Interference suppression, waveform optimization, and diversity combining in multiple -access communication systems
Abstract
In this work, we consider two closely related problems. First, we consider the problem of fitting two users into a channel normally occupied by one user. The users share the same physical channel, but the crosstalk is controlled. A linear receiver, which maximizes the signal-to-noise ratio (SNR), is employed. Several well-known time-limited symbol waveforms are tested. Furthermore, we use a numerical optimization algorithm to get near-optimum, time-limited symbol waveforms. Second, we consider chip waveform design. As we know, chip waveform design affects the error performance and spectral efficiency of a direct-sequence spread-spectrum multiple-access (DS/SSMA) system. Here, we propose an approach to design waveforms that may result in interchip interference (ICI). A novel performance metric including the effect of ICI is derived. Based on this metric, a closed form solution for optimal band-limited waveforms is obtained for excess bandwidth less than or equal to one. Numerical results are provided to evaluate the performance of the proposed waveforms for systems with conventional matched filter receivers and systems with linear adaptive receivers. In addition, we also investigate diversity combining techniques of a time-varying, correlated multipath fading channel for direct-sequence spread-spectrum (DS/SS) systems. The correlated paths are modeled and estimated jointly as a vector autoregressive (VAR) process. The joint estimation is shown to provide performance gain over separate estimations of the paths. The parameter matrices of the VAR model are estimated via the method of expectation maximization (EM) with two algorithms. The first algorithm provides a closed form solution to the maximization problem in the iterative EM procedure. However, the iterative EM-Kalman algorithm operates repeatedly on a batch of training data and results in large storage requirements and long processing delays. To overcome these disadvantages, a new algorithm with only forward-time recursions is proposed that can efficiently adapt to slowly changing Doppler spreads. Through computer simulations, a near ideal BER performance is found for both algorithms, and the efficacy of the new adaptive algorithm for channels with changing Doppler spreads is demonstrated.
Degree
Ph.D.
Advisors
Lehnert, Purdue University.
Subject Area
Electrical engineering
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