On the Construction of Orthogonal Spreading Code Groups for MC-CDMA with FDE in a Frequency Selective Channel

Abstract

The bit error rate (BER) performance of multicode multi-carrier code division multiple access (MC-CDMA) severely degrades due to the inter-code interference (ICI) in a strong frequency-selective channel. Recently a spreading code group construction method was proposed for MC-CDMA. The Walsh-Hadmard (WH) codes are divided into a number of code groups such that the code orthogonality can be maintained within each group even in a strong frequency-selective channel; any code pair taken from different groups is not orthogonal. The number of spreading codes in each group is determined by the maximum time delay difference of the channel. In this paper, we point out that the number of codes in each group is determined by the distribution of time delay differences among the propagation paths of the channel, not the maximum time delay difference. Based on that observation, we show that more orthogonal spreading codes can exist in each code group. The conditional BER is derived taking into account the interference from other code groups and the achievable downlink BER performance using the proposed spreading code group construction is numerically evaluated in a frequency-selective Rayleigh fading channel.