A Survey on Free Space Optical Link for Atmospheric Turbulence Strength Models

Transcription

1 A Survey on Free Space Optical Link for Atmospheric Turbulence Strength Models SOFIYA JENIFER.J Assistant Professor, Department of Electronics & Communication Engineering, Parisutham Institute of Technology and Science, Thanjavur, Tamilnadu, India. SARANYA.G Assistant Professor, Department of Electronics & Communication Engineering, Parisutham Institute of Technology and Science, Thanjavur, Tamilnadu, India. ABSTRACT: In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. Free Space Optical (FSO) communications is the only viable solution for creating a threedimensional global communications grid of interconnected ground and airborne nodes. The huge amount of data exchange between satellites and ground stations demands enormous capacity that cannot be provided by strictly regulated, scarce resources of the Radio Frequency (RF) spectrum. Free Space Optical (FSO) communications, on the other hand, has the potential of providing virtually unlimited bandwidth.various factors are affecting the performance of the communication channel in free space optical communication system. Developing model to get an accurate prediction of turbulence strength (C n ) become significant to understand the behavior of channel in different seasons. This dissertation deals with the survey of channel model such as Pamela, Hufnagel valley, Beam wandering, Polynomial regression, log-normal models. Keywords: Free Space Optical (FSO), Line of Sight (Los). I. INTRODUCTION Free-space optical communication (FSO) systems (in space and inside the atmosphere) have developed in response to a growing need for high-speed and tap-proof communication systems. Links involving satellites, deep-space probes, ground stations, unmanned aerial vehicles (UAVs), high altitude platforms (HAPs), aircraft, and other nomadic communication partners are of practical interest. Moreover, all links can be used in both military and civilian contexts. The FSO communication has been demonstrated at multi- Gbps data rates for few kilometer distances. FSO technology uses unlicensed optical wavelengths, which offers the high broadband communication capacity. The major impact on the quality of a laser beam propagating through the atmosphere over long distances is the atmospheric turbulences [1]. The received signal exhibits random intensity fluctuations in the presence of atmospheric turbulence. The greatest challenge of FSO is the performance evaluation under considering the effects of the atmospheric turbulences. Free space optical communications, is less expensive and high bandwidth access technique, which is receiving increasing attention with recent commercial application. Atmospheric turbulence is the major impact over FSO links [], which severely degrading the link performance. In this paper the detailed survey of channel models are presented. The rest of the paper is organized as follows: In Section II, experimental setup of free space optical (FSO) communication is explained. In section III, challenges of FSO are explained. In section IV, introduction to channel is explained. Section V introduces the analysis of different channel models. In Section VI, conclusion will be put forward. II. EXPERIMENTAL SETUP FSO communication technology is relatively simple. Based on the connectivity between FSO units, each consisting of an optical transceiver with a laser transmitter and a laser receiver to provide bi-directional capability. Each FSO unit uses a high-power optical source (i.e. laser or LED), added with a lens, which transmits light through the atmosphere to another lens receiving the information. The receiver lens connects to a high-sensitivity receiver through optical fiber. The major subsystems in an FSO communication system are illustrated in Figure 1.A data input produced by source is transmitted to a remote destination. The source output is modulated onto an optical carrier. Typically laser is then transmitted as an optical field through the atmospheric channel. Size, power and beam quality are the important aspects of the transmitter. Beam quality is used to determine the laser intensity and minimum divergence obtainable from the system [3]. The aperture size and the f-number are the important features on RES Publication 01 Page 58

2 the receiver side. The f/-number is used to determine the amount of the collected light and the detector field-of-view (FOV). The optical field is collected and detected generally in the presence of noise interference, signal distortion, and background radiation at the receiver side. The source data is modulated in three different ways: amplitude modulation (AM), frequency modulation (FM), or phase modulation (PM), each of which can be theoretically implemented at any frequency. noise (AWGN), whose spectral level is directly proportional to the receiver temperature. III. CHALLENGES OF FSO Free Space Optical communication (FSO) based wireless systems are not without challenges. The major limitation of free space optical communications emerges from the environmental factors through which it propagates. However relatively unaffected by rain and snow, free space optical communication systems can be severely affected by fog,beam wandering effects, scintillation and scattering effects [4]. The main design challenges in free space optical communications are depicted in figure-. Figure 1. BLOCK DIAGRAM OF FSO Intensity Modulation Direct Detection (IMDD) is the modulation scheme which is often used for an optical wave. Intensity is defined as flow of energy per unit area per unit time denoted in W/ m, and is proportional to the square of the optical field s amplitude. A collimated beam is produced by passing the light fields from the laser sources through the beam forming optics. There are two basic types of optical receivers: non-coherent receivers and coherent receivers. Whenever the transmitted information occurs in the power variation (i.e. IM) of the optical field the noncoherent receivers are used. Coherent receivers are also known as heterodyne receivers which optically mix a locally generated light wave field with the received field, and the combined wave is photo detected. It is used when the information is modulated onto the optical carrier using AM, FM, or PM, and are essential for FM or PM detection. The detected optical fields are affected by various noise sources present at the optical receiver. The three major sources in FSO communications are: background ambient light, photo detector induced noise, and electronic thermal noise in circuits [3]. Although background radiation may be reduced by the use of optical filtering, still it provides significant interference in the detection process. The detector quantum noise is originates from the randomness of the photon counting process at the photo detector. The thermal noise can be designed as additive white Gaussian Figure. Challenges in Free Space Optical Communication Factors affecting FSO: Many factors affect the performance of the FSO communication system. While designing the system, it is necessary to keep these factors and their effect on the system performance to achieve maximum performance. A. Scattering: Scattering is the form of radiation such as light passing through the atmosphere which is forced to deviate from a straight line trajectory due to localized nonuniformities present in the atmosphere. Light scattering can radically affect the performance of FSO communication systems. Rayleigh scattering: Rayleigh scattering arises from the electric polarizability of the particles. The oscillating electric field of a light wave which acts on the charges within a particle, causing them to move at the same frequency [5]. The particle therefore becomes a small radiating dipole whose radiation seemed to be as scattered light. The Figure 3 shows example of Rayleigh scattering on various particles present in air. RES Publication 01 Page 59

3 moderate fog with link attenuation potentials of approximately 3 db/km to 30 db/km. Figure 3.The beam of a 5mW green laser pointer is visible at night partly because of Rayleigh scattering on various particles and molecules present in air Mie Scattering: When the large particles in the atmosphere are able to scatter all wavelengths of light equally then the Mie scattering occurs. Particles denotes as an aggregation of material that constitutes a region with refractive index (n p ) that differs from the refractive index of its surroundings (n med ).The Figure 4 represents the example of Mie scattering of water droplets. Figure 4.The grey/white colour of the clouds is caused by Mie scattering by water droplets which are of a comparable size to the wavelength of visible light B. Absorption: Atoms and molecules are naturally characterized by their refractive index. The imaginary part of the refractive index, k, is related to the absorption coefficient, α, which is given by the following equation: α = 4πk = σ λ A N A (1) Where λ is the wavelength of the source is, σ a is the density and N A is the coefficient. C. Rain: Rain has a distance reducing nature in FSOC. When compared to fog, it has less significant impact. This is because the radius of raindrops ( μm) is significantly larger than the wavelength of typical FSOC light sources [5]. Typically the rain attenuation values are moderate in nature. D. Snow: Snowflakes are ice crystals that come in a different shapes and sizes. In general, however, snow tends are to be larger than rain. White out conditions might attenuate the beam, but scattering doesn't tend to be a big problem for FSOC systems because the size of snowflakes is larger when compared to the operating wavelength [6]. The impact of light snow to blizzard and white out conditions are falls approximately between light rains to E. Visibility Low visibilities will decrease the effectiveness and availability of FSO systems. Long-term weather observations show that some cities, such as Seattle, WA, have lower average visibilities than cities such as Denver, CO. This means that for the same distance, the same FSO system in Denver will experience a higher availability than a system installed in Seattle. Low visibility can occur during a specific time period within a year or at specific times of the day (such as in the early morning hours). Especially in coastal areas, low visibility can be localized phenomena (coastal fog). This means that for the same distance, the same FSO system in Denver will experience less downtime than in Seattle. One solution to the negative impact of low visibility is to shorten the distance between FSO terminals to maintain a specific statistical availability figure. This provides a greater link margin to handle bad weather conditions such as dense fog. Redundant path operation can improve the availability if the visibility is limited on a local scale. Examples are fog across a river or pond or an air conditioner's exhaust stream on top of a roof. Another solution is to use a multiple beam system to maintain higher link availability. Low visibility and the associated high scattering coefficients are the most limiting factors for deploying FSO systems over longer distances. F. Distance: Distance impacts the performance of FSO systems in three ways. First, even in clear weather conditions, the beam diverges and the detector element receives less power. For a circular beam, the geometrical path loss increases by 6 db when the distance is increased by a factor of two. Second, the total transmission loss of the beam increases with increasing distance. Third, scintillation effects accumulate with longer distances. Therefore, the value for the scintillation fade margin in the overall power budget will increase to maintain a predefined value for the BER.Most commercially available FSO systems are rated for operation between 5 5,000 m, with high-powered military and satellite systems capable of up to,000 km. Most systems rated for greater than 1 km incorporate three or more lasers operating in parallel to mitigate distancerelated issues. It is interesting to note that in the vacuum of RES Publication 01 Page 60

4 space, FSO can achieve distances of thousands of kilometers. G. Bandwidth: In standard O-E-O FSO systems, two elements limit the bandwidth of the overall system. These elements are the transmission source and the photo detector. When LEDs are incorporated into FSO systems, the bandwidth is typically limited to 155 Mbps. When laser sources are used, the speed can be much higher. Directly modulated lasers operating up to.5 Gbps are commercially available for use in FSO systems. At higher speed such as 10 Gbps or above, external modulators can be used to modulate the cw output of a laser source [6]. Where, P t is the transmitted power, G l is the product of the transmit and receive antenna field radiation patterns, λ is the wavelength and d is the link distance. Theoretically, the power falls off in proportion to the square of the distance. In practice, the power falls off more quickly, typically 3 rd or 4 th power of distance. The channel between transmitting and receiving antenna is shown in figure 5. Figure 5.Illustration of typical wireless data link IV. CHANNEL INTRODUCTION A. Channel: The channel refers to the medium between the transmitting antenna and the receiving antenna. The characteristics of wireless signal changes as it travels from the transmitter antenna to the receiver antenna. These characteristics depend upon the measure of distance between the two antennas, the path(s) taken by the signal and the environment (buildings and other objects) around the path [7]. The profile of received signal can be obtained from that of the transmitted signal if the medium between the two is modeled. This model of medium is called channel model. In general, the power of the received signal can be obtained by convolving the power of the transmitted signal with the impulse response of the channel. Convolution in time domain is equivalent to multiplication in the frequency domain. Therefore, the transmitted signal x after propagation through the channel H becomes y, which is given in equation (). Y(f) =H(f)x(f)+n(f) () Where H (f) is channel response, x (f) is the input, y (f) is the impulse response and n (f) is the noise. Note that x, y, H, and n are all functions of the signal frequency f. B. Path Loss: The simplest channel is the free space Line of Sight channel with no objects between the receiver and the transmitter or around the path between them. In this case, the transmitted signal attenuates since the energy is spread spherically around the transmitting antenna. For this LoS channel, the received power (P r ) is given below P r = P t G 1λ 4πd (3) V. OPTICAL TURBULENCE MODELS Atmospheric conditions are apparently affecting the performance of FSOC system making them highly susceptible to degrading effects of pointing errors and atmospheric turbulence strength (C n ). The main factors which can reduce the link performance of FSOC are aerosol, scattering effects caused by rain, snow and fog. The major factor causing pointing errors is sway of high rise buildings which in turn is caused due to thermal expansion, dynamic wind loads and big earthquakes. Another most important impairment in the FSOC system performance is the C n. In homogeneity in the temperature and pressure fluctuations leads to variations in the refractive index, results in C n and causes the random fluctuations of the phase and intensity of the received signal known as channel fading [7]. Intensity fluctuations caused by channel fading leads to an increase in the system s BER. The C n not only varies as a function of altitude(h), but also according to local conditions such as terrain type, geographic location, cloud cover and time of day. Several dozen turbulence profile models have been developed from experimental measurements made at a variety of locations [8]. Most are designed based on the structure of optical turbulence profile with the units of measurement for h and C n being m and m -/3 respectively. A number of statistical channel models have been proposed to describe weak or strong atmospheric-induced turbulence fading. In this aspect, the PAMELA, Hufnagel-valley model, Beam wandering model, Polynomial Regression models were derived and briefed. A. Lognormal Model: RES Publication 01 Page 61

5 Lognormal distribution is widely used model for the probability density function (pdf) of the irradiance due to its simplicity in terms of mathematical calculation. This turbulence model is only applicable to weak turbulence conditions and for propagation distances less than 100 m. Considering lognormal model, the pdf of the received optical field I is given as f (I ) in equation (4). [3]. f I = 1 exp ln i m i πσ f σ (4) i where mi is the mean and σi the standard deviation of ln(i). The scintillation index as a function of variance is given by σ SI = e σ i 1. For weak turbulence, SI falls in the range of [0, 0.75]. As the strength of turbulence increases, multiple scattering effects should be taken into consideration. In such cases, lognormal statistics exhibit large deviations compared to experimental data. The detection and fade probabilities which are mainly based on tails of the pdf are not accurately analyzed as lognormal pdf underestimates the behavior as compared with experimental results. This in turn affects the accuracy of performance analysis. B. PAMELA Model: The PAMELA model provides the atmospheric strength within the surface boundary layer. The latitude, longitude, date, time of day, percent cloud cover, and terrain type, as well as the single measurement of the atmospheric temperature, pressure, and wind speed at the desired height are the inputs [9]. PAMELA model was adapted from more complicated similarity based optical turbulence models which is given in equation 4, provides C n estimation within the surface boundary layers and it accepts all the above parameters of test fields as a inputs. C. Hufnagel-ValleyModel: The Hufnagel Valley model is one of the popular models for inland sites and daytime viewing conditions [10]. It permits variations in high altitude wind speed and near ground turbulence levels. In this model atmospheric turbulence strength is assigned as a sum of three exponential decay terms corresponding to a surface boundary layer, a strong layer caused by the high altitude jet stream, and a background tropopause layer. D. Beam Wandering Model: In the absence of turbulence, a laser beam exiting a transmitting aperture diameter D would have an angular spread θ o approximately λ/d, where λ is the wavelength of the transmitted optical beam. This spread becomes larger at the receiver for turbulent atmospheric case because moving turbulent eddies cause scattering of the optical fields. In addition to such beam spreading other effects such as beam wandering or even breakup of the beam in to multiple beams may occur. Turbulent eddies are tend to have smaller sections in which the refractive index varies faster compared to the overall turbulent region. As such, the beam spread may be shortterm or long term or both. By definition when a laser beam interacts with turbulent eddies whose dimensions are larger compared to the diameter of the laser beam, the deflection of the beam is relatively pronounced (although relatively low). Such beam spreading is termed as long-term spread or beam wandering [9]. The beam wander effect is related to the displacement of the instantaneous center of the beam the point of maximum irradiance of a travelling wave over the receiver plane. In contrast, those eddies which are smaller compared to the beam diameter tend to broaden the beam, but do not deflect it significantly. In general, the turbulent eddies continually flow across the laser beam with a transverse flow velocity and cause the deflection of the laser beam in different directions with time intervals in the order of D/ v. If we observe the super positioned beams over the interval greater than D/ v, then resultant beam seems to be a single beam with larger diameter which is greater than the short term spread. For Gaussian beam propagation over turbulent media, use with link lengths much shorter than k c n lo and ρo >> Dt, then mean square radius of the beam wander is approximated by Fante as < ρ L > 4L k D + D t t 4 RES Publication 01 Page 6 1 L F + 4L Where, ρ o = k L c n 3/5. k ρ o (5) The first two terms in the equation (5) represents the beam spread in vacuum, the last term represents the additional spread due to scattering of the laser beam by the turbulent eddies. The long term beam spread is a zero mean normal random variable with variance given by equation (5). For the case of weak turbulence and shorter link lengths (less than k c n lo 5 3 ).Where lo inner scale of the turbulent is eddies) the variance due to beam wandering effect is strongly dependent on receiver aperture diameter. In the D Cartesian or polar coordinate receiver plane, the stochastic polar variable of the beam is computed by coordination transformation given in equation (6)

6 γc = x c + y c (6) Where x and y are the Cartesian coordinate of the center of the Gaussian light spot on (OPD) and used to track the beam wander with respect to origin (0,0) mm. The beam wander can be statically characterized by the variance of γ and related with the atmospheric turbulence strength using the geometrical optics is given in equation (7). C n =< γ c >/.4R 3 W 1 3 (7) E. Polynomial Regression: This model is developed to obtain the best estimation of C n according to the macro scale meteorological data insitu. The concept of temporal and solar hour is introduced. Temporal hour at sunrise is 00:00, at noon is 06:00 and at the sunset is 1:00 in any day. Further it is allowed to have negative values [10]. The polynomial regression model is given in equation (8) C n = W T RH RH RH Ws Ws Ws (8) Where W is the temporal hour weight values taken for computations. This model is best one especially in practical manner since it requires only macro scale meteorological parameters which can be measured directly by a suitable weather station. This model is rehashed with the introduction of the solar radiation and aerosols loading in the atmosphere which is given in equation (9) C n = T RH RH 3.9 RH Ws Ws Ws SF TCSA TCSA (9) Where, SF is solar flux (kwm - ) and TCSA is total cross section area (cm / m 3 ).The TCSA can be determined by TCSA = RH RH RH RH RH ln RH SF (10) VI. CONCLUSION In this paper a detailed survey on atmospheric strength models prediction for FSO application is discussed. Obviously the model provides the basis for making predictions about the outcomes of experiments and/or measurements. The direct methods to practical atmospheric problems are usually given by sheer size and complexity of the atmosphere. Most of the existing models are derived from the data corresponding to their local atmospheric conditions; therefore they failed to attain the generalization on predicting the atmospheric turbulence strength. Furthermore these models do not offer any suitable means to tune their parameters to fit to new test fields. From this, it is concluded that, new models become significant to provide more accurate prediction on atmospheric turbulence strength. ACKNOWLEDGMENT Apart from my efforts, the success of any work depends on the support and guidelines of others. I take this opportunity to express my gratitude to the people who have been supported me in the successful completion of this work. I owe a sincere prayer to the LORD ALMIGHTY for his kind blessings without which this would not have been possible. REFERENCES [1]. H. Willebrand and B. S. Ghuman, Free Space Optics:Enabling Optical Connectivity in Today s Networks Indianapolis, IN: Sams, 00. []. X. Zhu and J. M. Khan, Free-space optical communication through atmospheric turbulence channels, IEEE Trans. Commun., vol. 50, pp , Aug. 00. [3].Hennes Henniger, Otakar Wilfert, An introduction to free space optical communications, RADIO ENGINEERING, VOL. 19, NO., JUNE 010. [4] John Kaufmann, Free Space Optical Communications: An Overview of Applications and Technologies, Boston IEEE Communications Society Meeting December 1, 011. [5]. Amarjeet Kaur, Ravinder Kumar Panchal, Analysis the Effect Atmosphere Turbulence in Free- Space Optical (FSO Communication Systems, IJEIT, Volume 3, Issue 11, May 014. [6].Brandon Rodenburg1, Mohammad Mirhosseini1, Mehul Malik1, Omar S Magaña-Loaiza1, Michael Yanakas1, Laura Maher1, Nicholas K Steinhoff, Glenn A Tyler and Robert W Boyd, Simulating thick atmospheric turbulence in the lab with application to orbital angular momentum communication, New Journal of Physics,vol 16 march 014. [7], Arun K. Majumdar, Jennifer C.Ricklin, Free Space Laser Communications: Principles and Advances: Optical and Fibre Communications Reports, (Springer, USA,008), RES Publication 01 Page 63

7 [8] A. Arockia Bazil Raj, J. Arputha Vijaya Selvi, Real time measurement of meteorological parameters for estimating low altitude atmospheric turbulence strength(c n ), IET Sci.Meas.Technol.,March 014. [9] A.Arockia Bazil Raj, J.Arputha Vijaya Selvi, Comparison of Different models for ground-level atmospheric turbulence strength (C n ) prediction with a new model according to local weather data for FSO applications, Applied Optics, Vol. 54, Issue 4, pp (015). [10].Murat Uysal, Jing (Tiffany) Li and Meng Yu, Error Rate Performance Analysis of Coded Free-Space Optical Links over Gamma-Gamma Atmospheric Turbulence Channels, IEEE Transactions on Wireless Communications, Vol. 5, No. 6, June 006. AUTHOR S BIOGRAPHIES J. Sofiya Jenifer completed her bachelor s degree in Electronics and Communication Engineering at Anjalai Ammal Mahalingam Engineering College affiliated Anna university in the year 013 and her Master Degree in VLSI Design at Kings College of Engineering affiliated Anna University in the year 015. She is working as Assistant Professor in the department of Electronics and Communication Engineering of Parisutham Institute of Technology and Science. Her area of interest includes, VlSI design, Communication systems and Optics. G. Saranya completed her bachelor s degree in Electronics and Communication Engineering at St. Joseph s College of Engineering and Technology affiliated Anna university in the year 01 and her Master Degree in Communication systems at Parisutham Institute of Technology and Science affiliated Anna University in the year 014. She is working as Assistant Professor in the department of Electronics and Communication Engineering of Parisutham Institute of Technology and Science. Her area of interest includes, Communication systems, Microprocessors and Optics. RES Publication 01 Page 64