Upper Bounds on the Free Distance for Trellis Codes Combined with CPFSK Signals

It is well-known that CPM schemes themselves exhibit attractive spectral efficiencies and that combining them with a rate m/(m + 1) external trellis encoder with 2m+1-level modulation results in better power-bandwidth performance. Since the free distance is a good measure of performance, the authors construct upper bounds on the ultimately achievable free distance for trellis codes of Ungerboeck type combined with constant-envelope CPFSK signals. Some encoders achieving the constructed upper bounds are also presented.