Beamforming from multiple transmission sites

1. An endpoint comprising:

an interface configured to:

receive a channel reference signal from each transmission site of a plurality of transmission sites, each of the plurality of transmission sites comprising a transmission site of a radio antenna system;

a processor coupled to the interface and configured to:

estimate a channel to each transmission site of the plurality of transmission sites using the channel reference signal from the respective transmission site;

select a first transmission site from the plurality of transmission sites, the first transmission site comprising the site with the highest quality estimated channel of each of the plurality of transmission sites;

determine a first precoding matrix corresponding to the first transmission site, the first precoding matrix to be applied to data transmitted by the first transmission site to the endpoint; and

determine a second precoding matrix corresponding to a second channel between a second transmission site of the plurality of transmission sites and the endpoint, the second precoding matrix determined using at least a first spatial direction, the first spatial direction comprising a spatial direction of a frequency domain product of the first precoding matrix and the first channel, and wherein a frequency domain product of the second precoding matrix and the second channel rotates the second channel to the first spatial direction; and

the interface further configured to:

communicate an indication of the first precoding matrix and an indication of the second precoding matrix to a transmission site;

receive a radio transmission transmitted from the first transmission site according to the first precoding matrix; and

receive a radio transmission transmitted from the second transmission site according to the second precoding matrix.

2. The endpoint of claim 1 , the processor further configured to determine a third precoding matrix for a third transmission site of the plurality of transmission sites, the third precoding matrix determined using at least the first spatial direction.

3. The endpoint of claim 1 , the processor further configured to determine a third precoding matrix for a third transmission site of the plurality of transmission sites, the third precoding matrix determined at least the first spatial direction.

4. The endpoint of claim 1 , the processor further configured to determine a third precoding matrix for a third transmission site of the plurality of transmission sites, the third precoding matrix determined using at least the first spatial direction and a second spatial direction comprising a spatial direction of a frequency domain product of the second precoding matrix and the second channel.

5. The endpoint of claim 1 , the determining the second precoding matrix further comprising:

determining a value indicative of the first spatial direction, the value based on the frequency domain product of the first precoding matrix and the first channel;

normalizing the value; and

selecting the second precoding matrix from a plurality of precoding matrices using a comparison of the normalized value to the second precoding matrix.

6. The endpoint of claim 1 , wherein the second precoding matrix is applied to data transmitted by the second transmission site to the endpoint to beamform the data transmitted by the second transmission site to the endpoint.

7. A method comprising:

receiving a channel reference signal from each transmission site of a plurality of transmission sites, each of the plurality of transmission sites comprising a transmission site of a radio antenna system;

estimating a channel to each transmission site of the plurality of transmission sites using the channel reference signal from the respective transmission site;

selecting a first transmission site from the plurality of transmission sites, the first transmission site comprising the site with the highest quality estimated channel of each of the plurality of transmission sites;

determining, by the endpoint, a first precoding matrix corresponding to the first transmission site, the first precoding matrix to be applied to data transmitted by the first transmission site to the endpoint; and

determining, by the endpoint, a second precoding matrix corresponding to a second channel between a second transmission site of the plurality of transmission sites and the endpoint, the second precoding matrix determined using at least a first spatial direction, the first spatial direction comprising a spatial direction of a frequency domain product of the first precoding matrix and the first channel, and wherein a frequency domain product of the second precoding matrix and the second channel rotates the second channel to the first spatial direction;

communicating an indication of the first precoding matrix and an indication of the second precoding matrix to a transmission site;

receiving a radio transmission transmitted from the first transmission site according to the first precoding matrix; and

receiving a radio transmission transmitted from the second transmission site according to the second precoding matrix.

8. The method of claim 7 , further comprising determining a third precoding matrix for a third transmission site of the plurality of transmission sites, the third precoding matrix determined using at least the first spatial direction.

9. The method of claim 7 , further comprising determining a third precoding matrix for a third transmission site of the plurality of transmission sites, the third precoding matrix determined using at least the first spatial direction.

10. The method of claim 7 , further comprising determining a third precoding matrix for a third transmission site of the plurality of transmission sites, the third precoding matrix determined using at least the first spatial direction and a second spatial direction comprising a spatial direction of a frequency domain product of the second precoding matrix and the second channel.

11. The method of claim 7 , the determining the second precoding matrix further comprising:

determining a value indicative of the first spatial direction, the value based on the frequency domain product of the first precoding matrix and the first channel;

normalizing the value; and

selecting the second precoding matrix from a plurality of precoding matrices using a comparison of the normalized value to the second precoding matrix.

12. The method of claim 7 , wherein the second precoding matrix is applied to data transmitted by the second transmission site to the endpoint to beamform the data transmitted by the second transmission site to the endpoint.

13. One or more non-transitory computer-readable storage media embodying logic that when executed by a processor is configured to:

receive a channel reference signal from each transmission site of a plurality of transmission sites, each of the plurality of transmission sites comprising a transmission site of a radio antenna system;

estimate a channel to each transmission site of the plurality of transmission sites using the channel reference signal from the respective transmission site;

select a first transmission site from the plurality of transmission sites, the first transmission site comprising the site with the highest Quality estimated channel of each of the plurality of transmission sites;

determine a first precoding matrix corresponding to the first transmission site, the first precoding matrix to be applied to data transmitted by the first transmission site to the endpoint; and

determine a second precoding matrix corresponding to a second channel between a second transmission site of the plurality of transmission sites and the endpoint, the second precoding matrix determined using at least a first spatial direction, the first spatial direction comprising a spatial direction of a frequency domain product of the first precoding matrix and the first channel, and wherein a frequency domain product of the second precoding matrix and the second channel rotates the second channel to the first spatial direction;

communicate an indication of the first precoding matrix and an indication of the second precoding matrix to a transmission site;

receive a radio transmission transmitted from the first transmission site according to the first precoding matrix; and

receive a radio transmission transmitted from the second transmission site according to the second precoding matrix.

14. The media of claim 13 , the logic further configured to determine a third precoding matrix for a third transmission site of the plurality of transmission sites, the third precoding matrix determined using at least the first spatial direction.

15. The media of claim 13 , the logic further configured to determine a third precoding matrix for a third transmission site of the plurality of transmission sites, the third precoding matrix determined using at least the first spatial direction.

16. The media of claim 13 , the logic further configured to determine a third precoding matrix for a third transmission site of the plurality of transmission sites, the third precoding matrix determined using at least the first spatial direction and a second spatial direction that is based at least on a second channel between the second transmission site and the endpoint.

17. The media of claim 13 , the determining the second precoding matrix further comprising:

determining a value indicative of the first spatial direction, the value based on the frequency domain product of the first precoding matrix and the first channel;

normalizing the value; and

selecting the second precoding matrix from a plurality of precoding matrices using a comparison of the normalized value to the second precoding matrix.