Modified Reduced Constellation PLL for Higher Order QAM

The carrier recovery is one of the most difficult problems for the communication systems applying the high-order QAM modulation technique. The decision-directed PLL is widely used carrier recovery technique for QPSK, which does not work for higher order QAMs. The reduced-constellation phase locked loop (RC-PLL) was proposed for the case of higher-order QAM. However, its performance in frequency offset detection still does not satisfy the system requirement especially when the offsets are large. In this paper, modified RC-PLL with tracking and hold algorithm (MRC-PLL) is proposed in order to improve the performance of RC-PLL in detecting phase offset as well as frequency offset in higher order QAM. Proposed MRC-PLL provides the acquisition range up to 45 degree phase offset and about up to 10 kHz frequency offset with the reduced acquisition time by half in comparison with RC-PLL.