Calibration of a downlink transmit path of a base station

1. A method for calibrating a transmit path of a base station, comprising:

generating a predefined waveform;

inserting the predefined waveform in a calibration symbol;

constructing a subframe;

transmitting, at a media access control (MAC) layer, downlink data to a user equipment during the subframe;

replacing, at a physical (PHY) layer, the downlink data of the subframe with the calibration symbol;

receiving, at the base station, through a dedicated feedback receive path, at least part of the subframe comprising the calibration symbol; and

calibrating the transmit path of the base station based at least in part on the received calibration symbol.

2. The method of claim 1 , wherein calibrating the transmit path comprises:

estimating a transmit path impairment based at least in part on the received calibration symbol;

determining a compensation value based at least in part on the transmit path impairment; and

calibrating the transmit path of the base station based at least in part on the compensation value.

3. The method of claim 1 , further comprising:

monitoring transmissions on the downlink channel through a dedicated feedback receive path.

4. The method of claim 1 , further comprising:

transmitting a control symbol of the subframe, wherein the control symbol is associated with one or both of a downlink reference signal and a downlink control channel.

5. The method of claim 4 , further comprising:

transmitting at least three additional symbols comprising downlink reference signals.

6. The method of claim 4 , wherein the downlink control channel comprises at least one of a Physical Downlink Control Channel (PDCCH), a Physical Control Format Indicator Channel (PCFICH), or a Physical Hybrid Automatic Request Indicator Channel (PHICH).

7. The method of claim 1 , further comprising:

transmitting a control symbol of the subframe to signal to the user equipment zero allocation of downlink resources during the subframe.

8. The method of claim 1 , further comprising:

receiving a negative acknowledgement (NACK); and

retransmitting, at the MAC layer, the downlink data according to a hybrid automatic repeat request (HARQ) procedure.

9. The method of claim 1 , wherein the predefined waveform comprises:

a representation of a maximum transmitted power signal of a power amplifier over maximum useful channel bandwidth.

10. The method of claim 9 , further comprising:

using the predefined waveform to estimate digital pre-distortion coefficients for linearization of the power amplifier.

11. The method of claim 1 , wherein the predefined waveform comprises:

an a priori tone to estimate an in-phase/quadrature (I/Q) imbalance of the transmit path.

12. The method of claim 11 , further comprising:

using the predefined waveform to estimate an image caused by the I/Q imbalance in the transmit path; and

estimating the I/Q imbalance based at least in part on the estimated image.

13. The method of claim 1 , wherein a first slot of the subframe comprises the calibration symbol and the transmit path is associated with a first antenna of the base station, the method further comprising:

calibrating a second transmit path of a second antenna of the base station during a second slot of the subframe, the second slot comprising a calibration symbol to carry a second predefined waveform to calibrate the second transmit path of the second antenna.

14. A base station configured to calibrate a transmit path, comprising:

a processor;

a memory in electronic communication with the processor; and

instructions stored in the memory, the instructions being executable by the processor to:

generate a predefined waveform;

insert the predefined waveform in a calibration symbol;

construct a subframe;

transmit, at a media access control (MAC) layer, downlink data to a user equipment during the subframe;

replace, at a physical (PHY) layer, the downlink data of the subframe with the calibration symbol;

receive, at the base station, through a dedicated feedback receive path, at least part of the subframe comprising the calibration symbol; and

calibrate the transmit path of the base station based in least in part on the received calibration symbol.

15. The base station of claim 14 , wherein the instructions executable by the processor to calibrate the transmit path comprise instructions executable by the processor to:

estimate a transmit path impairment based at least in part on the received calibration symbol;

determine a compensation value based at least in part on the transmit path impairment; and

calibrate the transmit path of the base station based at least in part on the compensation value.

16. The base station of claim 14 , wherein the instructions are executable by the processor to:

monitor transmissions on the downlink channel through a dedicated feedback receive path.

17. The base station of claim 14 , wherein the instructions are executable by the processor to:

transmit a control symbol of the subframe to signal to the user equipment zero allocation of downlink resources during the subframe.

18. The base station of claim 14 , wherein the predefined waveform comprises:

a representation of a maximum transmitted power signal of a power amplifier over maximum useful channel bandwidth.

19. The base station of claim 18 , wherein the instructions are executable by the processor to:

use the predefined waveform to estimate digital pre-distortion coefficients for linearization of the power amplifier.

20. The base station of claim 14 , wherein the predefined waveform comprises:

an a priori tone to estimate an in-phase/quadrature (I/Q) imbalance of the transmit path.

21. A method for calibrating a transmit path of a base station, comprising:

generating a predefined waveform;

inserting the predefined waveform in a calibration symbol;

preserving, at a media access control (MAC) layer, previously allocated downlink resources for the calibration symbol;

constructing a subframe, wherein the preserved downlink resources comprise the calibration symbol;

transmitting the subframe from the base station to a user equipment on a downlink channel;

receiving, at the base station, through a dedicated feedback receive path, at least part of the subframe comprising the calibration symbol; and

calibrating the transmit path of the base station based at least in part on the received calibration symbol.

22. The method of claim 21 , wherein calibrating the transmit path comprises:

estimating a transmit path impairment based at least in part on the received calibration symbol;

determining a compensation value based at least in part on the transmit path impairment; and

calibrating the transmit path of the base station based at least in part on the compensation value.

23. The method of claim 21 , further comprising:

monitoring transmissions on the downlink channel through the dedicated feedback receive path.

24. A base station configured to calibrate a transmit path, comprising:

a processor;

a memory in electronic communication with the processor; and

instructions stored in the memory, the instructions being executable by the processor to:

generate a predefined waveform;

insert the predefined waveform in a calibration symbol;

preserve, at a media access control (MAC) layer, previously allocated downlink resources for the calibration symbol;

construct a subframe, wherein the preserved downlink resources comprise the calibration symbol;

transmit the subframe from the base station to a user equipment on a downlink channel;

receive, at the base station, through a dedicated feedback receive path, at least part of the subframe comprising the calibration symbol; and

calibrate the transmit path of the base station based at least in part on the received calibration symbol.

25. The base station of claim 24 , wherein the instructions executable by the processor to calibrate the transmit path comprise instructions executable by the processor to:

estimate a transmit path impairment based at least in part on the received calibration symbol;

determine a compensation value based at least in part on the transmit path impairment; and

calibrate the transmit path of the base station based at least in part on the compensation value.

26. The base station of claim 24 , wherein the instructions are executable by the processor to:

monitor transmissions on the downlink channel through the dedicated feedback receive path.