Four-element Multiple-input Multiple-output Antenna Research Articles

High gain MIMO antenna with multiband characterization for terahertz applications

Recently, the relevance of multiple input multiple output (MIMO) technology has increased due to the growing user base and the surge in demand for high data rates. It provides an essential answer to the pressing requirement for high-capacity communication systems compliant with new standards. In this study, a four-element MIMO antenna with specific properties—a thickness of 0.0224 mm and a relative permittivity of 4.3 —is presented on a polyamide substrate. It is intended to operate in terahertz (THz) applications at 0.395 and 0.629 THz. The configuration of the antenna is based on a traditional fork antenna. At the resonant frequencies, this MIMO antenna can achieve bandwidths of 9.53 GHz and 24.19 GHz, with reflection coefficients of -16.19 dB and -24.27 dB, respectively. Moreover, a gain of 5.17 dB and 3.19 dB are obtained at the second and first resonant frequencies, respectively. In the effort to maintain the operational band and enhance gain, a significant challenge arises from the insufficient isolation between the four antenna elements within the limited space. A Defected Ground Structure (DGS) is constructed and the inter-element distance is carefully investigated in order to effectively address this issue. Mutual coupling effects are effectively reduced with this antenna, and an amazing MIMO diversity response is shown. Additionally, it achieves a mean effective gain of less than -3, envelope correlation coefficients below 0.0125, diversity gain of around 10 dB, and total active reflection coefficients below -4 dB. The suggested design was simulated using the ansys High-Frequency Structure Simulator program. These enhancements show that the recommended MIMO antenna is a suitable choice for terahertz applications like 5 G networks of the future and terahertz imaging systems used in medical settings for tissue imaging and cancer diagnosis.

Substrate analysis on the wideband, high gain flower-shaped MIMO antenna

This article presents a single-element,flower-shaped two and four-element MIMO(Multiple Input Multiple Output)antenna with enhanced bandwidth and gain. The array consists of identical elements on three commonly used textile substrates- cotton, jeans, and felt material. For each of the three substrates, the impedance bandwidth, reflection coefficient, and gain are used to assess the effectiveness of the recommended antenna.For the felt-based four-element MIMO antenna the outcomes of measurement and modelling are confirmed. Results reveal that the prototype provides a fractional bandwidth of 133.2%, spans the UWB range (3.1 to 10.6 GHz) and X-band (8 GHz to 12.23 GHz), and has a maximum gain of 16.96 dB.To investigate the MIMO performance, MIMO metrics were computed which include Envelope Correlation Coefficient (ECC), Diversity Gain (DG), Mean Effective Gain (MEG), Channel Capacity Loss (CCL), and Total Active Reflection Coefficient (TARC).

A low profile circularly polarized metasurface-based ultra-wideband 4x4 MIMO antenna for 5G NR band FR2 frequencies

This article presents a polarization conversion metasurface-based low-profile ultra-wideband circularly polarized (CP) multiple-input multiple-output (MIMO) antenna with high performance that is specifically designed for operating in 5G New Radio (NR) bands in frequency range 2 (FR2). For these frequency bands, the 4-port MIMO antenna is being proposed for the first time and is capable of being operated in all of the newly created radio frequency bands in the 5G FR2 region. The main advantage of the proposed configuration is that it requires no isolation/decoupling circuit and the antenna elements are placed orthogonal to their adjacent elements. The 4x4 MIMO antenna offers a good impedance bandwidth of 111.09 % ranging from (12.87–45.02) GHz below − 10 dB reflection coefficient with a minimum isolation of –23.77 dB and maximum isolation of −52.53 dB. The maximum peak realized gain of the 4x4 MIMO antenna without metasurface is 8.14 dBi in the operating band. An improvement of 47 % in the peak realized gain was observed after stacking the metasurface on top of the radiating patch. The proposed antenna produces a circular polarization (CP) in the desired operating band. The 3-dB axial ratio impedance bandwidth for the lower band in millimeter wave (mm-Wave) is 14.13 % ranging from (23.22 – 26.75 GHz) and for the upper band is 7.92 % ranging from (40.82 – 44.19 GHz). Initially, the proposed antenna is designed for a single element and it is extended for a 4-element MIMO with added 4x4 metasurface unit cells for improvement of gain and polarization conversion. The simulated and measured results are in good agreement. In addition to that, the envelope correlation coefficient (ECC), channel capacity loss (CCL), total active reflection coefficient (TARC), mean effective gain (MEG), and diversity gain (DG) are calculated and simulated for the proposed four-element MIMO antenna. The designed metsurface-based circularly polarized MIMO antenna covers all 5G new radio frequency bands in FR2 and hence the designed antenna is applicable for 5G NR band FR2 in mm-Wave communications.

Design of a Dual-Band WiFi Antenna Using the Theory of Characteristic Modes and Nested Chinese Characters

In this paper, a dual-band WiFi antenna and its Multiple Input Multiple Output (MIMO) system application is designed, fabricated, and measured based on the Chinese characters “Men” and “Wei”. The antenna uses a 40 × 40 × 1.6 mm3 FR4 substrate to analyze the combinatorial structure of Chinese characters using the theory of characteristic modes, to optimize the antenna dimensions by analyzing the mode current distribution, and to broaden the antenna bandwidth by etching rectangular slots on the ground. The measured and simulated results show that the four-element MIMO antenna covers 5.68–8.01 GHz, the isolation between the antennas is higher than 20 dB in the working band, the envelope correlation coefficient and the channel capacity losses of the simulation are lower than 0.001 and 0.18 bits/s/HZ, respectively. The efficiency of the antenna is higher than 90%, and it can be used for WiFi communication bands (5.8 GHz and 6 GHz).

Investigation of Parallel and Orthogonal MIMO Antennas with Two-Notched Structures for Ultra-Wideband Application.

Ultra-wideband (UWB) technology is widely used in many communication scenarios. However, narrowband systems can easily interfere with the UWB system, which generates multipath fading. In order to solve these interferences and meet the design requirements of high isolation of multiple-input multiple-output (MIMO) antennas, two MIMO antennas with double-notch structures are designed. Firstly, two U-shaped slots are etched on the radiating patch and feeder to achieve notch characteristics in WiMAX and ITU bands. Using this antenna element, a two-element antenna is put symmetrically in parallel, and two rectangular branches are loaded to improve the isolation. The size is 0.57λ × 0.32λ × 0.013λ (at 2.5 GHz). Then, a four-element antenna is designed to meet the requirements for another application; here, each element is placed orthogonally to each other, and the isolation is improved through loading a cross-shaped branch in the middle of these elements. The size is 0.57λ × 0.57λ × 0.013λ. Both antenna samples are tested to verify the design. Measurement results show that the working bandwidth is 2.45-14.88 GHz and 2.14-14.95 GHz, the isolation is greater than 17 and 20 dB, and the peak gain is 5.7 and 5.9 dBi for the two- and four-element MIMO antenna, respectively. Compared to the references, the designed antennas have a wider bandwidth and a higher gain and radiation efficiency. They are well-suited for diverse wireless applications.

A Compact Planar Monopole UWB MIMO Antenna for Short-Range Indoor Applications.

A compact, four-element planar MIMO (Multiple Input, Multiple Output) antenna that operates in an ultra-wideband is presented for diversity application. The orthogonal position of the unit cells replicates the single antenna thrice, thereby decreasing mutual coupling. A UWB MIMO antenna measuring 35 × 35 × 1.6 mm3 is built using a microstrip line (50 Ω impedance) on an FR4 substrate having a thickness of 1.6 mm. The ground plane and radiator of this antenna are adjusted in several ways to bring it within its operating constraints between the frequencies of 3.1 GHz and 10.6 GHz. This technique makes the antenna small and covers the entire ultra-wideband (UWB) frequency range. The NI USRP was used to test the proposed MIMO antenna to determine its real-time performance. Based on the computed results, we conclude that this proposed antenna has outstanding characteristics in terms of performance and is suitable for wireless ultra-wideband indoor communication and diversity utilization with a small size.

Miniaturized Four-Port MIMO Implantable Antenna for High-Data-Rate Wireless-Capsule-Endoscopy Applications

This study proposes a miniaturized four-port multiple-input multiple-output (MIMO) implantable antenna for wireless capsule endoscopy (WCE). The proposed MIMO implantable antenna operates in the 2.45 GHz industrial, scientific, and medical (ISM) band, has a compact size of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6.2\,\, \times 5.2\,\, \times 0.127~\text {mm}^{3}$ </tex-math></inline-formula> , and comprises four meandered resonators sharing a common slotless ground plane with an acceptable bandwidth (14.2%) and low mutual coupling. These features help the antenna achieve high data rates and countering multipath distortions, as confirmed using MIMO channel parameters. The calculated link budget analysis of the proposed antenna suggests that seamless high-rate (78-Mbps) information transmission can be efficiently achieved over a distance of up to 10 m (when placed in the large intestine). The proposed MIMO implantable antenna was optimized by placing it inside a capsule comprising sensors, batteries, and electronic components and was subsequently fabricated. The simulated ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula> -parameters, gain, and radiation patterns) and measured (in the minced pork) results were in close agreement with a measured bandwidth of 350 MHz and a peak measured gain of −20.1 dBi. To ensure patient safety, the specific absorption rate (SAR) was evaluated for all the four ports in different environments (stomach and small and large intestines), and satisfactory results were obtained. MIMO performance parameters, such as the envelope correlation coefficient (ECC), diversity gain (DG), and channel capacity (CC), were also evaluated and acceptable values were noted. To the best of our knowledge, this is the most compact four-element implantable MIMO antenna designed so far for biomedical applications.

A Compact High-Isolation Four-Element MIMO Antenna with Asymptote-Shaped Structure

The demand for high-speed wireless communication systems has led to the development of ultrawide-band (UWB) antennas with a compact size and high performance. In this paper, we propose a novel four-port multiple-input multiple-output (MIMO) antenna with an asymptote-shaped structure that overcomes the limitations of existing designs for UWB applications. The antenna elements are placed orthogonally to each other for polarization diversity, and each element features a stepped rectangular patch with a tapered microstrip feedline. The unique structure of the antenna significantly reduces its dimensions to 42 × 42 mm2 (0.43λ×0.43λ@ 3.09GHz), making it highly desirable for use in small wireless devices. To further enhance the antenna's performance, we use two parasitic tapes on the ground plane at the back as decoupling structures between adjacent elements. The tapes are designed in a windmill shape and a rotating extended cross shape, respectively, to further improve the isolation. We fabricated and measured the proposed antenna design on a single-layer substrate (FR4) with a dielectric constant of 4.4 and a thickness of 1 mm. The measured results show that the impedance bandwidth of the antenna is 3.09-12 GHz, with an isolation of -16.4 dB, an envelope correlation coefficient (ECC) of 0.02, a diversity gain (DG) of 9.991 dB, an average total effective reflection coefficient (TARC) of -20 dB, an overall group delay value less than 1.4 ns, and a peak gain of 5.1 dBi. Although there may be some antennas that have better performance in one or two specific aspects, our proposed antenna has an excellent trade-off among all the antenna characteristics including bandwidth, size, and isolation. The proposed antenna also exhibits good quasi-omnidirectional radiation properties, making it well-suited for a range of emerging UWB-MIMO communication systems, particularly in small wireless devices. In summary, the compact size and ultrawide-band capabilities of the proposed MIMO antenna design, coupled with its improved performance compared to other recent UWB-MIMO designs, make it a promising candidate for 5G and next-generation wireless communication systems.

Simple Structure E-Plane Decoupled Millimeter Wave Antenna Based on Current Cancellation Model

A simple structure E-plane decoupled millimeter wave (mmW) multiple-input multiple-output (MIMO) antenna based on current cancellation model is proposed in this communication. First, a coupling model of E-plane MIMO antenna is established by analyzing the coupling current. Then, a high-isolation two-element E-plane MIMO antenna loaded with decoupling branch Bar-L is designed by current cancellation method. Finally, a high-isolation four-element E-plane MIMO antenna is designed, fabricated, and measured to verify the applicability of the decoupling method in large arrays. The measured results show that the antenna achieves <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\vert \text{S}_{\mathbf {11}}\,\,\vert &lt; -10$ </tex-math></inline-formula> dB in band of 25.5–26.5 GHz, and the measured isolation of adjacent ports in this band is greater than 35 dB, which effectively improves the isolation without deteriorating other performance. The proposed antenna with a simple structure is suitable for mmW MIMO systems requiring high isolation.

FOUR-ELEMENT MIMO ANTENNA WITH HIGH ISOLATION AND DIVERSITY PERFORMANCE FOR UWB APPLICATIONS

A four-element multiple-input multiple-output (MIMO) antenna for wireless ultra-wideband (UWB) applications is proposed in this study. The single-element antenna is derived by conducting several design evolutions. This fundamental antenna achieves an UWB impedance bandwidth with good positive gain across the operating frequency ranges. In the investigation of MIMO antennas, dual and four-element MIMO antennas with variable layouts are designed and analyzed according to their S parameters, gain, radiation pattern, and diversity performance findings. Thus, a four-element MIMO antenna operating in the frequency range of 3.5-11.2 GHz with &amp;#62; -20 dB isolation in the majority of the band is proposed. The diversity performances in terms of envelope correlation coefficient and diversity gain are less than 0.006 and greater than 9.9, respectively, while the mean effective gain (MEG) and channel capacity loss (CCL) are within acceptable ranges. The simulated findings agree well with the measured results.

Design and Analysis of Wideband Flexible Self-Isolating MIMO Antennas for Sub-6 GHz 5G and WLAN Smartphone Terminals

A single radiator that is a part of four-port diversity Multiple-Input Multiple-Output (MIMO) antenna design is composed of four octagonal rings embedded between the two opposite sides of a T-shaped conductive layer surrounded by inverted angular edge cut L-shaped and E-shaped structures. The radiators are placed at the four corners with common ground at the center of a smartphone to form a four-element mobile MIMO antenna. The printing of the antenna is carried out on the flexible polyamide substrate (dielectric constant = 3.5 and loss tangent = 0.0027) with dimensions of 70 × 145 × 0.2 mm3. A wide impedance bandwidth of (84.12%) 2.39 to 5.86 GHz is achieved for all four radiators. The compact size of the radiators along with their placement enables the proposed MIMO antenna to occupy much less area while preserving the space for 2G/3G/4G antennas. The placement of the antennas results in self-isolation between antenna elements by achieving isolation greater than 17.5 dB in the desired operating bands. Furthermore, besides showing a high efficiency of 85% and adequate gain above 4 dBi, good diversity performances such as Envelope Correlation Coefficient (ECC) of less than 0.05, Diversity Gain (DG) of above 9.8 dB, Mean Effective Gain (MEG) of −3.1 dB, Channel Capacity of 21.50 bps/Hz, and Total Active Reflection Coefficient (TARC) of below −10 dB are achieved by the flexible MIMO smartphone antenna. The effect of bending along the X and Y-axis on the performance of the proposed MIMO antenna is also analyzed where decent performance is observed. This makes the proposed flexible four-element MIMO antenna a potential candidate to be deployed in future smartphones.

Compact Dual-Polarized Wideband Antenna With Dual-/Single-Band Shifting for Microbase Station Applications

A gain-enhanced wideband multiple-input multiple-output (MIMO) antenna with dual polarization and high isolation is proposed for microbase station application. It includes two mutually orthogonal bowtie dipole radiators, four bowtie parasitic elements with the switch function of ON-/OFF-state, four vertical stubs, and a cavity, which are fed by a pair of 50 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega $ </tex-math></inline-formula> coaxial cables. Shifting between the dual-band and single-band is realized by the switch. Four resonant frequencies are generated by cooperative operation of the radiators and cavity, and thus two wide bands are formed. C-shaped slots on radiators and switch slots on parasitic elements are analyzed for estimating the impact on resonant frequency and bandwidth, and the vertical stubs improve the impedance matching. The proposed wideband antenna achieves enhanced diversity performance in the MIMO antenna system. As a proof of concept, a prototype of the expanded four-element MIMO antenna is fabricated. Measured results match well with simulated data. When the switch is ON-state, it operates in 1.58–2.77 and 4.71–6.18 GHz with gains of 9.4–11.2 and 9.1–10.9 dBi, and radiation efficiencies of ≥87.5% and ≥87%, respectively. When the switch is OFF-state, it operates in 1.56–2.76 GHz with a gain of 8.9–10.1 dBi and radiation efficiency of ≥88%. Element isolation is ≥35.3 dB, and envelope correlation coefficient (ECC) is ≤0.002. The experiment of the established <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2 \times 2$ </tex-math></inline-formula> MIMO system demonstrates that it can operate with a lower bit error rate (BER), and the design shows enormous potential to be applied as massive MIMO microbase station antennas.

A four-element UWB MIMO antenna using SRRs for application in satellite communications

This paper proposes a method for designing a new &lt;em&gt;ultra wide band&lt;/em&gt; (UWB) multiple-input multiple-output (MIMO) antenna with two and four elements. First we presented an ultra-wide band antenna we studied these performances. Then, we studied the application of metamaterials to the design of MIMO antennas for miniaturization and the performance of antennas, in order to guarantee the proper functioning of the MIMO system with a much reduced separation distance between the radiating elements (λ/12), where the coupling can be very weak. The application of these circular double ring SRRs materials on the front plan of the antenna has contributed to the increasing of the antenna performance is studied in terms of S-Parameters, efficiency, diversity gain (DG), radiation properties and envelop correlation coefficient (ECC). It offers advantages such as the reduction of weight and congestion that is beneficial for their integration into satellite communications systems.

Wideband antenna pair and its applications for 5G metal frame mobile phones

A wideband antenna pair used for 5G mobile phones is proposed in this paper. Both elements of the antenna pair consisting of an open slot and a coupled feeding line can cover the band of 3.3–6 GHz, and the isolation is larger than 10 dB. The ground clearance and the total size are 1.5 mm and 30 mm × 1.5 mm × 7 mm, respectively. Using the antenna pair, four- and eight-element multiple-input–multiple-output (MIMO) antennas are constructed. The simulated results show that the proposed antenna pair can be used to construct MIMO antennas for 5G metal frame mobile phones. As an example, the four-element MIMO antenna is manufactured and tested. The measured results show that the four elements can cover the band of 3.3–6 GHz, and the n77, n78, n79, and 5 GHz WLAN bands are covered. At the working bands, the measured efficiencies and isolations are bigger than 46% and 11.4 dB, respectively.

Four-element pentaband MIMO antenna for multiple wireless application including dual-band circular polarization characteristics

AbstractIn this article, a meander line-shaped pentaband (2.18–2.24, 2.38–2.46, 2.65–2.70, 3.10–3.32, 3.38–3.46 GHz) four-element multiple-input–multiple-output antenna is presented. The proposed antenna is also circularly polarized in two bands (at 2.2 and 2.4 GHz) with dual-polarization like right-handed at port 1 or 3 and left-handed at port 2 or 4, which is widely used for mobile satellite services (MSS) and Internet of Things applications. This antenna is designed and fabricated with compact size 50 × 70 × 1.6 mm3 on the FR-4 substrate with good diversity performance in pentaband. Simulated results of antenna-like return loss, isolation, and parameters-related diversity have also been tested experimentally in a controlled environment, which is within the permissible limit. The designed antenna will be appropriate for MSS, industrial scientific and medical (ISM), broadband radio services and educational broadband services, WiMAX radio location services, and amateur radio services. Meanwhile, specific absorption rate of the designed antenna has been examined in an empirical environment for the Fresnel radiating near-field applications.

Four-element MIMO antenna for X-band applications

AbstractThis study presents a new design of a four-element multiple-input multiple-output (MIMO) antenna, which is suitable for X-band applications. The circular polarization (CP) operation in this work is attained by using Tai Chi-shaped patches and L-shaped feeds. The proposed four-element MIMO antenna achieves two resonant frequencies, the first one at around 7.75 GHz and the second one with CP radiation is at approximately 10.15 GHz frequency. The measured isolations between ports with 46.7 × 46.7 mm2 total dimensions are &lt;−20 dB. Due to the experiments, the envelop correlation coefficient for the orthogonal and parallel ports of the proposed MIMO antenna is &lt;0.003 and 0.005, respectively. Additionally, the presented antenna has a circularly polarized performance in the frequency band of 9.75–10.41 GHz and the average realized peak gain almost 2 dBic. The investigated antenna in this work is fabricated and tested in which the results are in a good agreement.

Coupling reduction using a novel circular ripple-shaped decoupling mechanism in a four-element UWB MIMO antenna design

A 4*4 multiple-input multiple-output (MIMO) antenna with compact size and good isolation is presented for UWB applications. The concept of polarization diversity is here discussed and employed to satisfy contemporary wireless communication systems requirements. In the proposed antenna, the utilization of a novel circular ripple-shaped decoupling structure and a matching structure are presented to minimize mutual coupling of the inter-elements and enhance adaptation. Experimental results show a bandwidth range from 2.96 to 11.4 GHz with a good reflection coefficient which is S 11 < −12 dB, high isolation S 21 and S 31 < −20 dB, good Envelope Correlation Coefficients ECC21 and ECC31 < 0.03, Diversity Gain DG21 and DG31 equal to 8.75, and a Total Active Reflection Coefficient (TARC) less than −12 dB. The antenna also exhibits a maximum gain of about 4 dBi with a maximum efficiency of 70% which proves that this antenna is a potential candidate for UWB applications.

A Six-Port Dual-Function RF Device With Four-Element MIMO Antenna Array and Bandpass Filter Operations

This article presents a dual-function RF device, with four antenna ports and two filter ports, which behaves either as a multiple-input multiple-output (MIMO) antenna array or a bandpass filter. It consists of four parallel-coupled microstrip lines as the dual-function part and two feed parts for the two channels. The former part simultaneously performs wideband array radiator and fourth-order filter resonator characteristics. For MIMO antenna array operation, the parallel-coupled microstrip line structure is equivalent to a traditional patch, which is divided into four strips with three slots. Based on such a configuration, wide antenna impedance bandwidth is achieved. With regard to the filter operation, it features a conventional fourth-order microstrip parallel-coupled filter typology. As a result, high circuit integration is achieved, and although the two channels share the common part, the bandwidth improvement for both the channels is realized. For demonstration, the proposed dual-function device is implemented and fabricated. As an MIMO antenna array, the center frequency and the measured impedance bandwidth is 4.5 GHz and 27.3%, respectively. The isolation between the two channels in this band is more than 23.6 dB. In addition, it features ±45° continuous beam scanning capacity with less than 3 dB gain reduction. With respect to the bandpass filter operation, the center frequency and measured impedance bandwidth is 0.55 GHz and 15.6%, respectively. The measured in-band insertion loss is 2.12 dB and channel isolation is more than 58 dB.

Low-Profile and Closely Spaced Four-Element MIMO Antenna for Wireless Body Area Networks

A compact four-element multiple-input multiple output (MIMO) antenna is proposed for medical applications operating at a 2.4 GHz ISM band. The proposed MIMO design occupies an overall volume of 26 mm × 26 mm × 0.8 mm. This antenna exhibits a good impedance matching at the operating frequency of the ISM band, whose performance attributes include: isolation around 25 dB, envelope correlation coefficient (ECC) less than 0.02, average channel capacity loss (CCL) less than 0.3 bits/s/Hz and diversity gain (DG) of around 10 dB. The average peak realized gain of the four-element MIMO antenna is 2.4 dBi with more than 77 % radiation efficiency at the frequency of interest (ISM 2.4 GHz). The compact volume and adequate bandwidth, as well as the good achieved gain, make this antenna a strong candidate for bio-medical wearable applications.

Tightly Arranged Four-Element MIMO Antennas for 5G Mobile Terminals

In this paper, a compact multiple-input multiple-output (MIMO) antenna is presented for 5G mobile terminals. The antenna consists of four tightly arranged elements and three lumped components. The edge-to-edge distance between adjacent elements is only 1 mm. It is found that adding an inductor at the current minimum location or a capacitor at the current maximum location of two elements can significantly reduce the mutual coupling. Based on high-isolated dual-element structures, a four-element MIMO antenna is experimentally analyzed for 3400–3600 MHz band. The size of the antenna is only $40.8 \times 3$ mm2. The measured −6 dB bandwidth of the antenna is 135 MHz, with port isolation higher than 11.6 dB. In addition, a vertically placed four-element MIMO antenna is designed with a size of $38.2\times3.2$ mm2. No ground clearance is needed between the antenna and the chassis ground.