Low complexity multiple user detection for SC-FDE with OQPSK signals

It is widely accepted that SC-FDE (Single Carrier with Frequency Domain Equalization) schemes are preferable for the uplink of broadband wireless systems since these signals have lower PAPR (Peak to Average Power Ratio) than OFDM (Orthogonal Frequency Division Multiplexing) signals. However, when we want to employ power-efficient grossly nonlinear amplifiers, as in satellite communications, the SC signals must have quasi-constant envelopes which have a spectral occupation much higher than the minimum Nyquist band, resulting in low spectral efficiency. In this paper we consider a low complexity pragmatic FDMA (Frequency Division Multiple Access) system employing OQPSK (Offset Quaternary Phase Shift Keying) modulations with quasi-constant envelope that increases the overall spectral efficiency by considering total spectral overlapping of two different users. To mitigate the overall interference, we present an iterative frequency-domain receiver with parallel multiple user detection capable to cope with ISI (Inter-Symbol Interference), CCI (Co-Channel Interference) levels inherent to the spectral overlapping, and IQI (In-phase/Quadrature Interference), the interference between the in-phase (I) and quadrature (Q) components of the OQPSK signals, for severely time-dispersive channels.