is also considered. Two source models are used. Coding gains of as much as 4 dB are achieved.

The proposed codes are also simulated over channels with correlated fading; it is concluded that an interleaver span of 4v yields performance close to what is achieved with ideal interleaving.

We begin by quantifying the amount of ﲲesidual redundancy inherent in the LSP's of Federal Standard 1016 CELP. This is done by modeling the LSP's as first-and second-order Markov chains. Two models for LSP generation are proposed; the first model characterizes the intra-frame correlation exhibited by the LSP's, while the second model captures both intra-frame and inter-frame correlation. By comparing the entropy rates of the models thus constructed with the CELP rates, it is shown that as many as one-third of the LSP bits in every frame of speech are redundant.

We next consider methods by which this residual redundancy can be exploited by an appropriately designed channel decoder. Before transmission, the LSP's are encoded with a forward error control (FEC) code; we consider both (Reed-Solomon) codes and convolutional codes. Soft-decision decoders that exploit the residual redundancy in the LSP's are implemented assuming additive white Gaussian noise (AWGN) and independent Rayleigh fading environments. Simulation results employing binary phase-shifting keying (BPSK) indicate coding gains of 2 to 5 dB over soft-decision decoders that do not exploit the residual redundancy.