A Complete Receiver for a Multi-h Continuous Phase Frequently Shift Keyed Signal in the Presence of Doppler

A Complete Receiver for a Multi-h Continuous Phase Frequently Shift Keyed Signal in the Presence of Doppler

Utah State University, Logan, UT

In the absence of signal imperfections other than the presence of additive white Gaussian (AWGN) noise, Continuous Phase Frequency Shift Keyed (CPFSK) signals with a changing modulation index (referred to in the literature as "multi-h") will have only a few possible starting and end phases at each symbol interval. This allows them to be represented in a trellis diagram where they inherently possess coding gain that can be realized with a Viterbi decoder. However, with a low earth orbit altitude small satellite exhibiting a large time-varying Doppler, the Viterbi decoder fails to maintain this coding gain advantage if the receiver does not also track timing, phase, and frequency offsets. An effective receiver implementation calculates timing and phase offsets for every possible received symbol and stores them in their own structure of metrics indexed by symbol time and phase state. In parallel with this process, a non data-aided band-edge frequency locked loop continuously tracks and corrects time-varying Doppler. This paper presents a design for a CPFSK receiver and the insights into how its frequency, phase, and timing loops function. It also explores the control loop design choices that affect the overall system performance.