## 4 Power Supply and Wireless Communication Technologies
The telemetric link can be used for bi-directional transfer of information, including the sensed and recorded data about the patient and data regarding the condition of the indwelling module; the link also enables wireless re-programming and communication between the multiple implanted modules comprising a wireless network within the body of the patient. Typically, to enable powering/data transfer using the same link, magnetic field modulations are employed to impress the data signal onto the carrier signal used to power the device. The changes in the signal characteristics are detected and interpreted by the indwelling module. The amplitude, phase and frequency of the signal can be modulated. The amplitude modulation is one of the most popular techniques for short range communications, and involves varying the amplitude of the signal from high to low, thus emulating the zero/one logic of digital communication; it is described by the modulation depth, _i.e._, the extent to which the amplitude was altered. In addition to the aforementioned amplitude modulation (AM) and amplitude shift keying (ASK), frequency modulation (FM) and frequency shift keying (FSK) refers to altering the frequency of the carrier signal, and phased shift key (PSK) involves changing the phase of the carrier signal by 180\({}^{\circ}\) or less. Typically, the data rate attainable with the above modulations is approximately 10% of the carrier frequency, however higher data rates can be achieved with more sophisticated modulation approaches, e.g., by combining two modulation techniques. The choice of modulation methodology used will also depend on the data transfer requirements of the implant, with lower frequencies used for those with low data rate needs and higher frequencies for those demanding large volume, ongoing data transmission. The restrictions within the system, e.g., availability of power or bandwidth, will also affect the choice of the modulation approach. Appropriately chosen, modulations can enhance the quality of the signal, improve the security of the patient-related data, increase the quality of the signal, enable accurate transfer of data in the presence of noise and other disturbances, and increase communication channel capacity. Communication channels can be organized into additive white Gaussian noise channels (AWGN), band limited channels and fading channels [62]. The former represents a channel model where white noise of a constant spectral density and a Gaussian distribution of amplitude is added to the signal sent through the channel. In the band limited channel model, the band width of the channel is smaller than that of the signal, resulting in the elimination of the frequency components of the transmitted signal above the channel cutoff frequency. In the case of the fading channel, the amplitude and phase of the passing signal change rapidly, attributed to fading due to multipath propagation and shadowing. An appropriate demodulation technique is also selected to minimize power consumption, reduce interference, and ensure accurate translation of the message. The information transferred from the indwelling module to the extracorporeal device is also modulated, with the electrical impedance of the implanted electronic circuit being reflected back to the transmitter circuit via the same inductive coupling link. The load shift key modulation (LSK) is attained by electronically switching the impedance of the implant between two states. As with other modulation approaches, the data rate is dependent on the carrier frequency. It is important to note that as the distance between the implanted and external inductive coils increase, the magnetic field induced by the external coil progressively transforms into an electric field, which cannot be modulated using LSK [62, 86].