Channel Asymmetry in Cellular OFDMA-TDD Networks

Abstract

This paper studies time division duplex- (TDD-) specific interference issues in orthogonal frequency division multiple access- (OFDMA-) TDD cellular networks arising from various uplink (UL)/downlink (DL) traffic asymmetries, considering both line-of-sight (LOS) and non-LOS (NLOS) conditions among base stations (BSs). The study explores aspects both of channel allocation and user scheduling. In particular, a comparison is drawn between the fixed slot allocation (FSA) technique and a dynamic channel allocation (DCA) technique for different UL/DL loads. For the latter, random time slot opposing (RTSO) is assumed due to its simplicity and its low signaling overhead. Both channel allocation techniques do not obviate the need for user scheduling algorithms, therefore, a greedy and a fair scheduling approach are applied to both the RTSO and the FSA. The systems are evaluated based on spectral efficiency, subcarrier utilization, and user outage. The results show that RTSO networks with DL-favored traffic asymmetries outperform FSA networks for all considered metrics and are robust to LOS between BSs. In addition, it is demonstrated that the greedy scheduling algorithm only offers a marginal increase in spectral efficiency as compared to the fair scheduling algorithm, while the latter exhibits up to ≈20% lower outage.