Beamforming in antenna systems

1. A base station, comprising:

baseband precoding circuitry, wherein the baseband precoding circuitry receives K inputs and digitally multiplies the K inputs with a multiple-input multiple-output (MIMO) precoding matrix (F MU ) generating M outputs, and wherein K indicates a number of users communicating with the base station;

a compensation circuitry, wherein the compensation circuitry digitally multiplies the M outputs of the baseband precoding circuitry with a compensation matrix (F CM ) generating M compensation outputs;

M radio frequency (RF) chains, wherein each RF chain is configured to receive one of the M compensation outputs, and generate an analog frequency-up-converted signal; and

analog precoding circuitry, wherein the analog precoding circuitry receives the M analog frequency-up-converted signals and analog multiplies the M analog frequency-up-converted signals with a constrained analog precoding matrix (F R ) generating N output signals for transmission, wherein N is greater than M;

wherein the constrained analog precoding matrix (F RF ) is determined based on an unconstrained analog precoding matrix (F RF,UC ), and wherein the unconstrained analog precoding matrix (F RF,UC ) is determined based on dominant eigenvectors of the sum of spatial channel covariance matrices of the K users; and

wherein the compensation matrix (F CM ) is determined based on the constrained analog precoding matrix (F RE ).

2. The base station of claim 1 , wherein K is less than or equal to M.

3. The base station of claim 1 , wherein multiplication of the unconstrained analog precoding matrix (F RF,UC ) with any invertible matrix is substantially equal to the constrained analog precoding matrix (F RF ).

4. The base station of claim 1 , wherein the MIMO precoding matrix (F MU ) is determined based on an effective channel matrix that comprises one or more of the constrained analog precoding matrix (F RF ), the compensation matrix (F CM ), and a raw channel matrix.

5. The base station of claim 1 , wherein the constrained analog precoding circuitry comprises phase shifters, and wherein the analog multiplication of the unconstrained analog precoding matrix (F RF,UC ) controls phases of the analog frequency-up-converted signals.

6. The base station of claim 1 , wherein the constrained analog precoding circuitry comprises phase shifters, wherein multiplication of the constrained analog precoding matrix (F RF ) and the compensation matrix (F CM ) is substantially equal to the unconstrained analog precoding matrix (F RF,UC ).

7. A method, comprising:

determining an unconstrained analog precoding matrix (F RF,UC ), wherein the unconstrained analog precoding matrix (F RF,UC ) is determined based on M dominant eigenvectors of the sum of spatial channel covariance matrices of K users, and wherein K indicates a number of users communicating with a base station;

determining a constrained analog precoding matrix (F RF ) based on the unconstrained analog precoding matrix (F RF,UC );

determining a compensation matrix (F CM ), wherein the compensation matrix (F CM ) is determined based on the constrained analog precoding matrix (F RF );

digitally multiplying K inputs with a multiple-input multiple-output (MIMO) precoding matrix (F MU ) generating M outputs;

digitally multiplying the M outputs with the compensation matrix (F CM ) generating M compensation outputs;

generating M analog frequency-up-converted signals based on the M compensation outputs; and

analog multiplying the M analog frequency-up-converted signals with the analog precoding matrix (F RF ) generating N output signals for transmission, wherein N is greater than M.

8. The method of claim 7 , wherein K is less than or equal to M.

9. The method of claim 7 , wherein multiplication of the unconstrained analog precoding matrix (F RF,UC ) with any invertible matrix is substantially equal to the constrained analog precoding matrix (F RF ).

10. The method of claim 7 , wherein the MIMO precoding matrix (F MU ) is determined based on an effective channel matrix that comprises one or more of the constrained analog precoding matrix (F RF ), the compensation matrix (F CM ), and a raw channel matrix.

11. The method of claim 7 , wherein the constrained analog precoding circuitry comprises phase shifters, and wherein the analog multiplication of the unconstrained analog precoding matrix (F RF,UC ) controls phases of the analog frequency-up-converted signals.

12. The method of claim 7 , wherein the constrained analog precoding circuitry comprises phase shifters, wherein multiplication of the constrained analog precoding matrix (F RF ) and the compensation matrix (F CM ) is substantially equal to the unconstrained analog precoding matrix (F RF,UC ).

13. A system, comprising:

one or more processors; and

a non-transitory computer-readable storage device including one or more instructions for execution by the one or more processors and when executed operable to perform operations comprising:

determining an unconstrained analog precoding matrix (F RF,UC ), wherein the unconstrained analog precoding matrix (F RF,UC ) is determined based on dominant eigenvectors of the sum of spatial channel covariance matrices of K users, and wherein K indicates a number of users communicating with a base station;

determining a constrained analog precoding matrix (F RF ) based on the unconstrained analog precoding matrix (F RF,UC );

determining a compensation matrix (F CM ), wherein the compensation matrix (F CM ) is determined based on the constrained analog precoding (F RF );

digitally multiplying K inputs with a multiple-input multiple-output (MIMO) precoding matrix (F MU ) generating M outputs;

digitally multiplying the M outputs with the compensation matrix (F CM ) generating M compensation outputs;

generating M analog frequency-up-converted signals based on the M compensation outputs; and

analog multiplying the M analog frequency-up-converted signals with the analog precoding matrix (F RF ) generating N output signals for transmission, wherein N is greater than M.

14. The system of claim 13 , wherein K is less than or equal to M.

15. The system of claim 13 , wherein multiplication of the unconstrained analog precoding matrix (F RF,UC ) with any invertible matrix is substantially equal to the constrained analog precoding matrix (F RF ).

16. The system of claim 13 , wherein the MIMO precoding matrix (F MU ) is determined based on an effective channel matrix that comprises one or more of the constrained analog precoding matrix (F RF ), the compensation matrix (F CM ), and a raw channel matrix.

17. The system of claim 13 , wherein the constrained analog precoding circuitry comprises phase shifters, and wherein the analog multiplication of the unconstrained analog precoding matrix (F RF,UC ) controls phases of the analog frequency-up-converted signals.