Craig K. Rushforth

A family of suboptimum detectors for coherent multiuser communications

Consideration is given to a class of suboptimum detectors for data transmitted asynchronously by K users employing direct-sequence spread-spectrum multiple access (DS/SSMA) on the additive white Gaussian noise (AWGN) channel. The general structure of these detectors consists of a bank of matched filters, a linear transformation that operates on the matched-filter outputs, and a set of threshold devices. The linear transformations are chosen to minimize either a mean-squared-error or a weighted-squared-error performance criterion. Each detector can be implemented using a tapped delay line. The number of computations performed per detected bit is linear in K in each case, and the resulting detectors are thus much simpler than the optimum detector. Under typical operating conditions, these detectors will perform much better than the conventional receiver and often nearly as well as the optimum detector. >

Multiuser signal detection using sequential decoding

The application of sequential decoding to the detection of data transmitted over the additive white Gaussian noise channel by K asynchronous transmitters using direct-sequence spread-spectrum multiple access (DS/SSMA) is considered. A modification of R.M. Fanos (1963) sequential-decoding metric, allowing the messages from a given user to be safely decoded if its E/sub b//N/sub 0/ exceeds -1.6 dB, is presented. Computer simulation is used to evaluate the performance of a sequential decoder that uses this metric in conjunction with the stack algorithm. In many circumstances, the sequential decoder achieves results comparable to those obtained using the much more complicated optimal receiver. >

Joint signal detection and parameter estimation in multiuser communications

The problem of simultaneously detecting the information bits and estimating signal amplitudes and phases in a K-user asynchronous direct-sequence spread-spectrum multiple-access communication system is addressed. The joint maximum-likelihood (ML) estimator has a computational complexity that is exponential in the total number of bits transmitted and thus does not represent a practical solution to the problem. An estimator that combines a suboptimum tree-search algorithm with a recursive least-squares estimator of complex signal amplitude is considered. The complexity of this estimator is O(K/sup 2/) computations per decoded bit, and its performance is very close to that of the joint ML receiver. This receiver has the advantage that the transmitted signal powers and phases are extracted from the received signal in an adaptive fashion without using a test sequence. >

Resonant spectra of dielectric spheres

The natural resonant frequencies and poles associated with the electromagnetic modes of a dielectric sphere with a relative index of refraction of 1.4 have been calculated for size parameters ranging from 1 to 50. Determining pole locations in the complex plane entailed the computation of spherical Bessel functions for large complex arguments. The symbolic programming language reduce was used to provide independent verifications of the convergence and accuracy of the numerical Bessel function routines required in these computations. To determine pole locations, we used a standard zero-finding routine to find the zeros of the scattering coefficient denominators. In addition, we used a separate zero-counting routine in conjunction with the search routine to ensure that all poles within a given region of the complex plane were found. The real parts of the calculated poles agree with the location of peaks in the resonance spectrum (calculated for real frequency excitation), whereas the imaginary parts are related to the widths of these peaks. The intensity inside the sphere, averaged over all spherical angles, was computed as a function of radius. When the particle is excited at resonance, the internal intensity exhibits a sharp peak near, but not on, the surface. The intensity was found to be the strongest when the particle is driven at resonant frequencies whose poles have small imaginary components in the complex plane.

An adaptive local-search algorithm for the channel-assignment problem (CAP)

The channel-assignment problem (CAP) for cellular radio networks is an NP-complete problem. Previous techniques for solving this problem have used graph-coloring algorithms, neural networks, simulated annealing, and pattern-based optimization procedures. We describe an efficient two-phase adaptive local-search algorithm for the channel-assignment problem. This algorithm has been applied to several existing benchmark problems with encouraging results. In many cases it outperforms the existing algorithms in the quality of the solution obtained. When used in conjunction with structured preprocessing, the algorithm can be applied to large networks. It is thus a practical tool for the planning of cellular radio networks.

Sizing dielectric spheres and cylinders by aligning measured and computed resonance locations: algorithm for multiple orders

An algorithm for determining the size of dielectric spheres and cylinders by aligning measured and computed resonance locations is presented. The orders of the resonance locations need not be known a priori. The algorithm is applicable to several types of scattering and emission spectra of spheres and cylinders if the index of refraction including dispersion is known and uniform, or nearly uniform, throughout the sphere or cylinder. The algorithm performs well when tested with groups of computed resonance locations of spheres (synthetic data) and with measured fluorescence emission spectra of spheres exhibiting as many as 5 orders of resonance.

Parameter estimation in a multi-user communication system

Many of the previously-developed multiple-access detection algorithms have assumed knowledge of the time delay, the amplitude, and the phase of each user. This paper presents methods of estimating the amplitudes and phases, both when the time delays are known and when the delays are unknown. In neither case are the transmitted bits assumed to be known. The estimators are unbiased and consistent, and each has a computational complexity linear in the number of users. Simulations are provided to demonstrate performance. >

Fast FFT-based algorithm for phase estimation in speckle imaging

This paper describes a fast direct algorithm for obtaining least-squares phase estimates from arrays of noisy phase differences. The algorithm uses the fast Fourier transform to diagonalize and decouple the system of equations which results from the application of the least-squares criterion. It is accurate and stable, and is perhaps an order of magnitude faster than the best iterative method. The effectiveness of the algorithm has been demonstrated by using it in connection with the Knox-Thompson speckle-imaging procedure to restore an optical object perturbed by simulated atmospheric turbulence. Representative results are discussed in the paper.

An efficient soft-in-soft-out multiuser detector for synchronous CDMA with error-control coding

We consider synchronous code-division multiple-access (CDMA) communications incorporating an error-control code (ECC) for improved performance. We describe a reduced-complexity multiuser sequence detector and develop a simple method for computing reliability values for the bit decisions produced by this detector. This reliability information is used by a soft-input Viterbi algorithm to decode the ECC. We compare the uncoded and coded performance of this reduced-complexity detector to those of certain decision-feedback equalizer (DFE) multiuser detectors. Simulation shows that the reduced-complexity sequence detector works better than the DFE detectors.

Average acquisition time for SSMA channels

The acquisition time performance of a two-user spread-spectrum multiple-access system is examined. The acquisition is accomplished using conventional (single-user) correlating/energy-detecting acquisition methods. The intent of the analysis is to determine how much performance penalty is incurred because of the presence of the multiple-user interference. The analysis indicates that for good spreading codes the performance degradation is not severe when the users amplitudes are all similar. However, in the presence of significant near/far power differences, the acquisition time increases dramatically. This suggests the need for acquisition methods which explicitly account for the presence of multiple users.<<ETX>>