Free Space Optical Communications Professor Zabih GHASSEMLOOY Associate Dean for Research Optical Communications Research Group, NCRLab School of Computing Computing, Engineering and Information Sciences The University of Northumbria Newcastle, U.K. http://soe unn ac uk/ocr/ http://soe.unn.ac.uk/ocr/ fary@ieee.org 3rd ECAI – Romania, 3-5 July 2009 Outline ̇ Introduction to FSO ̇ FSO ̇ Applications ̇ Issues ̇ Results ̇ Simulation Si l ti ̇ Experimental ̇ Final remarks 2 3rd ECAI – Romania, 3-5 July 2009 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 3rd ECAI – Romania, 3-5 July 2009 Free Space Optical (FSO) C Communications i ti 3rd ECAI – Romania, 3-5 July 2009 When Did It All Start? 800BC 150BC 1791/92 - Fire beacons (ancient Greeks and Romans) - Smoke S k signals i l (A (American i IIndians) di ) - Semaphore (French) 1880 - Alexander Graham Bell demonstrated the photophone1 – 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 1Alexander Graham Bell, "On the production and reproduction of sound by light," American Journal of Sciences, Series 3, pp. 305 - 324, Oct. 1880. 5 3rd ECAI – Romania, 3-5 July 2009 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. 6 3rd ECAI – Romania, 3-5 July 2009 7 FSO - Features No trenches No electromagnetic interference Used in the following protocols: Ethernet, Fast Ethernet, Gigabit Ethernet, FDDI, ATM, Optical Carriers (OC)-3, 12, 24, and 48. Complements No other access Huge bandwidth license network similar to fibre fee technologies No multipath fading – Intensity Secure modulation and direct detection transmission Quick to install;; only y takes few hours R Requires i no right i ht off way Achievable range g limited by y thick fog g to ~500m Over 3 km in clear atmosphere steering and tracking capabilities 3rd ECAI – Romania, 3-5 July 2009 Access Network bottleneck (Source: NTT) 8 8 3rd ECAI – Romania, 3-5 July 2009 Access Network Technology xDSL ̇ Copper based (limited bandwidth)‐ 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 atmospheric attenuation(rain)/absorptionlimits link to ~1km 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 FSO 9 3rd ECAI – Romania, 3-5 July 2009 FSO - Basics SIGNAL PROCESSING Cloud Rain Smoke Gases Temperature variations Fog and d aerosoll PHOTO DETECTOR DRIVER CIRCUIT ̇ ̇ ̇ ̇ ̇ ̇ The transmission of optical radiation through the atmosphere obeys the Beer Beer-Lamberts Lamberts’s s law: Preceive = Ptransmit * exp(-αL) α : Attenuation coefficient POINT A This equation fundamentally ties FSO to the atmospheric weather conditions POINT B Link Range L 10 3rd ECAI – Romania, 3-5 July 2009 Optical Components – Light Source Operating Wavelength ((nm)) Laser type Remark ~850 VCSEL Cheap, very available, no active cooling, reliable up to ~10Gbps ~1300/~1550 1300/ 1550 Fabry-Perot/DFB Fabry Perot/DFB Long life, compatible with EDFA, up to 40Gbps ~10,000 Quantum cascade Expensive, very fast and highly laser (QCL) sensitive For indoor applications LEDs are used. 11 3rd ECAI – Romania, 3-5 July 2009 Optical Components – Detectors M t i l/St Material/Structure t Wavelength ( (nm) ) Responsivity Typical (A/W) sensitivity iti it Gain Silicon PIN 300 – 1100 0.5 -34dBm@ 155Mbps 1 InGaAs PIN 1000 – 1700 0.9 -46dBm@ 155Mbps 1 Sili Silicon APD 400 – 1000 77 -52dBm@ 52dB @ 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. 12 3rd ECAI – Romania, 3-5 July 2009 Existing System Specifications ̇ Range: 1-10 km (depend on the data rates) ̇ Power P consumption ti up to t 60 W ̇ ̇ ̇ ̇ ̇ ̇ ̇ ̇ ̇ ̇ 15 W @ data rate up to 100 mbps and λ =780nm, short range 25 W @ date rate up to 150 Mbps and λ = 980nm 60 W @ data rate up to 622 Mbps and λ = 780nm 40 W @ data rate up to 1.5 Gbps and λ = 780nm Transmitted p power: 14 – 20 dBm Receiver: PIN (lower data rate), APD (>150 mbps) Beam width: 4-8 mRad I t f Interface: coaxial i l cable, bl MM Fib Fibre, SM Fib Fibre Safety Classifications: Class 1 M (IEC) Weight: up to 10 kg 13 Safety Classifications - Point Source Emitter 500 √ with holography class 3B class 3B Total power in a 5cm Lens (mW) class 3B class 3B 50mW 45mW class 3A class 3A 5.0 1.0 02 0.2 class 3A class 2 class 1 650 visible 10mW 8 8mW 8.8mW 2.5mW 0.5mW class 3A class 1 880 class 1 class 1 indoor √ 1310 1550 indoor infra-red Source:BT Wavelength (nm) 14 3rd ECAI – Romania, 3-5 July 2009 Power Spectra of Ambient Light Sources Normalis sed powe er/unit wav velength 1.2 Pave)amb-light >> Pave)signal (Typically (T i ll 30 dB with ith no optical ti l filtering) filt i ) Sun 1 Incandescent 0.8 1st window IR 0.6 2nd window i d IR Fluorescent 0.4 x 10 0.2 0 1.5 1.4 1.3 1.2 Wavelength (μm) 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 15 3rd ECAI – Romania, 3-5 July 2009 16 FSO – System Requirement ̇ ̇ ̇ ̇ ̇ ̇ ̇ Link specifications / data rate Response time Timeliness / latency Data throughput Reliability Availability 3rd ECAI – Romania, 3-5 July 2009 17 FSO – System Requirement M. Löschnigg, P. Mandl, E. Leitgeb, 2009 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 H brid FSO/RF Link - High availability (>99.99%) - Much higher throughput than RF alone - For greatest flexibility need unlicensed RF band 3rd ECAI – Romania, 3-5 July 2009 LOS - Hybrid Systems Video-conference for Tele-medicine CIMIC-purpose and disaster recovery 19 3rd ECAI – Romania, 3-5 July 2009 FSO - Applications In addition to bringing huge bandwidth to businesses /homes FSO also finds applications in : Hospitals Others: Ü Inter‐satellite communication Ü Disaster recovery Ü Fibre communication back‐up Multi‐campus university Ü Video conferencing Ü Links in difficult terrains Ü Temporary links e.g. conferences FSO challenges… Cellular communication back‐haul 20 3rd ECAI – Romania, 3-5 July 2009 21 FSO - Applications Ring Topology Star Topology FSO - Challenges Major challenges are due to the effects of: CLOUD, GASES, SIGN NAL PROCE ESSING SMOKE, PHOT TO DETECTOR DRIV VER CIRC CUIT RAIN, TEMPERATURE VARIATIONS FOG & AEROSOL To achieve optimal link performance, system design involves tradeoffs of the different parameters. POINT A POINT B 22 3rd ECAI – Romania, 3-5 July 2009 FSO Challenges – Rain & Snow λ = 0.5 – 3 mm Effects Photon absorption Snow attenuation Options Remarks ̈Increase transmit Effect not significant optical power • A heavy y rainfall of 15 cm/hour causes 20 - 30 dB/km loss in optical p power p • Light snow about 3 dB/km power loss • Blizzard could cause over 60 dB/km power loss 23 3rd ECAI – Romania, 3-5 July 2009 FSO Challenges - Physical Obstructions Pointing g Stability y and Swaying y g Buildings g ̈ Effects Loss off signal L i l ̈ 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 g topology: p gy User’s n/w become nodes at least one hop away from the ring ̈ Fixed tracking (short buildings) ̈ Active tracking (tall buildings) 24 ̈ Remarks May be used for urban areas, campus etc. ̈ Low data rate ̈ Uses feedback 3rd ECAI – Romania, 3-5 July 2009 FSO Challenges – Aerosols Gases & Smoke Effects ̇ Mie scattering ̇ Photon absorption ̇ Rayleigh scattering These contribute to signal loss: γ (λ ) = α m Solutions ̇ Increase transmit power ̇ Diversity techniques Remarks ̇ Effect not severe ( λ ) + α a ( λ ) + β m ( λ ) + β a ( λ ). ) Absorption coefficient Scattering coefficient 25 3rd ECAI – Romania, 3-5 July 2009 FSO Challenges - Fog λ = 0.01 - 0.05 mm Using Mie scattering to predict fog attenuations m and r are the refractive index and radius of the fog droplets, respectively. Qext is the extinction efficiency and n(r) is the modified gamma size distribution of the fog droplets. 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 p power 26 3rd ECAI – Romania, 3-5 July 2009 Fog - Predicted specific attenuation at 10 ºC f moderate for d t continental ti t l fog f 27 FSO Challenges - Fog Weather condition Precipitation Amount ((mm/hr)) Visibility Dense fog Thick fog dB Loss/km Typical Deployment Range ((Laser link ~20dB margin) g ) 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 Cloudburst 100 770 m 1 km -12.65 -9.26 1565 m 1493 m Thin fog g Snow Heavy y 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 0.96 -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 19 -0.06 12112 m 13771 m Very clear (H.Willebrand & B.S. Ghuman, 2002.) 28 3rd ECAI – Romania, 3-5 July 2009 29 FSO – Fog g Experimental p Data City y of Nice – Jan –July y 2006 Ref: E Leitgeb et al 2009 City of Graze – Jan - July 30 FSO Attenuation 3rd ECAI – Romania, 3-5 July 2009 FSO Challenges - Others ̇ ̇ ̇ ̇ Background B k d radiation di ti LOS requirement L Laser safety f Turbulence (scintillation) 3rd ECAI – Romania, 3-5 July 2009 FSO Challenges - Turbulence Effects ̈ IIrradiance di fluctuation fl t ti (scintillation) ̈ Image dancing ̈ Phase Ph flfluctuation t ti ̈ Beam spreading ̈ Polarisation fl t ti fluctuation Options ̈ Diversity techniques ̈ Forward error control control ̈ Robust modulation techniques ̈ Adaptive optics ̈ Coherent detection not used due to Phase fluctuation 32 Remarks ̈ Significant Si ifi t ffor llong link range (>1km) ̈Turbulence and thick fog g 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: Atmospheric inhomogeneity / random temperature variation along beam path å changes in refractive index of the channel path. P: Channel pressure, Te: Channel temperature The atmosphere Th t h behaves b h like lik prism i of different sizes and refractive indices Phase and irradiance fluctuation • Zones of differing density act as lenses, scattering light away from its intended path. • Thus, Thus multipath. multipath Result in deep signal fades that lasts for ~1-100 μs Depends on: ̇ Altitude/Pressure, Wind speed, ̇ Temperature and relative beam size. 3rd ECAI – Romania, 3-5 July 2009 Turbulence – Channel Models Irradiance PDF: pI (I ) = ⎧⎪ (ln( I / I 0 ) + σ l 2 / 2) 2 ⎫⎪ 1 exp ⎨− ⎬ I 2π σ l ⎪⎩ ⎪⎭ 2σ l 2 1 I ≥0 Model Comments Log Normal Simple; tractable Weak regime only onl I-K Weak to strong turbulence regime K Strongg regime g onlyy Rayleigh/Negative Exponential Gamma-Gamma Saturation regime only p( I ) = 2(αβ ) I Γ (α )Γ (β) α +β ( ) −1 2 Κ α − β ( 2 αβ βI ) 2 ⎡ ⎛ ⎞ ⎤ 0.49σ l ⎟ − 1⎥ α = ⎢exp⎜⎜ 12 / 5 7 / 6 ⎟ ⎣⎢ ⎝ (1 + 1.11σl ) ⎠ ⎦⎥ 2 ⎡ ⎛ ⎞ ⎤ 0.51σ l ⎟ − 1⎥ ⎜ β = ⎢exp⎜ 12 / 5 5 / 6 ⎟ + σ ( 1 0 . 69 ) l ⎠ ⎦⎥ ⎣⎢ ⎝ −1 −1 I = I Based on the modulation process the received irradiance is Irradiance PDF by Andrews et al (2001): ( α +β) / 2 All regimes I >0 x I y Ix: due to large g 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 L iirradiance di variance i (turbulence strength indicator) To mitigate turbulence effect we, employ subcarrier modulation 3rd ECAI – Romania, 3-5 July 2009 with spatial diversity Turbulence Effect on OOK Using optimal maximum a posteriori (MAP) symbol-by-symbol detection with equiprobable OOK data: dˆ (t ) = argg maxd P (ir / d (t )) The threshold depends on the noise level and turbulence strength Λ Noise variance 0.5*10-2 ( ) 2 1 2 πσ ⎫ ⎪ ⎬ dI ⎪⎭ 2 l 10-2 3*10-2 0.4 Thres shold level, i th ⎧⎪ − (( i r − RI ) 2 − i r 2 ) ⎫⎪ ⎬ ∫ exp ⎨⎪ 2σ 2 ⎪⎭ 0 ⎩ ∞ ⎧ − ln(( I / I ) + σ 2 / 2 ⎪ 0 l exp ⎨ 2 2σ l ⎪⎩ 0.5 0.45 Λ = 5*10-22 0.35 OOK based FSO requires adaptive threshold to perform Optimally. 0.3 0.25 0.2 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 35 3rd ECAI – Romania, 3-5 July 2009 1 . I SIM – System Block Diagram DC bias m(t) d(t) Data in Serial/parallel converter t . . Subcarrier modulator d l t . . m(t)+bo Summing circuit i it Optical t transmitter itt Atmospheric channel ir d’(t) . . Parallel/serial Data out converter Spatial diversity combiner Subcarrier demodulator 36 Photodetector array 3rd ECAI – Romania, 3-5 July 2009 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 ̇ SIM with space-time space time block code with coherent and differential detection (H. Yamamoto, et al 2003) al 2007) ̇ However, error control coding introduces huge processing delays and efficiency degradation (E. J. Lee et al, 2004) 37 3rd ECAI – Romania, 3-5 July 2009 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 by birds and misalignment when combined with a wide laser beamwidth, no need for an active tracking ̇ provides independent aperture averaging with multiple separate aperture system, system than in a single aperture ̇ Provides gain and bit-error performance ̇ Efficient coherent modulation techniques (BPSK etc.) etc ) - bulk of the signal processing is done in RF that suffers less from scintillation ̇ In dense fog, g MIMO p performance drops, p therefore use alternative configuration such as hybrid FSO/RF ̇ Average transmit power increases with the number of subcarriers, thus may suffers from signal clipping ̇ Inter-modulation distortion 3rd ECAI – Romania, 3-5 July 2009 Subcarrier Modulation - Transmitter m (t ) = ∑ M j =1 A j g ( t ) cos( w cj t + θ j ) d (t ) 39 3rd ECAI – Romania, 3-5 July 2009 SIM - Receiver SNR ele ( IRA ) 2 = 2σ 2 dˆ (t ) Photo-current ir (t ) = R I (1 + βm(t )) + n(t ) R = Responsivity, R i it I = Average A power, β = Modulation index, m(t) = Subcarrier signal 40 3rd ECAI – Romania, 3-5 July 2009 SIM - Spatial p Diversity y ̇ Single-input-multiple-output ̇ Multiple Multiple-input-multiple-output input multiple output (MIMO) 41 3rd ECAI – Romania, 3-5 July 2009 SIM - Spatial Diversity Assuming identical PIN photodetector on each li k th links, the photocurrent h t t on each h lilink k iis: i1 (t ) i2 ((tt ) iN (t ) ∑ iT (t ) R ⎛⎜ i ri ( t ) = Ii 1 + N ⎜⎝ dˆ (t (t ) ⎞ ∑ A j g ( t ) cos( w cj t + θ j ) ⎟⎟ M ⎠ j + n i (t ) ai is the scaling factor Diversity Combining Techniques Maximum Ratio Combining (MRC) [Complex but optimum] a i α ii Equal Gain Combining (EGC) a1 = a 2 = ... = a N 42 Selection Combining (SELC). No need for phase iT ( t ) = max( i1 ( t ), i 2 ( t )... i N ( t )) information 3rd ECAI – Romania, 3-5 July 2009 SIM Spatial Diversity – Assumptions Made ̇ Spacing between detectors > the transverse correlation size ρo of laser radiation, (note ρo = a few cm in atmospheric turbulence) ̇ Beamwidth at the receiver is sufficiently broad to cover the entire field of view of all N detectors. ̇ S Scintillation i till ti b being i a random d phenomenon h th thatt changes h with time makes the received signal intensity time variant with coherence time τo of the order of milliseconds. ̇ With the symbol duration T << τo the received irradiance is time invariant over one symbol duration. 43 3rd ECAI – Romania, 3-5 July 2009 Subcarrier Modulation - Spatial Diversity One detector Two detectors Three detectors A typical reduction in intensity fluctuation with spatial diversity Eric Korevaar et. al 3rd ECAI – Romania, 3-5 July 2009 ̇ Free Space Optics ̇ Characteristics ̇ Challenges ̇ Turbulence - Subcarrier intensity multiplexing - Diversity schemes ̇ Results and discussions ̇ Final remarks 3rd ECAI – Romania, 3-5 July 2009 Error Performance – No Spatial Diversity Normalised SNR at BER of 10-6 against the number of subcarriers for various turbulence levels for BPSK Normalis sed SNR @ BER R = 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 07 0.7 -5 -10 1 2 3 4 5 6 7 Number of subcarrier 8 9 SNR gain compared with OOK 10 3rd ECAI – Romania, 3-5 July 2009 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 R 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(π / M ) p ( I )dI log 2 M ∫0 -10 10 20 25 30 SNR 35 40 (dB) 3rd ECAI – Romania, 3-5 July 2009 Spatial Diversity Gain Spatial diversity gain with EGC against Turbulence regime 2 Photodetectors 3 Photodetectors 70 Saturation Diveristy G Gain (dB) 60 50 40 Moderate 30 20 10 Weak Turbulence Regime 3rd ECAI – Romania, 3-5 July 2009 Spatial p Diversity y Gain for EGC and SeLC 25 Link margin (d dB) Log Intensity Variance 0 22 0.2 20 0.52 0.72 1 15 Link margin g 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 = 10-66 1 Pe( SelC) = 2 [w [1 + erf ( x )] ∑ π 3 N 4 5 6 No of Receivers n i i 7 N −1 8 .e 9 10 ( − K 0 2 exp(2 xi 2σl −σl 2 )) Zeros of the n order {w }n = Weight factor of the nth order {xi }n = Hermite i Hermite polynomial polynomial i =1 2 N i 1 i= th i =1 ] K 0 = RI 0 A 2 σ 2 N 3rd ECAI – Romania, 3-5 July 2009 Spatial Diversity Gain for EGC and MRC 30 BER = 10-6 25 Spa atial Diversity Ga ain (dB) Pe ( EGC ) = Log Intensity variance = 1 20 MRC EGC 2 0.5 10 0 2 3 Most diversity gain region 4 0 ∑ w i Q ( K 1e ( x 0 1 π 5 6 No of Receivers 7 8 9 ⎛ ⎞ K 12 2 ⎟ exp ⎜ − Z P ( Z ) d θ dZ ⎜ 2 sin 2 (θ ) ⎟ Z ⎝ ⎠ 2σ u + μ u ) m i ) 1 = 0.22 1 1 Pe ( MRC ) = 15 5 π /2 ∫π ∫ ∞ ∫Q ∞ 0 π 1 ( v ∫ [S (θ )] π /2 ) γ MRC / I PIv ( I ) d I N v v dθ , 0 10 The optimal but complex MRC diversity is marginally superior to the p practical EGC 50 3rd ECAI – Romania, 3-5 July 2009 51 Temporal Diversity Retransmission on different subcarriers Other possibilities: different wavelengths different polarisations Delay ≥ Channel coherence time This process is reversed at the receiver side to recover the data Temporal p Diversity y Gain No fading No TDD 1-TDD 3-TDD 5-TDD 2-TDD -2 10 -4 BER 10 -6 10 Single delay path is the optimum -8 10 BER =10-9 -10 10 Rb = 155Mbps Log irradiance var =0.3 03 -32 -30 -28 -26 -24 -22 -20 Receiver sensitivity, Io (dBm) -18 -16 No TDD -17.17 Io (dBm) (no fading: -27.05) Fading penalty (dB) 9.88 Diversity gain (dB) 0 (gain / path) (0) 1-TDD 2-TDD -19.17 -19.85 3-TDD -20.13 5-TDD -20.3 7.88 2 (2) 6.92 2.96 (0.99) 6.75 3.13 (0.63) 7.2 2.68 (1.34) Multiple-Input-Multiple-Output p p p p i1 (t ) ∑ i2 (t ) dˆ (t ) iN (t ) By linearly combining the photocurrents using MRC, the individual SNRe on each link SNRele− i ⎛ RA = ⎜⎜ 2 ⎝ 2 Nσ H 53 ⎞ ⎟ I ∑ ij ⎟ j =1 ⎠ H 2 3rd ECAI – Romania, 3-5 July 2009 MIMO Performance -3 10 At BER of 10-6: 1X5MIMO 1X8MIMO 4X4MIMO 2X2MIMO 1X4MIMO -4 10 ̇ -5 10 BER -6 10 ̇ -7 10 -8 10 -9 10 2 x 2-MIMO requires additional ~0.5 dB of SNR compared with 4photodetector single transmitter 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. log intensity variance= 0.52 12 14 1 Pe = π 16 18 ∫ [S (θ)] π/2 20 22 2 (dB) SNR (R*E[I]) / No N dθ, 24 26 S (θ) ≈ 2 ⎛ ⎞ K2 ⎜ ⎟ σ + μ exp − w x exp[ 2 ( 2 )] ∑ j j u u ⎟ 2 ⎜ π j =1 ⎝ 2 sin θ ⎠ 1 m K2 = 0 54 RI0 A 2Nσ 2 H 3rd ECAI – Romania, 3-5 July 2009 FSO – Turbulence Chamber Thermometers, T4 Laser Module (Direct Modulation) Power = 3mW λ = 785nm Reflecting mirror OOK & BPSK Modulator + Demodulator Turbulence chamber PIN Detector + Amplifier Optical power meter head Heaters + Fans Reflecting mirror BPSK modulator •Carrier •Data rate Turbulence chamber •Dimension •Temp. range 1.5 MHz A few kHz 140 x 30 x 30 cm 24oC – 60oC 3 ECAI – Romania, 3-5 July 2009 rd FSO – With Scintillation Effect Received mean signal + Noise + Scintillation Signal Distribution Histogram of mean signal - no scintillation Gaussian fit Mean = -0.0012 Variance = 5e-5 16 Gaussian fit Gaussian fit 14 Bin n Size 12 10 8 6 4 2 0 -0.05 2.5 Observations -0.04 -0.03 -0.02 -0.01 0 0.01 With scintillation Signal level Lognormal fit Mean =1 Variance = 9e-3 0.02 0.03 0.04 Lognormal fit 9.10-3 (V2) • Total fluctuation variance = • Weak scintillation obeys Lognormal distribution ((variance < 1)) • Simulated turbulence is very weak. Bin Size 2 Lognormal fit 1.5 1 05 0.5 0 0.85 3rd ECAI – Romania, 3-5 July 2009 0.9 0.95 1 1.05 Signal level 2.93 V 1.1 1.15 FSO – OOK With Scintillation Effect 8 7.5 7 No scintillation Received Signal Threshold position. ith B in P erc e nta ge 6.5 6 5.5 5 4.5 4 3.5 3 22.55 2 1.5 1 0.5 Received Transmitted 0 -0.2492 -0.1992 -0.1492 -0.0992 -0.0492 0.0008 0.0508 Signal Level 0.1008 0.1508 0.2008 0.2492 1.4 1.3 With scintillation 1.2 Received Signal ≈ 400mV p-p 1.1 Thresho old range B in P e rc e n t a g e 1 09 0.9 0.8 0.7 Observation: The optimum symbol decision position in OOK depends on scintillation level 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.3488 -0.2488 -0.1487 -0.0487 0.0513 Signal Level 0.1513 0.2513 0.35130.3987 3rd ECAI – Romania, 3-5 July 2009 FSO – BPSK-SIM With Scintillation Effect 3.5 3.4 3.2 Received Signal No scintillation 3 28 2.8 2.6 Demodulated No low Pass filtering BIn Percentage B 2.4 2.2 2 1.8 1.6 14 1.4 1.2 1 Before demodulation 0.8 0.6 0.4 0.2 0 -0.2 -0.15 -0.1 -0.05 3.5 3.4 3.2 0 0.05 Singal Level 0.1 0.15 0.2 Demodulated Signal ≈ 400mV p-p With scintillation 3 2.8 2.6 24 2.4 Bin Percentage 2.2 2 Observation: Scintillation does not affect the symbol decision position in BPSK SIM 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 -0.25 -0.2 -0.15 -0.1 -0.05 0 0.05 Signal Level 0.1 0.15 0.2 0.25 3rd ECAI – Romania, 3-5 July 2009 FSO Network – Linking Two Universities i Newcastle in N tl Agilent Photonic Research Lab Optical Fibre @1550 nm Specifications: • 4x4 Du-plex communication link (The FlightStrata 155E) • VCSEL @ 650 nm wavelength • Si APD • Data D t rate: t 155 Mb Mbps • Maximum length: 3.5 km • Automatic Power Control and Auto Tracking rd 3 ECAI – Romania, 3-5 July 2009 Collaborators • Graz Technical University, Austria • Houston University, USA • UCL • Hong-Kong g g Polytechnic y Universityy • Tarbiat Modares University, Iran • Newcastle University • Ankara University University, Turkey • Agilent • Cable Free • Technological University of Malaysia • Others • 3rd ECAI – Romania, 3-5 July 2009 61 Summary Ü Access bottleneck has been discussed Ü FSO introduced as a complementary technology Ü Atmospheric challenges of FSO highlighted Ü Ü Subcarrier intensity modulated FSO (with and without spatial diversity) discussed FSO is i a complementary l t tto RF Ü 61 3rd ECAI – Romania, 3-5 July 2009 62 Acknowledgements g ̇ To many colleagues (national and international) and in p particular to all my y MSc and PhD students (past and present) and post-doctoral research fellows http://soe.unn.ac.uk/ocr/ 3rd ECAI – Romania, 3-5 July 2009 63 Thank you! Iran 2008