Introduction

The development of advanced antenna systems that can fulfill demanding performance criteria has become necessary due to the rapid expansion of wireless communication technologies, especially with the introduction of fifth generation (5G/5G advance) networks. However, 6G technology is still in the development stage and is expected to enter the market commercially by 2030 according to 3rd Generation Partnership Project (3GPP) and International Telecommunication Union (ITU)1. According to 3GPP and Ericson, 5G advanced (5.5G) technology is still evolving and will be developing till release-21 of 3GPP that is expected in 20282. The advancement of 5G and 5G Advanced technologies has been driven by the growing needs for increased capacity, reduced latency, and faster data rates in wireless communication. The utilization of mmWave (millimeter-wave) frequencies has proven to be crucial in accomplishing these goals. As stated in their respective specifications for 5G NR (New Radio), the International Telecommunication Union (ITU) and the Third Generation Partnership Project (3GPP) have acknowledged the critical importance of millimeter-wave (mmWave) spectrum, particularly for delivering ultra-reliable low-latency communication (URLLC), enhanced mobile broadband (eMBB), and massive machine-type communication (mMTC). MIMO (Multiple Input Multiple Output) antenna systems at mmWave frequencies have become indispensable to meet these demanding requirements, allowing increased spatial diversity, better data throughput, and superior spectral efficiency3,4. Since 5G advanced (5.5G) can accommodate extremely high data rates and wide bandwidths, the millimeter-wave (mmWave) frequency bands, namely n260 (37- 40 GHz), have become more and more popular among the spectrum bands designated for 5G New Radio (NR)5. However, there are certain obstacles associated with operating at mmWave frequencies, mainly related to path loss, interference control, and signal propagation6.

In order to overcome the difficulties associated with high-frequency communication and guarantee seamless user experiences across a variety of 5G applications, the design and optimization of mmWave MIMO antennas will continue to be crucial as 5G develops into 5G Advanced, which is focused on even higher reliability, expanded coverage, and improved energy efficiency. In literature, many techniques such as plus-sign & U-shaped geometry7,8, split ring resonator9, continuous transverse stub-based waveguide10, dielectric resonator11, machine learning12, elliptical slot as a mmWave radiator13, characteristic mode theory14, defected connected ground Geometry (DCGG)15, P-shaped radiator16, photonics based all-dielectric quarter-wave stack Bragg mirror17, stacked-metasurface18, surface plasmon polariton19, graded index lens20 and substrate-integrated waveguide (SIW)21 have been used. MIMO system in 5G and 5G advanced plays a vital role to improve diversity and spatial multiplexing. Several mmWave MIMO antennas such as two-port antenna with parasitic decoupling technique22, four-port MIMO antenna for wearable applications23, six-element antenna with open ended decoupling structure for V2V communication24 and 8-port metamaterial inspired antenna25 have been reported in literature. The 8-port planar antenna is unique among the several configurations because of its small size, simplicity in integrating with planar circuit technology, and capacity for multi-mode operation.

Owing to survey, it has been noticed that the mmWave MIMO antennas, specifically 8-port and 16-port mmWave antennas, are rarely reported. As per the best of authors knowledge, only four 8-port MIMO antenna for mmWave applications25,26,27,28 have been reported in the literature yet, while no works of 16-port massive anetnna for mmWave have been seen. In terms of novelty and uniqueness of this work, the design, modeling, and analysis of 8-port antenna and an approach toward 16-port masssive antenna that is especially tailored for the 5G NR n260 band are presented. The suggested antenna offers substantial advantages in terms of broad bandwidth (6.1 GHz), high gain (8.3 dB), improved port isolation (40 dB) and excellent MIMO diversity characteristics while operating within the mmWave frequency range. To guarantee optimum performance in a 5G communication environment, important design factors such as substrate material, element geometry, feeding network, and inter-port isolation are addressed. Furthermore, the antenna is ideally suited for deployment in base stations, mobile devices, and other applications where space is at a premium due to its small form factor and ease of manufacture. In this study, analysis of single element and MIMO antenna, geometry, reflection and transmission coefficients, radiation patterns and gain, MIMO diversity, an approach toward 16-port masssive antenna and conclusion of the work is discussed separately in foregoing sections.

Study of single and 8-port MIMO antenna structure

Literature is full of mmWave antenna designs, but it has been noticed that the 8-port MIMO antenna for mmWave (5G NR n260) with good performance is rarely reported. The annular ring is very popular in planar antenna design due to its compact size, ability to support multiple modes, and capability to reduce back radiation29,30. Four antenna designs (Designs 1–4) in 32–42 GHz range are shown in Fig. 1a, plotting the reflection coefficient against frequency. An appropriate geometric configuration is chosen for each design. Design-1 is a circular patch antenna, and the radius of the circular patch in terms of resonating frequency (\(f_0\)) is computed by Eq. 1 as reported in31, using the substrate (RT/Duorid (5870 tm): \(\varepsilon _r=2.33\), \(h=0.508\) mm) and design frequency of 36 GHz \(\in\) (5G NR n260). Due to power losses, Design-1 exhibits a narrow band and poor reflection coefficients in the 5G NR n260 band. In Design-2, an annular ring radiator is introduced for further improvement. The physical dimensions (\(r_1=\) inner circle radius and \(r_2=\) outer circle radius) of the annular ring are calculated in Eqs. ( 2– 4) based on the work reported in31:

$$\begin{aligned} & r_{1} = \frac{c k_{mn}}{2\pi f_0 \sqrt{\varepsilon _r}} \end{aligned}$$
(1)
$$\begin{aligned} & r_{1ef} = r_{1} \left[ 1 + \frac{2hX_a}{r_{1} \pi \varepsilon _r} \right] \end{aligned}$$
(2)
$$\begin{aligned} & r_{2ef} = r_{2} \left[ 1 + \frac{2hX_b}{r_{2} \pi \varepsilon _r} \right] \end{aligned}$$
(3)

where, \(k_{mn}\) is the root of Bessel functions31.

$$\begin{aligned} X_b = \ln \left( \frac{r_{2}}{2h} \right) + 1.41\varepsilon _r + 1.77 + \frac{h}{r_{2}} \left( 0.268\varepsilon _r + 1.65 \right) \end{aligned}$$
(4)

Putting \(r_{1} = r_{2}\) in Eq. ( 4) one can obtain the value of \(X_a\).

Fig.1
figure 1

(a) Systematic growth of single port proposed antenna (b) Single port antenna (Design-4).

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Design-2 exhibits good reflection coefficients as compared to Design-1, but it exceeds 5G NR n260 band due to its low radiating length. Design-2 is optimized using defected ground structure (DGS) and named it Design-3. In Design-3, resonating frequency is shifted from 41 GHz to 37 GHz due to DGS effect, but still more power losses are encountered due to impedance mismatch. In order to reduce power losses in Design-3, a modified DGS is used in Design-4, and it resonates at 36.4 GHz in 5G NR n260 band with reflection coefficient of about -50 dB, which indicates perfect impedance matching and little signal reflection at that frequency.

After a systematic study an optimized antenna (Design-4) as shown in Fig. 1b is chosen for eight port MIMO antenna design. Performance of 8-port antenna with different ground planes (full ground, partial ground and proposed novel ground structure) is illustrated in Fig. 2. From perusal of Fig. 2 it is clearly seen that the MIMO design with different ground planes is in consonance with Designs-(2-4) results and reflection coefficient with novel ground plane structure (proposed structure) seems better as compared to other ground planes. Isolation curves for different ground plane structures are also depicted in Fig. 2. From the perusal of Fig. 2 it is observed that the antenna with novel ground plane structure (proposed antenna) offers a good isolation (> 35 dB) as compared to partial ground plane isolation (> 20 dB) and full ground plane isolation (> 30 dB) in their entire operating band. Unlike a full ground plane or a simple rectangular ring plane, the novel ground plane structure decreases coupling between ports and breaks current paths, improving isolation. By reducing interference, this enhanced isolation improves signal performance and integrity. In the proposed MIMO design, a single element structure as shown in Design-4 is arranged in unique fashion and separated by rectangular conducting strips centered at rectangle structure to occupy less space and provide good MIMO diversity characteristics. Reflection and transmission coefficient curves of the proposed 8-port MIMO antenna are depicted in Fig. 3. From the figure it is observed that the reflection coefficient \((\left| S_{ij} \right| _{(i = j)}; \quad i, j = 1 \text { to } 8\)) curve of each port is same and superimpose to each other. The proposed antenna exhibits \(-\)10 dB wideband impedance bandwidth of 6.1 GHz (34.2–40.3 GHz) in mmWave band. In MIMO antennas, orthogonal elements reduce mutual coupling and interference, which results in better isolation than parallel elements. The reason for this is that with orthogonal configurations, there is less power transmission between neighboring antenna elements since the electric and magnetic field components do not greatly overlap. The efficiency of signal transmission and reception is increased by this separation, which also lowers crosstalk and enhances system performance overall. Orthogonal (P14, P35, P67, P28) and parallel (P12, P34, P57, P68) elements of the proposed antenna are clearly illustrated in Fig. 4a. In Fig. 3, it is observed that the isolation of orthogonal elements is larger than the isolation of parallel elements (isolation of P14 (48 dB) > P34 (35 dB)).Transmission coefficient curves \((\left| S_{ij} \right| _{(i \ne j)}; \quad i, j = 1 \text { to } 8)\) of the proposed antenna represents good degree of isolation (> 35 dB) in complete operating band and a maximum isolation of 40 dB is achieved at 36.4 GHz. A unique and effective isolator construction on the ground plane, which reduces the effect of closely spaced radiators, is responsible for the proposed antenna’s outstanding isolation.

Fig. 2
figure 2

Proposed 8-port MIMO antenna with different ground planes.

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Fig. 3
figure 3

Reflection and transmission coefficient curves of the proposed 8-port MIMO antenna.

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Antenna geometry and configuration

The analysis of single port antenna is performed and performance of single port and 8\(\times\)8 MIMO antenna \((52 \times 52 \times 0.508 \, \text {mm}^3)\) for mmWave applications is studied through HFSS EM tool and found it satisfactory as shown in Fig. 3. A well labeled top and bottom geometry of the proposed antenna is portrayed in Fig. 4a. An annular ring is used as a radiator and each side of substrate two parallel, and one orthogonal annular ring are placed at suitable distance.

Fig. 4
figure 4

(a) top and bottom geometry (b) top view fabricated photograph (c) bottom view fabricated photograph. surface current distribution at 39 GHz if port-1 is activated only for T time period (d) t = 0 (e) t = T/4 (f) t = T/2 (g) t = 3T/4.

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Ground plane structure (defected ground with two inverted L shape strips) for each annular ring is used and each ground is separated by rectangular strips that are centered at rectangular structure located at center of substrate. An edge feedline with 50\(\Omega\) impedance is used to provide excitation. The position and configuration of 8\(\times\)8 MIMO antenna as shown in Fig. 4a improves isolation of the ports. The antenna geometry as shown in Fig. 4a is etched over a dielectric substrate (RT/Duorid (5870 tm): \(\varepsilon _r\) = 2.33, h=0.508 mm, loss tangent \(\tan \delta\) = 0.0012). Thereafter, geometry of proposed 8\(\times\)8 MIMO antenna (cf. Fig. 4a) is fabricated for experiment and further study. A fabricated top and bottom photograph of the proposed prototype is shown in Fig. 4b & c, respectively. Surface current distribution images at 39 GHz of the proposed antenna for different period (t = 0, T/4, T/2, 3T/4) are shown in Fig. 4d–g, respectively. Only port-1 is considered for discussion of surface current distribution behaviour and rest ports (P2-P8) are omitted from the discussion as it makes manuscript more voluminous. At time t=0 (cf. Fig. 4d), a strong surface current appears on feedline, annular ring and edges of the ground plane to produce mmWave resonating frequency. Additionally, neighbouring elements are not affected by port-1 EM fields resulting isolation of the proposed antenna is excellent (> 35 dB) as clearly shown in Fig. 3. However, orthogonal element is totally free from the EM waves interference of port-1 as compared to parallel element which is also in consonance of results shown in Fig. 3. At time t=T/4 (cf. Fig. 4e), current on the radiating plane are slightly reduced whereas it increases on the edges of the ground plane. Resulting, electric and magnetic dipoles are excited at t=0 & t=T/4, respectively. At time t=T/2 (cf. Fig. 4f) & t=3T/4 (cf. Fig. 4g), current distributions are same as time t=0 & t=T/4 but the current direction is opposite. Thus, the novelty of the suggested geometry (cf. Fig. 4a) is demonstrated by the completion of one cycle of surface current distribution (cf. Figs. 4d–g).

Simulated and experimental results

The simulated and measured antenna performance in terms of reflection coefficients, transmission coefficients, radiation pattern, gain and radiation efficiency, ECC, TARC, MEG and CCL are presented and discussed in this section. Findings of the proposed work is also compared with recently published works26,27,28,32,33,34,35,36,37,38,39 as enumerated in Table 1. In Table 1, mmWave MIMO antennas performance is presented. The proposed mmWave antenna was to be compared with recently reported 8-port mmWave antennas only. But only three such antennas have been found in the literature26,27,28. As a result, Table 1 compares MIMO antennas intended for mmWave applications more broadly.

Table 1 A comparative overview of proposed antenna [P] and latest published mmWave antennas26,27,28,32,33,34,35,36,37,38,39.
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S-parameters

A simulated and measured results of reflection coefficient \((\left( |S_{ij \, (i=j)}|; \, i, j = 1 \text { to } 8 \right) )\) of the intended antenna are illustrated in Fig. 5. Moreover, a measured reflection coefficient curve and transmission coefficient curve image of Agilent N5247A vector network is portrayed in Fig. 6a & b, respectively. From the illustration it is observed that the proposed work exhibits a simulated and measured impedance bandwidth of 6.1 GHz (34.2–40.3 GHz) and 5.8 GHz (34.2–40 GHz), respectively. The bandwidth of the proposed antenna covers a complete band of 5G NR n260 (37-40 GHz) and the proposed antenna bandwidth (34.2-40.3 GHz) is larger than the bandwidth of antennas26 (24-29.5 GHz),28 (37.2–39.2 GHz),32 (36.5–39.28 GHz),33 (34.1–39.7 GHz),35 (27.1–28.8/35.2–38.9 GHz),36 (27.7-28.2/41.3-41.9 GHz) and39 (21.8–29.1 GHz) as reported in Table 1. However, bandwidth of the proposed antenna is slightly less than the bandwidth of antennas27,34,37 &38 as reported in Table 1. The antennas27,34,37,38 having a slightly large bandwidth but they compromise on other performance parameters such as no of ports, gain, isolation, efficiency, ECC and isolation. From the perusal of Table 1, it is also noticed that the max. isolation (40 dB) of the proposed antenna is better than the isolation of all the antennas as reported in Table 1 except isolation of antennas33 &35. However, antennas33 &35 offer high isolation with compromise of bandwidth and other parameters. Overall, the proposed antenna establishes a good trade-off between performance parameters.

Fig. 5
figure 5

Simulated and measured reflection coefficient curves.

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Fig. 6
figure 6

Measured images of Agilent N5247A vector network (a) reflection coefficient curve \(|S_{11}|\) (b) transmission coefficient \(|S_{12}|\).

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Gain and radiation efficiency

Radiation pattern depends on the antenna structure, MIMO elements arrangement and feeding type. For 5G advanced applications with mmWave frequency gain of the antenna is expected to high with narrower beamwidth. The antenna under test (AUT) is mounted on a rotating platform as shown in Fig. 7 that allows \(360^{\circ }\) azimuthal movement in order to measure the radiation pattern of an 8-port antenna in an anechoic chamber. To measure the broadcast or received power at different angles, a reference antenna (Horn antenna) that has been calibrated is placed at a known distance. The measurement is repeated for each port when the AUT is linked to a network analyzer or receiver. Because of the absorptive walls of the chamber, reflections are reduced, resulting in precise measurements of the antenna’s far-field pattern. The co and cross polarization of the proposed antenna at resonating frequency (36.4 GHz) in E and H-plane is shown in Fig. 8a & b, respectively.

Fig. 7
figure 7

Measurement setup image of the proposed antenna with terminated load in anechoic chamber.

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Fig. 8
figure 8

Co and cross polarization at 36.4 GHz in (a) E-plane (b) H-plane.

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The Fig. 8a illustrates the comparison of simulated and measured co-polarization and cross-polarization in the E-plane radiation pattern at 36.4 GHz. With side lobes on either side and a prominent main lobe centered at \(0^{\circ }\), the co-polarization (green and black lines) reveals a directed radiation pattern with a respectable front-to-back ratio. The simulated (solid green) and observed co-polarized patterns (dashed black and dashed black, respectively) are nearly identical; however, small differences may indicate real-world variations like fabrication tolerances. The observed cross-pol (red dashed) closely matches the simulated (solid blue), and the cross-polarization (blue and red lines) exhibits lower levels overall, indicating good polarization purity. The pattern indicates that, which is important for applications requiring high polarization isolation, the antenna operates well with little cross-polarization interference. The antenna radiates energy over a wider angular range in H-plane (cf. Fig. 8b), as evidenced by the co-polarization (green and black lines), which shows a broader main lobe with some visible side lobes and fluctuations. The simulated co-pol (solid green) and the measured co-pol (dashed black) exhibit similar basic trends, however there are some minor differences that are probably caused by real-world issues like manufacturing flaws or effects from the measurement equipment. In comparison to the co-pol, the cross-polarization (blue and red lines) is comparatively small, with the simulated cross-pol (solid blue) and the measured cross-pol (red dashed) nearly aligning. This suggests good isolation between the polarizations and efficient polarization performance with little cross-polarization. Overall, the antenna behaves well in the H-plane, showing negligible cross-pol interference and great directionality.

A 3D simulated gain at 36.4 GHz and 39 GHz is depicted in Fig. 9a & b, respectively. The gain of the proposed antenna is simulated at 36.4 GHz due to resonating frequency while 39 GHz frequency is chosen as it named 39 GHz band for 5G NR-n260. A gain of 7.2 dB and 8.3 dB is obtained at 36.4 GHz and 39 GHz, respectively. A simulated (8.3 dB) and measured (8.4 dB) peak gain of the proposed antenna as achieved at 39 GHz is larger than the antennas26 (6 dB),27(6 dB),33 (6.7 dB),34 (6.1 dB),35 (6.5 dB),36 (6.05 dB),37 (5.65 dB),38 (7 dB) and39 (5.76 dB) and less than the antennas28 (10 dB) and32 (10 dB) as reported in Table 1. The antenna28 has a large gain with compromise of antenna size, bandwidth, isolation, ECC and efficiency characteristics. All performance characteristics except gain of the proposed antenna are comparable than the antenna28. It is also seen that the proposed antenna offers less gain as compared to antenna32 but the proposed antenna has designed for eight ports with broad bandwidth (34.2–40.3 GHz) while antenna32 has four ports only with narrow bandwidth (36.5–39.28 GHz). The proposed antenna offers a high gain due to annular ring geometry and novel isolator structure. Annular ring geometry generates multiple resonant modes with high concentration of current distributions on the radiating plane resulting the structure reduces sidelobes and enhances main lobe and radiates more EM waves efficiently.

Fig. 9
figure 9

3D-gain of the proposed antenna at (a) 36.4 GHz (b) 39 GHz.

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Moreover, 2D gain of the proposed antenna is also calculated from measured radiation pattern data and depicted in Fig. 10. The Friis Eqs. ( 5 &  6) as reported in40 is used to calculate measured gain.

$$\begin{aligned} & Received\, power\, (P_{rp}) = \left( \frac{\lambda _0}{4\pi R_d} \right) ^{2} G_{rg} G_{tg} P_{tp} \end{aligned}$$
(5)
$$\begin{aligned} & Far-field\, distance\, (R_d) = \frac{2r^{2}}{\lambda _0}; \end{aligned}$$
(6)

where; \(P_{tp}\)-transmitted power, \(G_{rg}\) & \(G_{tg}\)- received & transmitted gain, r-distance between \((T_x)\) & \((R_x)\) antenna, \({\lambda _0}\)-wavelength

From the perusal of Fig. 10, it is observed that the simulated and measured gain are more or less same, and it is also in conformity of Fig. 9. The radiation efficiency of the proposed antenna is also shown in right hand vertical axis of Fig. 10. A maximum 85.2% radiation efficiency of the proposed antenna is achieved, and efficiency plot is in consonance of gain plot of the antenna. The radiation efficiency (85.2%) of the proposed antenna is larger than the antennas26(80%),27(84.9%),28(82%),34(80%),36(80%), and38(85%) and smaller than the antennas32(90%),33(96%),37(86%) and39(96.5%) as reported in Table 1. The antennas offer greater efficiency having less no of elements and compromise with other performance parameters. Overall, stable gain and good efficiency with 8-port elements of the proposed antenna makes it suitable for ultra-high-speed applications in 5G.

Fig.10
figure 10

2D-gain and radiation efficiency of the proposed antenna.

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MIMO characteristics

Single element of the proposed antenna is arranged on \(52 \times 52 \times 0.508 \, \text {mm}^3\) RT/Duorid (5870 tm) board in specific pattern to produce 8-port MIMO antenna. The proposed antenna offers MIMO diversity and spatial multiplexing that supports 5G communications. MIMO characteristics of the proposed antenna in terms of ECC, TARC, MEG and CCL are presented in this section and discussed accordingly. Indicating how closely the radiation patterns of the test antenna and its nearby antennas are associated, the ECC (Envelope Correlation Coefficient) plot draws attention to this relationship. ECC (\(\rho\)) has a value between 0 and 1, where 0 denotes totally independent radiation patterns, which are often attained with orthogonal polarization, and 1 denotes identical polarization and highly correlated patterns. Maximum isolation between antenna elements, which improves MIMO performance, is represented by an ECC value of zero, which is optimal. Utilizing the antenna system’s radiation patterns and scattering characteristics, the ECC may be computed. Simulated and measured ECC plot of the proposed antenna is computed using s-parameters and far-field parameters given in Eqs. ( 7 &  8), respectively as reported in41:

$$\begin{aligned} & E_{CC}^{ij} = \frac{\left| S_{ii}^{*} S_{ij} + S_{ji}^{*} S_{jj} \right| ^{2}}{(1 - |S_{ii}|^{2} - |S_{ji}|^{2})(1 - |S_{jj}|^{2} - |S_{ij}|^{2})} \end{aligned}$$
(7)
$$\begin{aligned} & ECC = \frac{\left| \int _{4\pi } f_{1}(\theta , \phi ) \cdot f_{2}(\theta , \phi ) \, d\Omega \right| ^{2}}{\int _{4\pi } |f_{1}(\theta , \phi )|^{2} \, d\Omega \cdot \int _{4\pi } |f_{2}(\theta , \phi )|^{2} \, d\Omega } \end{aligned}$$
(8)

Where, \(F_1 (\theta , \phi )\) symbolizes the component of the electric field when element 1 is energized and element 2 is terminated with a load that is matched. In a similar manner, when element 2 is activated and element 1 is terminated with a matching load, \(F_2 (\theta , \phi )\) represents the electric field component.

Fig.11
figure 11

Simulated and measured ECC curve of the proposed antenna.

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Using above equations, simulated and measured curve of the intended antenna is illustrated in Fig. 11. In Fig. 11, ECC for ports P(1, 2) P(3, 4), P(3, 5) & P(1, 4) is shown and rest ports ECC is omitted to avoid clustering in the diagram. Simulated results of the proposed antenna follow experimental results and a less than 0.005 & 1.2 \(\times 10^{-4}\) value is achieved for ports P(1, 2) & P(3, 4), and ports P(3, 5) & P(1, 4), respectively. ECC value of the proposed antenna is in acceptable limit (< 0.5)42 and found good as compared to antennas as enumerated in Table 1 except antennas32 and35. The ECC S-parameter equation is restricted to near-field effects and offers an approximation that might not precisely represent performance in various surroundings or free space. nonetheless, by taking into account far-field characteristics, the radiation pattern equation provides a more accurate measurement of ECC; nonetheless, it is difficult to compute and necessitates extensive computation and thorough radiation pattern data. However, for antennas with more than 85% radiation efficiency, the s-parameter may accurately forecast the ECC value.

The two most important performance criteria for assessing an 8-port MIMO antenna are TARC (Total Active Reflection Coefficient) and MEG (Mean Effective Gain). When all ports are excited at once, TARC measures the reflection coefficient, providing information on how well the antenna transmits signals when several elements are in use. Better efficiency and fewer signal reflections are indicated by a lower TARC score. In contrast, MEG measures the average power that the antenna receives or transmits when multipath propagation is present, taking into consideration real-world signal gains and losses. Optimizing both TARC and MEG is essential for achieving high efficiency, better isolation, and enhanced MIMO performance with an 8-port antenna. According to works25,39, following Eqs. ( 9 &  10) have been used to compute TARC and MEG plot of the proposed antenna.

$$\begin{aligned} & \text {TARC} = \frac{\sqrt{\left| \sum _{i=1}^{N} S_{i1} + \sum _{m=2}^{N} S_{im} e^{j \theta _{m-1}}\right| }}{\sqrt{N}} \end{aligned}$$
(9)
$$\begin{aligned} & \text {MEG}_{port_i} = 0.5 \left[ 1 - \sum _{j=1}^{K} \left| S_{ij} \right| ^2 \right] \end{aligned}$$
(10)
Fig. 12
figure 12

Simulated and measured TARC and MEG curve of the proposed antenna.

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Figure 12 displays the TARC and MEG curves for the suggested antenna. According to43, both the simulated and measured TARCs stay within allowable limit in operating band, that is, below − 10 dB. The simulated TARC closely matches the measured TARC. Additionally, the MEG curve of the suggested antenna falls within the permissible range of MEG values, which is between − 12 dB and − 3 dB, as stated in25,39, and the simulated and measured findings closely match.

A simulated and measured curve of channel capacity loss (CCL) of the proposed antenna is portrayed in Fig. 13. The CCL value of the proposed antenna is calculated using imperical formulas ( 11- 14) as given in44 as follows:

$$\begin{aligned} & \text {CCL} = -\log _2 \left( \det (\rho ) \right) \end{aligned}$$
(11)
$$\begin{aligned} & \rho = \begin{bmatrix} \rho _{11} & \rho _{12} & \rho _{13} & \cdots & \rho _{18} \\ \rho _{21} & \rho _{22} & \rho _{23} & \cdots & \rho _{28} \\ \rho _{31} & \rho _{32} & \rho _{33} & \cdots & \rho _{38} \\ \vdots & \vdots & \vdots & \ddots & \vdots \\ \rho _{81} & \rho _{82} & \rho _{83} & \cdots & \rho _{88} \end{bmatrix} \end{aligned}$$
(12)
$$\begin{aligned} & \rho _{ii} = 1 - \left( \left| S_{ii} \right| ^2 - \left| S_{ij} \right| ^2 \right) \end{aligned}$$
(13)
$$\begin{aligned} & \rho _{ij} = - \left( S_{ii} S_{ij} - S_{ji} S_{jj}^* \right) \end{aligned}$$
(14)

Minimal degradation in system capacity is ensured by the CCL values, both simulated and measured, staying well below the acceptable threshold of 0.5 bits/s/Hz44. This suggests that by preserving low mutual coupling between ports, the antenna efficiently supports multiple-input multiple-output (MIMO) systems, which makes it ideal for high-data-rate millimeter-wave communications.

Fig. 13
figure 13

Simulated and measured CCL curve of the proposed antenna.

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An approach toward massive antenna

For mmWave applications in 5G and 5G Advanced networks, a large antenna system with 16 ports provides a number of benefits, including increased capacity, spectrum efficiency, and high data rates. Utilizing mmWave frequencies in conjunction with a large antenna array allows for beamforming and spatial multiplexing, both of which are essential for meeting the demands of densely populated areas and high user density. The antenna array’s 16 ports enable advanced MIMO (numerous Input Multiple Output) techniques, which enhance signal robustness and enable simultaneous communication with numerous users even in difficult situations like non-line-of-sight (NLoS) circumstances45. The system’s precision beam steering capability is improved by the high port count, which maximizes signal coverage and minimizes interference. Furthermore, 5G Advanced networks can support the high throughput needed for applications like augmented reality (AR), virtual reality (VR), and autonomous cars because of the vast bandwidth made available at mmWave frequencies. To improve efficiency and system performance, the design of such an antenna must also provide low envelope correlation coefficient (ECC), high isolation, and little mutual coupling across ports. These antennas assist overcome the propagation issues associated with mmWave bands while providing high-capacity, low-latency connectivity, which is essential for achieving the full promise of 5G and beyond.

Owing to study of reported antennas1,2,3,4,5,6,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45 and a rigorous search in literature, as far as the authors are aware, no reports of 16-port massive antenna specifically designed for the 5G NR-n260 band have been made yet. In this section, eight port MIMO antenna (cf. Fig. 4) of this work is further modified without compromise of antenna size and converted into 16-ports massive antenna for mmWave applications. A well labeled top and bottom geometry of 16-port massive antenna is presented in Fig. 14.

Fig. 14
figure 14

Front and bottom view geometry of the proposed 16-port massive antenna.

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In this study, performance of 16-port massive antenna in terms of reflection coefficient and transmission coefficient is presented only. Other performance parameters of massive antenna are omitted from the discussion as it makes more voluminous of the manuscript and fabrication cost of 16-port massive antenna is also very high because of port for mmWave is more expansive as compared to low frequency port. Reflection and transmission coefficient of 16-port massive antenna is illustrated in Fig. 15. The proposed 16-port massive antenna exhibits two bands: first band in 30-33 GHz for ports P9-P16 and second band in 33.5–40.9 GHz for ports P1-P8. Roughly, it can be concluded that the antenna covers a large band 30–40.9 GHz for mmWave applications. Isolation in massive antenna is a big challenge [42], but the propose 16-port massive antenna offers a good isolation of more than 22 dB in entire band. The performance of 16-port massive antenna makes it more suitable candidate for 5G mmWave applications. In extended work of the proposed 16-port massive antenna can be addressed with metamaterial and array structure with all performance matrices separately.

Fig. 15
figure 15

Reflection and transmission coefficient of the proposed 16-port massive antenna.

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Conclusion

In this study, an eight port MIMO antenna for mmWave (5G NR-n260) applications is designed, fabricated and tested successfully. Simulated and measured results are in high degree of agreement. However, small deviation is observed in simulated and measured results due to fabrication and measurement tolerance. The proposed antenna offers a wideband bandwidth (6.1 GHz), excellent isolation (> 40dB) and high gain (8.3 dB) due to its annular ring structure on the radiating plane and a novel isolator structure on the ground plane. Annular ring enables multiple modes, high concentration of current distributions and capability to reduce back radiation whereas a unique and effective isolator construction on the ground plane reduces the effect of closely spaced radiators. MIMO characteristics such as ECC, TARC, MEG and CCL of the proposed antenna are also in well acceptable limit and supports carrier aggregation (more carriers can be transmitted simultaneously) with MIMO diversity and spatial multiplexing. Moreover, the proposed 16-port massive antenna exhibits high isolation (> 22dB) with two bands: first band in 30-33 GHz for ports P9-P16 and second band in 33.5-40.9 GHz for ports P1-P8. Ports P9-P16 generate new resonating band (30-33 GHz) as it comprises different and small ground plane resulting large current lengths appear on the radiating plane. We conclude that the massive antenna is a good choice for mmWave applications since it has greater than 22 dB isolation and efficiently spans a wide bandwidth of 30 to 40.9 GHz. This study can be concluded that the proposed 8-port MIMO antenna and 16-port massive antenna may contribute a vital role in 5G/5G advanced communication system.