Robust Symbol Rate Correction of Multi-h Continuous Phase Frequency Shift Keyed Signals in the Presence of Doppler

Robust Symbol Rate Correction of Multi-h Continuous Phase Frequency Shift Keyed Signals in the Presence of Doppler

Utah State University, Logan, UT

Communications with a low earth orbit (LEO) satellite require tracking and correcting both the Doppler frequency offset and the symbol rate. A constant envelope signal such as Continuous Phase Frequency Shift Keying (CPFSK) is a popular choice for driving a power amplifier close to its 1 dB compression point. Establishing a changing modulation index (“multi-h”) in the CPFSK signal produces coding gain, which requires a Viterbi decoder in the receiver. A simple way to create this coding gain is to alternate between two modulation indices. A receiver with a Viterbi decoder-assisted time and phase correction of a Continuous Phase Frequency Shift Keyed (CPFSK) signal with a changing modulation index (“multi-h”) can establish and maintain low symbol error rates post-lock, unless the Doppler-induced symbol error rate exceeds the timing loop’s ability to respond. This situation can lead to catastrophic loss of lock when the Viterbi decoder’s estimate of time exceeds one symbol period. This situation can occur within seconds in low earth orbit satellites. In this situation, the Viterbi decoder makes decisions about received symbols while using the wrong modulation index, and thus, the wrong matched filters to compute the path metrics. A way of detecting and correcting this situation in the Viterbi decoder is desired. This paper discusses two approaches to solve this problem. In one approach, when the estimate of time error exceeds one symbol period, the Viterbi decoder introduces dummy metrics into the metrics table to seamlessly change the timing reference. In another approach, several Viterbi decoders, each assuming a different state of the symbol-to-symbol transition, operate in parallel and an overall choice is made based on the best metrics measured.