Latency Aware Transmission Scheduling for Steerable Free Space Optics

Free space optics (FSO), which uses light as the carrier to transmit data in free space, has been demonstrated as a secure and high-speed solution for long distance and line-of-sight wireless communications. Applying FSO as fronthaul/backhaul communications between base stations (BSs) and the gateway can significantly increase the fronthaul/backhaul link capacity. Traditionally, the gateway has to be equipped with multiple FSO transceivers, each of which is used to communicate with a BS by establishing a dedicated FSO. In this paper, we propose to use a steerable FSO system, where the gateway is equipped with a steerable FSO transceiver to communicate with multiple FSO transceivers at different BSs in a time division multiplexing manner. Applying the steerable FSO system can reduce the number of FSO transceivers at the gateway, and thus reduce the capital cost of implementing an FSO based fronthaul/backhaul network. We formulate the transmission scheduling problem in the steerable FSO system to optimize the active time for each FSO link associated with the steerable FSO transceiver such that the overall delay of transmitting a packet from geo-distributed BSs to the steerable FSO transceiver at the gateway is minimized, while guaranteeing the latency requirements of the BSs. We propose the laTency aWare transmIssion Scheduling for sTeerable FSO (TWIST) algorithm, which is designed based on Sequential Quadratic Programming, to efficiently solve the proposed problem. The performance of TWIST is validated via extensive simulations.