1 NCRLabFree-Space Optical (FSO) Communications Employing Binary Polarization Shift Keying (2PolSK) Coherent Modulation in Atmospheric Turbulence ChannelXuan Tang1, Prof. Z. Ghassemlooy1 and Dr. C. G. Lee21: Optical Communications Research Group, NCRLab, Northumbria University, Newcastle upon Tyne, UK2: Department of Electronic Engineering, Chosun University, S. Korea
2 NCRLabFSO ChallengingThe laser beam propagating through the FSO channel suffers from the atmospheric turbulence induced fading ;Turbulence leads to random fluctuations in the direction, intensity and phase of the laser beam carrying the information ;It has been experimentally verified that polarization is less sensitive to the turbulence fluctuation experienced by the laser beam propagating through the channel .6.5 dB/km150 dB/km225 dB/kmIniguez, R.R., Idrus, S.M., and Sun, Z.: 'Atmospheric transmission limitations, in Optical Wireless Communications - IR for Wireless Connectivity', 2008, Taylor & Francis Group, LLC, London, pp. 25 – 42Pratt, W.K.: 'Atmospheric propagation', in Ballard, S.S. (Ed.): 'Laser communication systems' (John Wiley & Sons, Inc.,1969,), ppSaleh, A.A.M.: 'An investigation of laser wave depolarization due to atmospheric transmission', IEEE Journal of Quantum Electronics, June , (6), pp. 256
3 Why choose PolSK? NCRLab AM Disadvantages Requires adaptive thresholding scheme to perform optimally in the presence of turbulence ;PM DisadvantagesHighly sensitive to the phase noise;Requires a complex synchronization ;FM DisadvantagesBandwidth inefficient;Inferior BER performance compared to PM in the additive white Gaussian noise (AWGN) channel ;Alternative solution ─ PolSKHigh immunity to the laser phase noise ;Maintains SOPs over a long propagation link ;Doesn’t suffer from excess frequency chirp generated by the all-optical processing devices ;Attractive for the peak power limited systems because it’s a constant envelope modulation .Popoola, W.O. and Ghassemlooy Z.: 'BPSK subcarrier intensity modulated free-space optical communications in atmospheric turbulence', Journal of Lightwave Technology, 15 April 2009, 27, (8), pp. 967 – 973Betti, S., Marchis G.D., and Iannone E.: 'Coherent systems: structure and ideal performance', in Chang K. (Ed.): 'Coherent optical communications systems' (John Wiley & Sons, Inc., 1995), pp. 242 – 313Chi, N., et al.: 'Generation and transmission performance of 40 Gbit/s polarisation shift keying signal', Electronics Letters, 28 April 2005, 41, (9), ppZhao, X.: 'Circle polarization shift keying with direct detection for free-space optical communication', Optical Communications and Networking September 2009, 1, (4), pp
4 (No Spatial Diversity) NCRLab2PolSK System(No Spatial Diversity)DataSymbol ‘1’Symbol ‘0’LDPCPSPMAtmospheric ChannelxyVbVaVmatch (DC ONLY)LOLPFElo(t)Er(t)PDBPFSamplerV(t)Pr,lo : signal powerωr.lo: angular frequenciesФr,lo : phase noisesm(t): the binary informationLD, laser diode;PC, polarization controller;PS, polarizing beam splitter;LO, local oscillator;PD, photo detector;BPF, bandpass filter;LPF, lowpass filter.
5 2PolSK with Spatial Diversity NCRLab2PolSK with Spatial Diversitya1Elo(t)Er1(t)Ex1(t)Ey1(t)Samplera2Er2(t)Ex2(t)Ey2(t)anErn(t)Exn(t)Eyn(t)∑CombinerEqual Gain CombiningMaximum Ratio Combining
6 Results and Discussion NCRLabResults and Discussion3.9 dB0.92 dB8.94 dB5.94 dBWorst achievementBest achievement3 dBNo Spatial Diversity ̶̶̶ ̶̶ ̶ ̶ ̶ ̶ ̶̶̶ ̶ ̶ ̶ ̶ ̶ Weak RegimeEGC ̶ ̶̶̶ ̶ ̶ ̶ ̶ ̶ ̶ Moderate RegimeMRC ̶ ̶ ̶̶̶ ̶ ̶ Strong RegimeBER performances against the SNR for 2PolSK with single detector and spatial diversity N = 2 for weak, moderate and strong turbulence regimes.
7 Results and Discussion ─ contd. NCRLabResults and Discussion ─ contd.10.77 dB2.37 dB11.55 dB2.64 dB2.11 dB0.74 dBThe SNR requirement to achieve a BER of 10-6 against the number of photodetectors N with MRC for weak, moderate and strong turbulence regimes at a BER of 10-6.
8 NCRLabConclusionA novel 2PolSK system employing a spatial diversity with N -photodetector is proposed to circumvent the scintillation effect on a FSO link. My contributions in this work include:No need for synchronization at the receiver since the optical reference signal is transmitted at a different state of polarization;No error floor and no power penalty in the BER performance due to the intermediate angular frequency (IF) and the IF phase noise are eliminated by employing polarization modulation;Higher transmission data rates can be achieved by employing the external modulation.