Mutual coupling effect is an important electromagnetic interaction between the antennas, which cannot be neglected for massive multiple-input multiple-output (MIMO) antenna systems. In time division duplex (TDD) mode, it is necessary to perform reciprocity calibration, because the mismatches of transceiver radio frequency (RF) circuits at both sides of the link make the whole communication channel non-symmetric. Besides, there still exist disputes whether the mutual coupling disables the channel reciprocity or not, the result of which decides the choice of calibration methods. This paper focuses on the reciprocity of the mutual coupling and derives the coupling matrices for both transmitting and receiving. It is shown that the coupling matrices do not satisfy the principle of reciprocity, when the self-impedances and load-impedances of all antennas are not exactly equal respectively. Furthermore, theoretical analysis and simulation results are presented to evaluate the impact of non-reciprocal mutual coupling on the performance for massive MIMO systems with zero-forcing (ZF) precoding.
['Hao Wei', 'Dongming Wang', 'Xiaohu You']
Reciprocity of mutual coupling for TDD massive MIMO systems