X. Tang1, Z. Ghassemlooy1, S. Rajbhandari1, W. O. Popoola1 and 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: fary.ghassemlooy@unn.ac.uk

I. FSO INTRODUCTIONFeatures Applications Challenges

II. LOGNORMAL TURBULENCE MODEL

III. SYSTEM DESCRIPTION Transmitter

Receiver

IV. BIT ERROR PROBABILITY ANALYSIS

V. RESULTS AND DISCUSSIONS

VI. CONCLUSION

Benefits include Ultra High Wireless Bandwidth Most Secure Wireless Transmission License free operation Versatile Protocol Safe to Use All Major Cost Savings Reliable Communication High MTBF (Mean Time Between Failures)

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Typical Free-Space Optics deployments pictured above include point-to-point, multiple point-to-point, and mesh.

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Last-mile Connectivity

Clear Mesh Combines

FSO Mesh for Metro

NetsClearMesh Networks, a start-up

based in Pasadena, California, unveiled a wireless optical mesh networking solution capable of delivering business-class services at 5-100 Mbps without requiring licensed spectrum.

Ship-to-shore FSOUnder a Phase II SBIR program

sponsored by NAVSEA, LSA has developed a Free Space Optical Ship to Shore Communication System to address development of a Low Probability of Intercept/ Detection (LPI/LPD) communication capability for the littoral environment.

FSO Networks

Indoor FSO

ENTERPRISE APPLICATIONSEnterprise Connectivity •Health Care •Engineering & Design •Voice & Data •Video •Telco Bypass •Security •Disaster Recovery

MOBILE CARRIER APPLICATIONS•BTS Backhaul Connectivity

FSO transmission systems loose some of their energy from signal scattering, absorption and scintillation. Scattering: light signals are redirected as they pass through water particles. Absorption: some optical energy is converted to heat as it strikes particles

(such as smog). Scintillation: when heated (such as from smokestacks) air cause a bending

of the optical beam.

DEPENDS ON:• Altitude, Pressure, Wind speed• Temperature and relative beam size

Eddies of different sizesand refractive indices

The atmosphere behaves like prismof different sizes and refractive indices

Phase and irradiance fluctuation

Result in deep signal fades that

lasts for ~1-100 μs

0 22

2)2/2)/ln(exp

1

2

1)(

I

l

lnoII

Il

Ip

I: The received irradiance at the receiver

Ino: The received irradiance without scintillation.

σl: Log irradiance variance (turbulence strength

indicator)

Model Comments

Log Normal Simple; tractable

Weak regime only

I-K Weak to strong turbulence regime

K Strong regime only

Rayleigh/Negative

Exponential

Saturation regime only

Gamma-Gamma All regimes

The limitation of the log-normal model is defined by the Ryotov variance rage

The PolSK modulator is based on the LiNbO3 device of which the operating wavelength is 1550 nm [1]. LD: laser diodePBS: polarizing beam splitter

Vb controls the relative phase of the

two polarizations

Va controls the amount of light launched in x

and y polarizations

Vmatch applied to the 3 dB

coupler is used for wavelength

matching

x and y are the axes of polarization used to represent digital symbol ‘0’ and ‘1’, respectively.

[1] S. Benedetto, A. Djupsjobacka, B. Lagerstrom, R. Paoletti, P. Poggiolini, and G. Mijic, IEEE Photonics Technology Letters, vol. 6, pp. 949-951, August 1994.

LO: local oscillator; DC: directional coupler; BPF: bandpass filter; LPF: lowpass filter.

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

The 2PolSK modulation is based on the definition of the Stokes parameters S0, S1, S2 and S3 [1]:

{ni(t)}i=0,1,2,3 : the noise contribution which are independent of the received SOP and have the same variance.Note that the proposed 2PolSK refers only to the parameter S1. A digital symbol ‘0’ is assumed to have been received if S1 is above the threshold zero and ‘1’ otherwise.

[1] E. Collett, "The stokes polarization parameters," in Polarized light: fundamentals and applications New York: Marcel Dekker, Inc., 1993, pp. 33-66.

[1] M. Nazarathy and E. Simony, "Error probability performance of equi-energy combined transmission of differential phase, amplitude, and polarization," Journal of Lightwave Technology, vol. 25, pp. 249-260, January 2007.[2] M. N.-A.-S. Bhuiyan, M. Matsuura, H. N. Tan, and N. Kishi, "Polarization insensitive wavelength conversion for polarization shift keying signal based on four wave mixing in highly non-linear fiber " 14th OECC 2009, pp. 1-2, 13-17 July 2009.[3] X. Zhao, Y. Yao, Y. Sun, and C. Liu, "Circle polarization shift keying with direct detection for free-space optical communication " Optical Communications and Networking, , vol. 1, pp. 307-312, September 2009.

The conditional BER of the received irradiance:

The unconditional probability Pe is obtained by averaging Pec over the log normal irradiance fluctuation statistics:

This result is same as the BER expression of FSK. As regards the system sensitivity, PolSK and FSK techniques have complete equivalence [1].[1] R. Calvani, R. Caponi, F. Delpiano, and G. Marone, "An experiment of optical heterodyne transmission with polarization modulation at 140 Mbit/s bitrate and 1550 nm wavelength " GLOBECOM '91, vol. 3, pp. 1587-1591, 2-5 December 1991

0 5 10 1510

-8

10-6

10-4

10-2

100

SNR (dB)

BE

R

simulationtheory

Both simulated and theoretical curves match very closely which confirms the validity of the simulation.

For a fixed BER, the fading penalty increases with the turbulence varianceFading penalty is higher for lower values of BER at the same turbulence level

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

5

10

15

20

Turbulence Variance

Fad

ing

Pen

alty

(dB

)

BER=10-9

BER=10-6

BER=10-3

0 5 10 15 20 25 3010

-9

10-6

10-3

100

SNR (dB)

BE

R

ASKPSKPolSK

l2=0.9l2=0

3 dB 3 dB

7.1 dB