Improving the spectral efficiency of FDM-CPM systems through packing and multiuser processing

In frequency division multiplexed (FDM) systems, spectral efficiency can be increased by reducing the spacing between two adjacent channels, thus increasing the relevant interference and possibly accounting for it at the receiver. In this paper, we consider a FDM system where each user employs a continuous phase modulation (CPM), serially concatenated with an outer code through an interleaver, and iterative detection/decoding. We show that, by taking into account the increased interference using properly designed multiuser detection and synchronization schemes, it is possible to implement transmission schemes with unprecedented spectral efficiency at a price of a limited complexity increase with respect to a classical single-user receiver which neglects the interference.