Experimental evaluation of relative calibration in a MISO-TDD system

We study the transmit time reversal beamforming in a 8x1 MISO communication system at 2.68GHz. We consider the downlink time reversal transmission where a BS communicates with one user. A prototype composed by 8 antennas and designed by Orange labs acts as the BS while the user has a single antenna. The reciprocity property is destroyed by the non-symmetric characteristics of the RF electronic circuitry. We use relative calibration which is based exclusively on signal processing techniques to solve this issue. Utilizing a controlled test setup based on OpenAirInterface, the ExpressMIMO2 SDR boards, as well as a servo controlled rail, we show the feasibility of a relative calibration method through beamforming SNR measurements. We also evaluate the performance of an antenna selection scheme at the transmit side as a low-cost low-complexity alternative to capture many of the advantages of multi-antenna systems. The measurements show that the relative calibration method is performing almost optimal and that the complexity can be significantly reduced by using antenna selection.