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 epsiv_b/N_o, 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.