**5.5 Antenna alignment in wireless implantable devices**

One of the common challenges in wireless implanted systems is the misalignment in radiation direction and polarization. This issue can be easily addressed by increasing the transmitted power, however, there are safety risks limiting the amount of incident power. Utilizing circularly polarized antennas can only solve the problem of polarization misalignment keeping the radiation direction unaddressed. Thus, researcher paid significant efforts to design a universal solution for the aforementioned issues. One of the solutions is proposed in [40], where the harmonics yielded by the nonlinearity of rectifiers were exploited to align the

**Figure 22.** *Proposed intermodulation-based system for addressing the misalignment [40].*

transmitting and receiving antennas effectively. In this approach, two-tone (2T) waveform excitation is utilized to improve the rectification as well as to generate intermodulation as shown in **Figure 22**.
