Patent ID: 8588314

Claim:
A communication device comprising: a data communication unit that sends a communication signal to a power line, the communication signal being obtained by at least modulating transmission data; a zero-crossing-point detector that detects a zero crossing point of an alternating voltage applied from the power line; a coexistence-signal transmission unit that sends a coexistence signal to the power line in a coexistence region that starts at a time based on the zero crossing point detected by the zero-crossing-point detector, the coexistence signal having information for allowing a plurality of systems to be present on the power line; a coexistence-signal receiving unit that detects the coexistence signal from the power line in the coexistence region; a detection-period setting unit that sets a detection period for performing a detection operation for detecting a broadcast wave in accordance with the coexistence signal detected by the coexistence-signal receiving unit; and a broadcast-wave detector that detects a broadcast wave by performing frequency analysis on a signal on the power line in the detection period set by the detection-period setting unit, wherein the coexistence region includes a plurality of field areas, and wherein the coexistence-signal receiving unit performs detection based on an offset from a zero-crossing-point of the alternating voltage and a margin between field areas of the coexistence region.