Time-division duplex multiple input multiple output calibration

1. A time-division duplex (TDD) multiple-input multiple-output (MIMO) system comprising:

a plurality of nodes comprising a first group of antennas and a second group of antennas, each node of the plurality of nodes configured to wirelessly transmit and wirelessly receive signals; and

a baseband unit in communication with the plurality of nodes, the baseband unit comprising one or more processors configured to execute instructions stored by the baseband unit to:

cause the first group of antennas to transmit first reference signals to the second group of antennas over designated physical resources during a first time slot;

cause the second group of antennas to transmit second reference signals to the first group of antennas over the designated physical resources during a second time slot;

cause a third group of antennas to transmit third reference signals to the first and second groups of antennas during a third time slot;

generate channel estimates based on the first reference signals received by the second group of antennas, the second reference signals received by the first group of antennas, and the third reference signals received by the first and second groups of antennas; and

jointly process the channel estimates to generate, for at least each antenna of the first and second groups of antennas, a calibration coefficient representing a ratio for compensating for a difference between a transmit coefficient and a receive coefficient.

2. The TDD MIMO system of claim 1 , wherein the plurality of nodes comprises remote radio units, a first remote radio units of the remote radio units comprises an antenna of the first group of antennas, and a second remote radio unit of the remote radio units comprises an antenna of the second group of antennas.

3. The TDD MIMO system of claim 2 , wherein the plurality of nodes comprises a user equipment arranged to operate as a virtual network node.

4. The TDD MIMO system of claim 1 , wherein the plurality of nodes comprises a user equipment.

5. The TDD MIMO system of claim 1 , wherein the plurality of nodes comprises a first user equipment and a second user equipment, and wherein the first user equipment and the second user equipment are configured to communicate with each other via a peer-to-peer link.

6. The TDD MIMO system of claim 1 , wherein the designated physical resources are orthogonal physical resources.

7. A method of calibration in a time-division duplex (TDD) multiple-input multiple-output (MIMO) system, the method comprising:

causing transmission, from a first group of antennas, of first reference signals to a second group of antennas over designated physical resources during a first time slot;

causing transmission, from the second group of antennas, of second reference signals to the first group of antennas over the designated physical resources during a second time slot;

causing a third group of antennas to transmit third reference signals to the first and second groups of antennas during a third time slot;

generating channel estimates associated with the first and second groups of antennas based on the first reference signals received by the second group of antennas, the second reference signals received by the first group of antennas, and the third reference signals received by the first and second groups of antennas; and

jointly processing the channel estimates to generate, for each antenna of the first and second groups, a calibration coefficient representing a ratio for compensating for channel gain mismatch between a transmit coefficient and a receive coefficient.

8. The method of claim 7 , wherein a remote radio unit comprises an antenna of the first group.

9. The method of claim 7 , wherein a remote radio unit comprises an antenna of the first group, and wherein a user equipment arranged to operate as a virtual network node comprises an antenna of the second group.

10. The method of claim 7 , wherein a user equipment comprises an antenna of the first group.

11. The method of claim 7 , wherein a first user equipment comprises an antenna of the first group, wherein a second user equipment comprises an antenna of the second group, and the method further comprises transmitting information associated with the first reference signals received by the antenna of the second group of the second user equipment to the first user equipment via a peer-to-peer link between the first user equipment and the second user equipment.

12. The method of claim 7 , wherein the first group of antennas comprises a reference antenna, and wherein the jointly processing comprises a single link calibration that comprises determining the calibration coefficients based on a single link from the antennas of second group to the reference antenna.

13. The method of claim 7 , wherein the jointly processing comprises generating the calibration coefficients with an iterative calibration that comprises applying an iterative algorithm on a factor graph of multiple paths between antennas of the first and second groups.

14. The method of claim 7 , further comprising selecting an algorithm of a plurality of algorithms for the jointly processing, wherein the plurality of algorithms have different computational complexity and different accuracy.

15. The method of claim 7 , further comprising:

causing transmission, from the first group of antennas, of additional first reference signals to a third group of antennas during the first time slot; and

causing transmission, from the second group of antennas, of additional second reference signals to the third group of antennas during the second time slot; and

wherein the generating the channel estimates is based on the additional first reference signals and the additional second reference signals received by the third group of antennas.

16. The method of claim 7 , further comprising applying each of the calibration coefficients to a respective signal path associated with the antennas of the first and second groups to compensate for channel gain mismatches between respective transmit and receive coefficients.

17. The method of claim 7 , wherein the jointly processing comprises determining the calibration coefficients relative to a calibration coefficient associated with a reference antenna of the first group of antennas.

18. The method of claim 17 , wherein the reference antenna is included in a higher level subset of antennas, and wherein the higher level subset of antennas comprises a first higher level subset group of antennas and a second higher level subset group of antennas.

19. The method of claim 18 , further comprising:

causing the first higher level subset group of antennas to transmit first higher level subset reference signals to the second higher level subset group of antennas during a first higher level subset time slot;

causing the second higher level subset group of antennas to transmit second higher level subset reference signals to the first higher level subset group of antennas during a second higher level subset time slot;

generating higher level subset channel estimates based on the first higher level subset reference signals received by the second higher level subset group of antennas and the second higher level subset reference signals received by the first higher level subset group of antennas; and

jointly processing the higher level subset channel estimates to generate, for at least each antenna of the first and second higher level subset groups, a higher level subset calibration coefficient representing a ratio associated with a transmit coefficient and a receive coefficient.

20. The method of claim 19 wherein the jointly processing the higher level subset channel estimates is performed before jointly processing the channel estimates.

21. Non-transitory computer-readable storage comprising memory storing computer-executable instructions, wherein the computer-executable instructions, when executed by a baseband unit of a time-division duplex (TDD) multiple-input multiple-output (MIMO) system, cause the TDD MIMO system to perform the method of claim 7 .

22. A method of calibration in a time-division duplex (TDD) multiple-input multiple-output (MIMO) system, the method comprising:

causing transmission, from a first group of antennas, of first reference signals to a second group of antennas over designated physical resources during a first time slot;

causing transmission, from the second group of antennas, of second reference signals to the first group of antennas over the designated physical resources during a second time slot;

generating channel estimates associated with the first and second groups of antennas based on the first reference signals received by the second group of antennas and the second reference signals received by the first group of antennas; and

jointly processing the channel estimates to generate, for each antenna of the first and second groups, a calibration coefficient representing a ratio for compensating for channel gain mismatch between a transmit coefficient and a receive coefficient, wherein the jointly processing comprises generating the calibration coefficients with a least squares regression of determining the calibration coefficients for the antennas of the first and second groups.

23. The method of claim 22 , further comprising:

causing a third group of antennas to transmit third reference signals to the first and second groups of antennas during a third time slot;

wherein the generating the channel estimates is based on the third reference signals received by the first and second groups of antennas.