BS digital broadcasting has a high channel capacity, 52 Mbps per carrier, for transmission of two HDTV and variety of data programs in one carrier. One satellite transponder, which is assigned to one carrier, is usually shared by two or more broadcasters. Thus, in contrast with a conventional broadcasting system, the BS digital broadcasting system can simultaneously utilize more than one modulation scheme so as to meet to the transmission requirements of each broadcaster; a high C/N modulation scheme, such as TC8PSK, is employed to maximize channel capacity and a low C/N one, such as QPSK or BPSK, ensures reliable transmission during periods of heavy rainfall. A broadcaster could also choose to use two modulation schemes; simultaneous use of TC8PSK and QPSK realizes a hierarchical transmission scheme that extends the broadcaster's service availability.
Given some combination of modulation schemes, a packet that is modulated by a high C/N modulation scheme deteriorates quicker than one modulated by a low C/N scheme when the received C/N becomes degraded. When received C/N decreases due to heavy rain, a deteriorated packet transmitted by TC8PSK might become mislabeled with the packet ID of another packet group transmitted by QPSK or BPSK. This results in the deteriorated packet affecting the decoding process that is used for getting information via QPSK or BPSK.
In this invention, a receiver is composed of any combination of demodulator and bit error correction and threshold judgment circuits in order to get packet data from the multiplex transmission signal. The received conditions of the respective transmission schemes are detected by the error correction and threshold judgment circuits. When a transmission scheme is judged to be unusable due to bit errors, the relevant packet is replaced by a null packet that does not affect correctly transmitted packets in the decoder. Under a low C/N condition, data transmitted by a low C/N transmission scheme can be effectively prevented from being influenced by data transmitted by a high C/N transmitted scheme.
The figure illustrates an embodiment of the digital demodulator for the time division multiplexed signal in which the TMCC (Transmission and Multiplexing Configuration Control) signal that carries the configuration of multiplexing and modulation scheme of each packet is multiplexed. The demodulation process is controlled by the TMCC. The transmission conditions of respective transmission schemes are detected in the bit error detection and threshold judgment circuits. When the receiving condition is judged to be unusable because of a high bit error rate, a control signal that has information on error rate degradation is used to replace the relevant packet with a null packet or any other signal that does not affect correctly transmitted packets at the null packet inserting circuit.

Figure: Block diagram of digital receiver