1. 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

2. 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
Saleh, 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 [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
Zhao, 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)
NCRLab
2PolSK System
(No Spatial Diversity)
Data
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.

5. 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

6. 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.

7. 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.

8. 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.