Three-Mode Opportunistic Full-Duplex Switching

This paper proposes a novel opportunistic full-duplex (FD) scheme switching among three duplex modes (DMs) of half duplex (HD), three node FD (TNFD) and bidirectional FD (BiFD) for the cellular systems. To this end, we first derive the conditions, by which one of the three DMs achieves larger average spectral efficiency (ASE) than the other two DMs. Specifically, we derive the minimum required self inteference (SI) cancellation factors of user equipments (UEs)/base station for BiFD/TNFD to achieve the highest ASE among the three duplex modes respectively. We also show that they are given as an explicit function of the static system parameters: cell radius, path loss exponent and target SNR. Notably, for the typical small cell radii, BiFD with the state-of-art SI cancellation techniques significantly outperforms TNFD (which undergoes high inter-UE interference in small cell systems). Compared to the single mode or two mode duplex schemes, substantial ASE gains are achieved by three mode duplex switching according to the aforementioned static system parameters. Moreover, the proposed slow (i.e., a static system parameter-dependent) switching scheme achieves almost identical ASE to that of the fast switching scheme that requires high complexity to calculate and compare the instantaneous spectral efficiencies (SEs) of the three DMs.