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PERFORMANCE EVALUATION OVER FREESPACE OPTICAL (FSO) COMMUNICATION, USING APD DETECTOR Hamid Aslani Department of Telecommunication Multimedia (MMU) university Cyberjaya, Malaysia [email protected] Abstract Over last decade, free-space optical communication has become one of the more interesting schemes in long distance outdoor communication process. FSO improves solutions for RF communication in some special fields such as aerospace and military applications. This article gives an overview of the communication process, challenges of FSO application and also some compressions between RF and FSO concepts in the distance of 105 Km. Keywords Free-space optical communication, Radio frequency (RF) and free-space optical (FSO) compression, APD detector. I. INTRODUCTION Optical wireless system and FSO links provide higher performance to the communication system.FSO links improve data rate transmission and overall SNR of the system for deep-space communication applications. The most important parameter for an optical system is low weight and coast of system are improving fast. For a system by improving signal qualities some parameters such as coast, weight, and size will be reduced a lot. Transmitter part has some challenges in operation, size and the quality of the transmitter parts. . Based on these statements the (FSO) system becomes more and more interesting concept for these communication solutions. [1] In FSO concept, some parameters such as signal attenuation in the beam that affects by the atmospheric parameters and also the process of system adjustment and primary installation of the FSO unites have most significant effects on the system performance. [2] Also light propagation process which in some situation can be block by birds and limitation for light power or eye-safe range, and also the wavelength and f-number should be taking in to account. Improving these factors will increase the ability of transmitter to tolerate minimum divergence. In receiver side, aperture size and the f-number effect light collecting and also increase the quality of the system. F-number determines the field of view in receiver detectors. By considering these parameters system performance can be improved more and more. [3] Optical links are more preferable comparing with radio frequency (RF) for some specific applications with long distance communication. Some applications such as Unmanned Aerial Vehicles (UAVs), aircraft and satellite communications have wide range of usage in both civil and military aspects. Moreover, there is high amount of data which should be transferred between both sides of transmission at all moments. This article is including an overview about two models of RF and optical concepts. It also gives some compression between two sample of FSO and RF system to find out some of these application performances in data rate transmission, accuracy, and quality of received signals. II. THEORY OF THE OPERATION At the transmitter part data is converted to the light source to be for transmission through the FSO link. The drive circuit convert the electrical signal to the optical by changing the currents between its optical light sources. There are two types of light source which depends on the application can be used, they can be a light emitting diode (LED) or a Laser diode (LD). [4] At receiver side, the optical signals will convert to the electrical form again. Receivers are a combination of two devices; first part is the optical detector, and second part is the signal conditioning. At the receiver detectors has the most significant responsibility to detect the light signal. In this part signal is received by the detector in the light form and then a circuit which calls signal-conditioning will adjust the output of the detector and extract the original data from arrival signal with minimum distortion in electric form. Figure 1 introducing a basic of optical communication. [5] manner will be explained in following parts. There are so many types of photonic detector and also APD detectors in deferent size and shape and each one for specific purpose. Basically at the transmitter part the light is generated by the optical source which it can be generated by LED or LD. An exemplary link budget for a 105 Km for RF and FSO link are shown in Table 1 and 2. RF Link Transmitter power, Pt1 Pt_RF = 5; Transmitter power, Pt2 Pt_RF2 = 15; Watt Watt Transmitter power, Pt2 Pt_RF3 = 25; Watt Figur1. Block diagram of an optical wireless link showing the front end of an optical transmitter and receiver. Transmitter antenna diameter , Dt Receiver antenna diameter, Dr In some applications like UAVs APD detectors have higher performance; the reason of this manner will be explained in fallowing parts. There are so many types of photonic detector and also APD detectors in deferent size and operation for different proposes. Basically at the transmitter part the light is generated by an optical source which normally is LED or LD. The best choice from the two types of light generator also should be considered. The light will propagate from the light generator trough the FSO link and then will detect by the photo detectors.[6] Dt_RF = 1; m Dr_RF = 1.6; m 0.85 *10 -6 m Wavelength Transmitter transmission loss Lt Receiver transmission loss Alpha_t = 0.3 Alpha_r = 0.5 Noise figure Boltzmann constant 3 dB K 1.38 * 10 ^ (-23) System temperature (Kelvin) 290 K Table 1: an exemplary parameters for RF link systems. III. OPTICAL TRANSMISSION PROCEDURE Basically in transmitter part data is created in electric form, which will be converted to the light form. When signal received by the detector signal will detected and then converted to the electric form again for more processing. A sample block diagram of the transmission procedure is shown in figure 2.[7] Optical link Transmitter power, Pt1 1 mW Transmitter power, Pt2 5 mW Transmitter power, Pt2 10 mW Transmitter antenna diameter , Dt Receiver antenna diameter, Dr 0.305 0 .605 Quantum efficiency 0.3 Surface leakage current Equivalent resistance, Req 0 50 K Noise figure 3 dB Dark current noise Boltzmann constant 0.05 K Electron charge, q nA 1.38 * 10 ^ (-23) APD gain Planck’s constant, h m m 2 6.626068*10 -34 1.6*10 6 Figure 2: sample block diagram of optical transmission process. Table 2: exemplary parameters for FSO link systems. IV. LIGHT TRANSMISSION AND DETECTION EVALUATION In some applications such as UAV, APD detectors are more preferable, the reason of this A. RF link performance evaluation An exemplary system is evaluated it this part. RF system with project cost in table 2 are shown in figure 3,4, and 5. SNR, BER and also data rate are introduced as a function of distance. There are also compressions for different rates of transmission power. 8 B. Optical link performance over communication parameter variations. In this part the performance of optical system will be evaluated when APD detector and EDFA preamplifier are used at receiving process. Figure shows the SNR performance as a function of distance. An exemplary link cost also introduced in table 2.The system SNR will be introduce as equation 1,3 and 5. Signal and noise against distance with using APD detectors and EDFA amplifier and all equations for the overall received signal power SNR and received signal quality will be achieved by following equations: RF Link x 10 14 12 Data Rate 10 8 6 SNREDFA= 4 2 0 1 10 2 10 3 10 4 10 (1) 5 10 10 Distance (m) S Spontaneous emission (ASE) and self-mixing noise (ASE ASE) are depending on the system equipments, Figure3: RF link data rate as a function of distance. RF link with different transmit power 0 10 5 Watt 15 Watt -1 SNREDFA|back= 10 (2) -2 10 When S×ASE noise is not dominants, as it would be for ideal reception: -3 10 -4 10 BER SNREDFA|S×ASE=K3R-2 -5 (3) 10 Where -6 10 -7 10 (4) -8 10 -9 10 0 10 20 30 40 SNR (dB) 50 60 70 80 When ASE×ASE noise is dominant, SNREDFA|ASE×ASE=K4R-4, Figure 4: BER versus SNR with different transmit power (5 and 15 Watt). (5) Where, RF Link 5 Watt 15 Watt 25 Watt 160 (6) Primary parameters are introduced in Table 1 and 2. 140 120 Optical Link M=2 M=5 M=10 160 80 140 60 120 40 SNR (dB) SNR (dB) 100 20 0 1 2 3 4 5 Distance (m) 6 7 8 9 10 100 80 4 x 10 60 Figure 5 : SNR versus Distance for different RF transit power (5, 15, and25 Watt). 40 0 your table title. Run-in heads, such as “Abstract”, will require you to apply a style (in this case, italic) in addition to the style provided by the drop down menu to differentiate the head from the text. 1 2 3 4 5 Distance (m) 6 7 8 Figure6: SNR variation versus Distance. 9 10 4 x 10 system, which indicates that data rate transmission, is more than RF system. Optical Link Pr=0.001 Pr=0.005 Pr=0.010 160 RF Link Optical Link 160 140 140 120 SNR (dB) 120 100 SNR (dB) 100 80 80 60 60 40 40 0 1 2 3 4 5 Distance (m) 6 7 8 9 20 10 4 x 10 0 0 1 2 3 4 Figure7: optical link data rate with APD gain as a function of 5 Distance (m) 6 7 8 9 10 4 x 10 Figure 10: RF and optical SNR as a function of distance. distance. 8 x 10 8 15 RF Link Optical Link 14 Optical Link x 10 12 14 13 10 Data Rate 12 Data Rate 11 8 10 6 9 4 8 2 7 6 0 1 10 2 10 3 10 4 10 5 10 10 Distance (m) 5 4 0 10 1 2 10 3 10 4 10 Figure 11: RF and optical data rate compression. 5 10 10 Distance (m) 0 10 Figure 8: optical link data rate with APD gain as a function of distance. RF Link Optical Link -1 10 -2 10 Optical Link 0 10 Pr=0.001 Pr=0.005 Pr=0.010 -1 10 -3 10 -4 10 BER -2 10 -5 10 -3 10 -6 10 -4 BER 10 -7 10 -5 10 -8 10 -6 10 -9 10 -7 10 -8 0 10 20 30 40 SNR (dB) 50 60 70 80 Figure 12: SNR and bit error rate comparison between RF and FSO. 10 -9 10 0 10 20 30 40 SNR (dB) 50 60 70 80 Figure 9: SNR versus BER for an optical link equipped with APD gain All graphs from 1 to 9 showed that the system performance will be improved by using APD detector and EDFA amplifier. C. Optical and RF link performance compression. Conclusion Compression between RF and Optical system are shown in figure 7, 8 and 9.Figure 5.8 shows that the optical link has higher SNR compare with RF system. Figure 9 also shows the higher performance in optical Comparing with RF technology, FSO link has better performance by using APD detector and EDFA preamplifier. Optical system becomes able to transfer higher amount of data with higher SNR with 10 7 Km distance. This range of support can increasing performance in some applications such as UAV, Aircraft communications and also improves the accuracy of data transmission comparing with RF system. In FSO concept SNR can be significantly improved by applying APD detector and also the performance of the optical system can be improved rather than before by using an EDFA preamplifier at receiver side. REFERENCES [1] B. M. Folio and J. M. P. Armengol, “Radio frequency and optical inter satellite links comparison for future identified scenarios,” in CNES Workshop on Intersatellite Links, Toulouse, France 27–28November, 2003. [2] M. Sasaki, M. Fujiwara, M. Takeoka and J. Mizuno, “Quantum decoder for single photon communication,” Quantum Communication, Measurement, and Computing, Vol. 6, ed. J. H. Shapiro and O. Hirota (RintonPress,Princeton 2003), pp. 185–188. [3] S. Betti, G. D. Marchis, and E. Iannone, Coherent Optical Communications Systems, (John Wiley & Sons, Inc., New York, 1995), p. 308. [4] S. 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Araki, “Preliminary result on laser communication experiment using Engineering Test Satellite-VI _ETS-VI_,” Proc. SPIE 2381, 151–158 _1995_. [11] M. Sasaki, M. Fujiwara, M. Takeoka, and J. Mizuno, “Quantum decoder for single photon communication,” Quantum Communication, Measurement, and Computing, Vol. 6, ed. J. H. Shapiro and O. Hirota (RintonPress,Princeton 2003), pp. 185–188. [12] M. Aspelmeyer, T. Jennewein, M. Pfennigbauer, W. R. Leeb, and A. Zeilinger, “Long-distance quantum communication with entangled photons using satellites,” IEEE Journal of Selected Topics in Quantum Electronics, Vol. 9, No. 6, pp. 1541–1551, 2003.