Full Diversity Spreading code for Downlink Space-Time-Frequency Spreading CDMA

Recently proposed downlink space-time-frequency spreading CDMA (STFS-CDMA) is investigated in this paper. By analyzing, a spreading code design criterion of it is derived. From the criterion, we can see that the original two classes of spreading code adopted in STFS-CDMA, Walsh-Hadamard code (WHC) and double-orthogonal code (DOC), both can not always achieve the full space and frequency diversity. Then, a novel spreading code, zero padded rotary FFT code (ZPRFC), is proposed. Compared with WHC coded STFS-CDMA (WHC-STFS-CDMA) and DOC coded STFS-CDMA (DOC-STFS-CDMA), the novel ZPRFC coded STFS-CDMA (ZPRFC-STFS-CDMA) can obtain the full space and frequency diversity at the cost of the reduction of the number of supporting users.