Free-Space Optical (FSO) Communications Employing Binary Polarization Shift Keying (2PolSK) Coherent Modulation in Atmospheric Turbulence Channel Xuan Tang 1, Prof. Z. Ghassemlooy 1 and Dr. C. G. Lee 2 1: Optical Communications Research Group, NCRLab, Northumbria University, Newcastle upon Tyne, UK 2: Department of Electronic Engineering, Chosun University, S. Korea
6.5 dB/km150 dB/km225 dB/km FSO Challenging The 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 . 1.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 2.Pratt, W.K.: 'Atmospheric propagation', in Ballard, S.S. (Ed.): 'Laser communication systems' (John Wiley & Sons, Inc.,1969,), pp Saleh, A.A.M.: 'An investigation of laser wave depolarization due to atmospheric transmission', IEEE Journal of Quantum Electronics, June , (6), pp. 256
Why choose PolSK? AM Disadvantages Requires adaptive thresholding scheme to perform optimally in the presence of turbulence ; PM Disadvantages Highly sensitive to the phase noise; Requires a complex synchronization ; FM Disadvantages Bandwidth inefficient; Inferior BER performance compared to PM in the additive white Gaussian noise (AWGN) channel ; Alternative solution ─ PolSK High 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 . 1.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 – 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 – Chi, N., et al.: 'Generation and transmission performance of 40 Gbit/s polarisation shift keying signal', Electronics Letters, 28 April 2005, 41, (9), pp Zhao, X.: 'Circle polarization shift keying with direct detection for free-space optical communication', Optical Communications and Networking September 2009, 1, (4), pp
x y VbVb VaVa V match (DC ONLY) 2PolSK System (No Spatial Diversity) LD, laser diode; PC, polarization controller; PS, polarizing beam splitter; LO, local oscillator; PD, photo detector; BPF, bandpass filter; LPF, lowpass filter. P r,lo : signal power ω r.lo : angular frequencies Ф r,lo : phase noises m(t): the binary information V(t)
2PolSK with Spatial Diversity
Results and Discussion No Spatial Diversity ̶̶̶ ̶̶ ̶ ̶ ̶ ̶ ̶̶̶ ̶ ̶ ̶ ̶ ̶ Weak Regime EGC ̶ ̶̶̶ ̶ ̶ ̶ ̶ ̶ ̶ Moderate Regime MRC ̶ ̶ ̶̶̶ ̶ ̶ Strong Regime 3.9 dB 0.92 dB 8.94 dB 5.94 dB Worst achievement Best achievement 3 dB 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 dB 2.37 dB dB 0.74 dB 2.64 dB 2.11 dB The SNR requirement to achieve a BER of against the number of photodetectors N with MRC for weak, moderate and strong turbulence regimes at a BER of
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: 1. No need for synchronization at the receiver since the optical reference signal is transmitted at a different state of polarization; 2.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; 3.Higher transmission data rates can be achieved by employing the external modulation.