Jim K. Omura

On the Viterbi decoding algorithm

A new interpretation of the Viterbi decoding algorithm based on the state-space approach to dyamical systems is presented. In this interpretation the optimum decoder solves a generalized regulator control problem by dynamic programming techniques.

Trellis Encoding of memoryless discrete-time sources with a fidelity criterion

For memoryless discrete-time sources and bounded single-letter distortion measures, we derive a bound on the average per-letter distortion achievable by a trellis source code of fixed constraint length. For any fixed code rate greater than R(D^{\ast}) , the rate-distortion function at D^{\ast} , this bound decreases toward D^{\ast} exponentially with constraint length.

Coded Error Probability Evaluation for Antijam Communication Systems

We present a general union-Chernoff bound on the bit error probability for coded communication systems and apply it to examples of antijam systems. The key feature of this bound is the decoupling of the coding aspects of the system from the remaining part of the communication system which includes jamming, suboptimum detectors, and arbitrary decoding metrics which may or may not use jammer state knowledge.

An upper bound on the rate distortion function for source coding with partial side information at the decoder

An upper bound on the rate distortion function is obtained for source coding with partial side information at the decoder. Previous results were for complete side information, i.e. full knowledge of Y_{n} . below. A diagram given in the paper helps to describe the problem. The bound is given in

Process definitions of distortion-rate functions and source coding theorems

The standard definition of the distortion-rate function involves a limit of information-tbeoretic minimizations over distributions of random vectors. Several alternative definitions, each involving a single minimization over random processes, are presented here and verified. These definitions parallel Khinchines process definition of channel capacity, provide a new interpretation of block and nonblock source coding (with a fidelity criterion) theorems in terms of optimal stochastic codes, and provide a comparison between the optimal performance theoretically attainable (OPTA) using block and nonblock source codes. Coupling the process definitions with recently developed bounding techniques provides a new and simple proof of the block source coding theorem for ergodic sources.

Optimum linear transmission of analog data for channels with feedback

With feedback, the transmission of analog data over a channel can be regarded as a stochastic-control problem. Restricting ourselves to linear receiver operations and an average power constraint, we take this approach to find minimum mean-square error signals for multiplicative and additive noise channels with noiseless feedback and for additive noise channels with noisy feedback. Our solution for the additive Gaussian noise channel with noiseless feedback achieves the theoretical minimum mean-square error. For the noisy feedback problem, we use the result that the optimum signals are the minimum mean-square error estimates of the optimum noiseless feedback signals. This control-theoretic approach requires knowledge of only the first and second moments of all random variables and extends easily to multidimensional cases and to wide-sense Markov noise processes.

Generalized minimum shift-keying modulation techniques

The simultaneous data demodulation and phase tracking of an MSK signal using the Viterbi algorithm is described, and two variations of MSK modulation are studied. The MSK with overlay is a dual-rate modulation techniques in which low-rate and high-rate data are superimposed on an MSK signal. Here the demodulator uses the Viterbi algorithm to estimate both the low-rate and high-rate data simultaneously. The MSK with pseudorandom sequence spreading combats intentional or unintentional jamming. A simplified receiver for these spread-spectrum MSK signals is found that takes into consideration the effect of random phase perturbations. The performance of these demodulators is evaluated using transfer-function bounds for the bit error probability. For demodulation of the spread spectrum MSK signal, a simplified receiver is derived, and its performance in the presence of continuous jamming is evaluated. >

Decoding with approximate channel statistics for bandlimited nonlinear satellite channels

Expressions for the cutoff rate of memoryless channels and certain channels with memory are derived assuming decoding with approximate channel statistics. For channels with memory, two different decoding techniques are examined: conventional decoders in conjunction with ideal interleaving/deinterleaving, and maximum likelihood decoders that take advantage of the channel memory. As a practical case of interest, the cutoff rate for the band-limited nonlinear satellite channel is evaluated where the modulation is assumed to be M-ary phase shift keying (MPSK). The channel nonlinearity is introduced by a limiter in cascade with a traveling wave tube amplifier (TWTA) at the satellite repeater while the channel memory is created by channel filters in the transmission path.

A coding theorem for discrete-time sources

We present a new derivation of the source coding theorem for discrete-time sources. This proof parallels Gallagers [1] derivation of the random coding bound for channel coding theory and shows that the classical random coding exponent also emerges as a critical quantity for source coding. The major advantage of this approach is the simplicity of the derivation and its close relationship to the more familiar channel coding theory. The source coding theorem we derive here also yields a natural bound on the rate of convergence to the rate-distortion limit.

Modem Performance in VLF Atmospheric Noise

An atmospheric noise model for analyzing the performance of modulation/demodulation systems operating at VLF frequencies is presented. Performance analyses of various modems based on this noise model are compared with experimental measurements. Theoretical and experimental results are categorized by long integration time, short integration time, and nonlinear IF processing.