# Scalp-Implanted Ultra-Wideband Circularly Polarized MIMO Antenna for Biotelemetry Systems
## 1 Introduction
This antenna exhibits a fractional bandwidth (FBW) of 18.64% (ranging from 2.14 to 2.58 GHz) and demonstrates a mutual coupling of no more than \(-\)15.99 dB within the ISM band. In [15], a compact MIMO implantable LP antenna system is introduced, engineered for operation within the Medical Implantation Communication Service (MICS) band (402-405 MHz) and the ISM band (433.1-438.8 MHz). This design exhibits an FBW of 33.9% (355-500 MHz) and isolation exceeding 26 dB. In [16], a MIMO antenna with two ports was developed for the 403 MHz MICS band, featuring a spiral resonator design. This design achieves an analog FBW of 35.9%, in addition to a maximum isolation of 26 dB in the 433 MHz ISM band. Despite its performance, the antenna's size is relatively large, at 307 mm\({}^{3}\). In [17], a quadratic MIMO antenna was introduced, which includes a semicircular bent radiator sharing a common ground. This antenna is tuned to operate at 433 MHz and offers an FBW of 38.26% (355-523 MHz). A compact two-port implantable MIMO antenna with a bent resonator, operating at 2.45 GHz, is reported in [18]. This antenna features a bandwidth of 320 MHz and achieves an isolation of over 28 dB at the design frequency. In [19], a novel conformal MIMO antenna was introduced, marking the debut of eigenmode theory in MIMO antenna design. This antenna exhibited an FBW of 15.1% (ranging from 2.26 to 2.63 GHz) and achieved an impressive isolation level of 27.26 dB. All of the above MIMO antennas are LP antennas, and in complex multipath propagation environments, LP antennas may suffer from multipath interference, resulting in performance degradation. CP MIMO systems offer significant advantages in terms of multipath interference resistance, stable transmission, polarization mismatch resistance, and dynamic adaptability, and they have wider application prospects in mobile communications and dynamic environment applications, ensuring signal strength and connectivity stability without being significantly affected by changes in device attitude. To address the above challenges, [20] introduced a 3D MIMO ground-radiating cube antenna (CA) fed by a coplanar waveguide (CPW), which was circularly polarized by loading a pair of slots,but it was not suitable for IMDs due to its large size. Ref. [21] describes a MIMO antenna with directional mapping and polarization diversity characteristics for capsule endoscopy applications. It operates in the 915 MHz ISM band with 12.35% FBW and 10.35% 3 dB axial ratio bandwidth (ARBW). Though the above implantable MIMO antennas are circularly polarized, they suffer from the limitations such as excessive size or insufficient bandwidth. To overcome the aforementioned limitations, this paper presents a new, compact, dual-element, implantable, ultra-wideband, circularly polarized MIMO antenna designed for operation at 2.45 GHz. It enables high-speed data transmission without requiring additional power or frequency resources. The antenna has a bandwidth of 28.16% (2.14-2.83 GHz) and a wide effective 3 dB ARBW of 16.3% (2.23-2.63 GHz). It also has an isolation of greater than 27.5 dB and a peak gain of \(-\)24.1 dBi. The primary benefits of this MIMO antenna include (1) its highly compact design; (2) the availability of independent channels, which enhance CC; (3) an ultra-wide bandwidth and wide effective axial ratio (AR); and (4) the ability to support polarization diversity.