Murat Yuksel

Error analysis of multi-hop free-space optical communication

In this paper we analyze the error performance of free-space optical (FSO) communication over multiple hops. We first develop an error model for a single hop based on visibility, atmospheric attenuation, and geometric spread of the light beam. We model atmospheric visibility by Gaussian distributions with mean and variance values to reflect clear and adverse weather conditions. Based on this, we find the end-to-end bit error distribution of the FSO link for single hop and multi-hop scenarios. We present simulation results for decoded relaying, where each hop decodes the signal before retransmitting. We demonstrate that multi-hop FSO communication achieves a significant reduction in the mean bit error rate and also reduces the variance of the bit error rate. We argue that by lowering mean error and error variance, multi-hop operation facilitates an efficient system design and improves the reliability of the FSO link by application of specific coding schemes (such as forward error correction techniques).

Free-space-optical mobile ad hoc networks: Auto-configurable building blocks

Existence of line of sight (LOS) and alignment between the communicating antennas is one of the key requirements for free-space-optical (FSO) communication. To ensure uninterrupted data flow, auto-aligning transmitter and receiver modules are necessary. We propose a new FSO node design that uses spherical surfaces covered with transmitter and receiver modules for maintaining optical links even when nodes are in relative motion. The spherical FSO node provides angular diversity in 3-dimensions, and hence provides an LOS at any orientation as long as there are no obstacles in between the communicating nodes. For proof-of-concept, we designed and tested an auto-configurable circuit, integrated with light sources and detectors placed on spherical surfaces. We demonstrated communication between a stationary and a mobile node using these initial prototypes of such FSO structures. We also performed the necessary theoretical analysis to demonstrate scalability of our FSO node designs to longer distances as well as feasibility of denser packaging of transceivers on such nodes.

Building blocks for mobile free-space-optical networks

Existence of line of sight (LOS) and alignment between the communicating antennas are one of the key requirements for free-space-optical (FSO) communication. To ensure uninterrupted data flow, auto-aligning transmitter and receiver modules are necessary. We propose a new optical antenna design that employs spherical antennas covered with transmitter and receiver modules for maintaining optical links even when antennas are in relative motion. In this paper, for proof-of-concept, we design and test an auto-configurable circuit integrated with light sources and detectors placed on spherical surfaces. We also perform simulation-based analysis of these multi-element FSO modules that can enable mobility and high bandwidth in wireless, particularly indoor, networks. Broader impact of our work is to make FSO communication technology widely applicable in mobile, ad-hoc, and multi-hop wireless networks.

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.

Large-scale network simulation techniques: examples of TCP and OSPF models

Simulation of large-scale networks remains to be a challenge, although various network simulators are in place. In this paper, we identify fundamental issues for large-scale networks simulation, and porpose new techniques that address them. First, we exploit optimistic parallel simulation techniques to enable fast execution on inexpensive hyper-threaded, multiprocessor systems. Second, we provide a compact, light-weight implementation framework that greatly reduces the amount of state required to simulate large-scale network models. Based on the proposed techniques, we provide sample simulation models for two networking protocols: TCP and OSPF. We implement these models in a simulation environment ROSSNet, which is an extension to the previously developed optimistic simulator ROSS. We perform validation experoments for TCP and OSPF and present performance reuslts of our techniques by simulating OSPF and TCP on a large and realistic topology, such as AT&Ts US network based on rocketfuel data. The end result of these innovations is that we are able to simulate million node network tolopgies using inexpensive commercial off-the-shelf hyper-threaded multiprocessor systems consuming less than 1.4 GB of RAM in total.

Orthogonal rendezvous routing protocol for wireless mesh networks

Routing in multi-hop wireless networks involves the indirection from a persistent name (or ID) to a locator. Concepts such as coordinate space embedding help reduce the number and dynamism complexity of bindings and state needed for this indirection. Routing protocols which do not use such concepts often tend to flood packets during route discovery or dissemination, and hence have limited scalability. In this paper, we introduce Orthogonal Rendezvous Routing Protocol (ORRP) for meshed wireless networks. ORRP is a lightweight-but-scalable routing protocol utilizing directional communications (such as directional antennas or free-space-optical transceivers) to relax information requirements such as coordinate space embedding and node localization. The ORRP source and ORRP destination send route discovery and route dissemination packets respectively in locally-chosen orthogonal directions. Connectivity happens when these paths intersect (i.e., rendezvous). We show that ORRP achieves connectivity with high probability even in sparse networks with voids. ORRP scales well without imposing DHT-like graph structures (eg: trees, rings, torus etc). The total state information required is O(N3/2 for N-node networks, and the state is uniformly distributed. ORRP does not resort to flooding either in route discovery or dissemination. The price paid by ORRP is suboptimality in terms of path stretch compared to the shortest path; however we characterize the average penalty and find that it is not severe.

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.

Limited Scale-Free Overlay Topologies for Unstructured Peer-to-Peer Networks

In unstructured peer-to-peer (P2P) networks, the overlay topology (or connectivity graph) among peers is a crucial component in addition to the peer/data organization and search. Topological characteristics have profound impact on the efficiency of a search on such unstructured P2P networks, as well as other networks. A key limitation of scale-free (power-law) topologies is the high load (i.e., high degree) on a very few number of hub nodes. In a typical unstructured P2P network, peers are not willing to maintain high degrees/loads as they may not want to store a large number of entries for construction of the overlay topology. Therefore, to achieve fairness and practicality among all peers, hard cutoffs on the number of entries are imposed by the individual peers, which limits scale-freeness of the overall topology, hence limited scale-free networks. Thus, it is expected that the efficiency of the flooding search reduces as the size of the hard cutoff does. We investigate the construction of scale-free topologies with hard cutoffs (i.e., there are not any major hubs) and the effect of these hard cutoffs on the search efficiency. Interestingly, we observe that the efficiency of normalized flooding and random walk search algorithms increases as the hard cutoff decreases.

An implementation framework for trajectory-based routing in ad hoc networks

Routing in ad-hoc networks is a complicated task because of many reasons. The nodes are low-memory, low-powered, and they cannot maintain routing tables large enough for well-known routing protocols. Because of that, greedy forwarding at intermediate nodes is desirable in ad-hoc networks. Also, for traffic engineering, multi-path capabilities are important. So, it is desirable to define routes at the source like in source based routing (SBR) while performing greedy forwarding at intermediate nodes. In this paper, we investigate trajectory-based routing (TBR) which was proposed as a middle-ground between SBR and greedy forwarding techniques. We address various issues regarding implementation of TBR. We also provide techniques to efficiently forward packets along a trajectory defined as a parametric curve.

Automatic selection of parameters for vessel/neurite segmentation algorithms

An automated method is presented for selecting optimal parameter settings for vessel/neurite segmentation algorithms using the minimum description length principle and a recursive random search algorithm. It trades off a probabilistic measure of image-content coverage against its conciseness. It enables nonexpert users to select parameter settings objectively, without knowledge of underlying algorithms, broadening the applicability of the segmentation algorithm, and delivering higher morphometric accuracy. It enables adaptation of parameters across batches of images. It simplifies the user interface to just one optional parameter and reduces the cost of technical support. Finally, the method is modular, extensible, and amenable to parallel computation. The method is applied to 223 images of human retinas and cultured neurons, from four different sources, using a single segmentation algorithm with eight parameters. Improvements in segmentation quality compared to default settings using 1000 iterations ranged from 4.7%-21%. Paired t-tests showed that improvements are statistically significant (p<0.0005). Most of the improvement occurred in the first 44 iterations. Improvements in description lengths and agreement with the ground truth were strongly correlated (/spl rho/=0.78).