Reception of C4FM and CQPSK signals in APCO Project-25

In the first phase of APCO Project-25, Phase-I, the C4FM modulation is defined for the transmission, with a bandwidth of 12.5 kHz. However in Phase-II, CQPSK modulation based transmission is defined with the bandwidth of 6.25 kHz to increase spectral efficiency. This results in a receiver structure in Phase-II, which is capable of demodulating both C4FM and CQPSK signals so that backward compatibility is obtained with Phase-I signals. While this common demodulator facilitates a single receiver structure, on the other hand frequency demodulator based structure becomes unavoidable for CQPSK. In this paper, we propose a complete receiver structure for APCO P-25. The proposed receiver structure consists of four major units; frequency discriminator, frame synchronization, sampling phase and sample clock offset correction, and frequency offset recovery units. This structure is realized in computer environment and BER performances of both modulations are evaluated under various multipath fading channel, sampling clock error, and frequency offset error conditions.