Reduced state adaptive SISO algorithms for serially concatenated CPM over frequency-selective fading channels

Iterative detection (ID) has proven to be a near-optimal solution for concatenated finite state machines (FSMs) with interleavers over an additive white Gaussian noise (AWGN) channel. When perfect channel state information (CSI) is not available at the receiver, an adaptive ID (AID) scheme is required to deal with the unknown, and possibly time-varying, parameters. The basic building block for ID or AID is the soft-input soft-output (SISO) or adaptive SISO (A-SISO) module. The complexity of SISOs and A-SISOs increases exponentially with constraint length. Reduced state SISO (RS-SISO) algorithms have been applied to reduce the complexity of the A-SISO module. We show that serially concatenated CPM (SCCPM) with AID has turbo-like performance over fading ISI channels and also RS-A-SISO systems have large iteration gains. Various design options for RS-A-SISO algorithms are evaluated and performance and complexity are compared.