STAR: a carrier sense agnostic MAC scheme for a crowded NLoS-FSOC optical LAN

Abstract

Nondirect line-of-sight free-space optical communications (NLoS-FSOC) operate based on an optical wireless broadcast channel shared by multiple stations to communicate, thus forming an optical local area network (OLAN). Such a channel in the NLoS-FSOC is generated by a diffuse reflector (DR) that reflects light equally in all directions except toward itself. While the broadcast channel of an OLAN is accessible to many stations as in radio-frequency wireless LANs, the signal strength of the reflected beam in an OLAN depends not only on the receiver-DR distance, but also on the angle of incidence of the transmitted beam. An incident beam with a wide angle generates a weak reflected signal that may be undetectable by some stations in the communication range. The loss of the reflected power may render channel sensing ineffective. To address this challenge, we propose the use of the access point to indicate the time to attempt accessing the channel. We also propose a medium access control (MAC) scheme for OLANs using explicit start rather than the conventional channel sensing. The proposed MAC scheme, called STAR, not only enables stations to contend for the available channel, but also decreases collisions, and in turn, stations experience high throughput and low delay. Furthermore, we generalize STAR for the transmission of voice, video, and data traffic. We compare STAR to leading MAC schemes, adapted for their use on an OLAN. We show that STAR achieves 83% throughput for differential traffic and is a 15% higher throughput than the throughput of IEEE 802.11.