A Compact Dual-Band Implantable MIMO Antenna for Wireless Capsule Endoscopy

This article introduces a compact dual-element multiple-input multiple-output (MIMO) implantable antenna for high-data-rate wireless capsule endoscopy (WCE) applications. The proposed antenna works at 915 and 2450 MHz with the respective fractional bandwidths (FBWs) of 23.6% and 12.14%. The two-port MIMO system consists of two triangular-shaped radiators arranged face-to-face, forming a square configuration. The edge-to-edge distance between the radiators is 0.3 mm, indicating that the radiating elements are placed extremely close to each other. Three semicircular slots, three edge slots in the patch, a conducting via in each patch, and a high permittivity substrate are utilized to reduce the antenna’s volume (<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$9.8\times 9.8\times 0.3=28.81$ </tex-math></inline-formula> mm3). To minimize electromagnetic (EM) coupling between both patches, a diagonal slot measuring <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$12.5\times 0.3$ </tex-math></inline-formula> mm2 is etched in the ground plane, and a 6.5-nH inductor is added between the radiating patches. As a result, the mutual coupling values of −35.85 and −31.6 dB are observed at 915 and 2450 MHz, respectively. The effectiveness of the MIMO system is validated through the analysis of specific absorption rate (SAR) and link budget performance, both showing satisfactory results. With an input power of 1 W, the maximum SAR is 41.2 W/kg for 915 MHz and 43.2 W/kg for 2450 MHz. Channel parameters for the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\times 2$ </tex-math></inline-formula> MIMO configuration are calculated and validated, showing a channel capacity of 9.1 bps/Hz at SNR =20 dB. A prototype is developed and tested in the minced pork to validate simulations. Based on the results and performance, the proposed MIMO antenna system demonstrates high potential for high-data-rate capsule endoscopes.