Adaptive cancellation of asynchronous near-end crosstalk
A method for communication includes receiving first data timed by a first clock and receiving a signal including second data timed by a second clock, which is independent of the first clock, and generating a stream of data samples corresponding to the second signal. The received first data are resampled responsively to a time-varying phase shift between the first and second clocks to generate resampled data timed by the second clock. The resampled data are applied in estimating and subtracting an alien crosstalk component from the stream of data samples.
1. Communication apparatus, comprising:
a transmitter, which is coupled to transmit over a first communication link a first signal comprising first data timed by a first clock; and
a receiver, which is disposed in proximity to the transmitter and comprises:
a front end coupled to receive over a second communication link a second signal comprising second data timed by a second clock, which is independent of the first clock, and to generate a stream of data samples corresponding to the second signal;
an input buffer coupled to receive the first data, timed by the first clock, from the transmitter;
a phase detector configured to measure a time-varying phase shift between the first clock and the second clock;
an adaptive resampling circuit configured to resample the first data in accordance with the measured phase shift to generate resampled data timed by the second clock, and to adjust a rate of resampling the first data responsively to a fill level of the input buffer, so as to maintain the fill level within predefined bounds; and
a crosstalk cancellation circuit configured to apply the resampled data in estimating and subtracting an alien crosstalk component from the stream of data samples.
2. The apparatus according to claim 1 , wherein the first and second communication links are Ethernet BASE-T links.
3. The apparatus according to claim 1 , wherein the crosstalk cancellation circuit comprises a digital filter, which is configured to filter the resampled data to generate a crosstalk error signal, and a subtracter, which is coupled to subtract the crosstalk error signal from the data samples.
4. The apparatus according to claim 1 , wherein the adaptive resampling circuit comprises a Farrow filter, which is configured to apply resampling coefficients to the first data to generate the resampled data.
5. The apparatus according to claim 4 , wherein the time-varying phase shift measured by the phase detector includes an integer component and a fractional component, and wherein the adaptive resampling circuit is configured to adjust the resampling coefficients of the Farrow filter responsively to the fractional component of the time-varying phase shift.
6. The apparatus according to claim 5 , wherein the adaptive resampling circuit is configured to select the first data from the input buffer responsively to the integer component of the time-varying phase shift for processing by the Farrow filter.
7. The apparatus according to claim 1 , wherein the input buffer comprises first and second first-in first-out (FIFO) memories and a buffer input circuit, which is coupled to write samples of the first data in alternation to the first and second FIFO memories timed by the first clock, and
wherein the adaptive resampling circuit comprises a filter input circuit, which is coupled to read out the samples of the first data from the first and second FIFO memories in alternation, at times that are timed by the second clock and depend on the fill level.
8. The apparatus according to claim 1 , wherein the receiver comprises an equalizer, which is coupled to receive and filter the stream of data samples following subtraction of the alien crosstalk component.
9. A method for communication, comprising:
receiving first data timed by a first clock;
storing the first data in an input buffer;
receiving a signal comprising second data timed by a second clock, which is independent of the first clock, and generating a stream of data samples corresponding to the second signal;
resampling the received first data responsively to a time-varying phase shift between the first and second clocks to generate resampled data timed by the second clock;
adjusting a rate of resampling e first data responsively to a fill level of the input buffer, so as to maintain the fill level within predefined bounds; and
applying the resampled data in estimating and subtracting an alien crosstalk component from the stream of data samples.
10. The method according to claim 9 , wherein the first and second communication links are Ethernet BASE-T links.
11. The method according to claim 9 , wherein applying the resampled data comprises digitally filtering the resampled data to generate a crosstalk error signal, and subtracting the crosstalk error signal from the data samples.
12. The method according to claim 9 , wherein resampling the received first data comprises measuring the time-varying phase shift and adaptively resampling the first data in accordance with the measured phase shift.
13. The method according to claim 12 , wherein adaptively resampling the first data comprises applying resampling coefficients of a Farrow filter to the first data to generate the resampled data.
14. The method according to claim 13 , wherein measuring the time-varying phase shift comprises measuring an integer component and a fractional component of the time-varying phase shift, and wherein adaptively resampling the first data comprises adjusting the resampling coefficients of the Farrow filter responsively to the fractional component of the time-varying phase shift.
15. The method according to claim 14 , wherein receiving the first data comprises storing the first data in an input buffer, and wherein adaptively resampling the first data comprises selecting the first data from the input buffer responsively to the integer component of the time-varying phase shift for processing by the Farrow filter.
16. The method according to claim 9 , wherein storing the first data in the input buffer comprises writing samples of the first data in alternation to first and second first-in first-out (FIFO) memories, timed by the first clock, and
wherein resampling the first data comprises reading out the samples of the first data from the first and second FIFO memories in alternation, at times that are timed by the second clock and depend on the fill level.
17. The method according to claim 9 , wherein the first data are transmitted over a first communication link, and the signal is received over a second communication link, in proximity to the first communication link, and wherein applying the resampled data comprises reducing the alien crosstalk in the signal due to the first communication link.
18. The method according to claim 9 , and comprising applying an equalizer to filter the stream of data samples following subtraction of the alien crosstalk component.