Cellular radiotelephone signal which enables synchronisation at an entity of a supplementary channel by means of symbol numbering, and corresponding method, terminal and base station

A cellular radiotelephone signal is disclosed of the type that includes a main symmetrical bi-directional channel comprising a main up-link and a main down-link for the low- or medium-speed transmission of signaling and control information and data; and at least one supplementary channel which is assigned solely to the down-link and which is intended for high-speed data transmission using a multi-carrier technique, which ensures that the data are distributed over frequency/time space and which is structured in the form of pre-defined entities. Each entity contains a pre-determined number of symbols. At least one symbol from each entity of the supplementary channel includes a position identifier, which can be used to identify the position of the symbol within the entity, such that a terminal can read the symbol position identifier and deduce therefrom the start of the next entity, thereby obtaining synchronisation at the relevant entity of the supplementary channel.