Emulation and controlled testing of MIMO OTA channels
The present invention relates to techniques for OTA testing suitable for producing a test signal to emulate a wireless channel while using a limited number of transmit elements. The techniques described herein enable the number of transmit antennas used to emulate a given signal path in an emulated wireless channel to be less than the number of sub-paths used to characterize the angle spread of the given signal path. As a result, a test setup is provided having a relatively small number of transmit antennas which also accurately maintaining the desirable characteristics of the emulated wireless channel.
1. A system for emulating a wireless channel between a transmitter and a receiver, the wireless channel comprising one or more signal paths having respective amplitudes, angles of arrival and angle spreads, the system comprising:
a set of two or more receive antennas, the set having an effective, non-singular spatial correlation for a given signal path in the wireless channel;
a plurality of transmit antennas for transmitting transmit signals over the air to the set of receive antennas to emulate the given signal path in the wireless channel, the angle spread of the given signal path in the wireless channel represented as a plurality of N sub-paths, and the plurality of transmit antennas being less than N, and wherein the transmit antennas in the plurality are arranged at angular locations relative to the set of receive antennas in dependence upon said effective spatial correlation; and
circuitry coupled to the plurality of transmit antennas to generate the transmit signals and provide the transmit signals to corresponding transmit antennas, wherein the transmit signals provided to the transmit antennas have a signal power in dependence upon said effective spatial correlation.
2. The system of claim 1 , wherein the transmit antennas in the plurality are arranged in azimuth angles in dependence upon said effective spatial correlation.
3. The system of claim 2 , wherein the transmit antennas in the plurality are further arranged in elevation angles in dependence upon said effective spatial correlation.
4. The system of claim 1 , wherein the transmit signals provided to individual transmit antennas have a Doppler spectrum based on a virtual spatial movement parameter that represents spatial movement of the set of receive antennas relative to the plurality of transmit antennas, and wherein the Doppler spectrum of individual transmit signals are dependent upon the emulated path and the relative angular location of the corresponding transmit antennas.
5. The system of claim 1 , wherein the transmit signals provided to individual transmit antennas have a Doppler spectrum based on a virtual spatial movement parameter that represents spatial movement of the set of receive antennas relative to the plurality of transmit antennas, and wherein the Doppler spectrum of a given transmit signal provided to a given antenna in the plurality of transmit antennas is dependent upon the emulated path.
6. The system of claim 4 , wherein said circuitry:
generates a plurality of signal components of a transmit signal used to emulate the given signal path in the wireless channel; and
provides the signal components to a corresponding set of transmit antennas in the plurality of transmit antennas, wherein individual signal components have a Doppler spectrum dependent upon the emulated path and relative angular locations of each transmit antenna in the set of transmit antennas.
7. The system of claim 1 , wherein the effective spatial correlation of the set of receive antennas is based on measured data of the antennas in the set.
8. The system of claim 1 , including:
the angle spread of a second given signal path in the wireless channel represented as a plurality of M sub-paths;
the set of two or more receive antennas having a second effective, non-singular spatial correlation for the second given signal path in the wireless channel;
a second plurality of transmit antennas for transmitting second transmit signals to the set of receive antennas to emulate the second given signal path in the wireless channel, the second plurality of transmit antennas being less than M, and wherein the transmit antennas in the second plurality are arranged at angular locations relative to the set of receive antennas in dependence upon said second effective spatial correlation; and
said circuitry further coupled to the second plurality of transmit antennas to generate the second transmit signals and provide the second transmit signals to corresponding transmit antennas in the plurality of second transmit antennas.
9. The system of claim 8 , wherein at least one transmit antenna is common to both the first and second plurality of transmit antennas.
10. The system of claim 8 , wherein respective signal components for each of the first and second transmit signals are independently Rayleigh faded.
11. The system of claim 1 , wherein the plurality of transmit antennas includes antennas arranged to transmit a first polarization, and antennas arranged to transmit a second polarization different from the first polarization.
12. The system of claim 11 , wherein the second polarization is orthogonal to the first polarization.
13. The system of claim 11 , wherein the plurality of transmit antennas includes a first antenna arranged to transmit the first polarization, and a second antenna arranged to transmit the second polarization, the first and second antennas co-located at the same angular location.
14. The system of claim 13 , wherein a given transmit signal comprises a first signal component provided to the first antenna, and a second signal component provided to the second antenna.
15. The system of claim 1 , wherein the correlation of signals measured at the receive antennas include signals having different polarizations.
16. The system of claim 1 , wherein the transmit signals provided to the individual transmit antennas have a signal power dependent upon a polarization loss parameter that represents a difference in polarization between said individual transmit antennas and the set of receive antennas.
17. The system of claim 1 , further for emulating a second wireless channel between a second transmitter and a second receiver, the second wireless channel comprising one or more signal paths having respective amplitudes, angles of arrival and angle spreads, the angle spread of a given signal path in the second wireless channel represented as a plurality of M sub-paths, and wherein:
the set of two or more receive antennas have a second effective, non-singular spatial correlation for the given signal path in the second wireless channel;
the transmit antennas in the plurality are at said angular location relative to the set of two or more receive antennas further in dependence upon said second effective spatial correlation; and
said circuitry further generates second transmit signals and provides the second transmit signals to corresponding transmit antennas to the given signal path in the second wireless channel, wherein the second transmit signals provided to the transmit antennas have a signal power dependent upon said second effective spatial correlation.
18. The system of claim 1 , wherein:
the set of receive antennas have a measured spatial correlation for the relative angular location of the transmit antennas and the signal powers provided to the transmit antennas; and
a normalized least mean square error between the measured spatial correlation and said effective spatial correlation over a range of 0 to pi radians in azimuth is less than 0.05.
19. A method for emulating a wireless channel between a transmitter and a receiver, the wireless channel comprising one or more signal paths having respective amplitudes, angles of arrival and angle spreads, the method comprising:
determining an effective, non-singular spatial correlation of a given signal path for a set of two or more receive antennas;
computing angular locations for a plurality of transmit antennas relative to the set of two or more receive antennas in dependence upon said effective spatial correlation, the transmit antennas for transmitting transmit signals over the air to the set of receive antennas to emulate the given path in the wireless channel, the angle spread of the given signal path in the wireless channel represented as a plurality of N sub-paths, and the plurality of transmit antennas being less than N; and
generating the transmit signals and providing the transmit signals to corresponding transmit antennas, wherein the transmit signals have a signal power in dependence upon said effective spatial correlation.
20. The method of claim 19 , wherein computing angular locations for the plurality of transmit antennas comprises computing azimuth angles in dependence upon said effective spatial correlation.
21. The method of claim 20 , wherein computing angular locations for the plurality of transmit antennas further comprises computing elevation angles in dependence upon said effective spatial correlation.
22. The method of claim 19 , wherein the transmit signals provided to individual transmit antennas have a Doppler spectrum based on a virtual spatial movement parameter that represents spatial movement of the set of receive antennas relative to the plurality of transmit antennas, and wherein the Doppler spectrum of individual transmit signals are dependent upon the emulated path and the relative angular location of the corresponding transmit antennas.
23. The method of claim 19 , wherein the transmit signals provided to individual transmit antennas have a Doppler spectrum based on a virtual spatial movement parameter that represents spatial movement of the set of receive antennas relative to the plurality of transmit antennas, and wherein the Doppler spectrum of a given transmit signal provided to a given antenna in the plurality of transmit antennas is dependent upon the emulated path.
24. The method of claim 19 , wherein:
said generating comprises generating a plurality of signal components of a transmit signal used to emulate the given signal path in the wireless channel; and
said providing comprises providing the signal components to a corresponding set of transmit antennas in the plurality of transmit antennas, wherein individual signal components have a Doppler spectrum dependent upon the emulated path and the relative angular locations of each transmit antenna in the set of transmit antennas.
25. The method of claim 19 , wherein the effective spatial correlation of the set of receive antennas is based on measured data of the antennas in the set.
26. The method of claim 19 , including the angle spread of a second given signal path in the wireless channel represented as a plurality of M sub-paths, the method further comprising:
determining a second effective, non-singular spatial correlation of the second given signal path for the set of two or more receive antennas;
computing angular locations for a second plurality of transmit antennas relative to the set of two or more receive antennas in dependence upon said second effective spatial correlation, the second plurality of transmit antennas for transmitting second transmit signals to the set of receive antennas to emulate the second given path in the wireless channel, the second plurality of transmit antennas being less than M; and
generating the second transmit signals and providing the second transmit signals to corresponding transmit antennas in the second plurality of transmit antennas, wherein the second transmit signals have a signal power in dependence upon said effective spatial correlation.
27. The method of claim 26 , wherein at least one transmit antenna is common to both the first and second transmit antennas.
28. The method of claim 26 , wherein respective signal components for each of the first and second transmit signals are independently Rayleigh faded.
29. The method of claim 19 , wherein the plurality of transmit antennas includes transmit antennas arranged to transmit a first polarization, and antennas arranged to transmit a second polarization different from the first polarization.
30. The method of claim 29 , wherein the second polarization is orthogonal to the first polarization.