Method to improve active antenna system performance in the presence of mutual coupling

1. A method in a network node, comprising:

receiving a first input signal from a first antenna branch baseband processing unit of a first antenna branch of the network node;

receiving a second input signal from a second antenna branch baseband processing unit of a second antenna branch of the network node;

determining a first compensation coefficient;

determining a second compensation coefficient;

applying, using a mutual coupling processor, the first compensation coefficient to the second input signal received from the second antenna branch baseband processing unit to generate a first output signal;

applying, using the mutual coupling processor, the second compensation coefficient to the first input signal received from the first antenna branch baseband processing unit to generate a second output signal;

outputting the first output signal to a first power amplifier of the first antenna branch, the first output signal compensating for a mutual coupling from the second antenna branch to the first antenna branch, the first power amplifier operating in a non-linear region; and

outputting the second output signal to a second power amplifier of the second antenna branch, the second output signal compensating for a mutual coupling from the first antenna branch to the second antenna branch, the second power amplifier operating in the non-linear region.

2. The method of claim 1 , further comprising:

applying one or more pre-distortion techniques to transform a first output signal of the first antenna branch baseband processing unit to compensate for a non-linearity of the first power amplifier, the first output signal of the first antenna branch baseband processing unit corresponding to the first input signal;

applying the one or more pre-distortion techniques to transform a second output signal of the second antenna branch baseband processing unit to compensate for a non-linearity of the second power amplifier, the second output signal of the second antenna branch baseband processing unit corresponding to the second input signal.

3. The method of claim 2 , wherein the pre-distortion techniques include one of digital pre-distortion techniques and analog pre-distortion techniques.

4. The method of claim 2 , wherein:

the mutual coupling from the second antenna branch to the first antenna branch comprises a leakage signal power from the second power amplifier that is leaked in a backward direction and passes through the first power amplifier without being transformed by the one or more pre-distortion techniques; and

the mutual coupling from the first antenna branch to the second antenna branch comprises a leakage signal power from the first power amplifier that is leaked in a backward direction and passes through the second power amplifier without being transformed by the one or more pre-distortion techniques.

5. The method of claim 1 , wherein:

the first compensation coefficient and the second compensation coefficient are predefined;

the first compensation coefficient is based at least in part on an estimation of a mutual coupling from the second antenna branch to the first antenna branch; and

the second compensation coefficient is based at least in part on an estimation of a mutual coupling from the first antenna branch to the second antenna branch.

6. The method of claim 1 , wherein determining the first compensation coefficient and determining the second compensation coefficient comprises:

setting the first compensation coefficient to a first value;

setting the second compensation coefficient to a second value;

measuring an adjacent channel leakage power ratio based at least in part on a first power amplifier output from the first power amplifier and a second power amplifier output from the second power amplifier;

determining whether the adjacent channel leakage power ratio is less than an adjacent channel leakage power ratio threshold;

adjusting the first compensation coefficient and the second compensation coefficient upon determining that the adjacent channel leakage power ratio is more than the adjacent channel leakage power ratio threshold.

7. The method of claim 6 , wherein:

the first value is based at least in part on an estimation of a mutual coupling from the second antenna branch to the first antenna branch; and

the second value is based at least in part on an estimation of a mutual coupling from the first antenna branch to the second antenna branch.

8. The method of claim 6 , wherein the adjacent channel leakage power ratio threshold is −45 dBc (decibels relative to carrier).

9. The method of claim 1 , wherein the first antenna branch and the second antenna branch are two of a plurality of antenna branches of an antenna array of the network node, each of the plurality of antenna branches of the antenna array separated from each other by a distance, and compensation coefficients are only determined for pairs of antenna branches separated by a distance less than or equal to a threshold distance.

10. The method of claim 1 , wherein the network node is an active-array-antenna system base station.

11. A network node, comprising:

one or more processors, the one or more processors configured to:

receive a first input signal from a first antenna branch baseband processing unit of a first antenna branch of the network node;

receive a second input signal from a second antenna branch baseband processing unit of a second antenna branch of the network node;

determine a first compensation coefficient;

determine a second compensation coefficient;

apply, using a mutual coupling processor, the first compensation coefficient to the second input signal received from the second antenna branch baseband processing unit to generate a first output signal;

apply, using the mutual coupling processor, the second compensation coefficient to the first input signal received from the first antenna branch baseband processing unit to generate a second output signal;

output the first output signal to a first power amplifier of the first antenna branch, the first output signal compensating for a mutual coupling from the second antenna branch to the first antenna branch, the first power amplifier operating in a non-linear region; and

output the second output signal to a second power amplifier of the second antenna branch, the second output signal compensating for a mutual coupling from the first antenna branch to the second antenna branch, the second power amplifier operating in the non-linear region.

12. The network node of claim 11 , wherein the one or more processors are further configured to:

apply one or more pre-distortion techniques to transform a first output signal of the first antenna branch baseband processing unit to compensate for a non-linearity of the first power amplifier, the first output signal of the baseband first antenna branch processing unit corresponding to the first input signal;

apply the one or more pre-distortion techniques to transform a second output signal of the second antenna branch baseband processing unit to compensate for a non-linearity of the second power amplifier, the second output signal of the second antenna branch baseband processing unit corresponding to the second input signal.

13. The network node of claim 12 , wherein the pre-distortion techniques include one of digital pre-distortion techniques and analog pre-distortion techniques.

14. The network node of claim 12 , wherein:

the mutual coupling from the second antenna branch to the first antenna branch comprises a leakage signal power from the second power amplifier that is leaked in a backward direction and passes through the first power amplifier without being transformed by the one or more pre-distortion techniques; and

the mutual coupling from the first antenna branch to the second antenna branch comprises a leakage signal power from the first power amplifier that is leaked in a backward direction and passes through the second power amplifier without being transformed by the one or more pre-distortion techniques.

15. The network node of claim 11 , wherein:

the first compensation coefficient and the second compensation coefficient are predefined;

the first compensation coefficient is based at least in part on an estimation of a mutual coupling from the second antenna branch to the first antenna branch; and

the second compensation coefficient is based at least in part on an estimation of a mutual coupling from the first antenna branch to the second antenna branch.

16. The network node of claim 11 , wherein the one or more processors configured to determine the first compensation coefficient and determine the second compensation coefficient comprises one or more processors configured to:

set the first compensation coefficient to a first value;

set the second compensation coefficient to a second value;

measure an adjacent channel leakage power ratio based at least in part on a first power amplifier output from the first power amplifier and a second power amplifier output from the second power amplifier;

determine whether the adjacent channel leakage power ratio is less than an adjacent channel leakage power ratio threshold;

adjust the first compensation coefficient and the second compensation coefficient upon determining that the adjacent channel leakage power ratio is more than the adjacent channel leakage power ratio threshold.

17. The network node of claim 16 , wherein:

the first value is based at least in part on an estimation of a mutual coupling from the second antenna branch to the first antenna branch; and

the second value is based at least in part on an estimation of a mutual coupling from the first antenna branch to the second antenna branch.

18. The network node of claim 16 , wherein the adjacent channel leakage power ratio threshold is −45 dBc (decibels relative to carrier).

19. The network node of claim 11 , wherein the first antenna branch and the second antenna branch are two of a plurality of antenna branches of an antenna array of the network node, each of the plurality of antenna branches of the antenna array separated from each other by a distance, and compensation coefficients are only determined for pairs of antenna branches separated by a distance less than or equal to a threshold distance.

20. The network node of claim 11 , wherein the network node is an active-array-antenna system base station.