Throughput Comparison of Turbo-Coded HARQ in OFDM, MC-CDMA and DS-CDMA with Frequency-Domain Equalization

Abstract

OFDM, MC-CDMA and DS-CDMA are being researched vigorously as the prospective signaling technique for the next generation mobile communications systems, which will be characterized by the broadband packet technology. With packet transmissions, hybrid ARQ (HARQ) will be inevitable for error control. HARQ with rate compatible punctured turbo (RCPT) codes is one of the promising techniques. Data rate equivalent to the OFDM system can be attained with MC-CDMA and DS-CDMA by assigning all the available codes to the same user resulting in what is commonly referred to as multicode MC-CDMA and multicode DS-CDMA. A rake receiver is used for receiving the DS-CDMA signals. However, recently minimum mean square error frequency-domain equalization (MMSE-FDE) has been proposed for the reception of DS-CDMA signals. In this paper, we introduce RCPT HARQ to DS-CDMA with MMSE-FDE and compare its throughput performance with OFDM, multicode MC-CDMA and multicode DS-CDMA with rake combining. MMSE weight for packet combining is introduced and the soft value generation for turbo coding in MC-CDMA and DS-CDMA with MMSE-FDE is presented. The throughput is theoretically evaluated for the uncoded case. For RCPT-HARQ, the comparison is done by computer simulations. It is found that the throughput of HARQ using DS-CDMA with MMSE-FDE is the same as or better than using MC-CDMA. However, with higher level modulation, type I HARQ using OFDM is better than using either MC-CDMA or DS-CDMA; for type II HARQ without redundancy in the first transmission, however, MC-CDMA and DS-CDMA gives a higher throughput.