Orthogonal variable spreading factor code selection for peak power reduction in multi-rate OFCDM systems

Orthogonal Frequency and Code Division Multiplexing (OFCDM) systems are expected for next generation wireless multi-rate communications. OFCDM can realize variable bit rates by using the variable spreading gain technique. However, this technique requires the code generator of the variable spreading factors and the transmitter is thus complex. The other drawback of OFCDM systems is that OFCDM signals have high peak-to-average power ratio (PAPR). Therefore, reducing the PAPR is required. In this paper, we propose a multi-rate OFCDM system that can realize two classes of data rates by the Orthogonal Variable Spreading Factor (OVSF) code, where the OVSF code is generated by a single spreading factor code generator. The proposed multi-rate OFCDM can realize a high bit rate by dividing the spreading code and sending data in parallel with each of the divided OFCDM codes. The divided spreading code is selected to reduce the PAPR of the resultant OFCDM symbol. We show that in the proposed multi-rate OFCDM system, the high-rate users can achieve the better error rate performance than the users in the multi-code multi-rate OFCDM system at the same bit rate. We also show that the low-rate users can achieve the same error rate performance as the users in the multi-code multi-rate OFCDM system.