Optical Wireless: Benefits and Challenges

Transcription

1 Optical Wireless: Benefits and Challenges Maha Achour, Ph.D. President and CTO 1

2 About UlmTech.. Two Divisions: Free-Space optics and e-learning Free-Space Optics Division: Developing the first commercial software that simulates atmospheric propagation of optical wireless signals, the Simulight (release date: March 01, 2002); e-learning Division: Intelligent Real-Time Multimedia Platform for Online Learning and Collaboration; 2

3 Telecom Vertical Markets Component Products Cash Boards Emerging Industry Collapse of data delivery business model, Telecom Deregulation Act 1996, Lack of the killer innovative application led to end-user slow broadband adoption. Last mile connectivity. Will e-learning be the solution? Boxes Corporate Service Provider Added-Value Services Individual VoIP Games e-learning Video on Demand. 3

4 Broadband Access Service providers need to access paying customers quickly, cost effectively and reliably. Fiber often does not reach paying customers. Only 3% of worldwide Businesses are on fiber, and 75% are within a mile from fiber. Speed limitation, cost and asymmetrical properties of DSL, Cable, Satellite and other existing technologies. Broadband wireless communications, with Unlicensed Wireless Technologies UWT in particular will play major role in Broadband connectivity. At the end of the day someone, besides investors, needs to pay for all those tremendous optical and networking advances and improvements. 4

5 Unlicensed Wireless Technologies (UWT) Spectrum Band GHz B B B GHz GHz Technology Worldwide Coverage, indoor/outdoor, 11 and 22 Mbps, up to 15 miles, DS spreading and OFDM. Limited global coverage, Hiperlan in Europe (B1 and B2), indoor/outdoor (B2 and B3), 54 Mbps using OFDM on twelve non-overlapping 20 MHz bands, and some non-ieee radios support 450 Mbps using QAM. Open in Asia and part of Europe, DS spread spectrum, and some radios with 25 Mbps speed. Radios in this band are provided by Sierra Digital and are affected by rain. Regulations and Standards ISM band: FCC part and IEEE standard b,g UNII band: FCC part IEEE standard a ISM Band: FCC part and No IEEE standard FCC part No IEEE standards 5

6 Unlicensed Wireless Technologies (Cont.) Spectrum Band Technology Regulations and Standards GHz (90 GHz and 120 GHz in progress) Radios with Gigabit speeds, OOK modulation, no delay, few hundred meters in range due to Oxygen absorption. FCC part and No IEEE standards THz Free-Space Optics (FSO) using short ( nm), mid (1550nm) and long (10µm) wavelengths, OOK modulation, no delay, speeds up to 2.5 Gbps and few kilometers in distances. Eye-safety IEC, FDA and ANSI regulation No FCC regulation No standards 6

7 Optical Wireless: Benefits Quick Deployment No licenses required on a global scale, No frequency planning due to narrow Optical Beam, Practical size units that look like security cameras, Safe when units comply with the IEC safety standard, Protocol independent, Low Cost / Scalability Uses off-the-shelf components from the Fiber industry, Highly scalable in bandwidth, Low power consumption, with some do not require EOE conversion, Reliability Reliable hardware Communication link availability is based on the location, deployed unit and distance. 7

8 Wide Technologies Optical Wireless: Benefits Short Wavelengths: nm Mid Wavelengths: nm ex: 1550 nm AOptix (Adaptive Optics) Long Wavelengths: 10 µm ex: Maxima Corporation Terabeam Elliptica: About the size of a slightly deflated basketball. The laser is a CDRH Class 1 laser, meaning it is so eye-safe as to not require any warning labels. Source: Terabeam 8

9 Optical Wireless: Challenges Microwave signals above 10 GHz are mostly affected by rain. The availability of extensive global precipitation databases and rain fade (rainfall) simple modeling accurately estimates microwave deployment link availability. Free-Space Optics is affected by various weather conditions with Fog (visibility, fog type) in particular channel modeling (Simulight ) and weather databases. Formed in February 2001 to unify vendors and service providers efforts to bring proper awareness and understanding of the technology. 9

10 Meteorological Visual Range Background Contrast Visibility: Distance for which the Contrast transmission of the atmosphere is 2% in reference to the wavelength 550 nm that the eye has the greatest sensitivity. Using Camera Imagery to measure Visibility and fog, MIT Lincoln Lab Report

11 About Simulight.. Simulight considers the following optical propagation effects: Low altitudes propagation, Haze, rain, fog, low clouds and molecular scattering, Geometrical beam dispersion including diffraction effects, Water and carbon dioxide absorption, Absorption due to the presence of water vapor in the air, It supports wavelengths that span from 750 nm to 12 µm. 11

12 Atmospheric Propagation Modeling FSO atmospheric propagation: Deployment parameters: are related to the location and application of the FSO system installation: Range, Bandwidth, Wavelength FSO system parameters: are related to the deployed FSO system: location of the FSO system installation: number of transmitters (Tx) and Receivers (Rx), Tx diameter, Rx diameter, Tx power, Rx sensitivity, additional amplification, additional hardware losses.. Weather parameters: Meteorological Visual Range (Visibility), Temperature, Relative Humidity, fog model (non-selective, evolving, stable) 12

13 Atmospheric attenuation FSO systems are affected by the following weather conditions: Absorption: is a Quantum effect with H 2 O and CO 2 absorption bands defining the eight atmospheric windows 720 nm nm (fine line absorption). Increasing humidity cause additional water absorption. Rayleigh Scattering: due to scattering by air molecule. Very small compared to Mie scattering and is proportional to λ -4 Mie Scattering: Due to scattering by small particles of sizes comparable to wavelength. Turbulences (scintillation): Beam deviation, wander, broadening and power fluctuation. Rain Fade: Considered non-selective scattering. The attenuation is proportional to the rainfall rate (drop size distribution) 13

14 Scattering Effects Incident Beam with wavelength λ Incident Beam with wavelength λ (a) Small Particles with Multiple-order scattering diameter between < Tx λ/10and Rx Small Particles with diameter λ/4 Incident Beam with wavelength λ Small Particles with diameter > λ Earl J. McCartney, Optics of the Atmosphere: Scattering by Molecules and Particles, Wiley & Sons, New York, [1976]. Scattering patterns of electromagnetic waves by spherical particles. 14

15 Weather Parameters Absolute Humidity: Mass of water vapor in a unit volume of air. Saturation: Refers to the maximum possible amount of water vapor that air can hold (Temperature dependent) per unit volume. Dew Point: Is the temperature at which saturation occurs. Related to temperature and Dewpoint. Relative Humidity: The ratio of the absolute humidity to saturation. This parameter, along with temperature, is useful to determine additional water absorption. Visibility: Distance for which the Contrast transmission of the atmosphere is 2% in reference to the wavelength 550 nm that the eye has the greatest sensitivity. It is a function of the extinction coefficient β ext (λ). V = ln(0.02) / β ext (λ) = 3.91/ β ext (λ=0.55µ) 15

16 Meteorological Visual Range Relative contrast is defined as follows: C(V) L = C(0) max (V) L L (V) min min (V) L max Lmin (0) (0) - L min (0) L L max max (V) (0) = e -αv The above approximation holds when L min (0) << L max (0) and L min (0)=L min (V). If the background is the horizon, then L min (0)=L min (V). Complete understanding of visibility measurement is essential. Meteorological visual ranges V are defined with the above two approximations.. The problem is how to use V to derive β scat (λ). 16

17 Rainfall Attenuation Is considered non-selective scattering because the size of a raindrop (Diameter mm) is much larger than the incident wavelength. In 1920, F.W. Preston, in an almost forgotten paper, claimed that the obscuring power of falling rain is proportional only to the number of drops falling on unit area of the earth s surface per second. 17

18 Rainfall Attenuation Number of drops per m 3 Caption: mm/hr (+) 10 (o) 20 (*) 30 ( ) 40 Rainfall rate in mm/hr ( ) 50 Drop Diameter in mm Simulation Results: Rainfall between mm/hr Wavelength independent (x) 60 ( ) 70 ( ) 80 ( ) 90 ( ) 100 Rainfall attenuation db/km Drop Diameter in mm Rainfall rate in mm/hr 18

19 Mie Scattering Based on slide 14 assumptions, Visibility can be related to the extinction coefficient β(λ=0.55µ) by the following relation: V = ln(0.02) 3.91 = β (λ = 0.55µ ) β (λ = 0.55µ ) In most literatures, relating β(λ=0.55µ) to β(λ) was performed using the following equation: δ δ λ 3.91 λ β(λ) = β (λ = 0.55µ ) = (1) 0.55 V 0.55 The exponent δ = 1.6 for good visibility, 1.3 for V=6-50 Km and V 1/3 for visibility less than 6 Km. Problem: exponent value and the one-to-one relation between visibility and attenuation coefficient independent of droplet sizes and distributions. 19

20 Mie Scattering The general equation to derive scattering coefficient is: β scatt (λ)= a π a 2 N a Q scatt (x) where, x=2πa/ λ. λ dependence is not trivial due to the analytical expression of Q. Q(x) Q(x 850/550) x Q(x) Q(x) Q(x 1550/550) x x 20

21 Mie Scattering Build a virtual fog/haze model that reproduces the same results as the equation below at λ = 850 nm: β(λ) = β (λ λ = 0.55µ ) 550 1/ V 0.585V = 3.91 λ V 550 1/3 Concentration in cm Visibility 0.5 Km Drop Radius in µm Visibility 4 Km Attenuation in db/km Visibility in Km Black 785nm Red 1550 nm Blue 10 µm (+) Based on Drop Distribution Model 21

22 Mie Scattering Defining Drop-sizes distribution function by identifying the peaks and the slope of the curve at both ends. Use the general Mie definition of the scattering coefficient. More choices of haze/fog/cloud distributions based on visibility than raindrops distributions based on rainfall rates: Due to the behavior of large drops in the air. There are other constraints that small drops need to satisfy. Visibility along with fog type provide sufficient information to calculate FSO attenuations. 22

23 Mie Scattering Weather Conditions Clear: It includes light Haze and Rain: Visibilities over 4 Km. Rain: Weather conditions that accounts of rain only. Evolving Fog: Weather conditions between light haze, dense haze and stable fog: Visibilities between 1 and 4 Km. Stable Fog: Foggy weather conditions: Visibilities up to 1.5 Km. Selective Fog: Weather conditions between fog and low clouds: Visibilities up to 0.75 Km. 23

24 Mie Scattering Fog Model Visibility (Km) Wavelength (nm) Simulight / Experiments Simulight Mie Loss (db/km) Experiment* (db/km) Experiment* Day/Time Equation (1) (db/km) 1a Evolving / Dec 11, b 1230 / c 10,100 / 10, a Stable / Jan29/ b 1230 / c 10,100 / 10, a / Jan b 1230 / c 10,100 / 10, (*) Clay, M. R. and Lenham A. P., Transmission of electromagnetic radiation in fogs in the µm wavelength range, Applied Optics, Vol. 20, No. 22, 1981, page

25 Mie Scattering Sample of the fog droplet distribution for case 1: Evolving 1 Km Visibility Sample of the fog droplet distribution for case 2: Stable 75 m Visibility 25

26 Concluding Remarks Free-Space Optics offer quick, scalable and cost-effective solutions to the access network, Wide selection of products from T1 to OC48 and soon 10 Gbps speeds, Global deployments over the past decade, Need extensive weather database, Need to classify fogs for very low visibilities, Properly convey the technology limitations 26