Broadcast Technology

of digital transmission technology: digital modulation technology that converts a digital signal into a transmission signal, and error correction technology that overcomes errors caused by noise and/or distortion arising on a digital transmission path. These two fields have traditionally been researched independently. Coded modulation, on the other hand, can be viewed as an error correcting technology that integrates the above technologies giving particular consideration to nonuniform errors caused by multi-phase/multi-level modulation. In multi-phase/multi-level modulation, the separation between signal points (symbols) in a constellation used for transmitting information may vary. Figure 1 shows these differences between symbol distances for various modulation schemes. As can be seen from the figure, the symbol distances of each modulation scheme are different. In QPSK, for example, the transmission of symbol S0 is more likely to be erroneously received as S1 or S3 than as S2. In 8PSK, diagonally placed symbols can maintain a distance of at least 2.6 times that of adjacent symbols. Generally for coherent detection, the probability p(x) of the error distance x between the received symbol and transmitted symbol is a Gaussian distribution expressed as follows.