A Hybrid Phase/Data Viterbi Demodulator for Encoded CPFSK Modulation

By using an analysis technique based on the concept of transfer function bounds, we show that the Viterbi demodulator for correlative encoded CPFSK modulation is sensitive to carrier reference errors. To compensate for this sensitivity, we introduce a few extra phase states into the demodulation algorithm; this has been suggested earlier by Jackson and Omura. However, their demodulator is more complex than ours, as it is intended to compensate for rapidly varying phase offsets. Ours requires considerably fewer decoder states but uses a combination of external and internal phase estimation relative to the Viterbi demodulation algorithm, and is intended to track slowly varying phase offsets. We test the performance of our hybrid demodulation procedure, using an approximate analysis and a digital computer simulation which involves a simple vector structure and is based on orthogonal basis functions for the signal space under consideration. Our demodulation algorithm results in a modest increase in the number of Viterbi decoder states and no increase in the number of receiver matched filters. To track time-varying phase offsets, a truncated metric version of the Viterbi demodulation algorithm is introduced and tested.