Transcript Communications Employing Binary Polarization Shift Keying (2PolSK)

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: [email protected], [email protected], [email protected]
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].
1.
2.
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?
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].
1.
2.
3.
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
2PolSK System
(No Spatial Diversity)
Data 01001110
LD
PC
PSx
y
Vmatch (DC ONLY)
Symbol ‘0’
PM
Symbol ‘1’
PD
Er(t)
BPF
V(t)
LPF
Sampler
Va
Vb
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 power
LO Elo(t)
Er (t )  ei (st s (t ))
ωr.lo: angular frequencies
Фr,lo : phase noises
m(t): the binary information
 Pr / 2 eit   x 
Elo (t )  ei (lot lo (t ))
 Plo / 2  x 
Pr / 2  y
Plo / 2  y


2PolSK with Spatial Diversity
Ex1(t)
Er1(t)
Ey1(t)
Combiner
a1
a1
Elo(t)
Ex2(t)
Er2(t)
Ey2(t)
a2
∑
Sampler
a2
∑
Elo(t)
Exn(t)
Ern(t)
Eyn(t)
an
an
N

R 2 Plo 
SNR EGC 
Pri 


N 2 2n  i 1


Elo(t)
SNR M RC 
R 2 Plo
N

Pri
2
2 N n i 1
2
Results and Discussion
10
-3
Worst
achievement
BER
Best
achievement
10
-6
3 dB
8.94 dB
0.92 dB
3.9 dB
10
5.94 dB
-9
9
14
19
24
SNR (dB)
No Spatial Diversity
EGC
MRC
29
̶̶̶̶̶̶̶ ̶̶̶̶̶
̶̶̶ ̶̶̶ ̶ ̶
̶ ̶ ̶ ̶ ̶
34
39
Weak Regime
Moderate Regime
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.
weak
SNR at BER = 10 -6
35
10.77 dB
30
25
moderate
strong
11.55 dB
20
2.37 dB
15
2.11 dB
2.64 dB
0.74 dB
10
2
4
6
8
Number of photodetectors (N)
10
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.
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.