Optical wireless link for railway service

Proceedings of the 4th WSEAS Int. Conf. on Information Security, Communications and Computers, Tenerife, Spain, December 16-18, 2005 (pp128-130) Optical Wireless Link for Railway Service ZDENĚK KOLKA, OTAKAR WILFERT, VIERA BIOLKOVÁ, DALIBOR BIOLEK Department of Radio Electronics Brno University of Technology Purkyňova 118, 612 00 Brno CZECH REPUBLIC [email protected] Abstract: - Optical wireless links (OWL) have found many applications due to their specific properties. One of interesting applications is a 10Mb/s optical link connecting train wagons. The link design takes into account instability of mutual position and orientation of transmitter and receiver. The paper deals with analysis of extreme deviations, design, and modeling of transmitting and receiving optical systems for the link. Key-Words: - Free Space Optics, Communications, Networking o 1 Introduction ∆x The optical link is intended for modernization of classical train carriages to provide new services both for the crew and passengers. The optical system designed is based on inexpensive LEDs and plastic Fresnel lens. As the electronics is based on circuitry for fiber applications, the optical system must provide optical power at photodiode between -30dBm to 0dBm under all circumstances (carriage movements, optics smear, etc.). The extreme angular deviations of optical axes of transmitter and receiver can be determined from a simplified model in Fig. 1a that depicts situation in the horizontal plane. Axes of the wagons contain angle θA which depends on the track radius of curvature. However, the angle between optical axis o and abscissa [0,0][x0,y0] (the join of receiver and transmitter) is only θA/2. Movements of carriage caused by the rail irregularities could be expressed as side deviations ∆x. Points A and B represent the extreme cases. Angular effect of eventual vertical deviations ∆y is always smaller. The angles αA and αB can be determined as ⎛ L sin(θ A / 2) − ∆x cos(θ A ) ⎞ ⎟⎟ , ⎝ L cos(θ A / 2) + ∆x sin(θ A ) ⎠ α A = arctg ⎜⎜ A θA ⎛ L sin(θ A / 2) + ∆x cos(θ A ) ⎞ ⎟⎟ . ⎝ L cos(θ A / 2) − ∆x sin(θ A ) ⎠ Actual values of L, θA, and ∆x are given by railway standard specifications [1]. The values considered were: L from 0.3m to 1.5m, ∆x = 6cm, θA = 10° (the sharpest track curve). B θA/2 α [0,0] a) 18 16 14 alpha 12 αB 10 8 6 (1a) (1b) [x,y] L 4 |αA| 2 α B = arctg ⎜⎜ [x0,y0] 0 0.2 b) 0.4 0.6 0.8 1 1.2 1.4 1.6 L [m] Fig. 1 Simplified geometrical model for determination of αA and αB (θA – angular divergence of wagon axes; α – angular position of opposite device with respect to the optical axis o; L – distance between [0,0] and [x0,y0]). Proceedings of the 4th WSEAS Int. Conf. on Information Security, Communications and Computers, Tenerife, Spain, December 16-18, 2005 (pp128-130) DTXA oV (LED) (FL) SRXA,ef Def oP (PD) αmax F´ dFD DRXA (LED’) ∆ p f´ p´ Fig. 2 Design model of receiving optical system under extreme deviation (photodiode aperture is irradiated only by the outer LED); DTXA – diameter of transmitting aperture; DRXA – diameter of Fresnel lens; PD – photodiode; LED’ – image of the outer LED; F’ – second focal point Analysis of angular position of A and B (αA, αB) exhibits the maximum of |α|max = 16° for L = 0.3m, Fig. 1. This value determines required field of view of the receiver and required beam divergence of the transmitter. 2 Model of the Optical System Fig. 2 shows a design model of the receiver optical system. The transmitting aperture is composed from several properly directed LEDs without any optics. Using some basic principles of ray optics an analytical formula for the maximum distance ∆ between photodiode and lens can be obtained as ∆= pf ′ ( DRXA − d PD ) ⎛ D 2 pf ′ tg ⎜ α max − arctg TXA 2p ⎝ ⎞ . (2) ⎟ + DRXA ( p − f ′ ) ⎠ Because ∆ is smaller than p’, only a fraction of the receiving aperture participate on the power reception. It is denoted as effective aperture SRXA,ef (Fig. 2). Gain G of the optical system is defined as ratio of the actually received power to that received without any optics, i.e. ⎛D G = 20 log⎜⎜ ef ⎝ d PD ⎞ ⎛ ⎞ pf ' ⎟⎟ = 20 log⎜⎜ ⎟⎟ [dB], (3) ⎝ pf '−∆( p − f ' ) ⎠ ⎠ where Def is diameter of the effective aperture and dPD is diameter of active area of the photodiode. In the case of irradiation by several LEDs the received power is given as a sum of individual contributions PFD = π 4 2 d PD TRXA ∑ k I i ,k (l , m) 2 k L 100,1Gk cos α k , (4) where TRXA is transmission of lens, Ii,k (l,m) is the individual LED irradiance, l,m are direction cosines, Lk is distance between k-th LED and center of effective aperture, Gk is receiver gain (in dB) for k-th LED and αk is the incidence angle of radiation of k-th LED on effective aperture. 3 Computer Simulation Fig. 3 shows configuration of transmitting LEDs. The diodes (Vishay TSFF5200) are uniformly distributed around two circles (N1 diodes on the inner circle and N2 diodes on the outer one) and are deflected radially such way, that their axes make angles Θ1, Θ2 respectively with the normal line of TXA. Fig. 4 shows power (in dBm) received by a test aperture Dtest = 15mm in dependence on angular deviations αx, and αy and a section for αy = 0; both at L = 0.3m. The test aperture is always oriented perpendicularly to the beam going from the transmitter center. Its size is chosen similarly to the Proceedings of the 4th WSEAS Int. Conf. on Information Security, Communications and Computers, Tenerife, Spain, December 16-18, 2005 (pp128-130) size of effective aperture of the actual receiver to get a realistic image of field distribution. TXA The receiver optics consists of Fresnel lens DRXA = 50mm (E.F.L. 33 mm) and a photodiode with diameter of active area dFD = 4mm. Fig. 5 shows results of numerical analysis of received power for θA = 10° and ∆ = 25mm as a function of side deviations ∆x and ∆y for L = 1.5m. -0.1 -14.5 -0.05 ∆ y [m] D1 D2 -15 0 -15.5 0.05 -16 0.1 -0.2 -0.1 0 0.1 0.2 ∆ x [m] 0 Θ1 P [dBm] Θ2 -10 -20 TXA -30 -0.2 -0.1 L = 1.5m N1 Θ1 D1 [mm] 38 4 Θ2 D2 [mm] 75 N2 [°] 7 8 [°] 20 Fig. 3 Configuration of transmitting LEDs. -40 -4 -30 -5 -20 α y [deg] -10 -6 0 0 0.1 0.2 ∆ x [m] Fig. 5 Received power as function of ∆x and ∆y (see Fig.1) 5 Conclusions The optical system designed is based on inexpensive LEDs and plastic Fresnel lens and provides optical power at photodiode between -30dBm to 0dBm under all circumstances. The optical link is intended for modernization of classical train wagons to increase safety and provide new services both for the crew and passengers. -7 10 -8 20 6 Acknowledgements 30 40 -40 -9 -30 -20 -10 0 10 20 30 40 α x [deg] 10 This research has been supported by the Grant Agency of the Czech Republic under the contracts No. 102/05/0571 and No 105/05/0732 and by the Ministry of Education under the contract MSM0021630513. 5 P [dBm] 0 -5 -10 -15 -20 -40 -30 -20 -10 0 10 20 30 40 α x [deg] Fig. 4 Distribution of transmitter filed received by 15mm test aperture. References: [1] Standard TNŽ 281400: Carriage of length of 26,4m and gauge of 1435mm, Office for Standardization and Measurement Publishing, Prague, 1989. [2] SALEH, B.E.A. and TEICH, M.C.: Fundamentals of Photonics. John Wiley, New York, 1994. [3] SANTAMARIA, A. a LÓPEZ-HERNÁNDEZ, F.J.: Wireless LAN System. Artech House, London, 1994.