Techniques for determining uplink precoding matrix for a user equipment

1. A method for wireless communications by a user equipment (UE), comprising:

receiving a precoded reference signal (RS) from a base station (BS);

determining, by the UE, a first precoding used by the BS to transmit the RS;

calculating an effective channel based on the first precoding;

determining, by the UE, a second precoding based on the effective channel; and

transmitting a signal to the BS using the second precoding.

2. The method of claim 1 , wherein the RS comprises a channel state information reference signal (CSI-RS).

3. The method of claim 1 , wherein calculating the effective channel comprises:

estimating a channel based on the RS; and

multiplying the estimated channel by the first precoding to calculate the effective channel.

4. The method of claim 1 , further comprising:

transmitting a sounding reference signal (SRS) for use by the BS in selecting the first precoding.

5. The method of claim 1 ,

wherein determining the second precoding comprises performing a matrix decomposition on the effective channel to calculate the second precoding.

6. The method of claim 5 , wherein the matrix decomposition comprises singular value decomposition.

7. The method of claim 1 , wherein:

the UE receives the RS in a first subframe; and

the UE transmits the signal in a second subframe after the first subframe.

8. A method for wireless communications by a base station (BS), comprising:

receiving a sounding reference signal (SRS) from a user equipment (UE);

estimating a channel based on the SRS;

estimating a noise covariance matrix based on the estimated channel;

calculating, based on the noise covariance matrix, a first precoding to convey using a reference signal (RS), to the UE, wherein from the first precoding, the UE determines a second precoding as a function of the first precoding for use by the UE to transmit an uplink (UL) signal to the BS; and

transmitting the reference signal (RS) to the UE using the first precoding.

9. The method of claim 8 , wherein the RS comprises a channel state information reference signal (CSI-RS).

10. The method of claim 8 , further comprising:

receiving a signal transmitted by the UE using the second precoding.

11. An apparatus for wireless communications, comprising:

means for receiving a precoded reference signal (RS) from a base station (BS);

means for determining, by the apparatus, a first precoding used by the BS to transmit the RS;

means for calculating an effective channel based on the first precoding;

means for determining, by the apparatus, a second precoding based on the effective channel; and

means for transmitting a signal to the BS using the second precoding.

12. The apparatus of claim 11 , wherein the RS comprises a channel state information reference signal (CSI-RS).

13. The apparatus of claim 11 , wherein the means for calculating the effective channel comprises:

means for estimating a channel based on the RS; and

means for multiplying the estimated channel by the first precoding to calculate the effective channel.

14. The apparatus of claim 11 , further comprising:

means for transmitting a sounding reference signal (SRS) for use by the BS in selecting the first precoding.

15. The apparatus of claim 11 ,

wherein the means for determining the second precoding comprises means for performing a matrix decomposition on the effective channel to calculate the second precoding.

16. The apparatus of claim 15 , wherein the matrix decomposition comprises singular value decomposition.

17. The apparatus of claim 11 , wherein:

the means for receiving is configured to receive the RS in a first subframe; and

the means for transmitting is configured to transmit the UL signal in a second subframe after the first subframe.

18. A method for wireless communications by a user equipment (UE), comprising:

receiving a precoded reference signal (RS) from a base station (BS);

estimating a channel based on the RS;

determining a first precoding used by the BS to transmit the RS;

calculating an effective channel based on the estimated channel and the first precoding;

performing matrix decomposition based on the effective channel;

determining a second precoding based on the matrix decomposition; and

transmitting a signal to the BS using the second precoding.

19. The method of claim 18 , wherein determining the first precoding comprises:

measuring content of the received RS;

comparing the measured content to a known content; and

determining the first precoding based on the comparison.