  In time-division duplexing (TDD) systems, massive multiple-input
multiple-output (MIMO) relies on the channel reciprocity to obtain the downlink
(DL) channel state information (CSI) with the acquired uplink (UL) CSI at the
base station (BS). However, the mismatches in the radio frequency (RF) analog
circuits at different antennas at the BS break the end-to-end UL and DL channel
reciprocity. To restore the channel reciprocity, it is necessary to calibrate
all the antennas at the BS. This paper addresses the interconnection strategy
for the internal self-calibration at the BS where different antennas are
interconnected via hardware transmission lines. Specifically, the paper reveals
the optimality of the star interconnection and the daisy chain interconnection
respectively. From the results, we see the star interconnection is the optimal
interconnection strategy when the BS are given the same number of measurements.
On the other hand, the daisy chain interconnection outperforms the star
interconnection when the same amount of time resources are consumed. Numerical
results corroborate our theoretical analyses.
