A compact multi-frequency MIMO antenna

Abstract

This invention discloses a compact multi-frequency MIMO antenna, comprising two or four radiating elements arranged in a planar manner, with adjacent radiating elements arranged in opposite directions. Each radiating element includes an overlying dielectric layer, a serpentine radiating patch, a high dielectric constant substrate, and a ground plane. The serpentine radiating patch and the ground plane are provided with several slots. These slots regulate the current distribution and electromagnetic coupling of the radiating elements, enabling the antenna to operate in the 1.395-1.432 GHz, 2.400-2.480 GHz, and 5.725-5.875 GHz frequency bands. This invention achieves tri-frequency coordinated operation and energy / data band isolation through precise design of the serpentine radiating patch slot structure, balancing wireless power transmission and dual-band high-speed data communication. The use of foldback slots to extend the current path achieves an ultra-compact design, suitable for space-constrained implantable devices such as capsule endoscopes. The accompanying tri-frequency resonant design method features step-by-step slot optimization and synchronous ground plane adaptation, providing precise control and compatibility with MIMO self-decoupling structures, facilitating industrialization.

Description

Technical Field

[0001] This invention relates to the field of implantable biomedical antenna technology, specifically a compact multi-frequency MIMO antenna. Background Technology

[0002] In recent years, implantable medical devices (IMDs) have received widespread attention in biomedical engineering due to their importance in real-time monitoring, diagnosis, and treatment applications. These devices rely heavily on wireless communication for transmitting physiological data and receiving control signals. Therefore, implantable antennas play a crucial role in ensuring reliable high-speed data transmission and efficient wireless power delivery.

[0003] Currently available implantable multiple-input multiple-output (MIMO) antennas still have many technical shortcomings, including large antenna size, support for only single-frequency operation, limited isolation between antenna elements, and insufficient adaptability to different implantable platforms.

[0004] Previous approaches to achieving multi-band operation in implantable multiple-input multiple-output (MIMO) antennas typically relied on complex structures, electromagnetic bandgap (EBG) layers, or inductive decoupling methods, which could lead to complex manufacturing processes and increased overall device size. Furthermore, while three-dimensional or rigid antenna structures can provide three-dimensional radiation diversity, they may not be suitable for flexible or capsule-shaped implantable devices, thus limiting their practical application in space-constrained anatomical regions of the human body.

[0005] Furthermore, implanted antennas must meet stringent safety requirements, including compliance with specific absorptivity (SAR) limits, while maintaining reliable communication within highly depleted biological tissue. Achieving compact, multi-band, highly isolated antennas that provide robust, near-omnidirectional radiation in such environments remains a significant challenge. Summary of the Invention

[0006] The purpose of this invention is to overcome or at least partially solve the above-mentioned problems by proposing a compact multi-frequency MIMO antenna that can be effectively integrated into various implantable medical devices (IMDs), including wireless capsule endoscopes and leadless pacemakers. It overcomes the limitations of traditional implantable antennas, achieves simultaneous operation of multiple frequency bands, high isolation, and reliable wireless communication in space-constrained and high-loss biological environments, and meets human safety requirements.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a compact multi-frequency MIMO antenna, comprising two or four radiating elements arranged in a planar manner, with adjacent radiating elements arranged in opposite directions;

[0008] Each of the radiating elements includes a high dielectric constant substrate, an overlying dielectric layer, a ground plane, and a serpentine radiating patch;

[0009] The overlay dielectric layer covers the radiation unit, decoupling and isolating the radiation unit from the rest of the implantable medical device.

[0010] The serpentine patch and the ground plane are located on the front and back sides of the high dielectric constant substrate, respectively. Both the serpentine radiating patch and the ground plane are provided with several slots. The current distribution and electromagnetic coupling of the radiating element are controlled by the slots, so that the antenna can operate in the frequency bands of 1.395-1.432GHz, 2.400-2.480GHz and 5.725-5.875GHz.

[0011] Preferably, the serpentine radiating patch has an outer semi-enclosed groove, a folded groove, a power supply through-hole groove, and multiple additional grooves, which work together to achieve frequency tuning and three-frequency resonant impedance matching; the outer semi-enclosed groove has a structure that surrounds three sides and opens at the bottom along the edge of the patch, and an additional tuning groove is formed in the bottom groove section; the folded groove is nested in the outer semi-enclosed groove and is formed by continuous opening of an initial section, an extension section, and a final extension section; the additional grooves include a side additional groove provided next to the initial section of the folded groove, an extended additional groove provided along the side additional groove, and an independent rectangular additional groove provided next to the folded groove.

[0012] Preferably, the grounding plane has a grounding port through-hole groove and a plurality of transverse hollow grooves arranged longitudinally.

[0013] Furthermore, the slotted structure of the serpentine radiating patch and the ground plane described in this invention is optimized through a four-step systematic design evolution of a single radiating unit. The specific design evolution process is as follows:

[0014] Step 1: The antenna consists of a square radiating patch containing a rectangular slot formed by segments s1–s5, placed on a complete ground plane. This structure produces a single resonance at approximately 1.50 GHz, with a reflection coefficient of |S|. 11 |=-5.20dB indicates insufficient impedance matching and limited bandwidth performance.

[0015] Step 2 involves introducing additional slotted segments to enhance current distribution and increase the effective electrical length. Specifically, slots s6, s7, s8, and s are added to the chip. 11 and s 12 The process involves introducing a slotted g1 segment into the ground plane. These structural improvements resulted in multiple resonances at approximately 1.41 GHz, 3.12 GHz, and 4.26 GHz, with corresponding improvements in reflection coefficients of -11.64 dB, -25.51 dB, and -13.97 dB, respectively. This step validates the role of slotted loading in achieving multi-frequency characteristics.

[0016] Step 3, by introducing additional slotted structures s9 and s 14 s 1w The antenna was further optimized by improving the g3 segment and grounding structure. These adjustments altered the surface current path and electromagnetic coupling within the structure, shifting the resonant frequency to approximately 1.48 GHz and 2.81 GHz, with corresponding reflection coefficients of -13.61 dB and -27.48 dB, respectively. This stage was primarily used for frequency tuning and stabilization of the intermediate resonance.

[0017] Step 4, by introducing slots s 10 s 13 The section and grounding slot g7 were finely adjusted. These final adjustments optimized impedance matching and precisely aligned the resonant frequency with the target frequency band. Stable tri-frequency operation was finally achieved at 1.4GHz, 2.45GHz, and 5.8GHz, with reflection coefficients of approximately -16dB, -18dB, and -15.5dB, respectively.

[0018] Furthermore, each of the radiating elements is fed through a 50Ω coaxial port, and the feeding structure is adapted to the feeding reserved slot of the serpentine radiating patch. The antenna supports linear polarization in all three frequency bands, maintaining stable impedance matching.

[0019] Preferably, the four radiating elements are arranged symmetrically, and the spacing between the serpentine radiating patches of adjacent radiating elements is optimized to 0.5 mm. Self-decoupling is achieved through the spacing and arrangement of the serpentine radiating patches, resulting in inter-unit mutual coupling of less than -39.19 dB, -29.34 dB, and -20 dB in the 1.4 GHz, 2.45 GHz, and 5.8 GHz frequency bands, respectively. By rationally designing the arrangement of adjacent radiating elements, mutual coupling can be significantly reduced without the need for external decoupling components such as inductors, short-circuit supports, or electromagnetic band gaps (EBG). This strategy not only simplifies the manufacturing process and reduces system complexity but also maintains high isolation performance across all operating frequency bands.

[0020] Furthermore, the antenna has peak gains of -35dBi, -25dBi, and -15dBi at frequencies of 1.4GHz, 2.45GHz, and 5.8GHz, respectively, which can provide stable signal strength at distances of more than 20 meters and meet the requirements of high-speed data communication.

[0021] Furthermore, the compact multi-frequency MIMO antenna has a total volume of no more than 18.8 mm³, and is adapted to be integrated into the body of the capsule-type implantable device or cardiac implantable device. It is compatible with the battery, sensor and electronic circuit inside the device, and operates in three frequency bands: 1.395-1.432 GHz, 2.400-2.480 GHz and 5.725-5.875 GHz, to realize wireless power transmission and high-speed wireless communication of the capsule-type implantable device or cardiac implantable device.

[0022] Preferably, the additional tuning slot of the bottom slot section of the outer semi-enclosed slot has a vertical slot width that is twice the slot width of the bottom slot section, which is used to precisely tune the resonant characteristics of the 1.4GHz wireless power transmission frequency band and ensure the isolation between the power transmission and data communication frequency bands.

[0023] Preferably, the envelope correlation coefficient between any two of the radiating elements is less than 0.5, the diversity gain is greater than 9dB, and the antenna as a whole presents a radiation pattern similar to omnidirectional radiation, which is suitable for the movement of implanted devices in the human body.

[0024] Preferably, the specific absorption rate of the antenna meets the limit requirements of IEEE standards for 1 gram and 10 grams of human tissue, and the antenna surface is encapsulated with a biocompatible coating, so that the resonant stability, inter-unit isolation and overall MIMO communication performance can still be maintained when the dielectric constant and conductivity of human tissue change.

[0025] This invention also provides a design method for achieving tri-frequency resonance of an implantable multi-frequency MIMO antenna, comprising the following steps:

[0026] S1. Using a high dielectric constant substrate as the base, a square radiating patch and a complete ground plane are prepared. A semi-enclosed groove is opened at the edge of the square radiating patch to form an initial radiating structure. This structure generates a single resonance at 1.50 GHz, and its reflection coefficient is -5.20 dB.

[0027] S2. A folded groove is formed on the radiating patch inside the outer semi-enclosed groove, and an additional side groove is formed on the side of the folded groove. Simultaneously, two transverse hollow grooves are formed on the ground plane to enhance the current distribution on the surface of the radiating patch and increase the effective electrical length, so that the radiating structure generates multiple resonances at 1.41GHz, 3.12GHz and 4.26GHz, with corresponding reflection coefficients of -11.64dB, -25.51dB and -13.97dB, respectively.

[0028] S3. Extend the folding groove along the original path based on the initial section of the folding groove, and at the same time, open an independent rectangular additional groove on the radiating patch and open an additional tuning groove at the bottom groove section of the outer semi-enclosed groove; simultaneously, add a transverse hollow groove on the grounding plane to adjust the current path on the surface of the radiating patch and the electromagnetic coupling relationship within the structure, so that the resonant frequency shifts to 1.48GHz and 2.81GHz, corresponding to reflection coefficients of -13.61dB and -27.48dB respectively, thereby stabilizing the intermediate resonant frequency;

[0029] S4. Based on the extension section of the folding groove, continue to extend the folding groove to form a final extension section, forming an integrated continuous folding groove. At the same time, extend the folding groove along the side auxiliary groove to form an extended auxiliary groove. Simultaneously, add a transverse hollow groove on the grounding plane to finely adjust the resonant frequency and optimize the impedance matching, so that the radiating unit can achieve stable tri-frequency operation at 1.4GHz, 2.45GHz and 5.8GHz, with corresponding reflection coefficients of -16dB, -18dB and -15.5dB, respectively.

[0030] S5. The optimized individual radiating elements are arranged symmetrically in a plane with a spacing of 0.5mm. Self-decoupling is achieved through the combination of spacing and arrangement. The isolation of the entire operating frequency band is less than -29dB, forming a four-port MIMO antenna structure with a total volume of no more than 18.8mm³.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1. By precisely designing the slotted structure of the serpentine radiating patch, the antenna achieves tri-band coordinated operation and effective isolation between energy and data frequency bands, balancing 1.4GHz wireless energy transmission and 2.45GHz and 5.8GHz dual-band high-speed data communication, and avoiding electromagnetic interference between different frequency bands;

[0033] 2. By symmetrically arranging the four radiating elements in a plane and precisely optimizing the spacing of 0.5mm, a high isolation self-decoupling effect without external decoupling components is achieved. The isolation across the entire operating frequency band is less than -29dB, which greatly simplifies the overall antenna structure and reduces the difficulty of component integration.

[0034] 3. The antenna's ultra-compact design, achieved by extending the current path through a foldback groove, has a total volume of no more than 18.8 mm³, making it directly compatible with space-constrained implantable medical devices such as capsule endoscopes and leadless pacemakers.

[0035] 4. The three-frequency resonant design method adopts a step-by-step slotting optimization and ground plane synchronous adaptation approach. Through a four-step slotting design, it achieves gradual control from single resonance to precise three-frequency resonance. Each step has clear frequency and reflection coefficient indicators to ensure the three-frequency resonant performance and impedance matching effect of the radiating unit. Finally, it is adapted to a four-unit MIMO self-decoupling structure, which is easy to realize in industrialization. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the structure of a single radiating unit of the present invention, wherein... Figure 1 (a) in the image is a top view of the serpentine radial patch. Figure 1 (b) in the figure is a bottom view of the ground plane;

[0037] Figure 2This is a schematic diagram of the MIMO antenna configuration of the present invention, wherein... Figure 2 Image (a) is a schematic diagram of a dual-port MIMO antenna. Figure 2 (b) is a schematic diagram of the structure of a four-port MIMO antenna;

[0038] Figure 3 This is a schematic diagram of the scattering parameters of the four-port MIMO antenna of the present invention, wherein... Figure 3 In the figure, (a) represents the scattering parameters in the colon tissue model. Figure 3 (b) in the figure represents the scattering parameters in the cardiac tissue model;

[0039] Figure 4 This is the far-field radiation pattern of the four-port MIMO antenna of the present invention, wherein... Figure 4 (a) in the diagram is the radiation pattern for the 1.4 GHz band. Figure 4 (b) in the diagram shows the radiation pattern of the 2.45 GHz band. Figure 4 (c) in the diagram represents the radiation pattern of the 5.8 GHz band. Detailed Implementation

[0040] The present invention will be further described in detail below with reference to the accompanying drawings.

[0041] This specific embodiment is merely an explanation of the present invention and is not intended to limit the invention. After reading this description, those skilled in the art can make creative modifications to this embodiment as needed, but as long as they are within the scope of the claims of the present invention, they are protected by patent law.

[0042] This invention provides a compact multi-frequency MIMO antenna, which solves the technical problems in the prior art. The overall concept is as follows:

[0043] Example 1

[0044] Please see Figures 1-4 A compact multi-frequency MIMO antenna includes four radiating elements arranged in a planar anti-symmetrical manner. Each radiating element includes a high dielectric constant substrate, a top dielectric layer, a ground plane, and a serpentine radiating patch. The top dielectric layer is located on top of the serpentine radiating patch, the serpentine radiating patch is located on top of the high dielectric constant substrate, and the ground plane is located on the bottom of the high dielectric constant substrate. The antenna is configured to operate in the frequency bands of 1.395-1.432GHz, 2.400-2.480GHz, and 5.725-5.875GHz. The radiating elements achieve self-decoupling through spacing optimization, eliminating the need for external decoupling components.

[0045] This antenna is suitable for implantable medical devices (IMDs), including wireless capsule endoscopy systems and leadless pacemakers, and is designed to operate efficiently in a very small device space while supporting multi-band operation, high-speed wireless communication, and meeting human safety standards.

[0046] The antenna was designed considering both homogeneous and heterogeneous biological tissue environments to ensure robust performance after implantation. First, the implantable capsule device was modeled to simulate real-world usage conditions. The wireless capsule endoscope is a cylindrical device with a length of 24.65 mm and a radius of 5.5 mm, equipped with two end caps with a thickness of 5.5 mm; while the leadless pacemaker is 24.5 mm long, 11 mm in diameter, and has an upper end cap thickness of 1.2 mm. The capsule is made of alumina with a dielectric constant of 9.8 and a loss tangent of 0.006. Internal electronic components, including the battery and sensors, were modeled as a thin Rogers RT / duroid 6010 substrate sandwiched between PEC layers. =10.2, tanδ=0.0023). The antenna is placed inside the device to take into account its electromagnetic properties in relation to all components.

[0047] Specifically, the first step involves a single radiating element, which serves as the foundation of the MIMO structure. Measuring 4×4×0.26 mm³, it consists of a radiating patch with a serpentine and slotted structure, an overlay dielectric layer, a shorting pin, and a partially slotted ground plane. Miniaturization of the antenna is achieved by combining a high-dielectric-constant substrate, slotted loading, and a serpentine geometry. The antenna is excited by a 50Ω coaxial feed with a diameter of 0.4 mm.

[0048] Furthermore, the single radiating element is developed through a systematic four-step design evolution process to achieve the required tri-band operating characteristics and impedance matching performance. Step 1: The antenna consists of a square radiating patch containing a rectangular slot formed by segments s1–s5, placed on a complete ground plane. This structure produces a single resonance at approximately 1.50 GHz with a reflection coefficient of |S|. 11 |=-5.20dB indicates insufficient impedance matching and limited bandwidth performance.

[0049] Step 2 involves introducing additional slotted segments to enhance current distribution and increase the effective electrical length. Specifically, slots s6, s7, s8, and s are added to the chip. 11 and s 12 The process involves introducing a slotted g1 segment into the ground plane. These structural improvements resulted in multiple resonances at approximately 1.41 GHz, 3.12 GHz, and 4.26 GHz, with corresponding improvements in reflection coefficients of -11.64 dB, -25.51 dB, and -13.97 dB, respectively. This step validates the role of slotted loading in achieving multi-frequency characteristics.

[0050] Step 3, by introducing additional slotted structures s9 and s 14 s 1w The antenna was further optimized by improving the g3 segment and grounding structure. These adjustments altered the surface current path and electromagnetic coupling within the structure, shifting the resonant frequency to approximately 1.48 GHz and 2.81 GHz, with corresponding reflection coefficients of -13.61 dB and -27.48 dB, respectively. This stage was primarily used for frequency tuning and stabilization of the intermediate resonance.

[0051] Step 4, by introducing slots s 10 s 13 The section and grounding slot g7 were finely adjusted. These final adjustments optimized impedance matching and precisely aligned the resonant frequency with the target frequency band. Stable tri-frequency operation was finally achieved at 1.4GHz, 2.45GHz, and 5.8GHz, with reflection coefficients of approximately -16dB, -18dB, and -15.5dB, respectively.

[0052] By progressively introducing and optimizing slotted structures in the radiating patch and ground plane, effective control of current distribution, generation of multiple resonances, and impedance matching are achieved, thereby obtaining compact tri-band performance suitable for implantable MIMO antenna applications.

[0053] The SISO antenna was evaluated in a homogeneous human model with dimensions of 100×100×100 mm³. The model was enclosed within a radiating boundary box with dimensions of 200×200×200 mm³.

[0054] To obtain more accurate and realistic results, a heterogeneous tissue model was also constructed using Sim4Life to simulate human anatomy.

[0055] The dielectric properties of the tissue, including frequency-dependent relative permittivity and conductivity, were considered in the simulation to ensure that the simulated environment reflects the high-loss conditions encountered during implantation. The reflection coefficients measured in homogeneous and heterogeneous media showed good consistency, verifying the reliable resonance characteristics of the antenna in a real-world environment.

[0056] Two-port and four-port MIMO antennas are constructed from identical single radiating elements arranged in opposite directions. The spacing between the radiating elements is optimized to achieve minimal mutual coupling, such as... Figure 2 As shown.

[0057] The four-port MIMO antenna achieves mutual coupling levels below -39.19 dB, -29.34 dB, and -20 dB at 1.4 GHz, 2.45 GHz, and 5.8 GHz, respectively, while maintaining tri-frequency resonance. This structure demonstrates the feasibility of extending a single-element antenna design to a multi-element MIMO system while maintaining the desired resonance.

[0058] The four-port MIMO antenna achieves self-decoupling performance by optimizing the radiating element spacing to 0.5mm. This spacing significantly reduces mutual coupling between antenna elements without the need for external components or decoupling circuits. The four-port MIMO antenna measures 8.5×8.5×0.26mm³, with a total volume of 18.785mm³, and is compatible with wireless capsule endoscopes and leadless pacemaker devices.

[0059] Decoupling and frequency drift analysis were performed to ensure stable MIMO performance. Current distribution simulations showed that coupling between adjacent antenna elements could lead to frequency drift and reduced isolation. By introducing an optimized 0.5mm spacing between adjacent elements, mutual coupling was significantly reduced, and the resonant frequency was restored to the level of the single-element antenna Ant-I. The improvement in isolation was achieved through |S 21 The reduction in | was verified, decreasing from -7.64dB to -39.19dB at 1.4GHz and from -10.7dB to -29.34dB at 2.45GHz.

[0060] like Figure 3 As shown, the four-port MIMO antenna was tested in colon and heart models to evaluate the impact of implantation depth and surrounding tissue on antenna performance. In the colon model, the reflection coefficients ranged from -15.24 dB to -18.27 dB at 1.4 GHz, -18.76 dB to -27.09 dB at 2.45 GHz, and -18.03 dB to -20.26 dB at 5.8 GHz. In the heart model, the reflection coefficients ranged from -23.19 dB to -26.15 dB, -14.56 dB to -15.74 dB, and -30.72 dB to -36.32 dB, respectively. The cross-coupling coefficients remained below -31.56 dB, -27.78 dB, and -20.10 dB for all frequency bands.

[0061] like Figure 4 As shown, the far-field radiation pattern exhibits near-omnidirectional coverage, suitable for implantable devices in motion.

[0062] The peak gain is -35dBi at 1.4GHz, -25dBi at 2.45GHz, and -15dBi at 5.8GHz. The gain is slightly lower in the low-frequency band due to the higher dielectric loss in the tissue.

[0063] SAR assessment validated patient safety. Simulations using Sim4Life showed that, under real-world conditions with an operating power of -16 dBm per antenna element, the SAR values ​​for 1 gram and 10 grams of tissue were both below the IEEE C95.1-2019 standard threshold, thus ensuring compliance with safety standards for biomedical implantable devices.

[0064] Communication performance was evaluated through link budget calculations and simulations. The proposed MIMO antenna supports high-speed telemetry over distances exceeding 20 meters across all operating frequency bands, with data rates reaching 100 Mbps.

[0065] Key MIMO channel parameters, including envelope correlation coefficient (ECC) and diversity gain (DG), were evaluated using a radiation pattern-based approach. ECC remained below 0.5 across all frequencies, while DG exceeded 9 dB, validating robust diversity performance suitable for high-speed embedded communications.

[0066] In summary, the proposed four-port MIMO antenna combines compact size, tri-band operation, self-decoupling, near-omnidirectional coverage, and SAR compliance. The antenna design adapts to the geometry of real implantable devices and ensures reliable high-speed communication in heterogeneous biological tissues.

[0067] The above description of the embodiments is provided to facilitate understanding and use of the present invention by those skilled in the art. It is obvious to those skilled in the art that various modifications can be made to the embodiments, and the general principles described herein can be applied to other embodiments without creative effort. Therefore, the present invention is not limited to the above embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the present invention should be within the protection scope of the present invention.

Claims

1. A compact multi-frequency MIMO antenna, characterized in that, It includes two or four radiating units arranged in a planar manner, with adjacent radiating units arranged in opposite directions; Each of the radiating elements includes a high dielectric constant substrate, an overlying dielectric layer, a ground plane, and a serpentine radiating patch; The overlay dielectric layer covers the radiation unit, decoupling and isolating the radiation unit from the rest of the implantable medical device. The serpentine patch and the ground plane are located on the front and back sides of the high dielectric constant substrate, respectively. Both the serpentine radiating patch and the ground plane are provided with several slots. The current distribution and electromagnetic coupling of the radiating element are controlled by the slots, so that the antenna can operate in the frequency bands of 1.395-1.432GHz, 2.400-2.480GHz and 5.725-5.875GHz.

2. A compact multi-frequency MIMO antenna according to claim 1, characterized in that: The serpentine radiating patch has an outer semi-enclosed groove, a folded groove, a power supply through-hole groove, and multiple additional grooves, which work together to achieve frequency tuning and three-frequency resonant impedance matching. The outer semi-enclosed groove has a structure that surrounds three sides and opens at the bottom along the edge of the patch, and an additional tuning groove is formed in the bottom groove section. The folded groove is nested in the outer semi-enclosed groove and is formed by continuous opening of an initial section, an extension section, and a final extension section. The additional grooves include a side additional groove located next to the initial section of the folded groove, an extended additional groove extending along the side additional groove, and an independent rectangular additional groove located next to the folded groove.

3. A compact multi-frequency MIMO antenna according to claim 1, characterized in that: The grounding plane has a grounding port through-hole groove and multiple transverse hollow grooves arranged longitudinally.

4. A compact multi-frequency MIMO antenna according to claim 1, characterized in that: The spacing between the serpentine radiating patches of two adjacent radiating units is 0.5 mm. Self-decoupling between units is achieved through spacing optimization and reverse arrangement.

5. A compact multi-frequency MIMO antenna according to claim 2, characterized in that: The additional tuning groove of the bottom section of the outer semi-enclosed groove has a vertical groove width that is twice the groove width of the bottom section.

6. A design method for realizing tri-frequency resonance of an implantable multi-frequency MIMO antenna, applicable to the compact multi-frequency MIMO antenna described in any one of claims 1-5, characterized in that, Includes the following steps: S1. Using a high dielectric constant substrate as the base, a square radiating patch and a complete ground plane are prepared. A semi-enclosed groove is opened at the edge of the square radiating patch to form an initial radiating structure. This structure generates a single resonance at 1.50 GHz, and its reflection coefficient is -5.20 dB. S2. A folded groove is formed on the radiating patch inside the outer semi-enclosed groove, and an additional side groove is formed on the side of the folded groove. Simultaneously, two transverse hollow grooves are formed on the ground plane to enhance the current distribution on the surface of the radiating patch and increase the effective electrical length, so that the radiating structure generates multiple resonances at 1.41GHz, 3.12GHz and 4.26GHz, with corresponding reflection coefficients of -11.64dB, -25.51dB and -13.97dB, respectively. S3. Extend the folding groove along the original path based on the initial section of the folding groove, and at the same time, open an independent rectangular additional groove on the radiating patch and open an additional tuning groove at the bottom groove section of the outer semi-enclosed groove; simultaneously, add a transverse hollow groove on the grounding plane to adjust the current path on the surface of the radiating patch and the electromagnetic coupling relationship within the structure, so that the resonant frequency shifts to 1.48GHz and 2.81GHz, corresponding to reflection coefficients of -13.61dB and -27.48dB respectively, thereby stabilizing the intermediate resonant frequency; S4. Based on the extension section of the folding groove, continue to extend the folding groove to form a final extension section, forming an integrated continuous folding groove. At the same time, extend the folding groove along the side auxiliary groove to form an extended auxiliary groove. Simultaneously, add a transverse hollow groove on the grounding plane to finely adjust the resonant frequency and optimize the impedance matching, so that the radiating unit can achieve stable tri-frequency operation at 1.4GHz, 2.45GHz and 5.8GHz, with corresponding reflection coefficients of -16dB, -18dB and -15.5dB, respectively. S5. The optimized individual radiating elements are arranged symmetrically in a plane with a spacing of 0.5mm. Self-decoupling is achieved through the combination of spacing and arrangement. The isolation of the entire operating frequency band is less than -29dB, forming a four-port MIMO antenna structure with a total volume of no more than 18.8mm³.

7. A compact multi-frequency MIMO antenna according to claim 1, characterized in that: The envelope correlation coefficient between any two radiation units is less than 0.5, and the diversity gain is greater than 9dB.

8. A compact multi-frequency MIMO antenna according to claim 1, characterized in that: The antenna's specific absorption rate meets the IEEE standard limits for 1 gram and 10 grams of human tissue, and the antenna surface is encapsulated with a biocompatible coating.

9. The application of the compact multi-frequency MIMO antenna as described in claim 1 in the fabrication of capsule-shaped implantable devices or cardiac implantable devices, characterized in that, The compact multi-frequency MIMO antenna has a total volume of no more than 18.8 mm³ and is adapted to be integrated into the body of a capsule-shaped implantable device or a cardiac implantable device. It is compatible with the battery, sensors and electronic circuits inside the device and operates in three frequency bands: 1.395-1.432 GHz, 2.400-2.480 GHz and 5.725-5.875 GHz, to realize wireless power transmission and high-speed wireless communication of the capsule-shaped implantable device or cardiac implantable device.

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