On Subspace-based Blind Channel Estimation Algorithms for SFBC MC-CDMA systems

The issue of blind channel estimation for the downlink of Space-Frequency Block Coded Multi-Carrier Code Division Multiple Access (SFBC MC-CDMA) systems is ad- dressed. Specifically, we consider an intuitive subspace-based channel estimation method and we investigate the corresponding necessary and sufficient conditions under which the channel estimate is unique (within a complex scalar). The derived identifiability conditions are used to highlight two interesting properties of SFBC MC-CDMA systems: (i) there is no antenna order ambiguity even when only one spreading code is assigned to each user, (ii) channel identifiablility is guaranteed, regardless of the channel zeros location. We also establish the unbiasedness of the channel estimates and we derive closed-form expressions for the mean-square-error (MSE) of the estimates as well as the corresponding Cramer-Rao bound (CRB). For the derivation of the CRB, we follow a novel approach which assumes the knowledge of only the spreading code of the desired user and results in a much tighter bound than the CRB derived based on the knowledge of all users' signatures. I. INTRODUCTION Transmit diversity techniques, and particularly Space- Time/Frequency Block Codes (STBC/SFBC) (1), are known to be very effective for combating multipath fading in broad- band wireless communications. These schemes are even more attractive when applied to a robust multiple access technique such as Multi-Carrier Code Division Multiple Access (MC- CDMA). However, the diversity and coding gains that are possible in such systems can only be realized if the underlying channels are accurately acquired at the receiver (4). The issue of blind channel estimation for STBC MC-CDMA systems has been addressed in the past (2), (3); however, to the authors best knowledge, no channel estimation method has been proposed or analyzed for SFBC MC-CDMA. In this work, as a first step in filling that void, we investigate the problem of blind channel estimation for SFBC MC-CDMA systems. We begin by presenting a system model for complex modulation schemes that enables us to treat the links between the multiple transmit antennas and the single receive antenna as a single channel and, thus, reduces the multichannel estima- tion problem to a single-input single-output (SISO) problem. This allows us to develop a simple, intuitive, subspace-based