Spreading Code Design for Downlink Space-Time-Frequency Spreading CDMA

In this paper, we analyze the recently proposed downlink space-time-frequency spreading code-division multiple-access (STFS-CDMA) scheme. A spreading code design criterion is first derived for STFS-CDMA. From the spreading code design criterion, we can see that the two original spreading codes adopted in STFS-CDMA, i.e., the Walsh-Hadamard code (WHC) and the double-orthogonal code (DOC), both cannot achieve full space and frequency diversity, no matter how many users exist in the system. Then, a novel spreading code, i.e., permutated DOC (PDOC), is proposed. PDOC-coded STFS-CDMA (PDOC-STFS-CDMA) can obtain full space and frequency diversity when the number of users in the system is only one, but it cannot obtain full space and frequency diversity when the number of users is larger than one. To mitigate this problem, a zero-padded rotary fast Fourier transform code (ZPRFC) is proposed. Compared with WHC-coded STFS-CDMA (WHC-STFS-CDMA), DOC-coded STFS-CDMA (DOC-STFS-CDMA), and PDOC-STFS-CDMA, ZPRFC-coded STFS-CDMA (ZPRFC-STFS-CDMA) cannot only always obtain full space and frequency diversity but can also result in a low-complexity receiver at the cost of the reduction of the number of supporting users. Finally, the simulation results are given to compare with the performances of WHC-STFS-CDMA, DOC-STFS- CDMA, PDOC-STFS-CDMA, and ZPRFC-STFS-CDMA.