New Passenger Services in Railway Stations Based on Mode Group Diversity Multiplexing Optical Fiber Communications

Transcription

1 New Passenger Servces n Ralway Statons Based on Mode Group Dversty Multplexng Optcal Fber Communcatons D. LOUM 1,2,3, C. TATKEU 1,2, I. DAYOUB 1,3, M. HEDDEBAUT 1,2, J-M. ROUVAEN 1,3 1 Unversté de Llle Nord de France, F Llle, France, 2 IFSTTAR-LEOST, F Vlleneuve d Ascq Cedex, France 3 UVHC, IEMN-DOAE, F Valencennes Cedex, France dafa.loum@fsttar.fr Abstract Nowadays, because of ts wde bandwdth and hgh communcaton capablty, the optcal fber s more and more used for hgh data rate transmsson of nformaton n ralway envronments. Conventonally, only one servce s sent over the fber at a tme. However, many dfferent servces can be smultaneously conveyed n ralway statons such as passenger nformaton servce, cellular phone, W-F The objectve of the work proposed n ths paper s to demonstrate the potental benefts of transmttng rado sgnals over fber n a ralway envronment. The man dea s to explot the full capacty of the fber by transmttng multple servces usng the same fber. Snce, dfferent servces are operatng n dfferent frequency bands; we propose a new multplexng technque called - Mode Group Dversty Multplexng- (MGDM) to ensure the transmsson of multple servces usng the same fber, wthout addtonal nfrastructure. There are numerous advantages of the proposed technque e.g. faster and relable data exchange, hgh resoluton vdeo survellance capablty, hgh data rate nformaton exchange n ralway statons. We present, n ths paper, the physcal characterstcs of optcal fbers, performance of MGDM multplexng technque, and the nfluence of the laser exctaton condtons at the entrance of the fber on the performances of the system. Keywords: Optcal Fber, Passenger Servces, Ralway Staton, Mode Group Dversty Multplexng. 1. Introducton The development of cable networks wth optcal fbers has an mpact on a large number of applcatons n the cvl and mltary domans. Optcal fbers are known for ther large bandwdth and ther hgh data rate, superor to those offered by other technologes. However, ths bandwdth s not yet fully utlzed by the currently avalable technques. Generally a sngle servce s transmtted by fber, whle several servces could be combned wthn the same optcal fber. At present, sngle-mode (SMF-Sngle-Mode Fber) and mult-mode (MMF -Mult-Mode Fber) optcal fbers equp more and more buldngs, n partcular, shoppng malls, places and stes of publc transports stes such as statons, arports, hghways, ports or multmodal platforms. In these places, fbers connectons are often used as means of extenson of rado systems n partcular between buldngs. Furthermore, 90 % of the new buldngs n Europe are at present equpped wth mult-modes optcal fbers MMF [1]. To enable multple servces, (mult-frequency bands operaton), we propose the explotaton of a new multplexng technque called MGDM - Mode Group Dversty Multplexng, whch s a technque based on the transmsson and the spatal recepton of data. It conssts of usng 2 or 3 lasers sources smultaneously nsde the same fber. Our man objectve s to provde multple servces usng the same fber, wthout addtonal nfrastructure. In ths paper we present, n the frst part, the characterstcs of varous types of optcal fbers, before hghlghtng the advantages of the MGDM multplexng technque. We also study the nfluence of the axal delay (or offset) and of the spot sze at the entrance of the fber. We evaluate the transmsson performance n terms of communcaton Bt Error Rate (BER). Ths evaluaton study s performed by smulaton usng the VPI TM Systems tools. We operate at three conventonal wavelengths commonly used for rado transmsson over fber (850 nm, 1300 nm and 1550 nm). We consder fber optcs lengths from 10 m to 3 km. After several smulatons, we show that the optmal "spot sze" s around 8 µm and that up to three dfferent servces can be mplemented effcently n a sngle optcal fber. Consderng these dfferent servces, we then dscuss the 1-D and 2-D OCDMA codes used for the multplexng technques. In ths paper, we concentrate on the 2-D codes whch are assocated to the MGDM technque. Fnally, we conclude and gve drectons for future research.

2 2. Characterstcs of optcal fbers Optcal fber s consttuted of a very fne glass or plastc thread whch possesses the property of gudng lght. It serves the transmssons of ground and oceanc data and offers a data rate far superor than coaxal cables. It supports a "wde band" network through whch can be carred out varous types of servces (televson, telephony, vdeo conferencng ). There are varous types of optcal fber: sngle-mode, mult-mode and mcro-structured fber. The optcal fber presents many advantages: It offers wde bandwdth of the Terahertz order, and enables data rate of several Ggabts per second. It offers a hgh level of safety durng the transmssons, wth low attenuatons. It also has a long lastng of lfe, whch requres few nterventons accordng to mantenance. It provdes a thermal nsenstvty, a hgh degree of mmunty aganst electromagnetc radatons and jammng. However, optcsal fber possesses some dsadvantages such as dffcultes of adaptaton wth the opto-electronc transducers durng nstallaton. It requres precse mechancal algnment and specally adapted connectors. Moreover, the ntal nstallaton cost s hgh compared to conventonal networks and t s hard to fnd adequately traned personnel. Table 1 presents characterstcs of varous types of optcal fbers, and Fgure 1 presents an example of a mcro-structured optcal fber. Type of optcal fber Sngle-mode Mult-mode Nature Gradedndex fber Step-ndex fber Graded-ndex fber Step-ndex fber Dameter core / claddng 10 µm/125 µm 50-62,5 µm/125 µm 200 µm/300 µm Bandwdth 10 GHz/ km MHz/ km 25 MHz / km Wavelength / Frequency 1300 nm / THz 1550 nm / THz 850 nm / THz 1300 nm / THz Table 1: Characterstcs of varous types of optcal fbers. Fgure 1 presents an example of a mcro-structured optcal fber. Ar duct Ar duct Slca core d 137µm Slca core Ar duct dameter: d ~ 1-2µm Dstance between 2 ar duct:~ 1-3µm Mcro-structured fber Fg. 1: Example of a mcro-structured optcal fber. In spte of the hgh level of performance, sgnals whch cross across the optcal fber undergo attenuaton. Varous losses lke the absorpton, the modal dspersal, the mperfecton of the connectons, the presence of the curvatures and the mcro-curvatures nsde the fber are usually represented by Raylegh dstrbutons. The attenuaton of the sgnal depends essentally on the nature of the fber. The sngle-mode fber at wavelength 1550 nm (or a frequency of THz) has attenuaton of about 0.2 db / km, at 1300 nm (or a frequency of THz) attenuaton s about 0.4 db / km and the mult-mode fber of wavelength at 850 nm (or a frequency of THz) has attenuaton of about 3 db / km. In Table 2, we present the advantages and dsadvantages of varous types of optcal fber: snglemode, mult-mode and mcro-structured.

3 Types of optcal fber Advantages Dsadvantages Sngle-mode Very large bandwdth. No dstorton. Very hgh prce Mult-mode wth step-ndex Low prce Ease of mplementaton Hgh loss and dstorton of the sgnal Mult-mode wth Reasonable bandwdth graded-ndex Good qualty of transmsson Complexty of mplementaton Mcro-structured Large bandwdth Dffcultes of manufacturng. Low dstorton Lmted development. Table 2: Comparson of the varous types of optcal fber: advantages and dsadvantages. Havng presented the characterstcs of varous types of fbers, n the followng paragraph, we develop the mult-users servces aspects ntended for ralway statons applcatons. For ths purpose, we present and evaluate the performances of multplexng technques capable of mplementng multple access and managng mult-users. 3. Mode Group Dversty Multplexng (MGDM) The Mode Group Dversty Multplexng (MGDM) technque s based on the spatal transmsson and recepton of data [3]. It conssts of sendng packet nformaton correspondng to varous users. It s an optcal technque whch ams at creatng ndependent communcaton channels on a mult-mode optcal fber, usng subsets of propagatng modes. By assocatng a servce wth a propagated mode, the MGDM can offer same features as multplexng usng wavelength wth a lower cost. Ths technque s depcted n followng Fgure 2. At one extremty of the optcal fbre, several laser beams noted e carryng dfferent passenger servces, feed the transmsson lne. They are characterzed by a spot sze radus, noted w, and an offset radus n the plane xoy. At the opposte extremty, the energy correspondng to these dfferent laser beams s concentrated n dsks of radus r where the dfferent passenger servces sgnals can be smply recovered. y e 3 e 1 e 2 F z r 1 r 2 w x r 3 Fg. 2: Mode Group Dversty Multplexng (MGDM) technque mplementaton. To effcently manage MGDM multplexng technque, the generaton of certan number of optcal codes and the choce of mult-mode fber s necessary. Ths approach provdes several dstnct, ndependent propagaton channels n the optcal fber. Assocated wth these dfferent propagaton channels, we can also encode the dgtal sgnals on each of these channels. 4. Dfferent optcal codes used The prncple of data encodng not only ensures secure transmsson, but also allows several users or servces to communcate on the same transmsson medum. There are several types of optcal codes, ncludng the 1-D, 2-D and 3-D codes. Among the 1-D codes there are OOC-Optcal Orthogonal Code, PC-Prme Code and EPC-Extended Prme Code (whch s derved from PC code). In the followng paragraphs, we explan how these codes are constructed and generated.

4 OOC codes OOC codes are composed of unpolar sequences characterzed by four parameters (L, W, h a, h c ) where L s the length of the sequence, W s the weght of the code correspondng to the number of chps equal to "1", h and h c are respectvely the constrants for autocorrelaton and cross-correlaton. a L 1 The formula provdng the avalable number of users s: N ooc [2] W ( W 1) Table 3 provdes, for example, an OOC code of N = 5 users, length of code L = 71 and weght of code W = 4. OOC code N L W Poston of chps equal to Table 3: Example generaton of OOC codes for 5 users. The fgure 3 gves poston of chps equal to 1, the autocorrelaton and the cross-correlaton of OOC code. a) Postons of chps to 1 of OOC code b) Autocorrelaton of OOC code c) Cross-correlaton of OOC code Fg.3: a) Poston of chps, b) autocorrelaton and c) cross-correlaton of OOC code. Maxmum autocorrelaton s the weght of the code and the cross-correlaton s between 0 and 1.

5 PC codes The generaton of PC codes performed usng Galos s formula GF (p) of a prme number p such as: S p s, s,, s wth: et jgf( p), GF( p) 0,1,, p 1 ( j) 0 1 ( p1) s The followng formula s used: j. j (mod p) p p C ( n) 1 f ns ( j) jp, jgf( p) (2) 0 otherwse Usng equatons (1) and (2 ) we present n Table 4 an example of PC code. For N=5 users we obtan a code of length L = 25, wth weght of code W = 5. N L= p 2 W=p sj PC-Prme Code (1) Table 4: Example generaton of PC code for 5 users. a) Postons of chps to 1 of PC code b) Autocorrelaton of PC code c) Cross-correlaton of PC code Fg.4: a) Poston of chps, b) autocorrelaton and c) cross-correlaton of PC code.

6 In Fg. 4, for the PC code, maxmum autocorrelaton s the weght of the code but autocorrelaton values are duplcated. The cross-correlaton s stuated between 0 and 1. 2-D codes The 2D codes enable the spreadng of the data n both domans smultaneously, whch allows to decrease the constrants on the length of the codes (L, F), on the number of avalable codes N as well as mprovng the performances of the system n term of Bt Error Rate (BER) for a gven number of actve users. Fgure 5 presents an example of 2D system n both tme and frequency domans. Fg. 5: 2-D OCDMA, Tme and Wavelength. A famly of 2D codes used smultaneously n the temporal and spectral domans s characterzed by the parameters (L x F, W, h a, h c ), where W s the weght of the matrces of code, h a and h c are the values of autocorrelaton and cross-correlaton defned by: h a max L 1 F 1 0 j0 c j * c j h c max L 1 F 1 0 j wth: c and d are two matrces of a famly of code, c and j d 0,1, 0, L 1, j 0, F 1. represent the asynchronous gap between two codes, 0, F 1 et s a sum modulo-f. The objectve s to generate famles of codes whch allow to obtan a large number of sequences wth optmal propertes of autocorrelaton and cross-correlaton (h a =h c =1) n order to lmt the mpact of Multple Access Interference - IAM. - Example of FFH code t t1 t 2 t The followng fgure 6 presents the poston of chps and the evoluton of autocorrelaton and crosscorrelaton functons of 2-D code FFH- Fast Frequency Hoppng. We have represented n Table 5 an example of FFH code. We obtan, for N=20 users, a code of length L = 25 and weght of code W = 5. j0 c j * d j t FFH code N=p(p-1) L= p 2 W Table 5: Example generaton of FFH code for 20 users.

7 a) Postons of chps to 1 of FFH code b) Autocorrelaton of FFH code c) Cross-correlaton of FFH code Fg.6: a) Poston of chps, b) autocorrelaton and c) cross-correlaton of FFH code. The autocorrelaton of FFH code s best because we obtan a sngle peak correspondng to the maxmum of correlaton. The values of cross-correlaton are between 0 and 1. We conclude that 2D codes are qute nterestng to use because they ncrease the number of avalable users. 5. Smulatons results The followng smulaton results are obtaned by usng the VPI TM systems software. VPI TM Systems s a smulaton software for optcal transmsson channel, whch can be used to develop and support sophstcated desgn tools for photonc devces ncludng network components. It supports modelng several types of optcal fber by usng several opto-electronc components. In our case, t takes nto account the majorty of the optcal fber parameters. We study the evoluton of the Bt Error Rate (BER) wth respect to the emtted power for th ree typcal wavelengths used for rado transmsson over fber (850 nm, 1300 nm and 1550 nm), by usng fber lengths rangng from 10 m to 3 km and dfferent "spot sze" of 3 µm and 8 µm. In fgure 7 we have an example of a smulated channel of optcal transmsson wth wavelength of 850 nm and spot szes 3 µm and 8 µm.

8 a) Spot sze w=3 µm Wavelength 850 nm b) Spot sze w=8 µm Wavelength 850 nm Emtted power (dbm) Emtted power (dbm) Fg. 7 Evoluton of the BER vs. emtted power for spot sze w= 3 μm and w= 8 μm, Wavelength = 850 nm. It s better to use fber length lower than1 km because the gan obtaned s about 7 db compared to optcal fber of 3 km. Ths type of applcaton could be used nsde ralway statons. The varaton of spot sze permts to defne the number of laser source that could be sent accordng to the dameter of optcal fber. In practcal t s possble to send smultaneously up to 3 lasers sources nsde the same core of optcal fber and to manage up to 3 dfferent canals. These permt transmsson of much more nformaton at the same tme. Concluson and perspectves We presented the physcal characterstcs of dfferent types of optcal fbers, namely SMF, MMF and mcro-structured fbers. We develop the MGDM technque for the multple users and we gve some examples of optcal codes necessary for ths technque. We presented the results of our smulatons obtaned through VPI TM Systems software. These results lead us to conclude that the best results are obtaned for spot sze of 8 µm. Wth a spot sze of 8 µm, up to 3 lasers sources may be sent. Ths shows the potental benefts of transmttng rado sgnals over fber n a ralway envronment. In the future work we wll nclude a thrd dmenson of smulaton for augmentng the capacty and number of users to be sent. Références [1] I. Dayoub, A. Zaouche, J. M. Rouvaen, C. Lethen, J. P. Vlcot and D. Decoster, -Wreless Systems Rado-optc demonstrator for dstrbuted antenna system ndoor wreless applcatons usng low-cost VCSELs-, European Transactons on Telecommuncatons, 2007, Vol. 18 Issue 7, pp [2] Sahel, J. A., - Code Dvson Multple-Access Technques n Optcal Fber Networks-Part II, IEEE Transactons on communcatons, 1989, vol. 37 N 8, pp , Août. [3] M. Awad, I. Dayoub, J. M. Rouvaen and J.P. Vlcot, -RoF & Mult Servces In Sngle-MMF LAN Usng Mode Group Dversty Multplexng-, Invted paper to ICTTA 08, Aprl, Damas. [4] D. Loum -Transmssons rado haut débt sur fbres optques applquées aux transports terrestres-, Rapport d avancement de thèse : décembre 2008-décembre 2009, INRETS- LEOST FR.