Multi-carrier spread spectrum and its relationship to single-carrier transmission

The combination of orthogonal frequency division multiplexing (OFDM) and code division multiplexing (CDM) has found great interest. In contrast to a conventional OFDM system the information is spread over several subcarriers using orthogonal codes which leads to a diversity gain in frequency-selective channels. In this paper it is shown that such a system is technically equivalent to a single-carrier system with a guard interval if the columns of a Fourier matrix are used as spreading codes. The pros and cons of such a system compared to an OFDM-CDM system with arbitrary spreading matrix are described and discussed.