Comparison of carrier recovery techniques in M-QAM digital communication systems

In digital wireless systems the frequency uncertainty between the transmitter and the receiver can take large values. The carrier recovery loop must be able to acquire lock in the presence of phase noise on the synthesized carrier. However, high level modulation formats such as 16, 32 and 64 QAM require very small steady state phase jitter. Unfortunately, these requirements are often incompatible in carrier recovery circuits. Practically, acquisition aiding techniques such as frequency sweeping or frequency detectors are the main solution to this problem. This paper addresses decision feedback carrier recovery loops for coherent demodulation of the 16 QAM signal format. Two types of decision-directed phase detector referred as basic or original phase detector (PD) and phase-frequency detector (PFD) are used in order to extend the acquisition range of the loop. Extensive and realistic simulation tests are performed to investigate the acquisition behavior of both phase detectors in the presence of phase noise. The best performance is obtained with the PFD structure. Compared with the original phase detector, the PFD structure leads to a 15-fold increase of the loop acquisition range.