get 22 pdf

8/14/2019 Get 22 PDF

http://slidepdf.com/reader/full/get-22-pdf 1/8

ABSTRACT

Radio makes it poss ible to have communicat ion "wi thout wires " This is a highly at t ract ive propos i t ion for mobi le personal

communications, it is increas ingly an economic a l ternat ive to t radi t ional wired phone systems, and is a potent ia l t echno logy fo r h igh-

speed Internet access. But i n the terrest r ial env i ronm ent , radio s ignals are subject to scatter ing and mu l t ipath ef fects that l imi t the

quan t i ty of infor mat ion poss ible to t ran smit in a given band width , as wel l as the dis tances over which it can be commu nicated In

cellular and personal communication systems (PCS), radio coverage is del iberately restr icted further to al low fo r f requency reuse As a

consequence, terrest ria l wireless networks comprise numerou s antenna towers, base s tations, wired or microwave l inks, a nd mob i le

switch ing centers, al l dispersed over wid e geographical areas Satell i tes can provid e wireless coverage wit h m uch less terestrial

inf rast ructure, but only by int rod uc ing cons iderably p roblems of thei r ow n Geosynchronous satell ites require expensive and bulky userterminals a nd int roduce large s ignal delay because of thei r great dis tance Nongeosynchronous satell i tes , because of thei r mo t ion w i th

respect to points on the ground, great ly increase system complex i ty Proposed high-al t i tude aeronautical plat forms (HAAPs) are an

int r iguing al ternat ive From a communicat ions perspective, they wo uld have many advantages over bo th thei r terrest r ia l and satel li te

counterparts I f HAAPs prove to be reasonably s table, rel iable, and no t too costly , they w i l l of ferr cons iderable opportuni t ies fo r wireless

serv ices provis ion, and int roduct ion of innovat ive communicat ions concepts such as cell scanning and stratospheric radio relays.

I

I

Goran M. Djuknic and John Freidenfelds, Lucent Technologies

Yuriy Okunev, G eneral DafaComm, Inc.

here are two well-established methods for providing wire-

Tess commun ication services: terrestrially based systems,

as used in ce l lu la r and persona l co mm unica t ions sys tems(PCS), and satellites. Each concept h as its specific advantagesand disadvantages . Th e high-al t i tude aeronaut ical platform(HA AP) technology, we will argue, would have many of theadvantages of bot h terrestr ial and sa tellite systems, while atthe same tim e avoiding many of th eir pitfalls. Also, it wouldbring advantages of its own, not available in current systems.Table 1 summ arizes key points that will be m ade in this dis-cussion, concerning terrestrial systems, and mobile satellitesystems ba sed on geosynchronous Ear th orb i t (GEO), low-altitude Earth orbit (LEO), and medium-altitude Earth orbit(MEO) satellites.

~ G H L T ~ T U ~ ~R O N A U T ~ C A ~ctive and passive comm unications platform s at high alti-tudes are not a new idea -befo re the 1962 Telstar satel-

l i te , long dis tance te leph one cal ls were m ade by bouncingsignals from the Ec ho, a giant bal loon launched in 1960 topassively reflect broadcasts from the B ell Laboratories facilityat Crawford H il l . Th e focus of this art ic le is a i rborn e plat-forms - i rships, planes , hel icopters , or som e hybrid solu-t ions -which could ope ra te a t s t ra tospher ic a l t i tudes forsignificant periods of time, be low-cost, and be cap able of car-rying sizable, multipurpose commu nications payloads. Of p ar-t i c u la r i n t e r e s t a r e w a y s t o i m p l e m e n t c e l l u l a r iP C S o r

high-speed data networks using technological developments inairborne platforms which raise the p otential for combining theadvantages of geostationarity with terrestrial-systems-like cov-erage and signal delay. Da ta from worldwide measurem ents ofs t ratospheric wind veloci t ies indicate that their minimum,averaging 30 to 40 m/s, occurs between 65,000 and 75,000 ftdepending on latitude. These are long-term averages, and lit-t l e i s known a bou t t ime sca les on the ord er of minutes orhours . This is the bas is for the most often s tated cri t ique ofairborne platforms, namely that they will not be able to with-stand sud den wind gusts, resulting in a tem porary o r total lossof communica t ion . The ins tan taneous power , P, e e d e d t ocounter th e wind force exerted on an airship is

P = 112pC~ScV3

where p is the air density (which at stratosp heric altitudes isonly a few percent of the density at sea level,) Cd is the dragcoefficient, S , is the airship cross-sectional a rea, and v is theinstantaneou s wind velocity. Th e wind direc tion in this layerremains steady for mo st of the year, excep t for a twice yearlychange of 180". Typical wind velocity profiles ob tained by theHigh Resolu t ion Dopple r Imager (HRDI) on the Na t iona lAeronaut i cs and Space Admini s tra t ion ' s (NASA's ) Up perA t m o s p h e r i c R e s ea r c h S a t e l l i te ( U A R S ) c a n b e s e e n a thttp://www.sprl.umich.edu/HRDI/rdi-homepage.htm1.

Platform des igns in the h eavier- than-air c lass benefi tedfrom recent advances in the development of composite mate-rials; com puters and navigat ional sys tems; low-speed, high-

128 0163-6804/97/$10.000 997 IEEE IEEE Communications Magazine * September 1997

Authorized licensed use limited to: P D M College of Engineeirng Downloaded on February 5 2010 at 10:55 from IEEE Xplore Restrictions apply

http://www.sprl.umich.edu/HRDI

http://www.sprl.umich.edu/HRDI

8/14/2019 Get 22 PDF


,i Commu nicat ions technology r iskI

I

I

t Deployment t iming

II' System g row th

I

I

1 System complexity du e to

: mot ion o f components

Operational complexity and cost

I

! Radio channel "quality"

!j

I

' Indoor coverage

[ Breadth of geographical

, coverage

Shadowing f rom terrain

II Communicat ions and power

infrastructure; real estate

I Esthetic issues and health

concerns wi th towers and

I antennas

I

: Public safety concern about

i f ly ing objects

~ .~ .

Table 1. Summa ry comparison of wireless systems.

I

Matu re technology and wel l - Cons iderable new technology for

established indus try LEOs an d MEOs; GEOs sti l l lag supplem ented w ith spot-bea m 1cellular/PCS in volume, cost, and antennas; if widely deployed, I

performance opportuni t ies for spec ial ized

equipmen t (scarining beams to 'follow t raf f ic )

One p l a t f o rm and g round

init ial comm ercial service

Terrestrial wireless technology,

I

Deployment can be s taged;

substant ial in i t ia l bui ld-out to ent i re system i s deployed support typical ly enough for 1prov ide suf f ic ient coverage for

commercial service I

Cell-sp lit t ing to add capacity, System capacity increased only Capaci ty increase throug h spot- Irequ iring system reengineering; by add ing satell i tes; hard ware beam resizing, and add it ional ,

easy equipmen t updatehepa ir upgrade only wi th replacement platforms; equipmen t upgrades

Only user terminals are mobile

Service cannot start before the

satellites relatively easy

M o t i o n o f LEOs and MEOs a

major source of com plexity,

l inks are used I

High for GEOs, an d especially Some proposals require freque nt ILEOs due t o continu al launche s

to replace old or fai led satell i tes or to rest pi lots)

Mo t ion lo w to moderate (s tab i li ty

characterist ics to be proven)especially wh en intersatell i te I

Well -understood

landings of p lat forms (to refuel

Rayleigh fadin g l imits distance Free-space-l ike channel w ith Free-space-like channel at

and da ta rate; p ath loss up to Ricean fading ; p ath loss roug hly

qual i ty thro ugh proper antenna l imi ts spectrum ef fic iency

placement

Substantial coverage achieved Generally no t available (hig h- Substantial coverage possible

distances comparable toI

50 dbfdecade; go od s ignal 20 dB/decade; GEO distance terrestria l I

I

I

I

pow er s ignals in I r id ium to

calls)

Large regions in GEO; glob al for

LEO and ME0 p la t fo rm

Problem only at low look angles

t r igger r inging only for incoming I

A few kilometers per base station Hund reds o f ki lometers per

Causes gaps in coverage;

requires addi t ional equipmen t I

Numerous base stations to be

si ted, powered, and l inked bycables or m icrowave

Many sites required for coverage Earth stat ions located away from

and capac ity ; "smart" antennas popu lated areas

mig ht ma ke them mo re v is ible;

cont inued publ ic debates

expected

No t an issue

Similar t o satell ite

Single gateway collects traff ic

f rom a la rge area

Comparable t o satell i te

!Similar to satell i te I

I

Occasional concern about space

junk fal l ing to Earth overhead can raise s igni f icant ILarge craf t f loat ing or f ly ing

object ions. ~. ~ . -. . ~

IEEE Communications Magazine September 1997 12 9


8/14/2019 Get 22 PDF


U F i g u re 1. General NAAP-ba sed system layout.

altitude aerodynamics; propulsion systems, including internalcombustion and solar-powered; and the exceptional interest ofthe m ilitary in unmanned aerial vehicles (UAVs). T he goal oft h e D e f e n s e A d v a n c e d R e s e a r c h P r o j e c t s A g e n c y ' s(DARPA's) Airborne Communications Node (ACN) program

is to develop a 900-lb commun icat ions payload for a U AVhaving 42-hr endur anc e at altitudes above 60,000 ft.* Gen eralAtomics Aeron aut ical Systems (San Diego, Cal i fornia) hasalready developed several UAVs, an d it is likely that this p ro-gram will be the f i rs t to demon strate the airborn e-platformcommunica t ions concept [5]. More informat ion on re la tedprojects can be found by fol lowing the l inks from the E uro-pean S pace Agency (ESA) Web page on high-al t i tude longe n d u r a n c e (H A L E ) d a t f o r m s . a t ht tp: / /www.estec .esa .nl /

l

h a l e m / m / h a le .h t m .An early des ign from Jet P ropul-

sion Laboratories (JPL) involved theus e of pilotless planes circling abovet h e c o v e r a g e a r e a f o r w e e k s o rmon ths without landing, powered bymicrowave beams from the gro und.

T h i s a p p r o a c h d r aw s o n wire les spower t ransmiss ion research in thespace-s tat ion context , but the p rob-lems are in transmission efficiency,grou nd-sta tion cost, safety of flyingd r o n e s o v e r p o p u l a t e d a r e a s , a n ddanger to othe r a i rc ra ft from high-p o w e r m i c ro w a v e r a d i a t i o n . S o m ecurrent approaches involve pi loteda n d c o i i v e n t i o n a l l y p o w e r e dl ightweight planes in a holding pat-t e r n a t 7 0 ,0 0 0 f e e t , b u t t h e i r t i m ea l o f t w o u ld b e l i m i t e d to a b o u t 8h o u r s d u e t o f u e l c o n s t r a i n t s a n dhuman factors.

I n t h e l i g h t e r - t h a n - a i r a r e n a ,S k y sa t C o m m u n i c a t i o n s N e t w o r kCorpora t ion (New York) recent lyc a m e w i t h a p l a n f o r r e g i o n a ltelecommunications services over anarea up to 600 mi in diameter, using

Descnptions of the program are at ISX

Corporation and DARPA Web pages,

mac0 dc ia comlisolbattlelairborne html

and www dalpa milldocumentsl

procure97 liso html , respectively

airship-based communications platforms positioned at 70,000ft. Th e airships, a patente d design by AV-Intel Inc. from C ana-da, are unman ned, neutrally buoyant, an d conventionally pow-e r e d d i r i g i b l e s . T h e e n g i n e s w i ll h a v e t o b e o f s p e c i a lconstruction becau se of low air pressure at the planned al t i-tude. T hey will be 975 ft long and 150 ft in diam eter, have amaximum cruise spee d of 56 knots, and be able to carry up to2000 lbs of co mm unications payloa d. The se airships will be

capable of staying aloft for several mon ths, but actual flightduration will mostly depen d on th e wind speeds at a particularlocation.

The most ambitious plan so far has been developed by SkyStation International Inc. (http://www.skystation.com),hichin tends to p lace a ne twork of 250 s t ratosphe ric s tat ions infixed positions 100,000 ft above Ea rth, su ppo rted by severalthousand ground control and switching centers [ l ] . The net-work could se rve many mi l l ions of smal l and inexpens ivefixed, portable , and mobile communicat ions terminals forInternet access and video telephony service. Stat ions in theform of 30-ton helium-filled dirigibles will be maintained incorrect position using the G lobal Positioning System (GPS).Sola r pane l s in tegr a ted in to the d i r ig ib le sk in wil l supplypower to Corona Ion Engines (needed to counter stratospher-

ic winds and keep dirigibles in place). Frequency ban ds pro -posed for use are 47.2 to 47.5 G Hz (uplink) and 47.9 to 48.2GH z (downlink), and Sky Station Inc. has applied for interna-tional spectrum allocation in this band. The payload will con-sist of a beamform ing phased array antenna, and a bank ofregenerative processors for transmissionireception, modula-tion/demodulation, encodingidecoding, multiplexingidemulti-plexing, and switching functions.

Th e RotoStar concept develop ed by Silver Arrow (Israel) also

U F i g u re 2.Ai rbomeplat j fompay lo~d quipment in a C D M system. -_ _ ~ - - - ~ - - -I

I500 MHz

1 base stationequ ipment I

I i-.

I

I

I

Single-beaman ten n a

(to airborne-plat form)

t I

PSTN I

I- - - - - - - ----500 M Hz

- S O L - __IIU F i g u re 3 . Ground equipment in a M P - b a s e d CDMA system.

130 IEEE CommunicationsMagazine September 1997

Authorized licensed use limited to: P.D.M. College of Engineeirng. Downloaded on February 5, 2010 at 10:55 from IEEE Xplore. Restrictions apply.

http://www.estec.esa.nl/

http://www.skystation.com/

http://www.skystation.com/

http://www.estec.esa.nl/

8/14/2019 Get 22 PDF


involves an unm anned dirigible orbiting at 70,000f t , b u t w i th h e l i c o p t e r - l i k e “ s m a r t w i n gs ”(http:/lwww.weizmann.ac.illconsolar/spsp.htm1). Conventionally powered RotoStarswould be able to s tay aloft for fo ur days, andinternal combustion engines are currently underdevelopm ent; solar-powered platforms woulde n d u r e m a n y m o n t h s o n s t a t i o n , b u t t h e i r

e n g i n e s s t il l r e q u i r e a t e c h n o lo g i c a l b r e a k -through to be implemented.

Final ly, researche rs from the U nivers ity ofCalifornia at Los Angeles and Rockwell Cospora-tion, in partnership with NASA, are developing ap r o t o t y p e f o r a f l e e t o f s o l a r - p o w e r e dautonomous aircraft which would f ly at highal t i tudes in a geese-l ike V-formation. And inJapan, the Ministry of Posts and Telecommuni-cat ions has cond ucted a s tudy on t e a m i n g u pwith the private sector and un iversities to devel-O D a mobile phone network covering the en tire

4 Figure4. ttenuation in free-space and terrestrial-typepropagation.

IJapanese archipelago, based on 20 solar and/or fuel-cell-pow-ered airships positioned a t 20-km altitude.

Th e choice of energy source is of fundam ental im portance.

Fossil fuel is heavy, and therefore expensive to lift and main-t a i n a t a l t i t u d e . T h e v e r y l a r g e s iz e o f a l i g h t e r - t h a n - a i rHA AP u sing fossil fuel, for example, is dtermined by the ne edto lift the fuel th at will kee p it in place - roponent s havesuggested designs that would ope rate for anywhere from a fewdays to a few months between refueling. Solar energy has con-s iderable appea l , part icularly if we assum e that , for e i therbuoyancy or ae rody namic l if t in the th in a tmo sphere , th eHA AP will contain large surfaces suitable for collectors. U pto 1300 W/m2 are available at the equ ator, which is quite sub-stantial even if we assume solar cell efficiencies of 10-15 per-cent. The problem is that energy has to be stored for overnightuse. Adding bat teries , such as l i thium-ion at about 110 Whr/kg, makes for a very large (an d expensive) HAAP.

SYSTEM ARCHITECTUREND PARAMETERSGENERAL RCHITECTURE

A typical HAA P-based com municat ions system s tructu re isshown in Fig. 1.The platform is positioned above the cover-age area. L ighter- than-air HAAPs are kept s tat ionary, whileairplane-based HAA Ps are flown in a tight circle. For broad-cast applications, a simple anten na beams signals to terminalson the ground. Fo r individual ized commu nicat ions , such astelephony, “cells” are created on th e ground by some beam -forming technique in ord er to reuse channels for spatially sep-arated users, as is done in cellular service. Beamforming canbe as sophisticated as the use of phased-array antennas, or asstraightforward as the use of lightweight, possible inflatableparabol ic dishes with mechanical s teering. In t he case of a

m o v in g H A A P i t w o u l d a l s o b e n e c e ss a r y t o c o m p e n s a t emotion by electronic or mechanical means in orde r to keepthe cells stationary, or to “hand off” connections between cellsas is done in cellular telephony.

ONBOARDEQUIPMENT

Depending on the application, there are m any ways in which aHAAP-based com munications system could be implemented.A typical design would seek high reliability, low power con-sumption, and m inimum weight and size for the onb oard por-tion of the system. Tha t would lead to an architecture whichplaces most of the system on the ground , for example, by lim-i ting a i rborne comp onent s to a mul t i channel t ransponder ,

user-beam and feeder-beam antennas, and associated antennainterfaces.

Fig. 2 shows a code-division multiple access (CDMA) sys-

tem built around a standard satellite-like transponder band-width of 500 M Hz . T h e t r a n s p o n d e r b a n d w i d t h c a naccommodate up to 50 an tenna beams with 8 spread spectrumcarriers/beam (assuming 1.25 MHz carrier bandw idth). Carriers ignals coming from a ground cel l ( i .e . , f rom a part icularbeam) and received by the on board a ntenna a re f i rs t ampli-fied in low-noise amplifiers (LNA s). They are then limitqd tothe s tandard 10 MHz bandwidth by band-pass filters (BPFs),and frequency-division multiplexed (m uxed). Before transmis-sion to the ground station, multiplexed signals are amplifiedin the high-power amplifier (HPA), BPFed to the transponderbandwidth, and passed thro ugh the diplexer (D). Signal pathin the opposite direction is similar and includes an additionaldemult iplexing (demu x) s tage. If co mm ercial off-the-shelfequipmen t is to be used onboard, it will have to be placed in achamber w i th c l imate and a i r -pres sure cont ro l to preventfreezing, overhe ating (due to reduced heat convection), an ddielectric breakdown.

GROUND NSTALLATIONS

Comm unicat ions between the HA AP and the ground wouldtypically be concentrated into a single ground installation, orperhaps into two locations for redundancy. There would beconsiderable advantage to collocating RF units, base stations,and mobile switching centers (MSCs).

The ground system in Fig. 3 cor responds to the onboardequipment from Fig. 2. Carrier signals coming from the air-b o r n e s t a t i o n a r e f i l t e r e d by a B P F , a m p l i f i ed i n L N A s ,demultiplexed in the demux, and passed to CD MA base sta-tions. It should be emphasized here that a base station in thiscase consists only of a radio chan nel frame, since there is no

need for power-amplif ier and antenn a-interface frames forevery base station; a common wideband power amplifier andan antenna will serve al l the col located base s tat ions . Frombase s tat ions , the s ignals are passed in the usual ma nner tot h e m o b i le M S C a n d p u b l i c s w i tc h e d t e l e p h o n e n e t w o r k(PSTN). The return signal path toward the airborne station issimilar, except for the inverse multiplexing op eration in themux and high-power am plification by the HPA.

HAAP-BASEDCOMMUNICATIONS YSTEMPERFORMANCE

O ne of the most attractive features of an airbo rne platform-based wireless system is its very favorable path-loss ch arac ter-istic relative to eithe r terrestrial o r satellite systems. In Fig. 4,

IEEE Communications Magazine September 1997131


http://slidepdf.com/lwww.weizmann.ac.illconsolar

http://slidepdf.com/lwww.weizmann.ac.illconsolar

8/14/2019 Get 22 PDF


a typical pa th loss vs . dis tance is shown for terres tr ia l andnonterrestrial systems. For n onterrestrial systems, we assumefree-space path loss, inversely proport ional to the square of

the distance, 1/72, or 20 dB/decade. I n ter restria l systems pathloss is a s tochast ic variable (often de termined empirical ly,)and we t ake th e mos t commonly as sumed ra t io , l / r4 , or 40dB/decade [2]. Th e more favorable propagation characteristicsin satel l i te sys tems ar e more t ha n offset by their gre at dis -tance. Even LEO dis tances cause path losses comparab le tothose in a relatively large terrestrial cell: path loss to a LEO

satellite at 900 km a ltitude is equal to the p ath loss along thegroun d at 1 0 km distan ce. By contrast, from an airship at 22km a l t i tude to a point on the ground direct ly below i t , pathloss is the same as at th e edge of a relatively small terrestrial-system cell with approximately 2 km radius.

In addi t ion to free-space-l ike propagat ion, energy budgeto f t h e u s e r l i n k in a n a i r b o r n e - b a s e d s y s te m i s f u r t h e renhanced by Ricean, not Rayleigh type, fading and high-gainplatform antennas . As a resul t , the sys tem can o perate withconvent iona l ce l lu la r /PCS handse t s and re la tive ly s impleonboard equipment . To show tha t power requi rements foro n b o a r d e q u i p m e n t ( o f t h e f u l l -f l e d g ed s y st e m e x a m p l edescribed in this sect ion) ar e within l imits of the onb oard

amplifier and power supply, we compare the transmitter pow-ers of terrestrial and airborne systems. Consider the situationprese nted in Fig. 5, where th e ante nna tower of a terrestria lsystem is up to 150 ft in height, while H AA P is at an a ltitudeof 1 2 to 15 mi. Th e maximum coverage radius , d H , for a i r-bo rne sys tems is approximately 7.5 mi to m aintain th e lookangle, a, of at least 10”.The antenn a gain in terres tria l sys-tems is GT = 10-17 dB, while the gain of an airbo rne anten nais GH = 30-35 dB (we use capital G for anten na gain through-out, since the units will be obvious from the context).

I n o r d e r f o r a t e r r e s t r i a l a n d a HAA P-based sys tem tomaintain th e same qual i ty of service, i t i s required that thesignal-to-noise ratios (SNRs) be th e same at the e dge of theirrespective coverage areas. SN R in both cases is directly pro -por t iona l to t ransmi t t e r power P and anten na ga in G, an dinversely prop ort iona l to th e power of dis tance R be tweentransmitting and receiving antennas,

P x G

R nSN R __

The p ath loss exponent , n, has values from 2 to .5 [2]. Fo rfree-space propa gation n = 2, in a subu rban-type environmentn = 3.84, and for highly urb an areas the value is c lose to 5.This estimation obviously refers to the average SNR; takingfades into account gives additional gain for airb orne systemsdue to R icean fading d i s tr ibut ion . Equa t ing the respec t iveSNRs, we obtain the HAA P transmit ter power PH expressedin terms of the terrestrial-based transmitter power PT,

PH =--PTT Ri

GH RF

From the typical parameter values i t can be seen that therequired transmit power per voice channel is 50 mW . Conse-quent ly, we need 1.5 W/C DM A carrier (assuming 30 voicec h a n n e ls / C D M A c a r r i e r ) . T h e t o t a l a i r b o r n e t r a n s m i t t e rDower is therefore

carriers Watt

beam carrier50 beams x 8____ ~l.5-=600 W

Even at 10 percent efficiency the transmitter‘s power con-sumption is 6 kW, which is well within th e capabilities of anyairbo rne platform. T he capacity of this example system is cal-culated as:

carriers CDMA channels50 beams x 8 x 30 = 12,00 0 voice channels

beam carrier

meaning that t he system can serve 240,000 subscribers (assum-ing 0.05 Erlangs loadisubscriber).

It has a l ready been poin ted out tha t HA AP-based t e l e -phone systems would avoid the cost of communications linksrequired to con nect geographical ly dispersed base s tat ionsthat a re requ ired in terrestrial systems. This centralized archi-tecture can also result in improved efficiency of chann el uti-lization- large trunk group is more efficient tha n m ultiple

small ones . This is bes t d escribed in term s of an example .Consid er a subu rban-typ e coverage are a 1.50 mi in diam eter,serving 100,000 cus tomers . A t busy-ho ur t raff ic loading of0 .05 Er langs icus tomer , the to ta l of fe red load i s A = 5000Erlangs. A terrestria l cellular system serving the area wouldhave approximately 200 cells, assuming an average cell size of10 mi in d iam ete r . For s impl ic ity , we t ake tha t the of fe redtraffic is equally distributed amon g cells, and the traffic loadper cell is a = 2.5 Erlangs. Th e num ber of wireless channels,n , n e e d e d t o s e r v e t h is p o p u l a t i o n i s o b t a i n e d f r o m t h eEr lang-B formula , B(n, a ) [ 3 ] .F o r B = 0.02, the block ingp r o b a b i l i t y , t a b l e s s h o w t h a t n = 3 4 c h a n n e l s i c e l l a r erequired, or 6800 channels for the ent i re coverage a rea. If a

HA AP-ba sed system is used to provide cellular coverage, thetota l offered lo ad is served by a cen tral faci l ity. In such a

case , the num ber of channe l s does not have to be d im en-sioned according to the busy-hour traffic but rather accordingto the average traffic in the area, since all available channelscan be shared among all the cells and local traffic peaks ares m o o t h e d o u t ( p r o v i d e d , o f c o u r s e , t h a t t h e s y s t em h a sdynamic channel allocation capability). Using a conservativees t imate, the average t raff ic is taken to be 70 percent of itsp e a k v a l u e . To p r e s e r v e t h e s a m e g r a d e o f s e r v i ce , i t isrequi red tha t B ( N , 0 . 7 A ) = 0.02 , and th e to ta l number ofchannels for the ar ea is N = 3467. This saving in th e num berof channe l s requ i red means a reduc t ion in the num ber ofbase stations when using HAAP-b ased systems, or an increasein capacity for the same nu mber of base stations.

A po tential for CD MA system capacity increase is seen ini m p r o v i n g t h e a c c u r a c y of t h e p o w e r c o n t r o l a l g o r i t h m .Namely, the m ore e rrors the algorithm makes, the more chan-nel interferen ce occurs, and the less the achievable capacity(the number o f active voice channels). O ne decibel of err or inpower con trol, for example, is equivalent to 1 0 percent degra-dation in capacity 141. Two m ain factors in fluence the errorsin power control: the dynamic range of signal atten uatio n andthe dis t r ibut ion of fas t fades . Both factors are reduced in aHAAP-based system compared to the conventional terrestrialsystem. The dynamic range of signal attenua tion in a n ordi-nary ter res tria l cell is 60-80 dB; of that, 40-SO dB is prop aga-tion-induc ed difference, and 20-30 dB is du e to fading. In aHA AP system-formed cell, the dynamic range will be 12-22dB, where 2 dB is propag ation-indu ced difference an d 10-20dB are d ue to fading. In terrestrial cells, fast fades are typical-ly Rayle igh d i s t r ibuted . In con t ras t to th a t , channe l s in aHAAP-based system are characterized by Ricean distribution

of fades (like satellite channels,) yielding an additional energygain which is a function of the Ricean factor, K . Typically, therange of K is 0-20 dB, and th e larger its value, the higher th eenergy gain in HAAP-based systems compared to terrestrialones, where K is close to zero (due to Rayleigh fading).

H AW ’ ISSUES

The focus of this article is on the communica t ions sys temsthat might one day be placed on HA AP technology. The mostcritical issues, however, are rel ated t o th e platform itself- tstill remains to be demonstrated that placing a p la tf o m at strato-spheric altitude and “fi ing” t reliably above the coverage area ispossible, and that i t can be done in a cost-efficient, safe, and

1 3 2 IEEE Communications Magazine * September 1997


8/14/2019 Get 22 PDF


The othe r type of application in which a HAAP’slarge coverage area ought to be adv antageous is intelecommunications for areas having a low densityof customers, es pecially when prospective customers’specific geographic locations are unknown. Th e costpe r cus tomer of installing fixed facilities, such aswire , increases wi th decreasing customer densi ty .This is especially the case when facilities need to beinstalled in locations where custom ers may or maynot actually materialize. In cellular, PCS, and wire-less systems, cost per subscriber tends to be less sen-sitive ato traffic densities than it is for wireline. But

sustained manner. The se issues ap ply equally to high-flyingplanes and lighter-than-air craft.

Comm ercial aviation h as amply demons trated sustained,safe, efficient op eration of all manner of aircraft, but at whata l t i tudes? The legendary U-2 spy plane an d its derivativeshave shown that planes can fl y at stratospheric alt i tudes, butfor how many hours a t a t ime, and a t what cost? Dir igiblesfloat safely over football fields; but would th eir basic designbe extensible to stratospheric operation? Weather balloonsa r e r o u t i n e l y l a u n c h e d t o n e a r - s t r a to s p h e r i c h e i g h ts f o rmonths or even years at at time, but what if their position hadto be controlled precisely? Although considerable me asure-

ments of strato sphe ric winds have been mad e, how confidenta re we ab ou t gus t s l a st i ng a f ew hours o r a f ew minu te s?Wh at abou t taking off and landing in varied weather condi-tions? Wh at legal and regulatory hurdles m ight be erected bypubl ic pol icy bodies su ch as th e Fede ra l Av ia t ion Agency,concerned with safety, or the Fcderal Communications Com-mission, concerned with sp ectrum usage? W hat proof will berequ ired , and how long wi ll i t take , for even a successfulHAA P to gain wide market acceptance?

Clearly, a successful HAA P is not just the integ ration ofavailable technologies, as some platform designers would haveus believe, but rather a considerable extrapolation. Nonethe-less, the extrapolation is not so great as to remove our opt i -mism that solutions will be foun d, and H AA Ps will one daybecome reality.

Th ere are also som e communications issues that will need

to be addressed - nd there will undoubtedly be others thatare not now app arent. For example, design and implementa-tion of the required m ul t ibeam, steerable a i rborne antennawill not be an easy task. But th ere is a wealth of experience todraw upon from bui lding such antennas for L EO sa te l li tesand high-flying planes. The antenna will have to be capable ofp roduc ing up t o seve ra l hundred “ s t icky” beams , t ha t i s ,beams that will maintain their posit ion on the ground as th eHA AP flies in t ight circles or bobs in the wind. A challengethis is, but on e that should be doable through some combina-t ion of mechanica l and e lec t ronic control (a phased-arrayantenna mounted on a motorized chassis).

APPLICATIONS

e large coverage area of a HA AP would te nd to give it a

rompetitive advantage over terrestrial alternatives in two

types of applications. One is where many widely separated cus-tonzersreceive the same communication, as in entertainmentbroadcasting. Th e success of direct broadcast satellite (DBS )systems even in the pre sence of widely deployed cable T V inthe Uni t ed S t a t e s is indicative of t h is oppor tun i ty . HAA Ptechnology might be able to achieve many of the benefits ofGEO -based DBS without having to transmit quite so homoge-neously over so large an area. Unlike GEO-based technology,upstream chann els should also be possible with HA AP, whichwould enable interactive TV and Internet access capabilities.

cost per subscriber of terres trial wireless systems also riseswhen traffic densities get so low th at many underutilized basestations have to be installed to achieve geographic coverage.In such low-density situations, both satellite and HAAP-basedsolutions become relatively mo re com petitive vs. their te rres-trial counterparts. In these circumstances, satellites have theadvantage of even greater geographic coverage than HA APs.However, HAA Ps provide a quite large coverage area withoutgiving up indoo r signal penetrat ion as with satell i tes. Also,HA APs, unlike satell i tes, should b e able to use m uch of thesame equipm ent as terrestrial systems (especially the wirelessphones themselves).

In cer tain applications, a single HAAP’s coverage area ofabout 10 0 km ra dius would coincide nicely with a metropoli-tan ar ea. Fo r cellular or PCS services, for example, coverageof a metropo litan area is typically required in ord er to make itpractical to advertise and support commercial service. Terres-trial wireless systems fo r similar init ial coverage, althoughmuch quicker to deploy than traditional wireline networks,would require con siderably more t im e and investment thanthe HAAP-based alternative. In this kind of rapid-deploymentscenario, the H AAP -based system could even be temporary,to be rep laced by more cost-effective terrestrial wireless o rwireline technology as traffic volumes grow. Factors otherthan cost of build-out would also have to be weighed. HAAP swould eliminate highly visible anten na towers that sometimescause public resistance to terrestrial systems, but would intro-duce concerns about objects falling from the skies. Because of

the improved vantage point , HAA P-based systems shouldhave bet ter signal quality generally, and fewer “holes” in radiocoverage, particularly when com pared to ter restrial systemsthat have not ye t been “f ine- tuned.” However , coverage int u n n e l s a n d d e e p b a s e m e n t s w o u ld s t i ll r e q u i r e a d d e drepeaters or microcells.

A H AA P system whose coverage area is not too ambitious(e.g., look angle of 15”at th e edge of coverage) will affordsomething closer to line-of-sight com munications than a typi-cal terrestrial wireless system. In terms of applications, thismay be the best way to utilize som e of the higher frequenciesnow being considered, such as LMDS, 38 GHz, 47 GHz, andso on. Such applications as very wideband Internet access,entertainment video and audio, and videoconferencing mightbe enabled by this technology.

HA AP technology, because i t can-be made to cover large

areas quickly without having to rely on facilities in the servicearea, could be well suited to applications that are temporaryor limited in scope. Examples of such services would be cover-age for one-time or seasonal events, services for remote areas,tempo rary services in natural disasters or em ergencies, andthe maritime example discussed below.

Whatever the application, HAAPs could have some opera-tional advantages over their terrestrial or satell i te counter-p a r t s . S i n c e ty p i ca l H A A P s , u n l i k e s a t e l l i t es , w o u l d b eperiodically brought to ground , repairs and upg rades of hard-ware would be possible. Depending on the specific design, of

IEEE Comm unications Magazine September 1997 13 3

Authorized licensed use limited to: P.D.M. College of Engineeirng. Downloaded on February 5, 2010 at 10:55 from IEEE Xplore. Restrictions apply.

8/14/2019 Get 22 PDF


course, this accessibility may beless convenient than for terrestri-al systems. Assuming th at m osto f t h e c o m m u n i c a t i o n s e q u i p -m e n t i s c e n t r a l l y l o c a t e d i n aground s tat ion, system adm inis-t rat ion might actual ly be eas ierthan for typical dispersed terres-

trial systems. The single origin oft h e H A A P ’ s b e a m s t h a t f o r mc o v e r a g e c el ls o n t h e g r o u n dopens up the potent ial for f lexi-b l e c e l l c o n f i g u r a t i o n w i t h

Figure 6. ing-shaped cells for airbome systems.

onboard programmabil i ty- process that should be mu cheasier than splitting a terrestrial cell and redesigning radiopat terns to acco mm odate growth in terres tr ia l cel lular sys-tems. The fixed location of the HA AP could also be advanta-geous for situations in which en d-use r radios on the grounduse directional anten nas that ar e pointed to the signal source,as in a wireless local access system, for example. In that situa-tion, the end-user radios could be reassigned to different cells(beams) without having to redirect their antennas. Of course,any operat ional advantages in terms of the commu nicat ionswould have to be weighed agains t the cos t and operat ionalcomplexity of the H AA P itself.

OVEL SOLUTIONS

This section exploress some novel solutions and applicationsfor HAAP-based systems.

RING-SHAPED CELL CLUSTERING SIMPLIFIES THE

DESIGNF STEERABLE MULTIBEAMNTENNAS

Traditional arrang ement of cells in a hexagonal pattern cover-ing the plane is a natural consequence of the way wireless cov-e r a g e i s p r o v i d e d i n t e r r e s t r i a l s y s t e m s . H o w e v e r , w h e ncoverage is established from an an tenn a mounted on a circlingplane, or on an airship rotating arou nd its central axis due to

s tratospheric winds , the “n atural” cel l shape is a geom etricpattern invariant to such platform movements. Such coverage,m a d e up of a set of con cen tric rings, is shown in Fig. 6. Th isarrangem ent is possible s ince cell shapes and their relat ivepositions are of no consequence to the operation of a cellularsystem an d, in fact, even has certain advantages over th e tra-ditional p a t t e r n . In tha t case each ce l l has jus t o ne or twoneighbors, which simplifies handoff algorithms.

CELLSCANNING ELIMINATES

COMPLEX AIRBORNENTENNASND

SAVES POWER BY FOCUSINGON SMALLER AREAS

T h e H A A P i s p a r t i c u l a r l y s u i t e d t o t a k i n g a d v a n t a g e o femerging “smart antenna” technology. In fact, compared to itsterrestrial counterparts in which sectorized anten nas (beams)send a nd receive radio waves traveling along th e gro und, theHAAP’s favorable “look angle” means that its energy can bemo re readily focused onto a confined are a. A schematic viewof this appro ach is shown in Fig. 7; i t is similar to conceptsu s e d i n s o m e LEO s a t e l l i t e p r o p o s a l s ( e . g . , T e l e d e s i c ) .Depending on the appl i ca t ion , i t can be a r ranged th a t thebeam “visits” a particular cell at regular or irregular intervals.Regular visits are suitable for rea l-time applications an d ser-vices to m eet quality-of-service crite ria like delay and delayvariance. Rand om t imes between vis its can be used in non -time-critical applications such as Inter net access.

While th e beam is pointing to o ne of the cells, information

is exchanged between user ter-minals and the comm unicat ionsequipment on the platform: thet ra f fi c in ten ded for t ha t ce l l isbuffered in the interval betweensuccessive b eam visits, an d th enbeamed down in a burst man ner;likewise, the info rmation in use r

t e rmina l s is buffe red unt i l thecontrol signal from the platformindicates that th e beam is point-i n g t o t h e c e l l , t ri g g e r i n g t h eb e a m i n g u p o f i n f o r m a t i o n

burs ts . If on e beam is not eno ugh t o sat‘isfy the capaci ty ordelay requiremen ts , two or mo re beams can be used to scanthe cells in a staggered m anner. A v ariant of this approac h isa system in which beams have different roles: “scou t” beamsscan the cells in search of those in which there are da ta readyto send in user terminals; “ traffic” beam s visit only the cells“marked ” by scout beams, e i ther randomly or according to

some priority mechanism. All the described modes of opera-tion will require a n increase in buffering capabilities of end-use r equipm ent an d modi f i ca tions in cur rent a i r in te r faceprotocols.

STRATOsPHERIc RAD0- ELAY

MARITIMEOMIMUNICATIONSYSTEM

Providing high-quality telecommunications service, includingvoice and da ta t ransmiss ions , for m ari t ime vessels cross ingworld oceans is one of the mo st complex problems in telecom-munications engineering. At present, only GEO satellite sys-tems provide multichannel, long distance, reliable maritimecomm ercial commu nicat ion service. However, th e exis t ingmaritime satellite user terminals a re comparatively bulky, an dsatellite-based service is expensive. Use of the H AA P concep tcould solve the problem for a large part of th e world oceanfleet. Namely, there are scveral major world ocean shippinglanes: across the North A tlantic; connecting W est and S outh

Africa; between Africa arid Asia; between A frica and Aus-t r a l i a ; a n d b e t w e e n A s i a a n d N o r t h A m e r i c a . C h a i n s o fW s ositioned above these lanes would operate as strato-spheric radio-relay links, terminated by coastal radio centersat each en d of the transo ceanic link (Fig.e 8). Operat ing fre-quenc ies for use r , feeder . and in te r -HA AP l inks a re in thebands com monly used in s a te l l it e systems , cur rent or pro -posed, but a re only an example and are not essent ial to theope ration of the ma ritime system. The system could providemultichannel, reliable, cost-efficient maritime communicationservice, including voice, dat a (e.g., Intern et access) , video,paging, and broadcasting.

Figure 7 . Cell scanningfvom the airb omep latfom .

134 IEEE Communications Magazine 0 September 1997


8/14/2019 Get 22 PDF


Figure 8.Radio-relay maritime communication system.

P l a t f o r m s i n F i g . 8 a r e s h o w n a s s t a t i o n a r y , b u t t h esam e concep t wou ld be poss ib le t o imp lemen t even i f ap l a t f o r m m o v e d a t a r e l a t i v e l y l o w s p e e d a l o n g a r a c e -track-like path, for example, with endpoints close to l and-based gateways.

SUMMARY AND CONCLUSIONShis discussion has argued th at high alt i tude aerona uticalT latforms (HAAPs) would be of considerable interest for

wireless communications. Their position in the sky would givethem many of the favorable characterist ics of satell i tes, butwithout the distance penalty. Their position in the sky wouldalso let them avoid the r adio grou nd sc atter of terrestriallybased systems, while still being about as close, especially interms of path loss, as terrestrial an tennas. Thus, indoor cover-age should not be a problem, as it is with even LEO satellites,and technology designed for terrestrially based wireless sys-tems should be applicable. Since they collect traffic into a sin-gle point on the ground, HA APs would reduce the amo untand geographic extent of ground-based equipment vs. theirterrestrial counterparts. H AAP -based systems would generally

be mo re accessible for repai rs and upg rades than sa te l l itestha t have been launched , and , whi le the a i rb orne por t ionsmay be less accessible than terrestrially based systems, theHAA Ps' te rrest r ia l com ponen ts would be more accessiblesince they would be m ore centralized. Th e minimum systemsize for a s ingle HA AP corresponds well to a metropol i tanmarketing region, facilitating rapid initial deployment for cov-erage so that commercial service can be started. The vantagepoint of HAAPs and the centralization of their beamformingappara tus would open new possibi l i t ies for smart antennatechnology such as beam scanning.

Alas, if only there existed such technology as HA AP ! ASwe have already observed, it remains to be demonstrated thatplacing a platform at stratospheric altitude and "fixing" it reli-

a b ly a b o v e t h e c o v e r a g e a r e a i spossible, and th at it can be done ina cost-efficient, safe, and sustainedmanner . Nonetheless, consideringthe number and diversity of HAAPp r o p o s a l s , o n e is t e m p t e d t obelieve that s om e of them will besuccessful.

ACKNOWLEDGMENTSTh e authors wish to thank R. Myer,A. U . MacRae , F. Ludtke , and D.Hou for the i r va luable input andmany discussions on the topic; andt o t h e a n o n y m o u s r e v ie w e r s f o rt h e i r c a r e f u l r e a d i n g o f t h emanusc r ip t and t hough t fu l com-ments.

REFERENCES

[ I . Dicks, "Frequency Sharing Aspects o f N etworks Using StratosphericStations in th e Fixed and Fixed Satellite Services," Doc. no. USWP-4N32, Radiocommun. SG, ITU-R, June 27, 1996.

[2 ] W. C. Y. Lee, Mo bile Cellular Telecommunications, 2n d ed., New York:McGraw-Hill, 1995.

[3 ] R. R. Martine, Basic Traffic Analysis, AT&T and Prentice Hall, 1994.[4] A. J. Viterbi, CDMA: Principles of Spread Spectrum C ommunication,

[5 ] C.A. Robinson, Jr., "H igh-Capacity Ae rial Vehicles Aid Wireless Commu -Reading, MA: Addison-Wesley, 1995.

nications," Signal, Apr. 1997, pp. 16-20,

BIOGRAPHIES

GORANM. DJUKNICM I ([email protected]) received his Diploma and M.S.

degrees from the University of Belgrade, Yugoslavia, and a Ph.D. from CityCollege, New York, all in elec trical enginee ring. Since 1995 he has beenwit h Lucent Technologies, where he evaluates the po tential and op portun i-ties in satellite-based and other innovative schemes for establishing wirelesscommunications services. He also develops new wireless data applications,including telemedicine and multim edia. Previously, he worked as an assis-tan t professor a t Stevens In st itu te o f Technology, as a researcher in th earea o f wired and wireless com munications at th e Technical Institute, Bel-grade, and at Belgrade Telephone.

JOHN FREIDENFELDSs technology director for wireless in Lucent Technologies'Network Systems business, focusing on ne w marke t opportun ities. He hasheld po sitions in B ell Laboratories, AT&T, and N ew York Telephone; in tech-nology, strategic planning, and market assessment for telecomm unicationsservices and equipmen t. He has a Ph.D. in operations research from Stan-ford, an M.S. in electrical engineering and operations research from MIT,and a B.S.E.E. from the University of Connecticut.

YURIYOKUNEVMI obtained his M.S. and Ph.D. in electrical engineering fromth e St. Petersburg State University of Telecom munications. For more than20 years he held the position o f head of the D igital CommunicationsResearch Laboratory at the University. In 1995 he joined Bell Laboratories,Lucent Technologies, where he worked in the development and applica-tions o f wireless technology, and CDMA in particular, for satellite systems.He is currently with General DataComm Inc., where he develops high-bit-rate modems for data transmission systems. He is a member of the NewYork Academy of Sciences.

IEEE Communications Magazine September 1997 135

get 22 pdf

Documents