International Association of Scientific Innovation and Research (IASIR) (An Association Unifying the Sciences, Engineering, and Applied Research)

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1 International Association of Scientific Innovation and esearch (IASI) (An Association Unifying the Sciences, Engineering, and Applied esearch) International Journal of Engineering, Business and Enterprise Applications (IJEBEA) ISSN (Print): 79- ISSN (Online): Analysis Study of ain Attenuation on Optical Communications Link Mazin Ali A. Ali AL-Mustansiriyah Univ. / College of Science / Physics Department, Iraq - Baghdad. AL-Mustansiriyah Univ. P. O. Box Astract: This paper presents the attenuation due to rain on the free space optical communication systems. The work has compared the rain attenuation predicted y Marshal and Caronneau models. The performance is also done for receiver signal power, link margin, data rate and signal to noise ratio. The data rate 1M/s can e achieved for distance aout 8m. The two rain models have difference ehavior for distance larger than 3. On the other hand the receiver signal power, link margin, data rate and signal to noise ratio decreases with increasing distance and rain attenuation. Keywords: Attenuation, rain Attenuation, Free Space, Optical Communications. I. Introduction There is growing interest in the telecommunication community towards the use of Free Space Optics (FSO) links for short and very short path applications requiring the availaility of very large andwidth. In fact, the advantages of FSO communications over classical microwave links and fier systems are low equipment cost, and flexile, license free installation [1]. Laser eam propagation in the atmosphere is severely affected y aerosol scattering and atmospheric turulence which limit the path length for a given link availaility; these effect are much worse than in the case of microwave relay links. Specifically, impairments are due to (i): air turulence that produces eam spreading, eam wander, scintillation and degradation of the coherence of the wave front [], and (ii): hydrometeors (rain and snow) and suspended water particles (fog represents the worst meteorological situation for FSO) that introduce extra losses due to scattering [3]. II. ain attenuation model ain attenuation prediction is normally referred as specific attenuation which means attenuation per unit length. In Terahertz wave system like FSO, rain attenuation is particularly severe and greatly dependent on various models of raindrop-size distriution [4]. The most commonly used raindrop size distriutions that have een proposed are Marshal and Palmer [5]. Marshal and Palmer distriution proposed renowned empirical expression y fitting their data and the Laws and Parsons data. The specific attenuation of wireless optical link for rain rate of mm/hr is given y ( db / km) a. (1) Where a & are power law parameters equal to.365,.63 respectively. The power law parameters depend on frequency, rain drop size distriution and rain temperature. For the purpose of calculating the attenuation, it is adequate to assume that raindrops have spherical shape. This assumption makes a & independent of polarization [6]. Analysis on the effect of rain on FSO link can e done y knowing the rain attenuation on FSO links and corresponding rainfall intensity. The modeling of rain attenuation prediction can e done using two methods, namely empirical method and the physical method [7]. The empirical method is ased on correlation etween oserved attenuation distriution and corresponding oserved rain-rate distriution measured at 1 minute integration time [8, 9]. While the physical method is an attempt to use the physical ehavior involved in the attenuation process. The another model is proposed y Caronneau, this model predict is.67 ( db / km) () Caronneau s model proposed values to predict a & ased on measurement done in france [1]. However the measurement done was for very low rain intensities compare with rain intensity in tropical region. III. Communication Link Model We now consider three types of communication links: the receiver signal power, link margin, and data rate. In addition, we performed a signal to noise ratio calculation. 1. eceiver Signal Power We shall consider the situation of optical propagation etween points underwater. Consider a laser transmitting a total power P T at the wavelength 16nm. The signal power received at the communications detector can e expressed as [11] D. L/1 Prec P trans.1 trans rec (3) div L IJEBEA ; 13, IJEBEA All ights eserved Page 18

2 Mazin Ali A. Ali, International Journal of Engineering, Business and Enterprise Applications, 6(1), Septemer-Novemer., 13, pp Where D is the receiver diameter, θ is the divergence angle, γ is the attenuation factor (db/m), τ trans, τ rec are the transmitter and receiver optical efficiency respectively.. Link Margin Another important parameter in optical communications link analysis is "Link Margin", which is the ratio of availale received power to the receiver power required to achieve a specified BE at a given data rate. Note that the "required" power at the receiver P EQ (watts) to achieve a given data rate, (its/sec), we can define the link margin LM as [11]: ) L/1 LM [ P T /( N hc] [ D /( L ] 1 trans rec (4a) Or can e written as LM [ /( N hc]. P (4) rec Where N is the receiver sensitivity (photon/its) or (dbm), is a data rate, h is a plank constant and c is the light velocity. 3. Data ate Given a laser transmitter power P trans, with transmitter divergence of θ, receiver diameter D, transmit and receive optical efficiency τ trans, τ rec the achievale data rate can e otained from [1]. L/1 P T...1 D trans rec (5a) ( / ) L E p N Or can e written as 4.. P (5). E rec p N Where E p =hc/λ, is the photon energy at wavelength. 4. Signal to Noise atio (SN) The electrical power of the received optical signal is proportional to the mean squared avalanche photodiode APD current, which can e written as [13] i APD ( P rec M ) (6) and q h c where denotes the primary sensitivity of the APD, M is the APD gain, η is the quantum efficiency, q is the electron charge. The noise contriutions (i.e., the mean-square values of the APD current) are shot noise: x sig noise q( P rec ) M B (8) Surface leakage current noise: surface qi L B (9) multiplied dark current noise: x dark, m q( I D ) M B (1) And Johnson noise: 4kT B F T johnson eq (11) x where I D is the ulk dark current, I L is the surface leakage current, F( M) M ( x 1) is the excess noise factor, k is the Boltzmann constant, B is the equivalent noise andwidth, eq is the equivalent circuit resistance, F T is the noise figure of the electric circuit, and T is the system temperature. The SN for the optical communication system is thus given y (7) IJEBEA ; 13, IJEBEA All ights eserved Page 19

3 Mazin Ali A. Ali, International Journal of Engineering, Business and Enterprise Applications, 6(1), Septemer-Novemer., 13, pp SN APD P rec ( M ) x (1) q( P rec I D ) M B q I L B 4kT B F T / eq IV. Simulation results Simulation y matla carried out to show the effect of rain attenuation effect employing Marshal and Caronneau model on free space optical communication system (FSOWCS). The performance of (OWC) system can e evaluated y the receiver signal power, link margin, data rate and signal to noise ratio. We have investigated the high quality and the est performance of rain attenuation effect on optical communication link system for two models. The investigating ased on the modeling equations analysis and the assumed set of the operating parameters are shown in tale (1). Tale (1): Proposed operating parameters for optical communications links Operating parameter wavelength Transmitter power Transmitter divergence angle value 16 nm 5mw 1.5mrad Transmitter efficiency.5 eceiver efficiency.5 eceiver sensitivity eceiver diameter Bulk dark current, ID -dbm 1cm.5 na The APD gain 1 The excess noise factor, x.5 Electrical and, B 5MHz Surface leakage current, I L.1A System temperature, T Noise figure, F T Equivalent resistance, eq Simulations were performed to compare the value of rain FSO specific attenuation under Marshal and Caronneau models. Fig. (1) shows this comparison, and from, it can e inferred that the models is not closer ehaviour, accordingly Marshal model is etter than Caronneau model ecause it suffers less attenuation. 9K 3dB 5kΩ Fig. (1) Specific attenuation of the rain for the two models The received power is achieved for rain attenuation at a link distance less than 1m under different rain attenuation models as shown in fig. (). The received signal power decreases with increasing rain rate for oth Marshal and Caronneau rain models. It is also oserved that received signal power decreases with increasing distance under the same conditions. Also oserved that the Marshal & Caronneau curves have closer ehavior for low distances ut differ for distance larger than 3m under the same rain rate. IJEBEA ; 13, IJEBEA All ights eserved Page

4 Mazin Ali A. Ali, International Journal of Engineering, Business and Enterprise Applications, 6(1), Septemer-Novemer., 13, pp The link margin for receiver sensitivity -dbm is achieved for data rate 1M/s operating under rain models (Marshal, Caronneau) at a distance 1m. The link margin increases with decreasing rain rate for oth rain models and decreases with increasing distance as shown in fig. (3). Also noted that the link margin have a close ehavior curves for low distance. Figure () receiver signal power versus distance for rain attenuation under different rain rate IJEBEA ; 13, IJEBEA All ights eserved Page 1

5 Mazin Ali A. Ali, International Journal of Engineering, Business and Enterprise Applications, 6(1), Septemer-Novemer., 13, pp Figure (3) link margin versus distance for rain attenuation under different rain rate The range equation can e used to generate the communication data rate versus link range for varying rain rate under rain attenuation models. eceiver sensitivity -dbm which is equivalent to 37 photons/it. The data rate decreasing with increasing rain rate as shown in Fig (4), As can e seen a maximum data rate of 1 M/s can e achieved for a range large than 8 m under rain rate attenuation models. The signal no noise ratio(s/n) is increases with decreasing rain rate. It is also oserved that the signal to noise ratio (S/N) for rain attenuation models have very close ehavior curves for low distance. As well as high (S/N) has presented low rain rate compared with another values of rain rate. IJEBEA ; 13, IJEBEA All ights eserved Page

6 Mazin Ali A. Ali, International Journal of Engineering, Business and Enterprise Applications, 6(1), Septemer-Novemer., 13, pp Fig. (4) Data rate versus distance for rain attenuation under different rain rate Fig. (5) Signal to Noise ratio versus distance for rain attenuation under different rain rate V. Conclusion This investigation presents the simulation results of the effect of rain on a FSO link. The analysis focuses of Marshal & Caronneau Models which explain the effect of rain on FSO. These models have presented to oserved effects on receiver signal, link margin, data rate and signal to noise ratio for distance approximate to 1m. Simulation found that the rain attenuation affect for marshal model less than for Caronneau model ecause in marshal model used empirical expression ased on fitting data, while Caronneau model used physical method ased on measurements data. On the other hand the two rain models have very close ehavior for distance less than 3m.the receiver signal power, data rate, link margin and signal to noise ratio decreases with increasing rain rate for the two models under study. VI. eferences [1] C. Capsoni,. Neuloni and M. D' Amico, "Attenuation due to ain on FSO", XVI iunione Nazional di Elletromagnetismo, Genova, 6. [] L. C. Andrews,. L. Philips, "laser eam propagation through random media", SPIE Optical Engineering Press, [3] D. Deirmendjian, "Fra- Infrared and sumillimeter wave attenuation y clouds and rain", J. Appl. Meteorol., vol. 14, [4] S. Ishii, S. Sayama, et al., "ain Attenuation at Terahertz", Wireless Engineering and Technology, vol. 1, 9-95, (1). IJEBEA ; 13, IJEBEA All ights eserved Page 3

7 Mazin Ali A. Ali, International Journal of Engineering, Business and Enterprise Applications, 6(1), Septemer-Novemer., 13, pp [5] J. S. Marshall, W. M. Palmer," The distriution of raindrops with size", Journal of Meteorology, 5(4), , [6] W. Zhang, N. Moayeri, "Power-Law Parameters of ain Specific Attenuation". etrieved 9 March, 11. [7] J. S. Ojo, M. O. Ajewole, et al., "ain ate and ain Attenuation Prediction for Satellite Communication in KU and KA Bands over Nigeria", Progress In Electromagnetics esearch B, 5, 7-3, 8. [8]. K. Crane, P. C. oinson," ACTS propagation experiment: rain-rate distriution oservations and prediction model comparisons", Proceedings of the IEEE, 85(6), , [9] X.X. Zhou,Y. H. Lee, et al., "Conversion model of one-minute rainfall rate distriution in Singapore", Paper presented at the IEEE Antennas and Propagation Society International Symposium, 9 (APSUSI '9). [1] T. H. Caronneau, D.. Wisely, Opportunities and Challenges for Optical wireless, SPIE Conference on Optical wireless communication, Massachusetts, 1998, Pg [11] K. S. Shaik, "Atmospheric Propagation Effects elevant to Optical Communication", TDA Progress eport, pp: , [1] A. K. Majumdar, "Free-Space Laser Communication Performance in the Atmospheric Channel", Journal of Optical and Fier Communications eports, (4), pp: , 5. [13] G. Keiser, "Optical Fier Communications", McGraw-Hill, 3 rd edition,. IJEBEA ; 13, IJEBEA All ights eserved Page 4