FREE-SPACE OPTICAL COMMUNICATION USING SUBCARRIER INTENSITY MODULATION POPOOLA, Wasiu O. (2nd Year PhD student) Optical Communication Research Lab., CEIS [email: wasiu.popoola@unn.ac.uk ] Supervision Team: Fary Ghassemlooy – Director of studies Joseph Allen Erich Leitgeb ( University of Technology, Graz, Austria.) Steven Gao (now at University of Surrey)

Research Plan (1)

Research Plan (2)

Subcarrier Intensity Modulation (with and without diversity) Outline Problem definition FSO Introduction FSO challenges Subcarrier Intensity Modulation (with and without diversity) Results and Discussions Summary and Future work

FIBRE BASED RING NETWORK Problem Definition C O H U B REGIONAL FIBRE RING METRO FIBRE RING N FIBRE BASED RING NETWORK RF DOMINATED ACCESS NETWORK 75% of businesses are within one mile of fibre backbone, yet only 5% have access to it - RHK B Business C O Central office H Home U University Building N Network node OPTICAL FIBRE COPPER CABLE

Access Network bottleneck (Source: NTT)

Access network tech. xDSL: Radio link: Cable: FTTx: Copper based (limited bandwidth)- Phone and data combined 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

Free- space optical communication What is it? THE USE OF OPTICAL RADIATIONS TO COMMUNICATE BETWEEN TWO POINTS THROUGH UNGUIDED CHANNELS

Unguided channels SPACE WATER (H. Hemmati, NASA) ATMOSPHERE

FSO basics Link Range DRIVER CIRCUIT CLOUD, RAIN, SMOKE, GASES, PROCESSING SIGNAL PHOTO DETECTOR DRIVER CIRCUIT CLOUD, RAIN, SMOKE, GASES, TEMPERATURE VARIATIONS FOG & AEROSOL POINT A POINT B Link Range

Operating Wavelength (nm) FSO components FSO Optical (Laser diode) Sources 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 40Gbps ~10,000 Quantum cascade laser (QCL) Expensive, very fast and highly sensitive Selected FSO Detectors Material/Structure Wavelength (nm) Responsivity (A/W) Typical sensitivity Gain Silicon PIN 300 – 1100 0.5 -34dBm@ 155Mbps 1 InGaAs PIN 1000 – 1700 0.9 -46dBm@ 155Mbps Silicon APD 400 – 1000 77 -52dBm@ 150 InGaAs APD 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.

Preceive = Ptransmit * exp(-αR) FSO basics The transmission of optical radiation through the atmosphere obeys the Beer-Lamberts’s law which says: α -- Attenuation coefficient that results from absorption and scattering from the constituents of the atmosphere R – Link Range Preceive = Ptransmit * exp(-αR) This equation fundamentally ties FSO to the atmospheric weather conditions

FSO Features Similar bandwidth/data rate as optical fibre Very narrow beam – inherent security No EM interference No license issues Cheap (cost about $4/Mbps/Month according to fSONA)

Cost comparison Source:

FSO Features Fast to deploy (few hours) Transferable (no sunk cost) Suffers atmospheric effects most deleterious being thick fog Strictly line of sight - Pointing, Tracking and Alignment issues FSO can therefore complement/co-exist with all existing access network tech. to guarantee end users improved quality and more services

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 – 1st FSO (THE GENESIS) (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

Some areas of use In addition to bringing huge bandwidth to businesses /homes FSO also finds applications in : Multi-campus university Hospitals 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 Link Range Major challenges are due to the effects of: CLOUD, RAIN, SMOKE, GASES, TEMPERATURE VARIATIONS FOG & AEROSOL PROCESSING SIGNAL PHOTO DETECTOR DRIVER CIRCUIT POINT A POINT B Link Range

FSO challenges Aerosols Smoke Gases CHALLENGE EFFECTS OPTIONS REMARK GASES & SMOKE Mie scattering Photon absorption Rayleigh scattering Increase transmit power Diversity techniques Effect not severe

FSO challenges Rain CHALLENGE EFFECTS OPTIONS REMARK RAIN Photon absorption Increase transmit optical power Effect not significant

FSO challenges Fog CHALLENGE EFFECTS OPTIONS REMARK FOG Mie scattering Photon absorption Increase transmit power Hybrid FSO/RF Thick fog limits link range to ~500m Safety requirements limit maximum optical power Fog effect on performance…

Fog effect Weather condition Precipitation Amount (mm/hr) Visibility dB Loss/km Typical Deployment Range (Laser link ~20dB margin) Dense fog 0 m 50 m -271.65 122 m Thick fog 200 m -59.57 490 m Moderate fog Snow 500 m -20.99 1087 m Light fog Snow Cloudburst 100 770 m 1 km -12.65 -9.26 1565 m 1493 m Thin fog Snow Heavy rain 25 1.9 km 2 km -4.22 -3.96 3238 m 3369 m Haze Snow Medium rain 12.5 2.8 km 4 km -2.58 -1.62 4331 m 5566 m Light haze Snow Light rain 2.5 5.9 km 10 km -0.96 -0.44 7146 m 9670 m Clear Snow Drizzle 0.25 18.1 km 20 km -0.24 -0.22 11468 m 11743 m Very clear 23 km 50 km -0.19 -0.06 12112 m 13771 m (H.Willebrand & B.S. Ghuman, 2002.)

FSO challenges Turbulence CHALLENGE EFFECTS OPTIONS REMARK TURBULENCE Irradiance fluctuation (scintillation) Image dancing Phase fluctuation Beam spreading Polarisation fluctuation Diversity techniques Forward error control Robust modulation techniques Adaptive optics Significant for long link range (>1km) Turbulence and thick fog do not occur together Others: Building sway Background radiation LOS requirement Laser safety

Atmospheric turbulence CAUSE: Atmospheric random temperature variation along beam path. The atmosphere behaves like prisms of different sizes and refractive indices Phase and irradiance fluctuation (fading) Incoming optical radiation Eddies of different sizes and refractive indices Depends on: Altitude/Pressure, Wind speed, Temperature and relative beam size.

Turbulence models Model Remarks Log Normal Simple; tractable but for weak regime only K Strong regime only I-K Weak to strong turbulence regime Gamma-Gamma All regimes Negative Exponential Saturation regime only

Turbulence models Log Normal Simple; tractable but for weak regime only I : Received irradiance Io: mean irradiance without turbulence σl2 : Log irradiance variance (turbulence strength indicator) Irradiance PDF : Gamma-Gamma All regimes Based on modulation concept i.e. Irradiance PDF by Andrews et al (2001): I : Received irradiance Ix: due to large scale effects; obeys Gamma distribution Iy: due to small scale effects; Kn(.): modified Bessel function of the 2nd kind of order n σl2 : Log irradiance variance (turbulence strength indicator)

Turbulence effect on OOK Using optimal maximum a posteriori (MAP) symbol-by-symbol detection with equiprobable OOK data: OOK threshold level at various turbulence levels Increasing turbulence effect OOK based FSO requires adaptive threshold to perform optimally….

Turbulence effect on OOK No Intensity Fading No Pulse Bit “0” Threshold level A/2 Pulse Bit “1” A With Intensity Fading A All commercially available systems use OOK with fixed threshold which results in sub-optimal performance in turbulence regimes

Subcarrier modulation The need for adaptive threshold is circumvented through, subcarrier modulation + - Standard RF BPSK modulator

Subcarrier modulation TRANSMITTER

Subcarrier modulation 5-subcarriers b0 Drive current Output power m(t)

Subcarrier modulation RECEIVER R = Responsivity P = Average power = Modulation index m(t) = Subcarrier signal

Subcarrier modulation Performs optimally without adaptive threshold as is the case with optimal OOK Efficient coherent modulation techniques such as PSK, QAM can be easily used because the 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 It 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..

Selection Combining (Sel.C) Spatial diversity Diversity Combining Techniques Maximum Ratio Combining (MRC) Equal Gain Combining (EGC) Selection Combining (Sel.C)

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

Performance metrics System performance analysis is carried out considering the following metrics: 1. Average Bit-Error-Rate (BER) Models the number of bits received in error as a fraction of total transmitted bits SNRe = BPSK subcarrier signal-to-noise ratio p(I) = Irradiance PDF 2. Outage Probability (Po) Measures the probability that the instantaneous BER is greater than a pre-determined/specified threshold level BER*: Threshold BER

Some results Normalised SNR at BER of 10-6 against the number of subcarriers for various turbulence levels (No diversity) Increasing the number of Subcarrier/users, results In increasing SNR Gained SNR Compared with OOK

Some results BPSK based subcarrier modulation is the most BER against SNR for M-ary-PSK for log intensity variance = 0.52. (No diversity) BPSK based subcarrier modulation is the most power efficient

Some results Outage probability against power margin for various fading strength (No diversity) Power (dBm) needed to achieve outage probability, Po m (dBm)

Some results Spatial diversity gain with EGC against Turbulence regime

Some results Spatial diversity gain (EGC and MRC) against the number of receivers. The most diversity gain is obtained with up to 4 photodetectors The optimal but complex MRC diversity is marginally superior to the practical EGC Most diversity gain region

Access bottleneck has been discussed 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

Progress chat March 2008 On going Completed

Publications

Practical FSO implementation and data analysis Future work Practical FSO implementation and data analysis (Joint project with Newcastle University) FSO with forward error control SIM average power reduction

Appreciation Academic Staff: Prof. Fary Ghassemlooy ; J.I.H. Joe Allen; Dr. Erich Leitgeb; Dr. Steven Gao; Dr. Krishna Busawon and Dr. Wai Pang. My Colleagues: Wisit, Ming-Feng, Maryam, Sujan , Kamal, Rupak and every member of NCRLab

Research studentship Acknowledgement I will like to acknowledge Northumbria University for the following awards: ORSA and Research studentship

Questions and Comments Thank you. Questions and Comments

FSO availability Service Availability (%) Cities Range (m) 99.5 Phoenix – atmospherically excellent Denver – atmospherically good Seattle – atmospherically fair London – atmospherically poor 10,000+ 2400 1200 630 99.9 5200 850 420 335 99.99 460 290 255 185 Availability ranges are based upon two 125/155 Mbit/s FSO transceivers that are located outdoors and transmitting through clear air under normal operating conditions. (Bloom, S. et al. 2003)