Communications from High Altitude Platforms – a Complementary or Disruptive Technology?

With an ever increasing demand for capacity for future generation multimedia applications, service providers are looking to utilise the frequency allocations in the millimetre wave bands, e.g. those specified for Local Multi-Point Distribution Systems (LMDS). In these frequency bands signals are attenuated by rain and line of sight paths are required. A possible solution to these effects is to use High Altitude Platforms (HAPs). HAPs are either airships or planes that operate in the stratosphere, 17-22km above the ground. Such platforms have the potential capability to serve a large number of users, using considerably less communications infrastructure than required by a terrestrial network. They can be considered as either a complimentary or disruptive technology – complementary in the sense that they augment existing infrastructure, or disruptive in the sense that they replace existing infrastructure. This paper provides an overview of the HAP concept, discussing both advantages and critical issues for Broadband Fixed Wireless Access (B-FWA) delivery from HAPs with reference to conventional terrestrial/satellite technologies. Finally, a brief summary is provided of HAP projects underway at the University of York such as the European Framework V HeliNet Project, in which York is leading the broadband communications aspects. Introduction High Altitude Platforms (HAPs) are either airships or planes [1-14] that will operate in the stratosphere, 17-22km above the ground. Such platforms have the potential capability to be deployed rapidly, using considerably less communications infrastructure than that required if delivered by a terrestrial network. Recently, the ITU has recently licensed several frequency bands for communications from HAPs, one at around 2GHz and others in the mm-wave bands, at 48GHz, and additionally at around 30GHz for Asia. This will allow HAPs to be used for both narrowband and broadband communications, e.g. 3G mobile and Local Multi-Point Distribution System (LMDS) type services. The use of the mm-wave bands will require cutting edge technology. These frequency bands are capable of delivering considerably higher data rate services, due to the larger frequency allocations available. However, in these frequency bands, signals experience high attenuation due to Line of Sight (LOS) obstructions, and also to rain, which requires appropriate link margins to be used in order to guarantee availability and Quality of Service. This paper is divided into three sections. Firstly, we highlight some of the advantages of HAPs over terrestrial and satellite architectures and present an overview of possible communications applications. Secondly, some of the technical challenges that must be overcome before communications from HAPs can become reality are discussed. Finally, we provide a review of work underway on HAPs in which the University of York has involvement. Examples include HeliNet (a European Framework V project), and SkyLARC Technologies Ltd (a University spin-off company). 1 Communications Research Group, Dept. of Electronics, University of York, Heslington, York, YO10 5DD, UK 2 SkyLARC Technologies Ltd, Heslington Hall, Heslington, York, YO10 5DD, UK Advantages of HAPs over existing Terrestrial and Satellite Technologies The development of communications services from HAPs will lead to many possible applications. HAPs have the following potential advantages over terrestrial and/or satellite architectures: • Relatively low cost upgrading of the platform; • Rapid deployment; • Broadband capability using the mm-wave LMDS bands; • Large area coverage (compared with terrestrial) long range terrestrial links are severely affected by rain attenuation (see below) and obstructions to the line-of-sight paths; • Very large system capacity, smaller cells than satellite, with link budgets considerably more favourable; • Flexibility to respond to traffic demands through extensive and adaptable frequency re-use; • Ideally suited to multimedia services, broadcast, and multi-cast; • Low propagation delay, compared with satellites; • Fewer problems with obstructions in the Line of Sight (LOS) paths compared with terrestrial; • Less ground-based infrastructure required than with terrestrial; • Lower launch costs than satellites. The applications can be divided technologically into two major types: • Low data rate, mobile. The 3 Generation mobile (IMT-2000) standard includes provision for basestation deployment from HAPs. The technology required to deliver IMT-2000 terrestrially is already well under development by mobile equipment providers, such as Ericsson, Nokia, Motorola, Nortel Networks. This area will require further study before deployment from HAPs, particularly in area of cell planning and antenna development. The ITU [8] suggests that footprints larger than 150km radius can be served from a HAP (the maximum theoretical size caused by signals not propagating over the horizon is approximately 500km radius), potentially allowing one HAP to replace several thousand terrestrial base stations. • High data rate, potentially fixed terminals. For the broadband services, significant research and development is required before a service can be offered from HAPs (or terrestrially). The HeliNet Project (see below) is specifically addressing these areas. One of the main differences between the different HAP systems is the power available to the payload. Typically airship based schemes will have 10-20kW available for the payload, due to large surface area on which to deploy solar cells and large number of fuel cells that the airships can carry. Planes powered by conventional fuel sources such as the HALO scheme suggested by Angel Technologies will have similar power available. By comparison solar powered planes such as that developed by AeroVironment[9], and the HeliPlat (see below) is expected to have payload power of under 1kW. This does not mean that communications from solar powered planes are not feasible. Solar powered planes have much in common with communications satellites, in terms of available power from the solar panels, payload weight and space available on the platform. Whether HAP technology should be described as complementary or disruptive depends on application. Probably in the first instance HAPs will be used for niche market applications, providing new services to users that are not currently served by terrestrial wireless, fibre, cable or xDSL – i.e. they augment existing technology and are therefore complementary. As the technology develops further, mass-market applications will become available, for instance broadband communications from HAPs could be used to replace backhaul infrastructure for terrestrial mobile communications, especially in rural areas currently served by terrestrial microwave links. Clearly, that represents a threat to terrestrial wireless operators meaning that HAPs will also be a disruptive technology. As mentioned above HAPs are better for broadband services because it is easier to achieve a Line of Sight (LOS) path, allowing a much wider area to be served from a single base station [6]. At mm-wave frequencies rain causes considerable attenuation. For a given ground distance as shown in Figure 1, for all but the shortest distances rain attenuation on a HAP link is considerably less than a corresponding terrestrial link (assuming that it is possible to achieve a LOS path).