4.1 Introduction

The problem of estimation of DOA of coherent broadband sources has applications in wireless communication systems, especially systems with smart antennas [1]. DOA estimation and adaptive beamforming are the main issues in smart antenna systems. However, in a complex multipath environment, received signals from different directions may be correlated, which prevents the application of narrowband DOA estimation techniques to estimate DOA of broadband sources. This chapter introduces a novel coherent broadband DOA estimation technique based on modal decomposition of wavefields.

Wang and Kaveh [2] introduced the use of focusing matrices for the purpose of CSS processing for DOA estimation of far-field wideband sources. In this method, the wideband array data are first decomposed into several narrowband components. The focusing matrices are used for the alignment of the signal subspaces of narrowband components within the bandwidth of the signals, followed by the averaging of narrowband array data covariance matrices into a single covariance matrix. Then, any signal subspace direction finding procedure, such as MUSIC [3] or its variants, maximum likelihood (ML), or minimum variance (MV), can be applied to this averaged covariance matrix to obtain the desired parameter estimates. The design of focusing matrices in CSS method requires preliminary estimates of the direction of arrivals. Further, this method is applicable to only a pair of sources. In later years, the CSS technique was further developed and refined [4, 5] to account for multiple sources, but the problem of prior information about the DOA still remained.