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SkyBridge: a broadband access system using a constellation of LEO satellites

by Pascale Sourisse, Denis Rouffet and Hervé Sorre
Alcatel Espace (France)

New satellite solutions are currently being developed to provide access to interactive broadband services. Various types of architectures are considered using either geostationary satellites or innovative constellations of low Earth orbit (LEO) or medium Earth orbit (MEO) satellites. A first generation of such constellations is about to be launched to provide narrow-band mobile communication services (e.g. Globalstar).

Second generation constellations will include broadband capabilities which imply a large amount of spectrum demand. The limitation of available spectrum then becomes a major technical driver for the development of any new satellite system.

The role of satellites in the multimedia arena

Satellites will play a major role to offer a worldwide access to enhanced interactive broadband communication services.

Due to their large geographic coverage, cost-effectiveness, in particular in areas with low or moderate population density, and speed of installation, satellite systems will contribute to the extension of the reach of high-quality telecommunication networks.

Many initiatives have been announced to date providing broadband capabilities and using either geostationary satellites or constellations of LEO or MEO satellites. Geostationary systems and constellations of LEO satellites will have a complementary positioning in the multimedia world.

On the basis of their current very strong position in the market of broadcast services, geostationary systems will develop in new domains including both one-way and two-way asymmetric services with a low-speed terrestrial or space-based return link.

Constellations of LEO satellites will be a very efficient solution to offer highly interactive services. Due to a very short round-trip propagation time over the space segment (typically 20 ms as compared to 500 ms for geostationary systems), LEO systems will offer similar performances to terrestrial networks, thus allowing the use of common communication protocols, standards, and applications.

SkyBridge purpose

SkyBridge is a satellite-based broadband access system, allowing services such as high-speed Internet access and video conferencing to take place anywhere in the world. The system targets urban, suburban and rural areas which are not yet connected to broadband terrestrial infrastructure, or which are uneconomical to cover with traditional infrastructure. This effectively positions SkyBridge as a broadband wireless local loop system.

The system is based on a constellation of 64 LEO satellites which link professional and residential users equipped with low-cost terminals to terrestrial gateways. User terminals are not specific to the system and the architecture of the receiving sites may be adapted to the following configurations: individual reception, community reception (a SkyBridge terminal is shared between several subscribers) and professional configuration where the SkyBridge terminal is connected to a LAN or a PBX (see Figure 1).

Each spot beam (350 km radius) covers a set of SkyBridge terminals connected to one or several user terminals and one or several SkyBridge gateways. A user is registered in a single gateway where the traffic to and from the user is concentrated. The system is open to a wide range of multimedia user terminals designed for terrestrial applications.

A satellite has a 3000 km radius coverage divided into fixed spot beams of 350 km radius. Each user is attached to one and only one SkyBridge gateway. The traffic to and from any of the SkyBridge users will always be concentrated on the gateway where the user is registered. Each gateway acts as an autonomous access network due to the transparent payload of the satellites.

Each satellite is in a circular orbit at an altitude of 1457 km above the Earth. The constellation is divided into two symmetrical Walker sub-constellations of 32 satellites each. The low-path delay (20 ms typically) of the LEO constellation chosen for SkyBridge is compatible with TCP/IP-like protocols used for terrestrial applications. Thus, communications over SkyBridge are not disrupted by handshaking protocols which drastically reduce the data rate over high-delay links. Moreover, the low delay of LEO satellites contributes to high service quality links similar to terrestrial networks ensuring seamless integration into terrestrial networks. The SkyBridge system is fully adapted to interactive applications with real-time constraints (see Figure 2).

The SkyBridge system is a global telecommunication system designed to provide business and residential users with interactive multimedia application as well as LAN interconnection or classical ISDN applications. It is aimed at fixed terminals including those which will allow user mobility and terminal portability. The system accommodates highly interactive real-time applications aided by the low delay inherent to the system.

SkyBridge services and market

SkyBridge will deliver an asymmetrical broadband connection to the fixed network with, for residential terminals, up to 60 Mbit/s (in increments of 16 kbit/s) to the user and up to 2 Mbit/s (in increments of 16 kbit/s) on the return link via a gateway. Higher bit rates will be accommodated by professional terminals. This design is optimized for Internet communications which are characterized by random bursts of asymmetrical data transmission. In addition, the small size of the increments will provide the user with bandwidth on demand.

The basic concepts of its architecture position SkyBridge in the field of the highly interactive real- time applications such as:

• high-speed access to Internet, and more generally, on-line services;
• telecommuting through access to business servers and local area networks, electronic mail and file transfers;
• video conference and videotelephony;
• telemedecine;
• entertainment services: interactive video-on-demand and electronic games.

In addition, SkyBridge will provide:

• infrastructure links for interconnection of cells such as wireless loops;
• enhanced narrow-band services for voice, videoconference and data transmission.

SkyBridge positioning versus alternative technologies

Several land-line technologies will provide broadband access. However, SkyBridge, in many ways, will not compete with these systems, but rather provide an attractive complementary solution.

SkyBridge’s performances are comparable to ADSL/XDLS (asymmetric digital subscriber line) and HFC/FTTC (hybrid fibre coax/fibre to the curb) land-line technologies but several differences linked to cost and development speed suggest that SkyBridge will be more effective to target lower population density markets and will, in addition, provide an attractive "precursor" solution for higher population density markets.

SkyBridge a system that reuses the existing spectrum

The arising broadband networks will have to cope with the scarcity of the available spectrum resource. The first idea in order to offer the desired service can be to use higher frequencies, Ka band for instance.

But, SkyBridge provides a totally new approach. It is conceived for an operation in the Ku band allowing full frequency reuse. SkyBridge has been engineered since Day 1 of the programme to be able to reuse frequencies already used by the other services operating in the same band, including both geostationary system and terrestrial services. This, of course without creating interferences or additional operational constraints to those services.

SkyBridge, by the simple use of efficient mitigation techniques, makes additional spectrum available on a global basis.

How does SkyBridge protect geostationary satellite system?

To avoid creating interference with geostationary systems, a non-operating zone of the SkyBridge system is defined around the geostationary arc. This non-operating zone which covers 10° on both sides of the GSO arc as seen from the ground is calculated taking into account the antenna directivity and the different power levels. It is important to note that the non-operating zone is calculated for the whole geostationary arc in order to protect existing and future systems. The SkyBridge system will then be able to operate whatever the evolution of the GSO is. In addition, the operation of SkyBridge does not lead to any reduction of orbit and spectrum resources available for the implementation of geostationary systems.

Different techniques are applied to maintain the interference level at a non-visible level:

• When a SkyBridge satellite enters the non-operating zone as seen from a SkyBridge earth station, the beam coming from this satellite, and illuminating the zone under consideration, is extinguished and the traffic is handed over to another satellite of the constellation through a fully transparent procedure for the end user.
  
• The same technique applies to avoid interferences from SkyBridge gateway or terminal towards a geostationary terminal.

For other services such as terrestrial services, the SkyBridge system complies with pfd limitations included in the International Telecommunication Union (ITU) regulations thus protecting them.

SkyBridge is a very attractive solution to provide access in a cost effective manner on a global basis to a very wide range of high-quality broadband and narrow-band services, thus contributing to the objective of universal access to communication services.

This text is an extract from ITU News 8/97