ADVANCES in NATURAL and APPLIED SCIENCES ISSN: 1995-0772 Published BYAENSI Publication EISSN: 1998-1090 http://www.aensiweb.com/anas 2016 December10(17):pages 203-209 Open Access Journal Suppressing of Laser Signal Attenuation based FFSO Communication System 1 Shehab A. Kadhim, 2 Firas S. Mohammed, 2 Farouk. Kh. Shaker 1 Laser & Optoelectronics Research Center, Ministry of Science & Technology,Baghdad, Iraq 2 Department of Physics, College of Science,Al-Mustansiriyah University,Baghdad, Iraq Received 2 September 2016; Accepted 2 December 2016; Published 31 December 2016 Address For Correspondence: Firas S. Mohammed, Al-Mustansiriyah University, Department of Physics, College of Science, Baghdad, Iraq Copyright 2016 by authors and American-Eurasian Network for Scientific Information (AENSI Publication). This work is licensed under the Creative Commons Attribution International License (CC BY).http://creativecommons.org/licenses/by/4.0/ ABSTRACT Scattering of the light photons by different aerosols attenuate the free space optical (FSO) propagation that hinders network deployment due to loss of the line of sight (LoS). In this paper, the atmospheric turbulence effects on active receiving area of the Full Free Space Optical (FFSO) communication systems was investigated. From the results, it has been shown that if the number of integrated fiber optics receiver increases, the BER enhance and the performance of FSO links are improved both in the weak and strong turbulence condition. KEYWORDS: free space optics, atmospheric turbulence, OptiSystem INTRODUCTION outdoor optical wireless communication is commonly known as free space optical (FSO) communication [1]. FSO communication uses light propagating in a free space to transmit data between two points. FSO communication has recently gained a growing interest for both commercial and military applications [2] [3]. Instead of enclosing the data stream in a glass fiber, it is transmitted through the air. Free-space optical (FSO) communication is the most practical alternative to solve the bottleneck broadband connectivity problem, and as a supplement to conventional radio frequency (RF)/microwave links. FSO is all-optical, unlike the well-known RF wireless systems. FSO has become a high-bandwidth, viable wireless alternative to optical fiber cabling. So, one gets the speed of a fiber without the substantial costs of digging up sidewalks to install a fiber link. One of the solutions proposed to address the turbulence is by using hybrid system (FSO switches to RF) [4], or using optical fiber coupling with adaptive optics [5]. Single input multi output technique (SIMO)technology can significantly improve the data capacity through spatial multiplexing by increasing the numbers of receiving point as shown in Figure (1). The SIMO technology cannot only increase the data rate but also improve the system reliability through spatial diversity [6]. this work present the systems that use optical fiber as the receiver as full-optical free-space optical communication systems to overcome the atmospheric obstacles. Fig. 1: SIMO Communications Systems under turbulence condition. ToCite ThisArticle: Shehab A. Kadhim, Firas S. Mohammed, Farouk. Kh. Shaker., Suppressing of Laser Signal Attenuation based FFSO Communication System. Advances in Natural and Applied Sciences. 10(17);Pages: 203-209
204 Shehab A. Kadhim et al., 2016/Advances in Natural and Applied Sciences. 10(17) December 2016, Pages: 203-209 FSO communication Model: a FSO network involves atmosphere (turbulence, scattering) as communication channel for establishing optical wireless connection between transceivers. The LOS propagation path can range from hundreds of meters up to tens of kilo meters [7]. In the visible and IR wavelengths, light propagation through the atmosphere is affected by two phenomena: Absorption and scattering by air molecules and absorption and scattering by solid or liquid suspended particles present in the atmosphere. These are aerosols such as dust, haze, mist, and fog. According to Beer-Lambert s law, the received irradiance at a distance L from the transmitter is related to the transmitted irradiance by the following model [8]: τ( λ, L) = P R P T exp ( γ( λ), L) (1) Where:- γ( λ): The total attenuation coefficient m 1. P R : The received optical power at a distance L. P T : The transmitted optical power at the optical source. τ( λ, L): The transmittance of the atmosphere at wavelength λ. The total attenuation coefficient is change depending on the presence of downpour and this attenuation coefficient is the sum of the absorption and the scattering coefficients from aerosols and molecular constituents of the atmosphere, so: γ( λ)=α m ( λ)+α a ( λ)+β m ( λ)+β a ( λ) (2) The first two terms on the right hand side represent the molecular and aerosol absorption coefficients, and the last two terms are representing the molecular and aerosol scattering coefficients respectively. Neglect the attenuation contribution by molecular & aerosol absorption and molecular scattering as it is very small when compared with attenuations due to aerosol scattering, the equation (2) [9] is thus reduced to: γ( λ)=β a ( λ) (3) In order to compute attenuations caused by fog, smoke, dust, snow effects, mostly we rely on empirical approaches as they are convenient when approximation to very complex and time attrition theoretical approaches based on microphysical models. The most common empirical model is based on visibility range estimate. Established on the visibility range estimate with a 2% transmission threshold over the atmospheric path, attenuation resulting from scattering phenomenon can be estimated by [10]: γ(λ) B a 17.35 V ( λ 550 ) q (4) Where V is the visibility in km, λ is transmission wavelength in nm. γ(λ):is the total extinction coefficient for fog and q is the size distribution coefficient of scattering related to the size distribution of the fog droplets. The parameter q in equation(4) depends on the visibility distance range and is given by the following equation [11]: 1.6 if V > 50 km 1.3 if 6km < V < 50 km q= 0.16V+0.34 if 1km < V < 6 km (5) V- 0.5 if 0.5km < V < 1km 0 if V < 0.5 km
205 Shehab A. Kadhim et al., 2016/Advances in Natural and Applied Sciences. 10(17) December 2016, Pages: 203-209 FFSO -Design and Simulations: Although the first step in designing a wireless communication, such as (FSO) communication system in different media channels is to know what happens to an optical wave or a signal as it travels through that medium. The tremendous bandwidth favorable by FSO communications is available only under clear atmospheric conditions. Where there is no dispersion and power loss is practically zero. However, this is not a realistic situation and to exploit the great potentials of FSO communications, proper measures should be used in transmitter and receiver designs.simo communications systems are proven to be effective in RF or optical fading channels. Using (OptiSystem7.0) simulation software, three models of FSO systems have been designed. First, we design the main free space optical Link with 1Km rang, this system is equipped within (850nm) wavelength, and power about (24 mw) transmitter includes the PRBS (Pseudo Random Bit Sequence) generator. NRZ pulse generator, a laser source and MZM (Mach Zehnder Modulator). In the simulation, data generated by the PRBS generator at 10Gbps are encoded and light modulated using MZM, where the laser source acts as the carrier source as shown in Figure (2). Fig. 2: The main free space optical Link. The laser is Pass through the four air channels in free space, and this ability is divided by (power splitter), To be provided with all the air channel capacity of approximately (5 mw) as a simulation of the commercial system (LightPointe Flight Strata 155E 1.25Gbps/850nm Laser Link). In the simple main link design we studied all the loss effect on the free space optical communication system, resulting by scattering and the turbulence under different condition (clear, fog, smoke,dust and rain). Enhancement of the received signal by using a modified scatter receiver system, including a multi-mode optical sensor coupling to search for the scattered signal as shown in Figure (3), contains only one multi-mode optical fiber (3m) strapped across a transmitter integrated the same original transmitter specification, but is provided with minimal power only 1mw Which will be considered as a secondary source for the main system through using a power combiner element, in order to enhance received power and minimize generated losses due to turbulence weather.
206 Shehab A. Kadhim et al., 2016/Advances in Natural and Applied Sciences. 10(17) December 2016, Pages: 203-209 Fig. 3: A multi-mode optical sensor coupling to search for the scattered signal. Typical attenuation values of four conditions (clear, Low, mid, high) turbulence respectively, are given in Table (1). The optical signals from the FSO channel are received by (APD) photo detector. These simulations use three visualizers namely optical power meter to measure the power received in both (dbm) and Watts, optical spectrum analyzer which provide the facility to analyze the optical spectrum, finally BER analyzer automatically calculates the BER value. Table 1: Atmospheric attenuation in (db/km) as a function of visibilities for 850nm. Atmospheric Turbulence condition Attenuation value(db/km) Visibility(km) Clear 0.2-0.72 22.4 Low turbulence 0.73-7.93 1.92 Mid turbulence 7.99-21.5 1.51 High turbulence 21.7-33.6 0.62 A combination between the main FSO link with two integrated fiber optic coupling sensor sub-systems, as a demonstration of collecting scattered signals due to channel turbulence from more than one side and strengthened with the main FSO received signal. This design as shown in Figure (4), includes the two integrated fiber optic coupling sensor with main link. The two of side are the same transmitter wavelength and loaded data, but with minimal power about (1mW) per each. Which simulated to be considered as a secondary sources of the main system and is linked to each of the other side with main system by using the power combiner element.
207 Shehab A. Kadhim et al., 2016/Advances in Natural and Applied Sciences. 10(17) December 2016, Pages: 203-209 Fig. 4: FFSO system using two integrated fiber optic coupling sensor subsystems. RESULTS AND DISCUSSION We shall consider in detail the situation of optical propagation between two points in terrestrial applications. Therefore attenuation coefficient, received optical power, SNR, and BER using (850nm) laser beam was studying in this Simulations work for above three systems shows in Figures (3,4,5). FSO systems with a power of 24mw, range of 1 km were analyzed under different weather condition. Table (2) shows system parameters used in a simulation study. Table 2: System parameters which used a simulation study. Parameter Value Transmitter optical power (mw) 24 Transmitter divergence angle (mrad) 2 Transmitter efficiency 0.5 Receiver sensitivity (dbm) -20 Receiver diameter (cm) 8 Receiver efficiency 0.5 Figure (5) represents the FSO system attenuations under different weather conditions (high, mid, low and clear).where the received power inversely exponentially proportional with attenuation, while at low attenuation the received power will be look like constant. Since the turbulence leads to the reduced received optical power, hence increasing the bit error rate (BER) performance. Fig. 5: Total attenuation(db/km) verses Received Power (mw) for main link.
208 Shehab A. Kadhim et al., 2016/Advances in Natural and Applied Sciences. 10(17) December 2016, Pages: 203-209 Figure (6) represent the signal to noise ratio (SNR)was plotted versus Bit error rate7 (BER) for different attenuation values (high,mid,low and clear). Where ( BER) goes on increasing when the signal to noise ratio was decreased due to weather turbulence effect. Fig. 6: Bit error rate verses signal to noise ratio for main link. For a system which has very high losses in receiving signal due to turbulent weather effects, we will enhance the received power by using the integrated optical fiber sensor system to compensate for the losses generated as a result of scatter phenomenon. Fig.7 represents a comparison between coupling (one integrated optical fiber sensor) as ( photo-scatter receiver) link and coupling with (two integrated optical fiber sensor) as (Optimal design) technique to the main link. (a) (b) Fig. 7: Attenuation verses received power in (a) photo-scatter receiver and (b) optimal design system under different weather conditions. Fig.87 (a) represents a comparison between main link and (photo-scatter receiver) link under different weather conditions (high, mid, low and clear). When the attenuation increases, the received power decrease in the main link (original signal). But when adding the (photo-scatter receiver) link under the same weather conditions, especially in the case (high turbulence), It s clear the benefit of using design of scatter link through the increasing in receiving power. Also Fig.7 (b), represent a comparison between main link and Optimal design technique,where the same link under the same weather conditions, especially in the case (high turbulence). It s clear the benefit of using the optimal link through the high increasing in receiving power and thus increase the performance of the system. Fig.8 represents a comparison between coupling (one optical fiber sensor) as (photo-scatter receiver) link, Fig.8 (a) and coupling (two optical fiber sensor) as (Optimal design) main link, Fig.8 (b). It can be observed through Simulation results that as the signal to noise ratio increase, BER goes on decreasing when using design of scatter link with the main FSO system. Also the benefit of using
209 Shehab A. Kadhim et al., 2016/Advances in Natural and Applied Sciences. 10(17) December 2016, Pages: 203-209 Optimal design technique because the signal to noise ratio is high increasing and thus increase the performance of the system. (a) (b) Fig. 8: SNR verses BER in (a) photo-scatter receiver and (b) optimal design system under different weather conditions. Conclusion: As a conclusion, FFSO link was designed and simulated to investigate availability and error performance under the influence of different weather conditions. In this paper, we have analyzed the atmospheric parameters that affect the performance of FSO links. From the results, it has been shown that if the number of integrated fiber optics receiver increases, the BER enhance and the performance of FSO links are improved as well as the efficiency of adding diversity in error rate and outage capacity appears more in SIMO, both in the weak and strong turbulence condition. REFERENCES 1. Boucouvalas, A.C., 2005. Challenges in Optical Wireless Communications, Optics & Photonics News, 16(5): 36-39. 2. Goetz, P., W. Rabinovich, R. Mahon, J. Murphy, M. Ferraro, M. Suite, W. Smith, B. Xu, H. Burris, C. Moore, W. Schultz, B. Mathieu, W. Freeman, S. Frawley, M. Colbert, and K. Reese, 2010. Modulating retro-rector laser communication systems at the naval research laboratory, Proceedings of Military Communications Conference (MILCOM' 10), pp: 1601-1606. 3. Son, I.K., S. Kim and S. Mao, 2010. \Building robust spanning trees in free space optical networks, Proceedings of Military Communications Conference (MIL- COM' 10), pp: 1857-1862. 4. Zabidi, S., W. Al-Khateeb, M. Islam and W. Naji, 2010. The e_ect of weather on free space optics communication (FSO) under tropical weather conditions and 189 a proposed setup for measurement, Proceedings of International Conference on Computer and Communication Engineering (ICCCE' 10), pp: 1-5. 5. Kim, Issac I. and Eric Korevaar, 2002.Availability of Free Space Optics (FSO) and Hybrid FSO/RF Systems. Optical Access Incorporated. 6. Xueying Wu, Peng Liu, Mitsuji Matsumoto, 2010. 'A Study on Atmospheric Turbulence Effects in Full- Optical Free-Space Communication Systems ', 978-1-4244-3709-2. 7. Kartalopoulos, S.V., 2011. Free Space Optical Networks for Ultra-Broad Services, Wiley, USA. 8. Navidpour, S.M., M. Uysal and M. Kavehrad, 2007. BER performance of free-space optical transmission with spatial diversity, IEEE Trans. Wireless Commun., 6(8): 2813-2819. 9. Killinger, K.D., J.H. Churnside and L.S. Rothman, 1995. Atmospheric Optics, Chap. 44, OSA Handbook of Optics, pp: 44.1-44.50. 10. Shettle, E.P., 1989. Models of Aerosols, Clouds, and Precipitation for Atmospheric Propagation Studies, in Atmospheric Propagation in the UV, Visible, IR, and MM Wave Region and Related Systems Aspects, AGARD Conf. Proc. 454(15): 1-13. 11. Kim, I.I., B. McArthur and E. Korevaar, 2001. Comparison of Laser Beam Propagation at 785 nm and 1550 nm in Fog and Haze for Optical Wireless Communications, Proc. SPIE 4214: 26-37.