Broadcast preamble such as the P1 symbol in DVB-T2 carries signalling to inform the receivers of the transmission mode and ease initial synchronization. However, all the existing preamble schemes based on the channel correlation of adjacent frequencies may fail to work in practical channel conditions. In this paper, we propose a preamble scheme for OFDM broadcast system, exploiting the correlation of adjacent frequencies in the same time and the correlation of the same frequency in the adjacent time, with robust performance over dispersive fading channels.