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Factory Radio Communications 0 rf indoor collllllunicafions ___________ Factory Radio Communications Noise and propagation measurements reveal/imitations for UHF/microwave i'ndoor radio communication systems By Theodore S. Rappaport Virginia Polytechnic Institute and State University A glimpse into a typical factory re· any type of tether will require a radio In Japan, spectrum has been set aside veals a high degree of automation has system for control. Optical systems are for 300 mW, 4800 bps indoor radio .entered into the work place. Computer­ viable, but become inoperative when systems operating in the 400 MHz and driven automated test benches, wired­ obstructed. Furthermore, radio systems 2450 MHz bands (4). guided robots and PC-controlled drill will be us~ful for quickly and cheaply Accurate characterization of the oper­ presses are a few examples of the connecting often moved manufacturing ating channel is a mandatory prerequi­ proliferation of computer technology equipment and computer terminals. Ra­ site for the development of reliable and automation in manufacturing. The dio will also accommodate reconfig­ wideband indoor radio systems. Radio boom in automation has created a need urable voice/data communications for channel propagation data from factory for reliable real-time communications in other facets of factory and office building buildings have been· made available for factories. In 1985, the Manufacturing operation and may eventually be used the first time through a research pro­ Automation Protocol (MAP) networking in homes and offices to provide univer­ gram sponsored by NSF and Purdue standard was established by manufac- sal digital portable communications (1 ). University. As shown here, it is not . turing leaders to encourage commer­ Presently, communications· between environmental noise, but rather multi­ cialization of high data rate communica­ computers and automated machines are path propagation that limits the capacity tions hardware for use in computer­ conducted almost exclusively over ca­ of a radio link. The severity of multipath controlled manufacturing. MAP is capa­ ble. Narrowband radio systems (with is largely dependent upon factory inven­ ble of supporting 10 megabit per second data rates less than 10 kbps) are tory, and building structure and age. (Mbps) data rates for short periods. MAP currently used in some factories for networks (and TOP networks in office dedicated control of overhead cranes Factory Noise buildings) rely on coaxial cable or fiber and wire-guided vehicles and for inven­ Although much of the radio · noise optic cable to interconnect users. tory control. While narrowband systems encountered in factories arises from Twisted-pair interconnection of com­ are well-suited for human operation and weak emitting sources, measurements puter terminals is also commonly used. simple communications, it is anticipated have revealed that some types of indus­ This article discusses the use of radio that for a moderately sized AGV fleet trial equipment emit harmonic RF en­ links for communication in modern fac­ (greater than 25 vehicles), data rates of ergy and can radiate substantial noise tories. several hundred kbps will be needed to up to several hundred megahertz (8). accommodate real-time computer con­ Equipment such as RF-stabillzed arc he method for transporting parts-in­ trol and navigation of CIM-integrated welders, induction heaters and plastic Tprocess in a futuristic, but realistic, AGV systems employing multiple-ac­ bonders are acute sources of noise. just-in-time (Jin manufacturing environ­ cess radio networking. Although interference i~ significant at ment is one of the areas of research at In the United States, the FCC has HF and VHF, noise signatures of such the National Science Foundation (NSF) allocated. spectrum for narrowband in­ equipment fall off rapidly above 1 GHz Engineering Research Center (ERC) for dustrial radio communications in VHF (8). Recent empirical measurements Intelligent Manufacturing Systems. Analy­ (450 MHz) and UHF (900 MHz) bands. have confirmed that typical machine­ sis has shown that an inexpensive, agile More recently, the FCC has authorized generated noise levels in operational mobile robot fleet, capable of navigating the use of suitably designed spread­ factories are much less severe at higher I without any type· of pre-made track, spectrum systems for 900 MHz, 2400 frequencies (9). Figure 1 shows results could easily accommodate the type of MHz and 5725 MHz (3). If transmitters of peak noise power spectrum measure­ material flow required for a JIT manufac- meet with FCC approval, unlicensed 1 ments. made along an engine manufac­ . turing system. A truly autonomous W transmitter power levels may be used turing transfer line in full operation. guided vehicle (AGV) that does not use-- over bandwidths Qreater than 25 MHz; Worst-case noise levels are seen to be --- -~j- RF Design ...........................................................------~------~------------------------------..- rl indoor co~nlilunicafions ____________--+- 40 dB lower at UHF/microwave frequen­ cies and are only 5 dB above the thermal ·20.00 ·20.00 noise floor. These results are encourag­ ing and indicate that noise will not 'E severely hamper most factory radio -40.00 'E ·40.00 I :g.CD systems operating at UHF and above. I ·80.00 ·60.00 D. ~0 D. Multiple-Access Networking '2 ... 1 & L....al ..&. -L Considerations ·80.00 .i!: ·80.00 I ~ Future factory. radio communication a: systems will rely on multiple-access ·100.0 ·100.0 27.00 87.00 147.0 207.0 287.0 327.0 1.2 1.3 1.4 1.5 1.6 1.7 techniques to accommodate many fixed Frequency (MHz) Frequency (GHz) and portable/mobile terminals. Multiple­ access techniques such as frequency a) 27 MHz to 327 MHz (VHF) bl 1200 MHz to 1700 MHz lUHFl division multiple-access (FOMA), time­ division multiple access (TOMA), code Figure 1. Noise power spectrum measured 4 m from engine cylinder division multiple-access (COMA, also machining line (9). called spread-spectrum) and carrier­ sense multiole-access (CSMA. also Uctlit'Y fJI::tliM1i rc:tUIUJ f,Jc:ti ll\ltln Ur.i:tilnei users Into non-overlapping signal spaces. Because of non-Ideal condi­ tions, ~owever, there Is Inevitable over­ lapping of signals and the resulting co-channel interference can appear as lengthy message delays or as degrada­ tion of the desired received signal. Random access (CSMA, COMA) radio local area networks are attractive be­ cause they have relatively. few synchro­ nization requirements. Unlike fixed as­ signment networks (FDMA, TDMA) which assume all users require their own channel, random access techniques rely a) Transmitter b) Receiver on "bursty users" and assume that the likelihood of many users· using the network at one time Is small. For AGVs using reliable on-board dead-reckoning Figure 2. Block diagram of fac.tory multipath. measurement system. systems, infrequent position updates 60~------------------------~~~~ 99 ·.:·LOS light •••• •• n<=4 80 • OISS tight clutter, Site C x LOS heavy ..... ··· lOS heavy clutter, Site E 50 A LOS wall LOS light clutter, SiteD 50 * OBS light .. ···········• •' • OBS heavy ,•' ... ... ······· 20 All Sites ····:· ....... ········ 40 ....... .~ 10 .. ·· ..... ·;.. % Prob111blllly Signal ... ······< ......... ~~~ .., ** ,... .·><····· Signal Level Attenuation 30 n= 2.18 .. ·· * ...... ' ' .!!"-·· n=2 <Abscissa (d!J) o = 7.92 dB ·· • ·'' B · X •' '' 0 ..... / • .. ··'* •••* .rf..x•' ~ x ,• .. ~.······ . .. I t.'l'~· •€ . ~ X . X " ... / ::::.:.· .. ··.~*:·~~~---~····t··t .....~-~! ...~·········;:;· 0.5 10 ..•. .. ........ ····ei .. ·· ····· ······ 0.2 ····· ········· ·20 ·10 10 10 20 50 100 Distance (m) SIIJnal Level (dB about median) Figure 3. Large scale signal attenuation at all Figure 4. Cumulative distributions for three measure­ measurement sites. ments and their fit to various distributions. 68 January 1989 ... fRE UENCY SYN HESIS 11 c ..nk Silo" Woolt Uno In Silo F 10 .. ~ lloftlll#<ol <Aboclooo ·10 0 •• ·11 ~!~0----2~1--~50--~7~1--~100- .•••• lloYioltil m -- fUc:IM K•t0d8 TIME (o) 10 Figure 5. Temporal fading meas­ Figure 6. Cumulative signal level urement in engine assembly area distributions for temporal fading across crankshaft wash line (Site data In various factory location$. F). T·R separation is 25 m. suggest the use of random access high resolution graphics, maps) are networks for AGV control. On the other accommodated best by a fixed assign­ hand, direct remote control of a fleet of ment network. Selection of networking vehicles by a central dispatching station strategies of radio depends heavily upon (such as might be warranted for cleaning the number of users, the duration of a contaminated nuclear reactor) would transmissions, the limit of sophistication warrant a fixed assignment scheme. at each terminal, and the importance of At the ERC for Intelligent Manufactur­ real-time control. ing Systems at Purdue University, con­ siderable progress has been made in Multlpath Propagation determining realistic limitations on the Due to the large metal content of a delay characteristics of packet radio factory, multipath interference is created networks (10). Also, a powerful product by multiple reflections of the transmitted · form solution model for packet radio signal from the building structure and systems has been developed .(11 ). surrounding inventory. The resultant CSMA and COMA strategies can be received waveform is a sum of time- and merged to enhance multiple-access com­ frequency-shifted versions· of the origi­ munication performance
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