Channel state information at the transmitter (CSIT) can greatly improve
the capacity of a wireless MIMO communication system. In a time division duplex
(TDD) system CSIT can be obtained by exploiting the reciprocity of the wireless
channel. This however requires calibration of the radio frequency (RF) chains of the
receiver and the transmitter, which are in general not reciprocal. In this paper we investigate
different methods for relative calibration in the presence of frequency offsets
between transmitter and receiver. We show results of theses calibration methods with
real two-directional channel measurements, which were performed using the Eurecom
MIMO Openair Sounder (EMOS). We demonstrate that in a single-user MIMO
channel and for low signal-to-noise (SNR) ratios, the relative calibration method can
increase the capacity close to the theoretical limit
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