A carrier recovery scheme for 4-CPFSK with h=1/4

A carrier recovery scheme for 4-level CPFSK with h=1/4 is proposed. It is based on four frequency-switched Costas or in phase-quadrature (I-Q) loops, each synchronized on one of the signalling frequencies. The term "frequency-switched" refers to the way which one loop at a time is closed, depending on the data (frequency) estimate after decoding. From the I-Q loop's viewpoint, the CPFSK signal contains the correct signaling frequency on average only a quarter of the time. For the rest of the time, the signal is tracked by the other loops. This is not to be confused with data-aided or decision-feedback synchronization. Each loop resembles the conventional QPSK Costas loop synchronizer, with additional circuitry to rotate the signal space. Simulation results are presented.< >