ABSTRACT
The demand for
high-capacity wireless services is bringing increasing challenges, especially
for delivery of the ‘last mile’. Terrestrially, the need for line-of-sight
propagation paths represents a constraint unless very large numbers of
base-station masts are deployed, while satellite systems have capacity
limitations. An emerging solution is offered by high-altitude platforms (HAPs)
operating in the stratosphere at altitudes of up to 22 km to provide
communication facilities that can exploit the best features of both terrestrial
and satellite schemes. In this paper we outline the application of airships as
low cost alternative for HAPs for delivery of future broadband wireless communications.
NEED FOR HIGH ALTITUDE COMMUNICATION PLATFORM
Wireless solutions are becoming increasingly
important, because 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, telephony, TV, video-on-demand
and sound broadcasting.
LIMITATIONS OF
SATELLITES AND TOWER NETWORKING
Based on the studies carried out on the existing
methods used for communication using satellites and tower networking, it was
found that these methods have several limitations. Incase of Satellites,
revenue generation only begins with launch of entire constellation (for e.g.
Iridium has 66 satellites). It causes high latency (signal delay) and also
there is a limited signal penetration through buildings. Moreover only one
company can operate on the entire system and a technology set 3-4 years
prior to deployment is required. Any upgrade requires the launch of an entire
new constellation. A high infrastructure cost per subscriber, more than US $
100 million per satellite [1] is involved. Even incase of tower networks,
extensive lease and transmission systems are required. There is an inconsistent
penetration of buildings in the line of sight in case of tower networking. We cannot
shift capacities, and also the dead zones are quite common. Upgrades require a
modification at each site and coordinated cutover. Further a complex and
expensive installation and network management is needed. Network building
requires literally hundreds of towers. Environmental concerns also pose severed
restrictions on location of the towers.
AERIAL COMMUNICATION PLATFORMS
A potential solution to the wireless delivery problem lies
in aerial platforms, carrying communications relay payloads and operating in a
quasi-stationary position at high altitudes. 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
a modest FSPL (free space path 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. These platforms may be manned or unmanned with autonomous
operation coupled with remote control from the ground. Platforms under
investigations include Airship, Airplane, unmanned aerial vehicle and tethered
aerostat which reach up to 5 Km. Airships use very large semi-rigid
helium-filled containers. Another form of HAP is the unmanned solar-powered
plane, which needs to fly against the wind, or in a roughly circular tight
path. 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 programmes have already demonstrated flight endurance trials at up to
25km altitude 80000ft. 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.
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, we take it to mean a solar-powered
and unmanned aircraft, capable of long endurance on-station—e.g. 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 programmes
worldwide, 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.
BENEFITS OF HAPS OVER OTHER SYSTEMS
HAPs are highly suitable for broadband wireless communications. They
appear at a high elevation angle compared to terrestrial base stations, thereby
mitigating the terrestrial propagation effects. With a visibility of around 200
Km at 5 Deg elevations, they can replace a large number of terrestrial base
stations; and being considerably closer to the ground than satellites offer
much lower path loss than satellites - better by ~ 34 dB for LEO satellites and
~ 66 dB for GEO satellites. HAPs have been assigned frequency bands in 47/48
GHz and 28 GHz bands where at present spare spectrum is in plenty. HAP
telecommunication systems can be designed to respond dynamically to traffic
demands; they are relatively low cost compared to satellites; they can be
deployed incrementally and rapidly when necessary; the platforms and payload
are upgradeable; and they are environmentally friendly using solar power,
without need of launchers and eliminate the need of terrestrial masts. Typical
cell size of HAPs range between 1-10 Km and the communication throughout can
range between 25-155 Mb/s. The coverage is regional, though it is possible to
inter-link HAPs creating a national grid, or alternatively they can be
connected to distant gateways via satellites. They have been proposed for
broadband fixed wireless access (B-FWA), mobile communications as base
stations, rural telephony, broadcasting, emergency or disaster applications,
military communications, etc.
AIRSHIPS FOR HAPS
The past few years have seen a resurgence of
interest in balloons and airships, with technology developments such as new
plastic envelope materials that are strong, UV resistant and leak-proof to
helium, which is now almost universally used instead of the much cheaper
hydrogen. Such hi-tech airships have featured in high-profile attempts to
circumnavigate the globe (for e.g. the Breitling Orbiter10). Several high
technology airship programs have been launched recently for e.g. Zeppelin NT
and Cargolifter in Germany
Although the wind
profile with altitude may vary considerably with latitude and with season, a
form similar to that shown in Figure 1 will usually be obtained. This altitude
(>16 km) is also above commercial air-traffic heights, which would otherwise
prove a potentially prohibitive constraint
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 provides all the power requirements at
night. The overall long-term power balance of HAP is likely to be a critical
factor, and the performance and ageing of the fuel cells determine the
achievable mission duration. Within this height range, wind currents are low
and commercial aviation is unaffected. This is a very cost effective
alternative, which offers low technical risk and short development cycles.
Airship services can be configured to satisfy the traffic requirements in metropolitan
area. Further, the high elevation angles can substantially reduce the
occurrence of dead zones and multi path fading.
BENEFITS OF AIRSHIPS AS HAPS
Airships have a very long flight endurance, which ensures
continuous communications and surveillance coverage. Communication time delays
will also be low due to comparatively lower altitudes than satellites.
Technology upgrades during ground servicing is also possible. Airships can also
serve in multiple applications as communication system such as in 3rdgeneration
& 4th generation mobile 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 rollout covering a large region, relatively uncluttered propagation
paths, and elimination of much ground-station installation. Haps do not require
any launch vehicle, they can move under their own power throughout the world or
remain stationary, and they can be brought down to earth,
refurbished and re-deployed. Once a platform is in position, it can immediately
begin delivering service to its service area without the need to deploy a
global infrastructure or constellation of platforms to operate. Haps can use
conventional base station technology - the only difference being the antenna. The relatively low altitudes
enable the HAPs systems to provide a higher frequency reuse and thus higher
capacity than satellite systems. The low launching costs and the possibility to
repair the platforms gateway could lead to less expensive wireless
infrastructures per subscriber. Each platform can be retrieved, updated, and
re-launched without service interruption.
These platforms will be
environmentally friendly, since solar technology and non-polluting fuel cells
will power them. Studies carried out on the airships so far have shown that
they require lower overall cost than satellite or ground systems, as given in
Table 1. They have a larger payload weight and volume capability for multiple
payloads, antennas, and optics. The relatively low altitudes - compared to
satellite systems – provide subscribers with short paths through the atmosphere
and unobstructed line-of-sight to the platform. With small antennas and low
power requirements, the HAPS systems are suited for a wide variety of fixed and
portable user terminals to meet almost any service neede
PROBLEMS / CHALLENGES FACED BY
AIRSHIP HAPS
With the advantages mentioned, these have not been exploited
commercially because the technology has yet to arrive. There are a number of
open technical issues being actively pursued. A number of system issues are under
investigation including - system architecture, frequency planning, network
protocols, resource planning, etc. Propagation characteristics at 47/48 GHz are
not well defined yet; modulation/coding techniques have to be optimized for
such propagation conditions; 48 GHz antenna technology with multiple spot beams
is under development. Other issues include platform station-keeping, hand-off
considerations even for fixed stations due to platform movement and payload
power. HAPs have similar eclipse problem to satellite with regards to payload
power due to the use of solar cells. Reliable platforms are yet to be
developed.
THE TECHNOLOGICAL
CHALLENGES FACED TODAY CAN BE SUMMARIZED AS FOLLOWS
Winds in the
stratosphere are weak at approximately 20 km above sea level, and are believed
to be weakest at 22 km where the atmospheric pressure is approximately 40 hPa
in the middle latitudinal regions. Air density at this altitude is about 1/20
of that at sea level; therefore, the envelope needs to be large enough to yield
necessary buoyancy. If the atmospheric temperature and the buoyant gas
temperatures fluctuate drastically diurnally and annually, it will directly
affect the buoyancy; buoyant gas expands or contracts with temperature
fluctuations. If thermal variation is so large that the platform has to vent
excessive helium gas at high temperature conditions, then the platform will
lose buoyancy during sunset (lower temperatures) and possibly descend to the
ground.
Ø ENERGY SOURCE FOR PROPULSION
Solar energy can be harvested continuously in daytime in
the relatively continuous ‘fair weather’ stratosphere. However, if nighttime
becomes longer, as in the Polar Regions in
winter, the platform requires ground-based wireless power transmission systems
for continuous thrust powering due to the lack of solar energy.
Ø ENERGY STORAGE
Rapid progress in development of electrical automobile
batteries lends itself to LTA secondary batteries for nocturnal propulsion, and
there are good prospects that more energy efficient and lighter batteries can
be developed in the very near future.
Ø PROPULSIVE EFFICIENCY
Propulsive efficiency is one of the most important
parameters affecting total vehicle weight. Rigid airships in the past have had
total volume drag coefficients of 0.022 - 0.023. It is assumed in this study
that an optimized laminar flow body equipped with an aft propulsor achieves
0.020 as the total volume drag coefficient, considering recent research on the
optimized laminar flow body.
Ø AERODYNAMIC DESIGN
The airship is a pressurized balloon. Namely, the balloon
skin is made of strong fabric that confines gas expansion and prevents buoyancy
fluctuations from the buoyant gas temperature rise. The hull shape design was
adopted from a study on minimum drag hull shape optimization. The empennage
sizes are determined by existing airship data. To get maximum propulsive
efficiency, an aft propulsion three-bladed propeller is used. This example is
feasible structurally as well as from the point of power-balances.
Ø GROUND HANDLING, LAUNCH AND
RECOVERY
Launch and recovery is perhaps the most difficult phase of
airship flight. One important factor is the real estate required. This will
depend on the size of the airship, its controllability and type of launch. The
number of airships required to be moored or hangared at any one site would also
affect the acreage. Many companies are planning large fleets but the build,
launch and recoveries will need to be scheduled to optimize the ground area and
manpower available. However, the wind profile during the year may force all
launch and recovery operations to be conducted.
The Airborne Relay
Communications (ARC) System is the name of an airship platform planned by the
US Company Platforms Wireless International. The ARC system (refer figure 4) 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 (winds in excess of 145 km/h) or monthly servicing require the
temporary docking of the airship. 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. StratSat (refer figure 5)
is an airship system planned by the UK
CONCLUSION
Terrestrially,
the need for line-of-sight propagation paths represents a constraint unless
very large numbers of base-station masts are deployed, while satellite systems
have capacity limitations. Hence, airships may acts as a low cost alternative
for communication satellites for delivery of future broadband wireless
communications.
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