Techniques for beamforming in multi-user systems

1. A base station for communication with multiple user terminals, the base station comprising:

a digital precoder and an analog precoder which are configured to precode a first training signal;

a radio transceiver configured to transmit the precoded first training signal and to receive a precoded second training signal upon transmission of the precoded first training signal; and

a controller configured to, after each reception of the second training signal in a ping-pong process including multiple transmissions of first training signals and receptions of second training signals:

adjust a precoding of the digital precoder based on processing of the second training signal with respect to a normalization criterion;

cause the radio transceiver to transmit a subsequent first training signal, and

after transmission of the subsequent first training signal, adjust a precoding of the analog precoder based on processing of the second training signal using a codebook selection according to a multi-user beamforming criterion.

2. The base station of claim 1 ,

wherein the controller is configured to adjust the precoding of the analog precoder based on beam steering according to the multi-user beamforming criterion.

3. The base station of claim 2 ,

wherein the beam steering is based on directions of departure of most-significant multipath components transmitted by the transceiver.

4. The base station of claim 3 ,

wherein the beam steering is based on selecting beams according to a signal energy criterion.

5. The base station of claim 1 ,

wherein the multi-user beamforming criterion comprises one of the following: sum of beamforming gains; or Zero-Forcing (ZF) sum-rate.

6. The base station claim 1 ,

wherein the multi-user beamforming criterion is based on a plurality of analog beams transmitted by the radio transceiver to the multiple user terminals.

7. The base station of claim 1 ,

wherein the radio transceiver is configured to transmit a respective transmission based on an adjustment of the digital precoder with respect to a preceding reception.

8. The base station of claim 1 ,

wherein the controller is configured to adjust the precoding of the digital precoder based on conjugation and normalization of the second training signal.

9. The base station of claim 8 ,

wherein the normalization comprises correlation with a known pilot signal.

10. The base station of claim 1 ,

wherein the controller is configured to initialize the analog precoder to a state that enables the analog precoder scanning all directions.

11. The base station of claim 1 ,

wherein the controller is configured to initialize the digital precoder randomly.

12. A user terminal, comprising:

a digital precoder and an analog precoder which are configured to precode a second training signal;

a radio transceiver configured to transmit the precoded second training signal and to receive a precoded first training signal in response to transmission of the precoded second training signal; and

a controller configured to, after each reception of the first training signal in a ping-pong process including multiple transmissions of second training signals and receptions of first training signals:

adjust a precoding of the digital precoder based on processing of the first training signal with respect to a normalization criterion;

cause the radio transceiver to transmit a subsequent second training signal, and

after transmission of the subsequent second training signal, adjust a precoding of the analog precoder based on processing of the first training signal using a codebook selection according to a multi-user beamforming criterion.

13. A multi-user precoding method, comprising:

precoding, by a digital precoder and an analog precoder, a first training signal;

transmitting the precoded first training signal by a radio transceiver;

receiving a precoded second training signal by the radio transceiver upon transmission of the precoded first training signal; and

after each reception of the second training signal in a ping-pong process including multiple transmissions of first training signals and receptions of second training signals:

adjusting a precoding of the digital precoder based on processing the second training signal with respect to a normalization criterion;

transmitting a subsequent first training signal; and

after transmission of the subsequent first training signal, adjusting a precoding of the analog precoder based on processing the second training signal with respect to a multi-user beamforming criterion.

14. A precoding method, comprising:

precoding a second training signal by a digital precoder and an analog precoder;

transmitting the precoded second training signal by a radio transceiver;

receiving a precoded first training signal by the radio transceiver upon transmission of the precoded second training signal; and

after each reception of the first training signal in a ping-pong process including multiple transmissions of second training signals and receptions of first training signals:

adjusting a precoding of the digital precoder based on processing the first training signal with respect to a normalization criterion;

transmitting a subsequent second training signal; and

after transmission of the subsequent second training signal, adjusting a precoding of the analog precoder based on processing the first training signal with respect to a beamforming criterion.

15. A multi-user precoding device, comprising:

means for precoding a first training signal by a digital precoder and an analog precoder;

means for transmitting the precoded first training signal by a radio transceiver;

means for receiving a precoded second training signal by the radio transceiver upon transmission of the precoded first training signal; and

means for adjusting a precoding of the digital precoder based on processing the second training signal with respect to a normalization criterion; causing the means for transmitting to transmit a subsequent first training signal; and, after transmission of the subsequent first training signal, adjusting a precoding of the analog precoder based on processing the second training signal with respect to a multi-user beamforming criterion.

16. A precoding system for serving multiple user terminals, the precoding system comprising:

a digital precoder and an analog precoder component which are configured to precode a first training signal;

a radio transceiver component configured to transmit the precoded first training signal and to receive a precoded second training signal upon transmission of the precoded first training signal; and

a controller component configured to, after each reception of the second training signal in a ping-pong process including multiple transmissions of first training signals and receptions of second training signals:

adjust a precoding of the digital precoder based on processing the second training signal with respect to a normalization criterion;

cause the radio transceiver component to transmit a subsequent first training signal, and

after transmission of the subsequent first training signal, to adjust a precoding of the analog precoder based on processing of the second training signal using a codebook selection according to a multi-user beamforming criterion.

17. A computer readable non-transitory medium on which computer instructions are stored which, when executed by a computer, cause the computer to perform the method of claim 13 .