Channel Asymmetry and Random Time Slot Hopping in OFDMA-TDD Cellular Networks

This paper studies the performance of orthogonal frequency division multiple access - time division duplex (OFDMA-TDD) cellular networks when jointly applying dynamic channel allocation (DCA) and user scheduling under the assumption of asymmetric uplink (UL)/downlink (DL) loads. Specifically, a comparison between a fixed slot allocation (FSA) scheme, where the uplink UL/DL switching is synchronised across the network, and the random time slot opposing (RTSO) technique is made. The RTSO resembles an opportunistic interference mitigation technique. RTSO, however, does not obviate the need for user scheduling algorithms, but the combined use of RTSO and scheduling has not been studied. Therefore, two different scheduling algorithms, greedy and fair, are adapted to suit the OFDMA-TDD architecture. Their performance for various channel asymmetries under RTSO and symmetric FSA is evaluated, based on spectral efficiency and user outage. In order to account for the exposed location of base station (BS) antennas in a cellular environment, the effect of line-of-sight (LOS) propagation among BSs is considered. The results show that LOS among BSs in a system with unsynchronised switching points strongly hampers the network´s performance. This effect, however, is demonstrated to be substantially offset by DL-favoured asymmetries (dominant in data-centric networks) in combination with RTSO. Furthermore, it is shown that the greedy algorithm only offers a marginal increase in spectral efficiency as compared to the fair algorithm, while the fair algorithm exhibits up to ~20% lower outage.