A hybrid carrier-recovery system for high-order QAM signals

In digital wireless systems the white gaussian noise, multipath fading effect, transmission delay and several other factors can cause deviation of the receiver´s local carrier frequency from the transmitter´s modulated carrier frequency. The carrier recovery system should be able to acquire lock in the presence of phase noise on the synthesized carrier. However, high level modulation formats such as 16, 64 and 256QAM require very small steady-state phase jitter. Unfortunately, these requirements are often incompatible in carrier recovery systems. This correspondence addresses a hybrid carrier recovery system for the demodulation of high order QAM signals. The system proposed in this paper is based on two basic types of carrier recovery mechanism: the times four square loop is used in analog domain to acquire the carrier frequency, and the decision-directed PLL is used to compensate frequency and phase offset. Times four square loop can obtain relatively accurate carrier frequency and use it as receiver´s local oscillator, and DD-PLL can eliminate remanent frequency and phase offset. The combination of both the mechanisms can do better job than either one used alone. The system is simulated by Matlab, and the performance is examined under certain channel circumstances and phase noise condition. The result shows good efficiency and robustness.