Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Patent
1995-12-06
1998-07-07
Blum, Theodore M.
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
455 121, H04B 7185
Patent
active
057775770
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention concerns satellite selection.
The field of the invention is that of radiocommunications, to be more precise that of satellite radiocommunication networks in which a terminal connects to a satellite by means of radio wave to benefit from services offered by a network of this kind.
The network comprises a plurality of satellites travelling around the Earth, usually in more than one orbital plane, so that a terminal can always access at least one so-called proximity satellite. In the present context it is considered that radio transmission can be established effectively between the terminal and several proximity satellites Moreover, it is understood that the terminal has acquired synchronization to the satellites. If the terminal wishes to access the network, by a process known as logging on, it must choose a satellite from the proximity satellites. Since the satellite is not geostationary, transmission between it and the terminal will eventually be interrupted and at this time the problem arises of assigning a new satellite to the terminal, by a process known as changeover.
Similar problems of both types arise in terrestrial radiocommunication systems, for example the GSM pan-European digital cellular mobile radio system. In this case, a terminal and a base station communicate using communication channels carrying radio signals. These systems comprise a plurality of channels for transmission from the terminals to the base stations or for transmission from the base stations to the terminals.
These channels include a control channel that is transmitted continuously and that enables a terminal to access the system via the base station transmitting this channel in order to set up calls. The terminal must therefore identify this control channel in order to acquire the information enabling it to declare itself within the system. This information includes synchronization information and this procedure is therefore usually called the synchronization procedure.
In the solution generally adopted for such synchronization, which is covered by GSM Recommendations 4.08 and 5.08, the synchronization procedure is carried out in two stages. Initially the terminal measures the power of all the receive channels. The terminal then attempts to synchronize to the channel it receives at the highest power; if it fails to do this, it tries the other channels in decreasing received power order, until it is eventually able to synchronize.
This solution is well suited to terrestrial systems since the higher the power of a channel the better the connection. Furthermore, to a first approximation, the higher this power the shorter the distance between the terminal and the base station, with the result that this procedure tends to optimize the performance of the system.
This solution is not well suited to satellite systems since the signals transmitted by different proximity satellites are attenuated by similar amounts at the terminal. Moreover, the distance between a terminal and a satellite does not have a first order effect on the performance of the system.
Also, in a terrestrial network, when a terminal already connected to a base station receives the signals that the latter is transmitting at too low a power, the terminal is transferred to a new base station by a procedure known as handover. A selector device provided for this purpose selects the new base station, usually on the basis of several criteria. These criteria include the power level at which the terminal receives from the various neighboring base stations around it. An additional criterion often taken into account reflects the number of channels available in these neighboring stations, with the aim of ignoring a base station that is already overloaded.
The received power criterion cannot be used to decide to assign a new satellite to an already connected terminal, by the procedure known as changeover, for the reasons already mentioned.
SUMMARY OF THE INVENTION
Accordingly, an aim of the present invention is to provide me
REFERENCES:
patent: 4797677 (1989-01-01), MacDoran et al.
patent: 4876736 (1989-10-01), Kiewit
patent: 5446465 (1995-08-01), Diefes et al.
Alcatel Mobile Communication France
Blum Theodore M.
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