Research on Light Polarization FSO-OFDM System

Research on Light Polarization FSO-OFDM System Zhao Li1 Lei Zhi-yong1 Ren An-hu1Ke Xi-zheng2 Wu Ying-hui11. School of Electronic Information Engineering, Xi’an Technological University, Xi’an China, 7100322. Automation & Information School, Xi’an University of Technology, Xi’an China,710048e-mail: pilly_lily@Abstract—we analyzed the polarization characteristic and the influence of lithometeors on polarized light. We have proved out that it is doable to utilize the polarized light in FSO (Free Space Optical Communication) system. The scheme of optical polarization FSO-OFDM is advanced aimed at the format of the OFDM signals, and the principles of the transceivers are expatiated. At last, this scheme is confirmed by Monte Carlo approach, which shows that performance of the light polarization FSO-OFDM system is very stable.Keywords-Optical Communication; Free Space Optical Communication (FSO); Orthogonal Frequency Division Multiplexing (OFDM); Polarization ModulationI.F OREWORDWith the increasing development of semiconductor laser and photodetector, as well as the improvement in the optical communications technology, free space optical communication technology (FSO) has been widely used. Due to it has combined the superiority of fiber communication, microwave communication and other traditional wireless communication technologies, and it has the advantages of low cost, flexible networking, convenient installation and dispense with frequency permission, therebyit has became an emerging broadband wireless access and has been widely concerned. Although optical communicationis a kind of ultra-wideband technology, the bandwidth can be achieved to 1THz, transfer rate is often limited to 10Gb/s or less due to light dispersion and speed restriction of electronic devices [1]. Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation (MCM) technology, whose basic idea is to break down high-speed data stream into multiple orthogonal subcarriers to transmit, so that the symbol rate of the subcarriers dramatic declined, it is particularly suited high-speed wireless data transmission. The technology has been applied to atmospheric laser communications, and acquired some research results[2-5]. Yan Tang, etc defined two types of FSO-OFDM model [6], namely, the direct up/down-conversion model and the intermediate frequency (IF) model. Compared with the IF model, the direct up/down-conversion model does not need optical band-pass filter in the transmitter and receiver, but the anti-interference is lower than the IF mode. The method, which can achieve direct up/down-conversion model by employing the optical polarization properties, is proposed in this paper. As the horizontal polarized light and vertical polarized light pass through the same optical path when using light polarization modulation, and the atmospheric attenuation will be counteracted by each other in the solution of optical polarization angle, hence the FSO-OFDM polarization modulation can reduce or even eliminate the influence of atmospheric channel, and improve the system reliability, and at the same time it can also improve the system confidentiality.II.THE POLARIZED LIGHT PROPERTIES As we all know that only transverse wave has the phenomenon of polarization and longitudinal wave without that phenomenon. Light is a kind of transverse wave, whose vibration direction and propagation direction are vertical for each other, so the plane which is consisted of light’s propagation direction and vibration direction is polarizationFigure 1. The sketch map of the optical propagation Any components, which can produce linearly polarized light, can be referred to as polarization devices. It can be used to obtain or check the linearly polarized light. The polarizing disk has a special optical structure direction which is known as the vibration direction or the polarization direction. The process of the light polarizing and light analyzer are shown in Fig.2 and Fig.3, it can be seen that only the light which is parallel with vibration direction can permeate the polarizing disk. According to the Malus’ law, if the angle between the polarizing disk and polarization analyzer isα, the relation of the incident intensity and the emergent intensity can be written as:2/0SII=(1)αα22cos2cos×=×=SIII (2)IsFigure 2. Light polarizing process2010 International Conference on Electrical and Control EngineeringIFigure 3. Light analyzer processOptical vector satisfies the vector’s decomposition principle and superposition principle, shown in Figure 4, M and N are orthogonal linearly polarized light, the synthesis optical vector P will be obtained by changing the magnitude of M and N, and the amplitude and the polarization angle of the synthetic light will also be changed with the changing of M and N, thus the FSO-OFDM polarization modulation canbe progressed by using this principle [7].ωFigure 4. Optical vector compositionIII.T HE PROPAGATION CHARACTERISTIC OF POLARIZEDLASER IN ATMOSPHEREThe laser’s scattering and depolarization effects arecomplex physical phenomenon. The polarized laser’sdepolarization effect is mainly caused by the collision andscatter between the photons and the air-particles duringtransmitting in the atmosphere channel.The suspender particles and drift particles will arouse thelaser energy absorption and scatter. The reference [8] hasdescribed the scatter model of polarized laser. According tothis model, we calculated the distribution of the scatteredlaser’s polarization angle under the conditions of: the particleradius is mmmmrμμμμ8050205、、、=, the incidentpolarization angle is D0, the laser wavelength nm64.10=λ,the deflection angle D6.3=φ.0.040.080.123060901201501802102402703003300.000.040.080.120.00.10.20.30.43060901201501802102402703003300.10.20.30.4Figure.5(a) 5r mμ=Figure.5(b) 20r mμ=0.00.20.40.63060901201501802102402703003300.20.40.60.00.10.20.33060901201501802102402703003300.10.20.3Figure.5(c) 50r mμ=Figure.5(d) 80r mμ=Figure 5. Polarization angle distribution of different particle radiusFrom Figure 5 we can see that the locations with themaximum polarization angle are mainly concentrated in thebackward and lateral. We only care about the forwardscattering polarization angle in laser communication. Fromthe simulation data and simulation chart we can see clearlythat the forward scattering polarization angle is zero in theθ=0°,φ0°, namely, the spherical particles’ forwardscattering polarization angle is invariable. Many theories andpractices indicate that the air-particles are similar to sphericalparticles under the situation of which need lowprecision[9,10].According to the above analyses, it is feasible to employpolarized light to transmit signals in FSO-OFDM system.IV.FSO-OFDM SYSTEMFig.6 is the FSO-OFDM model. It is composed oftransmitting and receiving elements. The transmitter includesOFDM modulation module, optical up-converter module andatmosphere channel; the receiver includes optical down-converter module and OFDM demodulation module. Themain function of OFDM modulation module is to map thedata into OFDM symbols, then produce the baseband timeseries signals to process optical modulation. The mainfunction of optical modulation module is to linearly shift theOFDM spectrum from the RF domain to the optical domain,Figure 6. The FSO-OFDM Model(S/P—series/parallel, (I)FFT—(inverse) fast Fourier transform, GI—guardinterval, P/S—parallel/series, D/A—digital/analog, LPF—lowpass filter)The OFDM modulation module is to modulate themodulated (such as QAM, MPSK) signals onto severalorthogonal subcarriers, the baseband time series signals areas follows:()()tXNNkTsNnsmLlklmNkklm∑∑∑∝====⎟⎠⎞⎜⎝⎛=00,,1,,11ψ (3)()()()()⎪⎩⎪⎨⎧⋅+⋅+≤≤⋅⋅+=−−Δ−′else T m L l t T m L l e t S S LmT lT t Tk j k l m SS U 012,,πψ (4) Where N denotes the number of subcarriers, l denotes the number of OFDM symbols in one super-frame, m is the number of super-frames, S T , U T and Δ are the OFDM symbol period, inter-carrier distance and guard time, respectively, and satisfy Δ+=U S T T .k ′ represents the carrier index relative to the center frequency, 2/N k k −=′.k l m X ,, represents the coded signals of thelth information symbol at the kth subcarrier in mthframe.For the sake of simplicity, the OFDM signal in (5) only shows one OFDM symbol:()()UT t k j Nk k e X N t s /21,0,01Δ−′=∑=π (5) It is obvious that ()t s is the complex signal, but theoptical signals in IM/DD optical communication only can be described by intensity, that is to say the complex signals can not describe the optical signals[12]. Therefore, we propose to utilize the polarization characters of light to modulate the real /imaginary parts of the complex signals by two MZM, finally to complete the electro-optic transfer.V. THE PRINCIPLE OF FSO-OFDM POLARIZATIONMODULATION AND DEMODULATION The principle of optical modulation is shown in Fig 7. The laser signals are divided into two light beams with equal power after the beam splitter, then the two light beams passed through the polarizer 1 and polarizer 2 which are vertical each other, respectively. In this way, the two light beams will be the horizontally linearly polarized light and the vertically linearly polarized light. In this system, the RF electrical signals are divided into the real and imaginary parts to drive two MZM and obtain two optical signals. In the receiving end, two linearly light beams are obtained by holding the receiving optical signals pass through the horizontal polarizer 1 and the vertical polarizer 2. Finally, we will get the elliptical polarized light with special polarization angle after through the beam recombiner.The output signal of beam recombiner in modulation part can be written as()()()2/exp 2cos exp 2cos πωπωπππ+⋅⎟⎟⎠⎞⎜⎜⎝⎛+⋅⋅+⋅⎟⎟⎠⎞⎜⎜⎝⎛+⋅⋅=t j V V V A t j V V V A t P LD DC Q LD DC I (6) ()()()()⎩⎨⎧==t s V t s V Q I Im Re (7)A is proportional constant,，DC V is the bias voltage of themodulator, πV is the half-wave rectifier voltage, LD ωis thecarrier frequency for the laser.Figure 7. The schematic diagram of the FSO-OFDM polarizationmodulation (MZM—Mach Zehnder Modulator)Due to the horizontal polarized light and vertical polarized light are passing through the same optical path, they will with the same attenuation and delay caused by the atmospheric natural phenomenon. Hence, it will overcome the influence caused by atmosphere channel to a great extent[7].Figure 8. The schematic diagram of the FSO-OFDM polarizationdemodulationThe principle of optical demodulation is shown in Figure 8. The optical signals collected by optical antenna are divided into two-way signals after through the beam splitter, one of the signals enter into the horizontal polarization analyzer, the other into the vertical polarization analyzer. Finally, measure the horizontal polarized light intensity ∥P and the vertical polarized light intensity ⊥P by the photodetector.()()()t N t h V V V A t P DC I +⊗⎟⎟⎠⎞⎜⎜⎝⎛+⋅⋅⋅=ππρ2cos 1∥ (8) ()()()t N t h V V V A t P DC Q +⊗⎟⎟⎠⎞⎜⎜⎝⎛+⋅⋅=⊥ππρ2cos 2 (9) 1ρ，2ρare the photoelectric transformation efficient of photodector 1 and 2 respectively. The pulse response of the channel is represented by ()t h , ()t N is the Gaussian white noise with energy spectral density 0N , ()t P ⊥ and ()t P ∥ are the intensity of the vertical polarized light and the horizontal polarized light at receiver, respectively. At last, compound the signals to recover the complex OFDM signals, and then we will get the binary data after through the OFDM demodulation module.VI. CONCLUSIONIn order to analyze the performance of the light polarization FSO-OFDM system. We conduct a simulation by Monte Carlo method, and set the simulation parameters as follows: 105 OFDM symbols, the laser wavelength nm 64.10=λ, the channel is the Gaussian white noise ，the entrancing polarization angle is D 0, the number of subcarriersare N=128, 64, 32, 16, respectively. We chose PN sequence as the training sequence, and set the length of the training sequence and the length of the guard interval equal to 10 percent of the number of subcarriers. The simulation results indicated that the system’s BER is increased with the number of subcarriers increasing, but even if the number of subcarriers is 128, the system’s BER will below 10-6 when the SNR is 28dB.In this paper, we applied the OFDM technology to the free space optical communication system. According to the polarization characters of light and the influences to polarized light caused by air-particles, the air particles’ single forward scatter is invariable under the situation of which need low precision, it is feasible to employ polarized light to transmit signals in free space optical communication system. We proposed the light polarization FSO-OFDM system model, which solves the defect that the OFDM signals can not be easily practiced intensity modulation. Meanwhile, according to the simulation results the performance of the light polarization FSO-OFDM is very reliable.A CKNOWLEDGMENTThis work was supported by the National Natural Science Foundation (Grant No.60977054), The Research Foundation of Education Bureau of Shaanxi Province, China(Grant No.07JK332), The Natural Science Foundation of Shaanxi Province, China (Grant No. 2007F12). 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