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=====Bidirectional in-band communication in the Qi standard=====

====In-band communication====

The latest version of the Qi standard, designated as Qi2, facilitates power transfer of up to 15W through Magnetic Power Profile (MPP). This wireless energy transmission occurs via inductive close coupling between transmitter and receiver coils, utilizing a time-varying magnetic field as the transfer medium. The operational frequency spectrum for establishing effective power transfer typically ranges from 100 to 200 kHz. This frequency range is chosen based on electromagnetic principles: frequencies below this range produce insufficient magnetic field strength for effective coupling, while higher frequencies generate unwanted eddy currents and make electric currents flow only on the outside of the conductors. This last phenomenon is also know as the skin effect (Minnaert et al., 2018, pp. 102–103).
Additionally, the 100-150 kHz spectrum is internationally designated for industrial, scientific, and medical applications. This specific frequency range is therefore commonly referred to as the ISM-bands. Lower frequencies are preferentially used for power transmission due to their limited propagation characteristics, which confers advantages in electromagnetic shielding and minimizes interference with higher-frequency communications such as Wi-Fi (operating in GHz ranges). These higher-frequency signals are characterized by extended transmission ranges and structural penetration capabilities (Sagar et al., 2023, p. 83709).

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