Asymmetric multi-channel adaptive equalizer
An apparatus is disclosed to compensate for non-linear effects resulting from the transmitter, the receiver, and/or the communication channel in a communication system. A receiver of the communication system contains an image cancellation module that compensates for images generated during the modulation and/or demodulation process. The image cancellation module includes a fine carrier correction loop to correct for frequency offsets between the transmitter and receiver. The image cancellation module includes a coarse acquisition mode and a decision directed mode. The decision directed mode allows for a larger signal-to-noise ratio for the receiver when compared against the coarse acquisition mode.
1. An adaptive image canceler, comprising:
a feed forward equalizer (FFE) configured to receive a communication signal, the communication signal including a first signal of interest and noise;
an image canceler module configured to produce an output that is a substantially close replica of the noise; and
a summer module configured to combine an output of the FFE with the output of the image canceler module to substantially suppress the noise from the communication signal.
2. The adaptive image canceler of claim 1 , wherein the FFE is configured to adjust its impulse response based upon an output of the summer module.
3. The adaptive image canceler of claim 2 , wherein the FFE is further configured to adjust its impulse response through a least-squares algorithm.
4. The adaptive image canceler of claim 1 , wherein the image canceler module comprises:
an adaptive filter, having multiple equalization taps, configured to adjust its impulse response by adjusting equalization coefficients corresponding to the multiple equalization taps.
5. The adaptive image canceler of claim 4 , wherein the image canceler module is further configured to adjust its impulse response through a least-squares algorithm.
6. The adaptive image canceler of claim 4 , further comprising:
a switch configured to select between a coarse acquisition mode of operation and a decision directed mode of operation,
wherein the image canceler module is configured to adjust the equalization coefficients based upon an output of the summer module in the coarse acquisition mode of operation, and
wherein the image canceler module is configured to adjust the equalization coefficients based upon an estimation of the first signal of interest in the decision directed mode of operation.
7. The adaptive image canceler of claim 1 , further comprising:
a fine carrier correction loop configured to compensate for trequency offsets in an output of the summer module to provide a hard decision and to estimate data content of the hard decision to provide a soft decision.
8. The adaptive image canceler of claim 7 , wherein the fine carrier correction loop comprises:
a complex multiplier configured to multiply the output of the summer module by a fine carrier frequency adjustment to produce the hard decision;
a numerically controlled oscillator (NCO) configured to provide the fine carrier frequency adjustment based upon a phase detector output;
a slicer configured to estimate the data content of the hard decision to provide the soft decision; and
a phase detector configured to determine a phase difference between the soft decision and the hard decision to provide the phase detector output.
9. The adaptive image canceler of claim 7 , further comprising:
a second summer module configured to combine the soft decision and the hard decision to produce a least-squares error.
10. The adaptive image canceler of claim 9 , wherein the image canceler module and the FFE are configured to adjust their respective impulse responses based upon the least-squares error.
11. The adaptive image canceler of claim 1 , wherein the FEE and the image canceler module are configured to adjust their respective impulse responses based upon an output of the summer module.
12. The adaptive image canceler of claim 1 , wherein the communication signal is a multi-channel communication signal having a plurality of information channels, wherein the first signal of interest occupies a frequency range of a first information channel from among the plurality of information channels, and wherein the noise represents an image of the first signal of interest.
13. The adaptive image canceler of claim 12 , wherein the image canceler module is further configured to receive a reference residual signal, the reference residual signal being an estimate of the image of the first signal of interest.
14. A method for canceling noise embedded onto a communication signal, comprising:
(a) correcting, by an adaptive image canceler, for the noise present in the communication signal to provide an equalized communication signal, the communication signal including a first signal of interest and the noise;
(b) producing, by the adaptive image canceler, an output that is a substantially close replica of the noise; and
(c) combining, by the adaptive image canceler, the equalized communication signal with the output that is the substantially close replica of the noise to substantially suppress the noise from the communication signal to provide a noise corrected communication signal.
15. The method of claim 14 , wherein step (a) comprises:
(a)(i) adjusting an impulse response based upon the noise corrected communication signal.
16. The method of claim 15 , wherein step (a)(i) comprises:
(a)(i)(A) adjusting the impulse response through a least-squares algorithm.
17. The method of claim 14 , wherein step (b) comprises:
(b)(i) adjusting an impulse response by adjusting equalization coefficients to provide the output that is a substantially close replica of the noise.
18. The method of claim 17 , wherein step (b)(i) comprises:
(b)(i)(A) adjusting the impulse response through a least-squares algorithm.
19. The method of claim 17 , further comprising:
(d) selecting between a coarse acquisition mode of operation and a decision directed mode of operation; and
wherein step (b)(i) comprises:
(b)(i)(A) adjusting the equalization coefficients based upon the noise corrected communication signal in the coarse acquisition mode of operation; and
(b)(i)(B) adjusting the equalization coefficients based upon an estimation of the first signal of interest in the decision directed mode of operation.
20. The method of claim 14 , further comprising:
(d) compensating for frequency offsets in the noise corrected communication signal to provide a hard decision; and
(e) estimating data content of the hard decision to provide a soft decision.
21. The method of claim 20 , wherein step (d) comprises:
(d)(i) multiplying the noise corrected communication signal by a fine carrier frequency adjustment to produce the hard decision;
(d)(ii) generating the fine carrier frequency adjustment based upon a phase detector output; and
(d)(iii) determining a phase difference between the soft decision and the hard decision to provide the phase detector output.
22. The method of claim 20 , further comprising:
(f) combining the soft decision and the hard decision to produce a least-squares error.
23. The method of claim 22 , wherein step (a) comprises:
(a)(i) adjusting an impulse response based upon the least-squares error, and wherein step (b) comprises:
(b)(i) adjusting an impulse response based upon the least-squares error.
24. The method of claim 14 , wherein step (a) comprises:
(a)(i) adjusting an impulse response based upon the noise corrected communication signal, and wherein step (b) comprises:
(b)(i) adjusting an impulse response based upon the noise corrected communication signal.
25. The method of claim 14 , wherein the communication signal is a multi-channel communication signal having a plurality of information channels, wherein the first signal of interest occupies a frequency range of a first information channel from among the plurality of information channels, and wherein the noise represents an image of the first signal of interest.
26. The method of claim 14 , wherein the noise represents a mirror image of the first signal of interest that occupies a frequency range of the first information channel.