Signal cancellation using feedforward and feedback paths
A circuit that cancels a self-interference signal includes, in part, a pair of signal paths that are substantially in phase, each of which paths includes a passive coupler, a delay element and a variable attenuator. The circuit further includes, in part, a first group of P signal paths each of which is substantially in phase with the pair of paths, a second group of M signal paths each of which is substantially out-of-phase relative to the pair of signal paths, and at least a pair of feedback paths. Each of the P and M signal paths, as well as the feedback paths includes a delay element and a variable attenuator. Optionally, each of the M signal paths is optionally 180° out-of-phase relative to the pair of signal paths.
1. A circuit comprising:
a first signal path comprising a passive coupler, a delay element and a variable attenuator;
a second signal path comprising a passive coupler, a delay element and a variable attenuator, said second signal path being substantially in phase with the first signal path;
first P signal paths each being substantially in phase with the first and second signal paths, each of the first P signal paths comprising a delay element and a variable attenuator, each of (P−1) of the first P signal paths comprising a passive coupler, wherein P is an integer equal to or greater than 1;
second M signal paths each being out-of-phase relative to the first and second signal paths, each of the second M signal paths comprising a delay element and a variable attenuator, each of (M−1) of the second M signal paths comprising a passive coupler, wherein M is an integer equal to or greater than 1, and wherein a sum of M and P is an integer equal to or greater than two;
at least a first feedback path formed using an isolation port of the passive coupler disposed in the first signal path or an isolation port of a passive coupler disposed in a first one of the first P signal paths; said at least first feedback path comprising a delay element and a variable attenuator; and
at least a second feedback path formed via an isolation port of the passive coupler disposed in the second signal path or an isolation port of a passive coupler disposed in a first one of the second M signal paths; said at least second feedback path comprising a delay element and a variable attenuator.
2. The circuit of claim 1 further comprising:
at least one antenna for receiving or transmitting a signal.
3. The circuit of claim 2 wherein each of the first signal path, the second signal path, the first P signal paths and the second M signal paths is adapted to receive a sample of a transmit signal and generate a delayed and weighted sample of the transmit signal.
4. The circuit of claim 3 further comprising:
a control circuit configured to vary an attenuation level of the variable attenuators disposed in the first signal path, the second signal path, the first P signal paths and the second M signal paths;
a combiner adapted to combine the delayed and weighted samples of the transmit signal to generate a first signal representative of a self-interference signal; and
a combiner/coupler adapted to subtract the first signal from a received signal.
5. The circuit of claim 4 wherein the delay element disposed in the first signal path generates a delay shorter than an arrival time of a second sample of the transmit signal at the combiner/coupler, and wherein the delay element disposed in the second signal path generates a delay longer than the arrival time of the second sample of the transmit signal at the combiner/coupler.
6. The circuit of claim 5 wherein the first signal path, the second signal path, the first P signal paths and the second M signal paths form P/2+M/2+1 associated pairs of paths, the delays generated by the delay elements of each associated pair of delay paths forming a window within which the second sample of the transmit signal arrives at the combiner/coupler.
7. The circuit of claim 6 wherein the control circuit is configured to determine the attenuation levels of the variable attenuators in accordance with values of intersections of an estimate of the self-interference signal and P+M+2 sine functions centered at boundaries of the P/2+M/2+1 windows.
8. The circuit of claim 7 wherein a peak value of at least a subset of the P+M+2 sine functions is set substantially equal to an amplitude of the estimate of the self-interference signal.
9. The circuit of claim 8 wherein said circuit further comprises:
a splitter adapted to generate the sample of the transmit signal from the transmit signal.
10. The circuit of claim 9 further comprising:
an isolator having a first port coupled to the antenna, a second port coupled to a transmit line of the circuit, and a third port coupled to a receive line of the circuit.
11. The circuit of claim 10 wherein said isolator is a circulator.
12. The circuit of claim 1 wherein the second M signal paths are substantially 180° of-phase relative to the first and second signal paths.
13. The circuit of claim 1 further comprising a variable delay element.
14. The circuit of claim 1 further comprising at least one amplifier.
15. A method of reducing a self-interference signal, the method comprising:
delivering a first portion of a first sample of a transmit signal to a first passive coupler to generate a first through signal;
generating a first signal defined by a delayed and weighted sample of the first through signal;
delivering a second portion of the sample of the transmit signal to a second passive coupler to generate a second through signal;
generating a second signal defined by a delayed and weighted sample of the second through signal;
generating P signals each being substantially in phase with the first and second signals and each defined by a different delayed and weighted sample of either the first or the second through signals, wherein at least one of the P signals is a feedback signal;
generating M signals each being substantially out-of-phase relative to the first and second signals and each defined by a different delayed and weighted sample of either the first or the second through signals, wherein at least one of the M signals is a feedback signal; and
combining the first signal, the second signal, the first P signals and the second M signals to generate a combined signal representative of the self-interference signal.
16. The method of claim 15 further comprising:
receiving a second sample of the transmit signal via an antenna;
combining/coupling the combined signal with the second sample of the transmit signal received via the antenna.
17. The method of claim 16 further comprising:
setting the delay of the first signal to a value less than an arrival time of the second sample of transmit signal at the antenna; and
setting the delay of the second signal to a value greater than the arrival time of the second sample of the transmit signal at the antenna.
18. The method of claim 17 further comprising:
forming P/2+M/2+1 associated time windows defined by the delays of the first signal, the second signal, the P signals, and the M signals; and
selecting the delays of the first signal, the second signal, the P signals, and the M signals such that the arrival time of the second sample of the transmit signal at the antenna falls within each of the P/2+M/2+1 time windows.
19. The method of claim 18 further comprising:
determining weights of the first and second though signals in accordance with values of intersections of an estimate of the self-interference signal and P+M+2 sinc functions centered at boundaries of the P/2+M/2+1 time windows.
20. The method of claim 19 further comprising:
setting a peak value of at least a subset of the P+M+2 sinc functions substantially equal to an amplitude of the estimate of the self-interference signal.
21. The method of claim 20 further comprising:
receiving the first sample of the transmit signal from a splitter.
22. The method of claim 21 further comprising:
delivering a second portion of the transmit signal to an isolator;
delivering the transmit signal from the isolator to the antenna.
23. The method of claim 22 wherein said isolator is a circulator.
24. The method of claim 15 further comprising:
generating the M signals such that each of the M signals is substantially 180° out-of-phase relative to the first and second signals.
25. The method of claim 15 further comprising:
delaying the first sample of the transmit signal.
26. The method of claim 15 further comprising:
amplifying the first sample of the transmit signal.
27. The method of claim 15 further comprising:
amplifying the combined signal.
28. The method of claim 15 further comprising:
amplifying at least one of the first signal or the second signal.
29. The method of claim 15 further comprising:
amplifying at least one of the M signals.
30. The method of claim 29 further comprising:
amplifying at least one of the P signals.