SYNOPSIS

HIGH ALTITUDE PLATFORMS (Haps)

Terrestrial Systems
Satellite Systems

But there are a few limitation of these exiting systems mainly

No proper LOS propagation
No microcellular structures
High free space path loss
Lengthy propagation delay

Solution to these problems can be HAP. It can use

Very tall base station mast.
Geostationary system but at low altitude.

HAP is a potential and emerging solution to the wireless delivery problem lies in aerial platforms, at high altitudes, carrying communications relay payloads and operating in a quasi-stationary position at altitudes up to some 22km.

The following topics are covered in my seminar

High altitude platform technology
Advantages of HAP communication
Comparison with Satellite & Terrestrial system

1. INTRODUCTION

As the demand grows for communication services, wireless solutions are becoming increasingly important. Wireless can offer high-bandwidth service provision without reliance on fixed infrastructure and represents a solution to the ‘last mile’ problem, i.e. delivery directly to a customer’s premises, while in many scenarios wireless may represent the only viable delivery mechanism. Wireless is also essential for mobile services, and cellular networks (e.g. 2nd generation mobile) are now operational worldwide. Fixed wireless access (FWA) schemes are also becoming established to provide telephony and data services to both business and home users.

The emerging market is for broadband data provision for multimedia, which represents a convergence of high speed Internet (and e-mail), telephony, TV, video-on demand, sound broadcasting etc. Broadband fixed wireless access (B-FWA) schemes aim to deliver a range of multimedia services to the customer at data rates of typically at least 2 Mbit/s. B-FWA should offer greater capacity to the user than services based on existing wire lines, such as ISDN or xDSL, which are in any event unlikely to be available to all customers. The alternative would be cable or fiber delivery, but such installation may be prohibitively expensive in many scenarios, and this may represent a barrier to new service providers. B-FWA is likely to be targeted initially at business, including SME (small–medium enterprise) and SOHO (small office/ home office) users, although the market is anticipated to extend rapidly to domestic customers.

However delivering high-capacity services by wireless also presents challenges, especially as the radio spectrum is a limited resource subject to increasing pressure as demand grows. To provide bandwidth to a large number of users, some form of frequency reuse strategy must be adopted, usually based around a fixed cellular structure. Fig. 1a illustrates the cellular concept, where each hexagon represents a cell having a base station near its centre and employing a different frequency or group of frequencies. The same frequency or

frequency band is not utilized by the neighboring cells. These frequencies are reused only at a distance, the reuse distance being a function of many factors, including the local propagation environment and the acceptable signal-to-interference-plus-noise ratio.

To provide increased capacity, the cell sizes may be reduced, thus allowing the spectrum to be reused more often within a given geographical area, as illustrated in Fig. 1b. This philosophy leads to the concept of micro cells for areas of high user density, with a base-station on perhaps every street corner. Indeed, taking the concept to its extreme limit, one might envisage one cell per user, the evident price in either case being the cost and environmental impact of a plethora of base-station antennas, together with the task of providing the backhaul links to serve them, by fiber or other wireless means, and the cost of installation.

Fig. 1: Cellular frequency reuse concept. In b the smaller cells provide greater overall capacity as frequencies are reused a greater number of times within a given geographical area.

Pressure on the radio spectrum also leads to a move towards higher frequency bands, which are less heavily congested and can provide significant bandwidth. The main allocations for broadband are in the 28GHz band (26GHz in some regions), as well as at 38GHz1. Existing broadband schemes may be variously described as LMDS (Local Multipoint Distribution Services) or MVDS (Multipoint Video Distribution Services);

These are flexible concepts for delivery of broadband services, although they may be encompassed in the generic term B-FWA, or simply BWA.

The use of these millimeter wavelengths implies line of sight propagation, which represents a challenge compared with lower frequencies. Thus local obstructions will cause problems and each customer terminal needs to ‘see’ a base-station; i.e. a microcellular structure is essential for propagation reasons. This again implies a need for a very large number of base-stations. A solution to these problems might be very tall base station masts with line-of-sight to users. However, these would be not only costly but also environmentally unacceptable. An alternative delivery mechanism is via satellite, which can provide line-of-sight communication to many users. Indeed, broadband services from geostationary (GEO) satellites are projected to represent a significant market over the next few years4. However, there are limitations on performance due partly to the range of c. 40000 km, which yields a free-space path loss (FSPL) of the order of 200dB, as well as to physical constraints of on-board antenna dimensions. The latter leads to a lower limit for the spot-beam (i.e. cell) diameter on the ground, and these minimum dimensions constrain the frequency reuse density and hence the overall capacity. Additionally, the high FSPL requires sizeable antennas at ground terminals to achieve broadband data rates. A further downside is the lengthy propagation delay over a geostationary satellite link of 0·25s, which not only is troublesome for speech but also may cause difficulties with some data protocols.

Low earth orbit (LEO) satellites may circumvent some of these limitations in principle, but suffer from complexities of rapid handover, not only between cells but also between platforms. The need for large numbers of LEO satellites to provide continuous coverage is also a significant economic burden, and such schemes have yet to prove commercially successful.

2. High Altitude Platforms

A potential and emerging solution to the wireless delivery problem lies in aerial platforms, at high altitudes, carrying communications relay payloads and operating in a quasi-stationary position at altitudes up to some 22km. The position is Quasi-stationary because the platform or the station does not remain fixed at this altitude. A payload can be a complete base-station, or simply a transparent transponder, akin to the majority of satellites. Line-of-sight propagation paths can be provided to most users with the full coverage of sea or land and a modest FSPL (free space propagation loss), thus enabling services that take advantage of the best features of both terrestrial and satellite communications. A single aerial platform can replace a large number of terrestrial masts, along with their associated costs, environmental impact and backhaul constraints. Site acquisition problems are also eliminated, together with installation maintenance costs, which can represent a major overhead in many regions of the world. The platforms may be aero planes or airships (essentially balloons, termed ‘aerostats’) and may be manned or unmanned with autonomous operation coupled with remote control from the ground. Of most interest are craft designed to operate in the stratosphere at an altitude of typically between 17 and 22km, which are referred to as high-altitude platforms (Haps). While the term HAP may not have a rigid definition, it can be taken to mean a solar-powered and unmanned aero plane or airship, capable of long endurance on-station up to several months or more. Another term in use is ‘HALE’ High Altitude Long Endurance—platform, which implies crafts capable of lengthy on-station deployment of perhaps up to a few years.

HAPs are now being actively developed in a number of programs world-wide, and the surge of recent activity reflects both the lucrative demand for wireless services and advances in platform technology, such as in materials, solar cells and energy storage. Major business goal remains that of developing a stratospheric HAP capable of serving communication applications economically and with a high degree of reliability.

3. High Altitude Platform Technologies

3.1. Airship HAPs

Proposed implementations of airships for high-altitude deployment use very large semi-rigid or non-rigid helium filled containers, of the order of 100 m or more in length. Electric motors and propellers are used for station-keeping, and the airship flies against the prevailing wind. Prime power is required for propulsion and station-keeping as well as for the payload and applications; it is provided from lightweight solar cells in the form of large flexible sheets, which may weigh typically well under 400g/m2 and cover the upper surfaces of the airship. Additionally, during the day, power is stored in regenerative fuel cells, which then provide all the power requirements at night.

The overall long-term power balance of a HAP is likely to be a critical factor, and it will be the performance and ageing of the fuel cells that are likely to determine the achievable mission duration. However, this is mitigated by the fact that it is relatively easy to bring HAPs back to earth, unlike satellites, for service and/or replacement of the fuel cells as well as for other maintenance or upgrading. One of the major HAP airship projects is the Japanese SkyNet. Funded substantially by the Japanese government, and led by Yokosuka Communications Research Laboratory, this project aims to produce an integrated network of some 15 airships to serve most of Japan, providing broadband communication services operating in the 28GHz band, as well as broadcasting. A range of airships is being developed by Advanced Technologies Group, of Bedford, UK, at one time in collaboration with SkyStation International of the USA, who proposed an airship 150m in length supporting a communications payload of up to 800kg. HAP airships are also being proposed by Lindstrand Balloons. A novel design of HAP comprising several smaller airships joined together in an ‘Airworm’ configuration is being developed by The University of Stuttgart: this sausage-like formation aims to provide the lift while avoiding some of the structural and aerodynamic problems associated with very large airships.

3.1.1 Sky Station

Sky Station is the name of an airship system planned by the US Company “Sky Station International”. The number of platforms will depend on the demand (250 platforms are announced). The balloons will be covered with solar cells, giving energy to the electrical motors. The data rates foreseen for the fixed services are 2 Mbps for the uplink and 10 Mbps for the downlink. The data rates foreseen for the mobile services are 9.6 - 16 kbps for. The cost of the entire project for a worldwide broadband infrastructure is estimated at $2.5 billion. Sky Station has apparently chosen to use conventional electric motors and lightweight propellers, instead.

3.1.2 StratSat

StratSat is an airship system planned by the UK based company “Advanced Technology Group (ATG)” [1]. StratSat intends to offer a cost effective and safe solution for geo-stationary telecommunications payloads above large customer concentrations. With both civilian and military applications, the StratSat can be dispatched and kept there up to five years at a fraction of the cost of any alternative means. The airship in the stratosphere is well above conventional air traffic and presents no threat. Its cheap launch costs, compared to the conventional satellites allows those in the industry to talk of reducing the cost of calls from a mobile telephone, by an order of magnitude, thereby capturing a high proportion of the market. The solar array provides the sole source of renewable energy for the airship. The array is placed over the upper quarter of the hull and extends over approximately three-quarters of the length of the craft. The array can be realigned to the daily sun location/angle by the roll rotation of the whole airship. The airship is propelled and steered by means of a ‘Contra-Rotating Coned Rotor’ mounted on a tailcone at the rear of the envelope, as part of a compound propulsion system. This unit provides longitudinal thrust (to counter the prevailing stratospheric winds) and lateral force (for maneuvering) to enable the airship to hold station within a 1 km cube.

3.1.3 Stratospheric Platform System from Japan

The airship from Wireless Innovation Systems Group, Japan has a semi-rigid hull of ellipsoidal shape with an overall length of nearly 200 m. It is composed of an air-pressurized hull for maintaining a fixed contour, and internal bags filled with the buoyant helium gas. Two air ballonets are installed inside the hull to keep the airship at a required altitude. For a load balance to the lifting force, catenary curtains are connected to a lower rigid keel, directly attached to the envelope. Propulsive propellers are mounted on both the stem and stern of the airship, and tail wings are installed on the rear end of the hull. A solar photovoltaic power subsystem of solar cells and regenerative fuel cells is provided to supply a day/night cycle of electricity for airship propulsion

3.1.4 ARC System

The Airborne Relay Communications (ARC) System is the name of an airship platform planned by the US Company Platforms Wireless International [7]. The ARC system is designed to operate at lower altitudes, 3 to 10.5 km., originally known as “Aerostats”. These airships were designed as airborne defense platforms for low-level radar use. Inspired by the dirigibles that monitor the border between the US and Mexico, Platforms Wireless International develops a system which shall provide fixed wireless broadband as well as mobile services to areas of 55 to 225 km diameter per system and servicing up to 1’500’000 subscribers (depending on system configuration and antenna projection power). An ARC airship is a 46 m long helium-filled balloon, which can carry almost 700 kg of payload. An airship configuration is designed with two supporting aircrafts, which will be deployed to ensure uninterrupted service coverage when severe weather conditions or monthly servicing require the temporary docking of the airship. Unlike the three stratospheric platform stations described above, the ARC system is not using solar cells. Electricity is supplied to the payload via a 2.5 cm thick cable. It also incorporates a fiber-optic cable link that connects the airborne base stations to the rest of the network. A “no-fly zone” must also be created so other aircrafts do not fly into the airship or its cable.

Fig 2: The Sky Station Platform

Fig 3: The StratSat Platform

Fig 4: Japanese Stratospheric Platform System

3.2 Aero plane HAPs

Another form of HAP is the unmanned solar-powered plane, which needs to fly against the wind, or in a roughly circular tight path. Again, the prime challenge is likely to be the power balance, the craft having to be able to store sufficient energy for station-keeping throughout the night. The most highly-developed such craft are those from AeroVironment in the USA, whose planned Helios plane has a wing-span of 75m; their Pathfinder and Centurion programs have already demonstrated flight endurance trials at up to 25km altitude (80000ft). Funded initially by NASA, these programs have the goal of long-endurance operation for commercial communications and other applications. HeliPlat is a solar-powered craft being developed under the auspices of Politecnico di Torino in Italy, as part of the HeliNet Project funded by the European Commission under a Framework V initiative. The HAP project currently most near-market for communications is a manned aircraft with pilots operating on an 8-hour shift. Angel Technologies’ HALO project employs the specially designed Proteus aircraft operating at altitudes of 16–18km (51000–60000ft) to deliver broadband communication services over an area up to 40km in diameter. The aircraft will maintain a quasi stationary position by flying in a roughly circular path with a typical diameter of less than 13 km. The communications payload uses a pod below the fuselage housing up to 125 microwave antennas. The aircraft is well proven, and this may be considered a relatively low-risk solution, although ultimate commercial success will depend upon the economics of operation.

3.2.1 HALO-Proteus

Angel Technology Corporation (USA) is planning to offer broadband telecommunication service using manned aircraft [3]. A piloted, FAA (Federal Aviation Administration) certified High Altitude Long Operation (HALO) aircraft will provide the “hub” of the network operating continuously over each market in three eight hours shifts. Consumers will be able to access video, data, and the Internet at rates ranging from 1 to 5 Mbps.

The technology of high altitude manned aircraft is mature. A broadband wireless link at 52 Mbps has already been demonstrated.

3.2.2 SkyTower

Through funding support from NASA, AeroVironment has developed an unmanned, solar electric airplane called Helios which will be capable of continuous flight for up to six months or more at 60'000 feet in the stratosphere, above the weather and commercial air traffic (AeroVironment also developed Pathfinder Plus, Helios’ predecessor). Helios will provide a telecommunications platform from this position in the stratosphere, acting as an 11-mile tall tower—hence the name “SkyTower”. AeroVironment officially formed SkyTower, Inc. in October 2000 to pursue commercial telecom opportunities enabled by AeroVironment’s proprietary solar-electric aircraft technology. SkyTower’s stratospheric communications networks are comprised of airborne segments (or payloads) which communicate with user terminals and gateway stations on the ground. The ground gateway stations will serve as an intermediate interface between the aircraft and existing Internet and PSTN connecting systems. When a signal passes from the end users up to the airplane and then from the airplane to the ground gateway antenna, a ground switching router will determine whether the data should be directed to the Internet, a private data network, or the telephone network. These interactive network systems are being designed to maximize the overall throughput of the network. Fixed wireless broadband total throughput is projected to be approximately 10 to 20 Gbps per platform with typical user transmission speeds of 15 Mbps or higher (125 Mbps is feasible for a single user).

3.2.3 HeliPlat

The HeliPlat (HELIos PLATform) is being designed at Politecnico di Torino [8] under an ASI (Italian Space Agency) grant. HeliPlat is an unmanned platform with solar cell propulsion, which will be operated in the stratosphere. It will enable a payload of about 100 kg, and offers an available power of some hundreds watt. The present research proposal is devoted to the study of possible applications of such a platform, not only for cellular/personal communications, but also for localization and surveillance. The use of the platform as base station (GSM or UMTS) can provide cellular telephony service to rural areas with low user density, because large diameter cells can be easily implemented; to increase the capacity of the public switched network in case of natural disaster, easily moving the platform if needed; to provide reliable telecommunication services to the ships sailing transoceanic courses, using networks of aerial platforms placed on the most important navigation lanes.

Fig 5: The HALO-Proteus aircraft Fig 6: Sky Tower aircraft

Fig 7: Study of the HeliPlat configuration,

Showing its early stage

3.3 Some other aerial platforms

Another category of aerial platforms is the UAV—Unmanned Aerial Vehicle. This refers typically to small fuelled unmanned aircraft, having short mission durations and operating at generally modest altitudes. The main use of UAVs is for military surveillance, with some smaller craft being considered almost as disposable. The application of UAVs as communication relay nodes seems to be limited, no doubt due largely to their relatively short endurance. Larger military UAVs include Global Hawk and Predator, which can support large payloads and fly long distances, but have not generally been considered cost-effective for normal communication provision. Finally, the simplest and most available aerial platform is a tethered aerostat. This is an airship on a cable, whose length may reach up to 5km or more. Tethering partially deals with the major problem of station-keeping, although platform movement is still an issue. Power and communications backhaul may also be provided through the tether. The evident challenge is the hazard presented to air traffic, and although some aerostats are deployed in aircraft exclusion zones, their general application may be more suited to less developed regions. An important current program is that of Platforms Wireless International, which is developing a tethered aerostat for use in Brazil at an altitude of 4·6 km (15000ft)28. Its ARC (Airborne Relay Communications) system aims to deliver a range of cellular communication services to over 125000 subscribers.

3.4 Wind speed Profile and Footprint Diameter

For any HAP technology or the platform whether it be an airship or an aero plane, a major challenge is the ability of the HAP to maintain station keeping in the face of winds. An operating altitude of between 17 and 22 km is chosen because in most regions of the world this represents a layer of relatively mild wind and turbulence. Fig 2 illustrates a typical wind profile vs. height; although the wind profile may vary considerably with latitude and with season, a form similar to that shown will usually obtain. This altitude is also above commercial air-traffic heights, which would otherwise prove a potentially prohibitive constraint.

Fig. 8: Wind speed profile with height. Values vary with season

and location, but generally follow this rough distribution.

For a given platform altitude h, the diameter of the HAPS footprint can be computed using the formula:

Where, R = 6378 km.(Earth Radius),

h = Altitude

For minimum elevation angle of 15 degree the diameter is found to be 152 km. and for 0 degree the diameter is 1033 km.(Both values at 21 km platform altitude)

3.5 Communication applications

Fig 9: HAP Communication Scenario

Fig. 9 depicts a general HAP communications scenario. Services can be provided from a single HAP with up- and down-links to the user terminals, together with backhaul links as required into the fiber backbone. Inter-HAP links may serve to connect a network of HAPs, while links may also be established if required via satellite directly from the HAP.

The coverage region served by HAP is essentially determined by line-of-sight propagation (at least at the higher frequency bands) and the minimum elevation angle at the ground terminal. A practical lower elevation limit for BWA services might be 5°, while 15° is more commonly considered to avoid excessive ground clutter problems. From 20 km altitude above smooth terrain, 5° implies an area of .200km radius or 20000 square km, although for many service applications, e.g. to a city or suburban area, such wide coverage may not be required or appropriate. There is then opportunity to subdivide this area into a large number of smaller coverage zones, or cells, to provide large overall capacity optimized through frequency reuse plans. The size, number, and shape of these cells is now subject to design of the antennas on the HAP, with the advantage that the cell configuration may be determined centrally at the HAP and thus reconfigured and adapted to suit traffic requirements. Indeed, the HAP architecture lends itself particularly readily to adaptive resource allocation techniques, which can provide efficient usage of bandwidth and maximize capacity. Compared with geostationary satellite services, the cells can be considerably smaller, since the minimum spot-beam size from a satellite is constrained by the onboard antenna dimensions. Higher capacity with HAPs is also facilitated by the much more favorable link budgets compared to satellites, since the HAP is at relatively close range; this represents a power advantage of up to about 34dB compared to a LEO satellite, or 66dB compared to a GEO satellite. And compared with terrestrial schemes, a single HAP can offer capacity equivalent to that provided by a large number of separate base-stations; furthermore the link geometry means that most obstacles will be avoided.

BWA (Broadband Wireless Access) applications

The principal application for HAPs is seen as B-FWA (Broadband fixed wireless access), providing potentially very high data rates to the user. The frequency allocation for HAPs at 47/48GHz offers 2 * 300MHz of bandwidth, which might be apportioned 50:50 to user and backhaul links, and again 50:50 to up- and down-links. (An exception might be where links are mainly used for Internet traffic, which would warrant an asymmetric apportionment.) Studies undertaken based on the HeliNet scenario propose a scheme with an overall coverage region per HAP of diameter 60km, having 121 cells, each with a nominal ground diameter of 5km. Downlink HAP power is 1W per cell, and this can support data rates of up to 60Mbit/s which is well within the bandwidth required per cell of 25 MHz when using 16-QAM or higher order modulation schemes. The total payload throughput in this conservative demonstrator example is in excess of 7Gbit/s.

3G/2G applications

HAPs may offer opportunity to deploy mobile cellular services. A single base-station on the HAP with a wide-beam width antenna could serve a wide area, which may prove advantageous over sparsely populated regions. Alternatively, a number of smaller cells could be deployed with appropriate directional antennas. The benefits would include rapid roll-out covering a large region, relatively uncluttered propagation paths, and elimination of much ground-station installation.

HAP Networks

A number of HAPs may be deployed in a network to cover an entire region. Inter-HAP links may be accomplished at high EHF frequencies or using optical links; such technology is well established for satellites and should not present major problems.

3.6 Advantages of HAP communications

HAP communications have a number of potential benefits, as summarized below

1. Large-area coverage (compared with terrestrial systems): The geometry of HAP deployment means that long-range links experience relatively little rain attenuation compared to terrestrial links over the same distance, due to a shorter slant path through the atmosphere. At the shorter millimeter-wave bands this can yield significant link budget advantages within large cells.

2. Flexibility to respond to traffic demands: HAPs are ideally suited to the provision of centralized adaptable resource allocation, i.e. flexible and responsive frequency reuse patterns and cell sizes, unconstrained by the physical location of base-stations. Such almost real-time adaptation should provide greatly increased overall capacity compared with current fixed terrestrial schemes or satellite systems.

3. Low cost: Although there is to date no direct experience of operating costs, a small cluster of HAPs should prove considerably cheaper to procure and launch than a geostationary satellite or a constellation of LEO satellites. A HAP network should also be cheaper to deploy than a terrestrial network with a large number of base-stations.

4. Incremental deployment: Service may be provided initially with a single platform and the network expanded gradually as greater coverage and/or capacity is required. This is in contrast to a LEO satellite network, which requires a large number of satellites to achieve continuous coverage; a terrestrial network is also likely to require a significant number of base-stations before it may be regarded as fully functional.

5. Rapid deployment: Given the availability of suitable platforms, it should be possible to design, implement and deploy a new HAP-based service relatively quickly. Satellites, on the other hand, usually take several years from initial procurement through launch to on-station operation, with the payload often obsolete by the time it is launched. Similarly, deployment of terrestrial networks may involve time-consuming planning procedures and civil works. HAPs can thus enable rapid roll-out of services by providers keen to get in business before the competition. Furthermore, there is little reason why prepared HAPs should not be capable of being launched and placed on station within a matter of days or even hours. This will facilitate their use in emergency scenarios. Examples might include: natural disasters; military missions; restoration where a terrestrial network experiences failure; overload due to a large concentration of users, e.g. at a major event.

6. Platform and payload upgrading: HAPs may be on station for lengthy periods, with some proponents claiming 5 years or more. But they can be brought down relatively readily for maintenance or upgrading of the payload and this is a positive feature allowing a high degree of ‘future-proofing’.

7. Environment friendliness: HAPs rely upon sunlight for their power and do not require launch vehicles with their associated fuel implications. They represent environmentally friendly reusable craft, quite apart from the potential benefits of removing the need for large numbers of terrestrial masts and their associated infrastructure.

3.7 Comparison with satellite and terrestrial system

Table 1 summarizes the comparison of high altitude platform communication system to that of the existing systems such as the satellite and terrestrial communication system. The comparison is based upon operational height, lifetime, capacity, orbit type, coverage, fade margin, indoor reception.

Table 1: Comparison of the advantages and disadvantages of various wireless solutions

3.8 Some issues and challenges

1. System level requirements: HAP networks for broadband communication service delivery will require a rethink of the basic design of cellular-type services, with development focusing upon the frequency planning of different spot beam layouts, which are subject to wide angular variations and changes in link length, and frequency reuse patterns for both user and backhaul links. The network architecture will need to exploit opportunities of inter-terminal switching directly on the HAP itself as opposed to on the ground, and the use of inter-HAP links to achieve connectivity.

2. Propagation and diversity. Services from HAPs have been allocated frequencies by the ITU in the millimeter wave bands, at 47/48GHz (and also at 28GHz in ITU Region 3—predominantly Asia). Propagation from HAPs is not fully characterized at these higher frequencies: rain attenuation is significant in these bands, so one of the main requirements is to develop rainfall attenuation and scattering statistics. This will allow appropriate margins to be included and highlight any problems with frequency reuse plans developed at the system level.

3. Modulation and coding: In order to optimize network capacity, suitable modulation and coding schemes will be required to serve the broadband telecommunication services, with specified QoS (Quality of Service) and BER (Bit Error Rate) requirements, applicable under different link conditions. Adaptive techniques will provide overall optimum performance, using low-rate schemes involving powerful forward error correction (FEC) coding when attenuation is severe, up to high-rate multilevel modulation schemes when conditions are good.

4. Antennas: Antenna technology will be critical to BWA from HAPs. A large number of spot beams will be required, and these may be produced either by an ensemble of horn antennas or some form of phased array. Side lobe performance is an important issue, which will affect inter cell interference and, ultimately, system capacity. At a planned frequency of 48 GHz, this is demanding technology both for the HAP-based antenna and also for ground terminals.

5. Platform station-keeping and stability: The ability of HAP to maintain position reliably in the face of variable winds is a major challenge and will critically affect the viability of communication services. HAP positioning is likely to be represented as a certain statistical probability of remaining within a particular volume, e.g. a location cylinder. Stability is another critical issue. Inevitably, there will be roll, pitch and yaw of the platform, due to turbulence in the stratosphere; in this regard, larger craft are likely to exhibit greater stability.

6. Should ground-based antennas be fixed or steer able?: HAPs will vary in position, both laterally and vertically; the latter perhaps deliberately so in order to optimize altitude to minimize prevailing winds. This movement results in changes of look angle from the ground terminal, which can be used to determine whether fixed or steer able ground-based antennas are required. If the angular variation is greater than the beam width of the antenna, which will be a function of the gain required to operate the link, then it is necessary to use a steer able ground terminal antenna.

7. Payload power: An important distinction between the different types of HAP is the power available to the payload. Typically an airship may have in excess of 20 kW available for the payload, due to the large surface area on which to deploy solar cells. Planes powered by conventional fuel sources (e.g. the HALO scheme by Angel Technologies) will similarly have high power available. By comparison, solar powered planes (e.g. HeliPlat) may have significantly less available payload power; this is a limitation similar to that experienced by satellites, and means that the achievable downlink RF power, and hence overall capacity, will be constrained.

4. Conclusion

The novelty of HAP communications calls for some new concepts in terms of delivery of services raising critical issues for development. And the platforms themselves present some challenges and potential problems. But a combination of ‘technology push’ from the providers of platforms and ‘applications pull’ from the inexorable demand for communications may provide significant developments in HAPs for communication service delivery in the near future and although some of the goals for HAPs are a few years from realization, there can be little doubt that aerial platforms will play an increasingly important role in the delivery of wireless services means it is not so much a question of ‘if’ but of ‘when’.

With the evident opportunities for enhanced communication services, as outlined in this paper, it is to be anticipated that we shall see significant developments in HAPs for communication service delivery over the next few years.

REFERENCES

www.Google.com

www.extremetech.com

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www.21stcenturyairships.com

www.york.ac.uk

www.dataweek.co.za/news