The present invention relates to a method of selecting transmission channel in a time division multiple access protocol and to a communication system using the method of the invention.
In the state of the art, for time division multiple access protocols, methods have already been proposed for selecting a transmission channel from a plurality of transmission channels as a function of the transmission conditions measured on each of those channels. These methods apply in particular to the case of a radio transmission between a base station and a set of mobile telephones, the quality of transmission of each channel linking the base station to a mobile telephone varying independently according to the mobility of the telephone and the change in its radio environment.
Thus a method described in document U.S. Pat. No. B1-6,449,490 consists in:                receiving for each channel a value representing the transmission rate currently achievable on that channel;        determining for each channel the transmission rate achieved on that channel during a time window;        selecting the channel for which the ratio of transmission rate currently achievable to the transmission rate achieved during a time window is the highest.        
The value of this method relative to a selection method that does not take account of the variations in the quality of transmission of the channels is to increase the transmission rate of each channel by selecting the one on which the transmission conditions are the most favourable. The selection is made on the basis of the ratio of the transmission rate achievable to the transmission rate achieved to provide relatively equitable access to the transmission resource. The drawback of this method is that a channel on which the achieved rate would be limited for a reason other than the sharing of the transmission resource (a constraint linked to the reception capability of the mobile telephone for example) could be selected systematically, even when its transmission conditions are bad.
Another method aiming to take advantage of the independent variations in the transmission quality of the channels and described in the same document consists in:                receiving for each channel a periodic indication of the transmission quality of that channel;        computing for each channel the average transmission quality of that channel;        selecting the channel for which the ratio of the last transmission quality indication to the average transmission quality is the highest.        
When applying this teaching of the art, it emerges that certain channels would be allocated a much larger portion of the transmission resource than others. In fact, the distribution of the transmission resource depends on the distribution of the random variations of the transmission conditions of the set of channels. In addition, this distribution of the resource and the transmission quality of each resultant selected channel are extremely difficult to evaluate when the distributions of random variations of the transmission conditions of the channels differ, which is the case in practice.