Flexible UL/DL in Small Cell TDD Systems: A Performance Study with TCP Traffic

Time division duplex (TDD) systems offer a substantial amount of freedom to deal with downlink (DL) and uplink (UL) traffic asymmetries. Most TDD-based systems define either multiple static configurations or adaptive approaches to deal with such asymmetries. Our envisioned 5G concept embraces the flexibility brought along by TDD, and allows us to switch the link direction on a slot by slot basis. In this paper we study the interaction of Transmission Control Protocol (TCP) traffic, with a fully flexible UL/DL TDD allocation scheme. We show that flexibility is not only beneficial for exploiting the different instantaneous UL and DL traffic variations, but also performs well with TCP traffic, where the protocol behaviour plays an important role in throughput performance. The advantages of full flexibility compared to fixed static allocations for TCP traffic are reported for both small and large payloads, and for multi-cell scenarios where both DL and UL traffic are present.