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Stratospheric altitude: the key to delivering broadband wireless local loop service to consumers worldwide

by Martine Rothblatt, Jack Frohbieter and Huanchun Ye
Sky Station International, Inc.
(Washington, DC, United States)

There are four general telecommunications architectures which can be used to deliver broadband wireless local loop service to consumers. Two of these architectures are space-based — geostationary satellites and non-geostationary satellites. Two of these architectures are considered terrestrial — rooftop cellular-like millimeter wave repeaters and stratospheric relay platforms.

Intrinsic reasons of geometry and hardware engineering inevitably lead to a technical conclusion that the greatest amount of communications capacity over global metropolitan areas, for an equivalent investment in equipment and bandwidth, will come from the stratospheric architecture. Accordingly, the greatest amount of broadband Internet capacity for the lowest cost can be provided to consumers via stratospheric telecommunications.

Assessing the capacity of broadband wireless systems

Holding bandwidth, antenna aperture, power and other technical factors constant, the metropolitan capacity of a telecommunications system is equal to the number of spot beams that the system provides. The number of metropolitan area spot beams that a system can generate varies directly with the distance between the radio repeater and the coverage area until the line of sight approaches the outer boundary of the metropolitan area, and inversely thereafter. For example, a single building top repeater can generate at most six spot beams using a 60° sectorial antenna. At the other end of the continuum, a geostationary satellite cannot generate more than one spot beam per metropolitan area using typical state-of-the-art antenna apertures of 5 m at 20/30 GHz.¹ However, a single stratospheric telecommunications platform at 21 km altitude can generate approximately 700 to 1000 spot beams within a single metropolitan area, whereas a non-geostationary satellite at 500 km altitude would generate only six to nine spot beams out to 100 km from the centre of a metropolitan area.² The stratospheric architecture thus yields approximately 100 times greater metropolitan area capacity than the non-geostationary satellite orbit architecture (see Table 1).

Table 1

Broadband capacity in metropolitan areas

As an example, the sky station stratospheric platform system envisions 300 MHz of bandwidth in each direction in the 47.2–47.5 and 47.9–48.2 GHz bands. From a 21 km deployment altitude and with a 27.1 dBW e.i.r.p., the system generates 691 spot beams over a 77.6 km diameter coverage area, out to a 15° angle of elevation. Assuming a frequency reuse factor of 9, the total metropolitan capacity of the system is 7.68 Gbit/s times the number of covered metropolitan areas. With approximately 250 global metropolitan areas counting one million or more people, the global network of Sky Station platforms would offer a system-wide metropolitan capacity of 1.92 Tbit/s. This capacity would be time shared by subscribers at virtual T1/E1 rates and higher, resulting in a global subscriber capacity, at 0.1 erlang utilization, of over 250 million subscribers. This figure accords well with the projected demand for worldwide broadband consumer circuits by the end of 2010.

High-density and low-density market segments

Stratospheric and other broadband systems can be differentiated into high-density and low-density market segments. All space systems (geostationary and networked non-geostationary) are low-density architectures. They do an excellent job of providing some bandwidth everywhere, but cannot compete with terrestrial architectures in providing maximum capacity in metropolitan areas. Stratospheric and ground-based millimetre wave systems are high-density architectures. These designs excel at delivering metropolitan consumers the greatest value in terms of cost per unit bandwidth, but are not very cost-effective when it comes to rural service.

In planning for national broadband networks, it is wise to consider the complementary capabilities of high- and low-density system architectures. The best mix of service appears to come from layering a space-based system for rural low-density broadband service with a stratospheric system for metropolitan high-density broadband service. In addition, ground-based millimetre wave equipment should be considered for ultra-high reliability links, either in metropolitan or rural geographic areas.

It is important to not confuse low-density market segments with developing parts of the world. Indeed, most of the world’s most rapid metropolitan growth is expected to come from the developing world. The developing world’s mega-cities — from Cairo and Lagos to Jakarta and Bombay — are high-density market segments which need the stratospheric architecture in order to ensure mass access to the broadband channels which are essential to rapid economic development.

Wireless local loop and global information development

Cheap bandwidth will be as essential to economic development in the 21st century as cheap power was to industrialization in the 20th century. Developing countries must be assured of having ample, low-cost access to high-density broadband telecommunication links. In order to achieve this goal, it is critical for developing countries to include stratospheric platforms, such as the Sky Station stratospheric platform, in their national telecommunication plans. If developing countries rely solely on satellites for their broadband links, they will find themselves in the unfortunate situation of having only low-density broadband capability to meet high-density broadband demand. This would lock developing countries into an inferior information infrastructure which is inconsistent with the International Telecommunication Union’s (ITU) mandate for global information development.

Stratospheric telecommunications platform technology is now available for global deployment, subject to frequency allocation approvals and national business arrangements. The 1997 World Radiocommunication Conference (WRC-97) agenda offers an opportunity to establish stratospheric platform frequency allocations under the subject of "high-density fixed links above 30 GHz". Accomplishment of this agenda item at WRC-97 could be one of the ITU’s most long-lasting achievements, resulting in global information infrastructure (GII) parity in the 21st century, and correspondingly equitable economic growth worldwide.

This text is an extract from ITU News 8/97