Multiplex communications – Diagnostic testing – Determination of communication parameters
Reexamination Certificate
2000-08-03
2003-11-11
Ton, Dang (Department: 2666)
Multiplex communications
Diagnostic testing
Determination of communication parameters
C370S328000, C370S465000, C455S517000, C455S069000
Reexamination Certificate
active
06646995
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The field of the invention is that of digital mobile radio systems. The invention applies in particular to cellular mobile radio systems such as systems conforming to the GSM (Global System for Mobile communications) standard, for example.
More particularly still, the invention concerns the exchange of digital signals, whether they constitute data or speech, in time-division multiple access (TDMA) time-division multiplex systems.
2. Description of the Prior Art
The TDMA technique divides time into frames of fixed and predetermined duration, the frames being in turn divided into time slots. Each call is associated with one or more time slots.
Thus a frame comprises N time slots that can correspond to N calls. Each receiver is able to extract the time slots addressed to it in order to reconstitute the source signal. In this way N calls can be transmitted in the same frequency band.
Where data communications are concerned, mobile radio systems like the GSM system conventionally provide two services, corresponding to two different levels of quality. Thus, for transmission of data, the GSM system provides a first data communication mode called the full rate mode, in which a time slot is transmitted in each frame, and a second transmission mode called the half rate mode, in which the data signal is transmitted in one time slot only every two frames, on average.
In this second mode, the resource allocated to a call is halved compared to the first mode. This frees up resources for other calls.
This halving of the total bit rate of the call naturally requires modification of the channel coding used, to retain the same user bit rate, in other words, the half rate mode corresponds to channel coding with half the yield of that of the full rate mode. In the GSM system, the two modes respectively correspond to raw bit rates of 11.4 kbit/s and 22.8 kbit/s.
Consequently, the efficacy of half rate channel coding is less than that of full rate channel coding. For this reason the half rate mode can be used only when transmission conditions are good and/or the transmission quality required is average, in other words when a relatively high bit error rate can be tolerated. If the transmission channel is subject to interference and/or the data requires a higher transmission quality (i.e. a lower bit error rate) the full rate mode must be used.
According to the GSM standard, a transmission mode is chosen at the time the call is set up and is retained throughout the call. This technique has two drawbacks:
if the service in question requires a transmission quality corresponding to the use of the half rate mode under normal conditions of operability and if the half rate mode is adopted, should the channel then be subjected to a higher level of interference, exceeding the operability limit of the system (set at a C/I value of approximately 9 dB), the call in progress is suddenly cut off; under difficult coverage conditions a value of C/I below 9 dB may be encountered;
if the service in question requires a transmission quality corresponding to the use of the full rate mode under normal conditions of operability and if the full rate mode is adopted, should the channel subsequently be subject to a lower degree of interference, the channel coding employed is of higher quality than is required; the transmission channel is therefore unnecessarily occupied in alternate frames (causing unnecessary interference in neighboring cells).
In mobile radio systems these problems are major problems since the transmission channel changes continually with the movement of the mobile station and the movement and the activity of the sources of interference, etc. As a result, the full rate mode is usually chosen, for safety, and this leads to high and often unnecessary consumption of the transmission resource.
There are also two configurations in the case of speech signals (full rate mode and half rate mode), which correspond to the use of different speech encoders (source coding) and different channel encoders, the two pairs of encoders (source and channel) providing respective raw bit rates of 22.8 kbit/s (full rate) and 11.4 kbit/s (half rate).
Problems similar to those described above for data are also encountered with speech.
A major objective of mobile radio system designers is to limit the quantity of data transmitted, for a number of reasons and in particular:
to increase the number of calls in the multiplex;
to reduce the transmission time (in the case of transmitting data);
. . .
To this end, U.S. Pat. No. 5,327,576 proposes modification of the mode of transmission used, during a call, on the basis of the measured bit error rate.
To be more precise, in the method described in the above application, the base transceiver station (the station managing all calls with mobiles in a given cell) measures the bit error rate of a given call and selects one or other of the transmission modes according to the measured error rate.
This technique improves transmission resource use. It has a number of drawbacks, however, that the novel technical approach of the invention clearly highlights. In particular, it is based on an analysis of the transmission channel as seen from the base transceiver station only (or from the mobile station only), which leads to non-optimum resource use, as will emerge below.
One object of the invention is to overcome these various drawbacks of the prior art.
To be more precise, one object of the invention is to provide a method of adaptation of the air interface (essentially corresponding to layers
1
(physical) and
2
(link) of the ISO model) in a mobile radio system that minimizes the occupancy of transmission channels by reducing the quantity of resource allocated to a call on average and by limiting interference induced by a call in neighboring cells.
This object of limiting interference is crucial in cellular mobile radio systems in particular. In these systems, the same frequency band is allocated to several geographically dispersed cells. Although the distribution of the cells is defined to maximize the distance between them, it is by no means rare for the signals of a given cell to suffer interference from those of other cells using the same band to a degree that is above an acceptable limit for the system.
In a cellular system, a maximum interference level enabling the specified transmission quality to be provided is usually fixed. An object of the invention is therefore to provide a method of the above kind whereby the specified transmission quality continues to be provided if the interference exceeds this maximum level.
An object of the invention is therefore to provide a method of the above kind in which the untimely cutting off of calls is reduced.
In other words, an object of the invention is to expand the range of operability of the system, in particular under difficult transmission conditions.
Another essential and primordial object of the invention is to provide a method of the above kind whereby the number of calls can be increased. In other words, an object of the invention is to reduce, on average, the resource used to transmit a service in order to increase the number of users in the system, i.e. the number of calls per cell.
In one particular embodiment of the invention, a secondary object of the invention is to provide a method of the above kind for transmitting asynchronous data easily and as fast as possible, in particular when it is not possible to free up the same resource in both communication directions.
Another object of the invention is to provide a method whereby the network infrastructure is simplified. In the conventional way, network planning must offer acceptable operability (C/I~9 dB in the GSM system) everywhere (or over a certain portion of the area of each cell), which in some cases constitutes a very serious constraint.
An object of the invention is therefore to remove this constraint by widening the operability range (beyond 9 dB, for example in the case of the GSM system) to allow more efficient planning by
Delprat Marc
Le Strat Evelyne
Alcatel CIT
Ton Dang
Tran Phuc
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