JDT-014-2014 PERFORMANCE ANALYSIS OF OFDM EMPLOYING FREE SPACE OPTICAL COMMUNICATION SYSTEM Sambi. Srikanth 1, P. Sriram 2, Dr. D. Sriram Kumar 3 Department of Electronics and Communication Engineering, National Institute of Technology Tiruchirappalli, India, Email.Id 1 :srikanthsambi7@gmail.com, Email.Id 2 :sriram15vicky@gmail.com, Email.Id 3 :srk@nitt.edu Abstract:The free space optics (FSO) is considered as a suitable solution for optical fiber networks potentially solving the issues like high transmission capacity and ease of deployment of wireless links. The main aim of this paper is to analyze how far we can effectively transmit a RF signal using FSO link. This paper reports the performance of RF signal over optical FSO links for variable distances using Orthogonal Frequency Division Multiplexing (OFDM) Technique. Keywords: FSO, Psuedo Random Bit Sequence (PRBS), OFDM, Quadrature Amplitude Modulation (QAM). Introduction Free Space Optics (FSO) is a Optical communication Technology that uses light propagating in a free space to transmit data between two points. The increasing demand for high bandwidth in communication networks is persistent. The pursuit for a range of applications, including metro network extension, cellular backhaul, enterprise connectivity etc. has created an imbalance. The solution to this imbalance termed as last mile bottleneck was possible with FSO systems which facilitated solutions like bandwidth scalability, cost-effectiveness and portability. The various characteristics of these optical wireless systems also provide better security against wiretapping with the help of point to point communication and because high optical carrier frequencies were being used, no license fees are required for its usage, dealing with the growing scarcity of spectrum in the RF band. Also transmission losses are very less compared to baseband or RF technologies. Thus Free Space Optics (FSO) is a wireless technology that can be used as a last mile access capable to overcome the lack of fiber cables also making communication free from licensing. A maximum of up to 2.5Gbps data rate [1] transfer is possible with FSO communication, unlike the maximum data transfer rates of few hundreds of Mbps offered by RF communication systems. FSO soon found its hold in various applications like establishing quick temporary broadband links even where right of way was not available or was too expensive, multi campus communication networks, as fiber back up, cellular backhaul connections; inter satellite communications, low range links to a backbone network during crisis operation etc [2]. However, the performance of FSO links can be degraded by many atmospheric influences like rainfall, fog, snow, direct sunlight etc. This complex environment hinders the signal characteristics resulting in optical loss, phase fluctuation and turbulence induced amplitude and thus fading of the 158
received optical power. OFDM technique reduces the fading effect. It is a main problem of a signal while travelling in free space. OFDM technique is widely used in the field of wireless communication, because of its limited Inter Symbol Interference (ISI), able to reduce the fading effect and lower computational complexity. It is the best technique for high transmission rate and low cost optical components by using different types of M- array modulation, such as Quadrature Amplitude Modulation (QAM) or Mach- Zehnder Modulation (MZM). Recently, an equivalent optical domain multi-carrier format, called coherent optical OFDM (CO- OFDM) has been proposed for long haul transmission [3]. OFDM is established on the parallel transmission in frequency domain. OFDM eliminates the inter symbol interference (ISI) because the symbols are long. The modulator in the OFDM contains an M-point of inverse discrete Fourier transform (IDFT), subcarrier mapping, conversion from serial to parallel, and the addition of a cyclic prefix and filter before each OFDM symbol [4]. Moreover, the cyclic prefix code of the CO-OFDM system makes the system more resistant to intersymbol interference (ISI) and inter carrier interference (ICI). OFDM Transmitter, Free Space channel, OFDM Receiver diagrams are shown in fig 2, 3, 4 respectively. System design The System is designed by using a commercial free space optical system simulating tool, OptiSystem10.In this we can create environment, according to the normal outside conditions which are affecting the signal. So simulation settings are playing the most important role here. It takes parameters like distance between transmitter and receiver and weather conditions, attenuation etc. Main blocks in this system are OFDM transmitter, RF to Optical Converter, FSO link, Optical to RF convertor, OFDM Receiver. The block diagram is shown in the fig 1. It demonstrates a coherent 512- subcarrier 4-QAM OFDM system. On the transmission side, an inverse fast Fourier transform (IFFT) used to achieve modulation and multiplexing digital in nature. The subcarrier frequencies aremathematically orthogonal over one OFDM symbol period [5]. Two Mach -Zehnder Modulators, CW laser is used to up-convert the RF data to the optical domain. Aft er converting a signal from Electrical to optical then it is transmitted through the free space optical link and becomes degraded due to fading and weather conditions. A local oscillator and coherent receiver is used to down- convert the data from the optical to the RF domain, and at the final stage data is demodulated and sent to the detector and decoder. On the Transmitter side a pseudo ran dom sequence generator is used to generate a bit stream. The transmission of bit rate is 2.5Gbps.The data which is generating d by PRBS is mapped by a 4-QAM sequence generator. The data stream is further passed through OFDM which is having 512 subcarrier and processed by the IFFT and 64- Cyclic pre fix is added to ensure for a exact data recovery. At the transmitter side two Mach Zehnder modulators and a CW laser with a frequency of 193.1 THz and line width is 0.1 MHz is used to convert electrical signals to optical signals. We considered various distances for free space optical channel with attenuation= 0.5dB/km. 159
At the Receiver side, a local oscillator laser with power of -2dBm and line width equals to 0.1 MHz is used. It is assumed to be perfectly aligned. The In phase a d Quadrature components of the OFDM signal are recovered by two pairs of photo-detectors. The data fed to these two pairs of photo detectors are by 90 degrees of phase difference. Figure 2. OFDM Transmitter. Figure 3. Free Space Optical Channel. Figure 1. Block Diagram of FSO system. Photo-detector noise, such as dark current, thermal and shot noise is considered in the simulation. After that RF signal is passing through OFDM demodulator and the guarding interval is removed. The obtained signals are fed into a 4- QAM sequence decoder. Practical Design in Optisystem There are two main things are there in transmitter side. First one is that the generation of RF signal which is done by OFDM and the generated signal is converted into optical signal by using Mach-Zehnder modulator as mentioned before. Fig 3 shows that the FSO channel with an attenuation of 0.5 db/km. Variable ranges are to be considered here to propagate signal from transmitter to receiver. Ranges are 0.5 km, 1km, 1.5km, 2km. The receiver consists of two parts. First part is optical receiver, and this optical signal is converted into electrical signal for that the second part is RF receiver The optical signal is transmitted through free space and it is received by coherent detector at the receiver side. This block is having two pairs of photo-detectors, as mentioned before and which is shown in fig 5. And the signal will demodulated with OFDM demodulator and this demodulation signal is passing through a 4-QAM sequence decoder to get the original bits. Simulation Results and Discussion The performance of the system is analyzed by using Optisystem10 software. To analyze the system, here we considered simulation parameters to get constellation diagrams of both transmitter and receiver parts are given in table I. Simulation parameters are Link/Channel range, Attenuation, Spectral radiance of the sky N(λ). Fig 6 represents a 4-QAM constellation. It is observed after 4-QAM sequence generator (2 symbols/bit) at transmitter side that is shown in fig 2. Fig 7 shows optical spectrum analyzer, observed after signal is being converted from electrical to optical. It means it is observed just before transmission through the free space channel. Fig 8 shows a spectrum analyzer observed after receiving a signal from free space channel. If we observe 160
both transmitter and receiver optical analyzer figures, both are having maximum frequency at 193.1 THz. It shows receiver receiving almost proper signal. Fig 9 shows that constellation diagram at receiver side. It is observed after OFDM detector. Free space channel parameters like range and attenuation of the channel are changed to observe this constellation diagram. The range is adjusted to 0.5 km and attenuation factor taken as 0.5 db/km. System performance is appreciable for 0.5 km because it is having less degradation in it. So we can easily recover the data from the signal. To obtaining fig 10, free space optics channel range is adjusted to 1km and attenuation is taken as 0.5 db/km. signal is degradation is bit high compare to fig 9 but we can still recover data from this signal. Similarly for following fig 11 also observed after changing the channel parameters. Range is adjusted to 1.5 km and attenuation is taken as 0.5 db/km. So if increasing the range of the free space channel then the signal degradation will be more. In fig 12 the degradation of signal is more because range is increased to 2 km. Figure 5. Coherent Detection (Subsystem) Figure 6: 4-QAM Constellation Diagram Figure 4. OFDM Receiver. Figure 7: Optical Spectrum analyzer at Transmitter side 161
Figure 11: Receiver Constellation diagram at 1.5 km Figure 8: Optical Spectrum analyzer at Receiver side TABLE I: Simulation Parameters Parameters Values Link/Channel Range Attenuation 0.5,1,1.5,2 ( in km) 0.5 db/km Figure 9: Receiver Constellation diagram at 0.5 km Figure 12: Receiver Constellation diagram at 2 km Figure 10: Receiver Constellation diagram at 1 km 162
Conclusion The constellation diagrams shows that the increase in the distance causes an increase in the signal degradation. It is observed that fading effect is within acceptable limits till 1km and as the distance increases it becomes dominant. Hence using OFDM coherent detection in free space optics, we will recover the signals at 0.5 km and 1km. These signals are quite good compared to the higher distances where fading is high. References [1] R. Karthikeyan, Dr. R. Prakasam, OFDM Signal Improvement Using Radio over Fiber for Wireless System, International Journal of Computer Networks and Wireless Communications, 2013. [2] Fahad Almasoudi, Khaled Alatawi, Mohammad A. Matin Study of OFDM Technique on RoF Passive Optical Networks, Optics and Photonics Journal, 2013. [3] Manpreet Singh, Karamjit Kaur, Coherent Detection WDM Optical OFDM System, International Journal of Advanced Research in Computer and Communication Engineering, 2013. [4] Vishal Sharma, Gurimandeep Kaur, Modelling of OFDM-ODSB-FSO Transmission System under different Weather Conditions, Third International Conference on Advanced Computing & Communication Technologies, 2013. [5] Prabu K, Sumanta Bose, Dr. D. Sriram Kumar, Analysis of Optical Modulators for Radio over Free Space Optical Communication Systems and Radio over Fiber Systems, IEEE, 2012. [6] Pham Tien Dat, Alam Mohammad Shah, Kamugisha Kazaura, Kazuhiko Wakamori, Toshiji Suzuki, Koichi Takahashi, Mitsuji [7] Matsumoto, A Study on Transmission of RF Signals over a Turbulent Free Space Optical Link, IEEE, 2008. 163