Date:
Meeting:
6th October 2000
3rd FMAC meeting
Document reference:
Paper no.
prFMAC 0400 020 0
FMAC (00-03)/0020
Memorandum
subject:
Provision of backhaul links from High Altitude Platforms
from:
Steve Unger (Albera Networks, acting on behalf of Airship Technologies)
to:
FMAC
date:
6th October 2000
1
BACKGROUND
Interest is growing in the use of High Altitude Platforms (HAPS) to deliver UMTS services. HAPS
are defined by the ITU as “a station located on an object at an altitude of 20 to 50 km and at
a specified, nominal, foxed point relative to the Earth”. Several HAPS concepts are currently
under development in the US, Japan and Europe, including both airships and unmanned
aircraft.
HAPS allow network operators to provide wide geographic coverage much more rapidly
than would be possible from a terrestrial network. At the same time, the low operational
altitude compared even to low-Earth orbit satellites results in a very favourable link budget.
Standard terrestrial RF systems can be deployed on HAPS, resulting in lower costs and better
quality of service than equivalent satellite-based systems.
Much of the early drive to establish the HAPS concept came from the US company
SkyStation. As a result of this work, the ITU agreed at WRC1997 to make an allocation of
spectrum at 47 GHz to HAPS for the delivery of broadband fixed services. Airship
Technologies, a UK company based in Bedford (see www.airship.com), is the prime
contractor for the platforms required by SkyStation.
The most obvious application of HAPS is for mobile services. WRC2000 agreed that HAPS
could be used to deliver mobile services in the IMT2000 bands, and agreed a new ITU
recommendation setting out detailed operational constraints (ITU-R M1456: Minimum
performance characteristics and operational conditions for HAPS providing IMT2000).
Airship Technologies has started work on a new project, StratSat, intended to exploit this
opportunity. StratSat will be capable of providing service over a very wide coverage area,
perhaps 100 km in diameter. Within this area, a phased array antenna will be used to support
up to 1000 cells. The use of spot beams generated by this phased-array antenna will allow a
substantial reduction in adjacent cell interference compared to a terrestrial CDMA network,
giving a capacity increase of up to 80%.
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1
Date:
Meeting:
2
6th October 2000
3rd FMAC meeting
Document reference:
Paper no.
prFMAC 0400 020 0
FMAC (00-03)/0020
BACKHAUL ISSUES
A simplified picture of the StratSat system architecture is shown in the figure below:
The operational and regulatory constraints for the customer link (downlink) are well
understood (ITU recommendation M1456), and we are working on a more detailed design for
this part of the system.
There is however some uncertainty over how the backhaul link (uplink, feeder link) should be
provided. Backhaul links were not covered in ITU recommendation M1456, nor have they
been explicitly covered in any other ITU publication
Our proposal is to implement the backhaul link using standard point-point microwave radios,
exactly as for a terrestrial network. This allows us to coexist with other terrestrial point-point
links in the standard fixed service bands. We essentially treat the platform as a very tall tower
(!), with a high gain antenna deployed at each end of the link. The antenna at the platform
end has to be mounted on a stabilised platform, but the pointing accuracy required is not
particularly demanding, and we do not anticipate any difficulty in providing the necessary
degree of stability.
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Date:
Meeting:
6th October 2000
3rd FMAC meeting
Document reference:
Paper no.
prFMAC 0400 020 0
FMAC (00-03)/0020
We have analysed the link budget for a backhaul link of this type, using the RF parameters for
the STM1 radios currently manufactured by Nera and Ceragon (ex GigaNet). Although the
link operates over a range of about 20 km, only about 5 km of this link is assumed to be
subject to rain fade. Operation in the 23/26 GHz fixed service bands appears feasible,
providing an availability of 99.995% in rain zones E and F. Operation at 38 GHz is more
marginal, and we are currently carrying out a more detailed analysis of operation at this
frequency.
The proposed approach allows us to coexist with other terrestrial point-point links in the
standard fixed service bands. This results in high spectral efficiency compared to both
satellite-based and terrestrial networks:

An alternative option is to treat the backhaul from the HAP as a satellite groundlink, using
a relatively low gain antenna on the platform. This would however sterilise spectrum over
a much larger geographic area, and require complex coordination procedures to be
followed.

The equivalent terrestrial network would require a large number of low capacity links, in
order to provide backhaul between several hundred basestation sites. By co-locating all
these basestations onto a single platform, the HAPS concept allows us to concentrate this
into a much smaller small number of high capacity links.
We would welcome feedback from FMAC and the RA as to the appropriateness of our
proposed solution for HAPS backhaul. We would also welcome guidance as to how we
should now proceed in order to secure regulatory approval.
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