Combined Calibration Method to Time and Phase Synchronize MIMO Receive Channels and Perform Frequency Response Correction

1 . A method for calibrating a plurality of receivers in a multiple input multiple output (MIMO) communication system, the method comprising:

operating the MIMO communication system in a calibration mode comprising:

by respective ones of the plurality of receivers:

receiving a respective calibration sequence, wherein each received calibration sequence is a different channel-modified version of a common calibration sequence from a source;

deriving a respective equalizer based on the received calibration sequence;

aligning a time and a phase associated with at least one of the plurality of receivers, and correcting a non-uniform frequency response of at least one of the plurality of receivers;

wherein the aligning of the time and the phase and the correcting the non-uniform frequency response is based on the derived equalizers; and

wherein the aligning of the time and the phase and the correcting the non-uniform frequency response at least partially corrects for differences in channels and time delays experienced by different ones of the plurality of receivers.

2 . The method of claim 1 , wherein deriving the equalizer based on the received calibration sequence comprises performing a channel estimation calculation using the received calibration sequence and a known calibration sequence.

3 . The method of claim 2 , wherein performing the channel estimation calculation comprises performing a cross-correlation calculation between the received calibration sequence and the known calibration sequence.

4 . The method of claim 2 , wherein the equalizers are derived at least in part based on an inhomogeneity in a spectral decomposition of the received calibration sequence determined from the channel estimation calculation, and wherein correcting the non-uniform frequency response based on the derived equalizers comprises correcting for the inhomogeneity in the spectral decomposition.

5 . The method of claim 1 , the method further comprising:

subsequent to said calibration mode:

using a switch at each of the plurality of receivers to switch to an operation mode;

wherein, while in the operation mode, respective receivers of the plurality of receivers are configured to receive signals from respective antennas using the alignment and correction.

6 . The method of claim 5 , wherein the method is configured to be performed in real time by:

automatically repeating the calibration mode at preset intervals;

alternating between calibration mode and operation mode according to the preset intervals;

wherein a radio protocol is designed to have pre-scheduled gaps in data transmission such that the pre-scheduled gaps coincide with the repeating of the calibration mode.

7 . The method of claim 6 , wherein operating in operation mode comprises using calibration results from a most recent calibration.

8 . The method of claim 1 , the method further comprising:

aligning a time and a phase associated with at least a second one of the plurality of receivers, and correcting a non-uniform frequency response of at least the second one of the plurality of receivers, based on the derived equalizers, wherein the non-uniform frequency response of the second one of the plurality of receivers is different from the non-uniform frequency response of the first one of the plurality receivers.

9 . The method of claim 1 , wherein the reference sequence is a Constant Amplitude Zero Autocorrelation (CAZAC) sequence.

10 . A multiple-input multiple-output (MIMO) radio device comprising a plurality of receivers coupled to one or more processing elements, wherein the plurality of receivers and the one or more processing elements are configured to:

operate the MIMO radio device in a calibration mode, wherein while in calibration mode the MIMO radio device is configured to:

by each one of the plurality of receivers:

receive a calibration sequence, wherein each received calibration sequence is a different channel-modified version of a common calibration sequence from a source;

derive an equalizer based on the received calibration sequence;

align a time and a phase associated with at least one of the plurality of receivers, and correcting a non-uniform frequency response of at least one of the plurality of receivers;

wherein the aligning of the time and the phase and the correcting the non-uniform frequency response is based on the derived equalizers; and

wherein the aligning of the time and the phase and the correcting the non-uniform frequency response at least partially corrects for differences and time delays in channels experienced by different ones of the plurality of receivers.

11 . The MIMO radio device of claim 10 , wherein deriving the equalizer based on the received calibration sequence comprises performing a channel estimation calculation using the received calibration sequence and a known calibration sequence.

12 . The MIMO radio device of claim 11 , wherein performing the channel estimation calculation comprises performing a cross-correlation calculation between the received calibration sequence and the known calibration sequence.

13 . The MIMO radio device of claim 11 , wherein the equalizers are derived at least in part based on an inhomogeneity in a spectral decomposition of the received calibration sequence determined from the channel estimation calculation, and wherein correcting the non-uniform frequency response based on the derived equalizers comprises correcting for the inhomogeneity in the spectral decomposition.

14 . The MIMO radio device of claim 10 , wherein the derived equalizers are fractionally spaced frequency domain equalizers.

15 . The MIMO radio device of claim 10 , wherein the derived equalizers are time domain equalizers.

16 . A non-transitory computer-readable memory medium comprising program instructions executable by a processor to calibrate a plurality of receivers in a multiple-input multiple-output (MIMO) communication system, wherein the program instructions are executable to:

derive an equalizer for each of the plurality of receivers, wherein the equalizers for each of the plurality of receivers are based on a channel-modified version of a common calibration sequence received by each of the plurality of receivers;

align a time and a phase associated with at least one of the plurality of receivers, and correct a non-uniform frequency response of at least one of the plurality of receivers,

wherein the aligning of the time and the phase and the correcting the non-uniform frequency response is based on the derived equalizers.

17 . The non-transitory computer-readable memory medium of claim 16 , wherein the program instructions are further executable to:

use a shared start trigger to initiate reception of the calibration sequence by the plurality of receivers.

18 . The non-transitory computer-readable memory medium of claim 17 , wherein the program instructions are executable to configure the MIMO communication system to perform calibration in real-time without missing data packets received from antennas during the calibration.

19 . The non-transitory computer-readable memory medium of claim 16 , wherein the plurality of receivers share a single local oscillator, wherein sharing the single local oscillator enables the MIMO communication system to remain calibrated for an extended duration of time.

20 . The non-transitory computer-readable memory medium of claim 16 , wherein the common calibration sequence is a Constant Amplitude Zero Autocorrelation (CAZAC) sequence.