We propose a OFDM-CDMA system that employs polyphase codes to support variable spreading factors. A systematic approach for constructing the polyphase code sequences of variable spreading factors is developed. Polyphase codes exhibit better auto- and cross-correlation properties than Hadamard codes. When employed in OFDM-CDMA systems, polyphase codes result in certain structured multiple-access interference (MAI) caused by multipath. Analytical and numerical results show that OFDM-CDMA systems employing polyphase codes have better PAPR performance than those using Hadamard codes. The BER performance of the OFDM-CDMA system using polyphase codes is evaluated by numerical results and compared to that of the OFDM-CDMA system using Hadamard codes with and without clipping.
['Yingming Tsai', 'Guodong Zhang', 'Xiaodong Wang']
Polyphase codes for uplink OFDM-CDMA systems