For either of the encoding schemes, the highest modulation frequency of α2 is limited by the dynamical response speed of the WPT system. According to the small-signal analysis in [11], a dc-to-dc WPT stage usually has a pair of dominant poles whose imaginary part is close to kωs/2, where k is the coupling coefficient between the two resonant coils. The modulation frequency of α2 must be lower than kωs/2 to make the steadystate relationship (5) valid. Therefore, the maximum data rate of the binary encoding is