In the first and second embodiments, an artificial noise series AN1 is generated from a data series S, and an artificial noise series AN2 is generated from the artificial noise series AN1. By contrast, in the third embodiment, artificial noise series AN1 and AN2 are generated from a data series S.

FIG. 11 illustrates an example of a signal generator in accordance with the third embodiment. In the third embodiment, a signal generator 12a includes correlation series generators 21a and 22a in place of the correlation series generators 21 and 22 depicted in FIG. 5. The correlation series generator 21a is substantially the same as the correlation series generator 21 and generates an artificial noise series AN1 from a data series S. In the meantime, the correlation series generator 22a generates an artificial noise series AN2 from the data series S, unlike the correlation series generator 22.

The number of symbols of the data series S and the numbers of symbols of the individual artificial noise series AN1 and AN2 are equal. The threshold C_S-AN1 used by the correlation series generator 21a and the threshold C_S-AN2 used by the correlation series generator 22a may be the same or different from each other. The configurations and operations of the receiver 11, the signal combiner 13, the transmitter 14, and the mappers 23-25 are substantially the same in the first to third embodiments.