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PATENTS ACT 1953 :":^0 <br><br>
COMPLETE SPECIFICATION <br><br>
"MOBILE PHONE SYNCHRONIZING DEVICE" <br><br>
WE, ALCATEL AUSTRALIA LIMITED, ' (fKtf 000005 3&) A Company of the State of New South Wales, of 280 Botany Road, Alexandria, New South Wales, 2015, Australia, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: <br><br>
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This invention relates to a synchronisation device for radiocommunications terminal. <br><br>
In radiocommunications systems, such as for instance the digital cellular Paneuropean radiocommunication system known as GSM, communications transit between a terminal and a base station via communication channels carrying radio signals. Such systems include several channels designed either for transmission from terminals to base stations or for transmission from base stations to terminals. <br><br>
Among these channels is a control channel which is continuously transmitted, allowing the terminal to access the system via the base station transmitting this channel in order to establish communications. Therefore, the terminal must identify this control channel in order to acquire the data it needs to declare itself in the system. Among this data is found the synchronisation data, this is why it is common to talk about synchronisation procedure. <br><br>
The solution generally adopted for synchronisation occurs in two stages. Firstly, the terminal measures the power of all channels dedicated to reception. Then, the terminal tries to be synchronised with the strongest channel received, if it fails, it tries the other channels in descending order of received power, until it is successfully synchronised. This solution is in fact the one mentioned in GSM recommendations 4.08 and 5.08. <br><br>
Synchronisation procedure is systematically started when the terminal <br><br>
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is started, and, more generally, when there has been a loss of synchronisation, i.e. when the radio link, forwarded in particular by the control channel between base station and terminal, is interrupted. This interruption may be voluntary, the terminal may have been stopped, but it may also be involuntary. Indeed, it may temporarily be impossible for the terminal to be synchronised due to unsatisfactory radio reception conditions. For instance, this is the case when the terminal is in a tunnel, or more generally, when it is located in a shaded area (in the radio sense of the word). <br><br>
The terminal then successively tries without success to be synchronised with each of the receive channels during a synchronisation period, it then tries again during repeated synchronisation periods until reception conditions return to normal. Such a synchronisation period may be of a significant duration which depends on the number of receive channels and the time allocated to the terminal to try and be synchronised with a channel. As an example, this period can be estimated to last 14 seconds in the GSM system. During a synchronisation period whose duration is in this instance preset, the terminal is incapable of establishing a call, it is unavailable. <br><br>
It is an object of the present invention is to reduce this synchronisation period. <br><br>
According to the invention, there is provided a synchronisation device for a radiocommunications terminal, to identify a control channel among <br><br>
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several receive channels, including means of measuring the receive channel power, means of compiling a list where these receive channels are sorted in descending order of power and some means of finding the control channel among these receive channels by scanning this list, and also including means of selection allowing a receive channel to be entered in the list only if its power exceeds a preset value corresponding to a detection threshold. <br><br>
Thus, the synchronisation period is reduced proportionally to the number of channels exceeding this detection threshold over the total number of receive channels. An attempt at synchronisation occurs only with channels being powerful enough to allow terminal synchronisation. <br><br>
Furthermore, the terminal having a receive sensitivity, and the control channel being identifiable only if the difference between its power and this sensitivity is greater than an assessment limit, the value of the detection threshold is the sum of the sensitivity and this assessment limit. <br><br>
The invention will become clearer in the light of the description of a method of implementation given as an example to be read in conjunction with the attached figure representing the components of a terminal required for this implementation. <br><br>
Although it is a misuse of the term, for easier reading purposes, a channel will be likened to the radio signal it carries, in this instance each channel being characterised by the frequency of the carrier wave of the corresponding radio signal. <br><br>
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The terminal, as is common knowledge, mainly includes an antenna (1) designed for the reception of various channels, a receive circuit (2) such as a frequency synthetiser connected to the antenna (1) which selects among these channels a tuning channel (T) in response to a regulation signal (R), a power measurement circuit (3) which produces a power signal (P) <br><br>
representing the power of tuning channel (T), and a control circuit (4) such as a microprocessor which produces regulation signal (R) and receives power signal (P) to write it to working memory (5) by means of data signal (D) at an address allocated to tuning channel (T), this by means of an address signal (Ad). <br><br>
As is also common knowledge, control circuit (4) has access to the identity of all channels that can be received by the terminal. As an example, it may be a channel memory (6) which, for each of these channels, associates a reference value of regulation signal (R) with an address of working memory (5). Hence, control circuit (4) first produces the regulation signal (R) having one of the reference values and writes to the working memory (5) the value of power signal (P) at the address associated with this reference value in channel memory (6). The control circuit repeats this operation for all the reference values such that working memory (5) contains a power value for each of the channels. <br><br>
The invention proposes to eliminate the channels received with such a weak power that they cannot possibly permit terminal synchronisation. <br><br>
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To this end, control circuit (4) identifies in working memory (5) all channels whose power value is lower than a preset detection threshold below which synchronisation is impossible. This may be achieved by setting to 1 an indicator, i.e. a bit specific to the location of the corresponding address in working memory (5), for instance. <br><br>
It may also be envisaged that this threshold be set as a function of the terminal and control channel characteristics. Indeed, the terminal has a sensitivity which is the receive power lower limit below which it cannot differentiate between noise and a useful signal; we also talk about terminal noise level and this value may be likened to sensitivity. <br><br>
Furthermore, the control channel can only be identified if its power exceeds the noise level by a minimum value which shall be called assessment limit. Indeed, one must be able to differentiate the useful data contained in the control channel from the noise. <br><br>
This assessment limit is directly dependent on the control channel characteristics and therefore its general definition cannot be given. It corresponds to what the expert in the field usually calls the signal to noise ratio. It can thus be empirically or theoretically determined. <br><br>
In an advantageous method of implementation, the detection threshold is selected to be the sum of the terminal sensitivity and of the assessment limit. <br><br>
Once the channels received with a power lower than the detection <br><br>
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threshold have been identified in working memory (5) by means of indicators, control circuit (4) proceeds again as is common knowledge, but nevertheless it considers only the channels whose indicators are not set to 1. It stores these channels in working memory (5) in a descending order of power by recording for each channel a data indicating the corresponding reference value. Therefore, the result of this operation is a list where reference values are sorted according to the decreasing order of power of the corresponding channels. <br><br>
Control circuit (4) produces as regulation signal the first component of the list, it then controls the terminal as is common knowledge to attempt synchronisation on the corresponding channel. If the attempt is successful, the aim is reached; if it fails, it resumes the operation using the second component in the list and so on until the end of the list. If synchronisation has not occurred after the list has been exhausted, control circuit (4) then repeats all of the above operations, starting by measuring the power of all channels. <br><br>
The following numbered example is given for information only. The terminal sensitivity is equal to -113dBm, the assessment limit is equal to 7dB, such that the terminal will attempt synchronisation only on the channels having a power greater than -106dBm. <br><br>
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