Iran 2008

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Professor Z GHASSEMLOOY

Associate Dean for Research Optical Communications Research Group,

School of Computing, Engineering and Information SciencesThe University of Northumbria

Newcastle, U.K.http://soe.unn.ac.uk/ocr/

Free Space Optical Communications

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Northumbria University at Newcastle, UK

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Outline

Introduction Why the need for optical wireless? FSO FSO - Issues Some results Final remarks

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Optical Communications

Optical FibreCommunications

Photonic Switching

Indoor

Wired Wireless

Free-Space Optics(FSO)

OCRG - Research Areas

• Chromatic dispersion compensation using optical signal processing• Pulse Modulations• Optical buffers• Optical CDMA

• Pulse Modulations• Equalisation• Error control coding• Artificial neural network & Wavelet based receivers

• Fast switches• All optical routers

Subcarrier modulation Spatial diversity Artificial neural network/Wavelet based receivers

4HK Poly-Univ. 2007

Staff• Prof. Z Ghassemlooy• J Allen• R Binns• K Busawon• Wai Pang Ng

Visiting Academics• Prof. Jean Pierre, Barbot France • Prof. I. Darwazeh UCL• Prof. Heinz Döring Hochschule Mittweida Univ. of Applied Scie. (Germany) • Dr. E. Leitgeb Graz Univ. of Techn. (Austria)

OCRG - PeoplePhD• M. Amiri• M. F. Chiang:• S. K. Hashemi• R. Kharel • W. Loedhammacakra• V. Nwanafio• E. K. Ogah• W. O. Popoola • S. Rajbhandari (With IMLab)• Shalaby• S. Y Lebbe MSc and BEng• A Burton • D Bell• G Aggarwal • M Ljaz • O Anozie • W Leong

(BEng)• S Satkunam (BEng)

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Photonics - Applications

Long-Haul Metropolitan Home access

Board -> Inter-Chip -> Intra-Chip

• Photonics in communications: expanding and scaling

Health(“bio-photonics”)

Environmentsensing

Securityimaging

• Photonics: diffusing into other application sectors

RF & Optical Communications - Integration

TraditionalRadio

TraditionalOptics

Radio onFibre

OpticalWireless

Fibre Free Space

Ligh

twav

eR

F

Transmission Channel

Sou

rce

Free Space Optical (FSO)

Communications

….. BANDWIDTH when and where required.

AND THAT IS ?

Over the last 20 years deployment of optical fibre cables in the backbone

and metro networks have made huge bandwidth readily available to

within one mile of businesses/home in most places.

But, HUGE BANDWIDTH IS STILL NOT AVAILABLE TO THE END

USERS.

The Problem?

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Optical Wireless Communication

Abundance of unregulated bandwidth - 200 THz in the 700-1500 nm range

What does

It Offer

?

No multipath fading - Intensity modulation and direct detection

Secure transmission

High data rate – In particular line of sight (in and out doors)

Improved wavelength reuse capability

Flexibility in installation

Flexibility - Deployment in a wide variety of network architectures. Installation on roof to roof, window to window, window to roof or

wall to wall. 10

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Drawbacks

Multipath induced dispersion (non-line of sight, indoor) - Limiting data rate SNR can vary significantly with the distance and the ambient noise (Note SNR Pr

2)

Limited transmitted power - Eye safety (indoor)

High transmitted power - Outdoor

Receiver sensitivity

Large area photo-detectors - Limits the bandwidth

May be high cost - Compared with RF

Limited range: Indoor: ambient noise is the dominant (20-30 dB larger than the signal level . Outdoor: Fog and other factors

Optical Wireless Communication

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(Source: NTT)

Access Network bottleneck

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xDSL Copper based (limited bandwidth)- Phone and data combine Availability, quality and data rate depend on proximity to service provider’s C.O.

Radio link Spectrum congestion (license needed to reduce interference) Security worries (Encryption?) Lower bandwidth than optical bandwidth At higher frequency where very high data rate are possible, atmospheric attenuation(rain)/absorption(Oxygen gas) limits link to ~1km

Cable Shared network resulting in quality and security issues. Low data rate during peak times

FTTx Expensive Right of way required - time consuming Might contain copper still etc

Access Network Technology

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Optical Wireless Communications

Using optical radiation to communicate between two points through unguided channels

Types- Indoor- Outdoor (Free Space Optics)

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15

DR

IVE

R

CIR

CU

IT

SIG

NA

LP

RO

CE

SS

ING

PH

OTO

DE

TEC

TOR

Link Range L

FSO - Basics

Cloud Rain Smoke Gases Temperature variations Fog and aerosol

Transmission of optical radiation through the atmosphere obeys the Beer-Lamberts’s law:

α : Attenuation coefficient dB/km – Not controllable and is roughly independent of wavelength in heavy attenuation conditions.d1 and d2: Transmit and receive aperture diameters (m)D: Beam divergence (mrad)(1/e for Gaussian beams; FWHA for flat top beams),

This equation fundamentally ties FSO to the atmospheric weather conditions

10/22

1

22 10

)(L

tr LDddPP

Dominant term at 99.9% availability

FSO Link

Transmitter Lasers 780,850,980,1550nm, also 10 microns Beam control optics

o Multiple transmit apertures to reduce scintillation problems o Tracking systems to allow narrow beams and reduced geometric losses

Receiver Collection lens Solar radiation filters (often several) Photodetector - Large area and low capacitance (PIN/APD) Amplifier and receiver

o Wide dynamic range requirement due to very high clear air link margin o Automatic gain and transmitter power control

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Optical Components – Light Source

Operating Wavelength

(nm)

Laser type Remark

~850 VCSEL Cheap, very available, no active cooling, reliable up to ~10Gbps,

~1300/~1550 Fabry-Perot/DFB Long life, compatible with EDFA, up to 40Gbps50–65 times as much power compared with 780-850 nm

~10,000

Quantum cascade laser (QCL)

Expensive, very fast and highly sensitiveIdeal for indoor (no penetration through window)

For indoor applications LEDs are also used17Eye safety - Class 1M

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Optical Components – Detectors

Material/StructureWavelength

(nm)Responsivity

(A/W)Typical

sensitivityGain

Silicon PIN 300 – 1100 0.5 -34dBm@ 155Mbps

1

InGaAs PIN 1000 – 1700 0.9 -46dBm@155Mbps

1

Silicon APD 400 – 1000 77 -52dBm@155Mbps

150

InGaAs APD 1000 – 1700 9 10

Quantum –well and Quatum-dot (QWIP&QWIP)

~10,000

Germanium only detectors are generally not used in FSO because of their high dark current.18

Existing System Specifications

Range: 1-10 km (depend on the data rates) Power consumption up to 60 W

15 W @ data rate up to 100 mbps and =780nm, short range 25 W @ date rate up to 150 Mbps and = 980nm 60 W @ data rate up to 622 Mbps and = 780nm 40 W @ data rate up to 1.5 Gbps and = 780nm

Transmitted power: 14 – 20 dBm Receiver: PIN (lower data rate), APD (>150 mbps) Beam width: 4-8 mRad Interface: coaxial cable, MM Fibre, SM Fibre Safety Classifications: Class 1 M (IEC) Weight: up to 10 kg

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Power Spectra of Ambient Light Sources

Wavelength (m)

Nor

mal

ised

pow

er/u

nit w

avel

engt

h

0

0.2

0.4

0.6

0.8

1

1.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

Sun Incandescent

x 10

1st window IR

Fluorescent

Pave)amb-light >> Pave)signal (Typically 30 dB with no optical filtering)

2nd window IR

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FSO - Characteristics Narrow low power transmit beam- inherent security Narrow field-of-view receiver Similar bandwidth/data rate as optical fibre No multi-path induced distortion in LOS Efficient optical noise rejection and a high optical signal

gain Suitable to point-to-point communications only (out-door

and in-door) Can support mobile users using steering and tracking

capabilities Used in the following protocols:

- Ethernet, Fast Ethernet, Gigabit Ethernet, FDDI, ATM- Optical Carriers (OC)-3, 12, 24, and 48.

Cheap (cost about $4/Mbps/Month according to fSONA)21

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22Source:

Cost Comparison

Existing Systems

Auto tracking systems - 622 Mbps [Canobeam] TereScop - 1.5 Mbps to 1.25 Gbps (500m – 5km) Cable Free - 622 Mbps to 1.25 Gbps (High power class 3B

Laser at 100 mW) Microcell and cell-site backbone – GSM, GPRS, 3G and EDGE traffic

o No Frequency licenseo No Link Engineeringo Management via SNMP, RS232o or GSM connection

Last mileo 155 Mbps STM-1 linkso 622 Mbps ATM link for Banks etc

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800BC - Fire beacons (ancient Greeks and Romans)150BC - Smoke signals (American Indians)1791/92 - Semaphore (French)

1880 - Alexander Graham Bell demonstrated the photophone – 1st FSO (THE GENESIS)

(www.scienceclarified.com)

1960s - Invention of laser and optical fibre1970s - FSO mainly used in secure military applications1990s to date - Increased research & commercial use due to successful trials

When Did It All Start?

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In addition to bringing huge bandwidth to businesses /homes FSO also finds applications in :

Multi-campus universityHospitals

Others: Inter-satellite communication Disaster recovery Fibre communication back-up Video conferencing Links in difficult terrains Temporary links e.g. conferences

Cellular communication back-haul FSO challenges…FSO challenges…

FSO - Applications

RF wireless networks- Broadcast RF networks are not scaleable- RF cannot provide very high data rates- RF is not physically secure

- High probability of detection/intercept

- Not badly affected by fog and snow, affected by rain

A Hybrid FSO/RF Link- High availability (>99.99%) - Much higher throughput than RF alone- For greatest flexibility need unlicensed RF band

Hybrid FSO/RF Wireless Networks

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LOS - Hybrid Systems

Video-conference for Tele-medicine CIMIC-purpose and disaster recovery27

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DR

IVE

R

CIR

CU

IT

POINT A POINT B

SIG

NA

LP

RO

CE

SS

ING

PH

OTO

DE

TEC

TOR

Major challenges are due to the effects of:

CLOUD,

RAIN, SMOKE, GASES,

TEMPERATURE VARIATIONS FOG & AEROSOL

FSO - Challenges

To achieve optimal link performance, system design involves

tradeoffs of the different parameters.

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Effects Options Remarks

Photon absorption

Increase transmit optical power

Effect not significant

FSO Challenges - Rain

= 0.5 – 3 mm

FSO Challenges - Physical ObstructionsPointing Stability and Swaying Buildings

Effects Solutions Remarks Loss of signal Multipath induced Distortions Low power due to beam divergence and spreading Short term loss of signal

Spatial diversity Mesh architectures: using diverse routes Ring topology: User’s n/w become nodes at least one hop away from the ring Fixed tracking (short buildings) Active tracking (tall buildings)

May be used for urban areas, campus etc.

Low data rate Uses feedback

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FSO Challenges – Aerosols Gases & Smoke

Mie scattering Photon absorption Rayleigh scattering

Increase transmit power Diversity techniques

Effect not severe

Effects Solutions Remarks

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Effects Options Remarks

Mie scattering Photon absorption

Increase transmit optical power Hybrid FSO/RF

Thick fog limits link range to ~500m Safety requirements limit maximum optical power

FSO Challenges - Fog

= 0.01 - 0.05 mmIn heavy fog conditions, attenuation is

almost constant with wavelength over the 780–1600 nm region.

In fact, there are no benefits until one gets to millimeter-wave wavelengths.

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Weather condition

Precipitation Amount (mm/hr)

Visibility dBLoss/km

Typical Deployment Range (Laser link ~20dB margin)

Dense fog 0 m50 m -271.65 122 m

(H.Willebrand & B.S. Ghuman, 2002.)

Very clear 23 km50 km

-0.19-0.06

12112 m13771 m

Thick fog 200 m -59.57 490 m

Moderate fog Snow 500 m -20.99 1087 m

Light fog Snow Cloudburst

100 770 m1 km

-12.65-9.26

1565 m1493 m

Thin fog Snow Heavy rain 25 1.9 km2 km

-4.22-3.96

3238 m3369 m

Haze Snow Medium rain

12.5 2.8 km4 km

-2.58-1.62

4331 m5566 m

Light haze Snow Light rain 2.5 5.9 km10 km

-0.96-0.44

7146 m9670 m

Clear Snow Drizzle 0.25 18.1 km20 km

-0.24-0.22

11468 m11743 m

FSO Challenges - Fog

FSO Challenges - Beam Divergence

Beam width Typically, for FSO transceiver is relatively wide: 2–10-mrad

divergence, (equivalent to a beam spread of 2–10 m at 1 km), as is generally the case in non-tracking applications.

Compensation is required for any platform motion By having a beam width and total FOV that is larger than either

transceiver’s anticipated platform motion.

For automatic pointing and tracking, Beam width can be narrowed significantly (typically, 0.05–1.0 mrad

of divergence (equivalent to a beam spread of 5 cm to 1 m at 1 km)- further improving link margin to combat adverse weather conditions.- However, the cost for the additional tracking feature can be significant.

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Background radiation LOS requirement Laser safety

FSO Challenges - Others

Free Space Optics Characteristics Challenges Turbulence

- Subcarrier intensity multiplexing- Diversity schemes

Results and discussions

Wavelet ANN Receiver

Final remarks

Effects Options Remarks Irradiance fluctuation (scintillation) Image dancing Phase fluctuation Beam spreading Polarisation fluctuation

Diversity techniques Forward error control control Robust modulation techniques Adaptive optics Coherent detection not used due to Phase fluctuation

Significant for long link range (>1km)Turbulence and thick fog do not occur together In IM/DD, it results in deep irradiance fades that could last up to ~1-100 μs

FSO Challenges - Turbulence

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Cause: Atmospheric inhomogeneity / random temperature variation along beam path.

Depends on: Altitude/Pressure, Wind speed, Temperature and relative beam size. Can change by more than an order of magnitude during the course of a day, being the worst, or most scintillated, during midday (highest temperature). However, at ranges < 1 km, most FSO systems have enough dynamic range or margin to compensate for scintillation effects.

The atmosphere behaves like prismof different sizes and refractive indices

Phase and irradiance fluctuation

• Zones of differing density act as lenses, scattering light away from its intended path. • Thus, multipath.

Result in deep signal fades that

lasts for ~1-100 μs

FSO Challenges - Turbulence

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Gamma-Gamma All regimes

Model CommentsLog Normal Simple; tractable

Weak regime only

I-K Weak to strong turbulence regime

K Strong regime only

Rayleigh/Negative Exponential

Saturation regime only

Irradiance PDF by Andrews et al (2001):

0)2()()(

)(2)(1)

2(

2/)(

IIIIp

1

6/55/12

2

1

6/75/12

2

1)69.01(

51.0exp

1)11.11(

49.0exp

l

l

l

l

Ix: due to large scale effects; obeys Gamma distributionIy: due to small scale effects; obeys Gamma distributionKn(.): modified Bessel function of the 2nd kind of order n σl

2 : Log irradiance variance (turbulence strength indicator)

yx III Based on the modulation process the received irradiance is

Irradiance PDF:

02

220

2

)2/)/(ln(exp1

21)(

I

l

l

lI

III

Ip

To mitigate turbulence effect we, employ subcarrier modulation with spatial diversity

Turbulence – Channel Models

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40

A

No Pulse Bit “0” Pulse Bit “1”

No Intensity Fading

With Intensity Fading

A

Threshold level

A/2

All commercially available systems use OOK with fixed threshold which results in sub-optimal performance in turbulence regimes

Turbulence Effect on OOK

dIII

IiRIi

l

l

l

rr

2

220

20

2

22

22/)/ln(exp

.1

2

12

))((exp

))(/()(ˆ maxarg tdiPtd rd

Using optimal maximum a posteriori (MAP) symbol-by-symbol detection with equiprobable OOK data:

Turbulence Effect on OOK

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Log Intensity Standard Deviation

Thre

shol

d le

vel,

ith

0.5*10-2

10-2

3*10-2

5*10-2

Noise variance

OOK based FSO requires adaptive threshold to performoptimally….

….but subcarrier intensity modulated FSO does not

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Photo-detector

array

Atmosphericchannel

Serial/parallelconverter

Subcarrier modulator

.

.Data in

d(t)

Summing circuit

.

.

DC bias

m(t) m(t)+bo

Optical transmitter

Spatial diversity combiner

Subcarrierdemodulator

Parallel/serialconverter .

.

Data out

d’(t) ir

SIM – System Block Diagram

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Subcarrier Intensity Modulation

No need for adaptive threshold To reduce scintillation effects on SIM

Convolutional coding with hard-decision Viterbi decoding (J. P. KIm et al 1997)

Turbo code with the maximum-likelihood decoding (T. Ohtsuki, 2002) Low density parity check (for burst-error medium):

- Outperform the Turbo-product codes. - LDPC coded SIM in atmospheric turbulence is reported to achieve a

coding gain >20 dB compared with similarly coded OOK (I. B. Djordjevic, et al 2007)

SIM with space-time block code with coherent and differential detection (H. Yamamoto, et al 2003)

However, error control coding introduces huge processing delays and efficiency degradation (E. J. Lee et al, 2004)

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SIM – Our Contributions

Multiple-input-multiple-output (MIMO) (an array of transmitters/ photodetectors) to mitigate scintillation effect in a IM/DD FSO link overcomes temporary link blockage (birds and misalignment) when

combined with a wide laser beamwidth, therefore no need for an active tracking

provides independent aperture averaging with multiple separate aperture system, than in a single aperture where the aperture size has to be far greater than the irradiance spatial coherence distance (few centimetres)

provides gain and bit-error performance Efficient coherent modulation techniques (BPSK etc.) - bulk of the

signal processing is done in RF that suffers less from scintillation

In dense fog, MIMO performance drops, therefore alternative configuration such as hybrid FSO/RF should be considered

Average transmit power increases with the number of subcarriers, thus may suffers from signal clipping

Inter-modulation distortion

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45

M

jjcjj twtgAtm

1)cos()()(

Serial to Parallel

Converter

.

.

.

.

.

.

PSK modulator at coswc1t

PSK modulator at coswcMt

PSK modulator at coswc2t

Σ Σ Laserdriver

)(tdInput data

g(t)

g(t)

g(t)

A1

AM

A2

m(t)

DC bias

b0

Atmopsheric channel

Subcarrier Modulation - Transmitter

1'00,0 ][ ct NPRhModulation index is constrained to avoid over modulation

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0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-5

-4

-3

-2

-1

0

1

2

b0 Drive current

Outputpower

m(t)2maxP

P

5-subcarriers

M

jjcjj twtgAtm

1)cos()()(

Subcarrier Modulation - Transmitter

1'00,0 ][ ct NPRh

Photodetector

ir

x g(-t) Sampler

PSK Demodulator at coswc2t

PSK Demodulator at coswcMt

Parallel to Serial

Converter

PSK Demodulator

coswc1t

)(ˆ td Output data

.

.

.

SIM - Receiver

)())(1()( tntmIRtir

Photo-current

R = Responsivity, I = Average power, = Modulation index, m(t) = Subcarrier signaldi(t) = Data

2

2

2)(

IRASNRele

47

cN

iiiitir tntftdPhP

1, )(2cos()(1

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Performs optimally without adaptive threshold as in OOK Use of efficient coherent modulation techniques (PSK, QAM etc.)

- bulk of the signal processing is done in RF where matured devices like stable, low phase noise oscillators and selective filters are readily available.

System capacity/throughput can be increased Outperforms OOK in atmospheric turbulence Eliminates the use of equalisers in dispersive channels Similar schemes already in use on existing networks

The average transmit power increases as the number of subcarrier increases or suffers from signal clipping. Intermodulation distortion due to multiple subcarrier impairs its performance

But..

Subcarrier Modulation

SIM - Spatial Diversity

Single-input-multiple-output Multiple-input-multiple-output (MIMO)

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Selection Combining (SELC). No need for phase information

))()...(),(max()( 21 titititi NT ii ia

Maximum RatioCombining (MRC)[Complex but optimum]

Naaa ...21

Equal Gain Combining (EGC)

FSO CHANNEL

PSK Subcarrier

Demodulator....

)(ˆ td

)(1 ti

)(2 ti

)(tiN

a2

a1

aN

Combiner

)(tiT

Diversity Combining Techniques

ai is the scalingfactor

)()cos()(1)( tntwtgAINRti i

M

jjcjjiri

SIM - Spatial Diversity

Assuming identical PIN photodetector on each links, the photocurrent on each link is:

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SIM Spatial Diversity – Assumptions Made

Spacing between detectors > the transverse correlation size ρo of the laser radiation, because ρo = a few cm in atmospheric turbulence

Beamwidth at the receiver end is sufficiently broad to cover the entire field of view of all N detectors.

Scintillation being a random phenomenon that changes with time makes the received signal intensity time variant with coherence time o of the order of milliseconds.

Symbol duration T << o , thus received irradiance is time invariant over one symbol duration.

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Eric Korevaar et. alA typical reduction in intensity fluctuation with spatial diversity

One detector

Two detectors

Three detectors

Subcarrier Modulation - Spatial Diversity

Free Space Optics Characteristics Challenges Turbulence

- Subcarrier intensity multiplexing- Diversity schemes

Results and discussions

Wavelet ANN Receiver Final remarks

1 2 3 4 5 6 7 8 9 10-10

-5

0

5

10

15

20

Number of subcarrier

Norm

alis

ed S

NR @

BER

= 1

0-6

(dB

)

0.10.20.50.7

Log intensityvariance

Normalised SNR at BER of 10-6 against the number of subcarriers for various turbulence levels for BPSK

Increasing the number of subcarrier/users, resultsIn increased SNR

SNR gain compared with OOK

Error Performance – No Spatial Diversity

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20 25 30 35 4010

-10

10-8

10-6

10-4

10-2

SNR (dB)

BE

R

DPSKBPSK16-PSK8-PSK

Log intensityvariance = 0.52

0

22

)()/sin(loglog

2dIIpMMSNRQ

MBER e

BPSK based subcarrier modulation is the most power efficient

BPSK BER against SNR for M-ary-PSK for log intensity variance = 0.52

Error Performance – No Spatial Diversity

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10

20

30

40

50

60

70

Turbulence Regime

Dive

risty

Gai

n (d

B)

Weak

Saturation

Moderate

2 Photodetectors3 Photodetectors

Spatial Diversity GainSpatial diversity gain with EGC against Turbulence regime

Spatial Diversity Gain for EGC and SeLC

1 2 3 4 5 6 7 8 9 10-10

-5

0

5

10

15

20

25

No of Receivers

Link

mar

gin

(dB)

0.22

0.52

0.72

1

Log IntensityVariance

EGCSel.C

BER = 10-6

].)(1[2

))22exp((

1

1)(

220 llixK

n

i

NiiNSelCe exerfwNP

ni i

x1= Zeros of the nth order Hermite polynomial

ni i

w1

= Weight factor of the nth order Hermite polynomial

NARIK 200 2

Dominated by received irradiance,reduced by factor N on each link.

Link margin for SelC is lower than EGC by ~1 to ~6 dB

1 2 3 4 5 6 7 8 9 100

5

10

15

20

25

30

No of Receivers

Spat

ial D

iver

sity

Gai

n (d

B)

MRCEGC

Log Intensity variance

1

0.52

0.22

Most diversity gain region

The optimal but complex MRC diversity is marginally superior to the practical EGC

Spatial Diversity Gain for EGC and MRC

BER = 10-6

mx

i

ZEGCe

uuieKQw

dZdZPZKP

1

)2(1

0

2/

0

22

21

)(

)(1

)()(sin2

exp1

2/

0

0)(

,)(1

)(/

dS

IdIPIQP

N

IMRCMRCe

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Multiple-Input-Multiple-Output

BPSK Modu-Lator

and

Laser driver

d(t) ...

It1

It2

ItH

FSO CHANNEL

BPSK Subcarrier

Demodulator....

)(ˆ td

)(1 ti

)(2 ti

)(tiN

a2

a1

aN

Combiner

iT

By linearly combining the photocurrents using MRC, the individual SNRe on each link 2

122

H

jijiele I

HNRASNR

59

MIMO Performance

12 14 16 18 20 22 24 26

10-9

10-8

10-7

10-6

10-5

10-4

10-3

SNR (R*E[I])2 / No (dB)

BER

1X5MIMO1X8MIMO4X4MIMO2X2MIMO1X4MIMO

2/

0

,)(1 dSP Ne

m

juujj x

KwS

12

22 )]2(2exp[

sin2exp1)(

HN

ARIK

20

22

log intensity variance= 0.52

At BER of 10-6:

2 x 2-MIMO requires additional ~0.5 dB of SNR compared with 4-photodetector single transmitter-multiple photodetector system.

4 x 4-MIMO requires ~3 dB and ~0.8 dB lower SNR compared with single transmitter with 4 and 8-photodetectors , respectively.

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Free Space Optics Characteristics Challenges Turbulence

- Subcarrier intensity multiplexing- Diversity schemes

Results and discussions

Wavelet ANN Receiver Final remarks

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Transmission System - Receiver Models

TX Channel

Noise

…

+

Slicer

MF Equaliser Slicer Data out

CWT NN Slicer Data out

Data in

MMSE

Wavelet - NN

Data out

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PPM System – NN Equalization

PPMEncoder

h(t) ∑

NeuralNetwork

DecisionDevice

OpticalTransmitter

OpticalReceiver

n(t)

PPMDecoder

X(t)

MatchedFilter

ZjZj

Zj-1

.

Zj-n

.

Yj

Z(t)

M

0 0 1 0 Ts = M/LRb

XjM0 1 0 0

A feedforward back propagation neural network . ANN is trained using a training sequence at the operating SNR. Trained AAN is used for equalization

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Impulse Response of Equalized Channel

• Pulse are spread to adjust pulse .

• ISI depends on pulse spread

• Equalized response in a delta function which is equivalent to a impulse response of the ideal channel

Impulse response of unequalized channel

impulse response of equalized channel

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Results (1)

Adaptive linear equalizer with least mean square (LMS) algorithm is used.

The performance of ANN equalizer is almost identical to the linear equalizer.

Slot error rate performance of 8- PPM in diffuse channel with Drms of 5ns at 50 Mbps

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Results (2)

Unequalized performance at higher data rate is unacceptable at all SNR range Linear and neural equalization give almost identical performance.

Slot error rate performance of 8- PPM in diffuse channel with Drms of 5ns at 100 Mbps

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Results (3) - Wavelet-AI Receiver

SNR Vs. the RMS delay spread/bit duration

Wavelet

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68Wavelet-AI Receiver - Advantages and Disadvantages Complexity

- many parameters & computations. High sampling rates

- technology limited. Speed

- long simulation times on average machines. Similar performance to other equalisation techniques. Data rate independent

- data rate changes do not affect structure (just re-train). Relatively easy to implement with other pulse

modulation techniques.

Downlink

Uplink

Visible-light communication system

01

23

45

0

1

2

3

4

5200

400

600

800

1000

1200

1400

x[m]

Distribution of horizantal illuminance [lx]

y[m]

Illum

inan

ce[lx

]

Number of LEDs60 x 60 (4 set)

Distribution of illuminance

Visible Light Optical Wireless System with OFDM

FSO Network – Two Universities in Newcastle

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Agilent Photonic Research LabResearch Collaboration

Free space optical Du-plex communication

link (Northumbria

and Newcastle Universities)at a data rate of 155 MbpsOptical Fibre

A-Block

Agilent PhotonicResearch Lab

Collaborators

• Graz Technical University, Austria• Houston University, USA• University College London, UK• Hong-Kong Polytechnic University• Tarbiat Modares University, Iran• Newcastle University, UK• Ankara University, Turkey• Agilent, UK• Cable Free, UK• Technological University of Malaysia• Others•

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Final Remarks

Could the promise of optical wireless live up to reality? Yes!!

But Optical wireless must complement radio, not compete Industry must be bold in research and development Lower component cost, and single technology based

deviced Integration with existing systems Lover receiver sensitivity Of course more research and development at all

levels

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Summary Access bottleneck has been discussed

FSO introduced as a complementary technology

Atmospheric challenges of FSO highlighted

Subcarrier intensity modulated FSO (with and without spatial diversity) discussed

Wavelet ANN based receivers

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Acknowledgements

To many colleagues (national and international) and in particular to all my MSc and PhD students (past and present) and post-doctoral research fellows