ICTON 2007, Rome, Italy 1 Z. Ghassemlooy, W.O. Popoola, and E. Leitgeb* Optical Communications Research Group, NCRLab Northumbria University, Newcastle upon, UK. *Institute of Broadband Communications, TU, Graz, Austria

ICTON 2007, Rome, Italy 2  PROBLEM DEFINITON / MOTIVATION  FREE-SPACE OPTICS  ATMOSPHERIC TURBULENCE  SUBCARRIER MODULATION  SPATIAL DIVERSITY  RESULTS  CONCLUSIONS

ICTON 2007, Rome, Italy 3 THE NEED TO COMMUNICATE, EXCHANGE AND SHARE INFORMATION PROMPTED THE DEPLOYMENT OF DEVICES AND APPLICATIONS SUCH AS: THESE APPLICATIONS AND THE EMERGING ONES ALL HAVE ONE “COMMON EFFECT” ON THE END USERS/BUSINESSES……. MOBILE PHONES, HOME PHONES, BROADBAND, OFFICE PHONES, DIGITAL TV, VIDEO CONFERENCING, TELEMEDICINE, TELE- TEACHING AND MANY OTHERS ARE EMERGING BY THE DAY

ICTON 2007, Rome, Italy 4 …..INCREASED BANDWIDTH REQUIREMENT ! OPTICAL FIBRE, of course! HOW CAN USERS HAVE ACCESS TO THE REQUIRED HIGH BANDWIDTH ? AND THAT IS ? DEPLOYMENT OF OPTICAL FIBRE IN THE BACKBONE AND METRO NETWORKS HAVE MADE HUGE BANDWIDTH AVAILABLE TO WITHIN ONE MILE OF BUSINESSES/HOMES IN MOST PLACES. But, HUGE BANDWIDTH IS NOT AVAILABLE TO END USERS.

ICTON 2007, Rome, Italy 5 OPTICAL FIBRE COPPER CABLE B Business C O Central office H Home U University Building N Network node FIBRE BASED RING NETWORK REGIONAL FIBRE RING METRO FIBRE RING N N N N N N RF DOMINATED ACCESS NETWORK C O H U B H H B H

ICTON 2007, Rome, Italy 6  FIBRE TO THE HOME (FTTH)- NOT CHEAP  HYBRID FIBRE COAXIAL- STILL HAS COPPER  FIBRE TO THE HOME (FTTH)- NOT CHEAP  HYBRID FIBRE COAXIAL- STILL HAS COPPER  FREE SPACE OPTICS (FSO) (From NTT website)

ICTON 2007, Rome, Italy 7 TERRESTRIAL FSO BLOCK DIAGRAM 1010 DATA IN LED/LD DRIVER PHOTO DETECTOR SIGNAL PROCESSOR DATA OUT ATMOSPHERIC CHANNEL TRANSMITTER RECEIVER A PRACTICAL LINK HEAD IS CAPABLE OF FULL DUPLEX TRANSMISSION FEATURES:  HAS SAME BANDWIDTH AS FIBRE OPTICS  NO SUNK COST  CHEAP AND FAST TO DEPLOY  NO LICENSE  NO EM INTERFERENCE

ICTON 2007, Rome, Italy 8 REGIONAL FIBRE RING METRO FIBRE RING N N N N N N C O U B B ACCESS NETWORK WI-FI CAN BE USED WITHIN BUILDINGS FOR CONNECTIVITY

ICTON 2007, Rome, Italy 9 IN ADDITION TO BRINGING HUGE BANDWIDTH TO BUSINESSES /HOMES FSO ALSO FINDS APPLICATIONS IN : Multi-campus university Hospitals Others Disaster recovery Fibre communication back-up Video conferencing Links in difficult terrains Temporary links e.g. conferences Cellular communication back-haul

ICTON 2007, Rome, Italy 10 MAJORLY ATMOSPHERIC FACTORS (OPPORTUNITIES FOR RESEARCH !!!) OTHERS:  BUILDING SWAY  BACKGROUND RADIATION  LINE OF SIGHT (LOS) REQUIREMENT  LASER SAFETY AEROSOLS SMOKE GASES FOGRAINTURBULENCE

ICTON 2007, Rome, Italy 11 CHALLENGEEFFECTSOPTIONSREMARK RAIN Photon absorption Increase transmit optical power Effect not significant AEROSOLS GASES & SMOKE Mie scattering Photon absorption Rayleigh scattering Increase transmit power Diversity techniques Effect not severe TURBULENCE Irradiance fluctuation (scintillation) Image dancing Phase fluctuation Beam spreading Polarisation fluctuation Diversity techniques Coding Robust modulation techniques Adaptive optics Significant for long link range (>1km) Turbulence and thick fog do not occur together FOG Mie scattering Photon absorption 270 dB/km Increase transmit power Hybrid FSO/RF Thick fog limits link range to ~500m Safety requirements limit maximum optical power

ICTON 2007, Rome, Italy 12 CAUSE: ATMOSPHERIC INHOMOGENEITY / RANDOM TEMPERATURE VARIATION (0.1 degree) ON THE SPATILA SCALE OF 0.1 cm to 10 m. DEPENDS ON: Altitude, Pressure, Wind speed Temperature and relative beam size Eddies of different sizes and refractive indices The atmosphere behaves like prism of different sizes and refractive indices Phase and irradiance fluctuation Result in deep signal fades that lasts for ~1-100 μs

ICTON 2007, Rome, Italy 13 Gamma-GammaAll regimes ModelComments Log NormalSimple; tractable Weak regime only I-KWeak to strong turbulence regime KStrong regime only Rayleigh/Negative Exponential Saturation regime only Based on the modulation process where the fluctuation of light radiation traversing turbulent atmosphere is assume to consist of small scale (scattering) and large scale (refraction) effects.. Irradiance PDF by Andrews et al (2001): I x : due to large scale effects; obeys Gamma distribution I y : due to small scale effects; obeys Gamma distribution Kn(.): modified Bessel function of the 2nd kind of order n σ l 2 : Log irradiance variance (turbulence strength indicator) the received irradiance is

ICTON 2007, Rome, Italy 14 SINGLE SUBCARRIER MULTIPLE SUBCARRIER R = Responsivity P = Average power = Modulation index m(t) = Subcarrier signal b0b0 Drive current Output power m(t)m(t)

ICTON 2007, Rome, Italy 15 Why Subcarrier Modulation ?  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  Performs optimally without adaptive threshold as is the case with optimal OOK  Use of efficient coherent modulation (PSK, QAM etc. ) where signal processing is carried out in RF domain where devices such as stable, low phase noise oscillators and selective filters are readily available.  System capacity/throughput can be increased using multiple subcarrier  It outperforms OOK in atmospheric turbulence (data is hidden in subcarrier phase as against laser irradiance in OOK). But..

ICTON 2007, Rome, Italy 16 Selection Combining (SELC) Maximum Ratio Combining (MRC) [Complex but optimum] Equal Gain Combining (EGC) Diversity Combining Techniques The spacing between detectors > the transverse correlation size ρ o of the laser radiation, since ρ o = a few cm in atmospheric turbulence a i is the scaling factor

ICTON 2007, Rome, Italy 17 Average Bit-Error-Rate (BER) √ Without channels state information (CSI), SNR e = BPSK subcarrier signal-to-noise ratio p(I) = Irradiance PDF Outage Probability (P o ) Measures the probability that the instantaneous BER is less that a pre- determined threshold level Has no closed form solution. Have adopted a numerical evaluation.

ICTON 2007, Rome, Italy 18 BER performance against the normalized SNR with No Diversity σ l 2 =0.2 σ l 2 =1.6 σ l 2 =3.5 ~35 dB

ICTON 2007, Rome, Italy 19 BER performance against the normalized SNR when using two photodetectors for EGC and MRC spatial diversities MRC EGC 5 dB improvement compared with no diversity case 20 dB improvement compared with no diversity case

ICTON 2007, Rome, Italy 20 SNR at BER =10 -6 against the number of photodetector with MRC diversity The decrease in the diversity gain in strong turbulence can be attributed to the increasing loss of spatial coherence of the laser radiation resulting in deep fades.

ICTON 2007, Rome, Italy 21  FSO IS PROMISING IN SOLVING THE ACCESS NETWORK BOTTLENECK BY MAKING HUGE BANDWDITH AVAILABLE TO BUSINESSES/END USERS  FSO BASED ON SUBCARRIER MODULATION AND ITS ERROR PERFORMANCE HAVE BEEN PRESENTED  RESULTS SHOWING THE GAINS OF SPATIAL DIVERSITY IN MITIGATING ATMOSPHERIC TURBULENCE EFFECT ALSO PRESENTED  FSO SHOULD NOT BE VIEWED AS A SUBSTITUTE TECHNOLOGY TO RF BUT CAN ADEQUATELY COMPLIMENT IT.

22 Acknowledgment My PhD student Popoola Colleague at Erich Leitgeb at Graz University Austria Northunmbria University for the research funding

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