In this paper we consider a FDMA (Frequency Division Multiple Access) system employing OQPSK modulations (Offset Quaternary Phase Shift Key- ing) with quasi-constant envelope. To increase the over- all spectral efficiency we consider spectral overlapping between different frequency channels. We present an iterative frequency-domain receiver with parallel multiuser detection.
Our performance results show that our receiver is able to cope with the strong ACI levels (Adjacent Channel Interference) inherent to spectral overlapping, as well as with the interference between the in-phase (I) and quadrature (Q) components of the OQPSK signals, even for severely time-dispersive channels.