Prolate spheroidal functions: a general framework for MC-CDMA waveforms without time redundancy

Requests for higher data transmissions rates and system performance in multiuser radio communications, needed for the future mobile generations or wireless local area networks, have centered them in the focus of interest of investigations in the signal processing area. MC-CDMA systems with orthogonal user codes based on Hadamard codes do not have any control of the transmitted waveform time spreading. In this case, a time guard is necessary to avoid the ISI effect. We have designed an alternative set of non-binary codes based on the time-concentration capability of prolate spheroidal functions. This property allows the use of several waveforms with almost ISI-free behavior. The advantage of using discrete prolate functions (DPF), instead of Hadamard codes, as users´ codes is obvious; when the system is not fully loaded, users are able to transfer greater bit rates with better error performances. In a multiuser scenario, the receiver design is almost identical to the standard case. Exploiting also the DPF property of double orthogonality, implementation of a new set of matched filters in a shorter time interval is investigated.