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## Optical Wireless An Overview

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**Optical Wireless An Overview**Chintan Shah cmshah@cse.buffalo.edu December 9, 2004 University at Buffalo**Outline**• Introduction • What is Optical Wireless? • Applications • Transmitter and Receiver • Topologies • Challenges and Limitations • Topology Control and Routing • Conclusion University at Buffalo**What is Optical Wireless?**• Optical Wireless a.k.a. Free Space Optics (FSO) refers to the transmission of modulated light beams through the atmosphere to obtain broadband communication • Line-of-sight technology • Uses lasers/LEDs to generate coherent light beams University at Buffalo**What is Optical Wireless?**• Data rates of up to 2.5 Gbps at distances of up to 4km available in commercial products University at Buffalo**Last Mile problem**• Connecting the user directly to the backbone high speed fiber optic network is known as the Last Mile problem • FSO as the low cost bridging technology University at Buffalo**More Applications**• Allows quick Metro network extensions • Interconnecting local-area network segments spread across separate buildings (Enterprise connectivity) • Fiber backup • Interconnecting base stations in cellular systems University at Buffalo**Transmitter**• FSO uses the same transmitter technology as used by Fiber Optics • Laser/LED as coherent light source • Wavelengths centered around 850nm and 1550nm widely used • Telescope and lens for aiming light beam to the receiver University at Buffalo**Safety while using Lasers**University at Buffalo**Class 1 eye safety requirement for lasers used indoors**Array of LEDs are used Class 3B eye safety requirement for laser used outdoors 1550 nm lasers are generally chosen for this purpose Eye Safety • Classifies light sources depending on the amount of power they emit Table1: Laser safety classification for point-source emitter University at Buffalo**Receiver**• Photodiode with large active area • Narrowband infrared filters to reduce noise due to ambient light • Receivers with high gain • Bootstrap receivers using PIN diode and avalanche photodiode (APD) used University at Buffalo**Simplified Transceiver Diagram**University at Buffalo**Point-to-Multipoint Topology**University at Buffalo**Point-to-Point Topology**University at Buffalo**Ring with Spurs Topology**University at Buffalo**Mesh Topology**University at Buffalo**Typical Topology in a Metro**University at Buffalo**Challenges**• Physical Obstruction • Atmospheric Losses • Free space loss • Clear air absorption • Weather conditions (Fog, rain, snow, etc.) • Scattering • Scintillation • Building Sway and Seismic activity University at Buffalo**Physical Obstruction**• Construction crane or flying bird comes in path of light beam temporarily Solution: • Receiver can recognize temporary loss of connection • In packet-switched networks such short-duration interruptions can be handled by higher layers using packet retransmission University at Buffalo**Free space loss**• Proportion of transmitted power arriving at the receiver • Occurs due to slightly diverging beam Solution: • High receiver gain and large receiver aperture • Accurate pointing University at Buffalo**Clear Air Absorption**• Equivalent to absorption loss in optical fibers • Wavelength dependent • Low-loss at wavelengths ~850nm, ~1300nm and ~1550nm • Hence these wavelengths are used for transmission University at Buffalo**Weather Conditions**• Adverse atmospheric conditions increase Bit Error Rate (BER) of an FSO system • Fog causes maximum attenuation • Water droplets in fog modify light characteristics or completely hinder the passage of light • Attenuation due to fog is known as Mie scattering Solution: • Increasing transmitter power to maximum allowable • Shorten link length to be between 200-500m University at Buffalo**Scattering**• Caused by collision of wavelength with particles in atmosphere • Causes deviation of light beam • Less power at receiver • Significant for long range communication University at Buffalo**Scintillation**• Caused due to different refractive indices of small air pockets at different temperatures along beam path • Air pockets act as prisms and lenses causing refraction of beam • Optical signal scatters preferentially by small angles in the direction of propagation • Distorts the wavefront of received optical signal causing ‘image dancing’ • Best observed by the simmering of horizon on a hot day University at Buffalo**Scintillation (cont…)**Solution: • Large receiver diameter to cope with image dancing • Spatial diversity: Sending same information from several laser transmitters mounted in same housing • Not significant for links < 200m apart, so shorten link length University at Buffalo**Building Sway and Seismic activity**• Movements of buildings upsets transmitter-receiver alignment Solution: • Use slightly divergent beam • Divergence of 3-6 milliradians will have diameter of 3-6 m after traveling 1km • Low cost • Active tracking • Feedback mechanism to continuously align transmitter- receiver lenses • Facilitates accelerated installation, but expensive University at Buffalo**Empirical Design Principles**• Use lasers ~850 nm for short distances and ~1550 nm for long distance communication with maximum allowable power • Slightly divergent beam • Large receiver aperture • Link length between 200-1000m in case of adverse weather conditions • Use multi-beam system University at Buffalo**Limitations of FSO Technology**• Requires line-of-sight • Limited range (max ~8km) • Unreliable bandwidth availability • BER depends on weather conditions • Accurate alignment of transmitter- receiver necessary University at Buffalo**Topology Control and Routing**• Given: • Virtual topology: List of backbone nodes and potential links, Directed Graph G = (V, E) • Number of interfaces a node can have • Traffic profile of aggregate traffic demands between different source destination pairs • Required: • Optimal topology for maximizing the throughput from the traffic profile, i.e. subgraph G’ = (V, E’) so that interface and capacity constraints are met and network has maximum throughput University at Buffalo**Solution Strategy**• The algorithm consists of two parts: • Offline phase • It computes the sub-graph • Gives the routes and bandwidth reservation for every ingress-egress pair in the traffic profile • Online phase • Uses the topology computed in offline phase to exercise admission control • Routes individual flows University at Buffalo**Solution Strategy (cont)**• Task of finding sub-graph that will maximize throughput while restricting degree of each vertex is computationally prohibitive • Hence, Rollout algorithm is used to obtain near optimal solution • The order in which the traffic demands are considered for link formation decides the throughput of the system University at Buffalo**Basic Rollout Algorithm**• General method for obtaining an improved policy for a Markov decision process starting with a base heuristic policy • One step look ahead policy, with the optimal cost-to-go approximated by the cost-to-go of the base policy University at Buffalo**Basic Rollout Algorithm (Math)**• Consider problem: Maximize G(u) over set of feasible solutions U and each solution consist of N components u = (u1, u2, …, uN) • The base-heuristic algorithm (H) extends a partial solution (u1, u2, …, uk), (k < N) to a complete solution (u1, u2, …, uN) University at Buffalo**Basic Rollout Algorithm (Math)**• Thus, H(u1, u2, …, uk) = G (u1, u2, …, uN) • The rollout algorithm (R) takes a partial solution (u1, u2, …, un-1) and extends it by one component. Thus, R(u1, u2, …, un-1) = (u1, u2, …, un)where un is chosen so as to maximize H(u1, u2, …, un) University at Buffalo**Path Computation**• Find k paths for each entry in traffic profile • i = 0, d0 = 0, d = aggregate demand for this ingress-egress pair • Repeat following until we cannot find a path or whole demand is routed or i = k • Find a path using constrained shortest path first (CSPF) that accommodates (d-di)/(k-i), bandwidth and finalize links temporarily • Constraints are limited transmitter-receiver interfaces and limited link capacity • Route as much bandwidth of this demand on this route, call it di+1, • Decrement link capacity by di+1, and i = i + 1 This algorithm routes whatever we can on these paths University at Buffalo**Base Heuristic**• Partial topology by routing demands (t1,…,tn) is formed • The base heuristic routes the remaining demands in decreasing order of magnitude University at Buffalo**Index Rollout Algorithm**• Suppose demands (t1,…,tn) have been routed • For all possible next candidate demands, throughput is calculated using base heuristic • tn+1 is chosen as the one that produces maximum throughput when base heuristic is used to route remaining demands University at Buffalo**Comments**• Let: • N = # of nodes • M = # of communicating ingress-egress pairs • k = # of paths calculated for each communicating pair • Computational Complexity: • Offline phase: O(kM3N2), for constant number of communicating ingress-egress nodes • Online phase: O(k) • The base heuristic is such that the rollout works at least as good as the heuristic University at Buffalo**Conclusion**• This presentation gave an overview of Optical Wireless technology • We started with applications of FSO to provide motivation for its study • Transmitter and receiver designs were discussed • We looked at the challenges faced by this technology and techniques to deal with them • Finally had a brief look at the problem of Topology Control and routing of Bandwidth Guaranteed flows University at Buffalo**References**• D.J.T.Heatley, D.R.Wisely, I.Neild and P.Cochrane, “Optical wireless: The story so far”, IEEE Communications Magazine 36(12) (1998) 72-82 • H.A.Willebrand and B.S.Ghuman, “Fiber Optics Without Fiber”, IEEE Spectrum Magazine, August 2001, pp 40-45. • A.Kashyap, M.K.Khandani, K.Lee, M.Shayman, “Profile-Based Topology Control and Routing of Bandwidth-Guaranteed Flows in Wireless Optical Backbone Networks”, University of Maryland • http://www.freespaceoptics.org/ • http://http://www.fsona.com/ University at Buffalo