Diversity combining options and low-complexity MMSE equalization for spread spectrum OFDM systems in frequency selective fading channels

This paper compares diversity combining schemes for the downlink of spread spectrum orthogonal frequency division multiplexing (SS-OFDM) systems in frequency selective fading channels. In particular, symbol-level combining after despreading is compared to chip-level combining under maximal ratio combining (MRC) of signals from different diversity branches and minimum mean-square error (MMSE) equalization of spreading sequences. Symbol-level combining takes place after the operations of MMSE equalization and despreading, whereas the operations of equalization and despreading occur after MRC if chip-level combining is used. Chip-level combining combines diversity samples in an efficient mariner while reducing inter-code interference (self-interference) that results from the loss of orthogonality of spreading sequences due to a frequency selective channel. This method is shown to be superior to symbol-level combining when the diversity branches are uncorrelated, and when the branches differ only due to subcarrier interleaving. An MMSE equalization method with significantly reduced complexity for partially loaded systems is also presented, based on the premise of chip-level combining. Novel expressions for the bit error rate (BER) of the two methods, as well as the extension of the analysis to partially loaded systems are given. The extensions of chip-level combining and low-complexity equalization of a partially loaded system to an OFDM system with 2-dimensional spreading are also presented. The results are relevant to antenna diversity as well as temporal diversity achieved though re-transmission within an ARQ scheme.