# Scalp-Implanted Ultra-Wideband Circularly Polarized MIMO Antenna for Biotelemetry Systems
## 1 Introduction  
Recently, the demand for implantable medical devices (IMDs) in the healthcare industry has surged. Significant advancements have been made in applications like capsule endoscopy, blood glucose monitoring, cardiac pacemakers, intra-oral tongue actuation systems, and intracranial pressure monitoring [1, 2, 3, 4, 5]. This increase is driven by the capability of IMDs to detect biological signals within the body and convert them into microwave signals for transmission to an external receiver. IMDs typically incorporate a variety of components, such as sensors, circuit boards, power sources, and antennas [6]. Among these elements, the implanted antenna is particularly critical, as it facilitates effective data communication with a receiver that can be positioned several meters from the human body. Nonetheless, the design of implantable antennas involves overcoming multiple challenges. Most of the current IMDs use single-input single-output (SISO) antennas, including dual/multi-band circularly polarized (CP) antennas [7, 8] and ultra-wideband (UWB) antennas [9], but these designs have only one antenna for data transmission and reception; thus, their transmission rates are limited. In addition, due to the complexity and frequency dependence of the human body's organization, the electromagnetic (EM) characteristics greatly affect the antenna's performance, and the SISO system is relatively weak in terms of anti-interference capability in the face of signal interference. Since path loss increases with higher operating frequencies, this significantly impacts the communication range of SISO antennas, especially for high-data-rate communications. The newlydeveloped MIMO antenna system shows significant promise in overcoming the constraints of SISO antennas and improving the efficiency of spectrum utilization. The application of MIMO antenna technology in biotelemetry systems has the advantage of improving the communication capacity and reliability compared with the traditional SISO system, but it also faces some unique challenges, which need to be solved in the design of the space, power consumption, signal processing, and interference management in order to achieve stable and reliable communication. When employing MIMO antenna systems, it is possible to enhance channel capacity (CC) without the necessity for extra power or frequency resources. This is achieved by employing spatial multiplexing techniques, which enable the transmission and reception of data in parallel through multiple antennas. Consequently, this method boosts CC without requiring additional power or frequency resources. MIMO antenna technology has a large advantage in improving the wireless communication capacity and transmission rate. Although the traditional MIMO technology is mature, there are still some limitations in some dynamic or complex scenarios. Recently, some scholars have proposed realizing the channel estimation of the movable MIMO antenna system through tensor decomposition [10]. This technology brings new possibilities in the channel estimation of mobile MIMO antenna systems. More recently, millimeter-wave and terahertz bands have also become attractive for ultra-wideband transmission. A channel training-aided target-aware framework was developed in [11] to facilitate efficient resource sharing for THz-massive MIMO communication. In [12], the spatial broadband effect and the associated beam skew problem in MIMO communication were addressed by constructing a massive MIMO channel model. In addition, the MIMO antenna power leakage problem was significantly reduced and inter-user interference was mitigated by an energy-focused window approach in [13]. In [14], a four-element, implantable, linearly polarized (LP) MIMO antenna, designed for the 2.4 GHz industrial, scientific, and medical (ISM) band and utilizing electromagnetic bandgap (EBG) technology, is introduced.