UC San Diego

This dissertation considers the following question: "Should the sub-carriers in a multi-carrier system be spread or non-spread?" We approach this problem by comparing 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) for an asynchronous uplink. To ensure a fair comparison, we constrain both schemes to the same bandwidth, information rate, and energy-per-bit. Since MC-DS-CDMA uses direct sequence spreading at each sub-carrier, while each MC-CDMA sub-carrier is unspread, MC-CDMA employs a larger number of sub-carriers than MC-DS-CDMA over a given bandwidth. As a result, there potentially exists between the two schemes a trade-off between diversity gain and channel estimation errors. Initially, we compare MC-DS- CDMA and MC-CDMA under a single-input single-output (SISO) system and for two different channel scenarios (i.e., two different cases for the coherence bandwidth of the channel), but we then extend the comparison to a multiple- input multiple-output (MIMO) system employing dual transmit/receive diversity and space-time block coding. For both cases, we derive closed-form expressions of the bit error rates for both multi-carrier schemes, and we compare the numerical results for different information rates, number of users, and number of pilot symbols per channel estimate to determine the performance trade-offs that may exist between the two schemes. We also compare the MIMO results against those of the SISO system to determine the impact of additional diversity on these trade-offs