Adaptive interference nulling with pattern maintaining under mainlobe subspace and quadratic constraints

Abstract

A fast adaptive interference nulling algorithm with arbitrary pattern maintaining without direction of arrival is proposed under the framework of generalised sidelobe canceller (GSC). The problem is formulated to optimise the weight by minimising the total output power of the beamformer, while keeping the antenna gain accurately approximated to that of the quiescent pattern by the mainlobe subspace constraint (MSC) and the weight quadratic constraint. The MSC is constructed by the dominant eigenvectors of the mainlobe covariance matrix. The closed-form solution to this problem is obtained by two steps. First, the quadratic constraint is relaxed to obtain the weight in the form of a GSC beamformer with diagonal loading (DL) sample covariance matrix. Then, the stringent quadratic constraint is guaranteed by optimising the DL level, which is precisely estimated by a simple iterative equation. The maintenance of pattern mainlobe and the reduction of the computation complexity can be achieved simultaneously. Numerical experiments are performed to demonstrate the effectiveness and efficiency of the proposed algorithm, and discuss the influence of mutual coupling between antenna elements.