| The Continuous Phase Modulation (CPM) defined in the MIL-STD-188-181C Satellite Communications standard has a thirty-two state trellis structure. The complexity of the maximum-likelihood sequence estimator used to demodulate this signal is further increased because the CPM signal has two modulation indices (h-values) that require that the decoder store two sets of path metrics – one for each of the h-values. This paper describes a set of approaches which can be taken to reduce the complexity of the trellis structure of the CPM signal without reducing the demodulation performance for the range of signal to noise ratio values required by the UHF Satellite Communications standard. The first state reduction method uses an Ungerboeck-style set partitionin. The resulting decoder structure is described and the performance of the reduced state decoder is provided. Another approach utilizes a completely different, 4-state trellis structure with branch transitions defined that take advantage of the differential properties of the CPM modulation. Finally, a novel two-step approach is described which has the same complexity as the 4-state trellis structure, but may be easier to implement. Again, the resulting bit error rate performance is provided. |