Research On High-Order QAM Carrier Recovery Technology For C-DOCSIS Protocol

Data Over Cable Service Interface Specification(DOCSIS) is one of the widely supported standard protocol in Hybrid Fiber Coaxial(HFC). With the continuous development of application requirements, the American Cable-Labs officially released the latest C-DOCSIS protocol specification in 2014. High-order QAM modulation and demodulation are key modules in the downlink channel at physical layer of C-DOCSIS protocol, and the carrier recovery technology in it determines whether the system can successfully achieve good demodulation performance. At present, the requests for carrier recovery are becoming more and more strict, because the constellation points are particularly intensive with the rising of modulation order, and the system is sensitive to frequency offset and phase jitter at the moment.This article takes the downlink channel at physical layer of C-DOCSIS system as background to research some classic carrier recovery algorithms. The analysis shows that these classic algorithms could not give full consideration to frequency offset synchronization range, phase jitter and carrier recovery time. To solve this problem, a study of high-order QAM carrier recovery algorithm which orients the downlink channel at physical layer is carried out in this article. The main work includes:1. An improved Phase And Frequency Detectors(PFD) algorithm is put forward with its window structures designed. By changing the condition of “holding” into “tracking”, the output dc component is much bigger so that the improved PFD algorithm can achieve faster acquisition speed and short carrier recovery time.2. This article designs a multiple thresholds carrier recovery algorithm which is divided into three patterns, including coarse synchronization, transition stage and fine synchronization. It applies Automatic Frequency Control(AFC) to detect suitable pattern switching time. Firstly, the polarity-decision phase detector is employed to achieve coarse synchronization at first. Secondly, we make use of the Modified Reduced Constellation(MRC) algorithm to decrease frequency offset jitter further. Finally, we gain small phase jitter by using the Weighted Directed Decision(WDD) algorithm to complete fine synchronization.3. This article gets a multiple thresholds carrier recovery algorithm based on PFD eventually via combining 1 and 2. It draws the improved PFD into the multiple thresholds carrier recovery algorithm, so that it can adequately combines the advantages of large frequency offset synchronization range and short carrier recovery time in the improved PFD algorithm and small phase jitter in the multiple thresholds carrier recovery algorithm. It is a high-order QAM carrier recovery algorithm with good performance which gives full consideration to frequency offset synchronization range, phase jitter and carrier recovery time.