Flexible UL-DL Switching Point in TDD Cellular Local Area Wireless Networks

An advantage of time division duplex (TDD) wireless networks over frequency division duplex (FDD) is that the UL-DL switching point may be flexibly adapted to asymmetric traffic loads. This enables more efficient spectrum use, but on the other hand may lead to harmful cross-link interference between cells. As a result, the net gain (or loss) from flexible TDD depends on the traffic characteristics and network scenario. In this paper we address the problem in local area packet data networks, such as small- or femto-cell network, where high fluctuation in short term traffic loads is expected. We show through analysis and system level simulations that in such a scenario a significant gain in effective user throughput and packet delays may be achieved under low traffic loads. At high load the gain becomes smaller as packets accumulate in queues and there is both UL and DL traffic to transmit with high probability. When the UL and DL transmit powers are not balanced and switching point adaptation is applied the link direction with lower power may provide degraded performance. We show that introducing interference awareness at the scheduler provides more balanced performance and lowers the packet delays further.