Robert T. Short

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. >

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. >

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>>

A RASE approach to acquisition in SSMA systems

The rapid acquisition by sequential estimation (RASE) method of DS/SS acquisition is applied to acquisition in a multiple-access channel (MARASE). In MARASE, a set of equations is developed which are nonlinear, noisy, and have a solution over a finite domain. An effective solution method is given which involves finding separate solutions to each equation, and then combining them using the error-correction capability of the maximal length spreading codes. Average acquisition time is analyzed and compared with simulation results.<<ETX>>

A tree-search algorithm for signal detection and parameter estimation in multi-user 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 the use of a test sequence.<<ETX>>

Suboptimum coherent detection of direct-sequence multiple-access signals

The authors consider 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 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.<<ETX>>

Adaptive lattice bilinear filters

This paper presents two fast least-squares lattice algorithms for adaptive nonlinear filters equipped with bilinear system models. Bilinear models are attractive for adaptive filtering applications because they can approximate a large class of nonlinear systems adequately and usually with considerable parsimony in the number of coefficients required. The lattice filter formulation transforms the nonlinear filtering problem into an equivalent multichannel linear filtering problem and then uses multichannel lattice filtering algorithms to solve the nonlinear filtering problem. The first of the two approaches is an equation-error algorithm that uses the measured desired response signal directly to compute the adaptive filter outputs. This method is conceptually very simple however it will result in biased system models in the presence of measurement noise. The second approach is an approximate least-squares output-error solution. In this case the past samples of the output of the adaptive system itself are used to produce the filter output at the current time. This approach is expected to reduce the effect of measurement noise on the behavior of the system. Results of several experiments that demonstrate and compare the properties of the adaptive bilinear filters are also presented in the paper.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

DS/SSMA amplitude and phase estimation with unknown bits using a correlation matrix

An algorithm is proposed for the estimation of amplitude and phase in a spread-spectrum multiple-access (SSMA) environment in additive white Gaussian noise (AWGN) without prior knowledge of the data bits. The estimate is obtained from the autocorrelation of the matched filter outputs. Estimator complexity is O(K/sup 2/) for fixed delay. Convergence is discussed, and simulation results that show that the estimator compares favorably with a maximum likelihood estimator with known bits are presented.<<ETX>>