A simple over-the-air hardware calibration procedure in TDD systems

Accurate hardware calibration is necessary to achieve channel reciprocity and enable closed-loop MIMO operation in time-division-duplex mode. In this paper, a simple over-the-air procedure that efficiently estimates the base station (BS) hardware chains is presented. The procedure is to be performed between the BS and a dedicated user equipment (UE) experiencing a high signal-to-noise-ratio, and is based on a downlink (DL) and a following uplink (UL) signaling procedure. The UE first estimates the DL channel of each BS antenna and then sends, in the UL, normalized inverses of the estimated DL channels. In addition, the UE feeds back a set of normalization parameters to the BS. The obtained UL signals in addition to the normalization parameters allow for the estimation of the BS hardware chains and ensure channel reciprocity is achieved. Numerical results illustrate the performance of the proposed procedure when a zero-forcing precoder is used at the BS.