TECHNOLOGY DEVELOPMENT FOR WIRELESS COMMUNICATIONS SYSTEM USING STRATOSPHERIC PLATFORM IN KOREA

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1 TECHNOLOGY DEVELOPMENT FOR WIRELESS COMMUNICATIONS SYSTEM USING STRATOSPHERIC PLATFORM IN KOREA Jong-Min Park, Bon-Jun Ku, Yang-Su Kim and Do-Seob Ahn Broadband Wireless Communication Technology Department, Radio & Broadcasting Research Laboratory, ETRI 6 Gajeong-dong, Yuseong-gu, Daejeon, 3-3, Republic of Korea, jongmin@etri.re.kr Abstract Wireless communications system using stratospheric platform becomes the focus of world s attention, 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 practical use 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, multimedia mobile communication, fixed service, IMT-2 I. INTRODUCTION The need and importance of new wireless communication infrastructure which can provide high-speed multimedia mobile 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 innovative wireless communication networks using stratospheric platform, which is more often called HAPS (High Altitude Platform Station)[]. HAPS is defined as a station located on an object at an altitude of between 2km 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, wide coverage 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 satellite system 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 997 (WRC-97) already designated the bands GHz and GHz for use by HAPS in the fixed service [2]. As the result of WRC- 2, additional studies, limited to a number of countries in Region 3, within the 8-32 GHz frequency range, in particular the bands GHz and GHz were agreed. 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 22 was adapted to consider the use of HAPS providing IMT- 2 in the bands,88-,98 MHz, 2,-2,2 MHz and 2,-2,7 MHz in Regions and 3 and,88-,98 MHz and 2,-2,6 MHz in Region 2[4]. Since the stratospheric communications system using an airship can provide observation/monitoring/surveying service as well as communication service, considerable demands 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 2.6 km ( hpa) ~ 23.8 km (3 hpa) above the sea level. If we also consider other factors such as required weight and volume of a platform, 2.6 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 propagation delay, 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. km. Beamforming can be as sophisticated as the use of phasedarray antennas, or as straightforward as the use of multihorn antennas. When the platform is moving, it would also be 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. 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 /2/$7. 22 IEEE PIMRC 22

2 PSTN PSDN Comm. Link Bt. Platforms Broadcasting Gateway WWW High Speed Mobile Multimedia 64k ~ 2kbps Handheld, Mobile Leased Line 2k ~ 2Mbps, 2M ~ 4Mbps High Speed Internet 2k ~ 2Mbps Major Comm. Services Stratospheric Platform Additional Services Meteorological Observation Service Provider, Private Company Disaster Monitoring Radio Monitoring data service (2 kbps 2 Mbps) for home and office, hundreds of cell which an airship can cover are divided into some cells for IP and proper transmission methods were analyzed. Leased line service can provide lines for mass of data transmission between enterprises with the transmission speed of 2 Mbps 4 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-2 has been also conducted, since WRC-2 adopted new resolution 22. Because IMT-2 HAPS system can provide lots of cells more than one thousand with one airship platform to replace the existing terrestrial base stations, 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 for additional services. SOHO Fig.. Communications/additional services using HAPS router analyzed to clarify its availability for the integrated Transmit/Receive (T /R) antenna. Besides the additional elements 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 technological advantages 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 in the frequency bands of 48 GHz. And seven-beams antenna breadboard with positioner was fabricated and the performance was evaluated in the anechoic chamber. Elevation (deg.) 2 Controller Positioner Horn module Fig. 2. Horn module for frequency bands 48 GHz - III. ELEMENTARY TECHNOLOGY FOR SYSTEM A. Communications payload Since the beam coverage by the antenna loaded on an airship 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 in the 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 Azimuth (deg.) Level (db) Fig. 3. Radiated pattern of horn module in Fig. 2. For IMT-2 terrestrial system utilizing HAPS, the main approach to multibeam antenna utilizes APAA with DBF to satisfy the low sidelobe level resolved by WRC-2 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 -6-7

3 and phase errors in channels was examined through the statistical simulation. As the result, average envelop with account 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 4 x 4 cross dipole planar array antenna and circular array antenna with 73 cross dipole elements were utilized. It turns out that one panel of DBF antenna module having a diameter of 4.6 m needs about,6 elements to realize a cell having a diameter of km and about beams are required to provide service at elevation angle of 7. And Figure 4 and are the schematic diagrams of receiver and transmitter for digital beam forming respectively. Cross dipole element RF Subsystem Radiator Array IF Sampler ADC Rx Beam- Former To Baseband Modems (through the Switches) Fig. 6. Antenna module for IMT-2 frequency bands K N IF Sampler ADC - Fig. 4. Schematic of DBF receiver E, db High Quadrature Speed Up- Modulator DAC From Baseband Modems (through the Switches) K Tx Beam- Former Quadrature Modulator High Speed DAC Fig.. Schematic of DBF transmitter Up- 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 a linear 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 Radiator Array N deg. Fig. 7. Measurement results of radiation pattern with antenna module in Fig. 6. HAPS yet in the world. In Korea, the design was started from the total ground weight assumption, i.e. the sum of the mission 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 and required 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.2 ton (Communication Payload: ton, Additional Payload:.2 ton) and the required power of kw (Communication Payload: kw, Additional Payload: kw). Through optimization procedure, airship length, diameter, volume and the take-off weight of the total system were calculated. Three different pressure altitudes of hpa, 4 hpa, and 3 hpa are considered for the parametric study. The pressure altitude of hpa is considered as the best choice in the manufacturing and operation aspect. And small size airship

4 of 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-R study groups concerning HAPS issues. The major working parties 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 radiorelay route and the other the interference effects from HAPS ground stations into the radio-relay station[]. The former analysis results provide the PFD limit level of HAPS airship on the earth s 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 3 km of the HAPS altitude, but for the above 3 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 6 km up to 23 km for dbw/mhz of transmit power in any azimuth angles. Fig. 9 and show the interference environment for the study and the evaluation result respectively. Distance,r<km> P HG =(dbw/mhz) Distance,r<km> Fig.. Coordination area between radio-relay station and HAPS nadir For the study in IMT-2 service, we proposed a guidance to estimate the co-channel interference effects into the terrestrial cellular IMT-2 system, which is tower-based system from HAPS IMT-2 system within the boundary of an administration[6]. For the study on the sharing problems between HAPS IMT- 2 system and the cellular IMT-2 system, closed forms for the interference to a cellular mobile station and to a cellular base station were derived by approximating the HAPS IMT-2 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. shows the interference environment in IMT-2 forward link for the study. Cellular service area φ hn ÂÃÄ HAPS service area ÂÃÄÅÃÆÂÆÇÈÉ ÂÉÆÄÅÃÇÉÂ ÉÆÄÄÄÉÄÅÃÇÉÂ ÂÄÇÄ θ ϕ ÂÃÄÅÆÇÈÉÂ Fig. 9. Interference environment from HAPS to radio-relay Fig.. Interference environment in IMT-2 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-2 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 = db, in the

5 boundary between two services, the maximum gain of HAPS IMT-2 antenna should be 3 dbi rather than dbi, since the antenna pattern for the maximum gain of dbi has no margin to I/N = 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 the distance from HAPS nadir, HAPS IMT-2 service can be provided if its service area will not be overlapped with the cellular service area. Fig. 2 and 3 shows the calculated results of I/N versus distance from HAPS nadir with HAPS antenna gain in IMT- 2 forward link and reverse link respectively. I/N [db] I/N [db] I/N=% Fig. 2. I/N vs. distance from HAPS nadir with HAPS antenna gain in IMT-2 forward link Distance from nadir [km] I/N=% The number of cellular & HAPS users per cell : 2 cellular & HAPS power : mw cellular cell radius : km Only cellular -3dB point of Gm 23dBi -3dB point of Gm 23dBi -3dB point of Gm 3dBi -3dB point of Gm 3dBi -3dB point of Gm dbi -3dB point of Gm dbi The number of cellular & HAPS users per cell : 2 The number of interfered tier : HAPS cell radius : 2.7km cellular cell radius : km cellular power : mw Only cellular HAPS power : mw HAPS power : mw HAPS power : mw 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 of technology 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 validating unproven 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 [] Do-Seob Ahn et al., Conceptual Design of the Domestic Broadband Wireless Communication Network Using the Stratospheric Platform, Proceedings of KICS, 998, pp (in Korean) [2] ITU-R Resolution 22, Use of the bands GHz and GHz by high altitude platform stations (HAPS) in the fixed service and by other services and the potential use of bands in the range 8-32GHz by HAPS in the fixed service, 997 [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, 2 [4] ITU-R Resolution 22, Use of high altitude platform stations providing IMT-2 in the bands,88-,98mhz, 2,- 2,2MHz in Region and 3 and,88-,98mhz and 2,- 2,6MHz in Region 2, 2 [] ITU-R Document 9B/TEMP/7, 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.), WORKING DOCUMENT TOWARDS A PRELIMINARY DRAFT NEW RECOMMENDATION Interference analysis from HAPS systems to cellular systems to provide IMT-2 service Distance from nadir [km] Fig. 3. I/N vs. distance from HAPS nadir with HAPS antenna gain in IMT-2 reverse link