Reciprocity calibration for multiple-input multiple-output systems

1. A method for reciprocity calibration of a multiple-input multiple-output (MIMO) system, the method comprising:

transmitting, by a base station, a pilot symbol by transmitters (TXes) of the base station, wherein the transmitted pilot symbol is received by a receiver (RX) of at least one reference antenna, as a first set of r i0 , pilot symbols;

receiving, by the base station, a second set of pilot symbols by receivers (RXes) of the base station based on the first set of pilot symbols received and then transmitted from a transmitter (TX) of at least one reference antenna, wherein each of the second set of pilot symbols is received as an r 0,i pilot symbol by an individual antenna of the base station, where index “i” denotes individual antenna “i” of the base station, and index “0” denotes the at least one reference antenna;

calculating non-reciprocity compensation factors based on the first set of pilot symbols and the second set of pilot symbols, and

applying the non-reciprocity compensation factors to payload data;

wherein the non-reciprocity compensation factors are applied as r M =H DL C T WS, wherein r M denotes received symbols by the antennas of the base station, H DL denotes an observed transfer function, C T denotes a matrix combination of the non-reciprocity compensation factors, and WS denotes a beamformed payload data.

2. The method of claim 1 wherein the at least one reference antenna is one of the antennas of the base station.

3. The method of claim 1 wherein the at least one reference antenna is an antenna at a user equipment (UE) side.

4. The method of claim 1 wherein the at least one reference antenna is a dedicated auxiliary (AUX) antenna.

5. The method of claim 1 , wherein the non-reciprocity compensation factors are applied to payload data of the base station.

6. The method of claim 1 wherein the pilot symbol transmitted by the TX of the at least one reference antenna is outside of a frequency bandwidth of the payload data.

7. The method of claim 1 wherein the pilot symbol transmitted by the TX of the at least one reference antenna has a power spectral density that is generally comparable to that of symbols of the payload data.

8. The method of claim 1 wherein the pilot symbol transmitted by the TX of the at least one reference antenna is a first pilot symbol, the method further comprising transmitting a second pilot symbol by the TX of the at least one reference antenna.

9. The method of claim 1 , further comprising transmitting the payload data from the TXes of the base station to RXes of a user equipment (UE) side, wherein the pilot symbol and the data payload are transmitted within same time domain and within different frequency bandwidths.

10. The method of claim 9 wherein the pilot symbol and the payload data are within same frequency bandwidth.

11. The method of claim 10 wherein transmitting the payload data pauses while transmitting the pilot symbol.

12. The method of claim 1 , further comprising transmitting the payload data from the TXes of the base station to RXes of a user equipment (UE) side, wherein transmitting the payload data pauses when the pilot symbol is transmitted.

13. A multiple-input multiple-output (MIMO) system for wireless signals, comprising:

a base station comprising a plurality of base station antennas, wherein the plurality of base stations antennas and antennas of a user equipment (UE) side are configured to exchange the wireless signals, and wherein transmitters (TXes) of the plurality of base station antennas are configured to transmit a symbol;

a reference receiver (RX) and at least one reference antenna, the reference RX and the at least one reference antenna configured to receive a first set of pilot symbols based on to pilot symbol transmitted from the TXes of the base station, wherein each pilot symbol of the first set of pilot symbols is received as the r 0,i pilot symbol of a transmitter (TX) of individual antenna “i” of the base station; and

wherein receivers (RXes) of the plurality of the base station antennas are configured to receive a second set of pilot symbols from a transmitter (TX) of the at least one reference antenna, wherein the TX of the at least one reference antenna is configured to transmit the first set of pilot symbols that were received by the at least one reference antenna, wherein each pilot symbol of the second set of symbols is received as the r i,0 pilot symbol of the RXes of the base station, wherein index “i” denotes individual antenna “i” of the base station, and index “0” denotes the at least one reference antenna;

wherein the (MIMO) system is configured to calculate non-reciprocity compensation factors based on the first set of pilot symbols and the second set of pilot symbols, and

wherein the non-reciprocity compensation factors are applied to payload data;

wherein a computing module is further configured to apply the non-reciprocity compensation factors are as r M =H DL C T WS, wherein r M denotes received symbols by the antennas of the base station, H DL denotes an observed transfer function, C T denotes a matrix combination of the non-reciprocity compensation factors, and WS denotes a beamformed payload data.

14. The system of claim 13 wherein the at least one reference antenna is one of the antennas of the base station.

15. The system of claim 13 wherein the at least one reference antenna is an antenna of the user equipment (UE) side.

16. The system of claim 13 wherein the at least one reference antenna is a dedicated auxiliary (AUX) antenna.

17. The system of claim 13 wherein the pilot symbol is one of a plurality of pilot symbols.

18. The system of claim 13 wherein the pilot symbol transmitted by the TX of the at least one reference antenna is outside of a frequency bandwidth of the payload data.