A Compact Highly Isolated Four-Element Antenna System for Ultra-Wideband Applications

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Introduction
Tese days, there is a great deal of demand for needs and requirements that include fow of data without interference.A radio link's capacity may be increased using the multipleinput multiple-output (MIMO) technology by utilising several transmit and receive antennas to accomplish multipath propagation [1].Te deployment of multiple-input multiple-output (MIMO) systems has revolutionized wireless communication networks by signifcantly enhancing data throughput and spectral efciency [2].By exploiting spatial diversity and multiplexing gains, MIMO technology enables simultaneous transmission of multiple data streams over the same frequency band, thereby increasing the overall system capacity.In practical implementations, the efectiveness of MIMO systems heavily relies on the spatial separation of antennas and the mitigation of interference efects.To achieve optimal performance, antennas need to be strategically positioned to maximize diversity and minimize correlation between antenna elements [3,4].Tis spatial arrangement is crucial for combating multipath fading and improving signal quality, especially in environments with severe channel conditions.Ultra-wideband (UWB) technology complements the capabilities of MIMO systems by ofering enhanced data rates, low power consumption, and robust communication links.Operating across a wide range of frequencies, UWB enables high-speed data transmission while adhering to stringent regulatory constraints set by organizations like the Federal Communication Commission (FCC) [5].Te adoption of UWB further extends the applicability of MIMO systems in diverse wireless environments, ranging from personal area networks to industrial IoT deployments.However, the integration of MIMO and UWB technologies introduces challenges associated with antenna design and mutual coupling efects.In compact MIMO antenna confgurations, the proximity of antenna elements can lead to mutual coupling, where electromagnetic felds from one antenna infuence the performance of neighbouring antennas.Mutual coupling not only degrades the isolation between antenna elements but also afects the radiation pattern and impedance matching of the overall antenna array [6].
Addressing mutual coupling issues is essential for ensuring the robustness and reliability of MIMO-UWB systems, especially in scenarios where signal integrity is paramount.Techniques such as antenna decoupling, isolation enhancement, and adaptive array processing play a pivotal role in mitigating the adverse efects of mutual coupling and optimizing system performance.As we transition into the era of 5G connectivity, the demand for highspeed data transmission, ultra-low latency, and seamless connectivity continues to escalate.MIMO-UWB systems ofer a compelling solution to meet these evolving demands by leveraging advanced signal processing algorithms, beamforming techniques, and interference mitigation strategies [7].Tis is because radiations from free space, surface currents, and surface waves can all contribute to the formation of mutual coupling.It is possible for the total active refection coefcient of a MIMO antenna to be altered by a variety of signal processing methods.Tese techniques include those that reduce carrier frequency ofset (CFO), channel estimate, and angle of arrival estimation.When mutual coupling is present, it is possible for large outof-band (OOB) emissions from power amplifers (PAs) to interfere with communication on neighbouring channels.In the realm of digital communications, the mutual coupling has been reduced due to improvements made to MIMO precoding and decoding techniques [8].Before adaptive algorithms can calculate the weight vector, the received voltages must frst be adjusted to eliminate any mutual coupling that may have occurred.During postprocessing, one may improve the signal-to-interference noise ratio (SINR) by reducing the amount of relative interference or noise.In order to maintain the SINR, it is not essential to adjust the electromagnetic interactions.Using the approach described above, it is possible to achieve some degree of success in enhancing the electromagnetic suppression in the digital realm.When viewed from the point of view of the antenna, a technology that is efcient in decoupling is more successful at mitigating the consequences of mutual coupling.As a direct consequence of this, approaches for decoupling based on antennas need to be developed [9].
Tere are a variety of approaches that may be used to achieve decoupling.Examples of such decoupling methods include things like defected ground structures [10], decoupling networks [11], electromagnetic band gap structures (EBG) [12], neutralising lines [13], parasitic or slot components [14], and complementary split-ring resonator (CSRR) [15].Tere is a description of a UWB-MIMO antenna with twin ports in reference [16].Tis antenna has wideband features, including a frequency range of 3.0-11.0GHz as well as four rejection bands.According to the fndings presented in reference [17], slots with inverted L and U forms are able to give quadruple notch characteristics at frequencies of 3.3 GHz, 4.03 GHz, and 5.4 GHz.In addition to this, the antenna has an isolation of more than 25 decibels.In [18], a fexible CPW-fed MIMO antenna is shown; in order to achieve strong polarization diversity, a large impedance width of 125.83%, a maximum gain of 6.8 dBi, and an efciency of 89%, four identical elements are stacked symmetrically in a sequential rotating fashion.Tis allows the antenna to fulfl its goals.Te authors of [19] were able to reduce the size of a UWB-MIMO antenna by combining the radiation from a rhombic slot with that of four microstrip feeders.In order to achieve the notched properties, C-slots, L-slots, and an H-shaped EBG structure must frst be opened.Te antenna has a strong performance in terms of variety, with an isolation level of 40 dB, a peak gain of 5.5 dBi, and a radiation efciency range of 75%-90%.In a letter that was cited in [20], it was proven that a wideband impedance match and dual-band notch capability may be achieved by using a quasi-U-shaped patch on the upper plane and a stepped slot in the ground plane.Te use of defective ground structure for an elliptical planar antenna has been explored in [21], where the DGS is used to realise a UWB frequency response.Te purpose of this study was to investigate the use of DGS.In the research described in [22], two symmetrical half-slot components with CPWfed structures were used, together with a Y-shaped slot that was split at the bottom centre of the common ground.Both the current fow at low UWB frequency and the mutual coupling between the MIMO components are signifcantly reduced due to the slots' efcient blocking of both.Tere is a paper in reference [23] that describes a unique fourelement MIMO antenna that may be used for UWB applications; the antenna is CPW-fed and is made up of four orthogonal sections.An impedance width ranging from 2.1 to 20 GHz may be accomplished by using notch frequencies in the range of 3.3 GHz-4.1 GHz and 8.2 GHz-8.6 GHz with the antenna that has been presented.When notches are reached, it is possible to flter out interference from WiMAX and radar applications.In [24], the neutralisation line approach is discussed in relation to its use in the construction of MIMO antennas for UWB communications.Te bandwidths that were measured were 95.22% (3.51-9.51GHz) and 96.47% (3.52-10.0GHz), and the isolation was improved to 23 dB.Te MIMO antenna ofers a peak gain of 2.91 dBi and has superior diversity metrics than the other antenna types.In addition, the design of the antenna that is discussed in [25] is for an extremely compact 2-element planar antenna.In order to get the UWB response, a rectangular stepped slot and a z-shaped slot are etched into the ground plane.Te described construction has a geometrical dimension of 23 mm by 26 mm by 0.8 mm 3 , and the antenna has a peak gain of 5 dBi with an efciency of 80%.

International Journal of Antennas and Propagation
In this work, we have proposed a fractal-based compact four-element antenna for UWB applications operating from 3 to 12 GHz with high isolation and gain.By placing antenna elements orthogonally, isolation up to 30 dB is achieved which results in lower values of ECC below 0.25.Te antenna also achieves maximum gain up to 8 dB which is highly signifcant for UWB applications.Moreover, MIMO parameters diversity gain (DG) and channel capacity loss (CCL) have also been computed to verify presented MIMO antenna's diverse performance over the operating spectrum.Te results show that the antenna proposed in this research could be a viable candidate for UWB applications.
Te signifcance of the proposed work could be stated as follows: (1) Te presented antenna occupies only a minimum area of around 20 × 30 mm 2 which is relatively smaller in size compared to the other works in the literature.( 2) With such low-profle confguration, the antenna achieves ultra-wideband operation spanning between 3 and 12 GHz with 10 dB impedance bandwidth.
(3) Te designed antenna is implemented as a fourelement MIMO antenna system which provides high isolation better than 25 dB without seeking additional decoupling mechanism.(4) Most importantly, the antenna attains maximum gain up to 8 dB which is highly sufcient for UWB applications.(5) Furthermore, the antenna exhibits considerable MIMO features which include ECC, DG, and CCL over the operating spectrum.
1.1.Single Antenna Design and Analysis.Te proposed UWB antenna is illustrated along with its schematic representation as shown in Figures 1(a) and 1(b), respectively.Te radiating element of size L × W is printed on front side of 1.6 mm thick (H) FR4 substrate.Te radiating element consists of two main components, namely, modifed circular patch and feeding strip, as shown in Figure 1(a).To obtain the wide bandwidth, 15 equal steps of size L4 have been etched from the corners of the circular radiating element and connected with a 50-Ω feeding strip of size L1 × W1.On the one hand, defected ground has been printed on the backside of the substrate with the dimension of L1 × W. In order to achieve better impedance matching as well as wider bandwidth, portions of width W3 have been etched away from top of the ground plane.Furthermore, to improve the resonance over the desired band of spectrum, a slot of L3 × W2 has been removed from the centre of the ground plane as shown in Figure 1(b).Te detailed dimension of the single antenna element is listed in Table 1.
Te refection coefcient of the proposed UWB antenna is illustrated in Figure 2. It is clearly shown that the proposed antenna is capable of covering UWB spectrum ranging from 3 to 10 GHz over the 10 dB impedance scale.

Design Process.
To provide a better understanding of the proposed design, evolution stages have been studied in this section.Te design stages have been categorized as Antenna 1, Antenna 2, Antenna 3, and Proposed Antenna as shown in Figure 3. Antenna 1 consists of full ground plane with a full circle along with feeding strip.Ten, Antenna 2 is obtained after reducing the size of ground plane.Ground plane of the Antenna 2 is etched at top left and right edges and named as Antenna 3. Te proposed antenna is derived from Antenna 3 by incorporating 15 equal steps over the edges of the circular radiator.Te corresponding refection coefcients are illustrated in Figure 4. Antenna 1 is only able to provide dual and narrow band resonance at 8.5 and 10.25 GHz, respectively.Te reduction of ground plane size infuences resonance to be created at three spectrums that include 4 GHz, 7.8 GHz, and 10.5 GHz.It proves that defected ground could cause spectrum of operation with reasonable impedance matching.Even better impedance matching with wider spectrum of operation is obtained after making changes along top edges of the ground plane (Antenna 3).Te UWB performance is achieved by incorporating fractalbased structure (equal steps) over the circular radiating element as shown in Figure 4.

Parametric Study.
Te parametric analysis has been provided in this section to understand the design concepts of the proposed UWB antenna.However, the proposed antenna does have many parameters; only three have been taken for analysis including length of the feed (L1), length of the ground plane (L2), and length of square slot in the ground plane (L3).As shown in Figure 5(a), the parameter L1 has been altered from 10 mm to 13 mm with the equal step size of 1 mm.It is noticed that L1 has infuenced wideband resonance while it is progressively increased.Te proposed antenna is capable of exhibiting UWB resonance only at 11 mm with signifcant refection coefcient below 10 dB.Altering the stub length (L1) results in variations in the frequency of operation.Similarly, the efect of changing ground plane length (L2) is also studied and plotted in Figure 5(b).Increasing length (L2) afects impedance matching of the antenna throughout the bandwidth.On the other hand, variation in slot length (L3) in the ground plane is studied as shown in Figure 5(c).Ground slot length (L3) does not afect the wideband resonance but infuences the frequency shift as well as impedance matching of the designed antenna.

Surface Currents.
Te antenna surface current distributions at various frequencies are illustrated in Figures 6(a)-6(d).At 4 GHz, current concentration maximizes along the feeding strip as well as in the corners of circular radiating element as shown in Figure 6.It proves that length of the feeding strip afects performance of the antenna at lower frequencies.Figure 5      International Journal of Antennas and Propagation antenna at middle range of the spectrum.At 8 GHz, the current concentrated at the edges of the ground slot, whereas at 10 GHz, peak current is observed along the feeding line along with sliding edges on the ground plane as shown in Figure 6.
1.5.MIMO Antenna Structure.Te proposed single antenna element is converted into four-port MIMO antenna system which is printed on 65 mm × 65 mm FR4 substrate in orthogonal fashion.All the antenna elements in the MIMO system have covered UWB spectrum ranging from 3 to 12 GHz over the 10 dB impedance scale.Since all the antenna elements are placed orthogonally, signifcant isolation up to 20 dB is achieved between any adjacent pair.It results in considerable improvement radiation efciency and envelope correlation coefcient (ECC).Te structure of the four-port MIMO antenna is illustrated in Figure 7(a).Te fabricated prototype of the MIMO antenna is depicted in Figure 7(b).Te simulated and measured refection and transmission coefcients of the proposed MIMO antenna system are illustrated in Figures 8(a)-8(d).It is observed that both simulated and measured S-parameters of the antenna synchronized well with each other.It is mentioned earlier that isolation among antenna elements is better than 25 dB as all elements are placed orthogonally.It is verifed with surface current distribution of the proposed antenna excited at various frequency of operation as shown in Figures 9(a)-9(c).It is observed that when Antenna 1 is excited, other elements have not been afected by the current which fows in Antenna 1, thereby improving isolation signifcantly.
Similarly, measured two-dimensional radiation patterns at φ � 0 °and 90 °at 4 GHz, 6 GHz, and 8 GHz are illustrated in Figures 10(a)-10(c).At all the frequency ranges, the proposed antenna exhibits unique pattern over φ � 0 °and 90 °directions with cross-polarization better than 20 dB.Te simulated and measured gain of the proposed antenna is illustrated in Figure 11.Te proposed MIMO antenna achieves maximum gain up to 8 dBi over the desired operation spectrum which is highly signifcant for UWB applications.It is noteworthy that any dedicated structure or mechanism has not been included in this work to improve the gain.
1.6.MIMO Performance.In this section, various MIMO features have been discussed to validate the MIMO performance of the proposed antenna.Envelope correlation coefcient is an important parameter which reveals how far antenna elements are displaced, thereby achieving superior performance.Figure 12(a) illustrates the ECC of the proposed antenna which lies in the range between 0.1 and 0.2.Te obtained ECCs are lower than the acceptable threshold of 0.5.ECC values of the proposed antenna are calculated using the following equation: Also, diversity gain of the proposed MIMO antenna system has been computed using (2) and plotted in Figure 12(b).Generally, both DG and ECCs of the antenna are dependent parameters which represent gain of the antenna when radiated wave propagates in diverse directions.Generally, diversity gain of the ideal MIMO antenna system should be 10 dB over the operating spectrum.For the proposed antenna, it is as closer as 9.8 dB between 3 and 10 GHz.  ( Channel capacity loss (CCL) is an important metric being measured in terms of bits per seconds per Hz using (3) and (4).It measures loss of data during transmission of electromagnetic waves.For ideal antenna system, CCL should be zero but practically it could be less than 0.4 bits/ Sec/Hz.Te proposed antenna system exhibits CCL around 0.25 bits/Sec/Hz over the operating spectrum as shown in Figure 12(c) that ensures designed antenna has minimum transmission loss during radiation.

Channel Loss
where Figure 13(a) represents total active refective coefcient of the proposed antenna.It is obvious that TARC will be measured when phase of all ports will be changed when phase of the port 1 is kept constant.It is observed that frequency response of port 1 maintains same bandwidth when altering phase of all other ports.Similarly, multiplexing efciency has also been calculated plotted in Figure 13(b).Te proposed antenna exhibits multiplexing efciency lower than 2.8 dB over the operating bandwidth which is within the acceptable limit of the MIMO antenna system.Te overall performance of the designed antenna refects that it could be a suitable candidate for UWB applications.
State-of-the-art comparison is presented in Table 2.It is observed that size of an individual antenna has been comparatively minimized about 20 × 30 mm 2 with signifcant gain up to 8 dB.By placing antenna elements orthogonally, the proposed antenna achieves isolation up to 25 dB with signifcant MIMO parameters including ECC, DG, and CCL.International Journal of Antennas and Propagation

Conclusion
Tis study presents a quad-port antenna system for ultrawideband applications.Four modifed circular radiating components positioned orthogonally across the substrate and each supplied by a 50-microstrip line make up the planned antenna.By including a defective ground structure with two triangular slots at the top side borders of the ground plane and two square slots in the ground plane's centre, ultra-wideband response is made possible.By restricting the current fow between the antenna components through orthogonal designs, interport isolation up to 25 dB is accomplished.Each antenna element is printed on a conventional FR4 substrate with dimensions of 20 × 30 × 1.6 mm 3 and a dielectric constant and tangential loss of 4.6 and 0.02, respectively.Te suggested antenna has an impedance bandwidth of 10 dB and covers the frequency range of 3.1 to 12 GHz.Te suggested antenna has considerable radiation characteristics and a peak gain of up to 8 dBi for the operating frequency.Both the measured and the simulated outcomes agreed with one another extremely well.Te ECC < 0.25, DG > 9.9, and 2 3 CCL < 0.2 are within the allowed bounds of variation, which further suggests a strong overall performance in terms of variety.

Figure 1 :
Figure 1: (a) Structure and dimension of single element proposed antenna.(b) Schematic view of the proposed antenna.

Figure 3 :
Figure 3: Design stages of the proposed antenna.

Figure 4 :Figure 5 :Figure 6 :Figure 7 :
Figure 4: Performance analysis of the proposed antenna during design stages.

Table 1 :
Various dimensions of the proposed antenna.

Table 2 :
State-of-the-art comparison of the proposed antenna with the literature.
International Journal of Antennas and Propagation