Integrated Microwave and mm-Wave MIMO Antenna Module With 360° Pattern Diversity for 5G Internet of Things

The complete coverage of the operating frequency bands from microwave bands to millimeter (mm-wave) is required for the realization of the fifth-generation (5G) Internet of Thing (IoT) systems. Here, we present a multiband antenna operating at the microwave (2.5/3.5/5.5/7.5 GHz) and mm-wave bands (23–31 GHz), and its 12-port MIMO configuration with pattern diversity affording 360° coverage for 5G IoT applications. The multiband characteristics are obtained by adding well-designed quarter-wavelength stubs. The antenna operates at the important frequency bands from 2.37–2.65, 3.25–3.85, 5.0–6.1, and 7.15–8.5 GHz (|S11| <–10 dB), while it resonates from 23–31 GHz at the mm-wave band with the desired radiation characteristics. Moreover, the antenna has more than 95% radiation efficiency and a stable gain (> 2.5 dBi at microwave band and 6.5 dBi at mm-wave bands) characteristics. In addition, the single-element antenna is translated to design a <inline-formula> <tex-math notation="LaTeX">$2\,\,\mathbf {\times }\,\,2$ </tex-math></inline-formula> MIMO antenna. This MIMO unit is further utilized in the formation of <inline-formula> <tex-math notation="LaTeX">$2\,\,\mathbf {\times }\,\,4$ </tex-math></inline-formula> and the proposed <inline-formula> <tex-math notation="LaTeX">$3\,\,\mathbf {\times }\,\,4$ </tex-math></inline-formula> (12-port) MIMO configurations to achieve spectral and pattern diversity. Considering the unique three-dimensional arrangement of the antenna elements, the 12-port MIMO system is the only one of its kind that offers the codesign of microwave and mm-wave antenna, good isolation, and pattern diversity, providing complete 360° space coverage in elevation and azimuth planes. The proposed antenna module is suitable for 5G IoT, especially in an indoor scenario for smart houses, offices, and vehicle-to-everything communications.


I. INTRODUCTION
T HE COMBINATION of the fifth-generation (5G) communication and the Internet of Things (IoT) is going to wirelessly connect people, data, processes, infrastructure, and things around us with high data rates and low latency [1]. Manuscript  The 5G-enabled IoT, along with artificial intelligence, will revolutionize the world by realizing smart cities, smart healthcare, smart farming, smart factories, intelligent transportation system (ITS), and augmented/virtual reality [2]. To ensure seamless connectivity, 5G technology uses two frequency bands: 1) the existing sub 6 GHz bands and 2) a new, unused spectrum of millimeter waves (mm-waves). This wide spectrum allows the 5G technology by supporting the high traffic growth and growing demand for highbandwidth connectivity. A typical 5G network is shown in Fig. 1 [3]. The 4G and 5G macrocells working at sub-3 GHz and sub-6 GHz bands provide the essential wide coverage area, while the 5G small cells operating at mm-wave bands add ultimate capacity when needed. Cisco Systems recently reported that 29.3 billion networked IoT devices will operate by 2023, outnumbering humans by more than threefold [4]. These devices and sensors use various frequency bands, for example, the scientific and medical band (ISM, 2.4 GHz), Bluetooth band (2.4 GHz), and longterm evolution band (LTE, 2.5 GHz), the 5G sub-6 GHz band (2.5/3.5 GHz), and the radio-frequency identification (RFID, 2.54/5.8 GHz), x-band (7)(8), wireless local area network (WLAN, 2.4/5.2/5.8 GHz), downlink satellite communication (x-band, 7.36-8.06 GHz) band, and 5G mm-wave bands (24/26/28 GHz). The main challenge in the future IoT is reliable connectivity, which needs several frequency bands to avoid electromagnetic interference, energy efficiency to support continuous power to all devices, and the cost. This implies that antennas operating from sub-6 GHz to mm-waves are required for IoT communication. Various dedicated antennas for their applications in the IoT at sub-6 GHz and mm-wave bands have been widely studied in the literature [5]- [21]. These antennas operate either at sub-6 GHz [5]- [13] or mm-Wave bands [14]- [21]. Multiple antennas (operating at various frequencies) increase device size, cost, and complexity; new This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ 5G devices must be small, inexpensive, and simple. To avoid this issue, shared aperture antennas have been investigated [22]- [24]. These kinds of antennas simply put two antennas together in a shared aperture, one for the sub-6 GHz band and the other for the mm-Wave bands. For example, the antennas presented in [25] and [26] operate both at 2.4/5.2 GHz and 60 GHz to integrate the existing Wi-Fi connection with the future high data rate Wi-Gig connections. The joint implementation of WLAN band and Wi-Gig can provide long-and short-range radio accesses with highdata simultaneously. Since most of the countries have adopted 2.5/3.5 and 24/26/28 GHz bands for their 5G communications. Therefore, multiband antennas have been designed at sub-6 GHz and mm-wave bands [27]- [33]. The Fabry-Perot cavity-based folded-parallel-plate antenna provides coverage for 2.4-and 28-GHz bands [27]. In [28], slots and patches are combined in a leaky-wave antenna to realize the dualband function at 7 and 38 GHz. The aperture shared an array of dipole antennas that offer resonance at 3.5 and 28 GHz with stable radiation patterns [29]. A dielectric resonator slot antenna fed by a substrate-integrated waveguide operates at 5.2 and 24 GHz [30]. In [31], a dual function tampered slot structure is utilized to generate multiband characteristics at 2.5, 5.5, and 24/28 GHz. Furthermore, a planar antenna is presented using a mode-composite method [32]. In this design, a combination of a meander and an L-shaped SIW sections are used with multiple ports to obtain two bands at sub-6 GHz and two bands at mm-wave spectrum. Hussain [33] proposed a shared aperture square concentric slot-based 5G IoT antenna that operates simultaneously at various sub-6 GHz bands and 28-GHz bands. In the works mentioned earlier, the microwave antennas and mm-wave integrated antennas have been fed separately (at least two ports) at different points. This needs a lot of space for feed positions and a complex feeding network. The increased number of multiple ports restricts the MIMO configurations of these antenna designs, especially for the sub-6 GHz.
Recently, the problem of separately feeding the sub-6 GHz antenna and mm-wave antenna in the integrated shared aperture antennas has been addressed in [34], where a single-fed triple-band antenna operating at 3.5, 28, and 38 GHz and its MIMO configuration has been developed. This antenna has the advantages of single feed for both microwave and mmwave antenna. In this design, the higher frequency patch is fed directly through a microstrip feedline, while the meander line is attached to the patch for its operation at 3.5 GHz. However, the antenna is specified for 5G phones; the 2.4-, 2.5-, 5.5-, and 7.5-GHz bands, which ensure 5G IoT connectivity is not covered.
Overall, it remains challenging to realize an integrated microwave and mm-wave MIMO antenna having the advantage of pattern diversity and necessary isolation for future IoT applications. Here, we present a single-fed multiband antenna working at the microwave (2.5/3.5/5.5/7.5 GHz) and mm-wave (24/26/28 GHz) bands and its 12-port MIMO configuration to provide connectivity for the 5G IoT devices and sensors. To our knowledge, the MIMO system is the first of its kind that has the ability to cover all the key bands with polarization diversity having low coupling among the MIMO elements for 5G-enabled IoT standards.

II. SINGLE-ELEMENT ANTENNA FOR 5G IOT APPLICATIONS
The proposed multiband antenna operating at microwave and mm-wave bands for the 5G IoT application is discussed in this section. For easy understanding, the section is divided as follows.

A. Antenna Geometry
The configuration of the presented single-element antenna is shown in Fig. 2. The antenna consists of five metallic stubs connected to the triangular monopole at its front, while two stubs are connected to it at its backside through shoring vias. The antenna is designed on a high-frequency Rogers-5880 substrate (ε r = 2.2, tan δ = 0.0009). The front metallic stubs act as quarter-wavelength monopoles to provide multiple resonances at 2.5, 3.5, 5.5, 7.5, and 28 GHz. At the same time, the stubs at the backside connected to the main radiator through shorting pins are employed for the bandwidth improvements at 2.5-and 3.5-GHz bands. The length of each stub is optimized to get resonances at microwave and mm-wave bands. The optimized parameters of the antenna are as follows: A = 16, L = 21, h = 1.6, g m = 3, w s = 2, g w = 4.1, w p = 13, p w = 5, l 1 = 10.5, w 1 = 13.7, l 2 = 8.2, w 2 = 9.2, l 3 = 5.9, w 3 = 4.8,

B. Design Procedure
The design commenced with a basic triangular monopole that featured a partial ground plane operating at a resonating where c is the velocity of light in free space and ε r is the dielectric constant of the antenna substrate. Next, various stubs are attached to the main radiator to design bent monopoles with different resonating frequencies.
Except for the smallest stub designed for mm-wave bands, all other stubs consist of horizontal and vertical parts. The overall length of each stub is optimized carefully for extra resonances at 2.5, 3.5, 5.5, and 7.5 GHz, and 5G mm-wave bands. For easy understanding, the step-by-step design procedure of the proposed antenna and their corresponding s-parameters |S 11 | are depicted in Figs. 3 and 4, respectively. It is noted that all these antennas have the same lateral size.
First, a stub of width w is loaded to the extreme left of the main radiator to get the resonance at a lower frequency. This antenna (antenna-1) has two resonances: 1) the primary resonance due to the source antenna at 8.5 GHz and 2) the extra resonance at 2.5 GHz. The overall length of the monopole resonating at 2.5 GHz (L 2.5GHz ) equals Here, λ 2.5GHz is the wavelength at 2.5 GHz.
Similarly, more stubs, working as folded monopoles, are added next to the first one with a spacing of s = 1 mm to achieve resonances at 3.5, 5.5, and 7.5 GHz. For convenience, the antenna with two, three, and four stubs are denoted as antenna-2, antenna-3, and antenna-4, respectively. The overall lengths of these resonating stubs can be computed as Traditionally, the resonating monopole antennas are featured as narrow bandwidth [35], particularly at the lower frequencies, at 2.5 (7.2%) and 3.5 GHz (9.5%), as seen in Fig. 4. The limited bandwidth at these important bands, providing long-range connectivity to the IoT, can raise questions on the usage of the antenna. To overcome this problem, two additional stubs are attached at the backside of the antenna through the shorting pins (antenna-6 and antenna-7). These stubs are tuned carefully to give resonance at the neighboring frequencies (2.4 and 3.7 GHz) of 2.5 and 3.5 GHz to overlap these individual resonances for bandwidth improvement. It improved bandwidth to 11% at the 2.5-GHz band and 17% at the 3.5-GHz band.
Next, to provide a coverage for the 5G IoT at the mmwave band (24/26/28 GHz), a horizontal stub of length (l 5 ) is added at the right corner of the basic rectangular monopole, represented as antenna-5. The length of the mm-wave stub (L 26GHz ) is equal to Note that the antenna gives a resonance from 22 to 33.9 GHz at the mm-wave band. In summary, the proposed singleelement antenna design gives resonances at 2.5, 3.5, 5.5, 7.5, and 24/26/28 GHz; this fully coves the microwave and mm-wave bands to provide uninterrupted connectivity for 5G IoT.
For the further demonstration of the radiation mechanism of the proposed antenna, the surface current distribution on antenna at various resonant frequencies is shown in Fig. 5. As expected, the current is mainly concentrated in the first (largest stub at the extreme left-hand side) monopole at 2.4 GHz. While at 3.5 GHz, the current is prominent at the second resonant monopole, reflecting the origin of the resonance. Similarly, at 5.5, 7.5, and 28 GHz, respectively, the third, fourth, and fifth monopoles resonate (i.e., they exhibit the highest current concentrations).

C. Results and Discussion
The single-element antenna is fabricated through photolithography with high-precision level to validate the simulated results. The antenna is fed with a 2.92-mm wave end-launch connector. The snapshots of the fabricated prototype and the measurement setup are shown in Fig. 6. The antenna is measured in a far-field measurement (multiprobe anechoic chamber) facility at Electromagnetic Wave Technology Institute, Seoul, South Korea. The s-parameter is measured in open air condition in a network analyzer (E8364B) by Agilent Technologies. Overall, the measurement results show good agreement with the simulated results.
The gain and radiation efficiency characteristics of the antenna at microwave and mm-wave is plotted in Fig. 8. The antenna offers a gain level of more than 1 dBi at 2.5-GHz band, which increased to 2, 2.5, and 4 dBi at 3.5 GHz, 5.5 dBi, and 7.5 GHz, respectively. The gain increment with the increasing frequency is due to the increased effective antenna size at higher frequencies. At the same time, the gain is more than 6 dBi in the entire mm-wave bands. Moreover, the radiation efficiency is noted to be more than 95% at microwave and mmwave bands due to the usage of the low-loss high-frequency substrate. Fig. 9 shows the radiation pattern of the single-element antenna at 2.5, 3.5, 5.5, and 7.5 GHz. The antenna shows a radiation pattern of a conventional monopole antenna giving omnidirectional pattern at xoz-plane (phi = 0 • ) and dumbbell-shaped radiation patterns at yoz-plane (phi = 90 • ). Importantly, the radiation pattern is stable for all frequency bands due to the effective utilization of the individual resonances of the stubs. The radiation pattern of the antenna at mm-wave bands is shown in Fig. 10. It is interesting to note that the antenna has a stable end-fire radiation pattern at the mm-wave band, where the main direction of maximum radiation is parallel with the antenna structure. The end-fire nature of the antenna is due to its quasi-yagi antenna geometry. The perpendicular microwave stubs in front of the mm-wave monopole are acting as directors to give high-gain end-fire radiations [36].

III. MIMO ANTENNA FOR 5G IOT APPLICATIONS
The proposed single-element antenna is incorporated into various MIMO (2 × 2, 2 × 4, and 3 × 4 configurations) assemblies to exploit multipath propagation for the high-date rates, capacity, and link reliability through spatial and pattern diversity for its applications in future IoT communications.

A. 2 × 2 MIMO Antenna
First, the antenna is incorporated in 2 × 2 MIMO configuration by translating a single-element antenna orthogonally having a distance of d = 16 mm between the adjacent antennas. For the sake of isolation improvement, the adjacent antenna elements are printed on the backside of the substrate, while the diagonally positioned antennas are in the same plane of the substrate. The geometrical configuration and the fabricated prototype of the antenna are shown in Fig. 11. The simulated and measured transmission  coefficients (|S ij |) of the antenna revealed that the antennas offer high isolation of more than 20 dB (Fig. 12). Furthermore, the adjacent antennas offer the same simulated isolation characteristics (|S 21 |, |S 32 |, (|S 41 |, |S 43 |) due to the geometrical symmetry. Similarly, the orthogonally located antennas (|S 31 |, |S 42 |) have identical isolation among them. For brevity, only the measured value of |S 21 | and |S 31 | has been plotted. The average isolation between the diagonally located antennas is lower than the adjacent antennas as the distance among them is larger than the adjacent antennas. Here, the isolation for the mm-wave bands has not been shown, since it is very high because of the marked element spacing (16 mm ≈ 1.5λo at 27 GHz).

B. 2 × 4 MIMO Antenna
The 2 × 2 MIMO antenna is used for the design of the 2 × 4 MIMO system by flipping it 90 • along its axis. This configuration adds the pattern diversity of the MIMO antennas in both azimuth and elevation planes. The antenna design parameters are the same as the 2 × 2 MIMO antenna. For practical realization, two prototypes of the 2 × 2 MIMO elements are used having a slot of 1.6 mm (equal to the thickness of the substrate) as shown in Fig. 13.
As the isolation is the key parameter in the MIMO system, its numerically computed and measured plots at the microwave band are exhibited in Fig. 14. The antennas offer good isolation due to their unique arrangement; every antenna is perpendicular to its adjacent antennas. The antennas facing each other (e.g., port-5 and port 1, port-6, and port-2) have relatively low isolation (<15 dB) at lower bands, while all other antennas have transmission coefficients of less than −20 dB. Again, only some representative isolation plots between the antenna ports have been shown for a clear illustration. The measured data follows the pattern of the simulated curve and has a good agreement with its counterpart.

C. 3 × 4 MIMO Antenna
MIMO systems with several antenna elements with pattern diversity and low mutual coupling are the fundamental requirement for the 5G IoT communications. Therefore a 3 × 4 MIMO antenna is designed using the 2 × 2 MIMO designed previously in Section III-A. This design is obtained by rotating the 2 × 2 MIMO antenna, one time on the x-axis and another time on y-axis. This allows the arrangement of 12 antenna elements in a three-dimensional (3-D) geometry to form the proposed 3 × 4 MIMO antenna system. The proposed 3 × 4 MIMO antenna's geometry and fabricated prototype with its parts of the assembly are shown in Fig. 15.
It can be seen from the parts of the assembly that the proposed MIMO system consists of three units of planar 2 × 2 MIMO antenna; two units have a vertical slot, and one has a "plus-shaped" slot. These three units are arranged in the formation of the unique 3-D MIMO system. Therefore, the particular antenna has the advantages of mass production suitability with low-cost fabrication.
The simulated and measured isolation among antenna elements in this MIMO system is plotted as transmission coefficient in Fig. 16. Like the 2 × 4 MIMO antenna, the isolation among the antenna ports is more than 20 dB for all antennas except for those antennas which are facing each other (for example, port-5 and port-1, port-5 and port-2, port-11 and port-3, port-11 and port 2, etc.). Moreover, the antennas facing each other have an orthogonal arrangement, this gives radiation diversity and thereby have low coupling between the antennas. The isolation values of these antennas are more than 15 dB across the designated microwave bands, which are good enough for uninterrupted communications in 5G IoT.
To explore the radiation stability and beam diversity, the radiation patterns at selected ports (port-1, port-5, and port-6) are measured at various frequencies (2.5, 3.5, 5.5, and  7.5 GHz) and compared with the simulated pattern (Fig. 17). It is obvious from the antenna geometry (Fig. 15) that all the 12-antennas are identical, with the only difference being their direction of propagation. The antenna for port-1 shows a quasidirectional radiation pattern having a narrow halfpower-beamwidth (HPBW); the main beam toward 180 • at xoz-plane (phi = 0 • ). However, the radiations are mostly directed toward 120 • at yoz-plane (phi = 90 • ) with a wider HPBW. Interestingly, the radiation patterns of the antenna for port-5 show a mirror transformation of 180 • in both planes due to radiating orientation. That means, more radiations for port-1 are directed toward 0 • and 300 • in xoz-plane and yoz-plane, respectively. However, for port-6, the narrow beam is directed toward 90 • in xoz-plane and the radiation pattern with wide HPBW is around 60 • in xoy-plane. In the same fashion, the radiation patterns of other antennas can also be predicted. Due to identical monopoles, almost the same gain values are observed for all antennas at specific frequencies. It is worth noting that the radiation pattern of the antennas in the 3 × 4 MIMO system becomes quasidirectional at both planes, while it was omnidirectional at one plane and bidirectional (dumbbell shape) at another plane. This is because the neighboring facing antennas radiations are reflecting the main beam in the direction of open space. To understand this, let us consider the radiations of the antenna-1. This antenna is surrounded by antenna-5 and antenna-6 in the x-plane, while antenna-9 and antenna-12 in y-plane. Therefore, the antenna's main beam is prominent in the direction of the vacant space. Thereby, the omnidirectional radiations (in case of single-element) are tilted toward 180 • at xoz-plane, while the bidirectional beams are focused at 120 • at yoz-plane.
The multipath losses can be avoided if the antenna elements in the MIMO system offer pattern diversity, that is, propagation of radiation in different planes (directions). The pattern diversity also improves the quality and reliability of a wireless link of the IoT devices by providing coverage in all directions.
To demonstrate the pattern diversity of the proposed 3 × 4 MIMO module, the radiation patterns for some antenna elements (port-1 to port-4) at 3.5 GHz for xoy-and xoz-planes have been plotted in Fig. 18. It is evident from the figure that each antenna is radiating toward a specific direction. In both planes, the beams are directed toward 180 • , 90 • , 0 • , and 270 • for port-1, port-2, port-3, and port-4, respectively. In the same manner, the rest of the antennas also provide pattern diversity at various directions to offer 360-degrees coverage in azimuth and elevation planes. The separate ground planes and the orthogonal arrangement of antenna elements in this provide suppression of near-field coupling for the adjacent antenna elements for high port-to-port isolation and uncorrelated radiation patterns.
The radiation patterns of the mm-wave antennas (port-1 to 4) at 25 and 28 GHz have been shown in Fig. 19. Since  the mm-wave antennas have end-fire radiation patterns, each antenna radiates in different directions. For example, antenna-1 (port-1) has the main beam toward 270 • in xoz-and yoz-planes. Similarly, the antenna for Port-2 radiates toward 0 • . While antennas for Port-3 and 4 have more radiations in the direction of 90 • and 180 • , respectively. Again, the mm-wave antennas also offer radiation pattern diversity. Noting that the antennas for other ports (Port-5 to 12) have also specific directions for coverage based on their orientation.

IV. APPLICATION SCENARIOS
Since the proposed antenna covers the important IoT bands of microwave and mm-wave bands, the antenna can be used in various applications ranging from indoor communications in smart houses, offices, shopping malls, bus stops, and vehicle-to-everything (V2X) communications. Few application scenarios are explained below in detail.

A. Indoor Communication Systems
The indoor communication in future smart houses and shopping malls will be using microwave and mm-wave bands to support multisystem communication, as portrayed in Fig. 20. The microwave bands will provide long-range communications with medium speed to connect a huge number of IoT devices and multimedia gadgets, while the mm-wave bands will offer short-range high-data rates with ultralow latency communications for ultrahigh-definition multimedia, augmented reality, games, data devices, and more. Our antenna smoothly transmits and receives high-frequency signals in any direction of 360 • . There is no reception shadow area and broadband high-capacity reception is possible due to the realization of a high-isolation 12-port MIMO antenna with pattern diversity at both microwave and mm-wave bands.     [37], [38]. Moreover, using the 5G cellular bands (2.5, 3.5, and 24/26/28 GHz), the vehicles can communicate with a cellular network (V2N) and pedestrians (V2P). Because of the uncorrelated radiation pattern diversity and high-level isolation of the antenna elements, our antenna facilitates full-duplex operation, thus connecting everything in a V2X scenario.

B. V2X Communication Systems
The 12-port MIMO antenna module is simulated with a full car model to see the antenna behavior. The antenna is placed at a distance of 10 mm above the car body to isolate it from the conducting car body [39]. This can be realized in a practical scenario using a nonconducting aerodynamic antenna housing. The simulation setup and radiation pattern of the antenna for port 1 to 4 at 5.8 GHz at various ports are shown in Fig. 22. The antennas retain their radiation characteristics with a beam diversity, even when installed on the car body. In addition, the multiple antennas in the MIMO system are responsible to process transmission and reception simultaneously to provide full coverage for an uninterrupted link in V2X communications.
A link budget analyzes the performance of the communication channel considering all the losses that a communication link observes in a communication system. The MIMO antenna module's link budget is calculated at 5.9 GHz (ITS-G5/802.11p band) to determine its suitability for V2X communication, utilizing the following relation [40]: where P rec is the received power in dBm and P tran is the transmission power in dBm; T G is the gain of the transmitter antenna; L T is the loss at the transmitting end; L P is the overall free space path loss between the transmitting and receiving antennas; L M is miscellaneous losses, including the fade margin and polarization misalignment; R G is the receiver antenna gain; and L R is the total loss at the receiving end.  All other parameters for a given communication system are fixed [T G = R G = 3 dBi, L T ≈ 3.2 dB, L M ≈ 0.5 dB, and L R ≈ 2.35 dB], except L P , which varies for distance and frequency. The path loss L P can be estimated as [41] L P(dB/km) = 32.4 + 20log 10 (d km ) + 20Log 10 (f MHz ). (8) The minimum signal power (sensitivity) that the proposed antenna can detect is calculated using the method given in [42], which is found to be -82 dBm.
However, the Federal Communication Commission and World Health Organization are recommending antenna power up to 30 dBm for urban areas. The 5G automotive association (5GAA) is restricting transmit power to 21 dBm for the V2X communication [43]. The link budget of the proposed integrated microwave and mm-wave antenna at 5.9 GHz for various transmit powers is shown in Fig. 23. It shows that the antenna can offer a stable link for V2X communication over

V. PERFORMANCE COMPARISON
The proposed integrated microwave and mm-wave antenna module is compared with the related state-of-the-art works in Table I. Most of the integrated antennas [26], [27], [32]- [34] have few operating frequency bands with one direction of propagation. That is, the coverage area is limited to a fixed direction. The antennas proposed in [24], [25], [29], [31] offer a wide coverage area in mm-wave bands; however, the microwave band patterns have fixed beams toward a single direction. Although the antenna design [28] has flexible beam coverage, it is limited to a maximum of 80 • space coverage in the θ -direction. The multiband antenna [33] offers the advantages of the various operating microwave frequency bands (six sub-6-GHz bands) have the limitations of the single direction of propagation. Only our antenna has the capability of 360 • full space coverage in both theta and phi-directions with the additional advantages of wideband resonances at the essential microwave bands (2.5, 3.5, 5.5, and 7.5 GHz) and mm-wave bands (23)(24)(25)(26)(27)(28)(29)(30)(31). It is worth noting that none of the antennas in the literature support both microwave and mm-wave bands with a single feeding port, except [34]. Nevertheless, it has only three operating bands (3.5,28,and 38 GHz), and all antennas have the same propagation direction. In other words, our 12-port MIMO system is the only one of its kind that offers the codesign of microwave and mm-wave antenna with a single input port. Besides, it is capable of complete 360 • space coverage in elevation and azimuth planes for full-duplex operations to provide seamless connectivity in modern devices and sensors.

VI. CONCLUSION
The integration of microwave and mm-wave band antennas in a single design for the multistandard IoT system is a challenging task due to the large frequency ratio. We developed a multiband 12-port MIMO antenna operating at microwave bands (2.5/3.5/5.5/7.5-GHz bands) and mm-wave bands, offering high isolation and pattern diversity for 5G IoT applications. The single-element antenna features a printed triangular monopole with resonating stubs (five on the front and two on the back). The two stubs in the back are connected to the central radiator (with metallic pins) for the bandwidth improvements at 2.5-and 3.5-GHz bands. The multiband characteristics are obtained by adding five inverted L-shaped resonating stubs to a monopole on its front side. Two stubs are inserted at the backside of the antenna, through metallic pins for bandwidth improvements at 2.4-and 3.5-GHz bands. Simulation and measured results show that the antenna operates at key important bands of 2.5-GHz band (2.37-2.65 GHz), 3.5-GHz band (3.25-3.85 GHz), 5.5-GHz band (5.0-6.1 GHz), and 7.5-GHz band (7.15-8.5 GHz). Besides, it also gives resonance at 5G mm-wave bands (23-31 GHz) with suitable radiation characteristics.
The single-element antenna is translated to design 2 × 2 MIMO antenna, which is further used in the designing of 2 × 4 and the proposed 3 × 4 MIMO system to provide spectral and pattern diversity. The proposed 12-port MIMO module has the advantages of good isolation (less than 15 dB at the lowest frequency band of 2.5-GHz band) and pattern diversity for 360 • space coverage for full-duplex operations to provide seamless connectivity for modern devices and sensors. Furthermore, the antenna's application scenarios in indoor and V2X communication are discussed. The link budget is also calculated to find the antenna's suitability for V2X communications, which shows that the antenna can communicate within the acceptable noise margin over a distance of 1.25 km with a 21 dBm of transmit power. Owing to these qualities, the proposed MIMO antenna can be an excellent choice for V2X MIMO communications. To our knowledge, the proposed antenna is the first of its kind that has the ability to cover all the key frequency bands from microwave to mm-wave bands with polarization diversity and high isolation, making it a good candidate for 5G IoT applications.
Since the antenna elements has fixed radiation patterns that may trigger intercell interferences, future research will focus on beam steering and enhanced gain characteristics.