offset parameter indicating the next mbs_map_ie may be set as next mbs_map_ie ofdma symbol offset at block 105 an information element length parameter indicating the actual length of the next mbs_map_ie is set in the mbs_map_ie the information element length parameter indicating the actual length of the next mbs_map_ie is used to indicate the specific length of the next mbs_map_ie so that the mobile terminal may set a listening duration of the mobile terminal according to the specific frame length for example at this block the information element length parameter may be set as next mbs_map_ie length by performing actions at blocks 101 to 105 in the present invention the parameters relating to the next mbs_map message are deleted from the mbs_map_ie and the parameters indicating the specific receiving position of the next mbs_map_ie and that indicating the length of the next mbs_map_ie is added therefore a redefined mbs_map_ie is acquired accordingly and the specific format of the redefined mbs_map_ie is shown in table 2 at block 106 at the frame time point when the base station transmits the mbs data to the mobile terminal the base station transmits the downlink message carrying the mbs_map_ie and the mbs data to the mobile terminal the downlink message may be an existing dl_map.