AN ADAPTIVE FILTERING ALGORITHM AND ITS APPLICATION TO ADAPTIVE BEAMFORMING IN SPREAD-SPECTRUM SYSTEMS FOR INTERFERENCE REJECTION

Adaptive array and spread-spectrum techniques are widely used for the suppression of interference in many communications applications. In this dissertation, an improved adaptive filtering algorithm is investigated to synthetically overcome some of the problems associated with the conventional least-mean-square (LMS) algorithm, which requires generating a reference signal, and is not effective in coherent interference rejection. The results for the modified LMS algorithm, which we call the ZMS algorithm in this work, shows several advantages over the conventional LMS algorithm including: (1) reference signal generation is not required, (2) input interference signals are not required to be uncorrelated with the desired target signal, (3) convergence range is independent of the desired signal power, and (4) it converges rapidly and achieves small mean-square error. Two type of ZMS beamformers were investigated and simulated: the FIR notch filtered ZMS beamformer (FIR-ZMS) and the spatial notch filtered ZMS beamformer (SP-ZMS). Simulation results confirm the theoretical predictions of the ZMS algorithm. Signal leakage out of the input notch filter in the ZMS algorithm can result in degradation of the array performance. Simulations are also given to show this.; A combination of the ZMS adaptive beamforming and the frequency-hop spread-spectrum techniques is considered, providing more effective rejection of interferences. Analysis shows that for a given system processing gain of the spread-spectrum system, the main variable affecting the performance is the jammer-to-signal power ratio (JSR). It is shown that when the multiple jammer powers are rejected down to a level less than the target signal power by the adaptive nulling technique, the theoretical probability of error for detecting the target signal becomes zero. In practice the array performance in a frequency-hop signal environment is largely dependent on the convergence speed of the adaptive algorithm used and on the frequency-hop rate.; The FIR filter in the ZMS algorithm implementation is simulated to investigate the filter performance for decorrelating the target signal from the interference signal. Simulation results for a tap-switching algorithm using programmable FIR filter are shown to be effective in rejection or reception of the signal in a changing environment.