ADAPTIVE BEAMFORMING ALGORITHMS IN PRESENCE OF SIGNAL-COHERENT INTERFERENCES

Abstract

This thesis is dedicated to investigating signal cancellation phenomenon in adaptive beamformers in presence of signal-coherent interferences. Therefore, It compares spatially smoothed and redundancy averaged adaptive beamformers with the optimum interference plus noise.rejector INREJ. It is well known that spatial smoothing implies augmentation of array length while both redundancy averaging:and optimum INREJ do not. It is shown that in case of a single interference source the performance of the optimum INREJ is insensitive to any coherence between the interference and the desired signal sources. but is strongly dependent on their angular separation. Moreover, in case of mutually uncorrelated sources the redundancy averaged beamformer is asymptotically equivalent to the optimum INREJ. It is reconfirmed that coherence reduction by spatial smoothing depends on the angular separation between the desired signal and the interference sources. Meanwhile, redundancy averaged beamformer does not show such sensitivity. Yet, the latter is found to be also vulnerable to strong signal cancellation in presence of several signal-coherent interferences. Its pattern may not converge at all to the desired one of mutually uncorrelated sources. A new algorithm is proposed. It executes three main operations after estimating the spatial covariance matrix: redundancy averaging. decision about vulnerability to signal cancellation and estimate enhancement. Computer simulations indicated a remarkable performance improvement due to the proposed algorithm and they assisted all the presented theoretical arguments. The thesis also reinvestigates Luthra's solution suggested in 1985 to the adaptive nulling problem with a look-direction constraint in presence of signal-coherent jammers, but under the realistic assumption of independent numbers of array elements and of coherent jammers. Some limitations of this solution are pointed out. A generalized solution is proposed which has additional useful merits of- being less complex and of increased adaptation speed. A performance comparison of Luthr's ad hoc beamformer or equivalently the proposed one relative to the optimum interference notch is presented. Computer simulations validated the correctness of the presented theoretical arguments.