Bi-Directional Beamforming and its Capacity Scaling in Pairwise Two-Way Communications

Recent developments in various devices and applications require pairwise communications in an ad-hoc manner. Such systems are mostly interference-limited and suffer from capacity degradation caused by two-way communication. This paper investigates improvement of capacity performance in pairwise two-way communication through bi-directional use of spatial resources when beamforming is employed as the transmission scheme and instantaneous channel information is not available. This system is called a 'bi-directional beamforming' system. The primary approach to showing improvement in capacity performance in the bi-directional beamforming system involves focusing on the scaling of its achievable sum rate. Lower and upper bounds on the achievable sum rate are derived to evaluate the scaling of the achievable sum rate. Then, a condition is derived under which the bi-directional beamforming system outperforms the conventional beamforming system in the pairwise two-way communication environment. As a result, when the bi-directional beamforming system is employed, results show that the achievable sum rate scales twice as fast as that when conventional beamforming is employed.