Daniel M. Kohn <dan@teledesic.com>
Teledesic Corporation
USA
Using a constellation of several hundred low-Earth-orbit satellites--a global, broadband "Internet-in-the-sky," Teledesic will enable affordable access to fiber-like telecommunications capability anywhere in the world. The Teledesic Network will allow local service providers to extend their networks in terms of both scope of services and geographic reach. It will be a local service provided through a global network.
Keywords: wireless, broadband, worldwide, real-time, Internet access, satellites, low-Earth-orbit.
Access to information is becoming increasingly essential to all those things we associate with quality of life: economic opportunity, education, health care, and public services. Yet, most people and places in the world do not now have access even to basic telephone service. Even those who do have access to basic phone service get it through 100-year-old technology--analog copper wire networks--that for the overwhelming part will never be upgraded to an advanced digital capability.
While many places in the world are connected by fiber--and the number of places is growing--it is used primarily to connect countries and telephone company central offices. Even in the most developed countries, high cost will prevent the deployment of a significant amount of fiber for local access to most individual offices and homes. In most parts of the world, fiber deployment in the local access network likely never will happen.
This lack of broadband local access is a major problem for all of the world's societies. If these powerful technologies are available only in advanced urban areas, people will be forced to migrate to those areas in search of economic opportunity and to fulfill other needs and desires. It is no longer sound--economically or environmentally--to force people to migrate to increasingly congested urban areas in search of opportunity. The real potential of the information age is to find a means of allowing people to choose where they live and work based on things like family, community, and quality of life rather than access to infrastructure.
The one-way information dissemination made possible through broadcast technologies has created a means for nearly the entire world to view the benefits of advanced technology. But having created a means for everyone to see all the benefits of our societies we have also created expectations--legitimate expectations--that will seek fulfillment. Increasingly, even a sole proprietor in the developing world will need the same kind of connection to the "Global Village" available now only to the biggest, richest corporations. Through schools, community centers, and home access, individuals are beginning to use broadband connections for services such as Internet access, telemedicine, distance learning, videoconferencing, telecommuting, and many other applications. We need to create the two-way network links that allow people to participate economically and culturally with the world at large without requiring that they pick up and move to places with modern telecommunications infrastructure.
For more than three decades, geostationary satellites have been virtually the exclusive means of providing commercial space-based communications. Geostationary satellites will continue to play an important role, particularly for broadcast applications. However, these systems have a number of limitations for two-way communications, such as the need for high-power terminals and the signal delay caused by their high altitude. This delay means that a large number of applications, including essential Internet technologies such as the World Wide Web, are adversely affected--or simply don't work--over geostationary satellites. Because of their delay, geostationary satellites can never provide fiber-like delays to be seamlessly compatible with fiber-based networks on the ground. For natural economic reasons, these systems also tend to focus their capacity on the more economically developed areas. Via Satellite recently reported, for instance, that of over 200 geostationary commercial communications satellites, only one is on order to provide service to Africa.
New options are becoming available, however, with the development of non-geostationary communication systems, which primarily use low-Earth-orbit (LEO) satellites. LEO satellite systems can help meet the demand for information by providing global access to the telecommunications infrastructure currently available only in advanced urban areas of the developed world. The low altitude of LEO systems allows them to provide delays that are seamlessly compatible with terrestrial networks. Just as networks on the ground have evolved from centralized systems built around a single mainframe computer to distributed networks of interconnected PCs, space-based networks are evolving from centralized networks relying on a single geostationary satellite to distributed networks of interconnected low-Earth-orbit satellites.
The evolution from geostationary to LEO satellites has resulted in a number of proposed global satellite systems, which can be grouped into three distinct types. These LEO systems can best be distinguished by reference to their terrestrial counterparts: paging, cellular, and fiber.
|
System Type |
Little LEO |
Big LEO |
Broadband LEO |
|
Example |
ORBCOMM |
Iridium, Globalstar, ICO |
Teledesic |
|
Terrestrial Counterpart |
Paging |
Cellular |
Fiber |
Using a constellation of several hundred LEO satellites, Teledesic will enable affordable access to fiber-like telecommunications services to institutions and individuals anywhere in the world. This ability to deliver fiber-like, broadband, digital transmission capability at low cost, regardless of location, distinguishes the Teledesic Network from other existing and proposed communications systems.
Teledesic was founded in 1990 and is headquartered in Kirkland, Washington, a suburb of Seattle. Teledesic's principal shareholders are Craig O. McCaw and William H. Gates III. Mr. McCaw, who leads the company as its chairman, is the founder of McCaw Cellular Communications, which he built into the world's largest wireless communications company before its 1994 merger with AT&T. Mr. Gates is the co-founder, chairman, and CEO of Microsoft Corporation, the world's largest computer software company.
At the 1995 World Radio Conference, Teledesic received support from the developed and developing world alike, resulting in a new international satellite service designation for the frequencies necessary to accommodate the Teledesic Network. The action of the World Radio Conference mirrors Teledesic's success in obtaining a similar designation from the U.S. Federal Communications Commission (FCC). Teledesic is well positioned for FCC licensing in the near future.
Teledesic plans to begin service in the year 2002. Teledesic does not intend to market services directly to end users. Rather, it will provide an open network for the delivery of such services by others. The Teledesic Network will enable local telephone companies and government authorities in host countries to extend their networks, both in terms of geographic scope and in the kinds of services they can offer. Ground-based gateways will enable service providers to offer seamless links to other wireline and wireless networks.
Teledesic uses small, "Earth-fixed" cells both for efficient spectrum utilization and to respect countries' territorial boundaries. Within a 53 by 53 km cell, the network will be able to accommodate over 1,800 simultaneous 16 Kbps voice channels, 14 simultaneous E-1 (2 Mbps) channels, or any comparable combination of channel bandwidths. This represents a significant system capacity, equivalent to 20,000 simultaneous E-1 lines worldwide, with the potential for graceful growth to higher capacities. The network offers high-capacity "bandwidth on demand" through standard user terminals. Channel bandwidths are assigned dynamically and asymmetrically and range from a minimum of 16 kbps up to 2 Mbps on the uplink, and up to 28 Mbps on the downlink. Teledesic will also be able to provide a smaller number of high-rate channels at 155 Mbps to 1.2 Gbps for gateway connections and users with special needs. The low orbit and high frequency (30 GHz uplink/20 GHz downlink) allow the use of small, low-power terminals and antennas, with a size and cost comparable to a notebook computer.
The Teledesic constellation design supports the network requirements for quality, capacity, and integrity. To provide high-quality, high-speed wireless channels at the intended peak-user density levels requires substantial bandwidth. The only feasible frequency band internationally allocated to fixed satellite service that meets Teledesic's requirements is the Ka band. High rain attenuation, terrain blocking, and other terrestrial systems in this band make it difficult for earth terminals to communicate reliably with a satellite at a low elevation angle. The Teledesic constellation uses a high elevation mask angle to mitigate these problems. A low orbit altitude is used to meet the requirements for low end-to-end delay and reliable communication links that use low power and small antennas. The combination of low altitude and high elevation angle results in a small coverage area per satellite and a large number of satellites for global coverage. A high degree of coverage redundancy and the use of on-orbit spares support the network reliability requirements.
One or more local service providers in the United States and in each host country will serve end users. User terminals communicate directly with Teledesic's satellite-based network to other terminals and through gateway switches to other networks, such as the Public Switched Telephone Network and the Internet.
The network uses fast packet switching technology, with a packet design similar to asynchronous transfer mode (ATM). All communication is treated identically within the network as streams of short fixed-length packets. Each packet contains a header that includes address and sequence information, an error-control section used to verify the integrity of the header, and a payload section that carries the digitally encoded voice or data. Conversion to and from the packet format takes place in the terminals. The fast packet switch network combines the advantages of a circuit-switched network (low delay "digital pipes") and a packet-switched network (efficient handling of multi-rate and bursty data). Fast packet switching technology is ideally suited for the dynamic nature of a LEO network.
Each satellite in the constellation is a node in the fast packet switch network and has intersatellite communication links with eight adjacent satellites. Each satellite is normally linked with four satellites within the same plane (two in front and two behind) and with one in each of the two adjacent planes on both sides. This interconnection arrangement forms a non-hierarchical "geodesic," or mesh, network and provides a robust network configuration that is tolerant to faults and local congestion.
The topology of a LEO-based network is dynamic. Each satellite keeps the same position relative to Ether satellites in its orbital plane. Its position and propagation delay relative to earth terminals and to satellites in other planes change continuously and predictably. In addition to changes in network topology, as traffic flows through the network, queues of packets accumulate in the satellites, changing the waiting time before transmission to the next satellite. All of these factors affect the packet routing choice made by the fast packet switch in each satellite. These decisions are made continuously within each node using Teledesic's distributed adaptive routing algorithm. This algorithm uses information transmitted throughout the network by each satellite to "learn" the current status of the network in order to select the path of least delay to a packet's destination. The algorithm also controls the connection and disconnection of intersatellite links.
The network uses a "connectionless" protocol, similar to the routing of the Internet Protocol (IP). Packets of the same connection may follow different paths through the network. Each node independently routes the packet along the path that currently offers the least expected delay to its destination. The required packets are buffered, and if necessary resequenced, at the destination terminal to eliminate the effect of timing variations. Teledesic has performed extensive and detailed simulation of the network and adaptive routing algorithm to verify that they meet Teledesic's network delay and delay variability requirements.
All of the Teledesic communications links transport data and voice as fixed-length (512 bit) packets. The basic unit of channel capacity is the "basic channel," which supports a 16 kbps payload data rate and an associated 2 kbps "D-channel" for signaling and control. Basic channels can be aggregated to support higher data rates. For example, eight basic channels can be aggregated to support the equivalent of a 2B + D Integrated Services Digital Network (ISDN) link, or 97 channels can be aggregated to support an equivalent T-1 (1.544 Mbps) connection. A Teledesic terminal can support multiple simultaneous network connections. In addition, the two directions of a network connection can operate at different rates.
The links are encrypted to guard against eavesdropping. Terminals perform the encryption/decryption and conversion to and from the packet format. The uplinks use dynamic power control of the radio frequency transmitters so that the minimum amount of power is used to carry out the desired communication. Minimum transmitter power is used for clear sky conditions. The transmitter power is increased to compensate for rain.
The Teledesic Network accommodates a wide variety of terminals and data rates. Standard terminals will include both fixed-site and transportable configurations that operate at multiples of the 16 kbps basic channel payload rate up to 2.048 Mbps (the equivalent of 128 basic channels). These terminals can use antennas with diameters from 16 cm to 1.8 m as determined by the terminal's maximum transmit channel rate, climatic region, and availability requirements. Their average transmit power varies from less than 0.01 W to 4.7 W depending on antenna diameter, transmit channel rate, and climatic conditions. All data rates, up to the full 2.048 Mbps, can be supported with an average transmit power of 0.3 W by suitable choice of antenna size.
Within its service area, each satellite can support a combination of terminals with a total throughput equivalent to over 125,000 simultaneous basic channels.
The network also supports a smaller number of fixed-site GigaLink Terminals that operate at the OC-3 rate (155.52 Mbps) and multiples of this rate up to OC-24 (1.2 Gbps). Antennas for these terminals can range in size from 28 cm to 1.6 m as determined by the terminal's maximum channel rate, climatic region, and availability requirements. Transmit power will range from 1 W to 49 W depending on antenna diameter, data rate, and climatic conditions. Antenna site-diversity can be used to reduce the probability of rain outage in situations where this is a problem.
GigaLink Terminals provide gateway connections to public networks and to Teledesic support and data base systems including Network Operations and Control Centers (NOCCs) and Constellation Operations Control Centers (COCCs), as well as to privately owned networks and high-rate terminals. A satellite can support up to sixteen GigaLink terminals within its service area.
Intersatellite links (ISLs) interconnect a satellite with eight satellites in the same and adjacent planes. Each ISL operates at 155.52 Mbps, and at multiples of this rate up to 1.24416 Gbps depending upon the instantaneous capacity requirement.
One benefit of a small satellite footprint is that each satellite can serve its entire coverage area with a number of high-gain scanning beams, each illuminating a single small cell at a time. Small cells allow efficient reuse of spectrum, high channel density, and low transmitter power. However, if this small cell pattern swept the Earth's surface at the velocity of the satellite (approximately 25,000 km per hour), a terminal would be served by the same cell for only a few seconds before a channel reassignment or "hand-off" to the next cell would be necessary. As in the case of terrestrial cellular systems, frequent hand-offs result in inefficient channel utilization, high processing costs, and lower system capacity. The Teledesic Network uses an Earth-fixed cell design to minimize the hand-off problem.
The Teledesic system maps the Earth's surface into a fixed grid of approximately 20,000 "supercells," each consisting of nine cells. Each supercell is a square 160 km on each side. Supercells are arranged in bands parallel to the Equator. There are approximately 250 supercells in the band at the Equator, and the number per band decreases with increasing latitude. Since the number of supercells per band is not constant, the north-south supercell borders in adjacent bands are not aligned.
A satellite footprint encompasses a maximum of 64 supercells, or 576 cells. The actual number of cells for which a satellite is responsible varies by satellite with its orbital position and its distance from adjacent satellites. In general, the satellite closest to the center of a supercell has coverage responsibility. As a satellite passes over, it steers its antenna beams to the fixed cell locations within its footprint. This beam steering compensates for the satellite's motion as well as the Earth's rotation. (An analogy is the tread of a bulldozer that remains in contact with the same point while the bulldozer passes over).
Channel resources (frequencies and time slots) are associated with each cell and are managed by the current "serving" satellite. As long as a terminal remains within the same Earth-fixed cell, it maintains the same channel assignment for the duration of a call, regardless of how many satellites and beams are involved. Channel reassignments become the exception rather than the norm, thus eliminating much of the frequency management and hand-off overhead.
A database contained in each satellite defines the type of service allowed within each Earth-fixed cell. Small fixed cells allow Teledesic to avoid interference to or from specific geographic areas and to contour service areas to national boundaries. This would be difficult to accomplish with large cells or cells that move with the satellite.
Teledesic's engineering effort builds on previous work done in many advanced commercial and government satellite programs and was assisted by several government laboratories. The Teledesic system uses proven technology and experience from many U.S. defense programs, including the "Brilliant Pebbles" program, which was conceived as a similar orbiting global constellation of 1,000 small, advanced, semi-autonomous, interconnected satellites. Since 1990, Teledesic has drawn on the expertise of the contractors on that and many other programs for input into the early system design activities.
Design, construction, and deployment costs of the Teledesic Network are estimated at $9 billion. The Teledesic Network represents the first time that satellites and their associated subsystems will be designed and built in quantities large enough to be mass-produced and tested. These substantial economies of scale enable a cost structure comparable to that of wireline service in advanced urban areas.
The Teledesic Network emulates the most famous distributed network, the Internet, while adding the benefits of real-time connections, location-insensitive access, and broadband-on-demand capability. Because their low altitude eliminates the delay associated with traditional geostationary satellites, these networks can provide communications that are seamlessly compatible with terrestrial, fiber-based standards.
Because LEO satellites move in relation to the Earth, they all share a characteristic with profound implications: Continuous coverage of any point on Earth requires--in effect, global coverage. In order to provide service to the advanced markets, the same quality and quantity of capacity has to be provided to the developing markets, including those areas to which it would not be economically feasible to provide that kind of capacity for its own sake. In this sense, LEO satellite systems represent an inherently egalitarian technology that promises to radically transform the economics of telecommunications infrastructure to enable universal access to the Information Age.