INTER-CARRIER BANDWIDTH CONTROL FOR MITIGATING IQ IMBALANCE

• US 20110268232A1
• Filed: 05/02/2011
• Published: 11/03/2011
• Est. Priority Date: 05/03/2010
• Status: Active Grant

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Sundström, Lars, Park, Chester, Svensson, Jim
• Granted Patent: US 8,798,177 B2
• Time in Patent Office: Days
• US Class Current: 375/344
• CPC Class Codes: H04B 1/28 the receiver comprising at least one semiconductor device having three or more electrodes
• CPC Class Codes: H04B 1/30 for homodyne or synchrodyne receivers demodulator circuits H03D1/22
• CPC Class Codes: H04B 17/21 for calibration; for correcting measurements
• CPC Class Codes: H04L 27/2647 Arrangements specific to the receiver only equalisation H04L27/01
• CPC Class Codes: H04L 27/3863 Compensation for quadrature error in the received signal
• CPC Class Codes: H04L 5/001 the frequencies being arranged in component carriers

Abstract

Digital IQ imbalance estimation and compensation is facilitated by shaping the frequency response of receiver branches. In particular, in a multi-carrier receiver, the frequency response of signal processing elements in at least one receiver branch is set to not fully attenuate received signals in a frequency band of interest. The frequency band of interest is greater than the carrier bandwidth of the received signal processed by that receiver branch. In some embodiments, the received signal is not attenuated, and adjacent interfering signals are partially attenuated. This allows information regarding the interfering signals to appear in an IQ imbalance-induced, inter-carrier image of the signals in anther receiver branch, facilitating digital estimation and compensation of IQ imbalance.

Claims

1. A method of receiving and processing at least first and second communication signals concurrently in respective first and second branches of a receiver, wherein the first signal is modulated onto a first carrier having a first bandwidth, and the second signal is modulated onto a second carrier having a second bandwidth, the method comprising the steps of:

receiving the first and second signals;

setting the frequency response of signal processing elements in the first receiver branch to a frequency response sufficient to not attenuate at least the first signal;

processing the first signal in the first receiver branch;

setting the frequency response of signal processing elements in the second receiver branch to a frequency response sufficient to not fully attenuate received signals within a frequency band of interest, the width of which is greater than the carrier bandwidth of the second signal;

processing the second signal in the second receiver branch;

estimating the imbalance between In-phase (I) and Quadrature (Q) components of the first and second received signals; and

jointly compensating the estimated IQ imbalance over the first and second received signals.

2. The method of claim 1 wherein setting the frequency response of signal processing elements in a receiver branch comprises setting the frequency responses of at least an analog variable gain amplifier and an analog channel selection filter in the receiver branch.

3. The method of claim 1 wherein setting the frequency response of signal processing elements in a receiver branch comprises setting the frequency response of digital filters and interpolators in the receiver branch.

4. The method of claim 1 wherein setting the frequency response of signal processing elements in a receiver branch comprises configuring a digital baseband Fast Fourier Transformer (FFT) to operate with the determined frequency response.

5. The method of claim 1 wherein the first signal carrier bandwidth is broader than the second signal carrier bandwidth, and wherein setting the frequency response of signal processing elements in the second receiver branch comprises setting the frequency response to be at least as wide as the bandwidth of the first signal.

6. The method of claim 5 wherein a frequency response sufficient to not fully attenuate received signals within a frequency band of interest comprises a frequency response sufficient to not attenuate received signals within the frequency band of interest.

7. The method of claim 5 wherein a frequency response sufficient to not fully attenuate received signals within a frequency band of interest comprises a frequency response sufficient to not attenuate the second signal, and to partially attenuate interfering signals within the frequency band of interest.

8. The method of claim 7 wherein the degree of partial attenuation of interfering signals within the frequency band of interest is determined by the quantization error in the second receiver branch introduced by the interfering signals.

9. The method of claim 1 wherein the first and second signal carrier bandwidths are equal, and wherein setting the frequency response of signal processing elements in the second receiver branch comprises setting the frequency response to match the frequency response of signal processing elements in the first receiver branch.

10. The method of claim 1 wherein the first signal carrier bandwidth is broader than the second signal carrier bandwidth, and wherein setting the frequency response of signal processing elements in the second receiver branch comprises the steps of:

estimating the power level of interference in the first and second signals;

if the interference power level is sufficient to increase the interference power originating from IQ imbalance above a predetermined threshold but insufficient to increase quantization error in the second receiver branch above a predetermined quantization error threshold, setting the frequency response of signal processing elements in the second receiver branch to not fully attenuate interfering signals within the frequency band of interest; and

otherwise, setting the frequency response equal to the carrier bandwidth of the second signal.

11. The method of claim 1 wherein setting the frequency response of signal processing elements in the second receiver branch comprises setting the frequency response to be equal to the frequency response of signal processing elements in the first receiver branch;

and further comprising

calibrating the first and second receiver branches to have the same frequency response to within a predetermined frequency response error threshold.

12. The method of claim 11 wherein calibrating the first and second receiver branches comprises performing the calibration when the receiver is initially powered to an operative state.

13. The method of claim 11 wherein calibrating the first and second receiver branches comprises performing the calibration at predetermined intervals.

14. The method of claim 11 wherein calibrating the first and second receiver branches comprises injecting a test signal into at least one receiver branch and measuring its attenuation at the output of signal processing in the branch.

15. The method of claim 14 wherein the test signal comprises a single-tone signal.

16. The method of claim 15 further comprising repeating the calibration test while altering the frequency of the single-tone test signal over the first bandwidth.

17. The method of claim 16 wherein the frequency of the single-tone test signal is varied over a subset of subcarriers.

18. The method of claim 1 wherein the first and second signals are Orthogonal Frequency Division Multiplexed signals, the first signal is modulated onto a first plurality of subcarriers, the first plurality having a first bandwidth, the second signal is modulated onto a second plurality of subcarriers, the second plurality having a second bandwidth, and:

setting the frequency response of signal processing elements in the second receiver branch comprises setting the frequency response equal to the frequency response of signal processing elements in the first receiver branch; and

estimating IQ imbalance and compensating the estimated IQ imbalance on a per-subcarrier basis.

19. The method of claim 18 wherein estimating IQ imbalance on a per-subcarrier basis comprises injecting a test signal into at least one receiver branch on each subcarrier.

20. The method of claim 18 further comprising, for each subcarrier without a pilot signal, using decision-directed IQ estimation.

21. A receiver operative to receive at least first and second communication signals, wherein the first signal is modulated onto a first carrier having a first bandwidth, and the second signal is modulated onto a second carrier having a second bandwidth, comprising:

a first receiver branch comprising signal processing elements having a first frequency response and operative to receive and process the first signal; and

a second receiver branch comprising signal processing elements having a second frequency response and operative to receive and process the second signal;

wherein the frequency response of signal processing elements in the second receiver branch is sufficient to not fully attenuate received signals within a frequency band of interest, the width of which is greater than the carrier bandwidth of the second signal.

22. The receiver of claim 21 wherein a frequency response sufficient to not fully attenuate received signals within a frequency band of interest comprises a frequency response sufficient to not attenuate received signals within the frequency band of interest.

23. The receiver of claim 21 wherein a frequency response sufficient to not fully attenuate received signals within a frequency band of interest comprises a frequency response sufficient to partially attenuate interfering signals within the frequency band of interest.

24. The receiver of claim 23 wherein the degree of attenuation of interfering signals within the frequency band of interest is determined by the quantization error in the second receiver branch introduced by the interfering signals.

25. The receiver of claim 23 wherein the first and second signal carrier bandwidths are equal, and wherein the frequency response of signal processing elements in the second receiver branch is equal to the frequency response of signal processing elements in the first receiver branch.

26. The receiver of claim 21 further comprising:

an interference power estimator operative to estimate the power level of interference in the first and second signals; and

an adaptive frequency response function operative to adaptively the frequency response of signal processing elements in the second receiver branch to not fully attenuate interfering signals within the frequency band of interest if the interference power level is sufficient to increase IQ imbalance above a predetermined IQ imbalance threshold but insufficient to increase quantization error in the second receiver branch above a predetermined quantization error threshold, and operative to set the frequency response equal to the second bandwidth otherwise.

27. The receiver of claim 21 wherein the frequency response of signal processing elements in the second receiver branch equals the frequency response of signal processing elements in the first receiver branch, and further comprising:

a calibration unit operative to calibrate the first and second receiver branches to have the same frequency response to within a predetermined frequency response error threshold.

28. The receiver of claim 27 wherein the calibration unit is operative to perform the frequency response calibration when the receiver is initially powered to an operative state.

29. The receiver of claim 27 wherein the calibration unit is operative to perform the frequency response calibration at predetermined intervals.

30. The receiver of claim 27 wherein the calibration unit is operative to perform the frequency response calibration by injecting a test signal into at least one receiver branch and measuring its attenuation at the output of signal processing in the branch.

31. The receiver of claim 30 wherein the test signal comprises a single-tone signal.

32. The receiver of claim 30 further comprising repeating the calibration test while altering the frequency of the single-tone test signal over the first bandwidth.

33. The receiver of claim 21 wherein the first and second signals are Orthogonal Frequency Division Multiplexed, the first signal is modulated onto a first plurality of subcarriers, the first plurality having a first bandwidth, the second signal is modulated onto a second plurality of subcarriers, the second plurality having a second bandwidth, and the frequency response of signal processing elements in the second receiver branch equals the frequency response of signal processing elements in the first receiver branch, and further comprising:

a IQ imbalance estimation and compensation function operative to estimate IQ imbalance and compensate the estimated IQ imbalance on a per-subcarrier basis.

34. The receiver of claim 33 wherein the IQ imbalance estimation and compensation function is operative to estimate IQ imbalance on a per-subcarrier basis by injecting a test signal into at least one receiver branch on each subcarrier.

35. The receiver of claim 34 wherein the IQ imbalance estimation and compensation function is further operative to use decision-directed IQ estimation for each subcarrier without a pilot signal.