# Sub-Milliwatt Transceiver IC for Transcutaneous Communication of an Intracortical Visual Prosthesis
## 1 Introduction  
Visual impairment is a sensory challenge with a significant impact on the daily life of patients. Nearly 216 million people worldwide are visually impaired, of which 36 million are currently blind [1, 2]. For a lot of these blind people, stimulating the visual cortex is the treatment of last resort due to damage to the visual pathway. Such a system, an intracortical visual prosthesis, consists of implanted electrodes, a signal processor on the outside as well as a camera and a feedback loop. [3]. In the context of this work, the Dutch NESTOR project (NEuronal STimulation fOr Recovery of function) aims to implant 1024 electrodes for stimulation and recording in the brain [4, 5] (funded by the Netherlands Organization for fundamental research--NWO). Wireless connection between the implanted electrode and the external processor is desired to facilitate mobility and to avoid infections during long-term use [6]. On the implant side, this wireless connection involves uplink (for recording), downlink (for stimulation) and wireless power transfer. This work focuses on the implanted transceiver IC, which is the transmitter for sending out of the head (uplink) and the receiver for receiving stimulation data (downlink). There are a few possible configurations for the wireless system. The implanted electrodes are grouped into arrays of 64 electrodes. Our approach used in this work is to place the central transceiver beneath the skin. The electrode arrays are tethered to the central transceiver [7]. Figure 1 illustrates our solution. Our approach is more easily scalablethan placing an implanted transceiver on each electrode array beneath the skull, making them independent [8], but requiring multiple implanted transceivers. For instance, our 1024 electrodes are implemented by 16 arrays of 64 electrodes (typical of Utah arrays). Taking an approach with an independent transceiver per electrode array would require 16 transceivers in total. Our proposed tethered approach, which requires only one central transceiver as in Figure 1, is more scalable. Furthermore, in our approach, a single transceiver unit could be used with a smaller or larger amount of tethered electrode arrays. For the concept we propose, this does not matter. The electrode arrays need to be placed at the desired location, while communication with the outside world will not change, due to the central unit. Our approach is preferred over using a central transceiver placed beneath the skull, which will face more attenuation and path-loss by the bone tissue (up to 10 dB more) [9]. The main challenge in our tethered solution is the possible micro-motion of the implanted connecting wires; this can be partly alleviated with better packaging and implantation. In this paper, we focus on the implanted transceiver IC design and analyze its link budget in the context of a complete system. Generic biomedical telemetries have been proposed for the implanted transmitter for recording and also for an implanted receiver for stimulation. However, in this work, we combine them into a single implanted transceiver for bi-directional communication in an intracortical visual prosthesis. In [10], optical communication was attempted for the uplink. However, this faces a challenge of alignment sensitivity. Impulse radio ultrawideband (IR-UWB) was used in [11], showing a promising result. A high data rate was achieved in [12] using an IR-UWB transmitter. In our work, for the recording, we use IR-UWB for the uplink because of its low-power and high data rate potential at its transmitter, which will be at the implant side. For our implanted transmitter, we applied current control to make the impulse generator tunable for pulse width. A straightforward on-off keying modulation (OOK) scheme is implemented, based on a simple D-latch to achieve low-power consumption. Furthermore, a current-controlled variable oscillator is proposed to tune the number of pulses per bit.