Free-Space Optical (FSO) Free-Space Optical (FSO) Communications Employing Communications Employing

Binary Polarization Shift Binary Polarization Shift Keying (2PolSK) Coherent Keying (2PolSK) Coherent Modulation in Atmospheric Modulation in Atmospheric

Turbulence ChannelTurbulence 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. KoreaEmail: xuan.tang@unn.ac.uk, z.ghassemlooy@unn.ac.uk, c.ghiu.lee@ieee.org

6.5 dB/km 150 dB/km 225 dB/km

FSO FSO ChallenginChallengingg 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].

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. 128 - 1443. 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?Why choose PolSK?AM DisadvantagesRequires adaptive thresholding scheme to perform optimally in the presence of turbulence [1];PM DisadvantagesHighly sensitive to the phase noise;Requires a complex synchronization [2]; FM DisadvantagesBandwidth 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]. 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 – 9732. 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 – 3133. 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 -5494. 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

x

y

VbVa

Vmatch (DC ONLY)

2PolSK System 2PolSK System (No Spatial Diversity)(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.

Pr,lo : signal powerωr.lo: angular frequencies Фr,lo : phase noisesm(t): the binary information

yPxPetE

yPxePetE

lolotti

lo

rti

rtti

r

lolo

ss

2/2/)(

2/ 2/)(

))((

))((

V(t)

2PolSK with Spatial Diversity2PolSK with Spatial Diversity

2

122

2

EGCSNR

N

iri

n

lo PN

PR

N

iri

n

lo PN

PR

12

2

MRC2

SNR

Results and DiscussionResults and Discussion

No Spatial Diversity 000 00 0 0 0 0 000 0 0 0 0 0 Weak Regime EGC 0 000 0 0 0 0 0 0 Moderate Regime MRC 0 0 000 0 0 Strong Regime

9 14 19 24 29 34 3910

-9

10-6

10-3

SNR (dB)

BE

R

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 ─ Results and Discussion ─ contdcontd..

2 4 6 8 1010

15

20

25

30

35

Number of photodetectors (N)

SN

R a

t B

ER

= 1

0-6

weak

moderate

strong10.77 dB

2.37 dB

11.55 dB

0.74 dB

2.64 dB2.11 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.

ConclusionConclusion

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.