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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 OCRG - Research Areas Optical Communications Wired Optical Fibre Communications • Chromatic dispersion compensation using optical signal processing • Pulse Modulations • Optical buffers • Optical CDMA Wireless Photonic Switching Indoor • Pulse Modulations • Equalisation • Error control coding • Artificial neural network & Wavelet based receivers • Fast switches • All optical routers 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) 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 RF Radio on Fibre Traditional Radio Lightwave Source RF & Optical Communications Integration 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. 10 Iran 2008 11 Optical Wireless Communication Multipath induced dispersion (non-line of sight, indoor) - Limiting data rate D r a w b a c k s 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 11 Iran 2008 12 Access Network bottleneck (Source: NTT) 12 12 Iran 2008 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 FSO - Basics SIGNAL PROCESSING Cloud Rain Smoke Gases Temperature variations Fog and aerosol PHOTO DETECTOR DRIVER CIRCUIT Transmission of optical radiation through the atmosphere obeys the BeerLamberts’s law: 2 d2 L / 10 Pr Pt 2 10 d1 ( D L) 2 Dominant term at 99.9% availability 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 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 17 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 40Gbps 50–65 times as much power compared with 780-850 nm ~10,000 Quantum cascade Expensive, very fast and highly laser (QCL) sensitive Ideal for indoor (no penetration through window) For indoor applications LEDs are also used Eye safety -17 Class 1M Iran 2008 18 Optical Components – Detectors Material/Structure Wavelength (nm) Responsivity Typical (A/W) sensitivity Gain 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 Quantum –well and Quatum-dot (QWIP&QWIP) 10 ~10,000 Germanium only detectors are generally not used in FSO because of their high dark current. 18 Iran 2008 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 Power Spectra of Ambient Light Sources Normalised power/unit wavelength 1.2 Pave)amb-light >> Pave)signal (Typically 30 dB with no optical filtering) Sun 1 Incandescent 0.8 1st window IR 0.6 2nd window IR Fluorescent 0.4 x 10 0.2 0 1.5 1.4 1.3 1.2 Wavelength (m) 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 20 Iran 2008 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 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 24 When Did It All Start? 800BC 150BC 1791/92 - Fire beacons (ancient Greeks and Romans) - Smoke signals (American Indians) - 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 FSO - Applications In addition to bringing huge bandwidth to businesses /homes FSO also finds applications in : Hospitals Others: Inter-satellite communication Disaster recovery Fibre communication back-up Multi-campus university Video conferencing Links in difficult terrains Temporary links e.g. conferences FSO challenges… Cellular communication back-haul 25 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 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, GASES, SIGNAL PROCESSING SMOKE, PHOTO DETECTOR DRIVER CIRCUIT RAIN, TEMPERATURE VARIATIONS FOG & AEROSOL To achieve optimal link performance, system design involves tradeoffs of the different parameters. POINT A POINT B 28 Iran 2008 29 FSO Challenges - Rain = 0.5 – 3 mm Effects Photon absorption Options Remarks Increase transmit Effect not significant optical power 29 Iran 2008 FSO Challenges - Physical Obstructions Pointing Stability and Swaying Buildings Effects Loss of signal Multipath induced Distortions Low power due to beam divergence and spreading Short term loss of signal Solutions 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) 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 31 Remarks Effect not severe 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. Effects Options Increase transmit optical power Hybrid FSO/RF Mie scattering Photon absorption Remarks Thick fog limits link range to ~500m Safety requirements limit maximum optical power 32 Iran 2008 33 FSO Challenges - Fog Weather condition Precipitation Amount (mm/hr) Visibility Dense fog Thick fog dB Loss/km Typical Deployment Range (Laser link ~20dB margin) 0m 50 m -271.65 122 m 200 m -59.57 490 m 500 m -20.99 1087 m Moderate fog Snow Light fog Snow Cloudburs t 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 23 km 50 km -0.19 -0.06 12112 m 13771 m Very clear (H.Willebrand & B.S. Ghuman, 2002.) 33 Iran 2008 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 Free Space Optics Characteristics Challenges Turbulence - Subcarrier intensity multiplexing - Diversity schemes Results and discussions Wavelet ANN Receiver Final remarks FSO Challenges - Turbulence Effects Irradiance fluctuation (scintillation) Image dancing Phase fluctuation Beam spreading Polarisation fluctuation Options Diversity techniques Forward error control control Robust modulation techniques 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 38 FSO Challenges - Turbulence Cause: Atmospheric inhomogeneity / random temperature variation along beam path. The atmosphere behaves like prism of different sizes and refractive indices Phase and irradiance fluctuation Depends on: • 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 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: pI (I ) (ln( I / I 0 ) l 2 / 2) 2 1 exp 2 l I 2 l 2 1 I 0 Model Comments Log Normal Simple; tractable Weak regime only I-K Weak to strong turbulence regime K Strong regime only Rayleigh/Negative Exponential Gamma-Gamma Saturation regime only Based on the modulation process the received irradiance is x y I I I Irradiance PDF by Andrews et al (2001): ) / 2 ( 2()( p( I ) I ()() ) 1 2 (2 I ) 2 0.49l 1 exp 12 / 5 7 / 6 (1 1.11l ) 1 2 0.51l 1 exp 12 / 5 5 / 6 ( 1 0 . 69 ) l 1 All regimes I 0 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) To mitigate turbulence effect we, employ subcarrier modulation Iran 2008 with spatial diversity 40 Turbulence Effect on OOK No Intensity Fading No Pulse Bit “0” Threshold level Pulse Bit “1” A A/2 With Intensity Fading A All commercially available systems use OOK with fixed threshold which results in sub-optimal performance in turbulence regimes 40 Iran 2008 Turbulence Effect on OOK Using optimal maximum a posteriori (MAP) symbol-by-symbol detection with equiprobable OOK data: dˆ (t ) arg maxd P(ir / d (t )) 2 2 ((ir RI ) ir ) exp 2 2 0 ln( I / I ) 2 / 2 0 l exp 2 2 l 2 1 2 l 2 1 . I dI 0.5 Noise variance 0.5*10-2 0.45 10-2 3*10-2 0.4 Threshold level, i th 5*10-2 0.35 OOK based FSO requires adaptive threshold to perform optimally…. 0.3 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 1 41 SIM – System Block Diagram DC bias m(t) d(t) Data in Serial/parallel converter . . Subcarrier modulator . . m(t)+bo Summing circuit Optical transmitter Atmospheric channel ir d’(t) . . Parallel/serial Data out converter Spatial diversity combiner Subcarrier demodulator 42 Photodetector array 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) (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 A1 A2 Input data d (t ) Serial to Parallel Converter . . . . . . AM g(t) PSK modulator at coswc1t g(t) PSK modulator at coswc2t m(t ) M A j g (t ) cos(wcj t j ) j 1 Σ m(t) Σ Laser driver Atmopsheric channel DC bias b0 g(t) PSK modulator at coswcMt Modulation index is constrained to avoid over modulation [ Rh0 Pt ,00 Nc' ]1 45 Iran 2008 46 Subcarrier Modulation - Transmitter 2 1 0 -1 [ Rh0 Pt ,00 Nc' ]1 M m(t ) A j g (t ) cos(wcj t j ) j 1 -2 5-subcarriers Output power -3 P -4 -5 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Pmax 2 m(t) 1 b0 Drive current Iran 2008 SIM - Receiver SNRele Pr ( IRA ) 2 2 2 Nc PSK Demodulator x g(-t) Sampler coswc1t h P 1 d (t ) cos(2f t n(t ) i 1 i t ,i i i Photodetector ir PSK Demodulator at coswc2t . . . Photo-current PSK Demodulator at coswcMt ir (t ) R I (1 m(t )) n(t ) R = Responsivity, I = Average power, = Modulation index, m(t) = Subcarrier signal di(t) = Data 47 Parallel to Serial Converter dˆ (t ) Output data 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 SIM - Spatial Diversity Single-input-multiple-output Multiple-input-multiple-output (MIMO) 49 SIM - Spatial Diversity Combiner F S O i1 (t ) C H A N N E L i2 (t ) iN (t ) Assuming identical PIN photodetector on each links, the photocurrent on each link is: a1 a2 . . . . a N iT (t ) M R iri (t ) I i 1 A j g (t ) cos(wcj t j N j PSK dˆ (t ) ) ni (t ) Subcarrier Demodulator ai is the scaling factor Diversity Combining Techniques Maximum Ratio Combining (MRC) [Complex but optimum] ai ii Equal Gain Combining (EGC) Selection Combining (SELC). No need for phase a1 a2 ... a N iT (t ) max( i1 (t ), i2 (t )...i N (t )) 50 information 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. 51 52 Subcarrier Modulation - Spatial Diversity 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 Normalised SNR @ BER = 10-6 (dB) 20 15 Increasing the number of subcarrier/users, results In increased SNR 10 5 0 Log intensity variance 0.1 0.2 0.5 0.7 -5 -10 1 2 3 4 5 6 7 Number of subcarrier 8 9 10 SNR gain compared with OOK 55 Error Performance – No Spatial Diversity BPSK BER against SNR for M-ary-PSK for log intensity variance = 0.52 DPSK BPSK 16-PSK 8-PSK -2 10 10 BER BPSK based subcarrier modulation is the most power efficient Log intensity -4 variance = 0.52 -6 10 BER -8 10 2 Q SNRe log 2 M sin( / M ) p( I )dI log 2 M 0 -10 10 20 30 25 SNR 35 40 (dB) Iran 2008 56 Spatial Diversity Gain Spatial diversity gain with EGC against Turbulence regime 2 Photodetectors 3 Photodetectors 70 Saturation Diveristy Gain (dB) 60 50 40 Moderate 30 20 10 Weak Turbulence Regime Iran 2008 Spatial Diversity Gain for EGC and SeLC 25 Link margin (dB) Log Intensity Variance 0.22 20 0.52 0.72 1 15 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 EGC Sel.C BER = 10-6 1 Pe ( SelC) 2 3 N 2 N 4 5 6 No of Receivers n 7 [ w 1 erf ( x ) i 1 N 1 i i 8 .e 9 10 ( K 0 2 exp( 2 xi 2l l 2 )) Zeros of the n order w n = Weight factor of the nth order xi n = Hermite i Hermite polynomial polynomial th i 1 i 1 ] K 0 RI0 A 2 2 N Spatial Diversity Gain for EGC and MRC 30 BER = 10-6 Log Intensity variance 1 /2 0 25 Spatial Diversity Gain (dB) Pe( EGC) 1 20 0 1 m 2 u u ) wi Q( K1e ( x i ) 1 MRC EGC 15 Pe ( MRC ) 2 0.5 ( I ) dI Q / I P MRC I 0 10 5 0 K12 2 exp Z P ( Z ) d dZ 2 sin 2 ( ) Z 0.22 1 2 3 Most diversity gain region 4 6 5 No of Receivers 7 8 1 /2 S ( ) N d , 0 10 9 The optimal but complex MRC diversity is marginally superior to the practical EGC 58 Multiple-Input-Multiple-Output Combiner It1 It2 d(t) BPSK ModuLator and . . . Laser driver ItH F S O i1 (t ) C H A N N E L i2 (t ) iN (t ) a1 a2 . . . . a iT BPSK Subcarrier Demodulator dˆ (t ) N By linearly combining the photocurrents using MRC, the individual SNRe on each link SNRele i RA 2 2 N H 59 I ij j 1 H 2 MIMO Performance -3 10 At BER of 10-6: 1X5MIMO 1X8MIMO 4X4MIMO 2X2MIMO 1X4MIMO -4 10 2 x 2-MIMO requires additional ~0.5 dB of SNR compared with 4photodetector single transmittermultiple photodetector system. 4 x 4-MIMO requires ~3 dB and ~0.8 dB lower SNR compared with single transmitter with 4 and 8photodetectors , respectively. -5 BER 10 -6 10 -7 10 -8 10 -9 10 log intensity variance= 0.52 12 14 16 18 20 22 2 (dB) SNR (R*E[I]) / No 1 Pe / 2 S () N d, 24 26 S () 2 K2 w j exp exp[ 2 ( x 2 )] j u u 2 j 1 2 sin 1 m K2 0 60 RI 0 A 2 N 2 H Free Space Optics Characteristics Challenges Turbulence - Subcarrier intensity multiplexing - Diversity schemes Results and discussions Wavelet ANN Receiver Final remarks 62 Transmission System - Receiver Models Data in TX Channel + Noise Data out… Slicer MMSE Data out Slicer Equaliser MF Data out Slicer NN CWT Wavelet - NN Iran 2008 63 PPM System – NN Equalization n(t) M 0100 M 0010 PPM Encoder PPM Decoder Xj Optical Transmitter Decision Device X(t) Z(t) h(t) Yj Neural Network ∑ Optical Receiver Zj Zj-1 Zj Matched Filter . Zj-n Ts = M/LRb . A feedforward back propagation neural network . ANN is trained using a training sequence at the operating SNR. Trained AAN is used for equalization Iran 2008 64 Impulse Response of Equalized Channel Impulse response of unequalized channel impulse response of equalized channel • Pulse are spread to adjust pulse . • Equalized response in a delta function which is equivalent to a impulse response of the ideal channel • ISI depends on pulse spread Iran 2008 65 Results (1) Slot error rate performance of 8- PPM in diffuse channel with Drms of 5ns at 50 Mbps 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 66 Results (2) Slot error rate performance of 8- PPM in diffuse channel with Drms of 5ns at 100 Mbps Unequalized performance at higher data rate is unacceptable at all SNR range Linear and neural equalization give almost identical performance. Iran 2008 67 Results (3) - Wavelet-AI Receiver Wavelet SNR Vs. the RMS delay spread/bit duration Iran 2008 68 Wavelet-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. Iran 2008 Visible Light Optical Wireless System with OFDM Visible-light communication system Distribution of illuminance Distribution of horizantal illuminance [lx] Number of LEDs 60 x 60 (4 set) 1400 1200 Illuminance[lx] Down link Up link 1000 800 600 400 200 5 4 5 3 4 3 2 2 1 y[m] 1 0 0 x[m] FSO Network – Two Universities in Newcastle 71 Agilent Photonic Research Lab Agilent Photonic Research Lab Optical Fibre Research Collaboration A-Block 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 • 73 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 Iran 2008 74 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 75 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 Iran 2008