3-2 Wireless Communications System Using Stratospheric...

33Ryu MIURA and Masayuki OODO

3-2 Wireless Communications System UsingStratospheric Platforms

3-2-1 R&D Program on Telecom and BroadcastingSystem Using High Altitude Platform Stations

Ryu MIURA and Masayuki OODO

Telecommunication and broadcasting systems using radio-relay/base stations on boardthe high altitude platforms are expected to create the largest business market among itspossible applications. Telecommunications Advanced Organization (TAO) and Communica-tions Research Laboratory (CRL) is conducting R&D on the telecom and broadcasting sys-tem to demonstrate its feasibility. Prototype onboard equipment and ground equipment arebeing developed on schedule in the fixed, mobile, and broadcasting services. The perform-ance of them will be demonstrated and evaluated in the preliminary flight test planned in2002 using a helicopter, a jet and a stratospheric unmanned solar plane, Pathfinder Plus,which was developed by NASA, before the flight test using the low-altitude powered test air-ship being developed by National Aerospace Laboratory.

Keywords stratospheric platform, high altitude platform, airship, solar plane, multibeam antenna

1 Introduction

A stratospheric platform has been devel-oped wherein a large-scale airship will stay ata predetermined position approximately at thealtitude of 20 km in the stratosphere and willbe used for telecommunications, broadcasting,and environmental measurements. The proj-ect has been promoted through the ministerialcooperation of the Ministry of Education, Cul-ture, Sports, Science and Technology and theMinistry of Public Management, HomeAffairs, Posts and Telecommunications[1]～[5].Of the airship’s various applications, atelecommunication and broadcasting systemusing the stratospheric platform as a radiorelay or a base station is expected to demon-strate the largest economic impact for thefuture. For research and development of atelecommunications and broadcasting systemaiming to demonstrate the feasibility of thisaspect of the project, the Communications

Research Laboratory (CRL) and the YokosukaResearch Center of the TelecommunicationsAdvancement Organization of Japan (TAO)are playing a central role in developingonboard equipment and ground equipment forevaluation testing, centering on the three fieldsof fixed communications, mobile communica-tions, and broadcasting. The relevant equip-ment is to be installed on an alternate aerialvehicle to conduct a preliminary flight test andto evaluate its performance, prior to installa-tion on board the airship. On the other hand,the National Aerospace Laboratory (NAL) hastaken the initiative in developing the airshipitself, and the Mitaka Research Center of TAOis developing a TT&C (telemetry, tracking,and command) system. The structure of theresearch and development system in Japan isshown in Fig.1. Among the technologiesdepicted, this article introduces the progress ofthe development of the telecommunicationsand broadcasting system.

34

2 Candidate for the thirdtelecommunication infrastruc-ture equivalent to terrestrial andsatellite infrastructures

A terrestrial radio link faces a great deal ofobstacles in its path: Hence, the electromag-netic wave cannot travel great distances, whilea satellite link features an extremely longtransmission path, which weakens the waveand demands additional cost, although lessobstacles obstruct the signal. Thus, the cur-rent focus is on the use of an undevelopedspace, the stratosphere, that is located betweenthe terrestrial and the satellite domains. Theidea has existed for some time of an airship, aballoon, or the like which would be kept in thestratosphere, where the air stream is relativelysteady, and used for telecommunication sys-tems relay purposes. However, to make theairship remain stably in the stratosphere,where the atmospheric pressure is approxi-mately 1/20 times that of the ground, andwhere wind velocity is occasionally as high as30 m/s or more, it is necessary to circumventproblems relating to the supply of electric

power, flight control, launching and recovery,membrane materials, security, and so on; thereis so far no precedent of successful implemen-tation of such a system. On the other hand,the demand for an economical “broadband”wireless access system is increasing, due torapid popularization of mobile communication(such as mobile phones) and of the Internet,and it is expected that the stratospheric plat-form may be the infrastructure that will allowthese services to be offered economically tobusiness users as well as personal users. Aconceptual drawing of the service is shown inFig.2.

Features of the radio communications sys-tem offered by the stratospheric platform aresummarized as follows.(1) High line-of-sight probability and smallpropagation delay time: The stratosphericplatform can be considered an ultra-high-alti-tude tower, with a height of 20,000 meters (60times the height of the Tokyo Tower or more).That is, since the stratospheric platform caneasily provide line-of-sight paths with a highelevation angle, whereby the lines are relative-ly free of the influence of obstacles, a single

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

Research and development systemFig.1

35

platform can cover a large area with a radiusof 50-200 km, depending upon the frequency.Because the electric power required for trans-mission can be decreased, the antenna and theradio equipment can be made smaller. Fur-ther, there is minimal voice delay when usedfor voice telephone or the like, because thestratospheric platform is much closer to theground than satellites, and hence the propaga-tion delay time is less.(2) Electromagnetic environment: Today, thedemand for radio communication is rapidlyincreasing, and an analysis of the electromag-netic environment, including such details assuch as the sharing of frequencies betweendifferent systems and the influence of electro-magnetic waves on the human body, becomesimportant. Since mobile communicationlinks, broadcasting links, and the like of theterrestrial systems are built based on the prop-agation of electric waves traveling very closeto the ground, these lines are susceptible to theinterference of buildings, trees, terrain fea-tures, and the like; therefore, the electromag-netic waves are transmitted with an extremelylarge transmission power, to maintain linequality. For example, although the transmis-sion power of a VHF broadcasting electro-magnetic wave from the Tokyo Tower can beas high as 50 kW, the electromagnetic wavewill not necessarily reach as far as 100 km.

Further, there is an attenuation of electricpower as high as 40 dB (10,000 times)between the neighborhood of a transmittingstation and more distant areas of reception. Ifthe electric wave is transmitted from the strat-osphere, the transmission power can be as lowas a few tens of watts – sufficient to cover thesame area – as surplus electromagnetic wavesare minimally irradiated to the surroundings,and the attenuation in electric power over oneservice area can be mitigated to approximately10 dB (a few tens of times).(3) Economic efficiency and low risk: If an areawith a 100-km radius is intended to be cov-ered thoroughly with ground stations such assteel towers, more than 1,500 stations must beinstalled, whereas in the case of the stratos-pheric platform, the whole area could be cov-ered with one or two platforms. That is, in theterrestrial radio communication network, anarea that is covered by means of installation ofa great many number of base stations – steeltowers and the like – can be covered by asmall number of stratospheric platforms atonce, and consequently an extremely econom-ical network could be constructed. As anexample, a deployment plan for stratosphericplatforms that could cover the entire countryof Japan is shown in Fig.3. When the lowestservice elevation angle is assumed to be 10degrees, the number of necessary platforms


Conceptual drawing of service provided by the stratospheric platformFig.2

36

becomes 16. Further, unlike a satellite systemsuited to global service, the stratospheric plat-form is suited to local services, each centeringon an area where demand exists. High effi-ciency can thus be expected, compared to thelow earth-orbiting satellite systems and thelike that have little choice but to cover serviceareas including the sea and virtually unpopu-lated regions. That is, it can be said that theheight of the stratosphere is a naturally suit-able height from which users of a specific areamay be covered extensively and precisely.(4) Flexibility: The system can provide flexibili-ty in allocation of platforms and provision ofcommunication links according to need,responding to disasters and other such extraor-dinary events. It is also expected that in theevent the onboard equipment malfunctions orwhen upgrades become necessary, the onboardequipment can be temporarily retrieved on theground, therefore providing greater flexibilitythan a satellite system.

The telecommunication and broadcastingconcept using the stratospheric platform pro-vides that a number of stratospheric platformswill be arranged in the sky in a mesh manner,with a spacing of a few tens to a few hundredsof kilometers, set to serve as relay stations or abase station. With this concept, there can beenvisioned a variety of applications, not onlyfor fixed communications but also for mobile

communications, broadcasting, global envi-ronment measurements, radio monitoring, andso on. Among others, a broadband access net-work that uses high frequencies (in the mil-limeter wave band, for example) and providestransmission speed of 20 Mbps or more in asingle channel, aimed at personal users, isconsidered to be one of the applications thatwill make the most of the features of thestratospheric platform. It raises the possibilitythat high-quality digital services, such as ahigh-speed Internet and a high-quality digitalbroadcasting (whereby motion pictures and alarge volume of contents can be exchangedwithout taxing the user’s system) may be real-ized without relying on satellites or on the ter-restrial infrastructure, such as optical fibers.Further, by connecting the platforms withoptical wireless links, it will be possible toestablish links by wireless only over a widearea.

3 Comparison of platforms suit-able for telecommunicationsand broadcasting applications

The following can be listed as require-ments for a stratospheric platform with on-board radio equipment for telecommunica-tions and broadcasting applications.(1) Capable of reaching the stratosphere (analtitude of 20 km or so) and staying at a fixedpoint within a prescribed range.(2) Capable of round-the-clock continuousoperation for a long duration.(3) Capable of bearing large weights, so as toaccommodate numerous types of applicationapparatuses and at the same time supply suffi-cient power to these apparatuses.(4) Onboard radio equipment may be retrievedon the ground or on the sea, as necessary.(5) Is environmentally friendly in terms of thestratosphere.

As candidates that fulfill these require-ments, three categories of aircraft are consid-ered, classified broadly as: an unmanned air-ship with a propulsion system[1][6], a solarplane (solar cell-powered unmanned aircraft)


Example of an area deployment planin Japan

Fig.3

37

[7], and a manned aircraft (a jet)[8](Fig.4).Table 1 shows a rough comparison of theseaerial vehicles. Considering the missionequipment weight, influence on the environ-ment, and the like, the airship system is thecandidate with the most potential, as expected.However, the airship must be very large inorder to be capable of fulfilling these require-ments and at the same time reach the strato-sphere. Although the challenges are beingaddressed one by one, various technologicalproblems still remain. Therefore, although weare aiming at installing radio equipment on anairship as a final goal, we feel that it is notabsolutely necessary to insist upon a large-sized airship, nor to require that the airshipfulfill all of the above-mentioned require-ments, in the context of a partial preliminarydemonstration test representing the prior stage

to full development. There is also possibilitythat the solar plane, the manned aircraft, oreven a small-size airship may be suitablyapplied.

A conceptual drawing of the stratosphericairship that was created in the feasibility studycarried out by NAL in 1999 is shown in Fig.5[1][3]. The body of the airship is equippedwith a number of helium gas bags, a solar cellpanel on its top face, and three propulsionengines. A regenerative fuel cell is installedon the platform for operation at night. Analy-sis indicated that the length of the airshipwould be 245 m, the weight would be 32.4tons, and the power required to propel itwould be approximately 200 kW. At this time,it was calculated that the equipment loadcapacity (mission payload) was about 1 tonand the allowable power for maximum dissi-


Large-scale airship(unmanned)(*1)

Solar plane (unmanned)

Jet (manned)

Total length

All-in weight

Propulsion energysourceFlight duration time

Station keepingrange(radius)Mission equipmentweightMission allowableelectric powerSystem example

~200m

~30ton

Solar cell

About 3 years

~1km

~1000kg

~10kW

Japan, Korea, China,Sky Station and othercompanies in the U.S.

~70m

~1ton

Solar cell

Unknown

~1.5km

~100kg

~1kW

Helios, Pathfinder Plus(AeroVironment Inc., inthe U.S.)

~30m

~2.5ton

Fossil fuel

8 hours

~10km

~1000kg

~20kW

HALO (Angel Technolo-gy Corporation, in theU.S.)

(*1) from NAL feasibility study [1][3] (1999)

Candidates for the stratospheric platformFig.4

Comparison between main candidates for the stratospheric platformTable 1

38

pation was 10 kW. Note that these parametersare continually being revised, according to theactual state of technological development.

Initially as the energy source for such anairship, the feasibility of transmitting electricpower using a high- powered microwave out-put from the ground was investigated, anddevelopment and experimentation were con-ducted in the CRL[10], but the problem of dis-turbances in the electromagnetic environmentbecame a stumbling block, and consequentlythe strategy has been changed, focusing on acombination of the solar cell and the fuel cell.Table 2 shows the status of the present airshipdevelopment in Japan.

4 Research and development todate and preliminary demon-stration plan with CRL/TAO

The target of development of a govern-ment-run telecommunications and broadcast-ing mission is to demonstrate the conceptbehind it. Basic component technologies arein line with the technology developed forsatellite and terrestrial radio systems. Howev-er, especially in the high frequencies, there isno operational experience relating to environ-mental conditions and link requirements pecu-liar to the stratosphere, nor are there specificavailable technologies, and no system hasbeen implemented that could demonstrate thebenefits that may actually be provided totelecommunication users. Therefore, the mostimportant problem at present is to demonstratethese features as soon as possible, and toestablish the system’s future potential anddirectionality in terms of technology, business,and society. If these problems are achievedand a market appears in sight, the task willthen pass to those responsible for project exe-cution and manufacture.

The development of the telecommunica-tions and broadcasting project by CRL/TAObegan in 1998, and since then conceptualdesign, development of onboard equipment,and development of ground equipment havebeen carried out in the three fields of fixed


Developmentcompletion time Name

Year 2002-2003

2003-2004

In and after 2006

Stratosphere fighttesting aerial vehicle(presently underdevelopment)

Low-altitude stationkeeping flight

Technology demon-stration aerial vehi-cle

Target altitude15km

4km

20km

Feature

45-m class,non-powered,payload 40kg

65-m class,powered,payload250kg

150-m class,powered,payload 500-1,000kg?

Test item

Acquisition of basic data relat-ing to materials, heat control,and the like, as well as terrestri-al environment measurementsin the stratosphereAcquisition of basic data relat-ing to aerial vehicle control,and basic operations andtelecommunications experi-mentation at low altitudeAcquisition of general dataconcerning aerial vehicles inthe stratosphere and telecom-munication experimentation inthe stratosphere

Steps of present airship development and testing in Japan (Organization in charge: NAL;support: domestic heavy-industry manufacturers)

Table 2

Results of airship feasibility study (con-ducted by NAL in 1999)

Fig.5

39

communications, mobile communications, andbroadcasting[1][2][4][5]. To make the status ofthe project available to the public, we havepublished these achievements broadly indomestic and foreign academic conferences,newspapers, etc., and have applied for variousrelated patents, as part of our business strategyfor the future. In addition to these activities,execution of an evaluation test to check theperformance of the stratospheric platformwhen being used for radio monitoring of ille-gal radio stations on the ground is scheduledin fiscal year 2001. The outline of theonboard equipment for testing the applicationsis as follows.(1) Broadband fixed access: This is for provid-ing broadband access links of 2-150 Mbps orso to users at fixed locations by means ofsmall-sized antennas and radio equipment, toeffect the economical provision of movingpictures and a broadband Internet environ-ment. The use of a 48/47 GHz band and a31/28 GHz band has already been accepted bythe ITU (International TelecommunicationUnion) for fixed access[16][17]. As a proto-type antenna to be mounted for this purpose,two potential multibeam antennas have beendeveloped to date (as described below). Withthese antennas, the functions of space divisionmultiple access (SDMA) systems (where aplurality of users share the same frequencywithout interference), compensation for a plat-form’s oscillation, automatic acquisition ofground users, high-speed transmission ofmotion pictures, etc., may be evaluated.(2) Next-generation mobile access: This isintended to allow a small number of stratos-pheric platforms to take over the functions ofa number of base stations on the ground forW-CDMA, one of the third-generationportable telephone standards (IMT-2000). Incases where infrastructure must be built upfrom a nearly-zero state, this will be effectivein curtailing the cost to a great extent and indecreasing “dead spots,” where the strength ofelectromagnetic wave is weak. ITU hasapproved that part of the band for the terrestri-al IMT-2000 service may be used for the IMT-

2000 service using the stratospheric platform[16]. Two kinds of antennas have been devel-oped: a switching planar antenna and a multi-helical antenna, together with a test onboardrepeater of a through-repeater system. Theground user side will be able to access thestratospheric platform at 384 kbps using acommercially available third-generationportable phone terminal.(3) Digital broadcasting: With conventionalterrestrial broadcasting, a transmitting stationsends a broadcast wave of large electricpower, and a number of relay stations areinstalled in various places to compensate forattenuation of the electromagnetic wavecaused by buildings and terrain features, inorder to decrease the area where reception ispoor. With the stratospheric platform, oneplatform alone can provide a broad area ofcoverage, with a significantly lower transmis-sion power. Naturally the area of platformcoverage is smaller than that of the satellitebroadcasting, but precisely for this reason, theplatform is well-suited to broadcasting to acertain focused area. Although the frequen-cies of a broadcasting service using thestratospheric platform has not yet been estab-lished by the ITU, we have developed anonboard repeater for testing through-repeatersystem digital broadcasting that uses UHF in a400-500 MHz band. For the antenna, thedrooping dipole antenna is used. Through thisarrangement, performance of high-definitionTV broadcasting by an orthogonal frequencydivision multiplex (OFDM) scheme may beevaluated.(4) Radio monitoring: The observation andradio monitoring of illegal electromagneticwaves represent applications that could benefitfrom the strong line-of-sight capability of thestratospheric platform. Conventionally, radiomonitoring is conducted using monitoringantennas on the ground. When the antenna iselevated to the altitude of the stratosphere, theinfluence of multipath and shielding can bereduced; therefore, it is expected that high-accuracy and wide-area radio monitoring ofelectromagnetic wave sources may be real-


40

ized. To demonstrate this fundamental con-cept, we are developing a data-acquisitionapparatus with the use of mutually-orthogo-nalized two eight-element linear array anten-nas (2 GHz) to permit radio positioning of anelectromagnetic wave on the ground by usinga super-resolution DOA (direction-of-arrival)estimating algorithm and the ray-tracingmethod.

The components of the radio equipmentfor relay testing in the stratosphere describedabove need to be tested in the context of anenvironment as similar to that of the strato-sphere as possible, so that the results may beused to pave the way for conceptual demon-stration of a stratospheric platform-basedtelecommunications and broadcasting system.Therefore, CRL/TAO has devised a plan for apreliminary demonstration test, prior to instal-lation on board the airship in fiscal year 2002[2]. The aerial vehicles to be used in the pre-liminary demonstration test are expected to beof the following three types.(1) Helicopter (Kawasaki Vertol): This has anexcellent hovering ability and surpasses otherflying vehicles in the station keeping perform-ance (within a radius of 150 m). On the otherhand, it cannot reach the altitude of the strato-sphere; the highest attainable altitude is 3-4km or so. Therefore, this will carry multi-beam antennas developed for fixed access, anapplication requiring relatively high stationkeeping performance, and an access test is tobe conducted using a millimeter-wave band of30 GHz or more, with a high elevation angle.To confirm whether or not this vehicle canactually obtain flight permission over theYokosuka YRP, where the experiment isscheduled, the vehicle flew to the YRP andexecuted a hovering test at a height of 3,000m, and it was ascertained that there was noproblem (Fig.6). Note that for the basic radio-monitoring experiment, a smaller-sized heli-copter will be used to conduct the actual testof the radio positioning of electromagneticwave sources.(2) Small-sized jet (Gulfstream II): This is inferi-

or to the helicopter in the station keeping per-formance (turning radius of 10 km or more)and a bank angle of nearly 30 degrees is gen-erated when turning, but the jet can fly at analtitude of approximately 14 km, whichenables tests to be conducted in an environ-ment close to that of the stratosphere. There-fore, this machine is used to conduct a digitalbroadcasting test using low frequencies (suchas UHF frequencies) that do not require adirectional antenna for the onboard equip-ment.(3) Solar plane (Pathfinder Plus): This wasdeveloped by NASA, in the United States.Present models are the Helios (wingspan of 75m) and the Pathfinder Plus (wingspan of 37m), owned by AeroVironment Inc. in the U.S.These have the ability of soaring up to thestratosphere with electricity supplied only bythe solar cell. The Helios has successfullyreached an altitude of 30 km (August 2001)and the Pathfinder Plus has reached an altitudeof 24 km (August 1998) in Hawaii[7]. The sta-tion keeping performance can be attainedwithin a few kilometers, although it dependsupon the air stream. However, the missionpayload is limited, at 50-100 kg or so. Thesolar plane itself is assumed to be among thecandidates for the stratospheric platform, andit is said that NASA supports the use of thesolar plane for the platform. As the Helioscannot be used because of modifications andthe like scheduled in fiscal year 2002, we willuse the Pathfinder Plus to conduct tests of theW-CDMA signal relay (using a lightweight


Kawasaki Vertol flown to YokosukaYRP (March 2001)

Fig.6

repeater), and of UHF digital broadcasting.Regarding the roles of CRL and TAO,

TAO executes the conceptual design of anentire system and develops the onboardrepeater and the ground equipment, and CRLtakes charge of the development of the com-ponent technologies (centering on the antenna to be mounted) in respect of which researchremains to be conducted. Table 3 shows thepresent research and development schedule.After the preliminary demonstration test, weare going to install the telecommunicationequipment on the test airship currently beingbuilt by NAL, followed by an actual demon-stration test of the technology using the air-ship, as we push ahead with our plan todemonstrate the feasibility of the stratosphericplatform.

5 Overseas trends

Outside Japan, the movement toward com-mercialization of the stratospheric platformhas been active in recent years as well.

This movement has taken place against thebackdrop of the expansion of available fre-quencies at the World RadiocommunicationConference (WRC-2000), as discussed below,which expansion was expected to trigger sud-den business development of the platform.

The main developments are as follows.(1) Sky Station International Inc. (United States)

The first of its kind worldwide, this is aventure company that aims at developingcommunication business using a stratospheric

airship. The company successfully lobbiedthe ITU, for ITU permission to use frequen-cies in the 47/48 GHz band and in the 2 GHzband. The company is preparing to offer abroadband fixed access link (2-10 Mbps) inthe 47/48 GHz band and an IMT-2000 mobilecommunication link in the 2 GHz band,through the use of a 150-m class airship[6]. Itwas announced that a first test flight was to beexecuted at the end of 2000, but no publica-tion has yet been made. However, the compa-ny designated United States Lockheed MartinGlobal Telecommunications as a system inte-grator (according to a latest news, this contracthas been cancelled), and the company hasreportedly been recently working on thedesign of a new airship. Thus, it will be nec-essary to keep an eye on developments in thisarea in the meantime.(2) NASA/AeroVironment Inc. (United States)

NASA has proposed that a lightweightunmanned solar plane (with the above-men-tioned solar cell fixed on the upper-front por-tion of its main wing) be operated at an alti-tude of 17-30 km[7], and has invested over 80million dollars to date to develop the body ofthe airplane (Helios, and others) with a viewto use these mainly for earth observations.The AeroVironment Inc., a venture companythat deals in solar cars and the like, has sug-gested that this solar plane be used as atelecommunication base. This aerial vehicle isenvironmentally friendly and is nearing practi-cal use, but has problems, such as limitation ofthe mission weight at 100 kg and a limitedarea of flight at present (due to susceptibilityto wind damage). Note that SkyTower Inc., acommunication business proposing to use thisaerial vehicle, has already been established,with an agency in Japan (Japan StratosphereCommunications Inc.).(3) Angel Technologies Corporation/RaytheonCompany (United States)

The company has planned for a small-sizemanned jet to be operated at an altitude of 17-20 km[8]. The pilots will be on duty, forexample, over three shifts of eight hours each.This system offers a means for realizing the

41Ryu MIURA and Masayuki OODO

▲

Feasibility study Optimization of system design

Component technologies R&D

Stratosphere flight test

Low-altitude station keeping flight test

Stratosphere technology demonstration test

Conceptual desigh Optimization of system desigh

Onboard equipment and component technologies R&D

Ground equipment

Prelminary demonstration test

Test on board station keeping flight aerial vehicle demonstration test

▲　　　　　　　　　　　 ▲　　　　　　　　　　　 ▲WRC-2000 WRC-2003 WRC-2006

Equipmentimprovement

Technology

Install

▲▲ ▲

▲ ▲

▲▲

▲

▲

▲

▲

▲

▲

▲

▲ ▲

▲ ▲

▲

▲

Airship

Projectevaluation

Telecommuni-cations and broadcastingmission

1998 1999 2000 2001 2002 2003 2004 2005 2006~

Research and development sched-ule

Table 3

Install

stratospheric platform almost solely based onexisting technologies, and at the lowest cost.A special aircraft called the HALO-Proteushas already been developed, and succeeded inconducting an initial demonstration flight atan altitude of 15 km, in 1998. Angel Tech-nologies is in charge of the aerial vehicle andRaytheon Co. is in charge of the telecommuni-cations equipment.(4) European Space Agency (ESA)/LindstrandBalloons Ltd. (U.K.)

By contract with ESA, the balloon makerLindstrand carried out a feasibility study on astratospheric platform in the form of an airshipin 1998, but subsequently no contract wasconcluded.(5) Advanced Technologies Groups Inc. (U.K.)

This company has demonstrated actualresults in developing a variety of low-altitudeairships, such as the SkyCat series for trans-portation, sightseeing, monitoring, and AT-10for pilot training[1][9]. For the stratosphere(altitude of 20 km), the company has starteddevelopment of an airship called the StratSat(Fig.7). They had a contract with SkyStationtemporarily but there is no contract at present.The company aims at creating an airship of alength of 200 m, with resistance to wind dam-age of 35 m/s, and a payload of 2 tons. For apower source, a combination of the solar celland an additional diesel engine is proposed,where the latter is in preparation for the strongwind environments that may occur. The com-pany seems to manufacture a scale modelapproximately 120 m long and to conduct atest flight at an altitude of 10 km, with thecombined use of the solar cell and the dieselengine or the diesel engine’s alternative,microwave transmission of electrical energy in2003.

In addition, research activities have begunin Korea (Korea Aerospace Research Instituteand Electronics and TelecommunicationsResearch Institute) and in China (ShanghaiJiao Tong University and Beijing TsinghuaUniversity)and elsewhere; countries in South-east Asia, South America, Africa, and else-where are carefully following such research.

Further, interest is growing from all sides withrespect to individual research plans for thestratospheric platform: there is the “Space-bird” plan at Cranfield University, the“Helinet” plan at the University of York, andthe “Sirius” plan at the University of Surrey,all in the UK.; the U.S. Army has begun ahigh-altitude airship program, which Lock-heed Martin Naval Electronics & SurveillanceSystems was quick to apply for participationwith; etc. However, it is thought that Japancontinues to lead the world at present in thedevelopment of the applicable telecommuni-cations equipment.

6 Prototype of multibeam anten-na to be mounted

In fiscal year 1998, CRL began develop-ment of an antenna for broadband access thatinvolves the development of many difficulttechnological elements, necessitating the allo-cation of development time for the variouscomponents. For the broadband access anten-na to be mounted on the stratospheric plat-form, the following functions are required.

(i) Use of a high radio frequency (e.g., 10GHz or more) to secure a sufficient band-width for a broadband link.(ii) When using high frequencies, commu-nication quality is governed by thermalnoise, and hence a directional antenna witha high gain is necessary.

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

Conceptual drawing of StratSat TM,made by Advanced TechnologiesGroups Inc

Fig.7

43

(iii) Since the angle of view over the serv-ice area on the ground when viewed fromthe stratosphere may be as wide as 120degrees or more, a multibeam antenna thataccommodates 100 beams or more is nec-essary, both for transmission and forreception, in order to reconcile the wideangle of view and the high gain required,as well as to achieve effective use of thefrequencies involved.(iv) Since the position and attitude of theplatform vary due to variations in the airstream and in atmospheric pressure, theplatform must cancel out the influences ofattitude variation on the footprint on theground by means of beam control.Especially regarding (iii), note that at ITU-

R, where frequency allocation to the stratos-pheric platform is being discussed, multibeamfootprint patterns for broadband access werepresented, each of which would generate 300-400 cells in the Ka band, as shown in Fig.8.These patterns are required for the purpose ofmitigating the effect of the above-mentionedfrequency allocation on services already usingthe same frequency band (e.g., terrestrialfixed-communication service, satellite com-munication service, and the like)[17][18]. Fig.8(a) represents a model where footprints of thebeams on the ground form a circular pattern ofthe same size, regardless of the direction andthe elevation angle relative to the ground,while the cross-sections of the beams generat-

ed by the antenna are made to be elliptical.Further, Fig. 8 (b) represents a model where aservice area is divided into several zones,according to the elevation angle from theground, so that beam gain in each zone is thesame among the footprints and the footprintson the ground necessarily become elliptical.

For a multibeam antenna that can respondto these requirements, two types have beendeveloped to date: a multibeam horn (MBH)antenna of the mechanical-drive type, and adigital beamforming (DBF) antenna of theelectronic-scanning type[11]. Basic configura-tions of the antennas (in reception) are shownin Fig.9, basic specifications of the respectiveantennas are shown in Table 4, and photo-graphs of the antennas may be seen in Fig.10.We have developed a prototype antenna as alimited-function model, equipped with mini-mum functions, which permitted demonstra-tion of the concept. This prototype featured areduced number of antenna elements, due torestrictions of development time and budget,and was developed assuming it would be usedfor an onboard test on a 150-200 m classdemonstrator model. Although the strato-sphere presents a harsh environment (theatmospheric pressure is 1/20th that present onthe ground, and the temperature drops to -50degrees centigrade or lower), the temperatureand pressure inside the enclosure in which theequipment is installed are controlled, so thatas many pieces of ground equipment as possi-


Typical examples of multibeam footprints under consideration for formulation into an ITU-Rrecommendation

Fig.8

44

ble may be used, thus reducing costs.Features of each antenna are described as

follows.

(1) Mechanical drive multibeam horn (MBH)antenna

This is part of a multibeam system where a


Basic configuration of multibeam antenna to be mounted on demonstrator model underdevelopment

Fig.8

Prototype of multibeam antenna to be mountedFig.10

Item

Frequency band

Antenna typeSpot beam width

Number of multibeamsBandwidth

Ttansmission EIRPRecepion G/T

Compensation forplatform oscillation

Maximum transmissionrate

Power consumptionWeightOther

Multibeam horn antenna

Transmission: 47.2-47.5 GHzReception: 47.9-49.2 GHz

7-element corrugated horn12 degrees

Fixed beams:7300 MHz or more6.3 dBW or more

-15.4 dB/K or moreAttitude sensor and three-axis gimbal

mechanical drive control25Mbps or more

1.0 kW or less150 kg or less

Number of frequencies reused:7 or lessIsolation between the same frequency

beams: 30 dB or more

Digital beamforming antenna

Transmission: 28.5-27.35 GHzReception: 31.0-31.3 GHz

16-element (4 x 4) patch array10-13 degrees

Fixed beams: 9, adaptive beams: 33MHz or more

11-15 dBW-13 to -17 dB/K

Adaptive beamforming via digital signalprocessing of received signal

2-4 Mbps

1.6 kW or less74.2 kg

Sampling rete: 32 MHzResolution: 12 bits

Signal-processing device: FPGA (RX:100k gates x 61, TX: 100k gates x 31)

Main specifications of the multibeam antenna prototypeTable 4

45

number of horn antennas are fixed to a basethat is gimbal-controlled by a three-axismechanical drive, and each of the horn anten-nas forms a relatively fixed footprint on theground. The MBH antenna is specified to usea frequency in the 48/47 GHz band and fea-tures seven antenna elements. It is intended tobe used mainly for broadband feeder links,high-quality business links, and the like. Themain features of this antenna are as follows.

(a) A broadband antenna can be producedalmost exclusively relying on existingtechnologies.(b) The antenna has excellent wide-anglecharacteristics of gain, axial ratio, and thelike.(c) Development expense is comparativelylow.Since the footprints created by this anten-

na become fixed cells, it is mandatory to use adifferent frequency for each beam in such amanner that, for example, four or seven fre-quencies, are reused in the spatial domain;therefore this antenna lacks flexibility, relativeto the DBF antenna discussed below. Further,the antenna must feature a sufficiently lowside-lobe level. Moreover, when a grounduser stations moved or when the platformitself drifts in a vertical direction due to varia-tion in the atmospheric pressure or the like, ahandover action becomes necessary betweenthe beams. With this antenna, each elementrequires an aperture, as each element acts as adirectional antenna; when the antenna is com-posed of a few dozen or hundreds of elements,the antenna becomes oversized, and the num-ber of cables connecting the repeater and therespective antenna must also increase, makingthe gimbal difficult to rotate. Therefore, thisantenna is considered not to be particularlysuitable to a large-scale multibeam antennaapplication.(2) Digital beamforming (DBF) antenna

This represents a system where a beam isformed by a combination of the active arrayantenna and spatial digital signal processing; itis an “intelligent” next-generation antenna,referred to as a “smart antenna” or a “software

antenna.” The antenna effects automaticacquisition, tracking, interference separation,and the like by performing spatial parallel pro-cessing of signals received from the ground-user stations, thereby forming an antenna pat-tern. Further, the transmitting side forms abeam based on information on the direction ofarrival of the signal obtained by the receptionside, to effect spatial power combining. Sincethere is no mechanical drive component, thisantenna is considered to be suitable for multi-element, large-scale, multibeam formation.Recently, the DBF antenna has been adoptedfor cellular base station antennas because itsignificantly improves the capacity and qualityof mobile-communication systems, althoughthere are few examples of its use in a multi-element array antenna at high frequencies (10GHz or more). However, since the DBFantenna involves a number of parts untested inthe field and features a large number of com-ponents relative to the MBH system, thedevelopment cost is large, and it also con-sumes a great deal of electric power, in thecurrent configuration. Given this situation, forthe demonstrator model antenna, we focusedon improved beamforming functions, ratherthan on broadband characteristics, and speci-fied the antenna as an array antenna with alimited number of elements (16, in a 4-by-4square arrangement), in light of our desire toacquire basic data for the realization of amulti-element DBF antenna in the future. Asfor frequency, the newly allocated 31/28 GHzband was specified. It was assumed that agreat number of individual users will bedensely distributed on the ground and that thisantenna will be used in the access links bythose users. The main features of this antennaare as follows.

(a) Flexible, accurate transmit/receivebeam steering (or beam pointing), accom-modating a number of ground-user stationcommunication requests, which can beeffected individually (i.e., “beam on-demand,” with an adaptive multibeam).No fixed cell exists in the footprints, andthe handover frequency is less than that of


46

the MBH antenna.(b) To a specific ground user, the onboardantenna can always provide its maximumgain, and since the side lobe level is con-trollable, the link quality is stabilized;therefore it is possible to utilize the fre-quency effectively by space-division mul-tiple access (SDMA), thus increasing thecommunication capacity.(c) The antenna has the capability ofremoving undesired in-coming interferingwaves, reducing the interference on othersystems (such as satellite systems) thatshare the same band, and also can esti-mate, based on direction of arrival, thelocation of sources of communication sig-nals and unauthorized electromagneticwaves.(d) The antenna is robust against failure ofa number of components.(e) High-speed calibration of the transmit-receive array antenna is possible via signalprocessing.For a digital signal-processing device for

the DBF antenna, a FPGA (Field Programma-ble Gate Array, suited for high-speed parallelprocessing and featuring a reconfigurablealgorithm), and a general-purpose high-speedMPU (Micro Processing Unit, allowing foreasy program development) were adopted,with the aim of combining high-speed parallelprocessing performance with ease of programdevelopment. As remaining challenges,enhanced broadband characteristics are soughtthrough improvements in the processing speedof the digital devices, the problem of heat-dis-sipation must be overcome when active ele-ments are integrated in a large-scale array, andgain and axial ratio must be improved forcases of large scan angles, among others.

In the development of the beamformingalgorithm for the DBF antenna, it is necessaryto consider a multi-element array, power-limit-ed types of links, burst signals, SDMA, and soon, in the context of a practical future aerialvehicle. Therefore, the algorithm needs to bedesigned so as to allow a reduced number ofoperations, parallel-processing adaptability,

stable operations in a low-CNR environment,high-speed acquisition of the desired wave,sufficient suppression of interference, and soon.

A maximal ratio combining system[12][13], assisted by known referencesequences, was examined and implemented inthe prototype antenna. With this system, themaximum ratio combining previously used(within one of the existing diversity systems)was developed into a multi-user-compliantalgorithm, based on non-blind processing, andwas implemented within a simple arithmeticalsystem, so as to be applicable to the multi-ele-ment DBF antenna. Further, to accommodatethe low CNR signal, a beam-space processingmethod was adopted, whereby a spatial DFTforming a fixed multibeam in advance isassigned as a preprocessor. According to asimulation study, this method makes it possi-ble to conduct automatic acquisition of adesired wave with 10 symbols or so, evenwithin a low-CNR environment (not morethan 0 dB per antenna element), and also toperform separate reception for different userstations with a given degree of isolationbetween them, by instructing them to use dif-ferent reference sequences when their direc-tions of arrival are different (Fig.11). Further,it is also easy to convert weight coefficients ofa reception array into weight coefficients of atransmitting array on a different frequency.Although this method does not obtain the opti-mum solution in an environment where aninterference wave exists (in which case theWiener solution is preferable), this methodcan decrease the quantity of operations by oneor two orders of magnitude, relative to meth-ods that use complicated matrix operations orthe like (i.e., optimization methods such as therecursive least square method, or “RLS,” andthe direct matrix inversion, or “DMI”), pre-cisely because it does not employ such opera-tions. Further, since this method enables par-allel processing, it can be easily and effective-ly implemented in devices that utilize spatiallydistributed circuit resources, such as theFPGA and the ASIC. We are also developing


47

a high-speed calibration technique for thetransmit-receive array antenna, in which thisprinciple of beamforming is applied[14][15].

7 Summary

Here we have presented an outline ofresearch and development status of a stratos-pheric platform, with the Ministry of PublicManagement, Home Affairs, Posts and theTelecommunications and the Ministry of Edu-cation, Culture, Sports, Science and Technolo-gy both playing central roles. In CRL, theWireless Innovation Systems Group is produc-ing an analysis in conjunction with othergroups, such as the Broadcasting SystemsGroup and the Optical Space CommunicationsGroup, with a view to making use of theresearch accumulated thus far. Further, thisgroup is actively contributing technical sup-port to the Ministry of Public Management,Home Affairs, Posts and Telecommunications

in its attempt to obtain frequency allocation atthe ITU, in cooperation with the YokosukaResearch Center of TAO. To date, we havesubmitted a number of technical contributionsand obtained conditional approval for use ofthe 31/28 GHz band at the WRC (WorldRadiocommunication Conference) 2000 (inIstanbul), and in addition have formulatedseven preliminary draft new Recommenda-tions, continuing to refine these into actualRecommendations[19]. We can say that thetelecommunication technology based on thestratospheric platform involves a number ofproblems that have not been addressed world-wide in the fields of hardware, software, net-working, and protocols, and is one of very fewtechnologies in which Japan can play a lead-ing contributing role worldwide. For that pur-pose and others, we are making efforts toeffect a demonstration test in the stratosphereas soon as possible; this will represent a keyevent that will serve as the impetus to shift thenational initiative into the private sector.

Acknowledgments

We express our heartfelt gratitude to theTelecommunications Advancement Organiza-tion of Japan and the National Aerospace Lab-oratory, joint promoters of this project, fortheir kind guidance, as well as the broadcast-ing system group and the optical space com-munication group of the CRL, and also allcontributing personnel. In addition, we arethankful to Prof. Ronghong Jin (Professor atthe Shanghai Jiao Tong University) for sup-port in the evaluation of the algorithm for theDBF antenna, and Dr. Hiroyuki Tsuji of theWireless Access Group for cooperation in thebasic radio monitoring experimentation.


References1 Proc. The 1st ~ 3rd Stratospheric Platform Workshop (SPSW’99~01), 1999~2001.

2 R&D Report on the Wireless System Using Stratospheric Platform in 2000, TAO, May 2001.

3 Web Site of NAL, http://www.nal.go.jp/Welcome-e.html/

4 Web Site of CRL, http://www2.crl.go.jp/mt/b181/research/sbf/index-e.html/

Example of a simulation in which theDBF antenna pattern is formed bymaximal ratio combining, with the aidof already-known referencesequences (separate reception ofthree waves with a 16-element lineararray)

Fig.11

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

5 Web Site of TAO, http://www.yrp.tao.go.jp/

6 Web Site of Sky Station, http://www.skystation.com/

7 Web Site of AeroVironment, http://www.aerovironment.com/

8 Web Site of Angel Technologies, http://www.angelhalo.com/

9 Web Site of Advanced Technologies Group, http://www.airship.com/

10 Y. Fujino, M. Fujita, N. Gaya, M. Onda, S. Kunimi, and M. Ishii, "Wireless Power Receiving System for

Microwave Propelled Airship Experiment," Space Technology, Vol. 17, No. 2, pp. 89-93, Dec. 1997.

11 R. Miura, M. Oodo, and Y. Hase, "Development of Multibeam Antennas in the Millimeter-Wave Band On

Board Stratospheric Platform," Proc. AP-2000, Davos, Switzerland, Apr. 2000.

12 R. Miura, M. Oodo, A. Kanazawa, and Y. Koyama, "Maximal-Ratio-Combining Array Beamformer Assist-

ed by a Training Sequence for Space Division Multiple Access In Power-Limited Channels, " IEICE Trans.

Commun., Vol. E83-B, No. 2, pp. 394-405, Feb. 2000.

13 R. Miura, M. Oodo, Y. Hase, T. Inaba, T. Sakamoto, and M. Suzuki, "Digital Beamforming Array Antenna

On-Board Stratospheric Platform for Quick-Response SDMA in the Band 31/28 GHz," Proc. WPMC’01, Aal-

borg, Denmark, Sep. 2001.

14 M. Oodo and R. Miura, "A Novel Calibration Method for DBF Transmitting Array Antennas, " Proc. AP-

2000, Davos, Apr. 2000.

15 T. Nakamura, R. Miura, M. Oodo, and T. Ikegami, "Calibration of a DBF Receiving Array Antenna by Using

a Reference Sequence, " Proc. WPMC’00, Bangkok, Nov. 2000.

16 WRC-2000 Final Acts, ITU, Istanbul, Jun. 2000.

17 ITU-R Doc. 9B/44, Nov. 2000.

18 ITU-R Doc. 9B/114, Oct. 2001.

19 M. Oodo and R. Miura, "A Study of Frequency Sharing and Contribution to ITU for Wireless Communication

Systems Using Stratospheric Platforms," This Special Issue of CRL Journal.

Ryu MIURA, Ph. D.

Leader, Wireless Innovation SystemGroup, Yokosuka Radio Communica-tions Research Center

R&D Program on Telecom and Broad-casting System Using High AltitudePlatform Stations

Masayuki OODO, Ph. D.

Researcher, Wireless Innovation Sys-tem Group, Yokosuka Radio Communi-cations Research Center

Antennas Onboard Stratospheric Plat-form, Frequency Sharing

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