A reduced complexity hybrid receiver for the downlink multicell CDMA system

Abstract

In this paper, we propose a novel low-complexity hybrid receiver (HR) for the downlink of a multicell CDMA system with a transmit delay diversity scheme. We analyze the structures of the conventional receiver (CR) and decorrelating receiver (DR) as well as the conditional bit error probability of DR using the eigenvalues and eigenvectors of the channel auto-correlation matrix. This analysis guides us to design the HR combining the merits of DR and CR. Using the circulant approximation of the channel auto-correlation matrix, we develop the reduced-complexity HR and DR. Numerical study shows that HR is more stable than CR and DR in terms of performance variation with respect to system load. I. INTRODUCTION In practical CDMA systems, such as IS-95 and CDMA2000, under ideal conditions (no multipath), the downlink is free from intra-cell interference due to the use of orthogonal Walsh codes. However, in practice, the orthogonality of the codes is destroyed when the downlink intra-cell signals are received by a user through a multipath channel. As a result, downlink suffers from both intra-cell and inter-cell interferences. In this paper, we propose a low-complexity interference suppression scheme by designing a novel receiver, namely the hybrid receiver (HR) for the downlink of a multicell CDMA system with transmit delay diversity (TDD) scheme (1). We compare the performance of HR with that of the decorrelating receiver (DR) and the conventional receiver (CR). CR is simply the Rake receiver under which a user suffers from both intra-cell and inter-cell interferences, whereas DR elim- inates the intra-cell interference by first inverting the channel auto-correlation matrix from the maximum ratio combiner (MRC) output followed by despreading. We prove that in the downlink of a single-cell CDMA system, DR is the same as the conventional decorrelating receiver (2), when all the orthogonal codes in the system are in use. We design the proposed receiver (HR) by i) analyzing the conditional bit error probability (BEP) of DR as well as the structures of DR and CR using the eigenvalues and eigenvectors of the channel auto- correlation matrix and ii) efficiently combining the structures of CR and DR to achieve a desirable trade-off between the interference suffered by the former and the enhanced inter- cell interference and Gaussian noise suffered by the latter. At the same time, HR preserves the key advantageous feature of CR and DR; namely, it operates without knowledge of the other users' codes. Thus, to detect signals, it requires the