Initial and Post-Initial Code Acquisition in the Noncoherent Multiple-Input/Multiple-Output-Aided DS-CDMA Downlink

In this paper, we investigate the issues of both initial and post-initial acquisition schemes in the multiple-input/multiple-output (MIMO)-aided direct-sequence code-division multiple-access (DS-CDMA) downlink when communicating over spatially uncorrelated Rayleigh channels. The associated mean acquisition time (MAT) performance trends are characterized as a function of the number of MIMO elements. Furthermore, we characterize both the initial and post-initial acquisition performance as a function of the relevant system parameters. Our findings suggest that increasing the number of transmit antennas in a MIMO-aided CDMA system results in combining the low-energy noise-contaminated signals of the transmit antennas, which ultimately increases the MAT by an order of magnitude when the signal-to-interference-plus-noise ratio (SINR) is relatively low, regardless of whether single- or multipath scenarios are considered. This phenomenon has a detrimental effect on the performance of Rake-receiver-based synchronization schemes when the perfectly synchronized system is capable of attaining its target bit-error-rate performance at reduced SINR values, as a benefit of employing multiple transmit antennas. Based on our analysis justified by information-theoretic considerations, our acquisition design guidelines are applicable to diverse noncoherent (NC) MIMO-assisted scenarios.