The present invention relates to geostationary satellite telecommunications systems enabling information to be interchanged between terrestrial terminal (user terminals, servers, etc.) having a very wide variety of requirements concerning bandwidth and transmission delays.
More particularly, the invention relates to systems using one or more satellites having several tens of beams (generally 20 to 60), with it being possible for the number of beams to be different on up links and on down links. Such a system is in use in particular for providing telecommunications in Europe, with forty beams, each having an aperture of about 0.6xc2x0.
An attractive architecture consists in mounting a payload on the satellite for providing full connectivity and flexibility and also for regenerating messages.
Connectivity makes it possible to route traffic other than local traffic (i.e. between two terminals placed in the same beam) from one beam to another. It requires routing apparatus to be present on the satellite to send calls directly to the beam in which the receive terminal is to be found, and minimizes the use of terrestrial networks, which are particularly expensive for connections in connected mode.
Flexibility is constituted by the ability of the payload to interchange bandwidth and thus capacity between beams on the up link and/or down link.
A solution which appeared the most appropriate for providing high quality communication and for satisfying the above conditions consists in adopting an asynchronous transmission protocol (ATM) and in providing a payload on the satellite for full regeneration both on the forward link (from ground gateways for providing a connection from the terrestrial network to user terminals) and on the return link (from user terminals). However that approach makes it necessary to have a payload that is complex and not sufficiently mature, in particular because of the requirement for ATM switches to have memory of a type that is not available for satellite installation at present.
Another solution consists in providing the payload in such a manner that it uses the same beams for the up and down paths of a connection. It is based on the assumption that local traffic (within a single beam) between direct users and without intervention of a terrestrial segment is preponderant.
It is an object of the invention to provide a system that constitutes a satisfactory compromise between satellite payload complexity and reducing the use of terrestrial networks, i.e. seeking a minimum routing cost. An ancillary object is to provide a system terminals can be provided which, from the beginning, are suitable for subsequent use in a system having complete connectivity and flexibility with memory ATM switches on board the satellite.
To this end, the invention provides a system comprising a space segment having at least one satellite carrying a multibeam telecommunications payload, and a terrestrial segment comprising gateways giving access to the terrestrial network and that are distributed at a density of one gateway within the coverage of each beam or group of beams, and user terminals provided with means for radiocommunication with the payload of the satellite.
The payload comprises:
on a forward path from a gateway to a user terminal, a frame by frame routing unit for messages transmitted in time division multiplex (TDM); and
on a return path from a user terminal to a gateway, a routing unit operating within each group of carriers belonging to a given beam (the group of a beam possibly having only one carrier, but generally having at least five carriers in a commercial system).
The two routing units can be merged in the form of a common processor or they can belong to processors that are separate. A portion of the transmission capacity can be xe2x80x9cfrozenxe2x80x9d and reserved for local calls, i.e. calls within a given beam, whenever the quantity of local traffic justifies this approach. This reduces the mass of the payload and the power it consumes.
In a system of the above type, the return path is transparent (or in other words non-regenerative). In particular, it can make use of a mode of multiple access transmission relying on carrier frequency division with, for each carrier, each message being shared over time slots of the same rank in successive frames (F/TDMA mode). The absence of message regeneration on board the satellite makes the system open to future evolution and to new standards. It can be compensated by increasing the equivalent isotropic radiated power of the radio portions of the terminals.
In contrast, the forward path can be regenerative. In general, it will use a mode of transmission by time division multiplexing (TDM) with successive frames being carried without interruption and grouped together as superframes.
Under such circumstances, the satellite payload is designed to perform in succession, on the forward path, demodulation, switching, and remodulation. The switch can be of simple type serving merely to provide interconnections. In a solution that is more complex, the switch can have a TDM frame storage memory. It is then possible to send the data packets of the incoming frames to any outgoing modulator by modifying the order of succession of the packets.
The granularity of the system (defining its ability to subdivide the passband into fractions of greater or lesser size) can be high. On the forward link, it can be about 1.5 Mbps by virtue of the frame-by-frame switching. On the return path granularity can be about 5 MHz and can correspond to a data rate of 6xc3x97384 kbps under normal conditions.
The invention also provides a satellite suitable for use in a telecommunications system having a terrestrial segment with, gateways giving access to the terrestrial network, the satellite comprising:
on a forward path, a member for routing transmitted messages frame by frame in time division multiplex (TDM), and message-regeneration means; and
on a return path, a transparent routing member operating in F/DTMA mode within each group of carriers belonging to a given beam.
The above characteristics and others will appear more clearly on reading the following description of a particular embodiment, given by way or non-limiting example. The description refers to the accompanying drawings.