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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).

====Digital modulation====

According to Channi, modulation is “the process of varying some parameter of a periodic waveform in order to use that signal to convey a message”. Also, a digital modulation technique is defined as the discrete variation of the carrier wave (Channi, 2016, p. 43). The term "discrete" refers to changes occurring in distinct steps between fixed values. Within digital modulation, attributes like frequency or amplitude switch between set levels to convey binary information. This is in contrast to analog modulation, where the carrier signal varies continuously in accordance with the input signal.
The Qi standard uses digital modulation to communicate between Tx and Rx bidirectionally. In conclusion digital modulation restricts the carrier wave to a finite number of discrete frequencies, ensuring efficient transmission, high noise immunity (displayed by the signal-to-noise ratio), and enabling techniques like multiplexing and digital compression (Crecraft & Gergely, 2002, pp. 225, 226).

====Frequency Shift Keying====

Frequency Shift Keying (FSK) is used to communicate transmitter data to the receiver. To output a binary one, the frequency of the carrier wave is boosted, while a binary 0 is equal to a lower frequency of the carrier wave. The protocol is applied via an oscillator within the transmitter that oscillates between the two frequencies. The principle behind FSK modulation is shown in Figure 1 and a practical representation of the oscillator can be found in Figure 2 (Ciciora et al., 2004, pp. 138–142; Crecraft & Gergely, 2002, p. 227).

| {{:fsk_modulation_principle.gif?300}}  | {{:oscilator.png?300}}                    |
| Figure 1 FSK modulation principle      | Figure 2 Practical representation of FSK  |

FSK communication for Qi technology includes a typical packet structure between Tx and Rx engaged within [[power_transfer_architecture_of_the_qi_standard|the identification and configuration phase]]. The message begins with a preamble for synchronization. Secondly, a header containing the message type and the length information is given. This is followed by the payload data as a third step and finally the packet concludes a checksum for error detection.

It is not surprising that Qi implements FSK modulation for transmitter-receiver communication. In fact, this method offers many advantages:
  * High noise immunity and interference capability
  * Simplicity of implementation in the power transmission circuit
  * Ability to maintain power transfer during communication
  * High immunity to amplitude variations
  * Robust performance even with varying coupling coefficients

====Amplitude Shift Keying====

Amplitude Shift Keying (ASK) is the modulation protocol used for communication between the receiver and transmitter in the Qi standard. It works by varying the amplitude of the carrier wave. To transmit a digital one, the amplitude of the carrier remains at a constant level. To modulate a digital zero the amplitude is reduced. Similar to FSK the working principle is straightforward. ASK uses 2 different load resistances in the receiver circuit that can be switched on or off, resulting in different voltage drops across the circuit. Depending on the load impedance, the amplitude of the carrier changes accordingly. A visual representation of ASK modulation is provided in Figure 3 and Figure 4.
{{:ask_modulation_principle.gif?400|}}Figure 3 ASK modulation principle
{{:2-level_keying.png?200|}}Figure 4 2-level keying
ASK is preferred for receiver-to-transmitter communication because it consumes less power compared to FSK modulation. Also, switching between two loads can also be easily and compactly integrated into the receiver hardware.

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<color #808080>**References**</color>
  * <color #808080>Channi, H. K. (2016). A comparative study of various digital modulation techniques. International Journal in IT and Engineering, 4(03).</color>
  * <color #808080>Ciciora, W., Farmer, J., Large, D., & Adams, M. (2004). Digital Modulation. In Modern Cable Television Technology (pp. 137–181). Elsevier. https://doi.org/10.1016/B978-155860828-3/50006-0</color>
  * <color #808080>Crecraft, D. I., & Gergely, S. (2002). Radio communication techniques. In Analog Electronics (pp. 200–232). Elsevier. https://doi.org/10.1016/b978-075065095-3/50009-x</color>
  * <color #808080>Minnaert, B., Thoen, B., Plets, D., Joseph, W., & Stevens, N. (2018). Wireless energy transfer by means of inductive coupling for dairy cow health monitoring. Computers and Electronics in Agriculture, 152, 101–108. https://doi.org/10.1016/j.compag.2018.07.010</color>
  * <color #808080>Sagar, A., Kashyap, A., Nasab, M. A., Padmanaban, S., Bertoluzzo, M., Kumar, A., & Blaabjerg, F. (2023). A Comprehensive Review of the Recent Development of Wireless Power Transfer Technologies for Electric Vehicle Charging Systems. In IEEE Access (Vol. 11, pp. 83703–83751). Institute of Electrical and Electronics Engineers Inc. https://doi.org/10.1109/ACCESS.2023.3300475</color>

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