High Altitude Platforms for Wireless Commmunication

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H gh-altitude platformsfor wirelessrcommunications

by T. C Tozer and D. GraceThe demand for high-capacity wireless services is bringing increasing challenges,

especially for delivery of the last mile. Terrestrially, the need for line-of-sightpropagation paths represents a constraint unless very large numbers of base-stationmasts are deployed, whi le satellite systems have capacity limitations. An emerging

solution i s offered by high-al titude platforms (HAPs) operating in the stratosphere ataltitudes of up t o 22 km to provide communication facilities tha t 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 fo r delivery of future broadband wirelesscommunications.

1 The challenge for w ireless communicationsAs the demand grows for communication services,wireless solutions are becoming increasingly important.Wireless can offer high-bandwidth service provisionwithout reliance on fured infrastructure and represents asolution to the last mile problem, i.e. delivery directly toa customers premises, while in many scen arios wirelessmay represent the only viable delivery mechanism.Wireles s 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 hom euscrs.

Th e emerging marke t is for broadband data provisionfor multimcdia, which repres ents a convergence ol high-spe ed Internet (and e-mail) telephony, Tv video-on-demand, sound broadcasting ctc. Broadband fKedwireless access (R-FWA) sch em es aim to deliver a rangeof multimedia services to the customer at data rates oftypically at least 2 Mbit/s. R-FWA should ofier greatercapacity to the user than services based on existingwirelines, su ch as ISDN o r xDSL, which a re in any eventunlikely to be available to all customers. Th e alternativewould be cable or fibre delivery, but su ch installation maybe prohibitively expensive in many scenarios, and thismay represen t a barrier to ncw se rvic e providers. B-FWAis likely to be targe ted initially at business , including SM E(small-medium enterprisr) and S O H 0 (small office/hom e oilice) users, although the m arket is anticipated toextend rapidly to domestic custom ers.However delivering high-capacity services by wireless

also presents a challenge, especially as the radiospec trum is a limited resource subjcct to increasingpressure as demand grows. To provide bandwidth to alarge nuniber of users, so me form of frequency reusestrategy must be adopted, usually based around a fixedcellular structure. Fig. l a illustrates the cellular concept,where each hexagon represents a cell having a base-station near its centre and employing a dillercntfrequency or group of frequencies represented by thecolour. The se 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 atio.To provide increased capacity, the cell sizes may bcreduced, thus allowing the spectrum to be reused moreoften within a given geographical area, as illustrated inFig. l b .Thi s 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 ext rem e limit, one might envisage o ne 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 th e backhaullinks to serve them , by fibre or o ther wireless means, andth e cost of installation.Pressu re on the radio spe ctrum also leads to a movctowards higher frequency bands, which are less heavilycongested and can provide significant bandwidth. Thrmain allocations for broadband are in the 28GHz band(26GHz in some regions), a s wellbroadband sc heme s may be variously described as LMDS(Local Multipoint Distribution Services) or MVUS(Multipoint Video Distribution Services)2 3; thes e ar e

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Fig. 1a greater num ber o f t imes wit h in a given geographical area.Cellular frequen cy reuse concept. In th e smaller cells provid e greater ov erall capacity as frequencies are reused

flexible concepts for delivery of broadband services,although they may be encompa ssed in the generic termB-FWA, or sim ply BWA.

The use of thes e millimetre wavelengths implies line-of-sight propagation, which represents a challengecompared with lower frequcncics. Thus local obstruc-tions will cause problems and each customer terminalnee ds to see a base-station; i.e. a m icrocellular str uct ureis essential for propagation reasons. Th is again implies aneed for a very large numb er of base-stations.

A solution to these problems might be very tall basc-station mas ts with line-of-sight to us ers. How ever, thesewould be not only costly but also environmentallyunacceptable. An alternative delivery mechanism is viasatellite, which can provide line-of-sight commu nicationto many users, Indeed, broadband services lroingeostationary (GEO) satelli tes are projected to re presen ta 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) o l 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) dia meteron the ground, and these minimum dimensions constrainthe frequency reuse density and hence the overallcapacity. Additionally, the high FSPL requires sizeableantennas a t ground termiiials to achieve broadband datarates. Afu rther dow nside is the length y propagation delayover a ge ostationa ry satellite link of 0.25s.which not onlyis troublesome for speech b ut also may cause difficultieswith som e data protocols.Low ear th orbit (LEO) satellites may circumvent som eof these limitations in principle, but sufler k o mcomplexities of rapid handover, not only between cells butalso between platforms. Th e need lor large numb ers of

T,EO satellites to provide continuous coverage is also asignificant economic burden, and such schem es have yetto prove commercially successful.2 Aer ia l p la t fo rms: a solution?A potential solution to the wireless delivery pro blem liesin aerial platforms, carrying communications relaypayloads and operating in a quasi-stationary position ataltitudes up to som e 22 km . A payload can be a com plelebase-station, or simply a transparent transpond er, akin tothe majority of satellites. Line-of-sight propagation p ath scan be provided to most users, with a mo dest I W L , t husenabling services that take advantage of the bes t featuresof both terrc strial and satcllitc comniunicalions.A single aerial platform can replace a large nu mb cr ofterrestrial masts, along with their associated costs,environmental itnpact and backhaul constraints. Siteacquisition problems are also eliminated, together withinstallation maintenance costs, which can reprcsent 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 m ostinterest are cralt designed to operate in the stratosphereat an altitude of typically between 17 and 221tm, which arereferred to as high-altitude platforms (HAPs)~-7. Whilethc te rm HA P may not h ave a rigid definition, we take it tomean a solar-powered and uninanned aeroplane orairship, capable of long endurance on-station-e.g.several mon ths or InorE. 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.

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HAPS are now being activelydeve loped in a number o lprog a inm esworld-wide, and thc surge of recentactivity reflects both the lucrativedemand for wireless services andadvances in platform technology, suc hs in materials, solar cells and energy

storage.BalloonsTh e earliest aerial platforins wereballoons, whose history go es back tothe time of the an cient Chinese. In theWest, the first major use of lighter-than-air cralt was the Montgolfierbrothers manned hot air balloon inFrance in 1783. Subsequentdevelopment of balloons remainedlargely for military application,although their application for Luftschi f f technik GmbH ,Ocommunication purposes seems tohave been vcry limited. In the early 20th century, rigid, view to the scenic tourism m arket (Fig. 2)11. Anotherpowered airships were developed by thc Zeppelin major project, albeit low altitude, is CargolifterlL (Fig. 3 ;company in German y for passenger transport, and these this large airship is proposed to be som e 250m long andproved a remarlrable engineering feat, enabling aims to provide a flying crane service for transportationpassengers to travel from Europe to South A merica. of heavy goo ds over difficult terrain.Unlortunatcly, this e ra cam e to an abrupt cnd with the But a major business g oal remains t hat of developing aHindenburg disaster at Laliehurst, USA, in 1937. stratospheric HAP capable of serving communicationWhatcver the precise cause of that disaster, use of the applications economically and with a high deg rec ofhighly flammable hydro gen to fill the airship was no reliability. W heth er an airship or an aeroplane, a majorlonger considered acceptable, and with the development challenge is the ability of the HAP to maintain station-of commercial air travel after the Second World War large keeping in t he face of winds. An operating altitude ofairships seem ed to be consigned to history. between 17 and 22 kin is chosen because in most regionsSubsequent activity was mainly confined to hot-air of the world this rep rese nts a layer of relatively mild windballoons for recreational purposes, sniall balloons for and turbulence. Fig. 4llustrates a typical jet-s lrem i windmeteorological use, and tethered aerostats (balloons on profile vs. height; although the wind profile may varystrings). The latter may operate up to at least 5000m considerably with latitude and with seaso n, a form similaraltitude, although ther e ar e evident implications for air- Lo that show n will usually obtain. Ihis altitude (> 55000ft)traffic safety and their us e is highly restricted, the majority is also above com mercial air-traffic heights, w hich wouldoperating at muc h lower altitudes. (One major application otherwis e prove a potentially prohibitive constraint.of tethered aerostats is surveillance.Largc numbers are deployed alongthe US-Mexican border to detectunauthorised crossings.)The past few years have seen aresurgence of interest in balloons andairships, with technology develop-ments such as new plastic cnvelopematerials that are strong, Wresistant and leak-proof to helium,which is now almost universally usedinstead of the much cheaperhydrogen. S uch hi-tech airships havefeatured in high-profile attempts tocircumnavigatc the globe (e.g theRreitling O rbiter). And a newairship, the Zeppelin Nr a low-altitude craft, was launched by theZeppelin company in 2000, exactly100 Years alter its first airship, with a

Fig. 2 The Zeppel in NT airship, launch ed in June 2000 Courtesy of Zeppel in

Fig. 3 Art is t s impression of Carg ol i f ter Courtesy of Cargol i f ter G mb H )

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40 r

0 10 20 30 40 50 60 7windspeed. mis

Fig. 4 Windspeed prof i le wi th heigh t. Values vary wi thseason and locat ion, but general ly fo l lo w this rou ghdistribution. Source: NASA)Airship HAPsProposed implementations of airships for high-altitudedeployment u se very largc scnii-rigid or non-rigid hcliuni-filled containers, of the ord er of 100 m or m ore in length.Fig. 5show s an artists impression of a H A P froinLindstrand 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 shee ts, which m ay weighlypically well under 400g/m2 and cover lhc uppersurfaces of th e airship. Additionally, during the day, poweris stored in regenerative fuel cells, which then provide allthe power requiremcnts at night.Th e overall long-term power balance of a HAP is likely tobe a critical factor, and it will be the perforinance andageing of t he fuel cells that are likely to determ ine theachicvable mission duration. However, this is mitigated by

the fact that it isrelatively easy to bring HAPs back to ear th ,unlike satellites, for service and /or replac emen t of the fuelcells as well as for oth er m aintenance or upgrading.On e of the m ajor HA P airship projects is the JapaneseSkyNet?. Fun ded substantially by th e Japanesegovernment, and led by Yokosuka CommunicationsResearch Iabor atory, this project aims to produce anintegraled network of some 15 airships to serve most ofJapan, providing broadband communication serviccsoperating in th e 28GHz band, a s well as broadcasting.A range of airships is being developed by AdvancedTechn ologies G roup, of Bedford, UKI4, at one tim e incollaboration with SkyStation Internalional of the USA15,who proposed an airship 15Om in length supporting acomm unications payload of up to 8OOkg. HAP airships arealso being p roposed by Lindstrand Balloonsb. A noveldesig n of HA P comprising several sm aller airships joinedtogether in an Airwormconfiguration is bein g developedby The University of StuttgartL7? this sausage-likeformation aims to provide thc lilt while avoiding som e ofthe structural and aerodynamic problem s associated withvery large airshipsAeroplane HAPs

Another form of HAP is the unm anned solar-poweredplane, which ne eds to fly against the wind, or in a roughlycircular tight path. Again, the prime challenge is likely tobe the power balance, th e craft having to be ablc to s toresufficient 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 program mesLq Fig. 6) have already demoii-strated flight endurance trials at up to 25km alti tude(800OOft). Funded initially by NASA, these programmeshave the goal of longendurance operation for com inercialcomm unications and other applications.HeliPlatdu is a solar-powered craft being developedunder the auspices of Politecnico di Torin o in Italy,as partof the HeliNet ProjecP1,2funded by the EuropeanCommission under a Framework V

initiative. HeliNet is exam ining man yissues, from the acronauticalaspects-a scale size prototype planeis being developed- to three possibleapplications: broadband telecom-munication services, environmentalmonitoring and vehicle localisation.The broadband communicationsaspects arebeing led by th e Universityof Yorkz3 n d are described fur ther ina companion paper in this issueJ4.

But the HAP project currently mostnear-market for communications is aman ned aircraft with pilots operatingon an 8-hour shift. AngelTechnologies HALO projectL5mploysthc specially designed P roteus aircraft(Fig. 7) operating at altitudes of16-18km (51000-60000ft) to deliverFig. 5 Lindstrand HAP concept Courtesy o f Milk Design and LindstrandBalloons Ltd.l5)

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broadband communication servicesover an area up to 40kni in diameter.The aircraft will mainkin a quasi-stationary position by flying in aroughly circular path with a typicaldiameter o l less than 13 km. T h ecommunications payload uses a podbelow thc fuselage housing up to 125microwave antennas. The aircraft iswell proven, and this may beconsidered a relativcly low-risksolution, although ultimate commer-cial success will depend upon theeconomics of operation.Some other aerial platformsAn oth er categ ory of aerialplatforms is thc UAV-UnmannedAerial Vehicle. This refers typically tosmall fuclled unmanned aircraft,having short mission durations andOperating at grnerally modest operate up at 1OOOOO f t und er solar pow er Photo: NASA Dryden/Tom Tschida)altitudes. T he main use of IJAVs is formilitary survcillance, with some smaller cralt beingconsidered almost as disposable. Th e application of UAVsas communication relay nodes seeins to be limited, nodoubt due largely to thcir relatively short endurance.Larger military UAVs include Global H awkL6 and comm unication services to over 125000 subscribers.PredatorL7 Fig. S), which can s upp ort large payloads andflylong distances, bu t have riot generally been considered 3 C o m m u n i c a t i o n a p p l i c a t i o n scost-effective for normal communication provision.

Finally, the simplest and most available aerial platform is Fig. 9depicts a general H A P communications scenario.a tethered aerostat. Iliis is an airship on a cable, whose Services can be provided li-om a single HAP with up- andlength may reach up to 5k m or more. Tethering partially down-links to the user terminals, together with backhauldeals with the major problem ol station-keeping,although links as required into the fibre backbone. Inter-HAP linksplatform movement is still an issue. Power and may ser ve to connect a network of H A W 9 , while linkscommunicationsbackhaul may alsobe provided hrough the may also be established if rcquircd via satellite directlytether. Ihe evident challenge is the hazard presented to air from the HAP.traffic, and although someaerostats arc deployed in aircraft T he coverage region serve d by a HAP is essentiallyexclusion zones, their general application inay be more deter min ed by line-of-sight propagation (at least at thesuited to less developed regions. Ai important current higher frequency bands) and the minimum elevation

Fig. 6 Helios. AeroVironments craft has a wi ng span of 75 m and a ims to

programnic is that of Platforms Wireless International,which is developinga tethered aerostat for use in Brazil at analtitude of 4 6 km (15000H)28. ts ARC (Airb orne RelayComm unications) system aims to deliver a range of cellular

Fig. 7 HALO Proteus aircraf t . Note the p od for t he payload un dern eath. Courtesy o f Ang el Technologies Corp.)

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Fig. 8 Predator, a military UAV Courtesy of General Atomic s Aeronautical Systems Inc.)angle at the g round terminal. A practical lower elevationlimit for BWA services might be 5 ,while 15 i s morecommonly considered to avoid excessive ground clutterproblems. From 2 0km altitude above smooth terrain, 5implies an area of c.200km radius or 120000kmL,although for many service applications, e.g. to a city orsubur ban area, such wide coverage may not be requiredor appropriate.The re is then opportunity to subdivide this area into alarge number of smaller coverage zones, or cells, toprovide large overall capacity optiniiscd throughfrequency reuse plans. The size, number, arid shape ofthese cells is now subject to design oC the antennas on theHAP ith the advantage th at the cell conliguration inay bcdetermined centrally at the HAP and thu s reconfiguredand adapled to suit traffic requirements. Indeed, the HAParchitecture lends itself particularly readily to adaptiveresource allocation techniques, which can provideefficienl usag e of bandwidth and m aximise capacity.Compared with geostationa-y satellite services, thecells can be considerably smaller, since the minimumspot-beam size from a satellite is constrained by the on-

board antenna dim ensions. High er capacity with HAPS isalso facili tated by th e much more favourable link bud getscompared to satelli tes, since th e HAP s at relatively closerange; this rep res ents a power advantage of up lo aboul34dB compared to a LEO satellite, or 66dB com pared toa GEO satellite. And compared with terrestrial schemes,a single HA P can offer capacity equivalent to thatprovided by a large number o f separate base-stations;Furthermore the link gcom ctry mcan s that most obstaclcswill bc avoided.BWA pplications

?lie principal application for HAPS is seen as K-FWA, a sdescribed above, providing potentially very high datarate s to th e user. T he fr equency allocation Cor H PS t47/48GHz ol le rs 2 x 300MHz of bandwidth, whichmight be apportioned 50:50 to use r and backhaul l inks,and again 5050 to u p and down-links. (An exceptionmighl be where links are mainly used for Intern et traffic,which would warrant an asymmetric apportionment.)Studies undertaken based on the HeliNet scenariopropos e a sch etn e with an overall coverage region per

Fig. 9 HAP comm unications scenario

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HAP of tliamrter (iOktn, having 121 cells, each with anominal grou nd d iameter of 5kmJ4. )owulink HAI' poweris I W per cell, and this can su ppo rt data rate s of u p to 60Mbit/s which is well within the bandwidth required percell of 25 MHz when using 16-QAM or higher ordermodulation schem es. 'The total payload throu ghp ut in th isconservative dcniotistrator example is in excess ol7Chit/s, although the HALO sc he me is clainiing100Gbit/s2 .It is evident that the backhaul requircments are asstringent ab thc use r links themselves, since in principlewhat goes up must come down. No single wireless linkcan provide the full backhaul capacity, so this will alsoneed to b e handled via a cellular scheme . Th us the re willneed to be a number of distributed backhaul ground-stations, although th ese can be far fewer than the numberof use r cells served since the backhaul links will be higherspccification and handle gre ater capacity, with hig her-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 b c situated on the roofs of buildings.3G/2G applications

HAPS may offer opportunity to deploy ncxt generation3G) mobile cellular services, or indeed current (2G)

servicesi0 , and u se of the IMT-2000 ( 3 0 bands fromHAPS has been specifically authorised by the TT'lJ. Asingle base-station on the HAP with a wide-beamwidthantenna could ser ve wide area, which may proveadvantageous over sparsely populated regions.Alternatively, a numbe r 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 nctwork tocover an entire region. For example,Fig. 10shows severalH PS scrving the UK. Inter-HAP links may beaccomplishccl at h igh EHF frequencies or using opticallinks; such technology is well established for satellitesand should not present major problems.Developing world applications

H Ps offer a range of opportunities for services in thedeveloping world. 'These include rural telephony,broadcasting and data services. Such services may beparticularly valuable where existing groundinfrastruc ture is lacking o r difficult.Emergenry 01 disaster applications

HAPs can be rapidly deployed to supplement existingservices in the event o fa disaster (e.g. earthq uake, flood),or as restoration following failure in a c ore n etwork.Military conzmunicationsT he attractions of HAPs for military cominunicationsarc scll-cvident, with their ability for rapid deployment.

Fig. 10 Coverage o f the UK wi t h a n e t wo r k o f HAPsapproxiately l in e-of-sight propagat ion)They can act a s no des within existing military wirelessnetworks, or as 'surroga te' satellites-in this casecarrying a satellite payload and operating withconventional salconi terminals. Besides the ability toprovide comm unications where none might exist , there isa bcncfit in that their rrlativcly closc range dem and s onlylimited transmit power from the ground terminals, andthis provides enhanced LPI (Low Probability ofItitcrception) advantages.Within military scenarios, airships proper a re currentlyconfined largely to use a t very low he ights for mineclearance operations, and their application forcommu nications remain s 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 largelytranspar ent to microwaves and they present an extremelylow radar cross-section.4 A d v a n t a g e s of H A P c o m m u n i c a t io n sHAP commun ications have a num ber of potential benefits,as sum marised below.

Large-area coverage (compared with terrestrialsystems) . The geometry of H A P deploymcnt means thatlong-rangc links experience relatively little rainattenuation compared to terrestrial l inks over the samedistance, due to sho rter slant path through theatmosphere. At thc shortc r millimetre-wave bands thiscan yield significant link bud get advantages within largecells (see Fig. 11).

Flexibility to resjond to traffic demands. HAPS areideally suited to the provision of centralised adaptableresourc e allocation, i.e. flexible and responsive frequ encyreuse patterns and ccll sizes, unconstrained by thephysical location of bas e-stations. Suc h almost real-lime

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Fig. 11 Slant path inrain. The attenuate dpor t ion may be shorterf rom a HAP than f rom aterrestrial base-station.

Dlatform

userterminal

rainheigh

base station ground distance

adaptation should provide greatly increased overallcapacity compared with cu rrent fixed terrestrial sch emesor satellite systems.Low cost Although the re is to date no direct experienceof operating c osts, a small cluster of HAPs should proveconsiderably cheaper to procure and launch than ageostatio nary satellite or a constellation o LEO satellites.A HAP network should also be cheaper to deploy than aterrestrial network with a large number of base-stations.

Ixcrenzental deployment. Service may be providedinitially with a single platform and th e network expandedgradually as g reater coverage an d/or capacity is required.Th i s is in contrast to a LEO satellite network, whichrequires a largc number of satellites to achievecontinu ous coverage; a terrestrial network isalso likely torequir e a significant number of base-stations before it maybe regarde d as fully functional.Rapid deployment. Given th e availability of suitableplatforms, i t 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 th e payload often obsolete by the time it is

launched. Similarly, deployment ol 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 littlc rcason why prepared H A P Sshould not be capable of being launch ed 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; ovcrload due Lo a large concentration of users, e.g.at a major event.

Platform and payload upgrading. HAPS may be on-station for lengthy pcriods, with some proponentsclaiming 5 years or more . But they can be brought downrelatively readily for maintenance or upgrading of thepayload, and this is a positive featurc allowing a highdeg ree of 'future-proofing'.

Ewironnzentally 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 theTable 1: Compar ison of b road band te r res t r ia l , HAP an d sa te l l it e se rv ices: t y p ica l pa rame te rs

S ta t ion cove rage km up to 200k m > 5 0 0 k m(typical d iameter) u p t o g lo b alCell s ize (diameter) 0.1-1 km I- lO km c. 5 km 400 km m i n i m u mTotal service area spot service nation a lreg ona globa l quasi-globalMa xim um transmission 155MbiU s 25-1 55 Mbit/s ZOO mil l ionin f ras t ruc tureIn-service da te 2000 2003-2008? 2005 1998

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potential benefits of removing th e need for large num ber sof terrestrial m asts and their associated infrastructure .Table uminarises a comparison between terrestrial,

HAP and satellite delivery for broadband se rvices.5 Some i s sues and c ha l l eng es

The novelty of HAP communications calls for some newconcepts in terms of delivery o l serviccs (e.g. B-FWA),raising critical is suc s lor development. And the platformsthemselves present some challenges and potentialproblems.

System leuel requirements. HAP networks forbroadband coniinunication service delivery will require arethi nk of the basic des ign of cellular-type serv ices, withdevelopment focusing upon the frequency planning ofdifferent spot beam layouts, which are subject to wideangular variations and changes in link length, andfrequency rruse patterns for both user and backhaullinks. The network architecture will need to exploitopportun ities of inter-terminal switching directly on theHAP itself as opposed to on the grou nd, and the use ofinter-HAP links to achieve connectivity.

Propagation and diuersity. Services from HAPS havebecn allocated frequencies by the I?u in the millimetre-wave bands, at 47/48GHz (and also at 28GHz in ITURegion 3-predominantly Asia). Propagation from HAIsis not fully characterised a t these highe r frequencies: rainattenuation is significant in these bands, so one of thcmain requirements is to develop rainfall attenuation an dscattering statistics. This will allow appropriate ma rgins 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 an d coding. In order to optimise networkcapacity, suitable modulation and coding sch em es will berequired to serve the broadband telecommunicationservices, w ith specified QoS (Quality of Serv ice) an d BEK(Bit Er ro r Kate) requirem ents, applicable und er differentlink conditions. Adaptive techniques will provide overalloptimum perform ance, using low-rate sch em es involvingpowerful forward err or correction (FEC) coding whenattenuation is severe, up to high-rate multilevelmodulation sch emes wh en conditions are good.Resource allocation and network protocols. Channelassignm ent and resource allocalion sch em es will need tobe developed for the HAP scenario, which is essentiallydifferent froin either a terrestrial or a satellite cellularsccnario. The sch em es have to be lailored to multimediatraffic and also take into account the system topology andchoice of modulalion/coding scheme. T h e 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 UKAN could beused. Integration with terrestrial and/or satellitearchitectures will also require careful planning.Antenms Antenna technology will be critical to BWA

lrom HAPS. A large nuniber of spot beams will berequired, and these may be produced either by anensem ble of ho rn antennas or some form of phase d array.Sidelobe performan ce is an importan t issue, which willallect intercell iritcrfcrence and, ultimately, systemcapacity. At a p lanned fre quency of 48 GHz, this isdemanding technology both for the HAP-based antennaand also for ground terminals.Pla urm station-keepping and stability. The ability of aHAP o maintain position reliably in the face of variablewinds is a major challenge and will critically affect thcviability of communication services. H A P positioning islikely to be repre sented as a cer tain statistical probabilityof remaining within a particular volume, e.g. a locationcylinder. Exaniple figures from the HeliNet programmerelale to 99% arid 999% platform availability withinspecified location limits, and clearly such param eters arenot readily compatible with provision of co mmunicationservices having traditional four nines availability such as99.99%. Some new thinking will be required, perhapsbased on the use ol multiple HAPS arid diversitytechniques.

Stability is another critical issue. Inevitably, the re willbe roll, pitch and yaw of the platform , d u e to turbulence inthe stratosphere; in this regard, largrr craft arc likely toexhibit greater stability. Antcnna pointing on the HAPinay be maintained either through the use of amechanically stabilised sub-platform, which inay bebulky, or though use of electronic steering ol an arrayantenna. This latter technique ollers considerablepotential, but is also kchriologically demancling,especially at the millimetre-wave ban ds likely to be usedfor broadband services.

Handof t M o s t HAP schenics are proposing to usemultiple spol beams over the coverage area, yieldingcapacity through frequency re use . Although a BWAnetwork architecture is likely to have m ainly fixed user s,handoffs may occur as the antenna b eams move du e loplatform motion, depending on the HAP slabilisationtechniques. This is in contrast to conventional mobilecellular schemes, whcrc handoff is invariably due tomotion of the u ser. The size of the cells on the gro und , andthe physical svability o l the HAP antenna pointing, willgove rn how often the se will occur. It may be possible touse fixed antennas on the HAP nd to accommodatemotion simply thro ugh handoff procedures (which couldbe quite rapid). However, delay and jilter limitations forfuture m ultimedia services (cspccially video) may imposemuch m ore stringent constraints on the handoff processthan with conventional 2G or 3G services, and this is atopic of current research.Should ground-based antennas be f i x e or steemble?HAPS will vary in position, both laterally and vertically:the latter pcrhaps deliberately so in order to optimisealtitude to minimise prevailing winds. This movementresults in changes of look angle from the groun d terminal,which can be used to determine whether fixed orsk era ble ground-based antennas are required. Il theangular variation is greater than the beamwidth of theantenna, which will be a met ion of the gain required to

ELECTRONICS hi COMMUNICATION ENGINEERING JOURNAL JUNE 2001 135


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6N

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4 8 12 16 2 24time, h

IFiq. 12 Solar pow er f lux on a HAP a t a height of 17km as a func t ion ofseasonal extreme, t ime and lat i tudeoperate the link, then it is necessary to use a steerablegrou nd terminal antenna. Th e greate st angular variationis immediately below the HAP; however, at this closerange wider antenna beamwidths may be feasible due tofavourable link budgets. Chang es in vertical height inaybe more significant to antennas at the periphery ofcoverage if thes e are correspon dingly higher gain, andhence narrower beamwidth. C learly, the requirement forsteerable antennas will increase terminal costs, but maybe nec ess ary to achieve high link capacity.Payload powel: An important distinction between thedifferent types of HAP is the power available to thepayload. Typically an airship may have in ex ces s of20 kWavailable for the payload, due to the large su rface ar ea onwhich to deploy solar cells. Planes powered byconventional fuel sources (e.g. the HALO scheme byAngel Technologies) will similarly have high poweravailable. By com parison, 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 RI;power, and hence overall capacity, will be constrained.Indeed, solar powered planes have much in common withcomm unication satellites, in te rm s of available powerfrom t he solar panels, payload weight and spa ce availablcon thc platform. Power will need to be used mostefficiently, particularly through careful spot beam andantenna arra y design and power-efficient modulation andcoding schemes.Com pared with a satellite, a HAP will require a high erproportion of the power to char ge the batteries (fuel cells)becau se 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 ang le of the s un relativeto the solar panels between summ er and winter, and thes h o r t winte r day s will have an ad ditional significant effect.Fig. 12 shows the variation in incident solar power per

square metre as a function ofseasonal extreme, time of day, andlatitude.Advanced Technologies Group issuggesting the use o l airships inwhich solar-powered technology isaugmen ted with diesel engines thatcan also be used when station-keeping is a problemL5. Eut thcpower budget is most acute forsolar-powered plane technology,where the available area for thesolar cells and the permissible ma ssfor the fuel cells limit the power tothe payload; this may criticallydeterm ine the platforms viability.6 The w a y f o r w a r dAlthough some of the goals forHAPS are a few years fromrealisation, the re can be little doubtthat aerial platforms will play an increasingly important

role in the delivery of wireless serv ices. A com binatio n oftechnology push from the providers of platforms andapplications pull from the inexorable demand forcoininunications means that it is not so much a questionof if bu t w hen.Com mercial aerial platloriii projects already underw ayinclude the HALO programme, and the ethcrcdaerostat 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 wind sat their operating altitude. Th ere i s also the issue of long-term reliability, with ma nufacture rs postulating on-stationlifetim es of 5years or m ore, which is far longer than thoseof any cur rent aerial devices.

But these factors need to be bakanced with lhceconomic benefits of the service provided and the cost ofthrough-life operation. High link availability may not b erequired of a single HAP in many scenarios, wheth er dueto diversity techniques or because 99.99 ervice is notdemand ed. And w ith the opportunily to bring HAPS backto groun d lor maintenan ce, thc issue of long lilctime maybe less critical.One ol the technical hurd les lor Lhe inan uiacturcrs ofHAPS, whether solar-powered plane or airship, looksl ikely to be the performance of energy storage such a sfuel cells, and there is considerable on-going work in h i sarea. Although much can be learned from scale HAPprototypes, not all of the aerodynamic, structural andene rgy is sue s scale linearly, thus full-size HAP prototypeswill need to be built and tested in ord er to convince usersand investors of their commercial viability.

With the evident opportunities for enhancedconmiunication services, as outlined in this paper, it is tobe anticipated that we shall scc significant developm entsin HAPS for communication service delivery over th e nextfew years.

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A c k n o w l e d g m e n t sSupport for the HeliNet programme is acknowledgedlrom the EU Framework V Programme (IST-lC199-11214), and f rom mcntbers of the HeliNet teamboth alYork and amongour Par lners .Thanks particularlyto John Thorn ton , a nd lo John Jones for his airshipenthusiasm.R ef e renc es1Dudley Labs list of Vi-equency Allocations, May 2001, seehttp://www.dudleylab.corii/frec~aloc.htinl

2 GRACE, I .,DALY N. E., TO ZLR, I: C., and BURR, A. G.:LMLlSfroni high altitude aeronautical platforms. Pi-oc.IEEEGLOBECOM99, Rio de Janeiro, Brazil, 5th-9th December1999,5,pp.2625-2629

3 GRACE, D., DKY . E., IOZER, T. C., BtJRR, A. G., andPEARCE, D. A. J.: Providing multimcdia cominunicationsfrom high altitude platforms, Itzt J . Sat el l. Co w .(accepted for publication)

4 Global VSAT Forum, seelittp://www.gvf.org5 DJUKNIC, G. M., FKEIUENIELDS, J., and OKUNEV, Y.:

Establishing wireless communications services via higli-altitude aeronautical platkn-nix a concept whosc tiinc hascome?,ZEEE Conznzun. Mug September 1997,pp. 128-1356 STEELE, K.: Guest rditorial-an updatc on personalcommunications, Co~iznzur~ ag., Deceiiiber 1992,pp.30-3 1

R.: Aerial platforms: a promisingmeans of 3G coinmunications. IEEE Vehicular kc hnologyConf., Houston, TX 6th-20th May 1999,3 pp.2104-21088 TCOM, seehttp://www.tcomlp.com9 TxlckheedMartin upgrades first of six tethered aerostat radarsites and receives go-ahead on second sit[>. ackh eed MartinPress Release, October 2000. See liltl,://www.locktiee(l

martin.coin/ncws/articles/lO~9~~~2,tinl10 R1-eilling Orhitcr, see http: / /www.bre i tl in~.coin/enR/aero/

orbiter/11Zeppelin, seehttp://www.zcppclin-n t .coin12CargoLifter,sce httl,://www.cargoliLter.coin13Yokosulta CRL, see h t tp : / /w~2 . c r l . go . j p / t / t e~ i i i iZ /14 Aclvanccd Technologies Group, se ehttp://www.airship.corn15 SkyStation, svc hilI)://www.skystation.com16 Iindstrand Ealloons Ltd., see ~ittp://www.lindsirand.co.uk17REHMM: M. A., KRdILlN, E. H., EPPERLEIN, I

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High Altitude Platforms for Wireless Commmunication