Multiplex communications – Channel assignment techniques – Combining or distributing information via code word channels...
Patent
1996-02-06
1999-08-10
Ton, Dang
Multiplex communications
Channel assignment techniques
Combining or distributing information via code word channels...
370527, H04J 316
Patent
active
059369612
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention concerns a signalling packet for a communication system.
In communication systems, for example the GSM pan-European digital cellular mobile radio system, a terminal and a base station communicate by means of communication channels carrying radio signals. Systems of this kind include 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 a 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 usually therefore called the synchronization procedure.
In the solution generally adopted 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 that set out in GSM Recommendations 4.08 and 5.08.
The synchronization procedure is executed systematically when the terminal is switched on and, more generally, after any loss of synchronization, i.e. if the radio link carried by the control channel between the base station and the terminal is interrupted. This may be intentional, for example if the terminal is switched off, or unintentional. The terminal may be temporarily unable to synchronize because radio reception conditions are inadequate. This occurs in a tunnel, for example, or more generally when it is in a shadow area in the radio sense of this term.
Synchronization usually comprises two phases. The first or frequency synchronization phase consists in acquiring the frequency reference of the base station. The second or time synchronization phase consists in acquiring the time reference of the base station.
To this end, in the GSM system the control channel BCCH includes two sub-channels, namely a frequency control sub-channel FCH for the frequency synchronization and a synchronization sub-channel SCH for the time synchronization.
The frequency control sub-channel is in the form of a packet corresponding to a pure sinusoid transmitted at regular time intervals. The terminal must therefore look for this packet for a time period that substantially corresponds to the packet repetition period.
Because of the nature of the packet and the means employed to detect it, it is not possible to determine its start or its duration, and it is therefore necessary to use the synchronization sub-channel to achieve the necessary time synchronization.
The synchronization sub-channel follows the frequency control sub-channel with a known time-delay. It comprises a sequence of symbols having a suitable autocorrelation function. The terminal knows this synchronization sequence and correlates this sequence with the sequence of symbols received. Because of the inaccuracy of the time reference of the base station, the terminal does not know with certainty which of the symbols received corresponds to the first symbol of the synchronization sequence and many correlations are therefore required, shifting one of the sequences relative to the other, in order to identify the correlation peak.
A first aim of the invention is therefore to improve the performance of the synchronization procedure.
In most radio communication systems the radio signal transmission subsystem includes an element that is mobile. As a result, the frequency of the radio signal is modified because of the Doppler effect.
A Doppler shift naturally occurs in the GSM system if the terminal is moving, and its value is directly proportional to the speed of the terminal. Although it is r
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Chiodini Alain
Kumar Vinod
Thienpont Jean-Marc
Alcatel Mobile Phones
Ton Dang
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