Resolving interaction between channel estimation and timing recovery
System and method of timing recovery for recovering a clock signal with reduced interaction between an adaptive channel estimator and the overall timing loop for correcting clock phase. The channel response estimation in the timing recovery loop is dynamically adapted to the current channel response that varies over time. The channel estimator includes compensation logic operable to detect and compensate a correction of clock phase ascribed to the channel estimator. The compensation logic can calculate the offset between a center of filter (COF) value and a COF nominal value, the offset indicative of the amount and direction of clock phase correction contributed by the channel estimator. Based on the offset, the compensation logic adjusts the estimates channel response by adjusting the tap weights of the channel estimator to correct the offset, thereby compensating the clock phase correction.
1. A method of timing recovery for signals transmitted via serial links, said method comprising:
generating an equalized signal responsive to a digital input signal;
generating an estimated signal responsive to said equalized signal according to an estimated channel response;
detecting a correction in clock delay associated with said digital input signal, said correction caused by said generating said estimated signal; and
adjusting said estimated channel response based on said detecting to compensate said correction in clock delay,
wherein: said generating said estimated signal comprises applying a Finite Impulse Response (FIR) filter on said equalized signal; said FIR filter comprises a plurality of taps; and said adjusting said estimated channel response comprises adjusting tap weights of said FIR filter,
wherein said detecting comprises:
determining a difference of between two selected tap weights of said FIR filter; and
determining an offset between said difference and a predetermined nominal value, wherein said offset indicative of said correction in clock delay.
2. The method of claim 1 , wherein said adjusting said tap weights comprises updating tap weights for a first set of selected taps of said FIR filter based on said offset using an interpolation and/or extrapolation process, and wherein said updating is further based on tap weights of a second set of selected taps of said FIR filter.
3. The method of claim 1 further comprising determining said predetermined nominal value by:
accessing a plurality of candidate values;
adjusting said estimated channel response based on said plurality of candidate values respectively;
monitoring Signal-to-Noise Ratios (SNRs) of said equalized signal resulted from said adjusting based on said plurality of candidate values respectively; and
selecting said predetermined nominal value from said plurality of candidate values, wherein said predetermined nominal value results in an optimal SNR of said equalized signal.
4. The method of claim 2 , wherein said first set of selected taps comprise a reference tap, a tap before said reference tap and a tap after said reference tap, and wherein said second set of selected taps comprise said reference tap, two taps before said reference tap and two taps after said reference tap.
5. The method of claim 1 , wherein said two selected tap weights are associated with a tap before a reference tap and a tap after said reference tap.
6. The method of claim 2 further comprising:
determining if said updating causes a change in reference tap location; and
restoring tap weights for said first set of selected taps responsive to a determination of said change.
7. The method of claim 1 , wherein said detecting further comprises:
determining a predetermined nominal value of a selected tap weight of said FIR filter; and
determining a difference between said selected tap weight and said nominal value, wherein said selected tap weight varies with time during said generating said estimated signal, and
wherein further said adjusting said estimated channel response comprises updating tap weights of a first set of selected taps of said FIR based on said difference between said selected tap weight and said nominal value.
8. The method of claim 1 further comprising:
dynamically adapting said estimated channel response based on a channel response that varies in time, wherein said correction in clock delay is caused by said dynamically adapting; and
generating a recovered clock signal based on said estimated signal.
9. A device for signal processing, said device comprising:
an equalizer configured to generate an equalized signal responsive to a digital input signal;
a channel estimator coupled to said equalizer and configured to apply an estimated channel response responsive to said equalized signal to generated an estimated signal;
a compensation logic coupled to said channel estimator and configured to:
detect a correction in clock delay associated with said digital input signal, wherein said correction is introduced during channel estimation by said channel estimator; and
adjust said estimated channel response based on said correction in clock delay to compensate said correction,
wherein said channel estimator comprises a Finite Impulse Response (FIR) filter comprising a plurality of taps, each tap associated with a tap weight,
and further comprising adaptation logic coupled to said channel estimator and configured to dynamically adapt tap weights of said FIR filter based on a difference between said estimated signal and said digital input signal, and wherein said correction in clock delay is related to dynamic adaptation of said estimated channel response,
wherein said compensation logic comprises:
a center of filter (COF) logic configured to determine a current difference between two selected tap weights of said FIR filter;
a storage unit configured to store a nominal difference between said two selected tap weights; and
an adder configured to generate an offset signal between said nominal difference and said current difference, said offset indicative of said correction in clock delay.
10. The device of claim 9 , wherein said compensation logic further comprises interpolation/extrapolation logic configured to update a first set of selected tap weights of said FIR filter based on said offset and based on a second set of selected tap weights of said FIR filter.
11. The device of claim 9 , wherein said compensation logic is further configured to
determine if updating said first set of selected tap weights causes a change in reference tap location; and
restore previous tap weights for said first set of selected taps responsive to a determination of said change.
12. The device of claim 9 , wherein said COF logic is further configured to determine a current tap weight of an identified tap, and wherein said storage unit is further configured to store a nominal tap weight of said identified tap, and wherein further said adder is further configured to generate another COF offset between said nominal tap weight and said current tap weight, said another COF offset indicative of said correction in clock delay.
13. A receiver comprising:
an equalizer configured to generate an equalized signal responsive to a digital input signal;
a channel estimation filter coupled to said equalizer and configured to apply an estimated channel response responsive to said equalized signal to generate an estimated signal;
adaptation logic coupled to said channel estimation filter and configured to dynamically adapt said estimated channel response to a current channel response of a communication channel coupled to said receiver, wherein dynamical adaptation of said estimated channel response is operable to cause a correction of clock delay associated with said digital input signal; and
compensation logic coupled to said adaptation logic and configured to compensate said correction by updating said estimated channel response, wherein said compensation logic comprises;
a center of filter (COF) logic configured to determine a current COF value based on one or more tap weights of said channel estimation filter;
a storage unit configured to store a nominal COF; and
an adder configured to generate an offset of said current COF from said nominal COF.
14. The receiver of claim 13 , wherein said compensation logic further comprises:
interpolation/extrapolation logic configured to update said estimated channel response by updating tap weights of a first set of selected taps in said channel estimation filter based on said offset.
15. The receiver of claim 13 , wherein said current COF value is a function of a difference between two selected tap weights of said channel estimation filter.
16. The receiver of claim 13 , wherein said current COF value represents a current tap weight of an identified tap.
17. The receiver of claim 13 further comprising mode selection logic configured to selectively disable said adaptation logic or said compensation logic.