Method and apparatus for antenna array calibration using traffic signals

• US 9,094,254 B2
• Filed: 05/15/2013
• Issued: 07/28/2015
• Est. Priority Date: 11/15/2012
• Status: Active Grant

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Da Silveira, Marthinus Willem, McGowan, Neil
• Primary Examiner(s): HO, DUC CHI
• Application Number:
  US13/894,826
• Publication Number:
  US 20140133543A1
• Time in Patent Office: 804 Days
• Field of Search: None
• US Class Current: 1/1
• CPC Class Codes: H04B 17/12 of transmit antennas, e.g. of the amplitude or phase
• CPC Class Codes: H04B 7/06 at the transmitting station
• CPC Class Codes: H04L 25/03949 equalizer selection or adaptation based on feedback multiple signaling inclusive of a precoding command for adapting the transmitter H04L1/0031; feedback for transmit diversity systems H04B7/0619; selection of codebook or precoding matrix for MIMO diversity systems H04B7/0456

Abstract

An antenna array of a base station is calibrated using outbound traffic signals. The outbound traffic signals are captured for use as reference signals before the outbound traffic signals enter transmit paths in a radio unit of the base station. The outbound traffic signals are captured for use as a feedback after the outbound traffic signals exit the transmit paths. Each of the reference signals is one of the outbound traffic signals to be transmitted. An impairment estimator estimates the impairment for each of the outbound traffic signals based on the feedback signal and the reference signals. For each transmit path, a set of weights of an all-pass filter is determined, where the all-pass filter has a frequency response that approximates the impairment estimation in an occupied frequency region where reference signal power is above a threshold. The set of weights is inversed to obtain equalizer taps for each of the transmit paths.

Claims

1. A method for calibrating an antenna array of a base station to remove distortion incurred by transmit paths in the base station, the antenna array including a plurality of sub-arrays coupled to the transmit paths for transmitting outbound traffic signals to a wireless network, the method comprising the steps of:

capturing the outbound traffic signals for use as a feedback after the outbound traffic signals exit the transmit paths;

capturing the outbound traffic signals for use as reference signals before the outbound traffic signals enter the transmit paths, each of the reference signals being one of the outbound traffic signals that is to be transmitted via one of the sub-arrays;

performing an impairment estimation for each of the outbound traffic signals based on the feedback and the reference signals;

determining, for each of the transmit paths, a set of weights of an all-pass filter, wherein the all-pass filter has a frequency response that approximates the impairment estimation in an occupied frequency region where signal power of the reference signals is above a threshold;

inversing the set of weights to obtain equalizer taps for each of the transmit paths; and

applying the equalizer taps to the outbound traffic signals before the outbound traffic signals enter the transmit paths to thereby remove the distortion incurred by the transmit paths.

2. The method of claim 1, further comprising the step of:

capturing the outbound traffic signals in respective ones of the sub-arrays into a single feedback signal for the transmit paths.

3. The method of claim 1, wherein the feedback comprises a feedback signal for each transmit path.

4. The method of claim 1, wherein performing the impairment estimation further comprises the steps of:

applying a time-domain window to each of M blocks of the feedback and the reference signals, where M>

P, P being the number of transmit paths; and

generating and solving M equations for each of frequency bins in a frequency domain to obtain the impairment estimation in the frequency domain.

5. The method of claim 1, wherein performing the impairment estimation further comprises the step of aligning the feedback and the reference signals in time before performing the impairment estimation.

6. The method of claim 1, further comprising the steps of:

detecting a correlation between the reference signals; and

using the equalizer taps to add a phase variation as a function of time to the outbound traffic signals such that the outbound traffic signals have a relative phase that is changing at different points of time.

7. The method of claim 1, further comprising the steps of:

identifying the occupied frequency region where the signal power of the reference signals is above the threshold;

identifying a transition region at a boundary of the occupied frequency region; and

extrapolating phase values of the impairment estimation in the transition region .

8. The method of claim 1, wherein the step of applying the impairment estimation is performed before the outbound traffic signals are captured for use as the reference signals.

9. The method of claim 1, further comprising the steps of:

synthesizing a plurality of all-pass filters with each all-pass filter for one transmit path; and

removing a common phase response from the all-pass filters.

10. The method of claim 1, further comprising the step of:

computing a conjugate of the set of weights of the all-pass filter in a frequency domain to obtain the equalizer taps.

11. The method of claim 1, wherein one or more of the steps of capturing, applying, determining, inversing and applying are performed in a baseband unit of the base station.

12. The method of claim 1, wherein determining the set of weights of the all-pass filter further comprises the steps of:

transforming the impairment estimation W_d (p,k) in a frequency domain to a time domain to produce w_d (p,n) for transmit path p, wherein k is a frequency bin index and n is a time sample index;

setting samples (L/2+1) to (N−

L/2) of w_d (p,k) to zero to obtain an L-tap filter w_z(p,k), where L is the number of taps of the all-pass filter;

transforming w_z(p,k) to the frequency domain to obtain a frequency vector W_z(p,k);

determining the mean square error (MSE) between W_z(p,k) and W_d (p,k) for k frequency bins in the occupied frequency region;

if the MSE is not below a minimum value, setting magnitudes of W_z(p,k) to one and phases of W_z(p,k)equal to the phases of W_d (p,k) in the occupied frequency region, and keeping remaining values of W_z(p,k) as is; and

repeating the steps of transforming the impairment estimation, setting the samples, transforming w_z(p,k) to the frequency domain, determining the MSE and setting the magnitudes until the MSE is below the minimum value.

13. A base station that performs calibration of an antenna array to remove distortion incurred by transmit paths in the base station, the antenna array including a plurality of sub-arrays coupled to the transmit paths for transmitting outbound traffic signals to a wireless network, the base station comprising:

a feedback receiver adapted to receive a feedback of the outbound traffic signals after the outbound traffic signals exit the transmit paths;

an impairment estimator coupled to the feedback receiver, the impairment estimator adapted to receive the outbound traffic signals that are captured before entering the transmit paths for use as reference signals, and to perform an impairment estimation for each of the outbound traffic signals based on the feedback from the feedback receiver and the reference signals, wherein each of the reference signals is one of the outbound traffic signals that is to be transmitted via one of the sub-arrays;

a synthesizer coupled to the impairment estimator to determine a set of weights of an all-pass filter for each transmit path and to inverse the set of weights to obtain equalizer taps for each transmit path, wherein the all-pass filter has a frequency response that approximates the impairment estimation in an occupied frequency region where signal power of the reference signals is above a threshold;

a plurality of equalizers coupled to the synthesizer with one equalizer for each transmit path, each equalizer adapted to apply the equalizer taps to the outbound traffic signals for one of the transmit path before the outbound traffic signals enter the transmit paths to thereby remove the distortion incurred by the transmit paths.

14. The base station of claim 13, further comprising a combiner in the antenna array, the combiner adapted to combine the outbound traffic signals in respective ones of the sub-arrays into a signal feedback signal for the transmit paths.

15. The base station of claim 13, wherein the feedback comprises a feedback signal for each transmit path.

16. The base station of claim 13, wherein the impairment estimator is further adapted to:

apply a time-domain window to each of M blocks of the feedback and the reference signals, where M>

P, P being the number of transmit paths; and

generate and solving m equations for each of frequency bins in a frequency domain to obtain the impairment estimation in the frequency domain.

17. The base station of claim 13, wherein the impairment estimator is further adapted to align the feedback and the reference signals in time before performing the impairment estimation.

18. The base station of claim 13, wherein the plurality of equalizers are adapted to add a phase variation as a function of time to the outbound traffic signals when a correlation is detected between the reference signals, such that the outbound traffic signals have a relative phase that is changing at different points of time.

19. The base station of claim 13, wherein the synthesizer is further adapted to:

identify the occupied frequency region where the signal power of the reference signals is above the threshold;

identify a transition region at a boundary of the occupied frequency region; and

extrapolate phase values of the impairment estimation in the transition region.

20. The base station of claim 13, wherein the plurality of impairment estimation is applied to the outbound traffic signals before the outbound traffic signals are captured for use as the reference signals.

21. The base station of claim 13, wherein the synthesizer is further adapted to:

synthesize a plurality of all-pass filters with each all-pass filter for one transmit path; and

remove a common phase response from the all-pass filters.

22. The base station of claim 13, wherein the synthesizer is further adapted to compute a conjugate of the set of weights of the all-pass filter in a frequency domain to obtain the equalizer taps.

23. The base station of claim 13, wherein the synthesizer is further adapted to:

transform the impairment estimation W_d (p,k) in a frequency domain to a time domain to produce w_d (p,n) for transmit path p, wherein k is a frequency bin index and n is a time sample index;

set samples (L/2+1) to (N−

L/2) of w_d(p,k) to zero to obtain an L-tap filter w_z(p,k), where L is the number of taps of the all-pass filter;

transform w_z(p,k) to the frequency domain to obtain a frequency vector W_z(p,k);

determine the mean square error (MSE) between W_z(p,k) and W_d (p,k) for k frequency bins in the occupied frequency region;

if the MSE is not below a minimum value, set magnitudes of W_z(p,k) to one and phases of W_z(p,k) equal to the phases of W_d (p,k) in the occupied frequency region, and keep remaining values of W_z(p,k) as is; and

repeat operations of transforming the impairment estimation, setting the samples, transforming w_z(p,k) to the frequency domain, determining the MSE and setting the magnitudes until the MSE is below the minimum value.