Doubly-Iterative Equalization of Continuous Phase Modulation

Abstract

In this paper, a doubly-iterative receiver is proposed for joint turbo equalization, demodulation and decoding of coded binary continuous-phase modulation (CPM) for multipath fading channels. The proposed receiver consists of three soft-input soft-output (SISO) blocks: a front-end soft information aided minimum mean square error (MMSE) equalizer followed by a CPM demodulator and a back-end channel decoder. The MMSE equalizer, combined with an a priori soft-interference canceller (SIC) and an a posteriori probability mapper, forms a SISO processor suitable for iterative processing that considers discrete-time CPM symbols which belong to a finite alphabet. The SISO CPM demodulator and the SISO channel decoder are both implemented by the a posteriori probability (APP) algorithm. The proposed doubly-iterative receiver has the central demodulator coupled with both the front-end equalizer and the back-end channel decoder. A few back-end demodulation/decoding iterations are performed per each equalization iteration so as to improve the a priori information for the equalizer. As presented in the extrinsic information transfer (EXIT) chart analysis and simulation results for different multipath fading channels, this not only provides faster convergence to low bit-error rates, but also lower computational complexity. Index Terms Double turbo processing, continuous phase modulation, intersymbol interference, EXIT chart, minimum mean square error equalization