Capacity-Based Parameter Optimization of Bit-Interleaved Coded CPM With Differential Detection

The design of bit-interleaved coded continuous phase modulation (CPM) is characterized by the code rate, alphabet size, modulation index, and pulse shape. This paper outlines a methodology for determining the optimal values of these parameters under bandwidth and receiver complexity constraints. The cost function that is used to drive the optimization is the information-theoretic minimum εb/N0, which is found by evaluating the constrained channel capacity. The capacity can be estimated using Monte Carlo integration. A search for optimal parameters is conducted over a range of coded CPM parameters, bandwidth efficiencies, and channels. To limit complexity and allow any modulation index to be considered, the receiver is constrained to use a soft-output differential phase detector. Bit error rate curves using a binary turbo code confirm that the constrained capacity is a very good indicator of the performance of the complete system.