User-to-User Clustering, Channel Estimation & Cross-Link Interference Mitigation for Dynamic TDD Systems

In dynamic time-division duplexing (TDD) systems, half-duplex access points (APs) scheduled either in uplink (UL) or downlink (DL), simultaneously serve users operating in UL and DL on the same time-frequency resources. This incurs cross-link interference from APs operating in DL to APs in UL, and similarly from users operating in UL to users in DL. In this paper, we develop a scalable method for UL-user-to-DL-user interference (UUI) mitigation in dynamic TDD networks. To this end, we note that the UUI observed at each DL user will be predominantly caused by the UL users in its close vicinity. Hence, we propose to form local clusters of UL users for each DL user, including only the UL users that are expected to cause significant UUI to the DL user. Then, we present a graph-coloring-based pilot reuse algorithm that ensures orthogonal pilots among the UL users within the local clusters of each DL user, while maximizing the pilot reuse (i.e., minimizing the pilot length) for scalability. Further, we introduce beamformed DL pilots to estimate effective DL channels (the physical DL channel multiplied by the DL precoding matrix), which together with estimated user-to-user channels enable the multi-antenna DL users to design combining vectors maximizing their signal-to-interference-and-noise ratios, by taking both desired signal amplification and UUI suppression into consideration. We derive an achievable DL spectral efficiency with our proposed pilot schemes and combining vectors. Numerical results show the effectiveness of our proposed UUI mitigation scheme, both in terms of pilot overhead reduction and UUI suppression with our designed combining vectors.