A four-element compact broadband MIMO antenna based on metamaterials

Abstract

The present invention discloses a four-unit compact broadband MIMO antenna based on metamaterials, which belongs to the field of microwave antenna technology. The MIMO antenna of the present invention includes a MIMO antenna array and a metasurface array, the metasurface array is located above the MIMO antenna array and a spacing is left in the middle; the MIMO antenna array includes four antenna units, the antenna unit includes a radiation structure, a floor and a first dielectric substrate, the radiation structure and the floor are printed on the upper surface of the first dielectric substrate; the lower edge of the antenna unit constitutes a coplanar waveguide feeding interface; the metasurface array is a 3×3 arranged metasurface unit; the metasurface unit includes an open ring, an I-type structure and a second dielectric substrate. The antenna of the present invention has the characteristics of compact structure, high radiation gain, good diversity performance (ECC, DG, TARC) and high isolation, which makes it more powerful and suitable for WLAN, Sub‑6G and Ku-band communication applications, and also provides a possibility for the decoupling design of the antenna.

Description

Technical Field

[0001] The present invention belongs to the technical field of microwave antennas, and specifically relates to a four-element compact broadband MIMO antenna based on metamaterials, and more particularly to a compact four-element quasi-self-complementary (QSC) coplanar waveguide (CPW) fed ultra-wideband (UWB) multiple-input multiple-output (MIMO) antenna. Background Art

[0002] With the rapid development of wireless communication technology, devices are required to meet larger working bandwidth, faster transmission rate and better portability. Research shows that in UWB MIMO communication systems, combining multiple-input multiple-output technology with ultra-wideband technology is an effective solution to improve current communication technology. However, the coupling current generated between closely spaced antenna elements will affect the radiation performance of the antenna. Therefore, as a key component of the UWB MIMO communication system, one of the main design difficulties faced by the antenna is to achieve high isolation between radiating elements while ensuring a compact structure.

[0003] In recent years, various decoupling methods have been disclosed in the prior art, including the use of defective ground structures, the use of directional patterns and polarization diversity, the loading of parasitic units, the loading of electromagnetic bandgap structures, and the loading of metamaterials. Defective ground decoupling reduces the mutual coupling between MIMO antenna units by extending the path of the surface current on the floor or improving the transmission characteristics of the microwave circuit. Common defective ground structure designs mainly include: etching gaps directly on the metal floor of the MIMO antenna; designing defective ground structure filters and loading them between antenna arrays. Polarization diversity uses the polarization rotation characteristics to twist the electric field direction of the electromagnetic wave transmitted in the coupling field, reducing the electromagnetic energy radiated by other antenna units received by the antenna unit, thereby achieving coupling suppression between MIMO antenna units. Self-decoupling technology reduces mutual coupling by utilizing the different characteristics exhibited by the unique structure of the antenna unit, without adding additional decoupling structures, but requires continuous optimization of the antenna unit structure and array layout. Parasitic resonance decoupling allows the excited antenna unit to couple to the parasitic resonance structure and the non-excited antenna unit at the same time, and the parasitic resonance structure also couples to the non-excited antenna unit. Assuming that the two coupling currents received by the non-excited antenna unit are equal in amplitude and opposite in direction and cancel each other out, the coupling between antenna units can be reduced. Metasurface decoupling is to cover the metasurface structure at a certain distance above the array antenna to change the propagation wave number of the coupled electromagnetic waves between antenna units, so that the cutoff frequencies of the TE and TM modes approach infinity, and the surface waves cannot propagate along the dielectric interface, thereby reducing the mutual coupling between antenna units. How to structurally implement the above decoupling method has become a problem faced by technicians in this field.

[0004] The prior art “Design of a Six-Port Compact UWB MIMO Antenna With a Distinctive DGS for Improved Isolation” increases the isolation of the antenna to 20 dB by loading an improved fence-type decoupling structure on the ground plate to extend the current coupling path, but its size is large and is not suitable for miniaturized design; the prior art “Enhanced Quad-Port MIMO Antenna Isolation with Metamaterial Superstrate” discloses a new metamaterial superposition structure placed above the MIMO array to increase the isolation of the antenna to 23 dB, but its bandwidth is narrow and is not suitable for broadband applications; the prior art “High Isolation Compact UWB MIMO Antennas” loads cross-shaped parasitic branches between the four antenna units and circular branches in the center to increase the isolation of the antenna to 20 dB, but its size is large and the gain is low.

[0005] The above design solutions have achieved miniaturization, broadband or high isolation characteristics to a certain extent, but the overall performance of each antenna still has room for improvement. Therefore, how to achieve a MIMO array with good isolation performance and working in an ultra-wideband range in a compact structure is an important and challenging issue in the field of microwave antenna technology. Summary of the invention

[0006] The purpose of the present invention is to overcome the above-mentioned defects of the prior art and provide a four-unit compact broadband MIMO antenna based on metamaterials.

[0007] The technical problem proposed by the present invention is solved in this way:

[0008] A four-unit compact broadband MIMO antenna based on metamaterials, comprising a MIMO antenna array and a metasurface array, wherein the metasurface array is located above the MIMO antenna array with a gap in the middle;

[0009] The MIMO antenna array has a 90° rotationally symmetrical structure with the center of the array as the center, and includes four quasi-self-complementary ultra-wideband monopole antenna units arranged orthogonally and rotated; the antenna unit includes a radiation structure, a floor and a first dielectric substrate, and the radiation structure and the floor are printed on the upper surface of the first dielectric substrate; the lower edge of the antenna unit constitutes a coplanar waveguide feeding interface;

[0010] The metasurface array is a 3×3 arranged metasurface unit; the metasurface unit includes an open circular ring, an I-type structure and a second dielectric substrate, and the open circular ring structure and the I-type structure are printed on the upper surface of the second dielectric substrate; the center of the I-type structure coincides with the center of the open circular ring structure, is parallel to the edge of the second dielectric substrate, and has a length greater than the diameter of the open circular ring structure; the open circular ring structure has two symmetrical openings on the basis of the circular ring, and the connecting line of the two openings passes through the center of the open circular ring structure and is perpendicular to the I-type structure.

[0011] Furthermore, the radiation structure includes a microstrip branch and a half-cut hexagonal radiation patch. The microstrip branch extends inward from the lower edge of the first dielectric substrate, and the end is connected to the half-cut hexagonal radiation patch. The connection position is located at the narrow edge of the half-cut hexagonal radiation patch, and the left edge of the microstrip branch is collinear with the tangent of the half-cut hexagonal radiation patch.

[0012] Further, the floor comprises four grounding plates, which are respectively referred to as the first to fourth grounding plates;

[0013] The first ground plate is a rectangular patch with a cut corner, located on the right side of the microstrip branch with a gap, with the lower edge and the right edge reaching the edge of the first dielectric substrate respectively, with a height less than the length of the microstrip branch, and an arc cut corner at the upper left corner;

[0014] The second ground plate is a rectangular patch with a cut corner, located on the left side of the microstrip branch with a gap, with the lower edge and the left edge reaching the edge of the first dielectric substrate respectively, with the same height as the first ground plate, and an arc cut corner at the upper right corner;

[0015] The third ground plate is a right-angle trapezoidal patch, the right-angle waist coincides with the left edge of the first dielectric substrate, the bottom coincides with the upper edge of the second ground plate, and the hypotenuse forms an angle of 120° with the upper edge of the second ground plate;

[0016] The fourth ground plate is a flag-type patch, including a flag patch portion and a flagpole patch portion; the flag patch portion is the same as the third ground plate, the upper base of the flag patch portion coincides with the upper base of the third ground plate, and the right-angle waist coincides with the left edge of the first dielectric substrate; the flagpole patch portion is connected to the lower base of the flag patch portion, extending from the left edge of the first dielectric substrate to the right, and the length is the same as that of the second ground plate.

[0017] Furthermore, the impedance characteristic of the feeding interface is 50Ω.

[0018] Furthermore, the thickness of the dielectric substrate is 0.8 mm, and a FR4-epoxy dielectric substrate is selected.

[0019] Furthermore, the spacing between the metasurface array and the MIMO antenna array is 16 mm.

[0020] The beneficial effects of the present invention are:

[0021] Compared with the prior art microstrip-fed QSC UWB MIMO antenna, the present invention adopts coplanar waveguide feeding, which greatly reduces the size of the antenna; the unit antenna is a self-complementary antenna, and its input impedance does not change with the frequency, so it can achieve good impedance matching in a broadband range; the present invention adopts a hexagonal quasi-self-complementary structure to enhance its impedance matching characteristics; an arc groove is etched on the ground plane, which further expands the bandwidth of the antenna; the present invention arranges four antenna units in an orthogonal structure, and uses the polarization rotation characteristics to twist the electric field direction of the electromagnetic wave transmitted in the coupling field, thereby reducing the electromagnetic energy radiated by other antenna units received by the antenna unit, and enhancing the mutual coupling suppression between the MIMO antenna units; in the present invention, a metamaterial structure is loaded on the top of the MIMO antenna, and the propagation wave number of the coupled electromagnetic waves between the antenna units is changed, thereby reducing the coupled electric field and reducing the mutual coupling. In summary, compared with the antenna in the prior art, the antenna of the present invention has the characteristics of compact structure, high radiation gain, good diversity performance (ECC, DG, TARC) and high isolation, which makes it more powerful and suitable for WLAN, Sub-6G and Ku-band communication applications, and also provides a possibility for the decoupling design of the antenna. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the overall structure of the MIMO antenna of the present invention;

[0023] Figure 2 It is a structural schematic diagram of the antenna array in the MIMO antenna of the present invention;

[0024] Figure 3 It is a structural schematic diagram of the antenna unit in the MIMO antenna of the present invention;

[0025] Figure 4 It is a schematic diagram of the structure of the super surface array in the MIMO antenna of the present invention;

[0026] Figure 5 It is a schematic diagram of the structure of the super surface unit in the MIMO antenna of the present invention;

[0027] Figure 6 Graphs showing transmission characteristics of the MIMO antenna of the embodiment; wherein (a) is the S12 curve, (b) is the S13 curve, and (c) is the S14 curve;

[0028] Figure 7 is an ECC curve diagram of the MIMO antenna described in the embodiment;

[0029] Figure 8 is a DG curve diagram of the MIMO antenna described in the embodiment;

[0030] Fig. 9 FIG. 4 is a TARC curve diagram of the MIMO antenna described in the embodiment. DETAILED DESCRIPTION

[0031] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0032] This embodiment provides a four-unit compact broadband MIMO antenna based on metamaterials, and its overall structure schematic diagram is shown in FIG. Figure 1 As shown, it includes a MIMO antenna array and a super surface array, and the super surface array is located above the MIMO antenna array with a gap in the middle.

[0033] The structural diagram of the MIMO antenna array is as follows: Figure 2 As shown, the structure is 90° rotationally symmetrical with the center of the array as the center, including four orthogonally rotated quasi-self-complementary ultra-wideband monopole antenna units; the structural schematic diagram of the antenna unit is shown in Figure 3 As shown, it includes a radiation structure, a floor and a dielectric substrate, and the radiation structure and the floor are printed on the upper surface of the dielectric substrate.

[0034] The radiation structure includes a microstrip branch and a half-cut hexagonal radiation patch. The microstrip branch extends inward from the lower edge of the dielectric substrate, and the end is connected to the half-cut hexagonal radiation patch. The connection position is located at the narrow side of the half-cut hexagonal radiation patch. The left edge of the microstrip branch is collinear with the tangent of the half-cut hexagonal radiation patch.

[0035] The floor comprises four grounding plates, which are respectively denoted as the first to fourth grounding plates;

[0036] The first ground plane is a rectangular patch with a cut corner, located on the right side of the microstrip branch with a gap, with the lower edge and the right edge reaching the edge of the dielectric substrate respectively, with a height less than the length of the microstrip branch, and an arc cut corner at the upper left corner;

[0037] The second ground plane is a rectangular patch with a cut corner, located on the left side of the microstrip branch with a gap, with the lower edge and the left edge reaching the edge of the dielectric substrate respectively, with the same height as the first ground plane, and an arc cut corner at the upper right corner;

[0038] The third ground plate is a right-angle trapezoidal patch, the right-angle waist coincides with the left edge of the dielectric substrate, the lower base coincides with the upper edge of the second ground plate, and the hypotenuse forms an angle of 120° with the upper edge of the second ground plate;

[0039] The fourth ground plate is a flag-type patch, including a flag patch portion and a flagpole patch portion; the flag patch portion is the same as the third ground plate, the upper base of the flag patch portion coincides with the upper base of the third ground plate, and the right-angle waist coincides with the left edge of the dielectric substrate; the flagpole patch portion is connected to the lower base of the flag patch portion, extending from the left edge of the dielectric substrate to the right, and the length is the same as that of the second ground plate.

[0040] The antenna unit described in this embodiment is composed of a symmetrically halved hexagonal radiation patch and a complementary hexagonal slot ground plate with arc slots on both sides. The lower edge constitutes a coplanar waveguide feeding interface, and the impedance characteristic of the feeding interface is 50Ω.

[0041] In this embodiment, the thickness of the dielectric substrate is 0.8 mm, and a FR4-epoxy dielectric substrate is selected.

[0042] The structural diagram of the metasurface array is shown in Figure 4 As shown, it is a 3×3 arrangement of the super surface unit; the structural schematic diagram of the super surface unit is as follows Figure 5 As shown, it includes an open circular ring, an I-type structure and a dielectric substrate, and the open circular ring structure and the I-type structure are printed on the upper surface of the dielectric substrate; the center of the I-type structure coincides with the center of the open circular ring structure, is parallel to the edge of the dielectric substrate, and has a length greater than the diameter of the open circular ring structure; the open circular ring structure has two symmetrical openings on the basis of the circular ring, and the connecting line of the two openings passes through the center of the open circular ring structure and is perpendicular to the I-type structure.

[0043] In this embodiment, the dielectric substrate has a thickness of 0.8 mm. r =4.4, tanδ=0.02 FR4-epoxy dielectric substrate. The overall size of the unit antenna is 20×20mm 2 The overall size of the MIMO antenna is 40×40mm 2 The spacing between the metasurface array and the MIMO antenna array is 16 mm.

[0044] In this embodiment, the unit antenna is a self-complementary antenna, and the input impedance does not change with frequency, and can achieve good impedance matching within a wide bandwidth. In order to further improve the impedance matching of the antenna, an arc groove is etched on the ground plane. In order to increase the channel capacity, the quasi-self-complementary ultra-wideband monopole antenna is placed orthogonally rotated to form a four-port MIMO array, and the polarization rotation characteristics are used to twist the propagation direction of the electromagnetic wave in the coupled electric field, thereby reducing the electromagnetic energy radiated by other antenna units received by the antenna unit and enhancing the mutual coupling suppression between the MIMO antenna units.

[0045] When the MIMO antenna array is not loaded with a metasurface covering layer, the coupled electromagnetic waves between the unit antennas propagate along the surface direction of the dielectric substrate. Loading metasurfaces with different dielectric constants and magnetic permeabilities above the antenna array will change the propagation wave number of the coupled electromagnetic waves. After loading a metasurface with a negative dielectric constant (<0) and a positive magnetic permeability (>0), the coupled electric field decreases with increasing distance, and the electromagnetic waves mainly propagate in a direction perpendicular to the surface of the dielectric substrate. Therefore, loading a metasurface with a negative dielectric constant and positive magnetic permeability above the antenna array can effectively reduce the mutual coupling between the units.

[0046] Therefore, this embodiment designs a metasurface unit composed of an I-shaped structure and an open circular ring. The Floquet port excitation method is used to simulate the metasurface unit to obtain the reflection coefficient and transmission coefficient on the infinite surface. According to the S-parameter inversion algorithm, the intrinsic parameters of the metasurface structure are extracted through MATLAB, that is, the equivalent refractive index, equivalent impedance, equivalent dielectric constant and equivalent magnetic permeability of the metasurface structure are solved using the transmission matrix and S parameters of the equivalent model. In the frequency bands of 4.0-5.2GHz, 12.7-15.4GHz, and 18.5-20.0GHz, the real part of the dielectric constant of the metasurface structure is negative, and the real part of the magnetic permeability is positive. Therefore, placing it above the antenna array is conducive to suppressing the propagation of surface waves between units and reducing mutual coupling.

[0047] The transmission characteristic curve of the MIMO antenna loaded with the metasurface structure is shown in Figure 6 As shown; among them, Figure 6 (a) is the S12 curve, Figure 6 (b) is the S13 curve, Figure 6 (c) is the S14 curve. Figure 6 It can be seen that after loading the metasurface structure, the isolation is higher than 29dB in the low frequency band and higher than 23dB in the high frequency band. Compared with the case without the metasurface structure, the isolation in the two frequency bands is increased by 13dB and 4dB respectively. Therefore, loading the metasurface structure can effectively enhance the isolation performance of the MIMO antenna.

[0048] The envelope correlation coefficient (ECC) is a parameter index for measuring the diversity performance and coupling performance of a multi-antenna system, and is used to characterize the correlation of signal amplitudes between different antenna units. The ECC curve of the antenna described in this embodiment is shown in FIG. Figure 7 As shown, in the range of 2.49-6.90GHz and 12.85-20.00GHz, the ECC of the antenna is very small (<0.0002), and has good diversity characteristics.

[0049] Diversity gain DG (diversity gain) is a performance indicator that measures the degree of isolation or correlation between channels in a communication system. It is required that the diversity gain of a MIMO antenna be as close to 10 dB as possible. The DG curve of the antenna in this embodiment is shown in FIG. Figure 8 As shown, in the range of 2.49-6.90GHz and 12.85-20.00GHz, the diversity gain DG of the antenna is very close to 10dB, which has good diversity characteristics.

[0050] The total active reflection coefficient (TARC) indicates the effective working bandwidth when multiple antennas are excited simultaneously. Generally speaking, a MIMO antenna with a TARC lower than -10 dB indicates low return loss and good phase stability. The TARC curve of the antenna described in this embodiment is shown in FIG. Fig. 9 As shown, in the range of 2.49-6.90GHz and 12.85-20.00GHz, the TARC of the proposed MIMO antenna is lower than -10dB, which can ensure the reliability of signal transmission.

[0051] In this embodiment, after the MIMO array is loaded with the metasurface structure, a new resonant cavity is generated, which effectively improves the peak gain of the MIMO antenna. Its maximum gain is 7.44dB within the working frequency band, and it has good radiation performance.

Claims

1. A four-element compact broadband MIMO antenna based on metamaterials, characterized in that: It includes a MIMO antenna array and a metasurface array, wherein the metasurface array is located above the MIMO antenna array with a gap in the middle; The MIMO antenna array has a 90° rotationally symmetrical structure with the center of the array as the center, and includes four quasi-self-complementary ultra-wideband monopole antenna units arranged in an orthogonal rotation; the antenna unit includes a radiation structure, a floor and a first dielectric substrate, and the radiation structure and the floor are printed on the upper surface of the first dielectric substrate; The lower edge of the antenna unit forms a coplanar waveguide feeding interface; The metasurface array is a 3×3 array of metasurface units; the metasurface unit includes an open circular ring, an I-type structure, and a second dielectric substrate, and the open circular ring structure and the I-type structure are printed on the upper surface of the second dielectric substrate; The center of the I-shaped structure coincides with the center of the open circular ring structure, is parallel to the edge of the second dielectric substrate, and has a length greater than the diameter of the open circular ring structure; the open circular ring structure has two symmetrical openings on the basis of the circular ring, and the connecting line of the two openings passes through the center of the open circular ring structure and is perpendicular to the I-shaped structure; The radiation structure includes a microstrip branch and a half-hexagonal radiation patch. The microstrip branch extends inward from the lower edge of the first dielectric substrate, and the end is connected to the half-hexagonal radiation patch. The connection position is located at the narrow side of the half-hexagonal radiation patch. The left edge of the microstrip branch is collinear with the tangent of the half-hexagonal radiation patch. The floor comprises four grounding plates, which are respectively denoted as the first to fourth grounding plates; The first ground plate is a rectangular patch with a cut corner, located on the right side of the microstrip branch with a gap, with the lower edge and the right edge reaching the edge of the first dielectric substrate respectively, with a height less than the length of the microstrip branch, and an arc cut corner at the upper left corner; The second ground plate is a rectangular patch with a cut corner, located on the left side of the microstrip branch with a gap, with the lower edge and the left edge reaching the edge of the first dielectric substrate respectively, with the same height as the first ground plate, and an arc cut corner at the upper right corner; The third ground plate is a right-angle trapezoidal patch, the right-angle waist coincides with the left edge of the first dielectric substrate, the bottom coincides with the upper edge of the second ground plate, and the hypotenuse forms an angle of 120° with the upper edge of the second ground plate; The fourth ground plate is a flag-type patch, including a flag patch portion and a flagpole patch portion; the flag patch portion is the same as the third ground plate, the upper base of the flag patch portion coincides with the upper base of the third ground plate, and the right-angle waist coincides with the left edge of the first dielectric substrate; the flagpole patch portion is connected to the lower base of the flag patch portion, extending from the left edge of the first dielectric substrate to the right, and the length is the same as that of the second ground plate.

2. The four-element compact broadband MIMO antenna based on metamaterials according to claim 1, characterized in that: The impedance characteristic of the feeding interface is 50Ω.

3. The four-element compact broadband MIMO antenna based on metamaterials according to claim 1, characterized in that: The thickness of the first dielectric substrate and the second dielectric substrate is 0.8 mm, and FR4-epoxy dielectric substrates are selected.

4. The four-element compact broadband MIMO antenna based on metamaterials according to claim 1, characterized in that: The spacing between the metasurface array and the MIMO antenna array is 16 mm.

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