Overcoming alignment delay in RF+FSO networks

The use of highly directional antennae in wireless networks has been shown to increase network capacity. As the beamwidth decreases, near perfect alignment is required to achieve these capacity gains. This becomes particularly challenging in mobile adhoc networks, leading to high alignment delay. In this paper, we explore ways of mitigating alignment delay in practical delay tolerant networks (DTNs) where each node has both a radio frequency (RF) control and a free space optical (FSO) data channel. We show that overcoming alignment delay while multicasting data during contact opportunities is an abstraction of the minimum weighted set cover problem. An optimal multicast scheme is implemented in a DTN simulator over a new session-based MAC protocol. Thorough performance evaluation demonstrates that we can compensate for fixed alignment delay, paving the way for high capacity DTNs.