UL SRS Coordination in 5G Massive MIMO Systems

In massive multiple input multiple output (MIMO) systems operating in time division duplex (TDD), users connected to different base stations (BSs) transmit uplink (UL) pilot sequences to allow channel estimation for both UL and downlink (DL) directions. The ongoing standardization activity around the fifth generation (5G) of communication systems is allocating these pilots within the UL sounding reference signals (UL SRS). When the number of users is high, the use of orthogonal UL SRS between the active users becomes infeasible as the portion of the frame dedicated to the channel training phase proportionally increases with their increased orthogonality. Thus, a certain level of interference must be tolerated in practical systems and, as a consequence, a better understanding of all the factors playing a role in the achievements of higher spectral efficiency is a fundamental aspect to fully exploit the advantages of massive MIMO. In this paper we provide a comprehensive overview of the pilot contamination effects with particular focus on UL SRS allocation methods and reuse schemes, associated overhead and user multiplexing gain. Moreover, we introduce and describe the advantages of a new fractional UL SRS reuse strategy, referred as neighbor-aware, together with its flexibility in handling pilot sequence multiplexing.