Synchronization aspects while using reciprocity for precoding in MIMO-OFDM

Synchronization is a major topic in OFDM. Small remaining timing offsets, which are less than the cyclic prefix, are commonly compensated by the equalizer in the receiver. Therefore, separate detection and compensation of the timing offset, especially the fractional timing offset is not mandatory. Considering a reciprocal communication system, i.e., the uplink channel is assumed to be equal to the transposed downlink channel, for example, downlink precoding can be done by using the estimated uplink channel matrix. In this case a small remaining timing offset, effecting the transfer matrix, corrupts the assumption of reciprocity, and this can cause degradations in the BER. In this paper we consider this effect and show how it can be compensated within the transfer matrix. Furthermore, we analyze in which way the effect differs for different systems mainly caused by the D/A-converter and interpolation filter.