Method and apparatus for antenna calibration in a wireless communication system

1. A method in a licensed wireless communication network in a digital unit connected with a plurality of distributed antennas including a first antenna, a second antenna and a third antenna, the method comprising:

causing transmitting a first signal from the first antenna, a second signal from the second antenna, and a third signal from the third antenna in a same frequency resource in a time period during which no data or control signal is transmitted in the licensed wireless communication network;

obtaining a receiver and transmitter side loop-back phase difference between the first antenna and the second antenna based on the first signal received at the third antenna, the second signal received at the third antenna, the third signal received at the first antenna, and the third signal received at the second antenna; and

obtaining estimations of a time delay difference and an initial phase difference between the first and second antennas based on the obtained receiver and transmitter side loop-back phase difference.

2. The method of claim 1 , wherein the time period includes at least one of:

a Guard Period in a time divisional duplexing, TDD, frame structure, and

a subframe during which no data and control signaling is transmitted.

3. The method of claim 1 , wherein the first, second and third signals transmitted from the first, second, and third antennas are generated based on a same constant amplitude sequence.

4. The method of claim 1 , wherein the causing transmitting comprises:

causing transmitting the first signal, the second signal and the third signal in a time divisional multiplexing manner.

5. A method in a digital unit connected with a plurality of distributed antennas including a first antenna, a second antenna and a third antenna the method comprising:

causing transmitting a first signal from the first antenna, a second signal from the second antenna, and a third signal from the third antenna in a same frequency resource;

obtaining a receiver and transmitter side loop-back phase difference between the first antenna and the second antenna based on the first signal received at the third antenna, the second signal received at the third antenna, the third signal received at the first antenna, and the third signal received at the second antenna, wherein obtaining the receiver and transmitter side loop-back phase difference between the first and second antennas comprises:

obtaining a first loop-back phase for the first antenna by comparing a phase for the third signal received at the first antenna and a phase for the first signal received at the third antenna;

obtaining a second loop-back phase for the second antenna by comparing a phase for the third signal received at the second antenna and a phase for the second signal received at the third antenna; and

obtaining the receiver and transmitter side loop-back phase difference by determining a difference between the first and second loop-back phases; and

obtaining estimations of a time delay difference and an initial phase difference between the first and second antennas based on the obtained receiver and transmitter side loop-back phase difference.

6. The method of claim 1 , wherein obtaining the estimations of the time delay difference and the initial phase difference between the first and second antennas comprise:

estimating the time delay difference and the initial phase difference according to polynomial fit based on a Least Square criterion or a Recursive Least Square criterion.

7. The method of claim 1 , wherein the obtaining the receiver and transmitter side loop-back phase difference between the first and second antennas comprises:

obtaining the receiver and transmitter side loop-back phase difference between the first and second antennas for a subcarrier in the frequency resource.

8. The method of claim 1 , further comprising:

obtaining a loop-back amplitude difference between the first and second antennas based on the first signal received at the third antenna, the second signal received at the third antenna, the third signal received at the first antenna and the third signal received at the second antenna.

9. The method of claim 7 , wherein obtaining the loop-back amplitude difference between the first antenna and the second antenna comprises:

obtaining the loop-back amplitude difference between the first and second antennas for a subcarrier in the frequency resource.

10. An apparatus in a licensed wireless communication network, the apparatus connected with a plurality of distributed antennas including a first antenna, a second antenna and a third antenna, the apparatus comprising a processor and a memory, said memory containing instructions executable by said processor whereby said apparatus is operative to:

cause transmitting a first signal from the first antenna, a second signal from the second antenna, and a third signal from the third antenna in a same frequency resource in a time period during which no data or control signal is transmitted in the licensed wireless communication network;

obtain a receiver and transmitter side loop-back phase difference between the first antenna and the second antenna based on the first signal received at the third antenna, the second signal received at the third antenna, the third signal received at the first antenna, and the third signal received at the second antenna; and

obtain estimations of a time delay difference and an initial phase difference between the first and second antennas based on the obtained receiver and transmitter side loop-back phase difference.

11. The apparatus of claim 10 , wherein the time period includes at least one of:

a Guard Period in a time divisional duplexing, TDD, frame structure, and

a subframe during which no data and control signaling is transmitted.

12. The apparatus of claim 10 , wherein the first, second and third signals transmitted from the first, second, and third antennas are generated based on a same constant amplitude sequence.

13. The apparatus of claim 10 , wherein said memory contains instructions executable by said processor whereby said apparatus is further operative to cause transmitting the first, second and third signals in a time divisional multiplexing manner.

14. An apparatus connected with a plurality of distributed antennas including a first antenna, a second antenna and a third antenna, the apparatus comprising a processor and a memory, said memory containing instructions executable by said processor whereby said apparatus is operative to:

cause transmitting a first signal from the first antenna, a second signal from the second antenna, and a third signal from the third antenna in a same frequency resource;

obtain a receiver and transmitter side loop-back phase difference between the first antenna and the second antenna based on the first signal received at the third antenna, the second signal received at the third antenna, the third signal received at the first antenna, and the third signal received at the second antenna by:

obtaining a first loop-back phase for the first antenna by comparing a phase for the third signal received at the first antenna and a phase for the first signal received at the third antenna;

obtaining a second loop-back phase for the second antenna by comparing a phase for the third signal received at the second antenna and a phase for the second signal received at the third antenna; and

obtaining the receiver and transmitter side loop-back phase difference by determining a difference between the first and second loop-back phases; and

obtain estimations of a time delay difference and an initial phase difference between the first and second antennas based on the obtained receiver and transmitter side loop-back phase difference.

15. The apparatus of claim 10 , wherein said memory contains instructions executable by said processor whereby said apparatus is further operative to obtain the estimations of a time delay difference and an initial phase difference between the first and second antennas comprise:

estimating the time delay difference and the initial phase difference according to polynomial fit based on a Least Square criterion or a Recursive Least Square criterion.

16. The apparatus of claim 10 , wherein said memory contains instructions executable by said processor whereby said apparatus is further operative to obtain the receiver and transmitter side loop-back phase difference between the first and second antennas by:

obtaining the receiver and transmitter side loop-back phase difference between the first and second antennas for a subcarrier in the frequency resource.

17. The apparatus of claim 10 , herein said memory contains instructions executable by said processor whereby said apparatus is further operative to:

obtain a loop-back amplitude difference between the first and second antennas based on the first signal received at the third antenna, the second signal received at the third antenna, the third signal received at the first antenna and the third signal received at the second antenna.

18. The apparatus of claim 17 , wherein said memory contains instructions executable by said processor whereby said apparatus is further operative to obtain the loop-back amplitude difference between the first antenna and the second antenna by:

obtaining the loop-back amplitude difference between the first and second antennas for a subcarrier in the frequency resource.