Frequency detectors for CPM signals

Frequency tracking loops are used in digital links to control the carrier frequency of the received signals. A basic component in such loops is the frequency difference detector (FDD). This paper concentrates on a class of FDDs for use with binary continuous-phase modulation (CPM). Their derivation is based on two ideas: the maximum-likelihood principle for parameter estimation and Laurent´s (1986) decomposition of a CPM signal into the sum of linearly modulated components. The proposed FDDs are suitable for digital implementation and their operation is independent of clock recovery. Analysis and simulation are employed to investigate tracking operation and acquisition capability of frequency control loops endowed with such FDDs. Numerical examples are provided for modulation schemes of practical interest such as MSK and Gaussian MSK. The loop acquisition range is found to be fairly large, about 0.5-1.0 times the bit rate.<>