Analysis of the optimum spreading factor for MC-CDMA with RCPT-coded HARQ

In MC-CDMA, each user’s data-modulated symbol to be transmitted is spread over a number of subcarriers using an orthogonal spreading sequence defined in the frequency-domain. As the spreading factor SF becomes larger, the transmission performance improves due to increasing frequency diversity effect; however, the data rate per user decreases. The SF=1 system corresponds to an OFDM system. In an MC-CDMA system the same total throughput as in the OFDM system can be achieved if the number of users allowed to transmit simultaneously is the same as SF. In this paper, we investigate the optimum spreading factor for MC-CDMA when rate compatible punctured turbo coded hybrid ARQ (RCPT HARQ) is used for error control. It was found that when many strong paths exist, the total downlink throughput is highest when the data is spread over all the subcarriers available; however the highest total uplink throughput is attained with an OFDM system (SF=1). Keyword hybrid ARQ, rate compatible punctured turbo codes, frequency selective channel, MC-CDMA, mobile communications