communication via stratosphere

MOBILE COMMUNICATION VIA STRATOSPHERE -A REVOLUTION IN THE WIRELESS COMMUNICATIONS

g. Krishna teja, r. karthik, [e-mail] VEC, kavali.

ABSTRACT:

“ANTENNAS MAKE THE RELATIONSHIP AMONG THE PEOPLE THROUGH COMMUNICATION BUT BREAK THE INDIVIDUAL’S RELATIONSHIP WITH THE WORLD THROUGH RADIATION”

Today we are in danger due to health hazards caused by mobile radiation. Recent surveys and experts confirmed that continuous exposure to mobile radiation causes brain tumors, cancers, headache and stress feeling. Not only human beings but also birds, animals are facing lot of problems due to mobile radiation exposure ,but many Mobile Network operators establishing Antennas in and around us without meeting the guidelines of Government. Then how to reduce the effect of the radiation caused by the antennas, where to construct these antennas ? The answer is somewhat crazy i.e., in SKY. Recent research and experiments by NASA and JAPAN suggested that we can establish these antennas in the SKY i.e., in stratosphere where the velocity of wind is 10-15 miles/hr. These antennas in sky are termed as HAPS (High Altitude Platform stations). These platforms are established above the region of air-traffic control in stratosphere and are used not only

for mobile communication purpose but also in Broadband services, monitoring volcanic eruptions, flood effected regions with less investment compared to Satellites sent to outer space.
The focus of this paper is on high altitude platform stations (HAPS) such as solar platforms which could operate at stratospheric altitudes for significant periods of time, be low-cost, and be capable of carrying sizable, multipurpose communications payloads. Of particular interest are ways to implement cellular/PCS or high-speed data networks in airborne platforms. From the communications perspective, HAPs have many advantages over both their terrestrial and satellite counterparts. If HAPs prove to be reasonably stable, reliable, and not too costly, they will offer considerable opportunities for wireless services provision, and the introduction of innovative communications concepts.

INTRODUCTION
The HAPS, known also as HALE (High Altitude Long Endurance) stations or SPR (Stratospheric Platform Radio) are reusable unmanned aeronautical vehicles (UAV) at stratospheric altitudes that offer an alternative to the terrestrial and satellite telecommunication systems. They promise access to modern telecommunication services in many situations at lower costs and shorter deployment times. The HAPS can equally well be used in both, mobile and fixed services. High antenna masts have been used since the beginning of radio, and satellite technology has put the antennas at altitudes of hundreds or even thousands kilometers. The HAPS aim at exploiting potential benefits of intermediate altitudes between those used by the terrestrial and satellite technologies. The European Space Agency (ESA) and International Telecommunication Union (ITU) have been involved in this area since the late 1990s. The Wireless World Research Forum (WWRF), created to formulate visions on strategic future research directions and to generate, identify, and promote research, has included HAPS into their program in 2001. With all that involvement, the perspectives of practical application of that technology are becoming realistic.
On a basis of various concepts proposed, the ITU has elaborated a common understanding of HAPS. Accordingly, the HAPS are understood as stations “located on an object at an altitude of 20 to 50 km and a specified, nominal, fixed point relative to the Earth
HAPS are to be positioned well above commercial airspace at an altitude that is high enough to provide service to a large footprint, providing broadband (and narrowband) services with minimal ground network infrastructure. The common vision foresees that HAPS systems will

consist of one or more quasi-stationary HAPS, each associated with several ground stations, and with numerous mobile and fixed subscriber stations. Each HAPS can deploy a multi-beam antenna capable of projecting numerous spot beams within its potential coverage area. The HAPS stations will inter-work among themselves, and with other networks, terrestrial, satellite, public and private
The Stratosphere
The stratosphere is a part of the Earth’s atmosphere, which consists of several distinct layers. The lowest one is the troposphere. It extends from the Earth’s surface to the tropopause about 10 to 18 km in altitude, depending on the season and geographical position. In the troposphere, the air temperature generally decreases with height. The air pressure decreases from about 1000 hPa at the sea level to about 100 hPa at the tropopause altitude. Approximately 80% of the total air mass resides here, and almost all weather phenomena.
The stratosphere is the next layer, extending from the tropopause to the stratopause at about 50 km. The ozone layer resides here and more than 99% of the total air mass is concentrated in the first 40 km from the Earth’s surface. The stratosphere is characterized by a high static stability associated with increase of temperature with height. The pressure decreases further to reach about 1 hPa at the stratopause.

GENERAL ARCHITECTURE
A typical HAPS-based communications systems structure is shown .
The platform is positioned above the coverage area. There are basically two types of HAPS. Lighter-than air HAPS are kept stationary, while airplane-based HAPS are flown in a tight circle. For broadcast applications, a simple antenna beams signals to terminals on the ground. For individualized communication, such as telephony, "cells" are created on the ground by some beam forming technique in order to reuse channels for spatially separated users, as is done in cellular service. Beam forming can be as sophisticated as the use of phased-array antennas, or as straightforward as the use of lightweight, possible inflatable parabolic dishes with mechanical steering. In the case of a moving HAPS 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.
HAPS-BASED COMMUNICATIONS SYSTEM PERFORMANCE
One of the most attractive features of an airborne platform-based wireless system is its very favourable path-loss characteristic relative to either terrestrial or satellite systems.

A typical path loss vs. distance is shown for terrestrial and non-terrestrial systems. For nonterrestrial systems, free space path loss is inversely proportional to square of distance. In terrestrial systems, path loss is a stochastic variable(often determined empirically) and ratio is 1/r4. The more favourable propagation characteristics in satellite systems are offset by the great distance. Even LEO distances cause path losses comparable to those in a relatively large terrestrial cell. path loss to a LEO at 900 km altitude equal to path loss along ground at 10 km. An airship at 22 km altitude to a point on ground directly below it, path loss is same as at the edge of a relatively small terrestrial system cell with approximately 2km radius.

ON BOARD EQUIPMENT

Depending on the application, HAP-based communications system could be implemented in many ways.

Fig:A code-division multiple access (CDMA) system built around a standard satellite

A typical design will seek high reliability, low power consumption and minimum weight and size for the onboard portion of the system. That would lead to an architecture which places most of the system on the ground by limiting airborne components to a multi channel transponder, user-beam and feeder-beam antennas and associated antenna interfaces.
The figure shows a code-division multiple access (CDMA) system built around a standard Satellite-like transponder bandwidth of 500 MHz. The transponder bandwidth can accommodate upto 50 antenna beams with 8 spread spectrum carriers/beam (assuming 1.25 MHz bandwidth). Carrier signals coming from a ground cell (ie., from a particular beam) and received by the onboard antenna are first amplified in low-noise amplifiers (LNAs). They are then limited to the standard 10MHz bandwidth by band-pass filters(BPFs), and frequency division mulitplexed. Before transmitting to the ground station, multiplexed signals are amplified in the high-power amplifier (HPA), BPFed to the transponder bandwidth and passed through the diplexer (D). Signal path in the opposite direction is similar and includes an additional demulitplexing stage. If commercial off-the-shelf equipment is to be used onboard, it will have to be placed in a chamber with climate and air-pressure control to prevent freezing, overheating due to reduced heat convection) and dielectric breakdown.

GROUND INSTALLATIONS
Communications between the HAPS and the ground would typically be concentrated into a single ground installation or perhaps into two locations for redundancy.

Fig: The ground system
There would be considerable advantage to collocating RF units, base stations and mobile switching centers (MSCs).
The ground system in figure corresponds to the onboard equipment from the previous figure.Carrier signals coming from the air-borne station are filtered by a BPF, amplified in LNAs, demultiplexed in the demux and passed to the CDMA base stations. In this case the base station consists only of a radio channel frame, since there is no need for power- amplifier and antenna interface frames for every base station; a common wide band power amplifier and an antenna will serve all the collocated base stations. From the base stations, the signals are passed in the usual manner to the mobile switching center (MSC) and public switched telephone network (PSTN). The return signal path towards the airborne station is similar except for the inverse multiplexing operation in the MUX and high power amplification by HPA.

COMPARISONS BETWEEN TERRESTRIAL SYSTEMS AND HAPS

1.The antenna gain in terrestrial systems is
GT =10-17dB while an airborne antenna gain is GH = 30-35 Db
2.In terrestrial call dynamic range of signal attenuation is 69-80dB while it is 12-22 dB in HAPS.
3.The range of terrestrial system is upto 10-15 km but for the HAPS the range extends upto a radius of 200 km

Fuelling
The HAPS are fuelled by the following ways
Carrying fuel reserves on the board,
Capturing the solar energy.
SOLAR PLATFORMS
The Sun is the most natural energy source for stratospheric platforms located above clouds. The efficiency of photovoltaic cells has been significantly improved, making solar energy an attractive alternative to power high altitude platforms. As quasi-stationary stations experience the same day-night cycle as any point on the Earth surface, solar cells can produce electricity only 12-hours a day. Energy drawn from solar radiation during the daytime must thus be stored for use at night to assure continuous operation of station. The regenerative fuel cells (RFC) which use water as fuel are to be used for that purpose. During the day, the water is decomposed into hydrogen and oxygen in the electrolytic process. At night, the chemical reactions run backwards producing electricity and water with no pollution. Based on electrochemical processes, they are not limited by the efficiency of thermodynamically reversible engines ).

Operational Frequencies
WRC-97 has already designated in Resolution 122 a pair of 300 MHz frequency bands around 47
GHz (downlink: 47.2 - 47.5 GHz and uplink: 47.9 - 48.2 GHz) for the fixed services (FS) of highaltitude platform stations (HAPS). Due to the higher rain attenuation in certain areas, WRC-2000 proposed to study an additional frequency allocation for HAPS between 18 and 32 GHz for ITUR
Region 3 (Asia), focussing particularly, but not exclusively, on the bands 27.5 - 28.35 GHz and
31.0 - 31.3 GHz . Studies have been started within ITU-R to achieve the most efficient use of the spectrum and to define the technical sharing criteria.
It must be noted that the FS (Fixed Service) is under national responsibility and that also the operation of such an aircraft requires the authorization from the local aviation administrations

ADVANTAGES
Environmentally friendly
Solar power, fuel cells
Lower cost
No launch vehicle
Less demanding than space systems
Better propagation in many scenarios
Unobstructed line of sight paths
Less affected by rain attenuation
Replace extensive ground-based infrastructure
1 HAP can provide multi-cellular services over an area >60 km radius
Potential Applications
The interest in HAPS technology is justified by a variety of missions currently handled by satellites, aircrafts, or terrestrial systems that the technology is capable to perform in a more practical way. These include a number of communication and non-telecommunication applications:
Point-to-point, point-to-multipoint and broadcasting services
Surveillance of persons, goods, and areas
Monitoring
Data collection and remote sensing
Mapping
Weather
Location services including air and maritime traffic control.

Concluding Remarks
The High Altitude Platform Stations are expected to provide, in a cost-effective manner, a multitude of telecommunication and other services over large areas. Known also as stratospheric repeaters, they may operate individually or be interconnected with other similar HAPS and/or terrestrial and satellite-based stations. Compared to terrestrial telecommunication systems, the HAPS offer high signal arrival angle, largely unobstructed signal path, and large coverage area - like satellite-based systems but at lower cost. They can use most of conventional base station technology and terminal equipment. Compared to satellites, they do not require any launch vehicles, they offer much shorter signal path. They can be brought down to Earth for upgrading or repairing, and be re-deployed again. They can be kept quasi-stationary or be moved from one place to another on their own. Solar-powered platforms are environmentally friendly compared to the terrestrial antennas.
References
Tozer T C, Grace D: High-altitude Platforms for Wireless Communications;
Colella N J, Martin J N, Akyildiz I F: The Halo Network; IEEE Communications
Gavan J, Haridim M: Stratospheric Quasi-Stationary Platforms: Can They Replace Communication Satellite Systems?,
Radio Regulations, ITU 2001
High Altitude Platform Stations: An Opportunity to Close the Information Gap
Courtesy-
1).NASA( National Aeronautics and Space Administration)

2).Prof. Ryszard Struzakpast (Ex Vice-Chairman of the ITU Radio Regulations Board)

References: Tozer T C, Grace D: High-altitude Platforms for Wireless Communications; Colella N J, Martin J N, Akyildiz I F: The Halo Network; IEEE Communications Gavan J, Haridim M: Stratospheric Quasi-Stationary Platforms: Can They Replace Communication Satellite Systems?, Radio Regulations, ITU 2001 High Altitude Platform Stations: An Opportunity to Close the Information Gap Courtesy- 1).NASA( National Aeronautics and Space Administration) 2).Prof. Ryszard Struzakpast (Ex Vice-Chairman of the ITU Radio Regulations Board)