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Professor Z GHASSEMLOOY Associate Dean for Research Optical Communications Research Group,

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/.

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Professor Z GHASSEMLOOY Associate Dean for Research Optical Communications Research Group,

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  1. 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

  2. Northumbria University at Newcastle, UK 2 Iran 2008

  3. Outline • Introduction • Why the need for optical wireless? • FSO • FSO - Issues • Some results • Final remarks 3 Iran 2008

  4. Wireless Wired Indoor Free-Space Optics (FSO) OCRG - Research Areas Optical Communications Optical Fibre Communications Photonic Switching • Pulse Modulations • Equalisation • Error control coding • Artificial neural network & • Wavelet based receivers • Fast switches • All optical routers • Chromatic dispersion • compensation using • optical signal processing • Pulse Modulations • Optical buffers • Optical CDMA • Subcarrier modulation • Spatial diversity • Artificial neural network/Wavelet based receivers 4 HK Poly-Univ. 2007

  5. OCRG -People • 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) • 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)

  6. Photonics -Applications • Photonics in communications: expanding and scaling 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

  7. Radio on Fibre Traditional Radio RF Source Traditional Optics Optical Wireless Lightwave Free Space Fibre Transmission Channel RF & Optical Communications - Integration

  8. Free Space Optical (FSO) Communications

  9. 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 What does It Offer ? Abundance of unregulated bandwidth - 200 THz in the 700-1500 nm range No multipath fading - Intensity modulation and direct detection 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  Pr2) 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 Iran 2008 11

  12. Access Network bottleneck (Source: NTT) 12 Iran 2008 12

  13. 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

  14. Optical Wireless Communications • Using optical radiation to communicate between two points through unguided channels • Types - Indoor - Outdoor (Free Space Optics) 14 Iran 2008

  15. Dominant term at 99.9% availability FSO - Basics • Cloud • Rain • Smoke • Gases • Temperature variations • Fog and aerosol DRIVER CIRCUIT SIGNAL PROCESSING 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), PHOTO DETECTOR This equation fundamentally ties FSO to the atmospheric weather conditions Link Range L 15

  16. FSO Link • Transmitter • Lasers 780,850,980,1550nm, also 10 microns • Beam control optics • Multiple transmit apertures to reduce scintillation problems • 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 • Wide dynamic range requirement due to very high clear air link margin • Automatic gain and transmitter power control

  17. Optical Components – Light Source For indoor applications LEDs are also used 17 • Eye safety -Class 1M Iran 2008

  18. Optical Components – Detectors Germanium only detectors are generally not used in FSO because of their high dark current. 18 Iran 2008

  19. 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 19

  20. 1.2 Sun Incandescent 1 0.8 1st window IR Normalised power/unit wavelength 0.6 Fluorescent 0.4 x 10 0.2 0 0.7 0.8 0.9 1.0 0.3 0.4 0.5 0.6 1.1 1.2 1.3 1.4 1.5 Wavelength (m) Power Spectra of Ambient Light Sources Pave)amb-light >> Pave)signal (Typically 30 dB with no optical filtering) 2nd window IR Iran 2008 20

  21. 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

  22. Cost Comparison 22 Source: Iran 2008

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

  24. 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 24 Iran 2008

  25. Hospitals FSO - Applications In addition to bringing huge bandwidth to businesses /homes FSO also finds applications in : • Others: • Inter-satellite communication • Disaster recovery • Fibre communication back-up • Video conferencing • Links in difficult terrains • Temporary links e.g. conferences Multi-campus university Cellular communication back-haul FSO challenges… 25 Iran 2008

  26. 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

  27. LOS - Hybrid Systems Video-conference for Tele-medicine CIMIC-purpose and disaster recovery 27 Iran 2008

  28. FSO - Challenges • Major challenges are due to the effects of: • CLOUD, • RAIN, • SMOKE, GASES, • TEMPERATURE VARIATIONS • FOG & AEROSOL DRIVER CIRCUIT SIGNAL PROCESSING To achieve optimal link performance, system design involves tradeoffs of the different parameters. PHOTO DETECTOR POINT A POINT B 28 Iran 2008

  29. FSO Challenges - Rain  = 0.5 – 3 mm 29 Iran 2008

  30. FSO Challenges - Physical ObstructionsPointing Stability and Swaying Buildings 30

  31. FSO Challenges –Aerosols Gases & Smoke 31

  32. 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 Iran 2008

  33. FSO Challenges - Fog 33 (H.Willebrand & B.S. Ghuman, 2002.) Iran 2008

  34. 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.

  35. FSO Challenges - Others • Background radiation • LOS requirement • Laser safety Iran 2008

  36. Free Space Optics • Characteristics • Challenges • Turbulence - Subcarrier intensity multiplexing - Diversity schemes • Results and discussions • Wavelet ANN Receiver • Final remarks

  37. FSO Challenges - Turbulence 37

  38. The atmosphere behaves like prism of different sizes and refractive indices Phase and irradiance fluctuation FSO Challenges - Turbulence Cause:Atmospheric inhomogeneity / random temperature variation along beam path. Result in deep signal fades that lasts for ~1-100 μs • Zones of differing density act as lenses, • 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. Irradiance PDF: Based on the modulation process the received irradiance is Irradiance PDF by Andrews et al (2001): Ix: due to large scale effects; obeys Gamma distribution Iy: due to small scale effects; obeys Gamma distribution Kn(.): modified Bessel function of the 2nd kind of order n σl2 : Log irradiance variance (turbulence strength indicator) Turbulence – Channel Models To mitigate turbulence effect we, employ subcarrier modulation with spatial diversity Iran 2008

  40. Turbulence Effect on OOK Threshold level No Intensity Fading No Pulse Bit “0” Pulse Bit “1” A/2 A A With Intensity Fading All commercially available systems use OOK with fixed threshold which results in sub-optimal performance in turbulence regimes 40 Iran 2008

  41. Turbulence Effect on OOK Using optimal maximum a posteriori (MAP) symbol-by-symbol detection with equiprobable OOK data: OOK based FSO requires adaptive threshold to perform optimally…. ….but subcarrier intensity modulated FSO does not 41

  42. . . . . SIM – System Block Diagram DC bias m(t) m(t)+bo d(t) Summing circuit Optical transmitter Subcarrier modulator Serial/parallel converter Data in Atmospheric channel ir d’(t) Subcarrier demodulator Spatial diversity combiner Photo- detector array Parallel/serial converter . . Data out 42

  43. 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

  44. 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

  45. Subcarrier Modulation -Transmitter Modulation index is constrained to avoid over modulation 45 Iran 2008

  46. Output power m(t) b0 Drive current Subcarrier Modulation -Transmitter 5-subcarriers Iran 2008

  47. Photo-current R = Responsivity, I = Average power,  = Modulation index, m(t) = Subcarrier signal di(t) = Data SIM -Receiver 47

  48. 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

  49. SIM - Spatial Diversity • Single-input-multiple-output • Multiple-input-multiple-output (MIMO) 49

  50. DiversityCombiningTechniques Selection Combining (SELC). No need for phase information Equal Gain Combining (EGC) Maximum Ratio Combining (MRC) [Complex but optimum] SIM - Spatial Diversity Assuming identical PIN photodetector on each links, the photocurrent on each link is: ai is the scaling factor 50

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