Method and system for synchronizing in a frequency shift keying receiver

• US 20040190663A1
• Filed: 03/25/2003
• Published: 09/30/2004
• Est. Priority Date: 03/25/2003
• Status: Active Grant

• Current Assignee: Google Technology Holdings LLC (Alphabet Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Goldberg, Mark A., Carsello, Stephen R.
• Granted Patent: US 7,203,254 B2
• Time in Patent Office: Days
• US Class Current: 375/354
• CPC Class Codes: H04L 2007/047 using a sine signal or unmodulated carrier
• CPC Class Codes: H04L 2027/0065 Frequency error detectors H04L2027/0067 takes precedence
• CPC Class Codes: H04L 2027/0095 in a preamble or similar structure
• CPC Class Codes: H04L 27/10 Frequency-modulated carrier systems, i.e. using frequency-shift keying H04L27/32 takes precedence
• CPC Class Codes: H04L 7/042 Detectors therefor, e.g. correlators, state machines digital correlators in general G06F17/15
• CPC Class Codes: H04L 7/08 the synchronisation signals recurring cyclically

Abstract

The invention provides a method and apparatus for achieving timing synchronization during signal acquisition and for achieving frequency synchronization in a digital communication receiver after signal acquisition. The apparatus comprises: 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 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 small offset frequencies, according to the FSK modulation index and known sync symbol pattern; a peak detector, which forms a sync correlation signal according to the maximum combiner output amplitude, among the candidate offset frequencies, at each time instant; a threshold detector, which, at each time instant, compares the sync correlation signal to a threshold; a symbol timing estimator, which, upon detecting that the sync correlation has exceeded the threshold, establishes symbol timing in accordance with the peak of the sync correlation signal. The apparatus additionally comprises a frequency offset estimator for formulating an offset estimate using known and estimated symbols and a quality estimating tool for determining a quality of the frequency offset estimate using a signal-to-noise ratio calculation.

Claims

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;

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 frequency offset from the received signal.

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

4. The method of claim 1, further comprising implementing said multiple correlations using a recursive Goertzel algorithm.

5. The method of claim 1, further comprising allowing secondary distortions to enter the sync correlation in order to manage complexity.

6. 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 the peak of the sync correlation signal upon detecting that the sync correlation signal has exceeded the threshold.

7. The digital communication receiver of claim 6, further comprising a frequency offset estimator for estimating frequency offset of the received signal in order to track frequency synchronization after signal acquisition.

8. The digital communication receiver of claim 7, further comprising an error tracking mechanism for tracking out error in the received signal based on the estimated frequency offset in order to track frequency synchronization after signal acquisition.

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

coupling to the symbol timing mechanism for the digital communication receiver;

formulating a vector of complex valued symbol correlations, where each vector element is a correlation of a received signal with a sinusoid of 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.

10. The method of claim 9, 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.

11. An apparatus for achieving frequency synchronization in a digital communication receiver after acquisition of a signal, the apparatus comprising:

a frequency offset estimator for coupling to a symbol timing mechanism and 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.

12. The apparatus of claim 11, wherein the quality estimating tool includes means for determining the signal-to-noise ratio of 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 predetermined threshold.

13. 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 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 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 candidate 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; and

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.

14. A method for achieving timing synchronization for acquisition of a signal and for achieving frequency synchronization after acquisition of the signal, the method comprising:

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

forming multiple correlations at each time instant, wherein each of the multiple correlations corresponds to a selected sync symbol interval relative to a current time instant;

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 and 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 Fourier transform of the vector of phase-corrected sync symbol correlations over a set of candidate offset frequencies;

forming a sync correlation signal at each time instant according to a maximum discrete-time Fourier transform amplitude;

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

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

coupling to the symbol timing mechanism for the digital communication receiver;

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

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

computing a discrete-time Fourier transform vector of the vector of phase-corrected symbol correlations;

estimating the 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 Fourier transform vector.