Mehmet Bilgi

LIGHTNETs: Smart LIGHTing and Mobile Optical Wireless NETworks — A Survey

Recently, rapid increase of mobile devices pushed the radio frequency (RF)-based wireless technologies to their limits. Free-space-optical (FSO), a.k.a. optical wireless, communication has been considered as one of the viable solutions to respond to the ever-increasing wireless capacity demand. Particularly, Visible Light Communication (VLC) which uses light emitting diode (LED) based smart lighting technology provides an opportunity and infrastructure for the high-speed low-cost wireless communication. Though stemming from the same core technology, the smart lighting and FSO communication have inherent tradeoffs amongst each other. In this paper, we present a tutorial and survey of advances in these two technologies and explore the potential for integration of the two as a single field of study: LIGHTNETs. We focus our survey to the context of mobile communications given the recent pressing needs in mobile wireless networking. We deliberate on key challenges involved in designing technologies jointly performing the two functions simultaneously: LIGHTing and NETworking.

Multi-transceiver optical wireless spherical structures for MANETs

Due to its high bandwidth spectrum, Free-Space-Optical (FSO) communication has the potential to bridge the capacity gap between backbone fiber links and mobile ad-hoc links, especially in the last-mile. Though FSO can solve the wireless capacity problem, it brings new challenges such as frequent disruption of wireless communication links (intermittent connectivity) and the line-of-sight (LOS) requirements. In this paper, we study a multi-transceiver spherical FSO structure as a basic building block for enabling optical spectrum in mobile ad-hoc networking. We outline optimal designs of such multi-transceiver subsystems such that coverage is maximized and crosstalk among neighboring transceivers is minimized. We propose a low-level packaging architecture capable of handling hundreds of transceivers on a single structure. We also present MANET transport performance over such multi-element mobile FSO structures in comparison to legacy RF-based MANETs.

Multi-element Free-Space-Optical spherical structures with intermittent connectivity patterns

Due to its high bandwidth spectrum, free-space-optical (FSO) communication has the potential to bridge the capacity gap between backbone fiber links and mobile ad-hoc links, especially in the last-mile. Though FSO can solve the wireless capacity problem, it brings new challenges, like frequent disruption of physical link (intermittent connectivity) and the line of sight (LOS) requirements. In this paper, we study a spherical FSO structure as a basic building block and examine the effects of such FSO structures to upper layers, especially to TCP behavior for stationary and mobile nodes.

Prototyping Multi-Transceiver Free-Space Optical Communication Structures

Wireless networking has conventionally been realized via radio frequency (RF) based communication technologies. However, the capacity of these networks are limited by the availability of the RF spectrum. Free-Space-Optical (FSO) communication has the potential to deliver wireless communication links at optical-level speeds. Although it has the advantage of high-speed modulation, maintenance of line-of-sight (LOS) between transceivers during an on-going transmission is an important issue since FSO transmitters are highly directional. In this paper, we present a prototype implementation of such multi-transceiver electronically-steered communication structures. Our prototype uses a simple LOS detection and establishment protocol and assigns logical data streams to appropriate physical links.We show that by using multiple directional transceivers we can maintain optical wireless links with minimal disruptions that are caused by relative mobility of communicating nodes.

Automatic realignment with electronic steering of free-space-optical transceivers in MANETs: A proof-of-concept prototype

Free-Space-Optical (FSO) communication has the potential to achieve very high wireless communication rates at tens of GHz. Although it has the advantage of high-speed optical modulation, FSO communication is prone to mobility and it requires establishment and maintenance of line-of-sight (LOS) between FSO transceivers since FSO transceivers are highly directional. We consider FSO structures with multiple transceivers placed on a spherical shape with angular diversity and tackle the problem of automatically detecting and maintaining LOS alignment among neighbor multi-transceiver FSO structures. We present a prototype implementation of such multi-transceiver electronically-steered communication structures. Our prototype uses a simple LOS detection and establishment protocol and assigns logical data streams to appropriate physical links. We show that by using multiple directional transceivers and an auto-alignment mechanism, it is possible to maintain optical wireless links in a mobile setting with minimal disruptions and overhead.

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.

3-D Optical wireless localization

In this paper, we explore the possibility of using directionality of free-space-optical (a.k.a. optical wireless) communications for solving the 3-D localization problem in ad-hoc networking environments. Range-based localization methods either require a higher node density (i.e., at least three other localized neighbours must exist) than required for assuring connectedness or a high-accuracy power-intensive ranging device such as a sonar or laser range finder which exceeds the form factor and power capabilities of a typical ad-hoc node. Our approach exploits the readily available directionality information provided by a physical layer using optical wireless and uses a limited number of GPS-enabled nodes, requiring a very low node density (2-connectedness, independent of the dimension of space) and no ranging technique. We investigate the extent and accuracy of localization with respect to varying node designs (e.g., increased number of transceivers with better directionality) and density of GPS-enabled and ordinary nodes as well as messaging overhead per re-localization. We conclude that although denser deployments are desirable for higher accuracy, our method still works well with sparse networks with little message overhead and small number of anchor nodes (as little as 2).

Capacity scaling in free-space-optical mobile ad hoc networks

Wireless networking has conventionally been realized via radio-frequency-based communication technologies. Free-space-optical (FSO) communication with an innovative multi-element node design leverages spatially-diverse optical wireless links; making it a viable solution to the well-known diminishing per-node throughput problem in large-scale RF networks. Although it has the advantage of high-speed modulation, maintenance of line-of-sight between two FSO transceivers during a transmission is a challenge since FSO transmitters are highly directional. In this paper, we present our simulation efforts to make high-level assessments on throughput characteristics of FSO-MANETs while considering properties of FSO propagation and existence of multiple directional transceivers. We identify the intermittent connectivity problem that is caused by the relative mobility of nodes with multiple directional transceivers. We propose two cross-layer buffering schemes to remedy this problem and present their performance results. We conclude that sophisticated buffering mechanisms are required to properly buffer a packet during the misalignment period of two communicating nodes to avoid negative effects of this intermittency on the transport layer.

Optical wireless localization

In this paper, we explore the possibility of using directionality of free-space-optical (a.k.a. optical wireless) communications for solving the 3-D localization problem in ad-hoc networking environments. Range-based localization methods either require a higher node density (i.e., at least three other localized neighbors must exist) than required for assuring connectedness or a high-accuracy power-intensive ranging device such as a sonar or laser range finder which exceeds the form factor and power capabilities of a typical ad-hoc node. Our approach exploits the readily available directionality information provided by a physical layer using optical wireless and uses a limited number of GPS-enabled nodes, requiring a very low node density (2-connectedness, independent of the dimension of space) and no ranging technique. We investigate the extent and accuracy of localization with respect to varying node designs (e.g., increased number of transceivers with better directionality) and density of GPS-enabled and ordinary nodes as well as messaging overhead per re-localization. Although denser deployments are desirable for higher accuracy, our method still works well with sparse networks with little message overhead and small number of anchor nodes (as little as 2). We also present a proof-of-concept prototype of our FSO-based localization techniques and show the validity of our approach even with three transceivers per node.

Multi-transceiver simulation modules for free-space optical mobile ad hoc networks

This paper presents realistic simulation modules to assess characteristics of multi-transceiver free-space-optical (FSO) mobile ad-hoc networks. We start with a physical propagation model for FSO communications in the context of mobile ad-hoc networks (MANETs). We specifically focus on the drop in power of the light beam and probability of error in the decoded signal due to a number of parameters (such as separation between transmitter and receiver and visibility in the propagation medium), comparing our results with well-known theoretical models. Then, we provide details on simulating multi-transceiver mobile wireless nodes in Network Simulator 2 (NS-2), realistic obstacles in the medium and communication between directional optical transceivers. We introduce new structures in the networking protocol stack at lower layers to deliver such functionality. At the end, we provide our findings resulted from detailed modeling and simulation of FSO-MANETs regarding effects of such directionality on higher layers in the networking stack.