COMMUNICATION SYSTEM, TRANSMISSION APPARATUS, RECEPTION APPARATUS, AND DIGITAL TRANSMISSION METHOD

A communication system using hierarchical modulation and capable of flexibly adjusting line quality includes a transmission apparatus that hierarchically modulates and transmits signal sequences, a reception apparatus that hierarchically demodulates a received signal, and a hierarchization-parameter setting unit that sets hierarchization parameters. The transmission apparatus includes a hierarchical modulation unit that superimposes signals from layers while maintaining gray codes. The reception apparatus includes a hierarchical demodulation unit that demodulates and separates hierarchically modulated signals and detects signals from layers. The hierarchization-parameter setting unit sets a hierarchization ratio which is a signal power ratio used for superimposing a higher layer and a lower layer, a mapping pattern capable of selecting two patterns, and a mapping pattern mixing ratio indicating a ratio with which two mapping patterns are mixed.