Linear MIMO chip equalization for the CDMA downlink

We consider an MxN MIMO system transmitting orthogonal multicodes from each antenna. A time-dispersive channel causes the codes to de-orthogonalize, complicating the detection problem. A chip-level linear equalizer is used to simultaneously restore the code orthogonality and provide a useful degree of spatial signal separation. A closed-form expression for SINR post-despreading is obtained and used for performance comparisons. We review and compare the finite-length and truncated infinite-length MMSE equalizer solutions. We then investigate two forms of the adaptive NLMS algorithm, one version including a correlator in the error signal path. For a 4×4 system with a 5 MHz UMTS CDMA signal over a TU multipath channel, we find that an equalizer of length 16-chips is near-optimal, and convergence is possible in 5 msec.