US Patent 7791478 Dual bit encoding / decoding of RFID signal

In some embodiments of a radio frequency identification (RFID) system, multiple bits of data may be encoded at the same time based on a frequency shift key (FSK) modulation technique using multiple frequencies, by permitting any or all of the frequencies to be modulated on a carrier wave at the same time. In some embodiments a parallel but asymmetric down-conversion of the frequencies may allow both frequencies to be detected with identical filters.