# Low-Power Wireless Data Transfer System for Stimulation in an Intracritical Visual Prosthesis
## 2 System Considerations
### System Approach  
The goal for the downlink is to have a low-power, robust communication system meeting the requirements stated in the previous section. Here we focus on the receiver side because this will be in the implant and therefore, poses the most challenging design. To consume less power, system components which can be power hungry such as phase locked loops (PLLs) and high-Q oscillators should be avoided where possible. In addition, high frequencies (above 100 MHz) should be avoided because they usually make devices consume more power and face more signal attenuation through the skin. This has motivated the use of alternatives to classical demodulation techniques, which use some of the power-hungry components mentioned earlier. As a result, Figure 2 shows the proposed downlink system blocks for the external transmitter and the implanted receiver. It contains the following features:  
* Low frequency <100 MHz: the required downlink data rate is below 10 Mbps. Thus, modulating on carrier frequencies below 100 MHz will provide sufficient bandwidth. In addition, the electric and magnetic field will face minimal attenuation through the skin [22]. Lower frequencies also imply lower power consumption for the digital components.
* Inductive link: to eliminate the bulky antennas that are needed if low frequencies are used, an inductive link is proposed. The inductive link allows for short transmission ranges in the order of centimeters. This fits well with the application of the visual prosthesis, where transmission through a small layer of tissue (skin), at an implantation depth of 3 to 7 mm [23] is sufficient (see also Figure 1c) and security may be a concern.
* Phase shift keying (PSK): has a better theoretical bit error rate performance than other modulation schemes, such as amplitude shift keying (ASK) and frequency shift keying (FSK). Since the PSK modulation scheme is not sensitive to amplitude variations, it can easily cope with misalignment between transmitter and receiver coils, which has mainly effect on the signal amplitudes. Its transmitter is of similar complexity as in ASK. It is also spectrally more efficient than FSK which may require wideband inductive links [9].
* Bandpass sampling: at low frequencies (<100 MHz), the entire modulated signal (carrier and information) can be sampled or simply digitized and processed in the analog or digital domain.
* Non-coherent demodulation: using bandpass sampling at the receiver side, the entire received signal is sampled (digitized). It is possible to recover the information from the digitized received signal using a non-coherent digital technique. This avoids PLLs and allows for the use of low-power ring oscillators which have relatively large phase noise. The poor phase noise of the ring oscillator is not usually tolerated in classical demodulation techniques. Non-coherent analog demodulation for FSK and PSK generally uses envelope detection as part of the core demodulation process, and this has a poorer bit error rate (BER) performance than coherent demodulation [24]. However, this degradation in performance does not apply to the non-coherent digital demodulation proposed here as it does not use envelope detection. To overcome a weakness of our bandpass sampling approach, namely that it requires proper channel design, a coupled inductive link is used, to provide sufficient bandwidth for the channel.
* Differential encoding (optional): by using differential encoding and by matching the data rate to the power carrier frequency, we can improve the robustness to power carrier interference. This is based on the assumption that the power carrier interference is periodic and that each period of the signal will encounter the same interference when differentially encoded [25].