Method and system for synchronizing in a frequency shift keying receiver

1. A method for using a receiver to achieve timing synchronization during acquisition of a signal, the method comprising the steps of:

receiving a signal from the transmitter containing a plurality of data slots and a sync word within each transmitted data slot;

forming multiple correlations at each of a plurality of time instants, each of said multiple correlations corresponding to a selected sync symbol interval relative to a current time instant, by correlating the received signal with a sinusoid of an expected frequency for each of the selected sync symbol intervals;

gathering the multiple correlations at each time instant;

phase correcting the multiple correlations in accordance with an FSK modulation index and a known sync symbol pattern, thus forming a vector of multiple phase corrected sync symbol correlations;

computing at each time instant a discrete-time Fourier transform of the vector over a set of candidate offset frequencies;

forming a sync correlation signal according to a maximum discrete-time Fourier transform amplitude at each time instant among the set of candidate offset frequencies;

comparing the sync correlation signal to a threshold at each time instant, wherein the threshold is proportional to energy in the received signal over the time span of a sync signal relative to a current time instant; and

upon detecting that the sync correlation signal has exceeded the threshold, locating a peak of the sync correlation signal and establishing symbol timing in accordance with the peak.

2. The method of claim 1 , further comprising tracking a transmitter's frequency by estimating a frequency offset from the received signal.

3. The method of claim 2 , further comprising the step of tracking frequency synchronization in the received signal based on the estimated frequency offset.

4. A digital communication receiver capable of achieving time synchronization in the presence of frequency error, the receiver comprising:

a receiving mechanism adapted to receive a signal containing a plurality of data slots with a sync word within each transmitted data slot;

a correlator for forming multiple correlations, each of said multiple correlations corresponding to a selected sync symbol interval, and for correlating the received signal with a sinusoid of an expected frequency for each of the selected sync symbol intervals;

a combiner for gathering and combining the multiple correlations over a set of candidate offset frequencies according to an FSK modulation index and a known sync symbol pattern;

a peak detector for forming a sync correlation signal according to a maximum combiner output amplitude among the candidate offset frequencies at each time instant;

a threshold detector for comparing the sync correlation signal to a threshold at each time instant; and

a symbol timing estimator for establishing symbol timing in accordance with a peak of the sync correlation signal upon detecting that the sync correlation signal has exceeded the threshold.

5. The digital communication receiver of claim 4 , further comprising a frequency offset estimator for estimating a frequency offset of the received signal in order to track frequency synchronization after signal acquisition.

6. The digital communication receiver of claim 5 , further comprising an error tracking mechanism for tracking frequency synchronization after signal acquisition.

7. A method for achieving frequency synchronization in a digital communication receiver having a symbol timing mechanism, the method comprising:

formulating a vector of complex valued symbol correlations, where each vector element is a correlation of a received signal with a sinusoid of a known or estimated symbol frequency during each of a plurality of symbol intervals, and wherein each symbol interval is determined according to the symbol timing derived from the symbol timing mechanism;

basing symbol frequencies on known sync symbols and estimated data symbols during a remainder of a time slot;

phase-correcting the vector of complex valued symbol correlations according to an FSK modulation index and estimated symbol pattern for each time slot;

computing a discrete-time Fourier transform vector of the vector of complex valued symbol correlations;

estimating a frequency offset of the received signal according to a location of the peak magnitude of the Fourier transform vector; and

computing a quality metric for the frequency offset estimate according to a signal-to noise ratio computed from the magnitude of the Fourier transform vector.

8. The method of claim 7 , further comprising:

determining the signal-to-noise ratio for the frequency offset estimate by summing up the signal in a selected narrow bandwidth about the peak and discarding the frequency offset estimate if the signal-to-noise ratio is below a pre-determined threshold.

9. An apparatus for achieving timing synchronization during acquisition of signal and for achieving frequency synchronization after acquisition of the signal, the apparatus comprising:

a receiving mechanism adapted to receive a signal containing a plurality of data slots with a sync word within each transmitted data slot;

a correlator for forming multiple correlations, each of said multiple correlations corresponding to a selected sync symbol interval, and correlated with a sinusoid of an expected frequency for each of the selected sync symbol intervals;

a combiner for gathering and combining the multiple correlations over a set of small offset frequencies according to an FSK modulation index and a known sync symbol pattern;

a peak detector for forming a sync correlation signal at each time instant according to a maximum combiner output amplitude among the offset frequencies;

a threshold detector for comparing the sync correlation signal to a threshold at each time instant;

a symbol timing estimator for establishing symbol timing in accordance with a peak of the sync correlation signal upon detecting that the sync correlation has exceeded the threshold;

a frequency offset estimator for formulating an offset estimate using known and estimated symbols and for calculating a frequency offset estimate; and

a quality estimating tool for determining a quality of the frequency offset estimate using a signal-to-noise ratio calculation.