Method and apparatus for beam pattern stabilization

1. A method of stabilising at least one Multi User Multiple Input Multiple Output MU-MIMO beam pattern formed by an array of antenna elements at an access point of a point to multi-point wireless communication network comprising a plurality of subscriber modules, the method comprising:

generating an output of a gyroscopic sensor to indicate a rate of change of an orientation of the array of antenna elements;

processing the output of the gyroscopic sensor to generate an azimuth angle;

converting the azimuth angle to a respective radiofrequency phase angle for each respective element of the antenna array to give a linear phase front across the array having an incremental phase shift per element related to the azimuth angle by a respective pre-determined factor;

applying a respective phase correction having the respective radiofrequency phase angle to each respective element output of a MU-MIMO digital beamforming weights matrix having pre-determined weights, whereby to apply the linear phase front across the array;

applying respective further correction factors to each output of the MU-MIMO digital beamforming weights matrix to which the respective phase correction has been applied to correct for respective signal transmission characteristics between each respective output of the MU-MIMO digital beamforming weights matrix and the respective antenna element; and

generating the respective further correction factors by measuring respective signal propagation characteristics between each respective output of the MU-MIMO digital beamforming weights matrix and the respective antenna element by a process comprising:

coupling a sample from each signal transmitted from each output of the beamforming weights matrix to each respective antenna element;

removing the respective phase correction from each coupled sample; and

updating the respective further correction factors on the basis of the coupled samples with the respective phase correction removed.

2. A method according to claim 1 , comprising:

generating respective first phase corrections for a first time interval and respective second phase corrections for a second time interval based on a single approximation factor relating a change in array orientation to a change in radiofrequency phase.

3. A method according to claim 2 , comprising forming the at least one MU-MIMO beam pattern by:

measuring respective signal phase and amplitude propagation characteristics between each antenna element of the array and each of a first and second subscriber module for the first time interval;

correcting each measured respective signal phase and amplitude propagation characteristic by the respective first phase correction; and

forming a MU-MIMO beam pattern for the second time interval having a main lobe towards the first subscriber module and a null towards the second subscriber module by:

generating weights for the MU-MIMO digital beamforming weights matrix based on the corrected measured respective signal phase and amplitude propagation characteristics.

4. A method according to claim 1 , wherein the array of antenna elements is an integral part of the access point, and the gyroscopic sensor is mounted within the access point.

5. A method according to claim 4 , wherein the access point comprises a calibrated integrated module comprising transmit chains and the array of antenna elements.

6. A method according to claim 4 , wherein the access point is mounted on an antenna tower and said change in orientation of the array is due to wind.

7. A method according to claim 6 , wherein said change in orientation of the antenna array is a change in azimuth angle.

8. A method according to claim 1 , wherein the gyroscopic sensor is a rate gyro, and said output of the gyroscopic sensor is an angular rotation rate.

9. A method according to claim 8 , wherein processing the output of the gyroscopic sensor comprises:

integrating the angular rotation rate and removing a mean value to determine the azimuth angle; and

determining the respective phase correction for each antenna element of the array from the azimuth angle and from stored data regarding the geometry of the array.

10. A method according to claim 2 , wherein the first time interval is a first time division duplex period and the second time interval is a subsequent time division duplex period.