# Sub-Milliwatt Transceiver IC for Transcutaneous Communication of an Intracortical Visual Prosthesis
## 2 System Requirements and Considerations  
For the implanted transceiver, which comprises of the transmitter for uplink (sending out data from the implanted side) and its receiver for downlink (receiving stimulation data), the following essential system requirements are considered:  
1. Power consumption: Based on power consumption reported in [11; 20], the projected power consumption of the implant side of the 1024 electrode visual prosthesis without a wireless interface is in the order of 100 mW. Considering the wireless power transfer, and possible battery constraints at the implant side, it is desired that the wireless system adds no more than 10-30% extra power to the power budget at the implant side. This implies that only a low-power solution for the implanted transceiver will create a viable system. The external transceiver above the skin will be allowed to consume more power. Our proposed non-coherent BPSK receiver will fulfill this due to its simple architecture and predominantly digital components. For the uplink, on-off keying (OOK) is used for the IR-UWB transmitter because of its simpler architecture making for a low-power transmitter.
2. Transmission data rate: It is required to transmit a minimum of 23 Mbps for compressed data with electrode recordings [13]. The receiver is required to handle a minimum of 200 kbps of stimulation data [19]. IR-UWB for the uplink has the potential to deliver high data rate for short distances. Using an inductively-coupled link with BPSK at low frequency will provide a sufficient data rate, while yielding a low-power solution [21].
3. Bit-error-rate (BER): the uplink and downlink system should give a BER of at least \(10^{-3}\) over the full communication chain with an external side. BPSK is used for the downlink because it has a better theoretical bit error rate performance than other modulation schemes, such as amplitude shift keying and frequency shift keying [22]. The BER target is checked by using the IC results in the overall link budget. The focus of this work is on a low-power implanted side transceiver IC. Any need for improved BER could be taken care of by adjusting the link budget through the coil design with the external side. The case of the uplink, for example, where the IR-UWB receiver is external, reported receivers [23; 24] at similar sensitivities already meet the BER target of \(10^{-3}\). Similarly, for the downlink case, where the transmitter is external, the transmit voltage can be scaled easily to meet the implanted receiver sensitivity of 50 mV. In addition, straightforward coil design can already realize the required channel bandwidth.
4. Security: with the increasing risk of communication security breaches, the wireless link needs to be secure, especially at the physical layer. Therefore, short range transcutaneous communication is proposed from beneath the skin to the receiver just outside the head. The expected transmission path through skin is expected to be in the range of 3-7 mm [25].  
Co-existence with other sub-systems: in the overall wireless system of the visual prosthesis, the downlink and wireless powering are also present. The wireless link should be able to cope with other sub-systems in terms of frequency spectrum use, interference, and cross talk. Using the 3-5 GHz band for the uplink and the 4-12 MHz band for downlink provides sufficient frequency spacing to avoid interference. In addition, we propose the use of a coupled inductive link. Furthermore, the lower 3-5 GHz band of the 3-10 GHz for UWB is preferred due to lower attenuation through the skin [9]. In principle, pulse-based systems like IR-UWB are carrierless, and our target frequency band is 3-5 GHz as opposed to a carrier frequency, which is about 4-12 MHz for the BPSK communication in the downlink. Interference will be minimal due to its localization and near-field nature.  
The above considerations and requirements are summarized in Table 1.