Abstract: When the received symbol is located (FIG. 4) within one of regions G, H, I, L, M, N, Q, R, and S, 2 bits are decided to be most reliable to obtain their soft decision values=0 or 7. As for the remaining 2 bits, a soft decision value=0 to 7 is decided by soft decision in the I- or Q-axis direction. When the received symbol is located within one of regions A, E, U, and Y, all the 4 bits are decided to obtain their soft decision values=0 or 7. When the received symbol is located with one of regions B, C, D, F, J, K, 0, P, T, V, W, and X, 3 bits are decided to be most reliable to obtain their soft decision values=0 or 7. As for the remaining 1 bit, a soft decision value=0 to 7 is obtained by soft decision in the I- or Q-axis direction. A soft decision method that can implement soft decision in multilevel (amplitude and/or phase) modulation and can fully exhibit the correction performance of maximum likelihood coding can be provided.

Abstract: Data sequences correspond to L different parameter sets (effective symbol length, guard interval length, the number of carrier waves) and transformed into parallel data by respective serial/parallel converters, which are allocated to respective carrier waves for OFDM and subjected to inverse discrete Fourier transform by inverse discrete Fourier transformers to produce sampled values for the transmission waveform in the time domain. The sampled values are transformed into serial sequences of sampled values by parallel/serial converters and then into a single temporal sampling sequence by a temporal sampling sequence switching unit. A frame synchronizing symbol is added to the temporal sampling sequence and then transformed into an analog base band OFDM signal before it is converted up to a transmission signal. The frequency bandwidth of the OFDM signal is made smaller than a predetermined value defined by the bandwidth of the available transmission channel.