Simplified Robust Adaptive Detection and Beamforming for Wireless Communications

5Quadratically Constrained Simplified Robust Adaptive Detection

5.1 Introduction

Among different detection techniques, linear receivers are of great significance due to their ease of practical implementation. A linear receiver can be designed by minimizing some inverse filtering criterion [1, 2]. Appropriate constraints are imposed to avoid the trivial all‐zero solution. The detector's output variance is minimized subject to appropriate constraints, which depend on the multipath structure of the signal of interest [3] or the steering vector direction in beamforming applications. Constrained optimization methods have received considerable attention as a means to derive blind multiuser receivers with low complexity [4–6] and robust minimum variance distortionless response (MVDR) beamformers. Minimum output energy (MOE) detection has been proposed as a blind adaptive technique for multiuser detection in direct‐sequence code‐division multiple‐access (DS/CDMA) systems [5] as described in Chapter 4. The MOE detector is a scaled version of the MMSE detector. Since scaling does not affect the output SINR, MOE has the same optimal performance as MMSE if the channel is estimated properly. An extension to space‐time processing using MOE has been conducted [7]. In addition, a rake receiver using blind adaptive MOE detection has been used for DS/CDMA over multipath channels [8].

Most of the proposed adaptive approaches in the literature [4, 9–11] are based on the well‐known RLS algorithm. In ...