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TECHNOLOGY DEVELOPMENT FOR WIRELESS COMMUNICATIONS SYSTEM USING

STRATOSPHERIC PLATFORM IN KOREA

Jong-Min Park1, Bon-Jun Ku, Yang-Su Kim and Do-Seob Ahn

1Broadband Wireless Communication Technology Department, Radio & Broadcasting Research Laboratory, ETRI

161 Gajeong-dong, Yuseong-gu, Daejeon, 305-350, Republic of Korea, jongmin@etri.re.kr

Abstract Wireless communications system using

stratospheric platform becomes the focus of worldsattention, since it would have lots of advantages over the

conventional wireless communication infrastructure such as

terrestrial and satellite communications systems. The

research and development for putting the system to practicaluse is ongoing in some countries. This paper describes the

results and the status of research on the technology for

stratospheric communications system in Korea.

Keywords stratospheric platform, HAPS, multimediamobile communication, fixed service, IMT-2000

I. INTRODUCTION

The need and importance of new wireless communication

infrastructure which can provide high-speed multimediamobile communication service to users who are not satisfied

with low-speed data and voice service provided by existing

wireless network are rapidly increasing. Under the

circumstances, there are some efforts to pioneer innovativewireless communication networks using stratospheric

platform, which is more often called HAPS (High Altitude

Platform Station)[1]. HAPS is defined as a station located

on an object at an altitude of between 20-50km and at a

specified, nominal, fixed point relative to the earth. The

stratospheric communications system has the advantages of

both satellite communication system featuring flexibility of

network planning and construction, wide bandwidth, widecoverage and so on, and terrestrial communication system

featuring timely supply meeting the demands, easy

maintenance and so on. And the cost for fabrication and

operation of airship is competitive compared with satellitesystem and it is a great advantage that airship can be

recovered and repaired when system failure occurs. On

account of the active development of the HAPS in some

countries and its potential interference to other countries, the

matters including technical and regulatory issues are treated

as the international common issues.

The World Radio Conference in 1997 (WRC-97) already

designated the bands 47.2-47.5 GHz and 47.9-48.2 GHz for

use by HAPS in the fixed service [2]. As the result of WRC-

2000, additional studies, limited to a number of countries in

Region 3, within the 18-32 GHz frequency range, in

particular the bands 27.5-28.35 GHz and 31-31.3 GHz wereagreed. Furthermore, a new Resolution 734 was developed,

which requested studies on the feasibility of using HAPS

within the fixed service and the mobile service in the bands

above 3 GHz allocated exclusively for terrestrial

radiocommunications[3]. In addition, a new Resolution 221

was adapted to consider the use of HAPS providing IMT-

2000 in the bands 1,885-1,980 MHz, 2,010-2,025 MHz and

2,110-2,170 MHz in Regions 1 and 3 and 1,885-1,980 MHzand 2,110-2,160 MHz in Region 2[4].

Since the stratospheric communications system using an

airship can provide observation/monitoring/surveyingservice as well as communication service, considerabledemands can be expected in the near future. And, if the

mobility of airship can be properly utilized, communication

network can be easily configured at limited area in the case

of disaster and especially in the case of unification of Korea.

This paper describes the results and the status of research on

the technology for stratospheric communications system in

Korea.

II. TECHNOLOGY OF SERVICES AND SYSTEM

According to the analysis of the stratospheric weather

including wind velocity in Korea, it turns out that the proper

operating altitude range of the airship is 20.6 km (50 hPa) ~

23.8 km (30 hPa) above the sea level. If we also consider

other factors such as required weight and volume of a

platform, 20.6 km is the most suitable for the altitude of

domestic stratospheric platform. It can offer a line of sight

and a free-space-like channel path with short propagationdelay, and it may allow the use of low-power, small size of

terminals. Lighter-than-air airship should be kept stationary

within a sphere approximately with a radius of 0.5 km.

Beamforming can be as sophisticated as the use of phased-array antennas, or as straightforward as the use of multi-

horn antennas. When the platform is moving, it would alsobe necessary to compensate motion by electronic or

mechanical means in order to keep the cells stationary, or to

hand-off connections between cells as is done in cellular

telephony.

The major communication services for the domestic

terrestrial communication system using HAPS would be as

shown in Figure 1.

In the fixed service, there are high speed internet service and

leased line service. For the high speed internet service which

IP(Information Provider) provides high speed internet and

0-7803-7589-0/02/$17.00 2002 IEEE PIMRC 2002

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data service (512 kbps 2 Mbps) for home and office,

hundreds of cell which an airship can cover are divided intosome cells for IP and proper transmission methods were

analyzed. Leased line service can provide lines for mass ofdata transmission between enterprises with the transmission

speed of 2 Mbps 45 Mbps. In order to accomplish it, high

gain antenna and high power should be used for user

terminal. Considering the matters, proper transmission

methods are also analyzed. Mobile service using HAPS can

provide mobile internet access and videophone service.Required transmission speed is 64 kbps 2 Mbps and the

proper transmission methods were analyzed. Research and

development for the use of HAPS providing IMT-2000 has

been also conducted, since WRC-2000 adopted newresolution 221. Because IMT-2000 HAPS system can

provide lots of cells more than one thousand with one

airship platform to replace the existing terrestrial basestations, it has high advantage economically over the others.

Additional services include broadcasting, remote

sensing/monitoring and spectrum monitoring. Transmission

parameters including transmission rate, modulation,

bandwidth, power, antenna size and so on are established foradditional services.

III. ELEMENTARY TECHNOLOGY FOR SYSTEM

A. Communications payload

Since the beam coverage by the antenna loaded on anairship platform is composed of numerous small cells,

frequency reuse is possible and high gain beam makes small

antenna be able to be used for terminal at ground station.

Two types of horn antennas can be used principally for the

HAPS in the frequency bands 48/47 GHz: a dual-mode

conical horn and a corrugated one. If two separate antennas

are used to transmitter and receiver and the operating

frequency band of every antenna is sufficiently narrow inthe V-band, simple dual-mode horn of these can be chosen

to meet the requirements. The bandwidth of corrugated horn

has more wide band performance than that of dual mode

horn. Concrete parameters of horns should be carefully

analyzed to clarify its availability for the integrated

Transmit/Receive (T /R) antenna. Besides the additionalelements in the feeder system are required to create an

integrated T /R antenna. There are a lot of difficulties in

realizing a high isolation between T /R channels. The

separate transmitting and receiving antennas are considered

for our system. A dual mode horn has technologicaladvantages in the manufacture and low cost in comparison

with a corrugated horn. Dual-mode conical horn was

fabricated and the radiation beam patterns were measured inthe frequency bands of 48 GHz. And seven-beams antenna

breadboard with positioner was fabricated and the

performance was evaluated in the anechoic chamber.

Fig. 3. Radiated pattern of horn module in Fig. 2.

For IMT-2000 terrestrial system utilizing HAPS, the main

approach to multibeam antenna utilizes APAA with DBF to

satisfy the low sidelobe level resolved by WRC-2000 and to

reduce the degradation of Equivalent Isotropic Radiated

Power (EIRP) and gain to noise temperature ratio (G/T) due

to the station keeping variation of the airship by adjusting

the beam direction and shape easily. The behavior of

radiation beam patterns by the variations of the amplitude

PSDNPSTN

Gateway

WWW

StratosphericStratospheric

PlatformPlatform

Comm. Link Bt. PlatformsComm. Link Bt. Platforms

DisasterMonitoring

RadioMonitoring

MeteorologicalObservation

AdditionalAdditionalServicesServices

Service Provider,Private Company

router

BroadcastingBroadcasting

High SpeedHigh Speed

InternetInternet

512k ~ 2Mbps512k ~ 2Mbps

High SpeedHigh Speed

Mobile MultimediaMobile Multimedia64k ~ 512kbps64k ~ 512kbps

Leased LineLeased Line

512k ~ 2Mbps,512k ~ 2Mbps,

2M ~ 45Mbps2M ~ 45Mbps

SOHOHandheld, Mobile

Major Comm. Services

PSDNPSTN

Gateway

WWW

StratosphericStratospheric

PlatformPlatform

Comm. Link Bt. PlatformsComm. Link Bt. Platforms

DisasterMonitoring

RadioMonitoring

MeteorologicalObservation

AdditionalAdditionalServicesServices

Service Provider,Private Company

routerrouter

BroadcastingBroadcasting

High SpeedHigh Speed

InternetInternet

512k ~ 2Mbps512k ~ 2Mbps

High SpeedHigh Speed

Mobile MultimediaMobile Multimedia64k ~ 512kbps64k ~ 512kbps

Leased LineLeased Line

512k ~ 2Mbps,512k ~ 2Mbps,

2M ~ 45Mbps2M ~ 45Mbps

SOHOHandheld, Mobile

Major Comm. Services

Fig. 1. Communications/additional services using HAPS

-20 -15 -10 -5 0 5 10 15 20

Azimuth (deg.)

-20

-15

-10

-5

0

5

10

15

20

Elevation

(deg.)

-15-10-8-7-6-5-5-4-3-2-1Level (dB)

Positioner

Controller

Horn module

Fig. 2. Horn module for frequency bands 48 GHz

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and phase errors in channels was examined through the

statistical simulation. As the result, average envelop withaccount of deviation above average, and worst realization

radiation beam pattern with maximum sidelobe level are

compared to the required radiation beam pattern for HAPS.

It was assumed that 14 x 14 cross dipole planar array

antenna and circular array antenna with 73 cross dipoleelements were utilized.

It turns out that one panel of DBF antenna module having a

diameter of 4.6 m needs about 1,600 elements to realize acell having a diameter of 1km and about 500 beams are

required to provide service at elevation angle of 70. And

Figure 4 and 5 are the schematic diagrams of receiver and

transmitter for digital beam forming respectively.

Fig. 4. Schematic of DBF receiver

Fig. 5. Schematic of DBF transmitter

A cross dipole microstrip antenna, which has the simple

radiating element and feeding structure of quadrature hybrid,

which can achieve the circular polarization, was fabricated

and the radiation beam pattern was measured. From the

measured value of it, the radiation beam pattern for the

linearly array antenna with 4 elements was simulated by

using antenna array factor about the central and two offset

beams. And a linear array antenna in which the radiators

(each radiator is a pair of crossed dipoles) are arranged in alinear array to form three beams: one central beam on the

boresight and two offset beams was fabricated and the

performance was evaluated as shown in Fig. 6 and 7.

B. Stratospheric airship

Since the development of the airship requires state-of-the-art

technology such as ultra-light weight, durability against an

extreme environment, high confidence, and high

effectiveness, there is no airship put to practical use for

HAPS yet in the world. In Korea, the design was started

from the total ground weight assumption, i.e. the sum of themission payload and the weight of the stratospheric airship

itself. The airship is assumed to be a non-rigid type with a

pressurized balloon. The configuration of the airship is

determined by the drag minimization requirement andrequired buoyancy. The design iteration starts with the

initial guess for the weight and size of the airship and

payload, and stops when the total weight is equal to the

buoyancy on the ground. The payload specification used for

the design iteration includes the weight of 1.2 ton(Communication Payload: 1 ton, Additional Payload: 0.2

ton) and the required power of 11 kW (Communication

Payload: 10 kW, Additional Payload: 1 kW). Throughoptimization procedure, airship length, diameter, volume

and the take-off weight of the total system were calculated.

Three different pressure altitudes of 50 hPa, 40 hPa, and 30

hPa are considered for the parametric study. The pressure

altitude of 50 hPa is considered as the best choice in the

manufacturing and operation aspect. And small size airship

ToBaseband

Modems(through the

Switches)

K

RadiatorArray

N

1 AnalogDown-

Converter

IFSampler

ADC

DigitalDown-

Converter

Rx

DigitalBeam-

Former

Analog

Down-Converter

IF

Sampler

ADC

Digital

Down-

Converter

1

11

K

FromBaseband

Modems

(through

the

Switches)

Tx

DigitalBeam-Former

DigitalQuadrature

Modulator

DigitalQuadratureModulator

HighSpeed

DAC

HighSpeedDAC

AnalogUp-

Converter

AnalogUp-

Converter

N

Radiator

Array-90 -60 -30 0 30 60 90

deg.

-25

-20

-15

-10

-5

0

E,

dB

Fig. 7. Measurement results of radiation pattern with

antenna module in Fig. 6.

RF Subsystem

Cross dipole element

Fig. 6. Antenna module for IMT-2000 frequency bands

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of 5 m was fabricated for the proof of concept as shown in

Fig. 8.

Fig. 8. Small size airship for the proof of concept

IV. ITU-R ACTIVITIES FOR HAPS ISSUES

Korea has also been participating the activities in ITU-Rstudy groups concerning HAPS issues. The major workingparties in ITU-R dealing with HAPS issues include WP9B,

WP9D, JWP4-9S and WP8F.

For the study within fixed services, we evaluated

interference from HAPS to radio-relay station in the above

3GHz bands with two aspects; one is the interference

distribution from the PFD of HAPS airship into the radio-

relay route and the other the interference effects from HAPS

ground stations into the radio-relay station[5]. The formeranalysis results provide the PFD limit level of HAPS airship

on the earths surface as specified that of satellite system,

and show that the existing PFD criteria, which is specified

for satellite systems, is completely satisfied up to 30 km of

the HAPS altitude, but for the above 30 km, new criteria

should be established. However, the latter analysis results

provide the coordination criteria as a function of the azimuth

angle to provide sharing between the HAPS ground stations

and the radio-relay station, and show that the distance

between the radio-relay station and HAPS nadir can be

specified from 60 km up to 253 km for 50 dBW/MHz of

transmit power in any azimuth angles.

Fig. 9 and 10 show the interference environment for the

study and the evaluation result respectively.

Fig. 10. Coordination area between radio-relay station and

HAPS nadir

For the study in IMT-2000 service, we proposed a guidance

to estimate the co-channel interference effects into the

terrestrial cellular IMT-2000 system, which is tower-based

system from HAPS IMT-2000 system within the boundaryof an administration[6].

For the study on the sharing problems between HAPS IMT-

2000 system and the cellular IMT-2000 system, closed

forms for the interference to a cellular mobile station and toa cellular base station were derived by approximating the

HAPS IMT-2000 antenna pattern. Using the forms, the I/N

values were calculated according to the number of users per

cell, maximum antenna gain and transmission power.

Fig. 11 shows the interference environment in IMT-2000

forward link for the study.

Fig. 11. Interference environment in IMT-2000 forward link

As the results of the study, in the case of interference to a

mobile station, it turns out that the station is susceptible to

the effects of HAPS as a base station to provide IMT-2000

service and that the longer separation distances from HAPS

nadir should be prescribed with the above parameters. If the

interference from the same and adjacent cells in cellular

system becomes stronger up to I/N = 10 dB, in the

-300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300

-300

-250

-200

-150

-100

-50

0

50

100

150

200

250

300P

HG=-50(dBW/MHz)

Distan

ce,r

Distance,r

Cellular

service areaHAPS

service area

hn

Fig. 9. Interference environment from HAPS to radio-relay

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boundary between two services, the maximum gain of

HAPS IMT-2000 antenna should be 35 dBi rather than 50dBi, since the antenna pattern for the maximum gain of 50

dBi has no margin to I/N = 10 dB even though the

separation distance is far away from HAPS nadir. However,

in the case of interference to a base station, since the

interference from HAPS mobile stations is susceptible to thedistance from HAPS nadir, HAPS IMT-2000 service can be

provided if its service area will not be overlapped with the

cellular service area.

Fig. 12 and 13 shows the calculated results of I/N versusdistance from HAPS nadir with HAPS antenna gain in IMT-

2000 forward link and reverse link respectively.

Fig. 12. I/N vs. distance from HAPS nadir with HAPSantenna gain in IMT-2000 forward link

Fig. 13. I/N vs. distance from HAPS nadir with HAPS

antenna gain in IMT-2000 reverse link

V. CONCLUSION

The stratospheric communications system definitely has

advantages not only in the satellite communication aspect

but also in the terrestrial mobile communication aspect. And,

if HAPS proves to be reasonably reliable and stable,

considerable and various wireless services could be

expected worldwide. In this paper, the R&D status oftechnology required for wireless communications system

using stratospheric platform in Korea is presented.

Conceptual design of wireless communication system using

stratospheric platform and some experiments for

performance evaluation have been conducted. In order to put

HAPS system into practical use as a new telecommunication

infrastructure in Korea, lots of problems, such as validatingunproven technologies and driving up-to-date technologies

in some parts have to be solved.

ACKNOWLEDGEMENT

Korean Ministry of Information and Communications

sponsored this study.

REFERENCES

[1] Do-Seob Ahn et al., Conceptual Design of the Domestic

Broadband Wireless Communication Network Using the

Stratospheric Platform,Proceedings of KICS, 1998, pp.1183-

1187.(in Korean)

[2] ITU-R Resolution 122, Use of the bands 47.2-47.5GHz and

47.9-48.2GHz by high altitude platform stations (HAPS) in

the fixed service and by other services and the potential use of

bands in the range 18-32GHz by HAPS in the fixed service,

1997[3] ITU-R Resolution 734, Feasibility of use by high altitude

platform stations in the fixed and mobile services in the

frequency bands above 3GHz allocated exclusively for

terrestrial radiocommunication, 2000

[4] ITU-R Resolution 221, Use of high altitude platform stations

providing IMT-2000 in the bands 1,885-1,980MHz, 2,010-

2,025MHz in Region 1 and 3 and 1,885-1,980MHz and 2,110-

2,160MHz in Region 2, 2000

[5] ITU-R Document 9B/TEMP/75, WORKING DOCUMENT

TOWARD A PRELIMINARY DRAFT NEW

RECOMMENDATION Interference analysis from high

altitude platform stations (HAPS) to radio-relay stations in the

above 3GHz

[6] ITU-R Document 8F/TEMP/233(Rev.1), WORKING

DOCUMENT TOWARDS A PRELIMINARY DRAFT NEW

RECOMMENDATION Interference analysis from HAPS

systems to cellular systems to provide IMT-2000 service

55 65 75 85 95 105 115 125 135 145

-30

-25

-20

-15

-10

-5

0

5

10

15

20

I/N=10%

I/N

[dB]

Distance from nadir [km]

The number of cellular & HAPS users per cell : 200The number of interfered tier : 5

HAPS cell radius : 2.75kmcellular cell radius : 1kmcellular power : 1mW

Only cellular

HAPS power : 1mW

HAPS power : 100mWHAPS power : 1000mW

55 60 65 70 75 80

-30

-25

-20

-15

-10

-5

0

5

10

15

2025

30

35

40

45

50

I/N=10%I/N

[dB]

Distance from nadir [km]

The number of cellular & HAPS users per cell : 200

cellular & HAPS power : 1mW

cellular cell radius : 1km

Only cellular

-3dB point of Gm 23dBi

-13dB point of Gm 23dBi-3dB point of Gm 35dBi

-13dB point of Gm 35dBi

-3dB point of Gm 50dBi

-13dB point of Gm 50dBi