Interference Suppression for Spread Spectrum Signals Using Adaptive Beamforming and Adaptive Temporal Filter

This is a AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OH SCHOOL OF ENGINEERING report procured by the Pentagon and made available for public release. It has been reproduced in the best form available to the Pentagon. It is not spiral-bound, but rather assembled with Velobinding in a soft, white linen cover. The Storming Media report number is A563123. The abstract provided by the Pentagon follows: Interference and jamming signals are a serious concern in an operational military communication environment. This thesis examines the utility and performance of combining adaptive temporal filtering with adaptive spatial filtering (i.e. adaptive beamforming) to improve the signal-to-jammer ratio (SJR) in the presence of narrowband and wideband interference. Adaptive temporal filters are used for narrowband interference suppression while adaptive beamforming is used to suppress wideband interference signals. A procedure is presented for the design and implementation of a linear constraints minimum variance generalized sidelobe canceler (LCMV-GSC) beamformer. The adaptive beamformer processes the desired signal with unity gain while simultaneously and adaptively minimizing the output due to any undesired signal. Using the LCMV-GSC beamformer with a least mean squares (LMS) adaptive algorithm, it was shown that the tapped delay line (TDL) adaptive antenna array is more effective for the suppression of wideband jammer suppression than the linear array sensors (LAS) adaptive antenna array. Also a new technique for adaptive beamforming is presented which improves wideband interference suppression in a frequency-hopped environment. The output SJR improvement for the new technique compared to the conventional technique is as much as 15dB. Sometimes, multipath signals and jammers generated by a smart enemy are correlated with the desired signal which destroys the traditional beamformer's performance.