An adaptive subcarrier demodulator or an adaptive subcarrier modulator may be coupled to the OFDM physical layer interface to implement adaptive bit loading (ABL).

A decoder or an encoder may be coupled to the adaptive subcarrier demodulator or to the adaptive subcarrier modulator to implement low density, parity checking.

The apparatus comprises principally an ion sensing transducer, whose ouptut is coupled directly into a subcarrier modulator.

A subcarrier modulator (2) modulates the frequencies of a plurality of subcarriers having frequencies different from one another by using the output of the spreading modulator (1).