university at buffalo optical wireless an overview chintan shah [email protected] december 9,...

University at Buffalo

Optical Wireless An Overview

Chintan [email protected]

December 9, 2004


Outline Introduction

What is Optical Wireless? Applications

Transmitter and Receiver Topologies Challenges and Limitations Topology Control and Routing Conclusion


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


What is Optical Wireless?

Data rates of up to 2.5 Gbps at distances of up to 4km available in commercial products


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


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


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


Safety while using Lasers


Eye Safety

650 nm (visible)

880 nm (infrared)

1310 nm(infrared)

1550 nm(infrared)

Class 1 Up to 0.2 mW Up to 0.5 mW Up to 8.8 mW Up to 10 mW

Class 2 0.2-1 mW N/A N/A N/A

Class 3A 1-5 mW 0.5-2.5 mW 8.8-45 mW 10-50 mW

Class 3B 5-500 mW 2.5-500 mW 45-500 mW 50-500 mW

Table1: Laser safety classification for point-source emitter

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

Classifies light sources depending on the amount of power they emit


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


Simplified Transceiver Diagram


Point-to-Multipoint Topology


Point-to-Point Topology


Ring with Spurs Topology


Mesh Topology


Typical Topology in a Metro


Challenges Physical Obstruction Atmospheric Losses

Free space loss Clear air absorption Weather conditions (Fog, rain, snow,

etc.) Scattering Scintillation

Building Sway and Seismic activity


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


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


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


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


Scattering Caused by collision of

wavelength with particles in atmosphere

Causes deviation of light beam

Less power at receiver Significant for long range

communication


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


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


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


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


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


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


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


Solution Strategy (cont)

The order in which the traffic demands are considered for link formation decides the throughput of the system

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


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


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)


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)

Basic Rollout Algorithm (Math)


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


Base Heuristic

Partial topology by routing demands (t1,…,tn) is formed

The base heuristic routes the remaining demands in decreasing order of magnitude


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


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


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


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 optical wireless an overview chintan shah [email protected] december 9,...

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