optical wireless: benefits and challenges -...

CLEO / LEAP 2002, May 22, Long Beach, California Maha Achour, Ph.D. 1

Optical Wireless: Benefits and Challenges

Maha Achour, Ph.D.President and CTO

[email protected]


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;


Telecom Vertical Markets

Component Boards

Emerging Industry

Products

CashBoxes Service

Provider

Corporate

• 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 ?

Added-Value

Services

Individual

VoIP Games Video on Demand

e-Learning

….


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.


Unlicensed Wireless Technologies (UWT)

FCC part 15.249. No IEEE standards

Radios in this band are provided by Sierra Digital and are affected by rain.

24.05-24.250 GHz

ISM Band: FCC part 15.247 and 15.249. No IEEE standard

Open in Asia and part of Europe, DS spread spectrum, and some radios with 25 Mbps speed.

5.725-5.85 GHz

• UNII band: FCC part 15.407 • IEEE standard 802.11a

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.

B1 5.15-5.25 B2 5.25-5.35 B3 5.725-5.825

• ISM band: FCC part 15.247 and 15.249 • IEEE standard 802.11b,g

Worldwide Coverage, indoor/outdoor, 11 and 22 Mbps, up to 15 miles, DS spreading and OFDM.

2.4 – 2.4825 GHz

Regulations and StandardsTechnologySpectrum Band


Unlicensed Wireless Technologies (Cont.)

• Eye-safety IEC, FDA and ANSI regulation•No FCC regulation• No standards

Free-Space Optics (FSO) using short (785-850nm), mid (1550nm) and long (10µm)wavelengths, OOK modulation, no delay, speeds up to 2.5 Gbps and few kilometers in distances.

200-300 THz

FCC part 15.255 and 15.249. No IEEE standards.

Radios with Gigabit speeds, OOK modulation, no delay, few hundred meters in range due to Oxygen absorption.

57-64 GHz (90 GHz and 120 GHz in progress)

Regulations and StandardsTechnologySpectrum Band


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,

Optical Wireless: Benefits

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.


Wide Technologies

• Short Wavelengths: 750-850 nm

• Mid Wavelengths: 1300-1550 nm ex: Terabeam @ 1550 nm

AOptix (Adaptive Optics)

• Long Wavelengths: 10 µmex: Maxima Corporation

Optical Wireless: Benefits

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


• 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.

www.fsoalliance.com

Optical Wireless: Challenges


Meteorological Visual Range

“Using Camera Imagery to measure Visibility and fog”, MIT Lincoln Lab Report 2001

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.


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.


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)…


Atmospheric attenuation

FSO systems are affected by the following weather conditions:

Absorption: is a Quantum effect with H2O and CO2 absorption bands defining the eight atmospheric windows 720 nm – 12000 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)


Scattering Effects

Scattering patterns of electromagnetic waves by spherical particles.

(a) Small Particles with diameter < λ/10

Small Particles with diameter ≅ λ/4

Small Particles with diameter > λ

Incident Beam with wavelength λ Incident Beam with wavelength λ

Incident Beam with wavelength λ

Earl J. McCartney, Optics of the Atmosphere: Scattering by Molecules and Particles,Wiley & Sons, New York, [1976].

Multiple-order scattering between Tx and Rx


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 βext(λ).

V = |ln(0.02)|/ βext(λ) = 3.91/ βext(λ=0.55µ)


Meteorological Visual Range

• Relative contrast is defined as follows:

• The above approximation holds when Lmin(0) << Lmax(0) and Lmin(0)=Lmin(V).

• If the background is the horizon, then Lmin(0)=Lmin(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 βscat(λ).

V-

max

max

minmax

min

min

minmax e (0)L(V)L

(0)L - (0)L(0)L

(V)L(V)L (V)L

C(0)C(V) α=≅

−=


Rainfall Attenuation

• Is considered non-selective scattering because the size of a raindrop (Diameter 0.01-10 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.


Rainfall Attenuation

Simulation Results:

• Rainfall between 10-100 mm/hr

• Wavelength independent

Num

ber o

f dro

ps p

er m

3

Drop Diameter in mm Drop Diameter in mm

Rainfall rate in mm/hr

Rai

nfal

l rat

e in

mm

/hr

Rai

nfal

l atte

nuat

ion

dB/k

m

Caption:

mm/hr

(+) 10

(o) 20

(*) 30

(◊) 40

(●) 50

(x) 60

(□) 70

( ) 80

( ) 90

( ) 100


Mie Scattering

• Based on slide 14 assumptions, Visibility can be related to the extinction coefficient β(λ=0.55µ) by the following relation:

• In most literatures, relating β(λ=0.55µ) to β(λ) was performed using the following equation:

• The exponent δ = 1.6 for good visibility, 1.3 for V=6-50 Km and 0.585 V1/3 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.

)0.55(λ

3.91 )0.55(λ

ln(0.02) V

µβµβ ==

==

55.0

3.91 55.0

)0.55(λ β(λ)δδ λλµβ

−−

=

==

V(1)


Mie Scattering

• The general equation to derive scattering coefficient is:

βscatt (λ)= ∑a π a2 Na Qscatt (x)

where, x=2πa/ λ.• λ dependence is not trivial due to the analytical expression of Q.

Q(x)

x

x

x

Q(x 850/550)

Q(x)

Q(x 1550/550)

Q(x)


Mie Scattering

• Build a virtual fog/haze model that reproduces the same results as the equation below at λ = 850 nm:

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20

10

20

30

40

50

60

70

80

90

100

550

3.91 550

)0.55(λ β(λ)1/31/3 V585.0V585.0 −−

=

==

λλµβV

0.5 1 1.5 2 2.5 3 3.5 40

5

10

15

20

25

30

Atte

nuat

ion

in d

B/K

m

Con

cent

ratio

n in

cm

3

Drop Radius in µm Visibility in Km

Black 785nm

Red 1550 nm

Blue 10 µm

(+) Based on Drop Distribution Model

Visibility 0.5 Km

Visibility 4 Km


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.


Mie Scattering

Weather ConditionsClear: 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.


Mie Scattering

18.118.4512.6710,100 / 10,1003c

41.368.465.781230 /12303b

50.13Jan24 1812-1824

62.957.72750 / 6550.303a

110.42263.4213.6510,100 / 10,1002c

185.64247.33239.21230 /12302b

209.73Jan29/30 2106-2306

226.2227.06750 / 6550.075Stable2a

3.17.30.9310,100 / 10,1001c

10.617.3718.81230 / 12301b

14.16Dec 11, 222015.217.09750 / 6551Evolving1a

Equation (1) (dB/Km)

Experiment*Day/Time

Experiment*(dB/Km)

Simulight™Mie Loss (dB/Km)

Wavelength (nm)Simulight / Experiments

Visibility(Km)

FogModel

(*) Clay, M. R. and Lenham A. P., Transmission of electromagnetic radiation in fogs in the 0.53-10.1 µm wavelength range, Applied Optics, Vol. 20, No. 22, 1981, page 3831


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


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

optical wireless: benefits and challenges -...

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