# A 28 nm Bulk CMOS Fully Digital BPSK Demodulator for US-Powered IMDs Downlink Communications
## 1 Introduction  
In recent years, a considerable portion of the electronics field has been focused on bioengineering applications. The modern internet of medical things (IoMTs) or the body area networks (BANs), as well as the less recent implantable neural prostheses, are proof that more and more attention is being paid to these kinds of applications. Looking to neurological and mental disorders, their impacts can often be alleviated by bioelectronics and neuroprosthetics medicine. Implantable neural prostheses, such as deep brain the Ref. [1], cortical [2] or nerve [3] stimulators and cochlear implants [4] are examples of clinically adopted neuro-technology to revert impaired or lost functions.  
In general clinically adopted implantable neural prostheses, a micro-electrode array is kept in contact with the tissue and it is electrically linked, often by using cables, to a unit aimed at recording neuronal activities and/or for neural stimulation. The design principle has its roots back to the first fully implantable battery-powered pacemaker [5]. Since then, several variations of this design principle were used in many successful applications, despite having several critical limitations. For instance, the excessive intrusiveness and mechanical stresses that cables and connectors can have on human tissues are weak points of the system, often leading to failure. Moreover, due to its size, the implanted unit cannot be located close to the target tissue. Instead, it is placed in a remote location and wires are used to connect them to micro-electrode arrays for stimulation.  
To get rid of this drawback, more recently adopted implantable neurostimulators have employed a wireless link for power and data transfer from/to the implanted IC. This constitutes a technological challenge in neurostimulation, whose state-of-the-art culminates with arrays of free-standing smart electrodes, not relying on powered units and wired connections [6]. Such smart electrodes should integrate all the required elements to receive power and operational commands (downlink communication), while the preserve size should be compatible with the intended application, ideally smaller than 1 mm\({}^{3}\). For this reason, the miniaturisation of the implantable device is crucial to achieve safe and noninvasive surgical implantation. Such a system is better-known with the acronym IMD, which stands for an \"implanted medical device\".  
In this framework, as a part of wireless IMDs, we propose a new solution of a Binary Phase-Shift Keying (BPSK) demodulator in order to design a robust, low-area and low-power downlink for ultrasound (US)-powered implanted units. The implemented system is fully digital and PLL-free, thus reducing area occupation and making it fully synthesizable. Simulation results are reported using a 28 nm Bulk CMOS technology provided by TSMC.  
The paper is organized as follows. Section 2 reports a system overview of US-powered implanted medical devices. Then, starting from the US-link characterization shown in Section 3, the proposed fully digital BPSK demodulator is introduced and described in each single block in Section 4. Section 5 shows post-layout simulation results of the proposed circuit as also compared with the state-of-the-art. Finally, the last section reports the conclusions of this work.