Packet-Based Simulation for Optical Wireless Communication

This paper presents packet-based simulation tools for free-space-optical (FSO) wireless communication. We implement the well-known propagation models for free-space-optical communication as a set of modules in NS-2. Our focus is on accurately simulating line-of-sight (LOS) requirement for two communicating antennas, the drop in the received power with respect to separation between antennas, and error behavior. In our simulation modules, we consider numerous factors affecting the performance of optical wireless communication such as visibility in the medium, divergence angles of transmitters, field of view of photo-detectors, and surface areas of transceiver devices.

Wireless communication has traditionally been realized via omnidirectional radio frequency. Radio frequency has the major advantage of propagating in all directions enabling a receiver to roam inside the transmission sphere without experiencing a link disruption, although, it may encounter fading and hidden nodes as obstacles hurting the uniformity of the signal and new communicating nodes present in the propagation medium. Nevertheless, a typical RF-enabled node will have a large throughput gap with optical backbone of the network which reveals the last mile problem [1]–[3]. Pushing more aggressive medium access control (MAC) protocols that operate in much finer grained time scales and employing innovative multihop hierarchical cooperative MIMO [4] techniques remedy the issue partially in the cost of increased complexity. Marginal benefit of such approaches have become smaller due to the increased saturation of the RF spectrum. The throughput gap between optical backbone and the wireless last-mile calls for more radical approaches involving wireless spectrum bands physically much larger than the RF. Free-space-optical (FSO) (i.e., optical wireless) communication provides an attractive approach complementary to the legacy RF-based wireless communication. Most significant difference between FSO and RF is the requirement of line-of-sight in FSO, adding space-division multiplexing (i.e., spatial reuse) to already known multiplexing techniques such as wave-length and time division multiplexing. RF suffers from increased power consumption per interface compared to FSO because of the significantly larger volume of medium that needs to be covered by an individual interface. RF-based communication LED Photo Detector also has a greater need to employ complex security protocols to address security concerns that rise because of the higher risk of interception especially in military applications. A typical FSO transmitter (e.g., LASER, VCSEL or LED) forms a cone shaped volume in 3 dimensions (Figure 1) in which a potential receiver equipped with a photo detector can receive the signal. The exact shape of this cone is determined by the transmission power (for range) and divergence angle. A LASER has the smallest (in micro radian range) and an LED has the widest (a few hundred milli radians) divergence angle of the three types of transmitters. FSO can operate in large swathes of unlicensed spectrum reaching speeds up to ∼1 Gbps. Additionally, FSO transceivers have much smaller form factors, are less power-consuming (100 microwatts for 10-100 Mbps), very reliable (lifetime of more than 10 years), cheap and offer highly directional beams for spatial reuse and security.

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