Patent Publication Number: US-11664589-B2

Title: 5G MIMO antenna array with reduced mutual coupling

Description:
BACKGROUND 
     Modern wireless communication demands transmission of a large amount of data within a short duration. Also, exponential growth in mobile users requires high data throughput, low latency and enhanced channel capacity. Needs of uninterrupted high definition (HD) video streaming, online gaming as well as high data traffic for mobile terminal user have boosted the development in wireless communication systems. Future 5 th  generation (5G) mobile communication technologies require an even broader spectrum and higher energy efficiency to support data rates of 1 to 10 Gbps and latency times as low as 1 ms. Single antenna/antenna arrays with broad bandwidth are not always sufficient to fulfill these requirements. Also multipath propagation of the signal between transmitter and receiver causes shadowing and fading of the received signal. 
     Multiple antenna elements at both the transmitter and the receiver are typically used to resolve these problems, avoid fading losses, and overcome the data throughput limit faced by conventional single-input single-output (SISO) wireless communication systems. This is commonly referred to as a multiple-input multiple-output (MIMO) communication system. MIMO technology uses multiple data streams sent and received by the multiple antenna elements on the transmitter and receiver ends. Integration of a large number of antenna elements within the limited space of a MIMO system (such as a base station, mobile terminal, or both) on the scale required for 5G applications is typically referred to as Massive MIMO. 
     However, the MIMO system is subject to problems, such as the multipath propagation problem due to high correlation in multiple signal broadcasting, as well as mutual coupling within the MIMO system. Mutual coupling refers to the amount of cross talk between the independent radiating sources. It can be caused due to surface wave propagation and space wave coupling in the MIMO antenna near field, and it can have a significant effect on the performance of the system. For instance, mutual coupling can cause any one or combination of changes in input impedance of the system phased array antenna with scanning angle (known as scan blindness), a reduction in channel capacity, and bandwidth losses. 
     Some existing decoupling techniques include neutralization lines, parasitic scatters, eigen-mode decomposition, complementary split ring resonators (CSRR), polarization isolators, metamaterial superstrates, and asymmetrical coplanar strip (ACPS) walls, electronic band gap (EBG) and defected ground structures (DGS). However, past decoupling attempts using these techniques have not achieved decoupling at high frequencies, such as frequencies above 12 GHz, and thus have not resulted in elements that are suitable for 5G applications or other high-frequency applications. 
     SUMMARY 
     The present disclosure reduces mutual coupling between circuit elements of the MIMO system by providing an electronic bandgap (EBG) structure. 
     EBG structures are artificially engineered two-dimensional or three-dimensional periodic structures that include metallic, dielectric or metallic-dielectric materials that exhibit unique properties including wave dispersion characteristics, a frequency bandgap, to restrain the propagation of surface waves at the structure, and zero reflection phase due to high surface impedance, at a resonant frequency for incident electromagnetic (EM) waves. The EBG structures are capable of controlling electromagnetic wave propagation over specific frequency ranges, regardless of an incident angle and polarization state of the electromagnetic wave. EBG structures are typically used to avoid mixing between analog circuit and digital circuit components on a printed circuit board (PCB) due to ground noise coupling, or to improve radiation characteristics of a microstrip antenna. However, the EBG structures are limited to operate at the lower end of the microwave frequency range, as it is difficult to reduce the unit cell size to function at millimeter wave frequencies. By contrast, the present disclosure presents various embodiments of an EBG structure that can be used in millimeter wave frequency applications, such as MIMO antenna arrays. 
     In some embodiments, a sub-wavelength uniplanar EBG structure is provided. Unlike mushroom EBG structures that include metallic vias to provide a direct current path through the substrate between the adjacent unit cells arranged in a two-dimensional periodic arrangement, the uniplanar EBG structures do not include vias. The uniplanar EBG structures of the present disclosure have a unit cell periodicity of less than one half wavelength, and in some cases less than a quarter wavelength. These structures also have a relatively low profile. 
     In some embodiments, the EBG structure is coupled with a defected ground structure (DGS) in the ground plane. This causes resonance for frequencies within the frequency bandgap of the EBG structure. The effect of the DGS can be understood by treating it as an equivalent magnetic current. The increase in magnetic coupling between the top EBG lattice and the DGS in the ground causes high reduction of the surface wave at the resonance frequency. 
     The example EBG structures and antenna arrays of the present disclosure are suitable for operation at millimeter wave frequencies, including but not limited to 9 GHz and 28 GHz. These frequencies are in demand for current and future 5G wireless communication devices and systems. The reduced effect of coupling MIMO channels between the elements improve dense packing of the elements in a limited space. The reduced coupling also permits for size miniaturization of the MIMO system, as well as increased scanning range of the phased array due to isolation enhancement between the MIMO channels. 
     One aspect of the present disclosure is directed to an electromagnetic bandgap (EBG) unit cell structure including: an electrically conductive ground plane; a first dielectric substrate formed on a top surface of the electrically conductive ground plane and having a first dielectric constant; a second dielectric substrate formed on a top surface of the first dielectric substrate and having a second dielectric constant different from the first dielectric constant, wherein the EBG unit cell structure is configured to reflect propagating millimeter waves at an interface between the first and second dielectric substrates; and a top metal layer formed on a top surface of the second dielectric substrate and including an EBG pattern formed in the top metal layer. 
     In some examples, the EBG pattern may have a periodicity that is between 2 mm and 10 mm. 
     In some examples, the EBG pattern may include a ring divided into four sections separated from one another by respective gaps, and each ring section may include a first end connected to a first metal strip extending radially in a direction toward a center of the EBG pattern and a second metal strip extending radially in a direction away from the center of the EBG pattern. 
     In some examples, each second metal strip may be connected to a respective patch configured to connect to a corresponding coplanar patch of an adjacent EBG unit cell, and each patch of the EBG pattern may include a slit extending radially in a direction toward the center of the EBG pattern. 
     In some examples, a structure of the EBG pattern may be defined by no more than: (i) a radial distance from the center to the ring; (ii) a radial distance from the center to the patch; (iii) a periodicity; (iv) a gap width; (v) a ring and metal strip width; and (vi) a slit length. 
     In some examples, an irreducible Brillouin zone of the EBG pattern may be a triangular region designated by vertices Γ, X and M. 
     In some examples, the EBG pattern may exhibit a frequency bandgap centered between 27-29 GHz and having a fractional bandwidth (FBW) of at least 20%. 
     Another aspect of the disclosure is directed to an EBG lattice structure including a plurality of interconnected EBG unit cell structures according to any of the embodiments described herein, wherein an overall inductance of the EBG lattice structure is greater than an individual EBG unit cell structure. 
     In some examples, the interconnected EBG unit cell structures may be a 3×2 matrix of interconnected EBG unit cell structures. 
     In some examples, a periodicity of the plurality of interconnected EBG unit cell structures may satisfy a periodic boundary condition. 
     In some examples, the EBG lattice structure may further include: a first open ended transmission line formed in the top metal layer; and a second open ended transmission line formed in the top metal layer, and the EBG pattern formed in the top metal layer is positioned between and loaded by the first and second open ended transmission lines. 
     In some examples, the EBG lattice structure may further include: a middle metal layer positioned between the first dielectric substrate and the second dielectric substrate; a substrate integrated waveguide (SIW) formed in the middle metal layer; and a defected ground structure (DGS) formed in the ground plane. 
     In some examples, the DGS may be positioned directly underneath the EBG pattern. 
     Another aspect of the disclosure is directed to a multiple-input multiple-output (MIMO) antenna array including: a first proximity-coupled microstrip antenna; a second proximity coupled microstrip antenna; and an EBG lattice structure according to any of the embodiments described herein. 
     In some examples, the first and second open ended transmission lines may be configured to create a transmission zero at or near the center of the frequency bandgap exhibited by the EBG lattice structure. 
     In some examples, the center of the frequency bandgap may be 27.5 GHz. 
     In some examples, the first and second open ended transmission lines may be configured to reduce coupling between the two microstrip antennas by at least 10 dB over a range from 27 GHz to 29 GHz. 
     Another aspect of the present disclosure is directed to a multiple-input multiple-output (MIMO) antenna array including: a first proximity-coupled microstrip antenna; a second proximity coupled microstrip antenna; and an EBG lattice structure according to any of the embodiments described herein. 
     In some examples, the SIW may be configured to embrace the first and second microstrip antennas and to confine propagation of surface waves to a space within the walls of the SIW cavity. 
     In some examples, the SIW may include sidewalls comprising two rows of vias, and may be configured to direct radiation of the first and second microstrip antennas in a single direction, wherein a front-to-back ratio of radiation in the MIMO antenna array is between 20 dB to 30 dB. 
     In some examples, a vertical length of the DGS may be 1.2 mm. 
     In some examples, the antenna array may be configured to operate at a millimeter wave operating frequency, and each of a length and width of the EBG unit cell structure may be about a quarter wavelength to a half wavelength of the operating signal wavelength of the MIMO antenna array. 
     In some examples, the DGS may have a length and width less than a wavelength of the operating signal wavelength of the MIMO antenna array. 
     In some examples, the EBG lattice structure and the DGS may be configured to reduce coupling between the two microstrip antennas by at least 12 dB over a range from 27 GHz to 29 GHz. 
     In some examples, the EBG lattice structure and the DGS may be configured to reduce S 11  and S 22  parameters of the MIMO antenna array by about 20 dB to 30 dB over a range from 28.75 GHz to 29.25 GHz. 
     In some examples, the MIMO antenna array may have a radiation efficiency of between about 82%-84% and an envelope correlation coefficient of about 0.00015. 
     In some examples, the DGS may be configured to improve an S 21  parameter of the MIMO antenna array by about 30 dB to 40 dB for at least one frequency between 27 GHz and 29 GHz. 
     Another aspect of the disclosure is directed to an EBG unit cell structure including: an electrically conductive ground plane; a dielectric substrate formed on a top surface of the electrically conductive ground plane; and a top metal layer formed on a top surface of the second dielectric substrate and including an EBG pattern formed in the top metal layer, wherein the EBG pattern has a 90 degree rotational symmetry and a periodicity between 3-6 millimeters, and the EBG pattern exhibits a frequency bandgap for a range of frequencies at least in part between 8-12 GHz. 
     In some examples, the EBG pattern may include four corner patches positioned at respective corners of the top metal layer, and a central patch positioned at a center of the top metal layer, and the central patch may be connected to each corner patch by a respective path, and the respective paths may be configured to form a spiral resonator. 
     In some examples, an irreducible Brillouin zone of the EBG pattern may be a triangular region designated by vertices Γ, X and M. 
     In some examples, the frequency bandgap may have an FBW of about 50%. 
     Another aspect of the disclosure is directed to an EBG lattice structure including a plurality of interconnected EBG unit cell structures according to any of the embodiments described herein, wherein an overall inductance of the EBG lattice structure is greater than an individual EBG unit cell structure. 
     In some examples, a periodicity of the plurality of interconnected EBG unit cell structures may satisfy a periodic boundary condition. 
     Another aspect of the disclosure is directed to a MIMO antenna array including: a plurality of antennas; and one or more EBG lattice structures according to any of the embodiments described herein, wherein each pair of adjacent antennas is separated by a corresponding EBG lattice structure. 
     Another aspect of the disclosure is directed to a partially reflective surface (PRS) including: an EBG lattice structure according to any of the embodiments described herein, wherein the EBG lattice structure is positioned over a patch antenna. 
     In some examples, the EBG lattice structure may be a 7×7 array of EBG unit cell structures. 
     Another aspect of the disclosure is directed to a frequency selective surface (FSS) including an EBG lattice structure according to any of the embodiments described herein. 
     In some examples, a resonant frequency of the EBG lattice structure may be consistent for electromagnetic waves of either TE or TM modes of propagation for an angle of incidence up to 60°. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a side view schematic of an electronic bandgap (EBG) unit cell structure in accordance with an aspect of the disclosure. 
         FIG.  2    is a top view schematic of the EBG unit cell structure of  FIG.  1   . 
         FIG.  3    is a Brillouin dispersion diagram showing dispersion characteristics of the EBG unit cell structure of  FIG.  1   . 
         FIG.  4    is a top view schematic of an EBG lattice structure in accordance with an aspect of the disclosure. 
         FIG.  5    is circuit diagram showing an equivalent circuit of the EBG lattice structure of  FIG.  4   . 
         FIG.  6    is a graph showing simulated S-parameters of the EBG lattice structure of  FIG.  4   . 
         FIG.  7 A  is an exploded perspective view schematic of a multiple-input multiple output (MIMO) antenna array loaded with an EBG lattice structure and a defected ground structure (DGS) in accordance with an aspect of the disclosure. 
         FIGS.  7 B- 7 D  are top view schematics of first, second and third planes of the MIMO antenna array of  FIG.  7 A . 
         FIGS.  8 A-B  are graphs illustrating simulated S-parameters for each of a standard MIMO antenna array, a MIMO antenna array including the EBG lattice structure, and a MIMO antenna array including an EBG lattice structure and DGS. 
         FIG.  9    is a graph illustrating simulated isolation characteristics of antenna arrays of differing DGS lengths. 
         FIGS.  10 A-C  are graphs illustrating simulated radiation patterns in both the E-plane and the H-plane for a MIMO antenna array. 
         FIGS.  11 A-B  are graphs illustrating measured S-parameters for a prototype antenna array in accordance with an aspect of the disclosure. 
         FIGS.  12 A-C  are graphs illustrating measured radiation patterns in both the E-plane and the H-plane for a prototype antenna array in accordance with an aspect of the disclosure. 
         FIG.  13    is a graph illustrating efficiency and gain of MIMO antenna arrays in accordance with an aspect of the disclosure. 
         FIGS.  14 A-B  are graphs illustrating an envelope correlation coefficient (ECC) in accordance with an aspect of the disclosure. 
         FIG.  15    is a side view schematic of another example EBG unit cell structure in accordance with an aspect of the disclosure. 
         FIG.  16    is a top view schematic of the EBG unit cell of  FIG.  15   . 
         FIG.  17    is a graph showing dispersion characteristics of the EBG unit cell of  FIG.  15   . 
         FIG.  18    is a top view schematic of a transmission line loaded using the EBG unit cell of  FIG.  15   . 
         FIG.  19    is a circuit diagram showing an equivalent circuit of the transmission line of  FIG.  18   . 
         FIG.  20    is a graph showing transmission characteristics of the transmission line of  FIG.  18   . 
         FIG.  21    is a graph showing transmission characteristics of a frequency selective surface (FSS) including the EBG unit cell of  FIG.  15    for a plane wave incident to the FSS. 
         FIGS.  22 A and  22 B  are graphs showing transmission characteristics of a frequency selective surface (FSS) including the EBG unit cell of  FIG.  15    for electromagnetic (EM) waves at different angles of incidence to the FSS. 
         FIGS.  23 A and  23 B  are block diagrams of a respective transmitter and receiver in a MIMO communication system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows a side view of the structure of a uniplanar EBG unit cell structure  100 . The structure includes a substrate  110 , a lower metallic layer  120  formed on a top surface of the substrate  110 , a composite dielectric substrate  130  formed on a top surface of the lower metallic layer  120 , and an upper metallic layer  140  formed on a top surface of the composite dielectric substrate  130  and having a thickness “t.” The composite dielectric substrate  130  includes a first substrate  132  having a first relative permittivity ε r1  and a first height h 1 , and a second substrate  134  formed on a top surface of the first substrate  132  and having a second relative permittivity ε r2  and a second height h 2 . The first relative permittivity ε r1  may be different from the second relative permittivity ε r2 . Additionally or alternatively, the first height h 1 , may be different from the second height h 2 . Particularly, the second height may be greater than the first height. An effective dielectric constant of the composite dielectric substrate  130  may be calculated according to the following equation: 
     
       
         
           
             
               
                 
                   
                     
                       ε 
                       eff 
                     
                   
                   = 
                   
                     ( 
                     
                       
                         
                           
                             
                               ε 
                               
                                 r 
                                 ⁢ 
                                 1 
                               
                             
                           
                           ⁢ 
                           
                             h 
                             1 
                           
                         
                         + 
                         
                           
                             
                               ε 
                               
                                 r 
                                 ⁢ 
                                 2 
                               
                             
                           
                           ⁢ 
                           
                             h 
                             2 
                           
                         
                       
                       h 
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     The difference in relative permittivity between the first and second substrates  132 ,  134  creates a discontinuity that results in reflection of propagating waves. In effect, this causes an additional attenuation of forward propagating waves at the interface between the substrates  132 ,  134 . 
     In one example of the structure of  FIG.  1   , the parameters of the structure are: ε r1 =2.2 (e.g., relative permittivity of Taconic (TLY-5) material), h 1 =0.51 mm, ε r2 =4.4 (e.g., relative permittivity of Fiber reinforced glass epoxy (FR-4) material), h 2 =0.8 mm, and t=35 μm. The effective dielectric constant ε eff  of this structure, as calculated according to equation (1) above, is about 3.45. Additionally, the unit cell structure has a total profile height “h” of about 1.31 mm. 
     Also in the example of the structure of  FIG.  1   , it can be seen that the upper metallic layer  140  is planar, and does not include metallic vias. This reduces the fabrication complexity of the upper metallic layer  140 , as compared to fabrication of mushroom EBG structures. 
       FIG.  2    shows a top down view of an example uniplanar EBG unit cell structure, such as the structure  100  of  FIG.  1   . A design  200  of the upper metallic layer  140  of  FIG.  1    can be seen more clearly in  FIG.  2   . The design  200  has a 90 degree rotational symmetry, which allows solving of the eigen frequencies within a triangular irreducible Brillouin zone designated by vertices Γ, X and M. More particularly, the design  200  includes four split sectoral rings. Each sector  210  of the ring is connected at one end  212  to the center (C) of the layer through first metal strips  216  having a radial length r i , and extends outward away from the center (C) from the other end  214  by second metal strips  218 . Each of the ring sectors has a width “w,” and the gaps  220  separating the ring sectors  210  have a width “g.” The width and gap may be configured to respective values based on minimum tolerance during the fabrication of the design  200  in the upper metallic layer  140 . In one example of the structure of  FIG.  2   , each of w and g may be equal to 0.15 mm. 
     Patches  230  at each corner of the design  200  of the upper metallic layer  140  are a radial distance r 0  from the center (C) of the design  200  and are capable of being connected with adjacent unit cells. The patches are coplanar with the rest of the Additionally, a slit  232  extending toward the center (C) of the design  200  is provided at each corner patch  230  to enhance inductance of the unit cell structure  100 . The length “S” of the slit  232  may configured to control the center frequency of the bandgap, such as for the center frequency to be at or near to 28 GHz for 5G applications. Periodicity “P” of the design  200 , and by extension the EBG unit cell structure  100 , may be selected in order to satisfy a periodic boundary condition, so that only a few unit cells are needed to create the effect of an infinite array of cells. The periodicity of the design  200  may be as small as 2.05 mm or 0.35λ g  in both the length (x) and width (y) directions, where λ g  is the guided wavelength inside the composite dielectric substrate at a center frequency of the bandgap of the EBG unit cell structure. λ g  is defined by the following equation: 
     
       
         
           
             
               
                 
                   
                     λ 
                     g 
                   
                   = 
                   
                     
                       
                         λ 
                         0 
                       
                       
                         
                           ε 
                           eff 
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     in which λ 0  is the wavelength of the propagating waves. In the example of  FIG.  2   , the 2.05 mm periodicity of the design  200  is equal to 0.191λ 0 , which is less than the approximately 10 mm length corresponding to λ 0 , although in other examples, a periodicity of between 2-10 mm may be used. Also, perfect match layers of the electric and magnetic fields (E t =0/H t =0) are enforced along the z-axis of the unit cell structure, which is orthogonal to the direction of propagating waves. 
     The design  200  has a relatively simple geometry, which can be defined using only a few parameters, particularly r 1 , r 0 , q, w, S, and P. Simplification of the geometry is advantageous in that it permits for optimization of the structure to be achieved more easily since there are fewer parameters to control. Additionally, having fewer parameters means that frequency scalability is also simpler, making it is easier to scale down the structure for high frequency applications. In one example arrangement, the following parameters were selected: r 1 =0.525 mm, r o =0.825 mm, q=0.15 mm, w=0.15 mm, s=0.26 mm, P=2.05 mm, although in other arrangements, different parameters may be chosen according to simulation techniques, such as a full wave simulator. 
       FIG.  3    is a Brillouin dispersion diagram showing dispersion characteristics of the EBG unit cell structure  100  over the irreducible Brillouin zone of the structure. As noted herein, the irreducible Brillouin zone is a triangular region designated by the vertices Γ, X and M. The dispersion characteristics of the structure can be numerically simulated using an eigenmode solver, such as finite element eigenmode (FEEM) solver, with periodic boundary conditions to mimic a two-dimensional plane. The eigenmode solver may be included in a full wave simulator for simulating electromagnetic properties of the unit cell structure. The diagram of  FIG.  3    confirms the presence of a frequency bandgap between the first and second modes for the parameters of the EBG unit cell structure  100  of  FIG.  1   . Furthermore, the diagram shows that the frequency bandgap that extends beyond 27-29 GHz, and particularly from 26.2-32.03 GHz, which corresponds to a fractional bandwidth (FBW) of between 15-25% (in the example of  FIG.  3   , the FBW is 20.02%). Having a bandwidth in the 28 GHz range is particularly advantageous for millimeter wave operations, and the high fractional bandwidth makes the EBG unit cell structure suitable. 
       FIG.  4    shows a top down view of an example EBG lattice structure  400  including a plurality of EBG unit cell structures  410  arranged in an M×N lattice between first and second open end transmission lines  420 ,  430 . In the example of  FIG.  4   , the M×N lattice is a 3×2 lattice. Each unit cell structure of the plurality  410  shown in  FIG.  4    is designed according to the design  200  shown in  FIG.  2   , although other designs may be used and parameters may be adjusted in order to meet specific needs of a given application. Each transmission line  420 ,  430  has a length l m  and a width w m . Additionally, each transmission line  420 ,  430 , is separated from the EBG lattice structure  410  by a gap  440  having a gap width g m . In one example arrangement, the following parameters were selected: l m =7.8 mm, w m =3.5 mm, g m =0.15 mm, although in other arrangements, different parameters may be chosen according to simulation techniques, such as a full wave simulator. 
       FIG.  5    is a circuit diagram showing an equivalent circuit  500  of the example EBG lattice structure  400  of  FIG.  4   . The structure is modeled as an EBG lattice  510  positioned between a first open end transmission line  520  and a second open end transmission line  530 . As seen from both  FIGS.  4  and  5   , the transmission lines  520 ,  530  are symmetrical with one another, and the EBG lattice structure  510  is symmetrical with itself. 
     The EBG lattice structure  510  is modeled as a parallel LC tank circuit having a bandwidth that is equal to 
                 1   z     ⁢       L   C         ,         
in which z is the wave impedance and L and C represent the inductance and capacitance of the modeled tank circuit, respectively. The bandwidth of the structure is a width of the bandgap between the first two propagating modes in the modeled dispersion characteristics.
 
     The EBG lattice structure is configured as a T-network including serially connected branch components and a shunt component positioned between the branch components. Each branch component is modeled as a parallel resonant circuit having an inductance L e1  and capacitance C e1 . The shunt component is modeled as series resonant circuit having an inductance L e2  and a capacitance C e2 . Interconnection of each unit cell&#39;s ring sectors at the center results in a long surface current path over a surface of the unit cell. Additionally, interconnection of the EBG unit cells with one another further increases the current path of the EBG lattice structure as compared to an individual EBG unit cell, which further increases the length of the current path. The increase in current path leads to an increase the inductance L e1  of the structure, which in turn increases the bandwidth of the tank LC circuit. The overlapping area between the top metallic layer  140  and lower metallic layer  120  accounts for the capacitance C e1 . C g1  is an additional capacitance contributed by the gap  440  shown in  FIG.  4   . The fringing effect at the edges of the metallic layers  120 ,  140  further contribute to the overall capacitance, which may also be thought of as an increase in the effective overlapping area of the metallic layers  120 ,  140 . The reduction in capacitance due to the multilayer configuration and increase in inductance due to cell interconnection has a greater effect on the bandwidth of the structure than the increase in capacitance due to the fringing effect and gap. Therefore, since bandwidth is equal to 
                 1   z     ⁢       L   C         ,         
the lattice structure results in an overall increase in bandwidth.
 
     Each of the open ended transmission lines  520 ,  530  has an impedance of about 50Ω (P 1 , P 2 ). The transmission lines  520 ,  530  act as launcher and receptor, respectively, of propagating electromagnetic waves at the operating wavelength. The transmission lines  520 ,  530  are modeled in  FIG.  5    as a series inductor L s  coupled with a shunt capacitor C p2 . An additional shunt series RLC resonant circuit having values of R p1 , L p1  and C p1  is included in  FIG.  5    to represent radiation losses due to the aforementioned fringing effect at the edges of the metallic layers next to the transmission lines. 
     The transmission lines  420 ,  430  are provided in order to load the unit cell structures  410  in order to improve isolation characteristics of the EBG lattice structure  400 . For instance, at 27.5 GHz, coupling the EBG lattice structure with the above-specified parameters between two H-plane coupled patch antennas can result in an isolation improvement of up to 23.37 dB, as compared to an unloaded EBG lattice structure. 
       FIG.  6    shows simulations of the equivalent circuit shown in  FIG.  5    and of an unloaded EBG lattice structure and the EBG lattice structure with the transmission lines. The simulations were performed using both an electromagnetic simulator and a circuit simulator. For the simulations, 27.5 GHz was chosen as a center frequency. As shown in  FIG.  6   , the loaded transmission lines create a transmission zero near the center of the bandgap. Thus, loading the transmission lines reduces coupling between antennas by at least 10 dB over a range from 27 GHz to 29 GHz, and by as much as 50 dB according to the equivalent circuit simulation (40 dB according to the EM simulation) at the center frequency. These results confirm that the EBG lattice structure is capable of functioning as a surface wave suppression unit. 
     Aspects of the EBG lattice structures of the present disclosure, such as the EBG lattice structure  400  of  FIG.  4    described above, can be incorporated in a MIMO antenna array in order to decouple channels of the array at within a millimeter wavelength band. The MIMO antenna array may include an array of proximity-coupled microstrip antennas, as is found in 5G cellular applications. Proximity coupling is an electromagnetic coupling technique to achieve broadband impedance matching for the antenna element over a wide frequency range. The EBG lattice structure may be positioned between two microstrip antennas in order to reduce unwanted coupling effects between the antennas, thus permitting for better utilization of the millimeter wave operating frequency range, smaller antenna arrays, or a combination thereof. 
       FIG.  7 A  is a perspective view of such an example antenna array  700 . The antenna array  700  includes three metal planes: a ground metal plane  710 , a second metal plane  720  above the ground metal plane  710 , and a third metal plane  730  above the second metal plane  720 . The ground metal plane  710  is separated from the second metal plane  720  by a first dielectric substrate  750  having a first thickness h 1 , and the second metal plane  720  is separated from the third metal plane  730  by a second dielectric substrate  760  having a second thickness h 2 . The second dielectric substrate  760  may have a different relative permittivity than the first dielectric substrate  750 , may be relatively thicker than the first dielectric substrate  750 , or both. Features of each of the ground plane  710 , second plane  720  and third plane  730  shown in  FIGS.  7 B,  7 C and  7 D , respectively, and described in greater detail in connection with those figures. Parameters of the antenna array  700  layers may be the same as shown in  FIG.  1   , whereby the first dielectric substrate  750  may be a TLY-5 substrate with a height h 1  of 0.51 mm and a relative permittivity ε r1  of 2.2 and the second dielectric substrate  760  may be an FR-4 substrate with a height h 2  of 0.8 mm and a permittivity ε r2  of 4.4. 
     In  FIG.  7 B , the ground plane  710  is shown to include a defected ground structure (DGS)  712 . In the example of  FIG.  7 B , the DGS  712  is hairpin-shaped, and has a sub-wavelength effective slot length. For instance, for an operating frequency of 28 GHz, the effective slot length of the DGS  712  is less than 10.7 mm. In the example of  FIG.  7 B , the effective slot length is 6.81, or about 63% of the signal wavelength λ 0 . 
     In  FIG.  7 C , the second plane  720  is shown to include two substrate-integrated waveguides (SIW)  721 ,  722 . Each SIW  721 ,  722  may be a printed laminated waveguide. Metal vias  724  are evenly spaced in two rows in order to form the sidewalls of the waveguides  721 ,  722 . The spacing or pitch of the vias  724  is selected to be less than double the diameter of the vias  724 , in order that the vias  724  are effective at reducing radiation/leakage losses. The waveguide  722  is configured to form a cavity that confines propagation of surface waves from the antenna elements. In the example of  FIG.  7 C , the SIWs  721 ,  722  share a common wall  725  in between the input and output of the waveguide. A pair of microstrip feed lines  726 ,  728  are positioned at ends of the respective SIWs  721 ,  722 . In the example of  FIG.  7 C , each microstrip feed line  726 ,  728  is T-shaped stub having a mirror symmetry. The T-shaped stub provides for 50Ω impedance matching, although other structures may be provided to achieve different or similar impedance matching. This results in a unidirectional radiation pattern with a high FTBR so that gain of the antenna can be maximized over its operating range. This is accomplished all while maintaining sub-wavelength spacing between the two antenna elements, thus making the design practical for millimeter wave frequency applications such as 5G-compatible devices. In particular, the spacing of the respective microstrip feed lines  726 ,  728  to the antenna elements is only about 6.21 mm, or 58% of the operating signal wavelength λ 0  (10.7 mm, corresponding to a center frequency of 28 GHz). 
     A discontinuity in the lengthwise direction of the waveguide  722  limits the mode of propagation inside the waveguide to TE m0 . More particularly, in the example SIW arrangement of  FIG.  7 C , of the discontinuity created by the metal vias of the SIW (which suppress propagation of longitudinal current) results in only TE m0  mode (for which m≠0) waves being passed, and both TM mn  and TE mn  modes (for which m≠0 and n≠0) being suppressed. 
     The SIW cavity is configured to embrace the antenna elements  732 ,  734  such that the antenna elements  732 ,  734  are positioned within the respective SIW cavities and the top metal wall forms an outer periphery of the SIW cavity. This allows the propagation of surface waves to be confined to within a space within the walls of the respective SIW cavities. This configuration also ensures that the radiation patterns of the antenna array are not affected by incorporation of the EBG. In the example of  FIG.  7   , the waveguides  721 ,  722  are curved with a 90 degree bend in order to facilitate easier connection to the respective antenna elements  732 ,  734  through the feed lines  726 ,  728 , such as by attaching end launch connectors to the respective ports opposite ends of the plane  720 . The waveguides also exhibit lower losses due to reduced reflection from the presence of the curves. This, in turn, helps to achieve better impedance matching of the antenna elements  732 ,  734 . 
     The second plane includes a transition between each SIW to corresponding microstrips  727 ,  729 . The transition points are positioned close to the antenna feeds. Overall, presence of the SIWs helps to reduce radiation losses from the antenna feeds while also contributing to isolation between the ports. 
     In  FIG.  7 D , the third plane is shown to include first and second H-plane radiating antenna elements  732 ,  734  and an EBG lattice structure  736  positioned in between each of the radiating antenna elements  732 ,  734 . In the example of  FIG.  7 D , each radiating antenna element  732 ,  734  is a patch. Each radiating antenna element  732 ,  734  is positioned over a respective portion of the SIW  722  formed in the second plane  720 . The EBG lattice structure  736  may be the same as the EBG lattice structure  400  described in connection to  FIG.  4   . For instance, in  FIG.  7   , the EBG lattice structure is shown as a 3×2 lattice. The EBG lattice structure  736  provides isolation between the radiating antenna elements  732 ,  734 . For instance, 23.37 dB of isolation may be achieved at an operation frequency of 27.5 GHz, and an average isolation improvement of 12.1 dB may be achieved over the entire operating range. At this frequency, coupling between the antenna elements of the MIMO antenna array is −41.06 dB. 
     The DGS  712  formed in the first plane  710  is positioned directly underneath the EBG lattice structure  736 , and further enhances the isolation characteristics between the radiating antenna elements  732 ,  734 . 
     In one example arrangement of the antenna array of  FIGS.  7 A-D , the following parameters were selected: L=31.1 mm, W=34.7 mm, L u =21 mm, W u =22 mm, Py=8.325 mm, d x =1.29 mm, d y =1.35 mm, w r =0.18 mm, C=1.2 mm, g r =0.15 mm, l c =10.48 mm, w c =15 mm d r =6.2 mm, L p =1.8 mm, W p =1.8 mm, d e =2.79 mm, l e =5.75 mm, g e =0.35 mm, S 1 =1.42 mm, S 2 =2.56 mm, w a =6.2 mm, t 1 =2.02 mm, t 2 =0.85 mm, w 1 =1.1 mm, and w 2 =0.19 mm, although in other arrangements, different parameters may be chosen according to simulation techniques, such as a full wave simulator. Also, the example antenna array of  FIGS.  7 A-D  includes two antennas. However, in other antenna array arrangements, additional antennas may be included using the same or similar principles. For instance, an SIW corporate-feed network may be implemented with the SIWs in order to feed waves to the additional antenna elements through feeding branches of the corporate-feed network. In such an arrangement, the additional antenna elements may be patch antennas, comparable to those shown in  FIG.  7 D . 
       FIG.  8 A  is a graph illustrating simulated impedance matching characteristics (S 11 , S 22 ) for each of a standard MIMO antenna array, a MIMO antenna array including the above described EBG lattice structure, and a MIMO antenna array including both the EBG lattice structure and DGS, as a function of frequency. As shown in  FIG.  8 A , return loss at a first port of the antenna array (S 11 ) is about −40 dB and at a second port of the antenna array (S 22 ) is about −58 dB at 27.94 GHz in the standard MIMO antenna array. By contrast, in the MIMO antenna array including the EBG lattice structure, the return loss at both the first and second ports is reduced to about −20 dB at 27.94 GHz. Furthermore, in the MIMO antenna array including the EBG lattice structure and DGS, mutual coupling is reduced to about −47.72 dB at 27.94 GHz. An overall reduction in return loss can also be seen in  FIG.  8 A  for frequencies 28.75-29.25 GHz when the DGS is added to the EBG lattice structure. 
       FIG.  8 B  is a graph illustrating simulated isolation characteristics (S 21 ) for each of the same antenna arrays. As shown in  FIG.  8 B , insertion loss between the first and second ports of the standard MIMO antenna array (S 21 ) is no lower than −25 dB between 26 GHz and 29.5 GHz. Insertion loss is improved when an EBG lattice structure is added to the antenna array, and is reduced to as low as about −40 dB at 27.5 GHz. Insertion is even further improved by inclusion of a DGS, and is reduced to as low as about −63 dB at 27.94 GHz. This results in an overall isolation between antennas of the MIMO antenna array is improved by 35.23 dB at the operating frequency of 27.94 GHz when a DGS and EBG lattice array are included, as compared to the standard antenna array. 
     The amount of isolation between the ports of the antenna array can further be improved by optimizing a length of the DGS in the direction of the DGS slot in order to optimize capacitive coupling between the DGS and the EBG lattice structure.  FIG.  9    is a graph illustrating simulated isolation characteristics of antenna arrays including an EBG lattice structure and DGS as a function of frequency. The arrays plotted in the graph differ by the length of the DGS in the longitudinal direction of the DGS slot, also referred to as the provided underneath the EBG lattice structure. As can be seen from  FIG.  9   , the antenna array with a DGS having a vertical length of 1.2 mm has better isolation characteristics (about −58 dB at 27.94 GHz) compared to antenna arrays having a DGS with different vertical lengths, such as 0.4 mm (about −39 dB at 27.94 GHz), 0.8 mm (about −45 dB at 27.94 GHz) and 1.6 mm (about −40 dB at 28.2 GHz). 
     As can be seen in  FIG.  9   , isolation of the antenna array ports can be achieved using a sub-wavelength DGS, such as a DGS having a vertical length of 1.2 mm. Using a sub-wavelength DGS is also advantageous for maintaining a good front-to-back ratio (FTBR) of the antenna radiation.  FIGS.  10 A- 10 C  are graphs illustrating simulated radiation patterns in both the E-plane and the H-plane for a MIMO antenna array operated at different operating frequencies.  FIG.  10 A  shows the radiation pattern at 27.75 GHz,  FIG.  10 B  shows the radiation pattern at 28 GHz, and  FIG.  10 C  shows the radiation pattern at 28.25 GHz. As can be seen from the Figures, using a sub-wavelength size DGS allows for suppression of mutual coupling between patches of the EBG lattice structure without any significant change to the FTBR of the antenna radiation pattern. Specifically, the figures show FTBR to be about 26.1 dB over the operating frequency range. 
       FIGS.  8 A- 8 B,  9 , and  10 A- 10 C  illustrate properties of the above-described antenna array according to simulations conducted. To check the simulated properties, a prototype of the EBG and DGS loaded MIMO antenna array was fabricated using standard printed circuit board (PCB) fabrication techniques. The same properties were measured in the fabricated prototype over the operating range of 27.75 GHz-28.25 GHz. 
       FIG.  11 A  is a graph illustrating measured impedance matching characteristics (S 11 , S 22 ) for the prototype antenna array as a function of frequency.  FIG.  11 A  also illustrates the simulated characteristics from  FIG.  8 A  for the sake of comparison. The measurements confirm that return loss at both ports of the antenna array (S 11 , S 22 ) is only −20.57 dB or better over the entire operating frequency range. Thus, the combined EBG and DGS play the role of reducing return loss, while a wideband impedance matching for the antenna may be achieved at the same time through proximity feeding and through incorporation of T-shaped stubs. 
       FIG.  11 B  is a graph illustrating measured isolation characteristics (S 21 ) for the prototype antenna array as a function of frequency.  FIG.  11 B  also illustrates the simulated characteristics from  FIG.  8 B  for the sake of comparison. The measurements confirm that insertion loss between the first and second ports of the antenna array (S 21 ) is at least −35.39 dB over the entire operating frequency range, a value of −65.41 at 27.94 GHz, and a minimum value is found to be −75.08 dB at 27.76 GHz. The shift in resonance toward a lower frequency, as compared to the simulation results, may be explained by the presence of a thin air gap between the two substrates in the antenna array and further attributable to fabrication tolerances. 
       FIGS.  12 A- 12 C  are graphs illustrating measured radiation patterns in both the E-plane and the H-plane for the prototype MIMO antenna array operated at different operating frequencies.  FIG.  12 A  shows the radiation pattern at 27.75 GHz,  FIG.  12 B  shows the radiation pattern at 28 GHz, and  FIG.  12 C  shows the radiation pattern at 28.25 GHz. The radiation patterns were measured by positioning the MIMO antenna array in an anechoic chamber and matched terminating one of the input ports of the antenna to a 50Ω broadband load. The measurement results show FTBR to be 19.44 dB, which confirms overall good accuracy of the simulated results of  FIGS.  10 A- 10 C . 
       FIG.  13    is a graph illustrating efficiency of a simulated MIMO antenna array including an EBG lattice structure and a DGS as a function of frequency. As shown in  FIG.  13   , radiation efficiency is between about 82-84% within the operating frequency range of 27.75-28.25 GHz, and particularly 81.98% at 28 GHz. 
       FIG.  13    also illustrates gain according to simulations of a MIMO antenna array as well as measurements of the prototype MIMO antenna array. Loading the microstrip antennas of the MIMO antenna array with the EBG lattice structure forms a Fabry-Perot cavity resonance antenna, whereby multiple reflections inside the cavity of the antenna are capable of improving the antenna gain. The improved antenna gain can offset losses that typically occur through complex feed networks associated with large microstrip antenna arrays, particularly in millimeter wave frequency applications. 
     The prototype antenna can be measured by replacing the receiver with identical transmitting horns, such that the ratio of power for horn-to-horn (P S ) and horn-to-device under test (P T ) may be determined. Gain of the radiating patch of the device under test G T  may be calculated according to the following equation: 
                     G   T     =       G   S     +     10   ⁢            log   10     (       P   T       P   S       )                 (   3   )               
in which G S  is the gain of the horn. As shown in  FIG.  13   , at 28 GHz, the simulated peak gain is 9.29 dBi, and the measured gain is 8.99 dBi.
 
     Additionally, coupling between radiating elements of a MIMO antenna array may be characterized using an envelope correlation coefficient (ECC). A low ECC value is indicative of high channel capacity and good diversity of channels in the MIMO system. 
     A first method for calculating ECC is based on simulated or measured S-parameters, as shown in the following equation: 
     
       
         
           
             
               
                 
                   
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       FIG.  14 A  is a graph illustrating ECC according to the S-parameter based calculations of Equation (3). As shown in  FIG.  14 A , ECC of the antenna array is improved by 20.7 dB at 28 GHz by loading the radiation elements with thane EBG lattice structure and DGS. 
     A second method for calculating ECC is based on radiation field patterns {right arrow over (F)} l (θ, φ), in which i=1, 2 for the two elements of the MIMO antenna array. The ECC according to this method may be calculated according to the following equation: 
     
       
         
           
             
               
                 
                   
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       FIG.  14 B  is a graph illustrating ECC according to the radiation field pattern calculations of Equation (5). As shown in  FIG.  14 B , ECC of the antenna array is obtained to be 0.00015 using this equation. 
     The ECC calculations may further be used to determine diversity gain (DG) of the MIMO antenna array. DG may be determined according to the following equation:
 
DG=√{square root over (1−|ρ e | 2 )}  (6)
 
     Plugging the value of ECC from  FIG.  14 B  into Equation (6) yields an estimated DG of about 9.99 at 28 GHz. 
     The above described results are achieved while maintaining a relatively small size for the entire MIMO array. For instance, the MIMO antenna array shown in  FIGS.  7 A- 7 D  is only 34.7 mm (3.2λ 0 ) long by 31.1 mm (2.9λ 0 ) wide and only 1.31 mm (0.12λ 0 ) thick, whereby λ 0  corresponds to an operating wavelength at 28 GHz. Thus, the above described antenna array provides a significant improvement to reducing mutual coupling without sacrificing size of the array. This makes the above described antenna array particularly useful for applications in which antenna hardware must be kept to a minimum size, such as in 5G portable wireless devices and in millimeter wave phased array and radar tracking systems. 
       FIG.  15    is a schematic side view of an alternative EBG unit cell structure  1500 . The EBG unit cell structure  1500  includes a ground plane  1510 , a dielectric substrate layer  1520  formed on a top surface of the ground plane  1510 , and a metal layer  1530  formed on a top surface of the dielectric substrate layer  1520 . The dielectric substrate layer  1520  has a height “h” and the metal layer  1530  has a thickness “t.” The height and thickness of the layers may be the same as or different than the heights and thicknesses of layers described in connection to other EBG structures of the disclosure. The ground plane  1510  may be a continuous ground plane. In the example of  FIG.  15   , the dielectric substrate layer  1520  is a Taconic TLC-32 substrate, having a relative permittivity of ε r =3.2 and an dielectric loss tangent of tan(δ)=0.003). The substrate  1520  also has a height in the Z-axis direction of about 0.79 mm. The metal layer  1530  has a thickness of 35 μm. The unit cell structure  1500  has a periodicity P C  of 4.2 mm, which at an operational frequency of 9.88 GHz, amounts to 0.138λ 0 , which is less than a quarter but more than an eighth of the operational wavelength. The unit cell structure  1500  also has an overall height of 0.79 mm, which at the operational frequency of 9.88 GHz, amounts to 0.026λ 0 , which is less than a twentieth but more than a fortieth of the operational wavelength. 
     The metal layer  1530  may be patterned in order to form an EBG lattice structure.  FIG.  16    is a schematic top view of a pattern formed in the top layer  1530 . A square-shaped corner patch  1610  is formed in each corner of the unit cell. The corner patch  1610  has a length and width “1.” Each corner patch  1610  is connected to one another through a square-shaped central patch  1620  positioned at a center of the unit cell by a plurality of respective paths  1630 . Each respective path connects a corresponding corner patch  1610  to the central patch  1620 . The paths may have a uniform width “w,” and be the same as one another, thus giving the pattern a 90° rotational symmetry. The paths may also be arranged in a spiral fashion, such that gaps having a uniform width “g” are formed between adjacent segments of the paths  1630 . In the example of  FIG.  16   , a majority of the unit cell is filled entirely with the alternating paths and gaps. This results in a central region  1635  of the unit cell which contains the paths  1630  behaving as a spiral resonator. The corner patches  1610  provide for interconnection of adjacent cells. Intercell connection increases the surface current path between unit cells, which in turn increases the overall inductance of the lattice structure. Edges of the unit cell are occupied by additional paths  1640  that function as interdigital capacitors (IDCs). The IDCs increases capacitive coupling between nearby elements of the lattice structure. A geometry of the folds in the additional paths  1640  may dictate the overall capacitance of the structure. 
     Controlling either or both the inductance and capacitance of the lattice structure can enable control of the operating frequency of the structure. This is because the operating frequency of the structure is defined according to the following equation:
 
 f= 1/2π√{square root over ( LC )}  (7)
 
     Additionally, controlling the inductance of the lattice structure can control an overall bandwidth of a frequency bandgap of the EBG structure. This is because the bandwidth increases as the inductance increases. 
     The example EBG unit cell structure  1500  of  FIG.  15    may be particularly useful for applications within the X band (e.g., 8-12 GHz) of microwave frequencies, despite the relatively miniature size of the structure, both in terms of periodicity and height. 
     Dispersion characteristics of the EBG unit cell structure are determined using full wave simulations in an Eigen mode solver. A periodic boundary condition is imposed in order to simulate a structure with an infinite array size. Additionally, the 90° rotational symmetry of the structure facilitates the determination of Eigen frequencies, since the determination can be focused to within the irreducible Brillouin zone.  FIG.  17    is a graph showing the dispersion characteristics for the EBG unit cell structure of  FIGS.  15  and  16   . As shown in  FIG.  17   , the structure exhibits a frequency bandgap between 7.37-12.4 GHz between the first and second propagating modes. This amounts to an FBW of 50.89%. 
     An alternative way of characterizing the frequency bandgap of the EBG structure is to load a microstrip line with EBG structures symmetrically on both sides of the microstrip line. The result would be a 1 dimensional (linear) array of EBG unit cells on either side of the microstrip line. 
       FIG.  18    is a schematic top view of a simulation setup or arrangement  1800  of circuit elements including a microstrip line  1810  with a first linear array of EBG unit cells  1822  on a first side of the microstrip line  1810  and a second linear array of EBG unit cells  1824  on an opposing second side of the microstrip line  1810 . The microstrip line may be a 50Ω line. Each linear array of EBG unit cells may have the same number of unit cells, and may be aligned with one another to provide the setup  1800  with a 180 degree rotational symmetry. In the example of  FIG.  18   , seven EBG unit cells are included in each linear array. 
     Capacitive coupling between the linear arrays of EBG unit cells  1822 ,  1824  and the microstrip line  1810  interacts with propagating TEM waves in the microstrip line. The capacitive coupling restricts propagation of EM waves over the frequency bandgap of the EBG unit cells due to the resonant nature of the EBG unit cells. 
       FIG.  19    is a circuit diagram showing an equivalent circuit of the arrangement  1800  in  FIG.  18   . The transmission line  1810  is represented as having an impedance Z s , and the linear arrays of EBG unit cells  1822 ,  1824  are represented as a shunt LC circuit having a capacitance C p  and an inductance L p . The circuit diagram in  FIG.  19    is a simplified equivalent circuit when a plane wave impinges on the EBG with the back side ground plane removed. In such a circumstance, the EBG exhibits frequency selective characteristics with a transmission band at the resonance frequency. 
     A simulation of transmission characteristics for the EBG structure of  FIG.  15    is shown in  FIG.  20   .  FIG.  20    shows that the EBG structure produces two narrow stopbands on both sides of the frequency bandgap, in particular at 6.75 GHz and at 13.15 GHz. The 3 dB stopband bandwidths for these two bandgaps are seen to extend over 6.21-6.97 GHz (amounting to an FBW of 11.53%) and over 12.76-14.1 GHz (amounting to an FBW of 9.98%), respectively. Further simulation (not shown in  FIG.  20   ) demonstrated that the number of unit cells included in each linear array controls the magnitude of the insertion loss at both stopbands. 
     The frequency bandgap produced by the example arrangement of  FIG.  18    is capable of creating a mutual coupling between antenna elements of a MIMO antenna array. Due to the relatively miniature size of the EBG unit cell structure, the structure can be fit within a small space between antenna elements, enabling antenna elements and EBG structure of the MIMO antenna array to be packed together with a relatively higher packing density than in conventional arrays. 
     The example EBG unit cell structure  1500  of  FIGS.  15  and  16    can also be applied as a frequency selective surface (FSS).  FIG.  21    is a graph showing transmission characteristics of an FSS designed according to the EBG unit cell structure  1500 . In  FIG.  21   , it can be seen that the FSS yields a transmission pole for which the center frequency is 9.3 GHz and for which a 3 dB passband extends across 7.1-11.65 GHz (amounting to a FBW of 48.53%). This makes the FSS particularly useful for application within the X-band range of frequencies. 
       FIGS.  22 A and  22 B  are graphs showing transmission characteristics of an FSS designed according to the EBG unit cell structure  1500 .  FIG.  22 A  shows characteristics for TE modes of electromagnetic (EM) waves at different angles of incidence to the FSS.  FIG.  22 B  shows characteristics for TM modes of electromagnetic (EM) waves at different angles of incidence to the FSS. As can be seen from both  FIGS.  22 A and  22 B , varying the angle of incidence of the EM waves up to at least 60° does not cause any significant shift in the transmission pole. Thus, the free space transmission characteristics of the EBG structure are shown to be invariant to change in incident angle for both TE and TM modes of propagation up to at least 60°. 
     The above examples generally describe use of EBG structures in a MIMO antenna array in order to solve multipath propagation problems arising from close spacing between antenna elements of the MIMO array. An example system  2300  including a MIMO antenna array is shown in  FIGS.  23 A and  23 B . The system  2300  includes a transmitter component  2310  including a plurality of transmitter antennas  2311   a - 2311   n , and a receiver component  2320  including a plurality of receiver antennas  2321   a - 2321   n . At the transmitter end, input data is fed through a serial-to parallel to converter  2312  and then processed and transmitted in parallel by the respective transmitter antennas  2311   a - 2311   n  and supporting circuit elements (e.g., encoders, modulators). At the receiver end, the transmitted data is received in parallel by the receiver antennas  2321   a - 2321   n  and processed through an array processor  2322 . Further parallel processing is performed (e.g., using demodulators, decoders) before the received data is converted to output data by a parallel to serial converter  2324 . 
     A size of the transmitter component  2310  and of the receiver component  2320  is dependent on inter-element spacing of the respective transmitter antennas  2311   a - 2311   n  and receiver antennas  2321   a - 2321   n . The EBG unit cell structures described in the present disclosure are between an eighth and a quarter of an operating wavelength of the MIMO system in width, and have an even smaller profile in height, which means that the EBG structures positioned between antennas without taking up considerable space. This allows for the antennas to be positioned close to one another without resulting in mutual coupling (MC) between adjacent radiating elements due to free space radiations, surface currents, and surface waves. The avoidance of MC is particularly advantageous because MC can seriously degrade any one or combination of: signal-to-interference noise ratio (SINR) of an adaptive array; convergence of array signal processing algorithms; angle of arrival to estimate the carrier frequency offset; and impedance matching of an active antenna element. All of these unwanted effects due to MC have the additional unwanted effect of reducing channel capacity, data throughput, and spectral efficiency of the MIMO system, which in turn causes losses in bandwidth. 
     Consequently, the structures and techniques described in the present disclosure for achieving a reduction in MC (also termed as antenna decoupling) are a significant advancement in the field of MIMO, and particularly Massive MIMO. MIMO having a highly decoupled antenna array achieves a low value of envelope correlation coefficient (ECC), which in turn results in improvements to efficiency and diversity performance. This is especially necessary for the 5G frequency band at sub-6 GHz, since that band overlaps with both WLAN and WiMax frequencies, leaving limited available spectrum bandwidth for 5G applications. The use of MIMO at this and other 5G frequencies facilitates wider bandwidth for secure communications, such as short range communications and/or point to point communications. 
     Another application of the EBG unit cell structures of the present disclosure are as a partially reflective surface (PRS) over a patch antenna. The EBG-type metal layer superstrate, when loaded on a patch antenna, improves the radiation directivity of the antenna. This is often termed a Fabry-Perot cavity resonator antenna (FP-RCA). Tests have shown the gain of the antenna to be improved by 5-6 dB with the inclusion of a 7×7 array of EBG unit cell structures  1500  as shown in  FIGS.  15  and  16   . 
     The above described antenna array and components also have application to millimeter wave passive circuits such as filters and can be used to improve radiation performance in planar antennas. Including the antenna array of the present disclosure in PCBs can further help to reduce noise due to coupling between various analog and digital components. 
     The above examples generally describe applications within the X, Ku and Ka bands of the millimeter wave range. However, in other embodiments, applications within the V band may also be possible using the techniques and apparatuses described herein. For instance, the EBG-based structures can be upgraded to accommodate a 60 GHz band application, which is also useful for some 5G applications. This can be accomplished by further scaling down the structures using monolithic microwave integrated circuit (MMIC) or micromachining processes. For 60 GHz band communications, both the mutual coupling between adjacent radiating elements as well as limited transmission distance of the radiation elements impinge the performance of a MIMO antenna array. Performance metrics of a MIMO system rely on the overall MIMO element&#39;s gain and the coupling between ports. Therefore, the coupling between closely spaced antennas operating at or near a 60 GHz band should be reduced. This can be accomplished using the EBG structures of the present disclosure. A four-element or eight-element 60-GHz MIMO antenna array system can reasonably be developed for the 60 GHz applications using the EBG structures of the present disclosure with a scale factor. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.