Adaptive Spreading Code Assignment for Up-Link MC-CDMA

Multi-carrier (MC) code division multiple access (CDMA) is able to take the advantages of OFDM and CDMA and is a potential technique for future wireless communications. For an uplink MC-CDMA system, the symbols of different users are spread in the frequency domain. However, the frequency-selective fading of wireless channels destroys the othogonality of the spreading codes for different users and causes multiple access interference (MAI), especially for a network with full load. To reduce the impact of MAI, we adaptively assign spreading codes according to channel state information and MAI environments. Since it requires high computational complexity to find an optimal set of spreading codes for all active users, we develop several simplified approaches to search the suboptimal spreading code sets. It is demonstrated by the computer simulation that the adaptive spreading code assignment, even though suboptimal, can significantly improve the performance of MC-CDMA systems.