Theoretical Analysis of Multiple Transmitters/Receivers on the Performance of Free

Space Optics (FSO) Link Nur Haedzerin Md. Noor, A.W. Naji, Wajdi Al-Khateeb

IIUM-MIMOS Cyberspace Security Lab, Department of Electrical & Computer Engineering, Faculty of Engineering, International Islamic University Malaysia (IIUM),

53100, Kuala Lumpur, Malaysia haedzerain@yahoo.com

Abstract— The Free Space Optics (FSO) vulnerability towards atmospheric phenomena has introduced the usage of multiple transmitters/receivers in the FSO system as the optimal solution. With current interest in this technology as an alternative backup to the already established broadband technology, a proper understanding on the performance of FSO link has become vital and thus arise the need to examine the effect of these multiple transceivers on FSO link. This paper gives the preliminary result of the FSO link performance in terms of geometrical losses, power received and link margin using precise mathematical relations regardless of the various atmospheric conditions. Thus, this is the first step towards developing a comprehensive theoretical analysis of multiple transmitters/receivers on FSO link to be used as a benchmark in the practical implementation.

Keywords-component: Free Space Optics (FSO), line of sight (LOS), geometrical losses, link margin, multiple TX/RX FSO

I. INTRODUCTION Free space optical (FSO) communications is a line-of-sight

(LOS) technology that transmits a modulated beam of visible or infrared light through the atmosphere for broadband communications [1]. The concept remains relatively simple: a narrow beam of light which carries whatever optical signal is launched at a transmission station, transmitted through the atmosphere, and subsequently received at the receive station [6]. Therefore, this technique requires a clear line of sight between transmitter and receiver [7]. The transmission through FSO is fast as it offers the communications at the speed of light. FSO is often referred as “fiberless optics” or “optical wireless” transmission since it offers the speed of fiber with the flexibility of wireless.

The FSO communication system has a number of advantages. First, no licensing is required [2] in terrestrial communication link. Another advantage is the immunity to the radio frequency interference or saturation [3] has added the security features in this technology. The point-to-point laser signal is extremely difficult to intercept [4], making it ideal for stealthy communications, such as in quantum optics field known as Quantum Key Distribution (QKD). With a narrow beam angle for several miliradians, it is very hard to jam or tap the FSO. Environmental wise, FSO does not pollute the environment with electromagnetic radiation on RF since the wavelength of FSO is only from 850nm to 1500nm. Moreover,

economically, aside from the usage of laser which can contribute to the reduction in the equipment cost, the deployment of this wireless link is easier, faster and cheaper compared to optical cables.

Despite of all the advantages, FSO link performance is aggravated by atmospheric phenomena. The propagation medium of FSO link is in the troposphere region where most of the atmospheric phenomena occurred [5]. Since FSO is the LOS propagation, the transmission of light is greatly affected by these atmospheric phenomena. The link performance gets worse with the increment of the distance between the transmitter and receiver. Moreover, the climatology and the physical characteristics of the installation location also affect the performance of the FSO.

Commonly FSO systems used single transmitter and receiver to transmit the optical signal but it is possible to incorporate more than one transceiver to improve the system performance. This work proposes the usage of multiple transmitters/receivers (TX/RX) for terrestrial FSO links. The effort here is to focus on the most important parameters used to determine the FSO link’s performance of multiple transmitters/receivers and to simulate their behavior using the accurate mathematical relations that has been derived and improved by previous researches.

The remainder of the paper is organized as follows. Section

II presents the system overview of the multiple TX/RX FSO links and design. 4 transmitters and receivers are considered. Section III deals with the basic FSO link performance parameters used in this work and its mathematical modeling. Section IV describes the analysis done using a developed FSO link performance analysis model and comparison on the link performance. Finally, Section V gives the conclusion of the overall work.

II. SYSTEM OVERVIEW The main parameter in a FSO multiple transmitters/receivers

link design is the distance between the two terminals as seen in Fig 1. Each terminal, incorporate a link head which consists of multiple lenses of transmitters and receivers which will produce and collect the multiple laser beams. In [8], the multiple beams are independent beam when they leave the

Proceeding of the 2011 IEEE International Conference on Space Science and Communication (IconSpace) 12-13 July 2011, Penang, Malaysia

978-1-4577-0564-9/11/$26.00 ©2011 IEEE 291

transmitter’s units but over the distance, they begin to overlap and by the time they reach the receiver’s unit they become a single high powered spot of communication signal. The product in [13] used the multiple laser beams technology where the redundant signals are generated by the data splitter which splits the digital signal into 4 independent signals; each carries the same data input. These signals are then transmitted using the spatially diverse transmitters labeled as TX1, TX2, TX3, and TX4. Hence, each transmitter will transmit 4 independent laser beams to each of the receivers, labeled as RX1, RX2, RX3, and RX4. In total, there will 16 paths of laser beams altogether for the data to travel along the optical path.

By substituting the single transmitter with the several smaller transmitters with the same total power, it is expected to improve the link performance as it may increase the system diversity [11]. In this paper, the maximum number of transmitters/receivers involved is 4x4 combinations. Meaning that, there are a total of 16 combinations of these transmitters and receivers for both sites to be theoretically analyzed for its link performance.

III. FSO LINK PERFORMANCE PARAMETERSS In this section, the basic FSO link performance parameters

which are geometrical loss, received power, and link margin are investigated. These parameters are fundamental and can be used to quantify the FSO link performances which are either unaffected or affected by the atmosphere. Fig. 2, Fig. 3 and Fig. 4 illustrate the FSO link performance parameters of multiple transmitters/receivers with regards to the link distance. Notably, for this project, the FSO equipment used is FlightStrata 155 by Lightpointe. Therefore, all the theoretical calculations and simulation made are referenced to the manufacturer’s design specification.

A. Geometrical Losses Even though the atmospheric instability is said to be the

main contribution to the FSO link loses for the outdoor FSO system, the FSO device itself primarily adds to the loses. The geometrical losses, LGEO [dB] or also known as optical beam attenuation is the reduction in optical power directly resulting from the spreading of the laser beam as it propagates from one

end of the link to the other [9]. Some of the light beam will not hit the receiver’s lenses. As a result, the diverged light beam during the propagation along the optical path introduces the signal loss. As a solution to device shortcoming, a larger receiver aperture or the multiple receiver apertures which is equivalent to a larger diameter of one receiver aperture, is used.

The geometrical losses are calculated as in (1), for single receiver and (2), for quadruple receiver which is suggested in [6] and [9] respectively. Where dRX and dTX are diameter of the transmitter’s and receiver’s (m) lenses and ℓ is the link distance from site A to site B (km). ARX is the receiver aperture area and NRX is the number of receivers used. While θ is the transmit beam divergence (mrad) which is the angle of the light emitted from the transmitter. This angle determines the beam width of the laser beam as it travels across the optical path [9].

LGEO = 10 log10 [dRX / (dTX + θℓ)]2 (1)

LGEO = 10 log10 [4(ARXNRX) /π(θℓ)2] (2)

As seen in Fig. 2 the geometrical loss for both the single TX/RX and multiple TX/RX increases with the increment of the link range. However, the usage of multiple TX/RX can reduce the geometrical loss compared to single TX/RX. The geometrical loss keeps decreasing with the increment of the number of transmitter and receivers used by about 6.02 dB from a single TX/RX to quadruple TX/RX.

Figure 2. Geometrical losses for multiple transmitter/receiver

Figure 1. Multiple transmitters/receivers FSO link configuration.

292

Figure 3. Power received for multiple transmitter/receiver

Figure 4. Link margin for multiple transmitter/receiver

B. Received Power The power received, PRX at the optical receiver is the core

parameters to determine the quality of the FSO link. It depends on the transmit power, PTX (dBm), divergence angle, θ (mrad), atmospheric attenuation, α (dB/km), link distance, ℓ (km) as well as the system losses. There are two equations of received power (2) and (3) as suggested in [6] and [10] respectively. Both mathematical models are used to calculate the signal power received and supposedly produce the same value if used.

PRX = PTX * [(dRX )2/ dTX + (θℓ)] * 10(-αℓ/10) (3)

PRX = PTX * [ARX/ (θℓ)] * e(αℓ) (4)

Where in (3) dRX and dTX are diameter of the transmitter’s

and receiver’s (m) lenses, while ARX in (4) is the receiver’s aperture area (m2). Both equations, have taken into consideration the atmospheric attenuation, α (dB/km) as one of the factor that affect the power received at the receiver. However, in this paper, since most of the FSO design specification has been predetermined by the manufacturer, it is

possible to write the simplified version of the power received equation in the form of

PRX = PTOTAL - LSYSTEM - LGEO

(5) where LSYSTEM is the fix system loss which denote the optical system loss. Meanwhile, PTOTAL (dBm) is the total transmitted power which can be calculated as in (6) according to [9].

PTOTAL = PTX + 10 log10 (NTX) (6) Power received of the system can be improved by using multiple transmitters/receivers. It can be proved by looking at Fig. 3. where the 4x4 combinations of transmitter and receiver has the highest PRX value compared to 1x1 combinations at all link range value.

C. Link Margin Link Margin, LM (dB) is the difference between the mean

received optical power without atmospheric effects and the receiver sensitivity threshold [12]. Receiver sensitivity as defined in [9] is the minimum optical power that the receiver needs to detect in order to sustain a specified level of performance. For example, a 20dB link margin means that the link is available and communication between the transmitter and receiver is possible without failure up until 20dB of additional attenuation, else the link will completely down. The link margin equations according to [5] are based on the clarity of the weather. For clear weather:

LM = PTOTAL + |-SR| - LGEO – LSYSTEM (7)

Meanwhile, for non-clear weather:

LM = PTOTAL + |-SR| - LGEO – LATM – PSYSTEM (8) Where SR (dBm) is the receiver sensitivity threshold, LGEO (dB) is the geometrical loss, LSYSTEM (dB) is the equipment loss and LATM is the atmospheric attenuation which includes all losses that caused by atmospheric phenomena.

Initially, by using (7) to predict the FSO link margin, the graph as in Fig. 4 can be generated. By looking at the graph, one can see that the link margin decreases with the link range increment. 1x1 combination of transmitter and receiver can barely work at the link range of 7km compared to 4x4 combinations which can reach to a longer link range. Therefore, it can be said that, multiple transmitters/receivers can improve the link margin as well as increase the FSO link range. Hence, now FSO is no longer limited as the short distance applications.

293

IV. THEORETICAL ANALYSIS

A. FSO Link Performance Analysis Model The theoretical analysis was performed using the

programme developed in the MATLAB environment using the design specification specified by the FSO equipment manufacturer. Note that, FSO analysis in this paper only considered the clear weather. Fig. 5 shows the Graphical User Interface (GUI) of the FSO link performance analysis model. The predetermined equipment specification includes transmitted power (PTX), beam divergence (θ), receiver sensitivity (SR), transmitter’s diameter (dTX) and receiver’s diameter (dRX) as well as system loss (LSYSTEM). Number of transmitter and receiver for both sites can be chosen up to 4 transmitters and receivers respectively. Note that, the display button is to display the number of transmitter and receiver chosen by the user. The 3 plots displayed are the graph of geometrical loses versus link range, power received versus link range and link margin versus link range. The maximum link range that will be displayed on all the 3 plots depends on the value inserted by the user. This developed GUI can provide the basic platform to further investigate and predict the effect of using multiple transmitters/receivers on FSO link.

B. Results The modeling is carried based on the following design

parameters from the practical value available on the site i.e the manufacturer’s predetermined value. The transmitted power (PTX) is 6mW, approximately 7.78dBm; beam divergence (θ) is 2mrad; receiver sensitivity (SR) is -45dBm; transmitter’s diameter (dTX) is 0.025m; receiver’s diameter (dRX) is 0.08m and the overall equipment loss (LSYSTEM) is 8dB. From the FSO link performance analysis model of multiple transmitters/receivers it is possible to calculate the geometrical loss, power received, link margin and linear link margin shown in Table 1. The graphs in Fig. 6, Fig. 7 and Fig. 8 illustrate the FSO link performance parameters using multiple transmitters/receivers for link range of 1km.

Figure 5. FSO link performance analysis model of multiple

transmitters/receivers

TABLE I. Geometrical Losses, Power Received and Link Margin of Multiple Transmitters/Receiver for Link Range = 1km

SITE A SITE B LGEO (dB)

PR

(dBm) LM (dB) TX/RX TX/RX

1 1 1 1

1 2 3 4

27.959 24.949 23.188 21.938

-28.179 -25.169 -23.408 -22.158

16.821 19.832 21.592 22.842

2 2 2 2

1 2 3 4

27.959 24.949 23.188 21.938

-25.169 -22.158 -20.397 -19.148

19.831 22.842 24.603 25.852

3 3 3 3

1 2 3 4

27.959 24.949 23.188 21.938

-23.408 -20.397 -18.639 -17.387

21.592 24.603 26.367 27.613

4 4 4 4

1 2 3 4

27.959 24.949 23.188 21.938

-22.158 -19.148 -17.387 -16.138

22.842 25.852 27.613 28.862

Table 1 shows the result of the theoretical calculation done

with all those parameters for a link range of 1km. However, user can further use the model to obtain the result for various values of the link range and observed the extensive difference of the stated parameters as the link range changes in values. The usage of 4 receivers gives the lowest geometrical loss of 21.938dB. On contrary, 1 receiver gives the highest value which is 27.959dB. By looking at Fig. 6, the increment in the number of receiver can effectively reduce the geometrical loss, resulting in 3dB/octave of reduction. For two receivers, the loss drops by 3dB, from 28dB to 25dB and for 4 receivers, the loss drops is from 25dB to 22dB, another 3dB. Table 1 implies that, only the geometrical loss has the same values among the set. It means that, the geometrical loss only depends on the number of receiver used with condition that the values of link range and divergence angle are fixed. As for power received and the link margin, the increase in the number or transmitters and receivers will significantly increase its value. Note that in Figure 7, each time we varied the number of transmitter/receiver the power received will increased by 3dB, 2dB and 1 dB, it shows that, even though, we use the higher number of transmitter/receiver, it is only efficient at certain level, and in this case, it is either 4 transmitters or 4 receivers. Beyond this point, the achievable power received starts to saturate. In other words, if we further increase the number of transmitter/receiver of the system, we cannot see much different in the received power value. The same explanation applies to the value of the link margin without the atmospheric effect. In Fig. 8, the highest number of transmitters and receivers give the highest value of link margin which is 28.862dB. With this value of link margin, the achievable distance of FSO link can reach up to about 10km theoretically.

294

Figure 6. Geometrical loss for link range = 1km

Figure 7. Power received for link range = 1km

Figure 8. Link margin for link range = 1km

V. CONCLUSIONS The FSO link performance model developed in this paper is

the preliminary effort towards developing a more comprehensive analysis of multiple transmitters/receivers of terrestrial FSO links. From the result of the simulation, the multiple TX/RX provides a significant improvement in the link

performance without the effect of atmospheric condition. A higher combination of transmitters/receivers gives further improvement in terms of power received and link margin. This model will serve as the benchmark in the practical implementation that will be carried out using the experimental FSO system in International Islamic University, located in Kuala Lumpur, Malaysia. Since FSO link performance also depends on the local weather and atmospheric phenomena reduces the FSO link performance, the future work will include the analysis of the effect of using multiple TX/RX on FSO link under the tropical weather conditions which will involve the study of rain attenuation, haze and scintillations.

REFERENCES

[1] D. A Begun, “Scientist Test World’s Fastest Network”, 2008. [Online] Available at http://hothardware.com/News/Scientists-Test-Worlds-Fastest-Wireless-Network/

[2] D. Piscitello, “Free Space Optical: Extending Optical Networks Where No Fiber Has Gone Before”, 2010. [Online] Available at: http://www.isp-planet.com.

[3] E. Korevaar, I. I. Kim, B. McArthur , “Atmospheric Propagation Characteristics of Highest Importance to Commercial Free Space Optics”, Proc. SPIE (Atmospheric Propagation), vol. 4976, pp. 1–12, 2003.

[4] “Wiki: Free-space optical communication”, 2010. [Online] Available at: http://wapedia.mobi/en/Free_Space_Optics.

[5] O. Bouchet, H. Sizun, C. Boisrobert, F. de Fornel, P-N. Favennec, “Free-Space Optics Propagation and Communication”, ISTE Ltd, 2006.

[6] S. Bloom, E. Korevaar, J. Schuster, H. Willebrand, “Understanding the performance of Free Space Optics”, Journal of Optical Networking, vol. 2, pp. 178-200, 2003.

[7] B.S.S Naimullah, S. Hitam, N.S.M. Shah, M. Othman, S.B.A. Anas, M.K.Abdullah, “Analysis of the Effect of Haze on Free Space Optical Communication in the Malaysian Environment”, Proc. 2007 IEEE International Conference on Telecommunications and Malaysia International Conference on Communications, pp. 391-394, 2007.

[8] Korevaar, “Multiple Transmitter Laser Link”, United States Patent, patent number: 5777768, 1998.

[9] Nanyang University, Network Technology Research Center, Agilent Technologies, “A Trial-Based Study of Free Space Optics System in Singapore”, Info-Communications Development Authority of Singapore (iDA), pp. 1-72, 2002.

[10] H. Alma, W. Al-Khateeb, “Effect of Weather Conditions on Quality of Free Space Optics Links (with focus on Malaysia)”, Proc. of the International Conference on Computer and Communication Engineering 2008, pp.1206-1210, 2008

[11] Z. Hajjarian, J. Fadlullah, “MIMO Free Space Optical Communications in Turbid and Turbulent Atmosphere (Invited Paper)”, Journal of Communications, vol. 4, pp. 524-532, 2009

[12] Z. Kolka, O. Wilfert, V. Biolkova, “Reliability of Digital FSO Links in Europe”, Proc. World Academy of Science, Engineering and Technology 2007, vol. 25, pp. 444-447.

[13] H. Willebrand, “FlightPointe 2004 Product Overview”, April 2005. [Online] Available at: http://www.src.si/library/includes/file.asp?FileId=23.

295