Low complexity frequency domain equalization of MIMO channels with applications to MIMO-CDMA systems

CDMA and MIMO-CDMA systems using RAKE receivers are heavily limited by self- and multiple-access-interference. Linear equalization is a means to remove this interference, however it is often not practical due to the enormous complexity, especially in the MIMO case. This paper presents an approach to greatly reduce the complexity of linear MIMO equalizers. It discusses the complexity reduction of the equalizer itself and describes a suboptimal low-complexity method to compute its coefficients. The application of frequency domain equalization using the overlap/add FFT method to MIMO systems is suggested. The coefficients of the joint equalizer/MIMO detector are also derived in the frequency domain, based on an approximation of an MMSE criterion. Performance results in terms of BER are quantified through simulations of a MIMO-extended UMTS-FDD downlink.