Direct sequence spread spectrum (DS-SS) transmission may be used because of its well-known advantages such as resistance to interference, intentional jamming, interception, and robustness against fading. See, e.g., R. L. Pickholtz et al., “Theory of Spread-Spectrum Communications-A Tutorial,” IEEE Transactions on Communications, Vol. COM-30, No. 5, pp. 855-884, May 1982 the contents of which are fully incorporated herein by reference. Furthermore, code division multiple access (CDMA) can also be accomplished by assigning a unique spreading code to each user. See, e.g., A. J. Viterbi, CDMA: Principles of Spread Spectrum Communication, Addison-Wesley, Reading, Mass., 1995 the contents of which are fully incorporated herein by reference.
The combination of the DS-SS technique and Orthogonal Frequency Division Multiplexing (OFDM) results in three types of multiplexing schemes: multicarrier code division multiple access (MC-CDMA), multicarrier direct sequence code division multiple access (MC-DS-CDMA), and multitone direct sequence code division multiple access (MT-DS-CDMA). The MC-CDMA approach spreads the data stream in the frequency domain, and the MC-DS-CDMA the MT-DS-CDMA approaches spread the data streams in the time domain. If the multiple subcarrier data streams are obtained via a serial-to-parallel (S:P) conversion of a high-rate input stream, the symbol duration of each subcarrier signal is longer than the symbol duration of an equivalent single-carrier (SC) system. As a result of the S:P conversion, inter-symbol interference (ISI) can be reduced.
The performance of MC-DS-CDMA in a dispersive Rayleigh fading channel has been investigated and it has been shown that an MC-DS-CDMA system is robust in a multipath fading environment. The MC-DS-CDMA system retains the narrow band interference suppression features often desired of SC systems. For MT-DS-CDMA, good performance has also been obtained on a dispersive (Rician) channel.
The main differences between MC-DS-CDMA and MT-DS-CDMA are the frequency separation between adjacent subcarriers, and resulting processing gains. The subcarrier frequency separation of MC-DS-CDMA is the chip rate of the spreading code, whereas the subcarrier frequency separation of MT-DS-CDMA is the data rate of each subcarrier. Hence, MT-DS-CDMA has much more frequency overlap among subcarrier spectra than MC-DS-CDMA. It has been shown that an optimal spacing between adjacent subcarriers achieves better error probability performance for a given system bandwidth and given channel environment.