Free Space Optical Communications

Professor Z GHASSEMLOOY

Associate Dean for Research

Optical Communications Research Group, School of Computing, Engineering and Information Sciences The University of Northumbria Newcastle, U.K.

http://soe.unn.ac.uk/ocr/ Iran 2008 1

Northumbria University at Newcastle, UK

2 2 Iran 2008

Outline

     

Introduction Why the need for optical wireless?

FSO FSO - Issues Some results Final remarks

3 Iran 2008 3

OCRG -

Research Areas

Optical Communications

Wired Wireless Optical Fibre Communications • • • •

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

Photonic Switching • •

Fast switches All optical routers

Indoor • • • •

Pulse Modulations Equalisation Error control coding Artificial neural network & Wavelet based receivers Free-Space Optics (FSO)

 

Subcarrier modulation Spatial diversity



Artificial neural network/Wavelet based receivers

4 HK Poly-Univ. 2007

OCRG People

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)

PhD

• 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)

Photonics Applications

• Photonics in communications: expanding and scaling 6 Long-Haul Metropolitan Home access Board -> Inter-Chip -> Intra-Chip • Photonics: diffusing into other application sectors Health (“bio-photonics”) Environment sensing Security imaging Iran 2008

RF & Optical Communications Integration

Radio on Fibre Traditional Radio Traditional Optics Optical Wireless

Fibre Free Space Transmission Channel

Free Space Optical (FSO) Communications

The Problem?

AND THAT IS ?

….. BANDWIDTH when and where required.

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.

9

10

Optical Wireless Communication

Abundance of unregulated bandwidth -

200 THz in the 700-1500 nm range

No multipath fading -

Intensity modulation and direct detection

What does It Offer ?

High data rate –

In particular line of sight (in and out doors)

Improved wavelength reuse capability Flexibility in installation Secure transmission Flexibility -

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

Iran 2008 10

11

Optical Wireless Communication

D r a w b a c k s

Multipath induced dispersion (non-line of sight, indoor) -

Limiting data rate

SNR can vary significantly with the distance and the ambient noise (Note SNR 

P r

2 ) Limited transmitted power -

Eye safety (indoor)

High transmitted power -

Outdoor

Receiver sensitivity May be high cost -

Compared with RF

Large area photo-detectors -

Limits the bandwidth

Limited range:

Indoor:

ambient noise is the dominant (20-30 dB larger than the signal level

. Outdoor:

Fog and other factors

11 Iran 2008

Access Network bottleneck

12 12 12 (Source: NTT) Iran 2008

Access Network Technology

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 13 Iran 2008 13

Optical Wireless Communications



Using optical radiation to communicate between two points through unguided channels



Types

- Indoor

- Outdoor (Free Space Optics)

14 Iran 2008 14

FSO Basics

     

Cloud Rain Smoke Gases Temperature variations Fog and aerosol

Transmission of optical radiation through the atmosphere obeys the Beer Lamberts’s law: 2

P r



P t



d

1 2 (

D

2

L

) 2  10  

L

/ 10 Dominant term at 99.9% availability α

:

Attenuation coefficient dB/km –

Not controllable and is roughly independent of wavelength in heavy attenuation conditions.

d

1 and

d

2 : Transmit and receive aperture diameters (m)

D: B

eam divergence (mrad)(1/

e

for Gaussian beams; FWHA for flat top beams), This equation fundamentally ties FSO to the atmospheric weather conditions Link Range L 15

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

Optical Components – Light Source

Operating Wavelength (nm)

~850 ~1300/~1550

Laser type Remark

VCSEL Fabry-Perot/DFB Cheap, very available, no active cooling, reliable up to ~10Gbps, Long life, compatible with EDFA, up to 40Gbps 50 –65 times as much power compared with 780-850 nm ~10,000 Quantum cascade laser (QCL) Expensive, very fast and highly sensitive Ideal for indoor (no penetration through window)

For indoor applications LEDs are also used



Eye safety -

17 Class 1M Iran 2008 17

Optical Components – Detectors

Material/Structure Silicon PIN InGaAs PIN Silicon APD Wavelength (nm)

300 1000 400 – 1100 – 1700 – 1000

Responsivity (A/W) Typical sensitivity

0.5

0.9

-34dBm@ 155Mbps -46dBm@ 155Mbps 77 -52dBm@ 155Mbps 9

Gain

1 1 150

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

1000 – 1700 ~10,000 10

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

18 Iran 2008 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  25 W @ date rate up to 150 Mbps and  =780nm, short range = 980nm 60 W @ data rate up to 622 Mbps and  40 W @ data rate up to 1.5 Gbps and  = 780nm = 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 19

Power Spectra of Ambient Light Sources

1.2

1 0.8

0.6

0.4

0.2

0

P ave)amb-light >> P ave)signal (Typically 30 dB with no optical filtering) Sun Incandescent

1 st window IR

Fluorescent

x 10 2 nd window IR 20 20 Wavelength (  m) Iran 2008

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 Iran 2008 21

Cost Comparison

22 Source: 22 Iran 2008

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 license o No Link Engineering o Management via SNMP, RS232 o or GSM connection  Last mile o 155 Mbps STM-1 links o 622 Mbps ATM link for Banks etc

When Did It All Start?

800BC - Fire beacons (ancient Greeks and Romans) 150BC - Smoke signals (American Indians) 1791/92 - Semaphore (French) 1880 Alexander Graham Bell demonstrated the photophone –

FSO (

THE GENESIS)

1 st

24 (www.scienceclarified.com) 1960s - Invention of laser and optical fibre 1970s - FSO mainly used in secure military applications 1990s to date - Increased research & commercial use due to successful trials 24 Iran 2008

FSO Applications

In addition to bringing huge bandwidth to businesses /homes FSO also finds applications in : 25

Hospitals Cellular communication back-haul Others:



Inter-satellite communication



Disaster recovery



Fibre communication back-up



Video conferencing



Links in difficult terrains



Temporary links Multi-campus university e.g. conferences

25

FSO challenges…

Iran 2008

Hybrid FSO/RF Wireless Networks

 

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

LOS Hybrid Systems

27 Video-conference for Tele-medicine CIMIC-purpose and disaster recovery 27 Iran 2008

FSO Challenges

Major challenges are due to the effects of: CLOUD, RAIN, SMOKE, GASES, TEMPERATURE VARIATIONS FOG & AEROSOL

POINT A To achieve optimal link performance, system design involves tradeoffs of the different parameters.

28 POINT B Iran 2008 28

FSO Challenges - Rain

 = 0.5 – 3 mm

Effects

Photon absorption

Options Remarks

 Increase transmit optical power Effect not significant 29 Iran 2008 29

FSO Challenges Physical Obstructions Pointing Stability and Swaying Buildings

Effects Solutions

 Loss of signal  Spatial diversity  Multipath induced  Mesh architectures: using  Distortions Low power due to  beam divergence and spreading diverse routes Ring topology: User ’ s n/w become nodes at least one hop away from the ring  Fixed tracking (short  Short term loss of buildings) signal  Active tracking (tall buildings) 30

Remarks

 May be used for urban areas, campus etc.

 Low data rate  Uses feedback

FSO Challenges

–

Aerosols Gases & Smoke

Effects

 Mie scattering  Photon absorption  Rayleigh scattering

Solutions

 Increase transmit power  Diversity techniques

Remarks

 Effect not severe 31

FSO Challenges - Fog

 = 0.01 - 0.05 mm In 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.

32

Effects

 Mie scattering  Photon absorption

Options

 Increase transmit optical power  Hybrid FSO/RF

Remarks

 Thick fog limits link range to ~500m  Safety requirements limit maximum optical power 32 Iran 2008

FSO Challenges - Fog

Weather condition Dense fog Thick fog Moderate fog Light fog Thin fog Haze Light haze Clear Very clear Precipitation Snow Snow Snow t Cloudburs Heavy rain Snow Snow Snow Medium rain Light rain Drizzle Amount (mm/hr) Visibility dB Loss/km Typical Deployment Range (Laser link ~20dB margin) 100 25 12.5

2.5

0.25

0 m 50 m 200 m 500 m 770 m 1 km 1.9 km 2 km 2.8 km 4 km 5.9 km 10 km 18.1 km 20 km 23 km 50 km -271.65

-59.57

-20.99

-12.65

-9.26

-4.22

-3.96

-2.58

-1.62

-0.96

-0.44

-0.24

-0.22

-0.19

-0.06

122 m 490 m 1087 m 1565 m 1493 m 3238 m 3369 m 4331 m 5566 m 7146 m 9670 m 11468 m 11743 m 12112 m 13771 m

33 (H.Willebrand & B.S. Ghuman, 2002.) Iran 2008 33

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.

FSO Challenges - Others

  

Background radiation LOS requirement Laser safety

35 Iran 2008



Free Space Optics



Characteristics

 

Challenges Turbulence - Subcarrier intensity multiplexing - Diversity schemes



Results and discussions



Wavelet ANN Receiver



Final remarks

FSO Challenges Turbulence

Effects Options



Irradiance fluctuation



Diversity techniques (scintillation)



Forward error control

 Image dancing  Phase fluctuation

control



Robust modulation

 Beam spreading

techniques

 Polarisation fluctuation  Adaptive optics  Coherent detection not used due to Phase fluctuation 37

Remarks

 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

Cause: path.

Atmospheric inhomogeneity / random temperature variation along beam The atmosphere behaves like prism of different sizes and refractive indices

38

Phase and irradiance fluctuation

Result in deep signal fades that • Zones of differing density act as lenses, lasts for ~1 100 μs scattering light away from its intended path. • Thus,

multipath

.

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.

Iran 2008

39

Turbulence – Channel Models

Irradiance PDF:

p I

(

I

)  1 2  

l

1 exp

I

   (ln(

I

/

I

0 )  2 

l

2 

l

2 / 2 ) 2

I

 0

Model

Log Normal I-K K Rayleigh/Negative Exponential Gamma-Gamma

Comments

Simple; tractable Weak regime only Weak to strong turbulence regime Strong regime only Saturation regime only All regimes Irradiance PDF by Andrews et al (2001): Based on the modulation process the received irradiance is

I



I x y p

(

I

)  2 (   (  ) ( )     ) (  ) / 2

I

(    2 )  1     ( 2 

I

)

I

 0 I x : due to large scale effects; obeys

Gamma distribution

     exp   ( 1  0 .

49 

l

2 1 .

11 

l

12 / 5 ) 7 / 6      exp   ( 1  0 .

0 .

51 

l

69 

l

12 2 / 5 ) 5 / 6      1     1  1     1 To mitigate turbulence effect we, employ subcarrier modulation with spatial diversity I y : due to small scale effects; obeys Gamma distribution K

n

(.): modified Bessel function of the 2nd kind of order

n

σ

l

2 : Log irradiance variance (turbulence strength indicator) Iran 2008

Turbulence Effect on OOK

No Pulse Bit “0” No Intensity Fading Threshold level A/2 With Intensity Fading A Pulse Bit “1” A

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

40 Iran 2008 40

Turbulence Effect on OOK

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

d

ˆ (

t

)  arg max

d P

(

i r

/

d

(

t

))     0 exp exp      ((

i r

ln(

I



RI

) 2 

i r

2 ) 2  2 /

I

0 ) 2 

l

 2 

l

2 / 2  2 

dI

1 2 

l

2 1 .

I

0.5

0.45

0.4

Noise variance 0.5*10 -2 10 -2 3*10 -2 5*10 -2 0.35

0.3

OOK based FSO requires adaptive threshold to perform optimally….

0.25

0.2

….but subcarrier intensity modulated FSO does not

0.15

0.1

0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

Log Intensity Standard Deviation

0.8

0.9

41 1

SIM – System Block Diagram

d

(

t

) Data in Serial/parallel converter .

.

Subcarrier modulator DC bias

m

(

t

) .

.

Summing circuit

m

(

t

)+

b o

Optical transmitter Atmospheric channel

d

’(

t

) Data out Parallel/serial converter .

.

Subcarrier demodulator Spatial diversity combiner

i r

Photo detector array 42

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) 43

SIM – Our Contributions

Multiple-input-multiple-output (MIMO)

photodetectors) to mitigate scintillation effect in a IM/DD FSO link  overcomes

temporary link blockage

(an array of transmitters/ (birds and misalignment) when combined with a wide laser beamwidth, therefore no need for an active tracking  provides

independent aperture averaging

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

45

Subcarrier Modulation Transmitter

A 1 g

(

t

) PSK modulator at cosw c1 t Input data

d

(

t

) Serial to Parallel Converter .

.

A 2

.

.

.

.

A M g

(

t

) PSK modulator at cosw c2 t

m

(

t

) 

j M

  1

A j g

(

t

) cos(

w cj t

 

j

)

Σ

m

(

t

)

Σ

Laser driver Atmopsheric channel DC bias

b 0 g

(

t

) PSK modulator at cosw cM t Modulation index is constrained to avoid over modulation 45   [

Rh

0

P t

, 0  0

N c

' ]  1 Iran 2008

46 1 2

Subcarrier Modulation Transmitter

0 -1 -2

m

(

t

) 

j M

  1

A j g

(

t

) cos(

w cj t

 

j

) 5-subcarriers -3 -4 -5 0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1 Output power

P



P

max 2   [

Rh

0

P t

, 0  0

N c

' ]  1 m(t)

b 0

Drive current Iran 2008

SIM Receiver

SNR ele

 (

IRA

) 2 2  2

x

g(-t) PSK Demodulator Sampler

P r



i N

 

c

1

h i P t

,

i

 1  

d i

(

t

) cos( 2 

f i t

  cosw c1 t

n

(

t

) Photodetector PSK Demodulator at cosw c2 t

i r

.

.

.

Photo-current

PSK Demodulator at cosw cM t

i r

(

t

)

R



R I

( 1  

m

(

t

)) 

n

(

t

) = Responsivity,

I

= Average power,  = Modulation index,

m

(

t

) = Subcarrier signal di(

t

) = Data 47 Parallel to Serial Converter

d

ˆ (

t

) Output data

Subcarrier Modulation

  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

But..

 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 48 Iran 2008 48

SIM Spatial Diversity

 

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

49

F S O C H A N N E L

SIM Spatial Diversity

i

1 (

t

) a 1

i

2 (

t

)

i N

(

t

) a 2 .

.

.

a N .

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

:

i T

(

t

)

i ri

(

t

)  PSK Subcarrier Demodulator

d

ˆ (

t

)

R N I i

 1 

M



j A j g

(

t

) cos(

w cj t

 

j

) 

n i

(

t

)

a i

is the scaling factor

Maximum Ratio Combining (MRC)

a i i i

Diversity Combining Techniques

a

1 Equal Gain Combining (EGC) 

a

2  ...



a N

Selection Combining (SELC). No need for phase information

i T

(

t

)  max(

i

1 (

t

),

i

2 (

t

)...

i N

(

t

)) 50

SIM Spatial Diversity

–

Assumptions Made

 Spacing between detectors > the transverse correlation size ρ o of the laser radiation, because ρ o = 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.

51

Subcarrier Modulation Spatial Diversity

52 One detector Two detectors Three detectors A typical reduction in intensity fluctuation with spatial diversity Eric Korevaar et. al Iran 2008



Free Space Optics



Characteristics

 

Challenges Turbulence - Subcarrier intensity multiplexing - Diversity schemes



Results and discussions



Wavelet ANN Receiver



Final remarks

Error Performance – No Spatial Diversity

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

20 15 10 5 Increasing the number of subcarrier/users, results In increased SNR 0 -5 -10 1 2 3 4 5 6

Number of subcarrier

7 8 Log intensity variance 0.1

0.2

0.5

0.7

9 10 SNR gain compared with OOK

Error Performance – No Spatial Diversity

55

BPSK BER against SNR for M-ary-PSK for log intensity variance = 0.5

2

10 -2 10 -4 DPSK BPSK 16-PSK 8-PSK Log intensity variance = 0.5

2 BPSK based subcarrier modulation is the most power efficient 10 -6 10 -8

BER

 2 log 2

M

0  

Q



SNR e

log 2

M

sin(  /

M

 )

p

(

I

)

dI

10 -10 20 25 SNR (dB) 30 35 40 Iran 2008

Spatial Diversity Gain

Spatial diversity gain with EGC against Turbulence regime

Saturation

70 60 50 40 30 20 10

Weak

2 Photodetectors 3 Photodetectors

Moderate Turbulence Regime

Iran 2008 56

Spatial Diversity Gain for EGC and SeLC

25 20 15 Log Intensity Variance 0.2

2 0.5

2 0.7

2 1 Link margin for SelC is lower than EGC by ~1 to ~6 dB 10 5 0 Dominated by received irradiance, reduced by factor

N

on each link.

-5 -10 1 EGC Sel.C

BER = 10 -6

2 3

P e

(

SelC

)  2

N N

 4 5 6 7 8 9

i n

  1 [

w i



No of Receivers

1 

erf

(

x i

) 

N

 1 .

e

( 

K

0 2 exp( 2

x i

 

i i n

= Zeros of the  1

n

th order Hermite polynomial 10 2 

l

 

l

2 )) ]  

n

= Weight factor of the

n

th order

i

 1 Hermite polynomial

K

0 

RI

0

A

2  2

N

Spatial Diversity Gain for EGC and MRC

30

BER = 10 -6

25 Log Intensity variance 1

P e

(

EGC

)  0      / 0 2 exp    2

K

1 2 sin 2 (  )

Z

2  1 

m

 1

w i Q

(

K

1

e

(

x i

2 

u

 

u

) )

P Z

(

Z

)

d



dZ

20 MRC EGC 15 10 5 0.5

2 0.2

2

P e

(

MRC

)   1  0  

Q

 

MRC

/

I

 

P I

 (

I

 )

d I

  0  /  2

S

(  ) 

N d

 , 0 1 2 3

Most diversity gain region

4 5 6

No of Receivers

7 8 9 10

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

58

Multiple-Input-Multiple-Output

I t1 d

(

t

)

I t2

BPSK Modu Lator and Laser driver

I tH

.

.

.

F S O C H A N N E L

i

1 (

t

) a 1 Combiner

i

2 (

t

)

i N

(

t

) a 2 .

.

.

.

a N 

i T

BPSK Subcarrier Demodulator

d

ˆ (

t

) By linearly combining the photocurrents using MRC, the individual

SNRe

on each link

SNR ele



i

  

RA

2

N

 2 59

H j H

  1

I ij

  2

MIMO Performance

10 -3 10 -4 10 -5 10 -6 10 -7 10 -8 10 -9 log intensity variance= 0.5

2 1X5MIMO 1X8MIMO 4X4MIMO 2X2MIMO 1X4MIMO

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.

26

P e

12  1  14 16 18 20 22

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

/   0  2

S

(  ) 

N d

 , 24

S

(  )  1 

j m

  1

w j

exp 60

K

2  

K

2 2 2 sin 2  exp[ 2 (

x j RI

0

A

2

N

 2

H

2 

u

 

u

)]



Free Space Optics



Characteristics

 

Challenges Turbulence - Subcarrier intensity multiplexing - Diversity schemes

 

Results and discussions Wavelet ANN Receiver



Final remarks

Transmission System Models Receiver

Data in TX Channel

+

Noise Data out

…

Slicer MMSE Data out Slicer Equaliser MF Data out Slicer Wavelet - NN NN CWT

Iran 2008 62

PPM System – NN Equalization

n

(

t

)

M

0 1 0 0 PPM Encoder

X j

Optical Transmitter

X

(

t

)

h

(

t

) ∑

Z(t )

Optical Receiver

M

0 0 1 0 PPM Decoder Decision Device

Y j

Neural Network .

Z j Z j

-1 .

Z j

-

n Z j T s = M

/

LR b

Matched Filter    A feedforward back propagation neural network .

ANN is trained using a training sequence at the operating SNR.

Trained AAN is used for equalization 63 Iran 2008

Impulse Response of Equalized Channel

64 Impulse response of unequalized channel 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 Iran 2008

Results (1)

Slot error rate performance of 8- PPM in diffuse channel with D rms Mbps of 5ns at 50 65  Adaptive linear equalizer with least mean square (LMS) algorithm is used.

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

Iran 2008

Results (2)

Slot error rate performance of 8- PPM in diffuse channel with D rms Mbps of 5ns at 100 66  Unequalized performance at higher data rate is unacceptable at all SNR range  Linear and neural equalization give almost identical performance.

Iran 2008

Results (3) Wavelet-AI Receiver

67 Wavelet SNR Vs. the RMS delay spread/bit duration Iran 2008

Wavelet-AI Receiver Disadvantages Advantages and

      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.

68 Iran 2008

Visible Light Optical Wireless System with OFDM

Visible-light communication system

Down link Up link Distribution of illuminance Distribution of horizantal illuminance [lx]

Number of LEDs 60 x 60 (4 set)

1400 1200 1000 800 600 400 200 5 4 3 y[m] 2 1 0 0 1 2 x[m] 3 4 5

FSO Network – Two Universities in Newcastle

Agilent Photonic Research Lab

Agilent Photonic Research Lab Research Collaboration A-Block 71 Optical Fibre Free space optical Du-plex communication link (Northumbria and Newcastle Universities) at a data rate of 155 Mbps Iran 2008

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

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

levels research and development at all

Iran 2008 73

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

 74 Iran 2008 74

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

75 Iran 2008