Optical Communication Systems for Automobiles - …

1 Optical Communication Systems for AutomobilesTorsten Schaal, Thomas Kibler, and Eberhard ZeebDaimlerChrysler AG, Wilhelm-Runge-Str. 11, 89081 Ulm, Germanyemail: In this paper an overview of currently used optoelectronic Communication Systems in Automobiles isgiven and future applications as well as technological challenges are Systems are more and more attractiveto communicate both inside and outside cars. Whilefor in-car Systems the main driving factor to introduceoptical Systems is the need for high bandwidthchannels without electromagnetic interference, freespace car-to-car Communication Systems have thepotential to enhance the active and passive drivingsafety. Here we give an overview of fiber basedoptical data bus Systems for in-car networks, as wellas free space Communication Systems for car-to-carcommunication, using standard LED Data Bus SystemsIn 1998 the digital domestic bus (D B) was introducedas the first Optical data bus system [1] in Mercedes-Benz cars for an infotainment system .

2 The systemdata rate of Mbps is capable to transmit speech,audio, and control data for phones, mobiles, soundsystems, and speech recognition Systems arranged ina ring. In 2001 an international consortium of carmanufactures and suppliers set up an open standardfor infotainment networks, the media oriented systemtransport (MOST) [2]. The data rate of the MOST ringnetwork with Mbps allows the transmission ofcompressed video data. Until now MOST isintroduced in more than 20 series car models ofdifferent manufactures and over six million nodes areon the road. It is estimated that over 10 million nodeswill be sold per year from 2005 on. BMW additionallyhas introduced an Optical data bus system for anairbag sensor network, the so-called Byteflight system [3].

3 Here all network participants are connected bypoint-to-point links to an intelligent central starcoupler. All currently installed Optical data bussystems in cars, D B, MOST, and Byteflight, usebasically the same Optical and optoelectroniccomponents: polymethyl methacrylate (PMMA)polymer Optical fibers (POF) as waveguides, redemitting LEDs, and large areas silicon were chosen as waveguides because they aremore flexible than silica fibers, allowing the realizationof large and still flexible core diameter fibers ( mm). The main advantage of POFs are the relaxedconnector tolerances due to the large core diameterand the easy and efficient coupling of high NA lightsources to the fiber.

4 This simplifies the connectordesign and allows the use of plastic injection-moldedcomponents, even for ferrules. As transmissionwavelength the 650 nm low loss window of PMMAPOF is used, because in this wavelength regionlow-cost red light emitting diodes and efficient siliconphotodiodes are characteristics of typical transmitters andreceivers used in the MOST data bus system areshown in Table 1. Due to device fabricationtolerances and to the required wide temperaturerange the output power of the LEDs within anacceptance angle of a NA fiber is dBm and -10 dBm and the peak wavelength ofthe LEDs varies between 630 nm and 685 nm.

5 Thesensitivity of large area (1mm diameter) siliconphotodetectors is about 23 dBm with an overloadlimit of 2 dBm. These device parameters allow carmanufacturers to use a power budget of 13 dB for itscabling harness between two network 1: Specified data of MOST transmitter and receivermodules [4] under automotive worst case wavelength630 .. 685 nmspectral width 30 nmoutput .. 10 dBmrise/fall time8 nsreceiversensitivity-23 dBmoverload-2 dBmDue to the relatively high effective attenuation of thefiber of about dB/m (caused by the broad LEDemission spectrum and the emission wavelengthwhich is not exactly 650 nm) and the connectorinsertion losses of approx. 2 dB today only point-to-point links are realized both in the ring topology of theD2B or MOST Systems as well as in the active starconfiguration of the ByteFlight bus.

6 For example apoint-to-point link with a length of 15 m and twoconnectors at the interfaces of the connected devicesalready has a worst case attenuation of 10 dB,leaving a 3 dB margin for additional inline connectors,bending or installation losses, and a system margin hardly allows the insertion of additionalin-line connectors or other passive coupling second limitation of PMMA fiber based networks isthe maximum ambient temperature range of about85 C, so that they can only be used in the passengercompartment of the car and not in the roof or in theengine compartment. Last but no least the bandwidthof PMMA fiber Systems can not be enhanced easily todata rates well above 100 MBps, due to the limitedbandwidth of the fiber, of the LED as well as of largearea future automotive Optical data links is necessaryto have Optical wiring solutions with higher powerbudget and Optical wires, which would allow lowerbending radii, simpler handling procedures and higherambient temperatures.

7 This would dramaticallysimplify the harness pre-fabrication process at thesupplier side, the final installation and carmanufacturing process at the assembly lines, as wellas the maintenance and repair concept of the carscable harness in the workshops. In addition new kindof high speed data transmission networks will beneeded in cars for novel applications. Examples aresensor Systems for safety applications, enginemanagement Systems , drive-by-wire Systems , andvideo processing Systems for driver assistance andautonomous driving [5]. For these applications today soptical data bus technologies and Systems reach theirlimits mainly in terms of temperature stability, requiredpower budget and allowed data rates.

8 Therefore newphysical layer solutions have to be overcome all above mentioned limitations of thePMMA physical layer, we have proposed a newphysical layer concept based on polymer clad largecore silica fibers (PCS) fibers and 850 nm emittingvertical-cavity surface-emitting lasers (VCSELs).Using these components the fiber attenuation ofabout dB/m can be neglected for typical linklengths in cars and the fiber can be used up toambient temperatures of at least 125 C. For opticaldata links a fiber core diameter of 200 m is a goodcompromise between fiber flexibility and a sufficientlarge core diameter for efficient coupling to activecomponents and acceptable connector alignmenttolerances.

9 Compared to cables with 1 mm diameterPMMA POF, cables with 200 m PCS fibers providetwice the short-term tensile strength, one-third theminimum bend radius and up to ten times greaterlong-term tensile strength and flexing bending radii of standard PCS fibers are10 mm short term and 16 mm long term [6] and evencan be reduced by higher proof test levels during fiberdrawing. In a series of tests we have investigated thereliability of several PCS cables under the mostimportant environmental impacts, temperature andmechanical bendings. The tests have shown theirprinciple feasibility for automotive applications [7].The low loss of PCS fibers at 850 nm opens thepossibility to use GaAs vertical-cavity surface-emittinglasers (VCSELs) [8] as transmitters.

10 These lasers arewidely used in datacom applications and are veryattractive to be used in the automotive device has a large modulation bandwidth(>1 Gbps), a low threshold current, a high efficiency,and can be driven with simple driving studies and their use in large quantities inthe datacom market have shown lifetimes of severalten thousand hours even at high temperatures up to125 C [9].As first demonstration of the feasibility of aPCS/VCSEL physical layer for automotive applicationwe have build up several MOST networks based onthis new technology. We have replaced theoptoelectronic LED-transmitter and receiver modulesin standard series entertainment and have used PCScables in the cable harness.