Non-coherent and differentially coherent code acquisition in MIMO assisted DS-CDMA multi-path downlink scenarios

In this contribution we investigate both Differentially Coherent (DC) and Non-Coherent (NC) code acquisition schemes in the Multiple Input Multiple Output (MIMO)-aided Direct Sequence-Code Division Multiple Access (DS-CDMA) downlink, when communicating over uncorrelated Rayleigh channels. It is demonstrated that the employment of multiple transmit antennas has a detrimental impact on the achievable diversity gain at typical operational Signal-to-Interference plus Noise Ratios (SINR), as the number of transmit antennas is increased, regardless whether single-path or multi-path scenarios are considered. Our findings suggest that increasing the number of transmit antennas in a MIMO-aided CDMA system results in increasing the Mean Acquisition Time (MAT) by as much as an order of magnitude, when the SINR per chip value is relatively low. The main reasons for the performance trends are plausible, since we have to reduce each individual MIMO element's signal power for maintaining the same total power as in a single-antenna system and this will be further justified by information theoretic considerations in the NC MIMO-aided scenarios considered.