Uplink Symbol Detection in Dynamic TDD Mimo Systems with AP-AP Interference

We consider a MIMO communication system operating in dynamic time-division duplex, where one uplink (UL) access point (AP) detects information symbols transmitted from UL users (UEs) in the presence of AP-AP interference (AAI) caused by the signal a downlink (DL) AP transmits to a DL UE. We propose to mitigate the interference by jointly estimating the UL symbols and the AAI channel, but show that this problem is not uniquely solvable in the least-squares sense, and then present two methods for symbol detection that overcome this issue. Both methods let the UL UEs be silent in the first sample of each coherence interval. The first method performs joint symbol detection and AAI channel estimation for the remaining samples, while the second method does AAI channel estimation followed by interference subtraction. Despite the different nature of the two methods, we show that they are equivalent and that the variance of each symbol estimate is doubled compared to the genie case where the AAI is fully canceled. Numerical results verify our findings and show that our proposed methods offer a significant reduction in terms of the bit-error rate compared with the naive method where the AAI is not canceled.