On the Design of Reduced State Demodulators with Interference Cancellation for Iterative Receivers

We consider the problem of designing demodulators for channels with memory that use reduced-size trellis descriptions for the received signal. We assume an overall iterative receiver, and for the parts of the signal not covered by the trellis description, we use interference cancellation based on the soft information provided by the outer decoder. In order to reach a trellis description, a linear filter is applied as front-end to compress the signal structure into a small trellis. This process requires three parameters to be designed: (i) the front-end filter, (ii) the feedback filter through which the interference cancellation is done, and (iii) a target response which specifies the trellis. While (i) and (ii) can be found in closed form, a numerical search is required for (iii). The numerical search is, however, very efficient and stable. Demodulators of this form have been studied before under the name channel shortening (CS), but the interplay between CS and interference cancellation has not been adequately addressed in the literature.