NCRLab Free-Space Optical (FSO) Communications Employing Binary Polarization Shift Keying (2PolSK) Coherent Modulation in Atmospheric Turbulence Channel Xuan Tang1, Prof. Z. Ghassemlooy1 and Dr. C. G. Lee2 1: Optical Communications Research Group, NCRLab, Northumbria University, Newcastle upon Tyne, UK 2: Department of Electronic Engineering, Chosun University, S. Korea Email: xuan.tang@unn.ac.uk, z.ghassemlooy@unn.ac.uk, c.ghiu.lee@ieee.org

NCRLab FSO Challenging The laser beam propagating through the FSO channel suffers from the atmospheric turbulence induced fading [1]; Turbulence leads to random fluctuations in the direction, intensity and phase of the laser beam carrying the information [2]; It has been experimentally verified that polarization is less sensitive to the turbulence fluctuation experienced by the laser beam propagating through the channel [3]. 6.5 dB/km 150 dB/km 225 dB/km Iniguez, 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 – 42 Pratt, W.K.: 'Atmospheric propagation', in Ballard, S.S. (Ed.): 'Laser communication systems' (John Wiley & Sons, Inc.,1969,), pp. 128 - 144 Saleh, A.A.M.: 'An investigation of laser wave depolarization due to atmospheric transmission', IEEE Journal of Quantum Electronics, June 1967. 3, (6), pp. 256

Why choose PolSK? NCRLab AM Disadvantages Requires adaptive thresholding scheme to perform optimally in the presence of turbulence [1]; PM Disadvantages Highly sensitive to the phase noise; Requires a complex synchronization [2]; FM Disadvantages Bandwidth inefficient; Inferior BER performance compared to PM in the additive white Gaussian noise (AWGN) channel [3]; Alternative solution ─ PolSK High immunity to the laser phase noise [3]; Maintains SOPs over a long propagation link [4]; Doesn’t suffer from excess frequency chirp generated by the all-optical processing devices [3]; Attractive for the peak power limited systems because it’s a constant envelope modulation [4]. 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 – 973 Betti, 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 – 313 Chi, N., et al.: 'Generation and transmission performance of 40 Gbit/s polarisation shift keying signal', Electronics Letters, 28 April 2005, 41, (9), pp. 547 -549 Zhao, X.: 'Circle polarization shift keying with direct detection for free-space optical communication', Optical Communications and Networking September 2009, 1, (4), pp. 307-312

(No Spatial Diversity) NCRLab 2PolSK System (No Spatial Diversity) Data 01001110 Symbol ‘1’ Symbol ‘0’ LD PC PS PM Atmospheric Channel x y Vb Va Vmatch (DC ONLY) LO LPF Elo(t) Er(t) PD BPF Sampler V(t) Pr,lo : signal power ωr.lo: angular frequencies Фr,lo : phase noises m(t): the binary information LD, laser diode; PC, polarization controller; PS, polarizing beam splitter; LO, local oscillator; PD, photo detector; BPF, bandpass filter; LPF, lowpass filter.

2PolSK with Spatial Diversity NCRLab 2PolSK with Spatial Diversity a1 Elo(t) Er1(t) Ex1(t) Ey1(t) Sampler a2 Er2(t) Ex2(t) Ey2(t) an Ern(t) Exn(t) Eyn(t) ∑ Combiner Equal Gain Combining Maximum Ratio Combining

Results and Discussion NCRLab Results and Discussion 3.9 dB 0.92 dB 8.94 dB 5.94 dB Worst achievement Best achievement 3 dB No Spatial Diversity ̶̶̶ ̶̶ ̶ ̶ ̶ ̶ ̶̶̶ ̶ ̶ ̶ ̶ ̶ Weak Regime EGC ̶ ̶̶̶ ̶ ̶ ̶ ̶ ̶ ̶ Moderate Regime MRC ̶ ̶ ̶̶̶ ̶ ̶ Strong Regime BER performances against the SNR for 2PolSK with single detector and spatial diversity N = 2 for weak, moderate and strong turbulence regimes.

Results and Discussion ─ contd. NCRLab Results and Discussion ─ contd. 10.77 dB 2.37 dB 11.55 dB 2.64 dB 2.11 dB 0.74 dB The 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.

NCRLab Conclusion A 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.