111Katsuyoshi SATO et al.

3-4-2 Technologies of millimeter-wave road-vehiclecommunications

Katsuyoshi SATO, Hiroshi HARADA, Fumihide KOJIMA, and Masayuki FUJISE

In recent years interest in ITS (Intelligent Transport Systems) technology has been grow-ing, because it is thought that ITS can solve problems such as traffic jams and traffic acci-dents. Communication between roadside base stations and vehicles is a critical element ofan ITS. In this paper, we describe the road-vehicle communication (RVC) system using ROFtechnology and software radio. These systems can realize multimode service and highspeed data transmission. We developed RVC system using ROF technology which cantransmit three kind of services (ETC, PHS, BS) simultaneously. The software radio with smallsize for mobile terminal is also developed. The overview of these systems is shown. Inaddition, activities of YRP (Yokosuka Research Park) ITS Joint Research Group are shown.

Keywords road-vehicle communications, millimeter wave, Radio on Fiber, software radio, propa-gation

1 Introduction

Intelligent Transport Systems (ITS) areexpected to increase the availability of currenttransportation systems and help solve thecomplex problems they involve. The advan-tages of ITS are recognized in a wide range ofareas, including safety, efficiency, and envi-ronmental friendliness. For example, a typicaladvantage of ITS is lighter traffic, resulting inan increase in the efficiency of transportationsystems, higher fuel efficiency, and a reduc-tion in problems related to the supply of ener-gy. Wireless communications technologiesform the basis of the implementation of ITS;among the related technologies, road-vehiclecommunications technology, which serves as alink between the street-side infrastructure anddrivers, plays an important role in this imple-mentation. Applications of the road-vehiclecommunications technology have alreadybeen commercialized, with examples includ-ing ETS (Electric Toll Collection) and VICS(Vehicle Information and CommunicationsSystems). With a view toward further diversi-fying services and expanding DSRC (Dedicat-

ed Short-Range Communications), we shouldstrive to develop technologies for high bit ratedata transmission and multiservices. Undersuch circumstances, the CommunicationsResearch Laboratory has continued to conductresearch on the implementation of road-vehi-cle communications technology using theROF (Radio on Fiber) technique[1][2]. Thispaper describes our investigation of millime-ter-wave ROF road-vehicle communicationstechnology.

2 ROF Road-Vehicle Communica-tions

2.1 ROF communications system[3][4][5]

Road-vehicle communications and inter-vehicle communications play an essential rolein ITS. A variety of services are currentlyavailable in road-vehicle communications; asthese services are provided using different fre-quencies and methods, additional infrastruc-ture such as base stations are required if a newservice is to be offered. The vehicle stationalso requires the installation of antennas andother devices for individual services — it

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would look like a hedgehog with all of itsantennas, placing an under burden on users interms of service ability (Fig.1).

We have proposed the ROF (Radio onFiber) road-vehicle communications system asa solution to the above problems and a toolthat will be capable of properly handlingfuture services. This communications systemcan transmit different services by integratingthem into one; Fig.2 demonstrates its basicconcept. The ROF road-vehicle communica-tions system consists of a central base station,local base stations, and optical fibers to linkthem. The central base station converts anumber of frequencies for different servicesinto a common frequency band. The integrat-ed signal after conversion is further convertedfrom an electric signal to a light signal, andthen transmitted through optical fibers to localbase stations installed at roadsides. The use ofoptical fibers makes it possible to transmit amassive amount of data at a satisfactoryspeed. Electric/optical signal conversion iscarried out at a local base station, and the con-verted signals are emitted from a commonantenna. As these RF signals contain a con-

siderable amount of data for numerous servic-es, the employed frequency should be suchthat a broad bandwidth can be secured. Themobile station receives the integrated signalsusing a common antenna, and the signals aredistributed to individual service terminals. Ifsoftware radio technology (to be explainedlater) is employed, the structure of the vehiclestation can be greatly simplified. The systemconfiguration is inverted between uplink anddownlink.

2.2 Software radio technology[6]A wide range of systems are available for

use in current road-vehicle communications,and more will be developed in the future.Users will have to deal with the burden ofinstalling new hardware each time to haveaccess to a new service. It is therefore urgent-ly necessary to establish a system that canhandle different communications servicesusing a single terminal. A typical example ofsuch systems is software radio communica-tions technology that realizes a desired com-munications function by customizing the soft-ware program saved in the terminal hardware

Journal of the Communications Research Laboratory Vol.48 No.4 2001

Current situation of communications and broadcasts provided for carsFig.1

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and the base station by using an external con-troller. The target hardware of main interest isthe digital signal-proceeding unit, and the ele-mental techniques required for carrying out aspecific function are described in this pro-gram.

When this software radio communicationstechnology becomes available, differentmobile communications systems can be oper-ated using a single terminal, making it easy toemploy a wide range of modulation methods,transmission speeds, bandwidths, and connec-tion modes. In addition, as the system canflexibly adapt to changes in propagation con-ditions, the wireless terminal will be capableof handling multimode and multimedia. Thiswill make wireless base stations economicallyefficient, and eventually the frequencyresource will be used more efficiently as aresult. Additionally, in the development andoperation of a new communications system, itwill only be necessary to prepare the software;new hardware will not be required. This willallow a new system to be brought to opera-tional status quickly. In addition, it involves

the environmental advantage that industrialwaste is reduced upon system changeover.This technology is ecologically friendly insuch respect. The Communications ResearchLaboratory has been working intensivelytoward the smooth realization of this flexiblesystem since 1997.

3 36-GHz-Band ROF Road-VehicleCommunications System

3.1 36-GHz-Band ROF Road-VehicleCommunications System

The millimeter-wave band, which can pro-vide a broad band, will be a good choice fortaking advantage of the high capacity for datatransmission provided by ROF road-vehiclecommunications. We have developed an ROFroad-vehicle communications system utilizingthe millimeter-wave band that provides a largebandwidth. Fig.3 shows part of its basic struc-ture. As illustrated in the figure, the facilitiesare configured so as to provide functions forETC (Electronic Toll Collection) as an ITSservice, PHS (Personal Handy-Phone Sys-

Katsuyoshi SATO et al.

ROF-based multiservice road-vehicle communicationsFig.2

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tems) as a wireless communications service,and BS (Broadcasting Satellite) service as abroadcasting service. Our experimental facili-ties are composed of an integrated controlbase station, an optical-cable region, localbase stations, a wireless region, and mobilestation. The integrated control base station isinstalled in a laboratory of YRP (YokohamaResearch Park), and the local base stations areinstalled along an experimental course extend-ing along approximately 240 m of a publicroad. The interval between the local base sta-tions is 20 m, and as many as 10 local basestations have been built thus far. Fig.4 showsexternal views of the local base station andmobile station. In our experimental system,frequency bands of the RF signals for conven-tional wireless services are converted intocommon millimeter-wave bands, enabling thelocal base stations and vehicles to share thesame frequency band and unify the wirelesssections. The antenna for the mobile stationand RF unit can then be shared among severalservices, and the system configuration is con-siderably simplified. Fig.5 shows the frequen-cy allocation for wireless services sharing acommon frequency band. As common fre-quency bands for our experiment, a 500-MHzband has been assigned to each of the 36.00 to36.50-GHz uplink channel and the 36.75 to37.25-GHz downlink channel. As previouslydescribed, our experimental system is capableof offering ETC, PHS, and BS services simul-taneously. The control base station converts

the frequencies used for these services into theintegrated 36-GHz-band radio signals that areused for analog-based light intensity modula-tion. In providing radio signals for servicesconverged in a common frequency band in thewireless region, there is a method of transmit-ting signals via optical fiber by convertingthem into millimeter-wave signals in the con-trol base station. Another method is to sendsignals in the optical-fiber region by convert-ing service signals into integrated RF or inter-mediate frequencies, and then further convert-ing them into millimeter-wave signals at eachlocal base station. The configuration of ourexperimental system allows both methods tobe used. The mobile station converts thereceived RF signals into those of the respec-tive original frequencies, making it possible touse pre-existing terminals.

3.2 Influence of third-order intermodu-lation distortion (IM3)

Third-order intermodulation distortion(IM3) will be a great challenge in the imple-mentation of our system[7]. In the integratedcentral base station, the service signals forPHS, ETC, and BS are combined in the 5.8-GHz band and then used in the modulation oflight signals. The converted signals are sent tolocal base stations through optical cables.Noise may therefore occur in the service-sig-nal band due to, for example, distortion causedby amplifiers inserted along the transmissionpath. Figure 6 shows the frequency allocation

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Structure of the 36-37-GHz-band ROF multiservice experimental systemFig.3

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for service signals in the downlink channel,along with possible locations at which IM3may occur. The IM3 can be divided into thethree-tone-type distortion (A in Fig.6) causedby three signals, and the two-tone-type distor-tion (B, C, D) caused by two signals. Eithertype of IM3 may occur in the currently allo-cated band for service signals, as shown inFig.2. Affected by the signal distortion, thequality of each service signal will decline dueto a fall in the D/U ratio.

Fig.7 shows the spectra of service signalsand IM3 in the downlink channel. In the fig-ure, part of the IM3 (IM3) caused by ETC andBS signals is recognized in the band of thePHS signal, and the quality of PHS service ispredicted to degrade. However, a study exam-ining decay and other factors in the radio-wave propagation path of integrated servicesignals has shown that the extent of such aquality decline due to IM3 is sufficientlysmall.

3.3 Millimeter-wave propagation inROF road-vehicle communications[8]

As described above, our system employsthe 37-GHz band for signal transmission in theair. While the millimeter-wave band makes itpossible to realize high-capacity communica-tion, numerous propagation-related problemsremain to be solved due to its extremely highfrequency. For example, the millimeter waveis not suitable for use in long-distance com-munications, as the propagation loss becomestoo large. This may actually serve as anadvantage in terms of the frequency efficiencybut, in cases in which an antenna with a rela-tively wide directivity is used, the available

Katsuyoshi SATO et al.

Frequency allocation in the common frequency bandFig.5

Views of the central base station, localbase station, and mobile station

Fig.4

116

service area will be limited to a few tens ofmeters, depending on the employed communi-cations methodology. In addition, millimeterwaves are easily blocked by obstacles; forexample, if a large car cuts into the millime-ter-wave propagation path, communicationbecomes very difficult to continue. Similarly,communication using millimeter waves is eas-ily affected by rain. During road-vehicle com-munication, the Doppler shift will be extreme-ly noticeable, as the relative velocity betweenthe base station and mobile terminal station islarge.

Nevertheless, the broad bandwidth of the

millimeter waves is a great advantage. High-capacity data transmission is an essential ele-ment of ROF road-vehicle communications.Thus, the problems with the propagation ofmillimeter waves must be solved to realize oursystem. Interference is a crucial issue, partic-ularly for the millimeter-wave system usingROF technology. If, in the ROF system, twoor more local base stations linked to a singlecentral base station are allowed to emit mil-limeter waves of the same frequency, the sys-tem structure will be greatly simplified. At thesame time, the adjacent service areas of localbase stations linked to the common centralbase station may be regarded as a virtuallycontinuous single service area, even if theservice area of each local base station is small.However, in such a case, there will inevitablybe strong interference at the boundaries ofservice areas formed by local base stations.

Fig.8 illustrates an example of the meas-urement of received power. In this figure, theantenna of the base station uses the cosecantsquared-beam pattern, and thus the dynamicrange within the service area is relativelysmall compared with that provided by a stan-dard horn antenna. Large-level fluctuationsare recognized near the boundaries of serviceareas formed by local base stations. A close

Journal of the Communications Research Laboratory Vol.48 No.4 2001

Locations of third-order intermodulation distortion (IM3)Fig.6

Spectra of the signal and third-orderintermodulation distortion (IM3)

Fig.7

117

examination of the received power fluctua-tions at the boundaries (Fig.9) reveals a micro-interference pattern having a cycle of severaltimes the wavelength. Fluctuations of as greatas 20 dB are seen in some areas. This prob-lem must be solved if the independent serviceareas are to be regarded collectively as a virtu-ally continuous single area.

We adopt a technique using diversity as ameans of solving the problem. We have beeninvestigating a diversity technique usingpolarization to provide flat frequency charac-

teristics over a broad band, and to make fluc-tuation cycle affected by interference veryshort. Fig.10 shows the fluctuations inreceived power observed near the boundariesof service areas for the case using right-hand-ed circular polarization and left-handed circu-lar polarization in adjacent stations. Asdemonstrated in Fig.10, the amplitude of fluc-tuations decreases in the case of diversity,compared with the case in which the samepolarization is employed in adjacent stations.Although further improvements in the axis

Katsuyoshi SATO et al.

Measurement of received power (base stations located at the 0-m, 20-m, and 40-m positions)Fig.8

Received power near the boundariesof service areas (when polarization ofadjacent base stations was used ascircular polarization with the samerotational direction)

Fig.9

Received power near the boundariesof service areas (when polarization ofadjacent base stations was used ascircular polarization with oppositerotational directions)

Fig.10

118

ratio, for example, are required, we feel thatdiversity is a promising technique.

4 Multimode Vehicle Terminal byusing Software Radio Technology

Before the software radio technology isadopted in ITS, we must gain an understand-ing of the situation specific to ITS. Specifical-ly, there will be demand for the simultaneousreception of two or more services dependingon the time slot. In other words, we shoulddevelop a multi-process wireless communica-tions system that will enable multi-mode com-munications as well as simultaneous services.For this purpose, the CommunicationsResearch Laboratory has proposed Multi-Mode and Multi-Process Software Radio(MMSR) technology and has periodicallyexamined its utility. As part of this investiga-tion, we have created a prototype compactvehicle-borne terminal device using MMSRcommunications technology with enhancedefficiency. Fig.11 shows the external view of

the prototype terminal device. Table 1 con-tains a summary of its system specifications.

This compact software radio terminal canprovide services such as GPS, VICS, ETC,AM radio, FM radio, FM text broadcastingservice, and modulation/demodulation such asBPSK, QPSK, GMSK, ASK, and π/4QPSK,using only compact (17.5 cm in width, 19 cmin depth, 5 cm in height) hardware. Theseservices will be available simultaneously with-in the framework of the Intelligent Transporta-tion Systems. The signal-processing unit ofthis software radio system is composed ofonly two FPGAs of two million gates each,and the terminal can be driven by a powersupply of 12 V and 2-3 A. The software to beinstalled will be downloaded via wires as wellas wireless channels.

Fig.12 shows the system configuration ofthe compact software radio terminal. In prin-ciple, its digital signal-processing unit is com-posed of two FPGAs and one CPU serving asa controller. The software for FPGAs is savedin a PC connected using Ethernet cables,allowing it to be downloaded as desired. TheRF and IF units for ETC and GPS are notincluded in the terminal and can be addedupon request. The RF and IF units are pre-installed only for VICS and FM and AMradio.

The software to serve as a modem unit isdownloaded to one of the two FPGAs. Thismodem unit can provide modem functions forBPSK, QPSK, GMSK, ASK, and π/4QPSK,simply by changing the parameters that are

Journal of the Communications Research Laboratory Vol.48 No.4 2001

External view of the on-vehicle soft-ware radio terminal

Fig.11

Service

RF frequency bandIF frequency bandConfigurationMaximum number ofsimultaneous services

AD Conversion

Software download

GPS,VICS, ETC, Broadcasting (AM, FM),FM Multiple service, User mode (BPSK, QPSK, GMSK, ASK,π/4QPSK)5.8 GHz, 1.5 GHz, 76-100 MHz, 0.5-1.6 MHz70 kHz-5 MHz2FPGAs (2 million gates x2) + PCSR and MMSR technologies

5 (variable)

IF over-sampling20 Msps, 10 bitEthernetVICS radio channel (broadcasting-type)ETC radio channel (communication-type)

System specificationsTable 1

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differences among modulation and demodula-tion schemes. The parameters to be input inthis software are the system clock frequency,the employed modulation mode, the coeffi-cients of the waveform configuration filter forthe transmitter and receiver, the local oscilla-tion frequencies of the orthogonal modulatorand demodulator, the data clock frequencies ofthe transmitter and receiver, the coefficient ofthe loop filter used for clock recovery, and thethreshold level used for data demodulation.

As performance indices of the prototype

system, the size and download time of soft-ware constituting this system are shown inTables 2 and 3, respectively. The prototypesystem uses FPGA-1 for the ETC and USERmodes, while FPGA-2 is used for all otherpurposes, as illustrated in Fig.11. Regardlessof the number of downloaded communicationservices or used FPGAs, software of 1241 KBmust be downloaded when either FPGA-1 orFPGA-2 is used. As the required time fordownloading to an FPGA is approximately 8seconds, as indicated in Table 3, all software

Katsuyoshi SATO et al.

Software sizeTable 2

System configurationFig.12

Service

ETC

FM Multiple

VICS

USER mode

(full download)

ASK (parameter)

QPSK (parameter)

Time (byte)

1241 k

1241 k

1241 k

1241 k

1.73 k

1.73 k

Service

GPS

Broadcasting

Download

π/4QPSK (parameter)

BPSK (parameter)

GMSK (parameter)

Time (byte)

1241 k

1241 k

1241 k

1.73 k

1.73 k

1.73 k

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changes can be completed within 16 seconds.We compared the software volume and

download time between full downloading andparameter information downloading for sys-tem changeover, under the same conditions asused for the prototype system. The results areshown in Tables 2 and 3. These tables indi-cate that the software to be downloaded isreduced to approximately 1/1000 of its origi-nal size, and that its configuration time isshortened to approximately 1/8000 of its origi-nal length through the use of the parameter-information downloading software radio com-munications technology, which requires onlydifferential information for activation. Whenthe differential parameter-driven softwareradio communications technology (PDSR) isused, the time required for downloadingchanges little among different modulation-demodulation schemes. As a conclusion, theMMSR and PDSR technologies are importantkey technologies for realizing future software-radio-based intelligent transport systems.

5 Multimedia Lane and Multime-dia Station[9]

It takes time to introduce facilities such asbase stations, and to build up the infrastructurenecessary for implementation of the ROFroad-vehicle communications system. Thus,in the early stages, the availability of the infra-structure will be limited to major roads and afew other areas, and related service will beavailable only in such limited areas. It istherefore unlikely that a moving vehicle will

receive the RVC service over an extendedperiod on a continuous basis. The actual serv-ice areas will be limited to only parts of road-ways or to discontinuous sections of road-ways. These limitations will restrict the rangeof service contents, and the early-stage servic-es will consist of large-capacity simultaneousdownloading through media where real-timecharacteristics are not of great importance.

We propose the concept of a multimedialane and multimedia station serving as ITSroad-vehicle communications infrastructurecapable of providing a large-capacity, simulta-neous data transmission service. The multi-media lane and multimedia station are localfacilities intended to provide large-capacity,simultaneous data transmission services and todownload information from the installed ROFlocal base stations to the vehicle. A multime-dia station is defined as that using a singlelocal base station for the broad band packagetransmission, while a multimedia lane is thatusing two or more local base stations to down-load information. Their system specificationsare shown in Table 4. In our definitions, themultimedia lane assumes a situation in whichthe vehicle is operating during informationdownload. It will typically be employed inmain lanes and side lanes. In contrast, withthe multimedia station, there are no limitationson the speed of the moving vehicle. However,the amount of information a vehicle candownload from a local base station decreasesas the speed of the vehicle increases. Thus,when a user downloads information with alarge data size, static downloading will be

Journal of the Communications Research Laboratory Vol.48 No.4 2001

Software downloading timeTable 3Service

1.ETC

2.FM Multiple

3.Broadcasting

1+2

1+2+3+4

USER mode

(full download)

ASK (parameter)

QPSK (parameter)

time

8380 ms

8260 ms

8260 ms

16620 ms

16640 ms

8380 ms

less than

1 ms

Service

4.VICS

5.GPS

6.Download

1+2+3

1+2+3+4+5

π/4QPSK (parameter)

BPSK (parameter)

GMSK (parameter)

Time

8260 ms

8750 ms

16650 ms

16620 ms

16650 ms

less than

1 ms

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dominant under actual conditions, such aswhen information is downloaded from a localbase station installed in a parking lot.

Fig.13 shows a system model of the multi-media lane. The multimedia lane is composedof two or more local base stations installed by

the roadside, and an entrance gate. Theentrance gate senses the arrival of a vehiclethat will download service data, and the localbase stations transmit the service data to thevehicle. The vehicle downloads the data,passing by more than one local base station.In this case, the continuous cells used in datadownloading by a vehicle share a co-channelin order to reduce the load of handover on theterminal mounted in the vehicle.

Fig.13 shows a system model of the multi-media station. The configuration of the multi-media station is almost the same as that of themultimedia lane, except that only one localbase station is used each time data is down-loaded. Each local base station has equipmentfor sensing the arrival of a vehicle.

6 YRP ITS Joint Research Group

Yokosuka Research Park (YRP) conductsresearch and development on ITS communica-tions, having established a research groupcomposed of the Communications ResearchLaboratory and various private companies inFY1998. The ITS joint research group aredoing separate activities in the inter-vehicle

Katsuyoshi SATO et al.

Multimedia lane

Definition

Vehicle

Major sites of

application

Download information through

multiple connected local base

stations

Should be moving during infor-

mation download

Main lanes, side lanes

Multimedia station

Definition

Vehicle

Major sites of

application

Download information through

a single local base station

No vehicle speed requirements

during information download

(In principle, vehicle, should be

at a standstill.)

Parking lots

System featuresTable 4

System models of the multimedia lane and multimedia stationFig.13

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communications and road-vehicle communi-cations working groups.

The road-vehicle communications jointworking group has provided a base for thestandardization of road-vehicle communica-tions technology using ROF technology. Thisworking group comprises three sub-workinggroups (SWG) — propagation SWG, high-speed transmission SWG, and multi-serviceSWG. The sub-working groups have beenactively conducting their own experiments anddiscussions.

7 Conclusions

This paper has described the technologicalrequirements for the implementation of ROFroad-vehicle communications using millimeterwaves, and the facilities we have developedfor field tests on the ROF technology. Themillimeter-wave ROF road-vehicle communi-cations system incorporates a wide range ofrelated technologies. Currently, we are work-

ing to enable the transmission of more diverseapplications, and are investigating the techni-cal items that will serve as a base for standard-ization of the multimedia station. As a varietyof leading-edge technologies are incorporatedinto the millimeter-wave ROF road-vehiclecommunications system, many technologicalproblems remain to be solved before it can beimplemented. One of our challenges is todevelop a technique (such as IM3) that willbring together a number of applications on thesame frequency band, and another is to solvethe problems associated with the use of mil-limeter waves (in terms of propagation anddevices, for example). We will tackle suchtechnological issues on an individual basis.We plan to expand the range of available serv-ices and combine road-vehicle communica-tions technology with inter-vehicle communi-cations technology in order to provide an inte-grated road-vehicle, inter-vehicle communica-tions service in the near future.

Journal of the Communications Research Laboratory Vol.48 No.4 2001

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33, Mar. 2000.

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123Katsuyoshi SATO et al.

Katsuyoshi SATO

Senior Researcher, Mobile Communi-cations Group, Yokosuka Radio Com-munications Research Center

Propagation

Hiroshi HARADA, Ph. D.

Senior Researcher, Mobile Communi-cations Group, Yokosuka Radio Com-munications Research Center

Transmission

Fumihide KOJIMA, Ph. D.

Researcher Mobile CommunicationsGroup, Yokosuka Radio Communica-tions Research Center

Radio Communication System

Masayuki FUJISE, Dr. Eng.

Leader, Mobile CommunicationsGroup, Yokosuka Radio Communica-tions Research Center

Optical Fiber Communication, WirelessCommunication