Symbol timing tracking and method therefor
View Patent ↗A system and method in a communication device ( 10 ) uses timing tracking to correct timing drifting due to the difference in frequency of a transmitter clock and a receiver clock. With the timing tracking, correlation values of three consecutive samples are calculated using the receive signal and the recovered symbols and then summed. A timing signal, (n k , frac k ) is updated based upon a metric calculated from a previous correlation value, R −1 , present correlation value (R 0 ) and a next con-elation value (R +1 ). Adjustment of timing signal is based on the relative location with respect to the current timing of a peak of a second order polynomial curve formed by the first correlation value R −1 , the second correlation value R 0 and the third correlation value R +1 .
1. A timing tracker for use in a communication device that communicates data with symbols and uses a symbol recovery function, the timing tracker comprising:
a first correlator for producing a first correlation value, R +1 corresponding to a next signal sample in time relative to current timing for data recovery, as a function of received samples and recovered symbols;
a second correlator for producing a second correlation value, R 0 corresponding to a present signal sample in time relative to current timing for data recovery, as a function of received samples and recovered symbols;
a third correlator for producing a third correlation value, R −1 corresponding to an immediately past signal sample in time relative to current timing for data recovery, as a function of received samples and recovered symbols; and
a timing updater for updating a current timing, n k and frac k , to a new timing, n k+1 and frac k+1 , for data recovery on a next successive block of data, using the first correlation value, the second correlation value and the third correlation value and the current timing n k and frac k , where n k is a sample index for a kth block of data that defines a location of a symbol and where frac k defines a symbol center after the sample index n k .
2. The timing tracker of claim 1 , wherein the new timing, n k+1 and frac n+1 , is updated from current timing n k and frac k based on a relative location with respect to the current timing of a peak of a second order polynomial curve formed by the first correlation value R −1 , the second correlation value R 0 and the third correlation value R +1 .
3. The timing tracker of claim 1 , wherein the timing updater further comprises:
a metric calculator that calculates a value m, wherein
m=R +1 −2frac k [2 R 0 −R +1 ]
where the first correlation value is R +1 =R +1 −R −1 , the second correlation value is R 0 =R 0 −R −1 ; and
first means coupled to the metric calculator for determining a value of frac k+1 , wherein frac k+1 is frac k if m=0, frac k+1 is frac k +C if m>0, and frac k+1 is frac k −C if m<0, where C is a predetermined constant.
4. The timing tracker of claim 3 wherein initial values of n k and frac k are provided by a synchronizer.
5. The timing tracker of claim 3 further comprising:
second means coupled to the first means for adjusting timing values n k+1 and frac k+1 ; and
interpolator means coupled to the second means for receiving adjusted timing values, the interpolator means providing a received signal with adjusted timing.
6. The timing tracker of claim 5 wherein the second means further comprises decision logic for implementing decisions as follows:
if frac k+1 ≧1, frac k+1 =frac k+1 −1 and n k+1 =n k+1 +1;
if 0≦frac k+1 <1, then make no change to frack+1 and nk+1; and
if frac k+1 <0, frac k+1 =frac k+1 +1 and n k+1 =n k+1 −1.
7. The timing tracker of claim 5 wherein the first means and the second means are each implemented by either hardware circuitry, software code or a combination of both.
8. A communication device comprising:
a first transmitter for communicating via a first communication link;
a first receiver for communicating via the first communication link;
a second transmitter for transmitting data via a second communication link;
a second receiver for receiving data via the second communication link;
a controller coupled to the first transmitter, the first receiver, the second transmitter and the second receiver, the controller supporting communication flow between the first communication link and the second communication link through the communication device,
the second receiver comprising:
a synchronizer for periodically synchronizing the second receiver to received data;
a message recoverer producing recovered data for input to the controller, the message recoverer comprising an equalization filter, a shift register coupled to the equalization filter, a linear interpolator coupled to the shift register, a symbol recoverer coupled to the linear interpolator, and a symbol-to-bit converter for producing recovered information;
a timing tracker coupled to the message recoverer, the timing tracker comprising:
a first correlator and accumulator producing a first correlation signal r +1 ;
a second correlator and accumulator producing a second correlation signal r 0 ;
a third correlator and accumulator producing a third correlation signal r −1 ;
a first squaring circuit squaring the first correlation signal;
a second squaring circuit squaring the second correlation signal;
a third squaring circuit squaring the third correlation signal;
a first magnitude accumulator receiving an output of the first squaring circuit and producing a first correlation value R +1 ;
a second magnitude accumulator receiving an output of the second squaring circuit and producing a second correlation value R 0 ;
a third magnitude accumulator receiving an output of the third squaring circuit and producing a third correlation value R −1 ; and
a timing updater for generating updated timing n k+1 , frac k+1 from the first, second and third correlation and current timing n k , frac k , where n k is a sample index for a kth block of data that defines a location of a symbol and where frac k defines a symbol center after the sample index n k .
9. The communication device of claim 8 , wherein the updated timing, n k+1 and frac n+1 , is updated from current timing n k and frac k based on the relative location with respect to current timing of a peak of a second order polynomial curve formed by the first correlation value R −1 , the second correlation value R 0 and the third correlation value R +1 .
10. The communication device of claim 8 , wherein the timing updater further comprises:
a metric calculator that calculates a value m, wherein
m=R +1 −2frac k [2 R 0 −R +1 ]
where the first correlation value is R +1 =R +1 −R −1 , the second correlation value is R 0 =R 0 −R −1 ; and
first means coupled to the metric calculator for determining a value of frac k+1 , wherein frac k+1 is frac k if m=0, frac k+1 is frac k +C if m>0, and frac k+1 is frac k −C if m<0, where C is a predetermined constant.
11. A data receiver for maintaining clock alignments between data synchronization occurrences, comprising:
a synchronization circuit generating a current timing signal, n k , frac k corresponding to a center location of a first symbol of a packet header of a packet of data;
a message recoverer coupled to the synchronization circuit for producing recovered symbols;
a timing tracker coupled to the message recoverer, the timing tracker comprising:
a plurality of correlators for producing correlation signals; and
a timing updater for generating an updated timing signal n k+1 , frac k+1 from first, second and third correlation values and the current timing n k , frac k .
12. The data receiver as defined in claim 11 , wherein the message recoverer further comprises an interpolator to provide a received signal, x′(n), having adjusted timing.
13. The data receiver as defined in claim 12 , wherein the interpolator generates the signal x′(n) as a function of at least two samples and frac k .
14. The data receiver as defined in claim 12 , wherein x′(n) is
x ′( n )= x ( n+ 1)+frac k [x ( n+ 1)− x ( n )].
15. The data receive as defined in claim 12 , wherein the interpolator further comprises a symbol recovery circuit that operates in a phase shift keyed (PSK) channel.
16. The data receiver of claim 11 , wherein the plurality of correlators further comprises:
a first correlator and accumulator producing a first signal (r +1 );
a second correlator and accumulator producing a second signal (r 0 ); and
a third correlator and accumulator producing a third signal (r −1 ).
17. The data receiver as defined in claim 16 , wherein the plurality of correlators further comprises:
a first squaring circuit coupled to the first correlator and accumulator for squaring the first signal;
a second squaring circuit coupled to the second correlator and accumulator for squaring the second signal; and
a third squaring circuit coupled to the third correlator and accumulator for squaring the third signal.
18. The data receiver as defined in claim 17 , wherein the plurality of correlators further comprise:
a first magnitude accumulator coupled to the second squaring circuit for receiving an output of the second squaring circuit and producing a first correlation value (R +1 );
a second magnitude accumulator coupled to the second squaring circuit for receiving an output of the second squaring circuit and producing a second correlation value (R 0 ); and
a third magnitude accumulator coupled to the second squaring circuit for receiving an output of the second squaring circuit and producing a third correlation value (R −1 ).
19. A method of operating a receiver to adjust timing of receiver data following synchronization, the method comprising the steps of:
receiving a data signal in blocks via a data communication link;
synchronizing the data signal to provide an initial timing values n 0 and frac 0 , where n 0 is a sample index for a first block of data that defines a location of a first symbol and where frac 0 defines a symbol center after the sample index n 0 ;
recovering data symbols from each of the blocks of the data signal;
correlating first samples of the data signal with recovered data symbols to produce a first correlation value (R +1 );
correlating second samples of the data signal with recovered data symbols to produce a second correlation value (R 0 );
correlating third samples of the data signal with recovered data symbols to produce a third correlation value (R −1 );
deriving updated timing values, n k+1 and frac k+1 for a next data block from the first correlation value, the second correlation value, the third correlation value and current timing values, n k and frac k ; and
adjusting sampling of the data signal in the next data block using the updated timing values n k+1 and frac k+1 .
20. A method of operating a receiver to adjust timing of a receiver clock to align with symbols of a received data signal transmitted by a transmitter, comprising:
receiving a synchronization signal at a beginning of the received data signal;
generating a timing signal, n 0 and frac 0 , from the synchronization signal for a first block of data;
generating correlation signals (R −1 , R 0 , R +1 ) from recovered symbols in the first block of data correlated with samples of the received data signal of the first block; and
generating an updated timing signal, n 1 and frac 1 , for symbol recovery for a second block of data from the correlation signals and the timing signal n 0 and frac 0 based on the relative location with respect to the current timing of a peak of a second order polynomial curve formed by the correlation signals.
21. The method of claim 20 , further comprising:
repeating the generating of the updated timing signal by repeating the generating of correlation signals and an immediately prior timing signal.