James M. Ooi

Decentralized control of a multiple access broadcast channel: performance bounds

Optimal decentralized control of the multiple access broadcast channel is considered. A technique is presented for upper bounding the throughput of a slotted multiple access system with a finite number of users, immediate ternary feedback, retransmission of collisions, and no buffering. The upper bound is calculated for the two- and three-user cases, and it is shown that Hluchyj and Gallagers (1981) optimized window protocol is effectively optimal for these cases.

Fast iterative coding techniques for feedback channels

A class of capacity-achieving, low-complexity, high-reliability, variable-rate coding schemes is developed for communication over discrete memoryless channels with noiseless feedback. Algorithms for encoding and decoding that require computations growing linearly with the number of channel inputs used are developed. The error exponent associated with the scheme is shown to be optimal and implies that capacity is achievable. Simulations are performed and support the analytically predicted high performance and low complexity.

A separation theorem for periodic sharing information patterns in decentralized control

Optimal decentralized control of a discrete-time stochastic system is considered under a periodic sharing information pattern. In this scenario, controllers share information with one-step delay every K time steps. The periodic sharing pattern is a generalization of the previously studied one-step delay sharing pattern, which is known to possess a nonclassical separation property. It is proven that the periodic sharing pattern has an analogous separation property.

Fast iterative coding for feedback channels

A class of practical, very low-complexity, variable-rate coding schemes is developed for communication over channels with feedback. It is shown that for arbitrary discrete memoryless channels (DMCs) with noise-free feedback, these schemes achieve error probabilities that decay exponentially with block-length at any rate below the channel capacity. Extensions of the strategy for use on finite-state channels, known and unknown (universal communication), with feedback are also developed.

A computationally efficient wavelet transform CELP coder

A wavelet-transform-based CELP coder design is presented in this paper for high-quality speech coding at about 4.8 kbits/s. The coder quantizes the second residual using a wavelet transform approach instead of the stochastic-codebook-based vector quantization normally used in CELP coders, including the U.S. Federal Standard FS 1016 coder at 4.8 kbits/s. The wavelet coder improves the computational efficiency for encoding the second residual by requiring only 1.2 MIPS instead of 8.3 MIPS required by FS 1016. Subjective speech quality tests involving pairwise comparisons show that the wavelet coder was preferred 61% of the time over FS 1016.<<ETX>>

Parameter estimation for autoregressive Gaussian-mixture processes: the EMAX algorithm

The problem of estimating parameters of discrete-time non-Gaussian autoregressive (AR) processes is addressed. The subclass of such processes considered is restricted to those whose driving noise samples are statistically independent and identically distributed according to a Gaussian-mixture probability density function (PDF). Because the likelihood function for this problem is typically unbounded in the vicinity of undesirable, degenerate parameter estimates, a global maximum likelihood approach is not appropriate. Hence, an alternative approach is taken whereby a finite local maximum of the likelihood surface is sought. This approach, which is termed the quasi-maximum likelihood (QML) approach, is used to obtain estimates of the AR parameters as well as the means, variances, and weighting coefficients that define the Gaussian-mixture PDF. A technique for generating solutions to the QML problem is derived using a generalized version of the expectation-maximization principle.

Applications of wavelets to Speech processing: A case study of a celp coder

Wavelets are a new family of orthogonal basis functions for representing finite energy signals. In this chapter, we provide a brief review of wavelets and their properties. We cite a number of applications of wavelets to speech processing that have been proposed recently. As a detailed case study in wavelet applications, we present our work on a wavelet-transform-based CELP coder design for high-quality speech coding at about 4.8 kbits/s. The coder quantizes the second residual using a wavelet transform approach instead of the stochastic-codebook-based vector quantization normally used in CELP coders, including the U.S. Federal Standard FS 1016 coder at 4.8 kbits/s. The wavelet coder improves the computational efficiency for encoding the second residual by requiring only 1.2 MIPS instead of 8.3 MIPS required by FS 1016. Subjective speech quality tests involving pairwise comparisons show that the wavelet coder was preferred 61% of the time over FS 1016.

Further results on fast iterative coding for feedback channels: multiple-access and partial-feedback

The compressed-error-cancellation framework of Ooi and Wornell is extended for multiple-access channels with feedback and single-user channels with partial feedback.