Effect of AWGN & Fading (Rayleigh & Rician) Channels on BER Performance of Free Space Optics (FSO) Communication Systems Taissir Y. Elganimi Electrical and Electronic Engineering Department, University of Tripoli, Tripoli-Libya e.taissir@ee.edu.ly ABSTRACT Free space optics (FSO) communication systems are wireless point-to-point communication systems that use laser beams to transmit and receive communication signals using line-of-sight (LOS) directed laser beams. The atmospheric turbulence induced fading is one of the main impairments affecting FSO communications. The performance of modulation techniques such as Quadrature Phase Shift Keying (QPSK) and Differential Phase Shift Keying (DPSK) are studied in the Additive White Gaussian Noise (AWGN) channel and the Rayleigh and Rician fading channels for FSO communication systems using computer MATLAB 11 version. This model is a useful tool for Bit Error Rate (BER) performance for real data communication by FSO under the frequently used wireless channel models (AWGN, Rayleigh and Rician). The performance measures which are presented in this paper are: the Bit Error Rate (BER) versus the ratio of bit energy to noise power spectral density. The simulation model built for this research work demonstrates that the QPSK modulation provides better performance than the DPSK modulation in terms of data rate, and that the AWGN channel performs better than the Rayleigh and Rician fading channels. Synthetic data is used to simulate this research work. KEYWORDS: AWGN, RAYLEIGH, RICIAN, BER, FSO, QPSK, DPSK. CONCLUSION The BER performance for FSO communication systems using QPSK and DPSK modulation schemes over AWGN and fading (Rayleigh and Rician) channels are analyzed in this paper. By implementing the QPSK and DPSK modulation techniques, the main criterion is comparison of the variation of BER for different. It is observed that the QPSK modulation provides better performance than the DPSK modulation in terms of data rate. The performance of AWGN channel is the best amongst all channels as it has the lowest BER under QPSK and DPSK modulation schemes and the amount of noise occurs in the BER of this channel is quite lighter than the fading channels. The performance of Rayleigh fading channel is the worst of all channels as BER of this channel is highly affected by noise under QPSK and DPSK modulation schemes. From the overall analysis, it is clear that the performance of Rician fading channel is worse than that of AWGN channel but better than that of Rayleigh fading channel since the Rician fading channel has a higher BER than the AWGN channel and lower BER than that of the Rayleigh fading channel. Quick Response Code Access this article online Website: www.ijrws.org ISSN Citation Taissir Y. Elganimi, Effect of AWGN & Fading (Rayleigh & Rician) Channels on BER Performance of Free Space Optics (FSO) Communication Systems, International Journal of Research in Wireless Systems (IJRWS), Vol. 2, No. 2, pp. 34 39, June, 2013 ISSN: 2320-3617 International Journal of Research in Wireless Systems (IJRWS), Volume 2, Issue 2, June (2013) 34
Effect of AWGN & Fading (Rayleigh & Rician) Channels on BER Performance of Free Space Optics (FSO) Communication Systems I. INTRODUCTION Free space optical (FSO) communication systems has received much attention in recent years as a cost-effective, license-free and wide-bandwidth access technique for high data rates applications. However, the performance of FSO communication severely suffers from turbulence-induced fading caused by atmospheric conditions. A direct line-of-sight (LOS) link offers numerous advantages compared to the conventional wired and radio frequency (RF) wireless communications [1]. The performance of the FSO link is hampered by some atmospheric conditions such as fog, smoke, rain, snow etc. There are few other circumstances when the performance of the FSO system may get affected which includes buildings swaying during earthquake or some temporary blockage between LOS connections required for data transmission [1]. The main components of an FSO transmitter and receiver are modulator, optical source, optical detector and a demodulator. The basic block diagram of a Free Space Optical system is given in the Fig 1. Source Destination Modulator Driver Laser Atmospheric Channel Demodulator Amplifier Photo Detector Fig. 1. Basic Block Diagram of the FSO System. The modulation and the demodulation both are done in the electrical domain as the commonly used modulation technique in this system is Pulse Position Modulation (PPM) and On-Off Keying (OOK) [2]. Apart from these modulation techniques, the research is ongoing to implement other modulation techniques to achieve good system performance. Free-space point-to-point optical links can be implemented using infrared laser light, although low data rate communication over short distances is possible using LEDs. Infrared data association technology is a very simple form of FSO communication [2] [3]. Atmospheric turbulence induced fading is one of the main impairments affecting FSO communications [4]. Like other wireless communication systems, transmission medium faces two major problems in FSO communication system. These problems are AWGN noise Rayleigh and Rician Fading. AWGN noise When the transmitted signal passes through a channel, AWGN noise affects this signal. It contains a uniform continuous frequency spectrum over a particular frequency band. Rayleigh Fading When no LOS path exists between transmitter and receiver, and only has the indirect path then the resultant signal received at the receiver will be the sum of all the reflected and scattered waves. Rician Fading It occurs when there is a LOS as well as the non-los path in between the transmitter and receiver, i.e. the received signal comprises on both the direct and scattered multipath waves [5]. A. Bit Error Rate (BER) The BER, or quality of the digital link is an important parameter in digital communications, it is calculated from the number of bits received in error divided by the number of bits transmitted. BER = (Bits in Error)/(Total bits received) B. Eb/No (Energy per Bit to Power Spectral Density Ratio) This is also an important parameter in digital communication or data transmission. It is a normalized signalto-noise ratio (SNR) measure, also known as the SNR per bit. It is especially useful to evaluate the performance of different digital modulation schemes by comparing the Bit Error Rate (BER) performance without taking bandwidth in consideration [6]. II. DIGITAL MODULATION TECHNIQUES There are different options available for the digital modulation techniques which can be used in the field of Free Space Optical communication such as On-Off Keying (OOK), Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK). The OOK modulation provides good BER performance but data rate is low [2]. Modulation techniques are expected to have three positive properties [6]: Good Bit Error Performance: Modulation schemes should achieve low bit error rate in the presence of fading, Doppler spread, interference and thermal noise. International Journal of Research in Wireless Systems (IJRWS), Volume 2, Issue 2, June (2013) 35
Effect of AWGN & Fading (Rayleigh & Rician) Channels on BER Performance of Free Space Optics (FSO) Communication Systems Power Efficiency: Power limitation is one of the critical design challenges in mobile applications. Nonlinear amplifiers are usually used to increase power efficiency. However, nonlinearity may degrade the bit error rate performance of some modulation schemes. Spectral Efficiency: Spectrum efficiency is the optimized use of spectrum or bandwidth so that the maximum amount of data can be transmitted with the fewest transmission errors. Spectral efficiency is measured in units of bit/sec/hz. In this paper, QPSK and DPSK schemes are studied in the AWGN channel, Rayleigh and Rician fading channels in FSO communication systems. A. Quadrature Phase Shift Keying (QPSK) The Quadrature Phase Shift Keying is one of the variants of PSK modulation which uses four different points on the constellation diagram, equally spaced around a circle to represent the data bits. These four phases help the QPSK to encode two bits per symbol while representing the data. The QPSK can be used to double the data rate compared with BPSK system while it maintains the same bandwidth of the signal [7]. Generally, for analytical expressions used in the AWGN channel for M-PSK modulation, the symbol error probability is given by the single integral from equation 8.22 in [8] And the bit error probability is And the analytical expressions used in the fading channels for M-PSK modulation, the average symbol error rate from equation 9.15 in [8] From [8] and [9] Where And is the Hamming weigh of bits assigned to symbol i, and The following expression is very close, but not strictly equal, to the exact BER (from [9] and equation 8.29 from [8]) Where And is the Hamming weigh of bits assigned to symbol i, and B. Differential Phase Shift Keying (DPSK) Differential phase shift keying (DPSK) is a common form of phase modulation which conveys data by changing the phase of carrier wave. In Phase shift keying, high state contains only one cycle but DPSK contains one and half cycle. This modulation technique codes information by using the phase difference between two neighboring symbols. In the transmitter, each symbol is modulated relative to the previous symbol and modulating signal. Differential modulation is theoretically 3 db poorer than coherent. This is because the differential system has 2 sources of error a corrupted symbol, and a corrupted reference [6]. Special case of M = 4, e.g., QPSK, (from equations 5.2-59 and 5.2-62 from [10]) Generally, analytical expressions used in the AWGN channel for M-DPSK modulation from equation 8.84 in [8]. International Journal of Research in Wireless Systems (IJRWS), Volume 2, Issue 2, June (2013) 36
BER Effect of AWGN & Fading (Rayleigh & Rician) Channels on BER Performance of Free Space Optics (FSO) Communication Systems The following expression is very close, but not strictly equal, to the exact BER [9] Where And is the Hamming weigh of bits assigned to symbol i, and III. SIMULATION RESULTS In this section, one of the important topics in FSO communication systems, the concept of fading is demonstrated by the approach available in computer simulation MATLAB 11. It is necessary to explore what happens to the signal as it travels from the transmitter to the receiver. MATLAB 11 provides a simple and easy way to demonstrate fading taking place in wireless systems. A test case is considered with the synthetically generated data. The results are represented in terms of bit energy to noise power spectral density ratio and Bit Error Rate (BER) for practical values of system parameters. The simulation parameters used in the present study are shown in Table 1. TABLE I. SIMULATION PARAMETERS CONSIDERED FOR SIMULATION. Parameter Value / Type And Constant Length (L) k-factor Alphabet Size (for DPSK) range Carrier Modulation used QPSK and DPSK And the analytical expressions used in the fading channels for M-DPSK modulation from equation 8.165 in [8] From [8] and [9] Channel used AWGN, Rayleigh and Rician The plot of versus BER was drawn with the help of semilogy function. Simulation results in Fig 2, Fig 3, and Fig 4 shows the performance of the FSO system using QPSK and DPSK modulation schemes over AWGN, Rayleigh fading channel and Rician fading channel respectively. The Bit Error Rate (BER) plot obtained in the performance analysis showed that the QPSK modulation provides better performance than the DPSK modulation in terms of data rate. And QPSK DPSK 0 2 4 6 8 10 12 Fig. 2. BER performance of QPSK and DPSK modulation techniques in AWGN channel. International Journal of Research in Wireless Systems (IJRWS), Volume 2, Issue 2, June (2013) 37
BER BER BER BER Effect of AWGN & Fading (Rayleigh & Rician) Channels on BER Performance of Free Space Optics (FSO) Communication Systems QPSK DPSK Rayleigh Fading Channel Rician Fading Channel AWGN Channel 0 2 4 6 8 10 12 14 16 18 20 Fig. 3. BER performance of QPSK and DPSK modulation techniques in Rayleigh fading channel. QPSK DPSK 0 2 4 6 8 10 12 14 16 Fig. 4. BER performance of QPSK and DPSK modulation techniques in Rician fading channel. Finally, Fig 5 and Fig 6 shows the plot of BER versus over fading (Rayleigh and Rician) channels and AWGN channel under QPSK and DPSK modulation techniques respectively. 10-12 0 2 4 6 8 10 12 14 16 Fig. 6. Bit Error Rate (BER) Performance of Rayleigh, Rician and AWGN Channels for DPSK modulation technique. From Fig 5 and Fig 6, it is obvious that the BER value of can be achieved by the QPSK modulation in the AWGN channel with an value of 8.3 db. In Rayleigh and Rician channels the values of 10.9 db and 9.4 db are required for achieving similar performance respectively. In the DPSK modulation over AWGN channel, if the value is 10.8 db then a BER value of is achieved. In the case of Rayleigh channel if the is 13.4 db, a BER value of is achieved. The DPSK modulation provides lower data rate as against the QPSK modulation. Hence, the QPSK modulation provides better performance than the DPSK modulation in terms of data rate. The choice of modulation technique in a particular application will depend on the achieved value of i.e. if the value is more than 10 db, then the QPSK modulation can be used as it will provide higher data rate for signal transmission. For higher values of, the BER is decreasing in all the fading channels for different modulation schemes. Rayleigh Fading Channel Rician Fading Channel AWGN Channel ACKNOWLEDGMENT I would like to acknowledge all my family members for their continuous support and understanding. And I would also like to thank the anonymous reviewers for their comments, which have enhanced the quality of this paper. 10-12 0 2 4 6 8 10 12 14 Fig. 5. Bit Error Rate (BER) Performance of Rayleigh, Rician and AWGN Channels for QPSK modulation technique. REFERENCES [1]. S. Bloom, E. Korevaar, J. Schuster, and H. Willebrand, Understanding the performance of free space optics, Journal of Optical Networking, Optical Society of America, vol. 2, Issue 6, 178-200, 2003. [2]. B. Barua, Comparison the Performance of Free-Space Optical Communication with OOK and BPSK Modulation under Atmospheric Turbulence, International Journal of Engineering Science and Technology (IJEST), vol. 3, no.5, 2011. International Journal of Research in Wireless Systems (IJRWS), Volume 2, Issue 2, June (2013) 38
Effect of AWGN & Fading (Rayleigh & Rician) Channels on BER Performance of Free Space Optics (FSO) Communication Systems [3]. F. Xu, M. Khalighi, and S. Bourennane, Ecole Centrale Marseile, Institut Fresnel, Pulse Position Modulation for FSO Systems: Capacity and Channel Coding, 10th International Conference on Telecommunication, ConTEL, IEEE, 2009. [4]. E. Bayaki, Performance Analysis of MIMO Free-Space Optical Systems in gamma-gamma Fading, IEEE Transactions on Communications, November 2009. [5]. Nuzhat Tasneem Awon, Mizanur Rahmon, Ashraful Islam, and Touhidul Islam, Effect of AWGN & Fading (Rayleigh & Rician) channels on BER performance of a WiMAX communication System, International Journal of Computer Science and Information Security (IJCSIS), vol. 10-no.8, August 2012. [6]. A. Sudhir Babu and K. V Sambasiva Rao, Evaluation of BER for AWGN, Rayleigh and Rician Fading Channels under Various Modulation Schemes, International Journal of Computer Applications, vol. 26-no.9, July 2011. [7]. Tejbir Singh Hanzra, and Gurpartap Singh, Improvement in Performance of Free Space Optical Communication, International Journal of Applied Information Systems, vol. 2-no.4, May 2012. [8]. Simon M. K., and Alouini M. S., Digital Communication over Fading Channels - A Unified Approach to Performance Analysis, 1st ed, Wiley, 2000. [9]. Lee, P. J., Computation of the bit error rate of coherent M-ary PSK with Gray code bit mapping, IEEE Trans. on Commun., vol. COM-34, no. 5, pp. 488-491, 1986. [10]. Proakis, J. G., Digital Communications, 4th ed., McGraw-Hill, 2001. Biography "Taissir" was born in 1988 in Tripoli-Libya. He received his bachelors degree in Electronic and Communication Engineering from University of Tripoli in 2010. Currently he is a teacher assistant in electrical and electronic engineering department, University of Tripoli. His research activities were in the field of broadband wireless communications, which led him to write his bachelors thesis on Free Space Optics (FSO) communications. Areas of his research interests include optical communication systems, pulse and digital electronic circuits. International Journal of Research in Wireless Systems (IJRWS), Volume 2, Issue 2, June (2013) 39