digital television broadcasting using cofdm modulation is designed to modulate orthogonal frequency - division - multiplexed mid - band carriers with metadata including synchronization signals and transmission - mode signals . dtv signals modulate ofdm carriers occupying portions of the frequency spectrum of the transmission channel that extend in frequency both below and above these mid - band carriers . the ofdm midband carriers are capable of signaling when a new broadcast service is used that differs from the one disclosed . the signaling is provided by modulating the midband carriers with respective elements of signature sequences , each of which signature sequences is composed of zadoff - chu sequences and repetitive pseudo - random sequences scrambled by a zadoff - chu sequence .