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 2(m+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.