Symbol Synchronization in a CPM Communications Receiver Based on Energy Near Half the Symbol Rate
The primary objective of the invention is to provide a new method for timing recovery and symbol synchronization in a continuous phase demodulator that overcomes the problems of symbol synchronization for a general continuous phase modulation system.
The method is linear in the phase up to an inverse tangent function error detector. This linearity makes possible the application of the method of the invention in cases where the modulation is not binary.
In the method, the symbol phase error is determined by the output of a complex filter which must have an output magnitude that is nonzero if the phase is to be determined. The magnitude depends on the amount of overlap of the Fourier transform of a phase modulation function with a replica of the transform shifted by 2.pi./T where T is the time interval between symbol starts. Thus some of the energy in the phase modulation function must lay at frequencies that are more than half of the symbol rate or no overlap will exist.
In an example presented here, the error filter has been designed for a modulation function which is nonzero over an interval that is 16 times the time between successive symbol starts. It is assumed that 4 samples are collected during the time between each symbol start so that the length of the modulation is 64 samples.
The imaginary part of the filter output is zero at every offset that corresponds to an integral number of symbol start intervals. This is the basis for the symbol phase error.
The symbol synchronization procedure is independent of the modulation index and the size of the symbol set. There is no constraint on the modulation index to be a rational value.
In addition to tracking, acquisition of the symbol phase and frequency are also achieved by the methods of the invention.
The symbol phase synchronization procedure described hereby is tolerant to the presence of unremoved carrier phase. Such a phase will be present if the LO signal does not exactly match the carrier frequency in the I.F. channel input. The carrier phase tolerance comes about because the error detection filter produces a zero response for a DC input.
Patent Status: U.S. patent(s) 6,466,630 issued.CONTACT: