The effects of spatial diversity and imperfect channel estimation on wideband MC-DS-CDMA and MC-CDMA

In our previous work, we compared the theoretical bit error rates of multi-carrier direct sequence code division multiple access (MC-DS-CDMA) and multi-carrier code division multiple access (MC-CDMA). To ensure a fair comparison, we constrained both schemes to the same bandwidth, information rate, and energy-per-bit, and these constraints resulted in a possible trade-off between diversity gain and channel estimation errors between the two schemes. While only a single-input single-output (SISO) system was considered in our previous work, in this paper, we extend the comparison to a multiple-input multiple-output (MIMO) system which employs Alamouti space-time block coding at each sub-carrier frequency to achieve spatial diversity. We consider only those cases where MC-CDMA has higher frequency diversity than MC-DS-CDMA. Since increases in diversity yield diminishing gains, we conclude that the addition of spatial diversity to this multi-carrier comparison benefits MC-DS-CDMA more than MC-CDMA. To determine whether these gains for MC-DS-CDMA are enough to offset the difference in frequency diversity between the two schemes, we derive closed-form expressions for the bit error probabilities of both schemes, and we compare the MIMO results against those of the SISO system for different information rates, number of users, and number of pilot symbols per channel estimate.