Reciprocity calibration of TDD smart antenna systems

Calibration of the rf branches of the arrays at the two sides of the link is a necessary procedure for the channel estimation algorithms of smart antenna based wireless communication systems. Reciprocity calibration is a procedure adopted by Time Domain Duplex (TDD) systems such as WiMAX. This paper discusses the reciprocity calibration of an MxN TDD wireless system and uses a termination based formulation to identify a new design constraint. An integral part of the operation of TDD systems employing smart antennas is the process of estimating the downlink (DL) channel. This channel estimation has to be done efficiently so that valuable "over the air resources" are not wasted in the process. Typically, great attention is being paid to designing a wireless system so that the Signal to Noise Ratio (SNR) is maximized thus resulting in the best throughput from one side of the link to the other. Termination based formulations of the general MxN communication system problem have appeared in several recent works. In the present paper, we discuss the design constraints necessary for the implementation of a reciprocity based calibration and channel estimation. Ramifications of not meeting these design constraints will be studied and reported in the future.