Data-aided frequency recovery for SOQPSK signals based on the PAM representation

In this letter, a clock-aided and data-aided frequency recovery method for shaped offset quadrature phase shift keying (SOQPSK) signals is proposed. Based on the pulse amplitude modulation (PAM) representation of continuous phase modulation (CPM), we eliminate the modulation of the SOQPSK signals and thus the problem turns to be the frequency estimation of a complex sinusoid in complex noise. It is shown that the proposed detector is suitable for two typical types of SOQPSK, i.e., SOQPSK-MIL and SOQPSK-TG. Furthermore, as presented by numerical results, the proposed method is unbiased and its performance is close to the Cramer-Rao bound (CRB).