Phase constellation modulator

The present invention relates to phase constellation modulators. In particular, the present invention relates to reduced complexity phase constellation modulators for continuous phase modulation (CPM) communication systems. The present invention provides a method and apparatus for implementing a continuous phase modulation with an approximate phase constellation. The method reduces complexity by using fewer than all phase points of the nominal phase constellation. The method includes the steps of storing () an approximate phase constellation of phase points () selected from a nominal phase constellation of phase points, identifying () a next phase in the approximate phase constellation of phase points using an N-bit input word, and outputting () the next phase. In reducing the number of phase points used in the modulation, the method may store only unique phase points in said phase constellation, and may further reduce the number of phase points by storing only those phase points differing by at least a phase threshold. In identifying the next phase, the method may shift () an input bit into an N-bit shift register (with an N-bit output) and apply the N-bit output () as an address to a memory that stores the subset of phase points. The memory thereby outputs () the next phase.