High-altitude platforms for wireless communications - CiteSeerX

High-altitude platformsfor wireless

communicationsby T. C. Tozer and D. Grace

The demand for high-capacity wireless services is bringing increasing challenges,especially for delivery of the ‘last mile’. Terrestrially, the need for line-of-sight

propagation paths represents a constraint unless very large numbers of base-stationmasts are deployed, while satellite systems have capacity limitations. An emerging

solution is offered by high-altitude platforms (HAPs) operating in the stratosphere ataltitudes of up to 22 km to provide communication facilities that can exploit the bestfeatures of both terrestrial and satellite schemes. This paper outlines the application

and features of HAPs, and some specific development programmes. Particularconsideration is given to the use of HAPs for delivery of future broadband wireless

communications.

1 The challenge for wireless communications

As the demand grows for communication services,wireless solutions are becoming increasingly important.Wireless can offer high-bandwidth service provisionwithout reliance on fixed infrastructure and represents asolution to the ‘last mile’ problem, i.e. delivery directly toa customer’s premises, while in many scenarios wirelessmay represent the only viable delivery mechanism.Wireless is also essential for mobile services, and cellularnetworks (e.g. 2nd generation mobile) are nowoperational worldwide. Fixed wireless access (FWA)schemes are also becoming established to providetelephony and data services to both business and homeusers.

The emerging market is for broadband data provisionfor multimedia, which represents a convergence of high-speed Internet (and e-mail), telephony, TV, video-on-demand, sound broadcasting etc. Broadband fixedwireless access (B-FWA) schemes aim to deliver a rangeof multimedia services to the customer at data rates oftypically at least 2 Mbit/s. B-FWA should offer greatercapacity to the user than services based on existingwirelines, such as ISDN or xDSL, which are in any eventunlikely to be available to all customers. The alternativewould be cable or fibre delivery, but such installation maybe prohibitively expensive in many scenarios, and thismay represent a barrier to new service providers. B-FWAis likely to be targeted initially at business, including SME(small–medium enterprise) and SOHO (small office/home office) users, although the market is anticipated toextend rapidly to domestic customers.

However delivering high-capacity services by wireless

ELECTRONICS & COMMUNICATION ENGINEERING JOU

also presents a challenge, especially as the radiospectrum is a limited resource subject to increasingpressure as demand grows. To provide bandwidth to alarge number of users, some form of frequency reusestrategy must be adopted, usually based around a fixedcellular structure. Fig. 1a illustrates the cellular concept,where each hexagon represents a cell having a base-station near its centre and employing a differentfrequency or group of frequencies represented by thecolour. These frequencies are reused only at a distance,the reuse distance being a function of many factors,including the local propagation environment and theacceptable signal-to-interference-plus-noise ratio.

To provide increased capacity, the cell sizes may bereduced, thus allowing the spectrum to be reused moreoften within a given geographical area, as illustrated inFig. 1b. This philosophy leads to the concept of microcellsfor areas of high user density, with a base-station onperhaps every street corner. Indeed, taking the concept toits extreme limit, one might envisage one cell per user, theevident price in either case being the cost andenvironmental impact of a plethora of base-stationantennas, together with the task of providing the backhaullinks to serve them, by fibre or other wireless means, andthe cost of installation.

Pressure on the radio spectrum also leads to a movetowards higher frequency bands, which are less heavilycongested and can provide significant bandwidth. Themain allocations for broadband are in the 28GHz band(26GHz in some regions), as well as at 38GHz1. Existingbroadband schemes may be variously described as LMDS(Local Multipoint Distribution Services) or MVDS(Multipoint Video Distribution Services)2,3; these are

RNAL JUNE 2001 127

a b

Fig. 1 Cellular frequency reuse concept. In b the smaller cells provide greater overall capacity as frequencies are reuseda greater number of times within a given geographical area.

flexible concepts for delivery of broadband services,although they may be encompassed in the generic termB-FWA, or simply BWA.

The use of these millimetre wavelengths implies line-of-sight propagation, which represents a challengecompared with lower frequencies. Thus local obstruc-tions will cause problems and each customer terminalneeds to ‘see’ a base-station; i.e. a microcellular structureis essential for propagation reasons. This again implies aneed for a very large number of base-stations.

A solution to these problems might be very tall base-station masts with line-of-sight to users. However, thesewould be not only costly but also environmentallyunacceptable. An alternative delivery mechanism is viasatellite, which can provide line-of-sight communicationto many users. Indeed, broadband services fromgeostationary (GEO) satellites are projected to representa significant market over the next few years4. However,there are limitations on performance due partly to therange of c. 40000 km, which yields a free-space path loss(FSPL) of the order of 200dB, as well as to physicalconstraints of on-board antenna dimensions. The latterleads to a lower limit for the spot-beam (i.e. cell) diameteron the ground, and these minimum dimensions constrainthe frequency reuse density and hence the overallcapacity. Additionally, the high FSPL requires sizeableantennas at ground terminals to achieve broadband datarates. A further downside is the lengthy propagation delayover a geostationary satellite link of 0·25s, which not onlyis troublesome for speech but also may cause difficultieswith some data protocols.

Low earth orbit (LEO) satellites may circumvent someof these limitations in principle, but suffer fromcomplexities of rapid handover, not only between cells butalso between platforms. The need for large numbers of

128 ELECTRONICS & C

LEO satellites to provide continuous coverage is also asignificant economic burden, and such schemes have yetto prove commercially successful.

2 Aerial platforms: a solution?

A potential solution to the wireless delivery problem liesin aerial platforms, carrying communications relaypayloads and operating in a quasi-stationary position ataltitudes up to some 22km. A payload can be a completebase-station, or simply a transparent transponder, akin tothe majority of satellites. Line-of-sight propagation pathscan be provided to most users, with a modest FSPL, thusenabling services that take advantage of the best featuresof both terrestrial and satellite communications.

A single aerial platform can replace a large number ofterrestrial masts, along with their associated costs,environmental impact and backhaul constraints. Siteacquisition problems are also eliminated, together withinstallation maintenance costs, which can represent amajor overhead in many regions of the world.

The platforms may be aeroplanes or airships(essentially balloons, termed ‘aerostats’) and may bemanned or unmanned with autonomous operationcoupled with remote control from the ground. Of mostinterest are craft designed to operate in the stratosphereat an altitude of typically between 17 and 22km, which arereferred to as high-altitude platforms (HAPs)2,3,5 –7. Whilethe term HAP may not have a rigid definition, we take it tomean a solar-powered and unmanned aeroplane orairship, capable of long endurance on-station—e.g.several months or more. Another term in use is ‘HALE’—High Altitude Long Endurance—platform, which impliescrafts capable of lengthy on-station deployment ofperhaps up to a few years.

OMMUNICATION ENGINEERING JOURNAL JUNE 2001

HAPs are now being activelydeveloped in a number of programmesworld-wide, and the surge of recentactivity reflects both the lucrativedemand for wireless services andadvances in platform technology, suchas in materials, solar cells and energystorage.

BalloonsThe earliest aerial platforms were

balloons, whose history goes back tothe time of the ancient Chinese. In theWest, the first major use of lighter-than-air craft was the Montgolfierbrothers’ manned hot air balloon inFrance in 1783. Subsequentdevelopment of balloons remainedlargely for military application,although their application forcommunication purposes seems tohave been very limited. In the early 20th century, rigid,powered airships were developed by the Zeppelincompany in Germany for passenger transport, and theseproved a remarkable engineering feat, enablingpassengers to travel from Europe to South America.Unfortunately, this era came to an abrupt end with theHindenburg disaster at Lakehurst, USA, in 1937.Whatever the precise cause of that disaster, use of thehighly flammable hydrogen to fill the airship was nolonger considered acceptable, and with the developmentof commercial air travel after the Second World War largeairships seemed to be consigned to history.

Subsequent activity was mainly confined to hot-airballoons for recreational purposes, small balloons formeteorological use, and tethered aerostats (‘balloons onstrings’). The latter may operate up to at least 5000maltitude8, although there are evident implications for air-traffic safety and their use is highly restricted, the majorityoperating at much lower altitudes. (One major applicationof tethered aerostats is surveillance.Large numbers are deployed alongthe US–Mexican border to detectunauthorised crossings9.)

The past few years have seen aresurgence of interest in balloons andairships, with technology develop-ments such as new plastic envelopematerials that are strong, UVresistant and leak-proof to helium,which is now almost universally usedinstead of the much cheaperhydrogen. Such hi-tech airships havefeatured in high-profile attempts tocircumnavigate the globe (e.g theBreitling Orbiter10). And a newairship, the Zeppelin NT, a low-altitude craft, was launched by theZeppelin company in 2000, exactly100 years after its first airship, with a

Fig. 2 The ZeppeliLuftschifftechnik G

Fig. 3 Artist’s impr


view to the scenic tourism market (Fig. 2)11. Anothermajor project, albeit low altitude, is Cargolifter12 (Fig. 3);this large airship is proposed to be some 250m long andaims to provide a ‘flying crane’ service for transportationof heavy goods over difficult terrain.

But a major business goal remains that of developing astratospheric HAP capable of serving communicationapplications economically and with a high degree ofreliability. Whether an airship or an aeroplane, a majorchallenge is the ability of the HAP to maintain station-keeping in the face of winds. An operating altitude ofbetween 17 and 22 km is chosen because in most regionsof the world this represents a layer of relatively mild windand turbulence. Fig. 4 illustrates a typical jet-stream windprofile vs. height; although the wind profile may varyconsiderably with latitude and with season, a form similarto that shown will usually obtain. This altitude (>55000ft)is also above commercial air-traffic heights, which wouldotherwise prove a potentially prohibitive constraint.

n NT airship, launched in June 2000 (Courtesy of ZeppelinmbH)10

ession of Cargolifter (Courtesy of Cargolifter GmbH)11

RNAL JUNE 2001 129

Airship HAPsProposed implementations of airships for high-altitude

deployment use very large semi-rigid or non-rigid helium-filled containers, of the order of 100 m or more in length.Fig. 5 shows an artist’s impression of a HAP fromLindstrand Balloons. Electric motors and propellers areused for station-keeping, and the airship flies against theprevailing wind. Prime power is required for propulsionand station-keeping as well as for the payload andapplications; it is provided from lightweight solar cells inthe form of large flexible sheets, which may weightypically well under 400g/m2 and cover the uppersurfaces of the airship. Additionally, during the day, poweris stored in regenerative fuel cells, which then provide allthe power requirements at night.

The overall long-term power balance of a HAP is likely tobe a critical factor, and it will be the performance andageing of the fuel cells that are likely to determine theachievable mission duration. However, this is mitigated by

40

35

30

25

20

15

10

5

07060504030

windspeed, m/s

heig

ht, k

m

20100

Fig. 4 Windspeed profile with height. Values vary withseason and location, but generally follow this roughdistribution. (Source: NASA)

Fig. 5 Lindstrand HAP concept (Courtesy of Milk Design and LindstrandBalloons Ltd.15)

130 ELECTRONICS &

the fact that it is relatively easy to bring HAPs back to earth,unlike satellites, for service and/or replacement of the fuelcells as well as for other maintenance or upgrading.

One of the major HAP airship projects is the JapaneseSkyNet13. Funded substantially by the Japanesegovernment, and led by Yokosuka CommunicationsResearch Laboratory, this project aims to produce anintegrated network of some 15 airships to serve most ofJapan, providing broadband communication servicesoperating in the 28GHz band, as well as broadcasting.

A range of airships is being developed by AdvancedTechnologies Group, of Bedford, UK14, at one time incollaboration with SkyStation International of the USA15,who proposed an airship 150m in length supporting acommunications payload of up to 800kg. HAP airships arealso being proposed by Lindstrand Balloons16. A noveldesign of HAP comprising several smaller airships joinedtogether in an ‘Airworm’ configuration is being developedby The University of Stuttgart17,18: this sausage-likeformation aims to provide the lift while avoiding some ofthe structural and aerodynamic problems associated withvery large airships.

Aeroplane HAPsAnother form of HAP is the unmanned solar-powered

plane, which needs to fly against the wind, or in a roughlycircular tight path. Again, the prime challenge is likely tobe the power balance, the craft having to be able to storesufficient energy for station-keeping throughout thenight. The most highly-developed such craft are thosefrom AeroVironment in the USA, whose planned Heliosplane has a wing-span of 75m; their Pathfinder andCenturion programmes19 (Fig. 6) have already demon-strated flight endurance trials at up to 25km altitude(80000ft). Funded initially by NASA, these programmeshave the goal of long-endurance operation for commercialcommunications and other applications.

HeliPlat20 is a solar-powered craft being developedunder the auspices of Politecnico di Torino in Italy, as partof the HeliNet Project21,22 funded by the European

Commission under a Framework Vinitiative. HeliNet is examining manyissues, from the aeronauticalaspects—a scale size prototype planeis being developed—to three possibleapplications: broadband telecom-munication services, environmentalmonitoring and vehicle localisation.The broadband communicationsaspects are being led by the Universityof York23 and are described further ina companion paper in this issue24.

But the HAP project currently mostnear-market for communications is amanned aircraft with pilots operatingon an 8-hour shift. AngelTechnologies’ HALO project25 employsthe specially designed Proteus aircraft(Fig. 7) operating at altitudes of16–18km (51000–60000ft) to deliver

COMMUNICATION ENGINEERING JOURNAL JUNE 2001

broadband communication servicesover an area up to 40km in diameter.The aircraft will maintain a quasi-stationary position by flying in aroughly circular path with a typicaldiameter of less than 13 km. Thecommunications payload uses a podbelow the fuselage housing up to 125microwave antennas. The aircraft iswell proven, and this may beconsidered a relatively low-risksolution, although ultimate commer-cial success will depend upon theeconomics of operation.

Some other aerial platformsAnother category of aerial

platforms is the UAV—UnmannedAerial Vehicle. This refers typically tosmall fuelled unmanned aircraft,having short mission durations andoperating at generally modestaltitudes. The main use of UAVs is formilitary surveillance, with some smaller craft beingconsidered almost as disposable. The application of UAVsas communication relay nodes seems to be limited, nodoubt due largely to their relatively short endurance.Larger military UAVs include Global Hawk26 andPredator27 (Fig. 8), which can support large payloads andfly long distances, but have not generally been consideredcost-effective for normal communication provision.

Finally, the simplest and most available aerial platform isa tethered aerostat. This is an airship on a cable, whoselength may reach up to 5km or more. Tethering partiallydeals with the major problem of station-keeping, althoughplatform movement is still an issue. Power andcommunications backhaul may also be provided through thetether. The evident challenge is the hazard presented to airtraffic, and although some aerostats are deployed in aircraftexclusion zones, their general application may be moresuited to less developed regions. An important current

Fig. 6 Helios. Aerooperate up at 1000

ELECTRONICS & COMMUNICATION

Vironment’s craft has a wing span of 75 m and aims to00ft under solar power (Photo: NASA Dryden/Tom Tschida)

ENGINEERING JOU

programme is that of Platforms Wireless International,which is developing a tethered aerostat for use in Brazil at analtitude of 4·6 km (15000ft)28. Its ARC (Airborne RelayCommunications) system aims to deliver a range of cellularcommunication services to over 125000 subscribers.

3 Communication applications

Fig. 9 depicts a general HAP communications scenario.Services can be provided from a single HAP with up- anddown-links to the user terminals, together with backhaullinks as required into the fibre backbone. Inter-HAP linksmay serve to connect a network of HAPs29, while linksmay also be established if required via satellite directlyfrom the HAP.

The coverage region served by a HAP is essentiallydetermined by line-of-sight propagation (at least at thehigher frequency bands) and the minimum elevation

Fig. 7 HALO Proteus aircraft. Note the pod for the payload underneath. (Courtesy of Angel Technologies Corp.)

RNAL JUNE 2001 131

Fig. 8 Predator, a military UAV (Courtesy of General Atomics Aeronautical Systems Inc.)

angle at the ground terminal. A practical lower elevationlimit for BWA services might be 5°, while 15° is morecommonly considered to avoid excessive ground clutterproblems. From 20km altitude above smooth terrain, 5°implies an area of c.200km radius or 120000km2,although for many service applications, e.g. to a city orsuburban area, such wide coverage may not be requiredor appropriate.

There is then opportunity to subdivide this area into alarge number of smaller coverage zones, or cells, toprovide large overall capacity optimised throughfrequency reuse plans. The size, number, and shape ofthese cells is now subject to design of the antennas on theHAP, with the advantage that the cell configuration may bedetermined centrally at the HAP and thus reconfiguredand adapted to suit traffic requirements. Indeed, the HAParchitecture lends itself particularly readily to adaptiveresource allocation techniques, which can provideefficient usage of bandwidth and maximise capacity.

Compared with geostationary satellite services, thecells can be considerably smaller, since the minimumspot-beam size from a satellite is constrained by the on-

132 ELECTRONICS &

board antenna dimensions. Higher capacity with HAPs isalso facilitated by the much more favourable link budgetscompared to satellites, since the HAP is at relatively closerange; this represents a power advantage of up to about34dB compared to a LEO satellite, or 66dB compared toa GEO satellite. And compared with terrestrial schemes,a single HAP can offer capacity equivalent to thatprovided by a large number of separate base-stations;furthermore the link geometry means that most obstacleswill be avoided.

BWA applicationsThe principal application for HAPs is seen as B-FWA, as

described above, providing potentially very high datarates to the user. The frequency allocation for HAPs at47/48GHz offers 2 � 300MHz of bandwidth, whichmight be apportioned 50:50 to user and backhaul links,and again 50:50 to up- and down-links. (An exceptionmight be where links are mainly used for Internet traffic,which would warrant an asymmetric apportionment.)Studies undertaken based on the HeliNet scenariopropose a scheme with an overall coverage region per

alternative backhaul viasatellite for remote areas

local backhaul linksto base-stations, forless remote areas

remotehub

fibrenetwork fibre

network

LEO/MEO/GEO

inter-HAP link

Fig. 9 HAP communications scenario


HAP of diameter 60km, having 121 cells, each with anominal ground diameter of 5km24. Downlink HAP poweris 1W per cell, and this can support data rates of up to 60Mbit/s which is well within the bandwidth required percell of 25 MHz when using 16-QAM or higher ordermodulation schemes. The total payload throughput in thisconservative demonstrator example is in excess of 7Gbit/s, although the HALO scheme is claiming 100Gbit/s25.

It is evident that the backhaul requirements are asstringent as the user links themselves, since in principlewhat goes up must come down. No single wireless linkcan provide the full backhaul capacity, so this will alsoneed to be handled via a cellular scheme. Thus there willneed to be a number of distributed backhaul ground-stations, although these can be far fewer than the numberof user cells served since the backhaul links will be higherspecification and handle greater capacity, with higher-order modulation schemes. Fortunately these ground-stations can nevertheless be modest and unobtrusive andtheir location within the coverage region is non-critical;they will probably be situated on the roofs of buildings.

3G/2G applicationsHAPs may offer opportunity to deploy next generation

(3G) mobile cellular services, or indeed current (2G)services30, and use of the IMT-2000 (3G) bands fromHAPs has been specifically authorised by the ITU. Asingle base-station on the HAP with a wide-beamwidthantenna could serve a wide area, which may proveadvantageous over sparsely populated regions.Alternatively, a number of smaller cells could be deployedwith appropriate directional antennas. The benefits wouldinclude rapid roll-out covering a large region, relativelyuncluttered propagation paths, and elimination of muchground-station installation.

HAP networksA number of HAPs may be deployed in a network to

cover an entire region. For example, Fig. 10 shows severalHAPs serving the UK. Inter-HAP links may beaccomplished at high EHF frequencies or using opticallinks; such technology is well established for satellitesand should not present major problems.

Developing world applicationsHAPs offer a range of opportunities for services in the

developing world. These include rural telephony,broadcasting and data services. Such services may beparticularly valuable where existing groundinfrastructure is lacking or difficult.

Emergency or disaster applicationsHAPs can be rapidly deployed to supplement existing

services in the event of a disaster (e.g. earthquake, flood),or as restoration following failure in a core network.

Military communicationsThe attractions of HAPs for military communications

are self-evident, with their ability for rapid deployment.

ELECTRONICS & COMMUNICATION ENGINEERING JOUR

They can act as nodes within existing military wirelessnetworks, or as ‘surrogate’ satellites—in this casecarrying a satellite payload and operating withconventional satcom terminals. Besides the ability toprovide communications where none might exist, there isa benefit in that their relatively close range demands onlylimited transmit power from the ground terminals, andthis provides enhanced LPI (Low Probability ofInterception) advantages.

Within military scenarios, airships proper are currentlyconfined largely to use at very low heights for mineclearance operations, and their application forcommunications remains to be fully exploited. Although itmight be thought that airship HAPs are vulnerable toenemy attack, they do possess an advantage in thatdespite their large size their envelope is largelytransparent to microwaves and they present an extremelylow radar cross-section.

4 Advantages of HAP communications

HAP communications have a number of potential benefits,as summarised below.

Large-area coverage (compared with terrestrialsystems). The geometry of HAP deployment means thatlong-range links experience relatively little rainattenuation compared to terrestrial links over the samedistance, due to a shorter slant path through theatmosphere. At the shorter millimetre-wave bands thiscan yield significant link budget advantages within largecells (see Fig. 11).

Flexibility to respond to traffic demands. HAPs areideally suited to the provision of centralised adaptableresource allocation, i.e. flexible and responsive frequencyreuse patterns and cell sizes, unconstrained by thephysical location of base-stations. Such almost real-time

Fig. 10 Coverage of the UK with a network of HAPs(approxiately line-of-sight propagation)

NAL JUNE 2001 133

groundbase station

ground distance

userterminal

platformbase station

platformheight

rainheight

Fig. 11 Slant path inrain. The attenuatedportion may be shorterfrom a HAP than from aterrestrial base-station.

adaptation should provide greatly increased overallcapacity compared with current fixed terrestrial schemesor satellite systems.

Low cost. Although there is to date no direct experienceof operating costs, a small cluster of HAPs should proveconsiderably cheaper to procure and launch than ageostationary satellite or a constellation of LEO satellites.A HAP network should also be cheaper to deploy than aterrestrial network with a large number of base-stations.

Incremental deployment. Service may be providedinitially with a single platform and the network expandedgradually as greater coverage and/or capacity is required.This is in contrast to a LEO satellite network, whichrequires a large number of satellites to achievecontinuous coverage; a terrestrial network is also likely torequire a significant number of base-stations before it maybe regarded as fully functional.

Rapid deployment. Given the availability of suitableplatforms, it should be possible to design, implement anddeploy a new HAP-based service relatively quickly.Satellites, on the other hand, usually take several yearsfrom initial procurement through launch to on-stationoperation, with the payload often obsolete by the time it is

134 ELECTRONICS &

launched. Similarly, deployment of terrestrial networksmay involve time-consuming planning procedures andcivil works. HAPs can thus enable rapid roll-out ofservices by providers keen to get in business before thecompetition.

Furthermore, there is little reason why prepared HAPsshould not be capable of being launched and placed on-station within a matter of days or even hours. This willfacilitate their use in emergency scenarios. Examplesmight include: natural disasters; military missions;restoration where a terrestrial network experiencesfailure; overload due to a large concentration of users, e.g.at a major event.

Platform and payload upgrading. HAPs may be on-station for lengthy periods, with some proponentsclaiming 5 years or more15. But they can be brought downrelatively readily for maintenance or upgrading of thepayload, and this is a positive feature allowing a highdegree of ‘future-proofing’.

Environmentally friendly. HAPs rely upon sunlight fortheir power and do not require launch vehicles with theirassociated fuel implications. They represent environ-mentally friendly reusable craft, quite apart from the

Table 1: Comparison of broadband terrestrial, HAP and satellite services: typical parameters

Terrestrial HAP LEO satellite GEO satellite(e.g. B-FWA) (e.g. Teledesic31)

Station coverage <1km up to 200km >500km up to global(typical diameter)

Cell size (diameter) 0·1–1km 1–10km c. 50km 400km minimum

Total service area spot service national/regional global quasi-globalMaximum transmission 155Mbit/s 25–155Mbit/s <2Mbit/s up 155Mbit/srate per user 64Mbit/s down

System deployment several base flexible many satellites flexible, but longstations before before use lead timeuse

Estimated cost of varies $50million upwards? c. $9billion >$200millioninfrastructure

In-service date 2000 2003–2008? 2005 1998


potential benefits of removing the need for large numbersof terrestrial masts and their associated infrastructure.

Table 1 summarises a comparison between terrestrial,HAP and satellite delivery for broadband services.

5 Some issues and challenges

The novelty of HAP communications calls for some newconcepts in terms of delivery of services (e.g. B-FWA),raising critical issues for development. And the platformsthemselves present some challenges and potentialproblems.

System level requirements. HAP networks forbroadband communication service delivery will require arethink of the basic design of cellular-type services, withdevelopment focusing upon the frequency planning ofdifferent spot beam layouts, which are subject to wideangular variations and changes in link length, andfrequency reuse patterns for both user and backhaullinks. The network architecture will need to exploitopportunities of inter-terminal switching directly on theHAP itself as opposed to on the ground, and the use ofinter-HAP links to achieve connectivity.

Propagation and diversity. Services from HAPs havebeen allocated frequencies by the ITU in the millimetre-wave bands, at 47/48GHz (and also at 28GHz in ITURegion 3—predominantly Asia). Propagation from HAPsis not fully characterised at these higher frequencies: rainattenuation is significant in these bands, so one of themain requirements is to develop rainfall attenuation andscattering statistics. This will allow appropriate margins tobe included and highlight any problems with frequencyreuse plans developed at the system level. An importantobjective is to determine the most appropriate diversitytechniques (e.g. space, time and frequency) for eachtraffic type.

Modulation and coding. In order to optimise networkcapacity, suitable modulation and coding schemes will berequired to serve the broadband telecommunicationservices, with specified QoS (Quality of Service) and BER(Bit Error Rate) requirements, applicable under differentlink conditions. Adaptive techniques will provide overalloptimum performance, using low-rate schemes involvingpowerful forward error correction (FEC) coding whenattenuation is severe, up to high-rate multilevelmodulation schemes when conditions are good.

Resource allocation and network protocols. Channelassignment and resource allocation schemes will need tobe developed for the HAP scenario, which is essentiallydifferent from either a terrestrial or a satellite cellularscenario. The schemes have to be tailored to multimediatraffic and also take into account the system topology andchoice of modulation/coding scheme. The mostappropriate medium access control (MAC) and networkprotocols will be selected as a starting point. For BWAservices it is likely that a modified version of thebroadband standards IEEE 802.16/ETSI BRAN could beused. Integration with terrestrial and/or satellitearchitectures will also require careful planning.

Antennas. Antenna technology will be critical to BWA


from HAPs. A large number of spot beams will berequired, and these may be produced either by anensemble of horn antennas or some form of phased array.Sidelobe performance is an important issue, which willaffect intercell interference and, ultimately, systemcapacity. At a planned frequency of 48 GHz, this isdemanding technology both for the HAP-based antennaand also for ground terminals.

Platform station-keeping and stability. The ability of aHAP to maintain position reliably in the face of variablewinds is a major challenge and will critically affect theviability of communication services. HAP positioning islikely to be represented as a certain statistical probabilityof remaining within a particular volume, e.g. a locationcylinder. Example figures from the HeliNet programmerelate to 99% and 99·9% platform availability withinspecified location limits, and clearly such parameters arenot readily compatible with provision of communicationservices having traditional ‘four nines’ availability such as99·99%. Some new thinking will be required, perhapsbased on the use of multiple HAPs and diversitytechniques.

Stability is another critical issue. Inevitably, there willbe roll, pitch and yaw of the platform, due to turbulence inthe stratosphere; in this regard, larger craft are likely toexhibit greater stability. Antenna pointing on the HAPmay be maintained either through the use of amechanically stabilised sub-platform, which may bebulky, or though use of electronic steering of an arrayantenna. This latter technique offers considerablepotential, but is also technologically demanding,especially at the millimetre-wave bands likely to be usedfor broadband services.

Handoff. Most HAP schemes are proposing to usemultiple spot beams over the coverage area, yieldingcapacity through frequency reuse. Although a BWAnetwork architecture is likely to have mainly fixed users,handoffs may occur as the antenna beams move due toplatform motion, depending on the HAP stabilisationtechniques. This is in contrast to conventional mobilecellular schemes, where handoff is invariably due tomotion of the user. The size of the cells on the ground, andthe physical stability of the HAP antenna pointing, willgovern how often these will occur. It may be possible touse fixed antennas on the HAP, and to accommodatemotion simply through handoff procedures (which couldbe quite rapid). However, delay and jitter limitations forfuture multimedia services (especially video) may imposemuch more stringent constraints on the handoff processthan with conventional 2G or 3G services, and this is atopic of current research.

Should ground-based antennas be fixed or steerable?HAPs will vary in position, both laterally and vertically:the latter perhaps deliberately so in order to optimisealtitude to minimise prevailing winds. This movementresults in changes of look angle from the ground terminal,which can be used to determine whether fixed orsteerable ground-based antennas are required. If theangular variation is greater than the beamwidth of theantenna, which will be a function of the gain required to

RNAL JUNE 2001 135

1250

1000

750

500

250

00 24201612

time, h

sola

r po

wer

,W/m

2

84

22 June

22 December

36°N40°N45°N

36°N

40°N45°N

Fig. 12 Solar power flux on a HAP at a height of 17km as a function ofseasonal extreme, time and latitude

operate the link, then it is necessary to use a steerableground terminal antenna. The greatest angular variationis immediately below the HAP; however, at this closerange wider antenna beamwidths may be feasible due tofavourable link budgets. Changes in vertical height maybe more significant to antennas at the periphery ofcoverage if these are correspondingly higher gain, andhence narrower beamwidth. Clearly, the requirement forsteerable antennas will increase terminal costs, but maybe necessary to achieve high link capacity.

Payload power. An important distinction between thedifferent types of HAP is the power available to thepayload. Typically an airship may have in excess of 20 kWavailable for the payload, due to the large surface area onwhich to deploy solar cells. Planes powered byconventional fuel sources (e.g. the HALO scheme byAngel Technologies) will similarly have high poweravailable. By comparison, solar powered planes (e.g.HeliPlat) may have significantly less available payloadpower; this is a limitation similar to that experienced bysatellites, and means that the achievable downlink RFpower, and hence overall capacity, will be constrained.Indeed, solar powered planes have much in common withcommunication satellites, in terms of available powerfrom the solar panels, payload weight and space availableon the platform. Power will need to be used mostefficiently, particularly through careful spot beam andantenna array design and power-efficient modulation andcoding schemes.

Compared with a satellite, a HAP will require a higherproportion of the power to charge the batteries (fuel cells)because it must cope with long periods of darkness eachnight, the worst case being the shortest day (22ndDecember in the northern hemisphere). At higherlatitudes both the variation in the angle of the sun relativeto the solar panels between summer and winter, and theshort winter days will have an additional significant effect.Fig. 12 shows the variation in incident solar power per

136 ELECTRONICS &

square metre as a function ofseasonal extreme, time of day, andlatitude.

Advanced Technologies Group issuggesting the use of airships inwhich solar-powered technology isaugmented with diesel engines thatcan also be used when station-keeping is a problem15. But thepower budget is most acute forsolar-powered plane technology,where the available area for thesolar cells and the permissible massfor the fuel cells limit the power tothe payload; this may criticallydetermine the platform’s viability.

6 The way forward

Although some of the goals forHAPs are a few years fromrealisation, there can be little doubt

that aerial platforms will play an increasingly importantrole in the delivery of wireless services. A combination of‘technology push’ from the providers of platforms and‘applications pull’ from the inexorable demand forcommunications means that it is not so much a questionof ‘if’ but ‘when’.

Commercial aerial platform projects already underwayinclude the HALO programme, and the PWI tetheredaerostat project in Brazil. Solar-powered planes aredeveloping rapidly, as is technology for airships. The bigchallenge is whether HAPs can provide the requiredgrade-of-service, especially in the face of uncertain windsat their operating altitude. There is also the issue of long-term reliability, with manufacturers postulating on-stationlifetimes of 5 years or more, which is far longer than thoseof any current aerial devices.

But these factors need to be balanced with theeconomic benefits of the service provided and the cost ofthrough-life operation. High link availability may not berequired of a single HAP in many scenarios, whether dueto diversity techniques or because ‘99·99%’ service is notdemanded. And with the opportunity to bring HAPs backto ground for maintenance, the issue of long lifetime maybe less critical.

One of the technical hurdles for the manufacturers ofHAPs, whether solar-powered plane or airship, lookslikely to be the performance of energy storage such asfuel cells, and there is considerable on-going work in thisarea. Although much can be learned from scale HAPprototypes, not all of the aerodynamic, structural andenergy issues scale linearly, thus full-size HAP prototypeswill need to be built and tested in order to convince usersand investors of their commercial viability.

With the evident opportunities for enhancedcommunication services, as outlined in this paper, it is tobe anticipated that we shall see significant developmentsin HAPs for communication service delivery over the nextfew years.


Acknowledgments

Support for the HeliNet programme is acknowledgedfrom the EU Framework V Programme (IST-1999–11214), and from members of the HeliNet teamboth at York and among our Partners. Thanks particularlyto John Thornton, and to John Jones for his airshipenthusiasm.

References

1 Dudley Lab’s list of Frequency Allocations, May 2001, seehttp://www.dudleylab.com/freqaloc.html

2 GRACE, D., DALY, N. E., TOZER, T. C., and BURR, A. G.:‘LMDS from high altitude aeronautical platforms’. Proc. IEEEGLOBECOM’99, Rio de Janeiro, Brazil, 5th–9th December1999, 5, pp.2625–2629

3 GRACE, D., DALY, N. E., TOZER, T. C., BURR, A. G., andPEARCE, D. A. J.: ‘Providing multimedia communicationsfrom high altitude platforms’, Int. J. Satell. Commun.(accepted for publication)

4 Global VSAT Forum, see http://www.gvf.org5 DJUKNIC, G. M., FREIDENFELDS, J., and OKUNEV, Y.:

‘Establishing wireless communications services via high-altitude aeronautical platforms: a concept whose time hascome?’, IEEE Commun. Mag., September 1997, pp.128–135

6 STEELE, R.: ‘Guest editorial—an update on personalcommunications’, IEEE Commun. Mag., December 1992,pp.30–31

7 EL-JABU, B., and STEELE, R.: ‘Aerial platforms: a promisingmeans of 3G communications’. IEEE Vehicular TechnologyConf., Houston, TX, 16th–20th May 1999, 3, pp.2104–2108

8 TCOM, see http://www.tcomlp.com9 ‘Lockheed Martin upgrades first of six tethered aerostat radar

sites and receives go-ahead on second site’. Lockheed MartinPress Release, October 2000. See http://www.lockheedmartin.com/news/articles/100900_2.html

10 Breitling Orbiter, see http://www.breitling.com/eng/aero/orbiter/

11 Zeppelin, see http://www.zeppelin-nt.com12 CargoLifter, see http://www.cargolifter.com13 Yokosuka CRL, see http://www2.crl.go.jp/t/team2/

index.html14 Advanced Technologies Group, see http://www.airship.com 15 SkyStation, see http://www.skystation.com16 Lindstrand Balloons Ltd., see http://www.lindstrand.co.uk17 REHMET, M. A., KRÖPLIN, B. H., EPPERLEIN, F.,

KORNMANN, R., and SCHUBERT, R.: ‘Recent developmentson high altitude platforms’. Airship Convention, July 2000,ISBN 0-9528578-2-0

18 University of Stuttgart, see http://www.isd.uni-stuttgart.de/arbeitsgruppen/airship/halp/index.htm

19 AeroVironment, see http://www.aerovironment.com20 ROMEO, G., FRULLA, G., and FATTORE, L.: ‘HELIPLAT: a

solar powered HAVE-UAV for telecommunicationapplications. Design and parametric results. Analysis,manufacturing and testing of advanced composite structures’.Int. Technical Conf. on Uninhabited Aerial Vehicles,UAV2000, Paris, France, 14th–16th June 2000

21 TOZER, T. C., OLMO, G., and GRACE, D.: ‘The EuropeanHeliNet project’. Airship Convention 2000, July 2000, ISBN 0-9528578-2-0

22 HeliNet, see http://www.helinet.polito.it23 Communications Research Group at the University of York,

see http://www.elec.york.ac.uk/comms


24 THORNTON, J., GRACE, D., SPILLARD, C., KONEFAL, T.,and TOZER, T. C.: ‘Broadband communications from a high-altitude platform: the European HeliNet programme’,Electron. Commun. Eng. J., June 2001, 13, (3), pp.138–144

25 COLELLA, N. J., MARTIN, J. N., and AKYILDIZ, I. F.: ‘TheHALO network’, IEEE Commun. Mag., June 2000,pp.142–148

26 Global Hawk, see http://www.dsto.defence.gov.au/globalhawk/1_about.html

27 Predator, see http://www.gat.com/asi/aero.html28 Platforms Wireless International, see http://www.plfm.com29 DRCIC, U., KANDUS, G., MOHORCIC, M., and JAVORNIK,

T.: ‘Interplatform link requirements in the network of highaltitude platforms’. 19th AIAA Int. Communications SatelliteSystems Conf., Toulouse, France, 17th–20th April 2001

30 PENT, M., LO PRESTI, L., MONDIN, M., and ORZI, S.:‘Heliplat as a GSM base station—a feasibility study’.DASIA’99, Data Systems in Aerospace Conf., Lisbon,Portugal, May 1999

31 Teledesic, see http://www.teledesic.com/about/about.htm

©IEE:2001First received 17th April and in final form 17th May 2001.

RN

Tim Tozer is leader of theCommunications ResearchGroup at the University of York,and Senior Lecturer in theDepartment of Electronics. Hisresearch interests cover radiosystems, including satellite andterrestrial broadband, and HAP-based communications. Tim hashad numerous academic andtechnical papers published and isresponsible for a range ofresearch projects, includinginvolvement in the HeliNet programme. He also teachesextensively and has presented international workshops on VSATsystems. Previously he worked at RSRE on the design of satellitecommunications. Tim is active in the IEE Professional Networkon Satellite Systems & Applications and is a past Chair of IEEProfessional Group E9. He is a Director of SkyLARCTechnologies Ltd., a company developing wireless broadbandsolutions with emphasis on HAPs.

E-mail: [email protected]

David Grace received an MEngdegree in Electronic SystemsEngineering from the Universityof York, UK in 1993. Since 1994 hehas been a member of theCommunications Research Groupat York, where he is now aResearch Fellow, and has workedon a variety of research contracts.In 1999 he received a DPhil degreefor work on ‘Distributed dynamicchannel assignment for thewireless environment’ from theUniversity of York. His currentresearch interests include broadband communications from HAPs,and dynamic channel assignment/multiple access schemes formultimedia communications. He is now project manager for TheUniversity of York’s contribution to the EU Framework V HeliNetproject, namely the broadband communications programme. Davidis also a Director of SkyLARC Technologies Ltd.

E-mail: [email protected]

Address: Department of Electronics, The University of York,Heslington, York, YO10 5DD, UK.

AL JUNE 2001 137

Documents

High-altitude platforms for wireless communications - CiteSeerX