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1 Citation: Tang Xuan Rajbhandari Sujan Popooa Wasiu Oyewoe Ghassemooy Zabih Muhammad Sajid Sheikh Leitgeb Erich and Kandus Gorazd (1) Performance of BPSK subcarrier intensity moduation free-space optica communications using a og-norma atmospheric turbuence mode. In: Symposium on Photonics and Optoeectronics (SOPO) 19-1 June 1 Chengdu China. URL: < This version was downoaded from Northumbria Research Link: Northumbria University has deveoped Northumbria Research Link (NRL) to enabe users to access the University s research output. Copyright and mora rights for items on NRL are retained by the individua author(s) and/or other copyright owners. Singe copies of fu items can be reproduced dispayed or performed and given to third parties in any format or medium for persona research or study educationa or not-for-profit purposes without prior permission or charge provided the authors tite and fu bibiographic detais are given as we as a hyperink and/or URL to the origina metadata page. The content must not be changed in any way. Fu items must not be sod commerciay in any format or medium without forma permission of the copyright hoder. The fu poicy is avaiabe onine: This document may differ from the fina pubished version of the research and has been made avaiabe onine in accordance with pubisher poicies. To read and/or cite from the pubished version of the research pease visit the pubisher s website (a subscription may be required.)

2 Performance of BPSK Subcarrier Intensity Moduation Free-Space Optica Communications using a Log-norma Atmospheric Turbuence Mode X. Tang S. Rajbhandari; W. O. Popooa Z. Ghassemooy Optica Communications Research Group Schoo of Computing Engineering & Information Sciences Northumbria University Newcaste UK xuan.tang@unn.ac.uk S. S. Muhammad Nationa University of Computer and Emerging Sciences (FAST-NU) Lahore Pakistan sm.sajid@nu.edu.pk Abstract In this paper we present simuation resuts for the bit error rate (BER) performance and the fading penaty of a BPSK - subcarrier intensity moduation (BPSK-SIM) free-space optica (FSO) communication ink in a og-norma atmospheric turbuence mode. The resuts obtained are based on the Monte- Caro simuation. Mutipe subcarrier moduation schemes offer increased system throughput and require no knowedge of the channe fading in deciding what symbo has been received. In an atmospheric channe with a fading strength σ of.1 obtaining a BER of 1-6 using a -subcarrier system wi require a signa-tonoise (SNR) of 3.1 db. The required SNR increases with the fading strength and at a BER of 1-9 the fading penaty due to the atmospheric turbuence is ~ 41 db for σ =. 9. The comparative studies of the OOK and BPSK-SIM schemes showed that for simiar eectrica SNR BPSK-SIM offered improved performance across a range of turbuence variance. Keywords subcarrier intensity moduation BPSK turbuence og-norma channe mode. I. INTRODUCTION Transmission by ight (aser) over free space has received a significant attention as a compementary technoogy to the radio frequency (RF) based techniques by a number of researchers [1 ]. The ever increasing demand for higher bandwidth couped with ast mie botteneck imposed by the RF based systems have forced service providers to ook for aternative systems in certain dedicated appications. FSO with avaiabiity of unreguated bandwidth in excess of THz is seen as the utimate soution for overcoming the access network botteneck. In addition FSO inks offer a number of advantages incuding a secure transmission smaer and more compact E. Leitgeb Institute of Broadband Communications Graz University of Technoogy A-81 Graz Inffedg. 1 Te.: G. Kandus Department of Communication Systems Jožef Stefan Institute Jamova 39 Ljubjana Sovenia gorazd.kandus@ijs.si transceiver modues a ow deveopment and instaation cost and immunity to the eectromagnetic interference [3]. The reiabiity of an FSO communication system is greaty infuenced by the atmospheric conditions. The beam scattering caused by fog and haze can significanty reduced the received optica signa eve. Heavy fog causes attenuation greater than 3 db/km thus imits the ink ength to < 1m. Rain and snow affect mainy the radio and microwave frequencies but their effects are not deeterious for FSO systems. However FSO can encounter significant osses in a cear sky condition due to inhomogeneities in temperature and pressure [4 5]. Scintiation severey imits the reiabiity of FSO inks as it deteriorates the signa intensity at the receiver and can even resut in compete oss of communication inks []. The effect of scintiation is more severe for sma aperture receivers [4 6]. However increasing the detector coection aperture is not the best soution as there is an optimum aperture size to minimize the scintiation eve. Making the coector aperture diameter much arger than the minimum aperture diameter resuts in very itte SNR improvement [6]. The appropriate seection of the moduation technique is vita to circumvent turbuence induced fading. Though the onoff keying (OOK) scheme is the simpest and extensivey used moduation technique [7 8] it does not offer immunity to the turbuence induced fading [ 5 8]. The non-predictive behavior of turbuence eve gives rise to the random fuctuation of the optica intensity eve at the receiver. This impies that the OOK wi require an adaptive threshoding scheme to perform optimay. This adaptive threshoding is compex to impement and practicay not suitabe [5 9]. Since /1/$5. 1 IEEE Authorized icensed use imited to: University of Northumbria. Downoaded on August 181 at 14:56:1 UTC from IEEE Xpore. Restrictions appy.

3 scintiation affects the optica intensity eve it is a reasonabe approach to use moduation techniques that carries the information in the phase or the frequency of the carrier signa. The phase shift keying (PSK) based subcarrier intensity moduation requires no adaptive threshoding scheme thereby offering superior performance compared to the OOK in the presence of the atmospheric turbuence induced fading channes [7]. In this work BPSK-subcarrier intensity moduation (BPSK-SIM) is investigated where SIM is used to increase the system capacity by moduating mutipe sources at different subcarriers. However the penaty paid is the higher SNR to attain a desired BER performance. Hence mutipe SIM is the preferred choice when increased capacity is more important than the power requirement [1]. The rest of the paper is arranged as foows: the ognorma turbuence mode for FSO is introduced in Section II foowed by the detaied description of the proposed BPSK-SIM system in Section III. The simuation resuts for the BER for the BPSK-SIM scheme in turbuent atmospheric channe is presented in Section IV. The simuation resuts are verified using a theoretica BER performance and fading penaty is cacuated for a number of desired BERs. Finay concusions are presented in Section V. II. LOGNORMAL TURBULENCE MODEL The atmospheric turbuence impairs the performance of an FSO ink by causing the received optica signa to vary randomy thus giving rise to signa fading. The fading strength depends on the ink ength the waveength of the optica radiation and the refractive index structure parameter C of the channe. The og-norma distribution is generay used to mode the fading associated with the weak atmospheric turbuence regime [ 8 11]. This mode is mathematicay tractabe and it is characterised by the Ryotov variance σ. The turbuence induced fading is termed weak when σ <1. and this defines the imit of vaidity of the og-norma mode [8]. Beyond the weak turbuence regime other modes such as the gamma-gamma [11] and the negative exponentia [8] wi have to be considered. The Ryotov variance σ can be cacuated as [4]: 6 11 ( k ) 7 σ = 1.3C n L ; (1) where L is the propagation distance and k is the wave number. n The og-norma modes assumes the og intensity of the aser ight traversing the turbuent atmosphere to be normay distributed with a mean vaue of σ /. Thus the probabiity density function of the received irradiance is given by [11 1]: 1 1 (n( I / I) + σ / ) p = exp I ; () I πσ I σ where I represents the irradiance at the receiver and I is the signa irradiance without scintiation. III. PROPOSED BPSK-SIM SYSTEM Figure 1 shows the bock diagram of the BPSK-SIM system with two subcarriers if necessary more subcarriers can be used for increased throughput. Using BPSK the input data d are moduated onto the RF subcarriers whose { 1 d } ampitudes frequencies and phases are { a } and { ϕ } a { ω c ω } c 1 c ϕ c respectivey. In Fig. 1 g ( represents the puse shaping function. The combination of the two RF subcarrier signas is then used to moduate the intensity of the optica source. Prior to this a D.C. signa b is added to the composite RF signa to ensure that the optica source is appropriatey biased at the centre of its inear dynamic range so as to accommodate the fu swing of the sinusoida subcarrier signa. In the atmospheric channe turbuence effect modeed using the og-norma causes the intensity of the transmitted optica signa to fade. At the receiver the composite SIM signa superimposed on the enveope of the incoming optica signa is recovered via the direct detection (DD) scheme. Eectrica bandpass fiters are used to capture individua subcarriers i si ( foowed by the standard RF coherent detector to recovers the transmitted data sequence { d 1 d }. The system noise (therma and sho is modeed as an additive white Gaussian (AWGN) and no intersysmbo interference is considered since the ink under consideration is a direct ine of sight with no mutipath propagation. The received photocurrent can therefore be modeed as foow: i ( RI[ 1+ m( ] + n( = ξ ; (3) where I represents the optica irradiance R is the photodetector responsivity and n( ~ N( σ ) represents Figure 1. The bock diagram of subcarrier BPSK system. Authorized icensed use imited to: University of Northumbria. Downoaded on August 181 at 14:56:1 UTC from IEEE Xpore. Restrictions appy.

4 AWGN with σ = σ Therma + σ Bg. ξ is the optica moduation index and to keep the optica source within its dynamic range the condition ξm( 1must aways hod. IV. SIMULATION RESULTS DISCUSSION The system described above is simuated using the Monte- Caro approach in Matab. In the simuation the puse shaping function is assumed to be rectanguar and the ampitude { a ac } of each subcarrier signa is obtained from 1/ξ. The simuation parameters are given in Tabe I. TABLE I. SIMULATION PARAMETERS Parameters Vaues Data rate R b 1 Mbps 1 st Carrier frequency 4 MHz nd Carrier frequency 8 MHz Samping frequency MHz Laser waveength λ 85 nm Number of subcarriers N PIN photodetector reponsivity R 1 Optica moduation index ξ 1 Butterworth Bandpass fiter Butterworth Lowpass fiter Centre frequency Centre frequency 4 MHz MHz 8 MHz MHz 1 MHz Lognorma variance σ.1 σ. 9 W In order to show the effect of scintiation and noise on the system performance we wi be ooking at the BER metric and fading penaty under different channe conditions. It shoud however be mentioned that since both subcarrier channes are BPSK moduated their error performance wi be simiar and we wi therefore be presenting resuts for one of them ony. By adopting the approach given in [13] the theoretica unconditiona BER per subcarrier channe is obtained as: P = e = P p( I) di ec Q ; (4) 1 ( ()) [ n I / I + ] σ / γ I exp di I πσ σ where γ ( I ) represents the SNR at the input of the coherent demoduator. BER No Turbuence Theory Simuation σ =.1 σ = SNR (db) Figure : The theoritica and simued BER against the SNR in a weak atmospheric turbuence FSO channe with a scintiation σ {.1.5 } = Figure iustrates the predicted and cacuated (using (4)) BERs performance against the SNR in a weak atmospheric turbuence channe with σ = {.1.5}. The simuated and theoretica curves match very cosey for a scintiation eves thus confirming the vaidity of the simuation. It can ceary be observed that the SNR required to achieve a desired BER increases with the atmospheric turbuence eve. To achieve a BER of 1-3 an additiona ~3.4 db of SNR is required compared to the idea channe (without turbuence) when σ =. 1. However the SNR differences between the idea channe and σ =. 5 increases to ~ 1. db. Further increment in the SNR requirement to achieve a BER of 1-6 is observed with increasing scintiation eves. Further insignt into the effect of turbuance can be obtained by poting the fading varience against the fading penaty see Figure 3. The fading penaty represents the increment in SNR to acheive a desired BER in a tubuance channe compared to the idea channe. The figure demonstrates that the fading penaty increases with the turbuance variance irespective of the BER. For the same turbuence eve fading penaty is higher for ower BER. For exampe when the turbuence variance is.5 the fading penaty are ~ 1. db 1.1 db and 7.7 db for BER of and 1-9 respectivey. The Fading Penaty (db) BER=1-3 BER=1-6 BER= Turbuence Variance Figure 3: The fading penaty againt the og intensity for mutipe subcarriers FSO system in weak atmospheric turbuence under different BER condition. Authorized icensed use imited to: University of Northumbria. Downoaded on August 181 at 14:56:1 UTC from IEEE Xpore. Restrictions appy.

5 fading penaty can be as high as ~ 41 db when the turbuence variance is.9 thus demonstrating the vunerabiity of the system under high turbuance conditions. BER SIM-BPSK OOK with fixed threshod OOK with adaptive threshod σ =.5 No Turbuence SNR (db) Figure 4: The BER performance of OOK and SIM-BPSK schemes against the SNR under turbuence variances of and.5. The comparative studies of the BER perfromances of OOK and SIM-BPSK under a weak turbuance atmospheric condition is carried out and are demonstrated in Figure 4. A threshod eve of haf of the mean of the received intensity is seected for the fixed threshod eve in the OOK system. Though the threshod eve is optimum for OOK in channe without turbuance it is ceary demonstrated that a fixed threshod vaue is non-idea in the presence of turbuance. The fixed threshod can not optimize the performance of OOK scheme over weak turbuence whie the adative threshod can optimize the performance by reducing the BER. Based on simiar eectrica SNR vaues the penaty due to the turbuance is much higher for the optimum OOK scheme compared to the BPSK-SIM even in weak turbuance channe. The fading penaty at the turbuence variance of.5 is ~ 1.1 db for BPSK-SIM to achieve a BER of 1-6. However the fading penaty at the variance eve is much higher even for OOK with adaptive threshod indicating superior performance of the BPSK-SIM in fading channes. The difference in the performance is attributabe to the difference in the methods of representing information signas on the optica carrier.the received irradiance of OOK represents digita information and any fuctuation in the irradiance increases the error probabiity. However information is transmitted by moduating the subcarrier phase in BPSK which is ess affected by the irradiance fuctuation. v. CONCLUSIONS This paper iustrated the BER performance and fading penaty of a FSO communication ink through the weak atmospheric turbuence channe for the BPSK-SIM scheme using the Monte-Caro simuations. The resuts iustrated that fading penaty increases with the turbuence eve. The fading penaty of ~ 1.1 db is observed at a turbuence variance of.5 at a desired BER of 1-6 however a much higher penaty of ~ 41dB is incurred at a turbuence variance of.9. The comparative studies of the OOK and BPSK-SIM base on simiar eectrica SNR schemes showed that BPSK-SIM offers improved performance for a range of turbuence variance. REFERENCES [1] B. S. Naimuah M. Othman A. K. Rahman S. I. Suaiman S. Ishak S. Hitam and S. A. Ajunid "Comparison of waveength propagation for free space optica communications" in ICED 8 Maaysia 1-3 December 8 pp [] X. Zhu and J. M. Kahn "Free-space optica communication through atmospheric turbuence channes" IEEE Transactions on Communications vo. 5 pp August. [3] S. S. Muhammad T. Pank E. Leitgeb A. Fried K. Zett T. Javornik and N. Schmitt "Chaenges in estabishing free space optica communications between fying vehices " in CNSDSP 8 Graz Austria 5 Juy 8 pp [4] R. R. Iniguez S. M. Idrus and Z. Sun "Atmospheric transmission imitations" in Optica Wireess Communications - IR for Wireess Connectivity London: Tayor & Francis Group LLC 8 p. 4. [5] L. Jia J. Q. Liu and D. P. Tayor "Optica communication using subcarrier PSK intensity moduation through atmospheric turbuence channes" IEEE Transactions on Communications vo. 55 pp August 7. [6] D. L. Fried "Optica heterodyne detection of an atmosphericay distortion wave front" Proceedings of the IEEE vo. 55 pp January [7] R. R. Iniguez S. M. Idrus and Z. Sun "Moduation Coding and Mutipe Access" in Optica Wireess Communications - IR for Wireess Connectivity London: Tayor & Francis Group LLC 8 p. 1. [8] W. O. Popooa and Z. Ghassemooy "BPSK subcarrier intensity moduated free-space optica communications in atmospheric turbuence" Journa of Lightwave Technoogy vo. 7 pp Apri15 9. [9] S. M. Haas and J. H. Shapiro "Capacity of wireess optica communications" IEEE Journa on Seected Areas in Communications vo. 1 pp October 3. [1] T. Ohtsuki "Mutipe-subcarrier moduation in optica wireess communications" IEEE Communication Magazine vo. 41 pp March 3. [11] Z. Ghassemooy W. O. Popooa and E. Leitgeb "Free-space optica communication using subearrier moduation in gammagamma atmospheric turbuence" ICTON '7 vo. 3 pp Juy 7. [1] W. O. Popooa Z. Ghassemooy J. I. H. Aen E. Leitgeb and S. Gao "Free-space optica communication empoying subcarrier moduation and spatia diversity in atmospheric turbuence channe " Optoeectronics IET vo. pp February 8. [13] A. Jurado-Navas A. Garcia-Zambrana and A. Puerta-Notario "Efficient ognorma channe mode for turbuent FSO communications" Eectronics Letters vo. 43 pp February 1 7. Authorized icensed use imited to: University of Northumbria. Downoaded on August 181 at 14:56:1 UTC from IEEE Xpore. Restrictions appy.