Multifunctional twelve port frequency agile diversity antenna for indoor wireless applications

The recent resurgence of new-generation reconfigurable technologies delivers a plethora of various applications in all public, private and enterprise solutions over the globe. In this paper, a frequency reconfigurable polarization and pattern diverse Multiple-Input-Multiple-Output (MIMO) antenna is presented for indoor scenarios. The MIMO antenna is comprised of twelve radiating elements, and polarization and pattern diversity is obtained by arranging them in three different planes: Horizontal Plane (HP), Vertical Plane-I (VP-I), and Vertical Plane-II (VP-II). The proposed antenna operates in mode I (wideband) and mode II (multiband), by combining two different radiators using PIN diodes. The antenna dynamically switches between Mode I (wideband) and mode II (multiband). Mode, I cover the ultra-wideband (UWB) range from 2.3 to 12 GHz, while mode II covers GSM (1.85–1.9 GHz), Wi-Fi and LTE-7 (2.419–2.96 GHz), 5G (3.15–3.28 GHz and 3.45–3.57 GHz), public safety WLAN (4.817–4.94 GHz), and WLAN (5.11–5.4 GHz) frequency bands. The peak gain and efficiency of the MIMO antenna are 5.2 dBi and 80%, respectively.

Radiator I. The radiator-I demonstrates the UWB (mode I) operation of the proposed antenna element. Figure 2 depicts the evolution of the radiator-I and reflection coefficients during the development stages. The evolution begins with an elliptical-shaped radiator (stage I), which is fed by a microstrip line of 50 Ω, and a partial ground plane. In stage II, a rectangular patch is loaded on top of the elliptical radiator to achieve a wide impedance bandwidth. In stage III, a square slot is etched out from the top edge of the radiator to increase the current length, thereby covering the lower region frequencies and thus the UWB frequency range. Finally, in stage IV, a stub is added to the ground plane of the monopole antenna to improve impedance matching, and a diode D1 is integrated at the feed line with a biasing circuit, as shown in the Fig. 1a. The proposed modified U-shaped radiator resonates over the UWB range of 2.3 to 12 GHz.
Radiator II. The radiator-II demonstrates the multiband (mode II) operation of the proposed antenna element. The evolution of the radiator-II and reflection coefficients during the development stages are depicted in the Fig. 2a,b. The development of radiator-II begins with an L-shaped Meandering Resonator (LMR) (stage I) that resonates from 5.11 to 5.48 GHz. In stage II, the LMR is modified into an F-shaped Meandering Resonator (FMR) to achieve resonance at 2.4 GHz (2.419-2.96 GHz). In stage III, a Meandered Open-Ended Resonator (MOER) is integrated with the FMR to achieve resonance in the 1.85-1.9 GHz frequency range. Furthermore, in stage IV, an Open-Ended Outer Resonator (OEOR), which resonates at 3.5 GHz, is integrated with the stage III radiator. Also, the gap between the LMR, FMR, MOER, and OEOR is optimized to obtain an additional resonance at 4.9 GHz.
The proposed multiband radiator offers hexa-band resonance, covering a wide range of wireless applications such as 2G, 4G, 5G, Wi-Fi, Public safety WLAN, and WLAN. The proposed antenna element achieves dual functionality by integrating the diode D2. When diode D1 is forward biased and diode D2 is reverse biased, the antenna will radiate over the UWB spectrum. On the other hand, when diode D2 is forward biased and diode D1 is reverse biased, the antenna switches to mode II and radiates over the six bands. The current density plots of radiator I and radiator II are presented in Fig. 3A,B respectively to understand the resonating nature of the antenna.
The equivalent circuit of the proposed antenna element is presented in Fig. 4. The lumped parameters are interpreted by predicting the type of RLC circuit, either parallel or series, based on the impedance characteristics www.nature.com/scientificreports/ of the antenna as in 29 . The three parallel resonant circuits connected in series correspond to the UWB radiator and the six series resonant circuits connected in parallel correspond to the multiband radiator. The reflection characteristics of the equivalent circuit are shown in Fig. 5 for both modes.
Twelve port IMA. MIMO/diversity 30 techniques are becoming increasingly important in addressing the multipath fading effects that degrade signal quality. The probability of signal fading is higher in indoor scenarios due to the existence of multiple obstructions. Hence, the polarization of the signal may change, lowering the signal quality while receiving it. In such a situation, polarization-diverse antennas are strongly recommended to avoid polarization mismatches and increase link reliability. In this work, a twelve-port IMA is proposed with multiple polarization vectors to encounter signal losses. The schematic of the MIMO antenna is shown in Fig. 6a, and its fabricated prototype is shown in Fig. 6b. The proposed MIMO antenna consists of twelve monopole antenna elements arranged in three different planes: Horizontal Plane (HP), Vertical Plane-I (VP-I), and Vertical Plane-II (VP-II). In HP, the four resonating elements are arranged orthogonal to each other to achieve both horizontal and vertical polarization. Whereas in VP-I and VP-II, four resonating elements are interlocked in a cross-shape pattern to achieve vertical polarization. Further, the VP-I and VP-II are interlocked with the HP. However, the antenna elements in VP-I and VP-II are perpendicular to the antenna elements in HP. This particular arrangement helps in the generation

Results and discussion
The following subsection presents the reflection, coupling and gain characteristics of the twelve-port IMA.
Reflection coefficients. Figure 7a,b display the measured and simulated reflection coefficients of the proposed MIMO antenna in mode I and mode II, respectively. In mode I, the antenna achieves an impedance bandwidth of 135% (2.3-12 GHz) over the UWB spectrum. In mode II, the antenna resonates at six bands centered at 1.88 GHz, 2.47 GHz, 2.5 GHz, 3.24 GHz, 3.5 GHz, 4.9 GHz, and 5.25 GHz, with bandwidths of 2.66%, 20.4%, 1.53%, 4.3%, 3.61%, and 7.16%, respectively.
Mutual coupling. In the proposed MIMO antenna, the four radiators are arranged orthogonally in the HP, and the remaining eight elements are placed in VP-I and VP-II. The inter-element spacing is kept as 0.16λ 0 , where λ 0 is calculated at the lowest operating frequency. In both UWB and multiband modes, the proposed antenna achieves isolation greater than 14 dB, as shown in Fig. 8a,b.
Antenna gain and efficiency. Figure 9 depicts the gain and efficiency of the proposed antenna. In mode I, the antenna exhibits a peak gain of 5.2 dBi and an efficiency of 80%, as shown in Fig. 9a   www.nature.com/scientificreports/ Radiation characteristics. The radiation characteristics of the proposed twelve-port diversity antenna are measured in an anechoic chamber. Figure 10 represents the antenna radiation characteristics in mode I. The radiation characteristics for port-1, port-2, port-3, port-4, port-5, and port-12 are plotted at 3 GHz, 6 GHz, and 10 GHz frequencies. Similarly, the radiation patterns for mode II are evaluated and depicted in Fig. 11. The discrepancies in radiation patterns are due to loss introduced by bias lines.  Envelope correlation coefficient (ρ e ). ECC 31 is calculated using the far-field radiation characteristics of the antenna by Eq. (1).
where F denotes the radiated field between the two antenna elements, θ is the angle of elevation, φ is the azimuthal angle, and Ω is the solid angle. The ECCs for antenna elements-2, -5, and -12 with respect to antenna element-1 are calculated for different frequencies in both modes I (3 GHz, 4 GHz, 6 GHz, 8 GHz, and 10 GHz) and II (1.88 GHz, 2.47 GHz, 3.24 GHz, 3.5 GHz, 4.9 GHz, and 5.25 GHz) and are shown in Tables 1, 2, 3, 4, 5, 6. The ECC should be 0 in the ideal case, but practically, values up to 0.5 are acceptable 29 . In all cases, the proposed MIMO/diversity antenna achieves ECC < 0.1.

Apparent diversity gain, effective diversity gain and mean effective gain. Another important
parameter to consider is Apparent Diversity Gain (ADG), which measures link reliability and is calculated using Eq. (3).  www.nature.com/scientificreports/ The EDG values for mode I and mode II at ports-2, -5, and -12 with respect to port-1 are presented in Tables 1,  2, 3, 4, 5, 6. The EDG is lower than the ADG as it takes radiation losses into account. Another important MIMO parameter is the MEG ratio, which determines the average amount of power received by the antenna in a multipath fading environment, and it can be calculated using Eq. (7).
The MEG values for mode I and mode II at ports-2, -5, and -12 with respect to port-1 are presented in Tables 1, 2, 3, 4, 5, 6. It is found that the diversity performance metrics are within practical limits, confirming that the proposed MIMO antenna is a good candidate for wireless indoor scenarios. Figures 12 and 13 represent the MEG characteristics of the proposed antenna for both modes, and it is found to be less than 2 dB. Total active reflection coefficient and channel capacity loss. TARC and CCL 32 characterize the frequency, bandwidth, and radiation capability of multiport antennas and can be calculated using Eqs. (8) and (9) Figure 11. (continued) Table 1. Diversity performance of the antenna in HP: for mode-I (between port-1 and port-2).  Table 2. Diversity performance of the antenna in VP-I: for mode-I (between port-1 and port-5). www.nature.com/scientificreports/ Table 3. Diversity performance of the antenna in VP-II: for mode-I (between port-1 and port-12).  Table 4. Diversity performance of the antenna in HP: for mode-II (between port-1 and port-2).  Table 5. Diversity performance of the antenna in VP-I: for mode-II (between port-1 and port-5).  Table 6. Diversity performance of the antenna in VP-II: for mode-II (between port-1 and port-12).   Tables 1, 2, 3, 4, 5, 6 respectively. In both cases, the antenna achieves TARC less than − 13 dB. The antenna CCL is less than 0.2 bits/s/Hz in mode I and less than 0.25 bits/s/Hz in mode II, which is less than the acceptable limit of 0.4 bits/s/Hz. Figures 14, 15, 16, 17 represent the TARC and CCL characteristics under different modes.   Table 7 compares the performance of the proposed MIMO antenna with the existing antenna designs. The proposed antenna is smaller in size, has more resonating bands, and offers frequency agility due to the use of PIN diodes and polarization and pattern diversity. The salient features of the proposed antenna configuration are:

f (GHz) ECC Isolation (dB) ADG (dB) EDG (dB) MEG TARC (dB) CCL (bits/s/Hz)
• The antenna integrates two radiators in a small size of 26 mm × 26 mm, as compared to 10,19,20 , without any performance degradation. • The antenna supports a wide range of applications, including UWB, GSM, Wi-Fi, LTE-7, 5G, Public safety WLAN, and WLAN than [10][11][12][13][14][15][16][17][18][19][20] to provide high-speed communication without latency.  (a) (b) Figure 16. CCL characteristics at (w.r.t. other ports for mode-I: wideband) (a) port-1, (b) port-12.  www.nature.com/scientificreports/ • The antenna also offers frequency agility, which helps to reduce interferences by turning ON and OFF the respective switches between UWB and multiple bands based on the user's requirements, as compared to [10][11][12][14][15][16]18,19 . • The MIMO set achieves greater than 14 dB isolation without the use of complex decoupling structures, and the twelve radiating elements are oriented in a 3-D fashion, with a size of 62 × 62 mm 2 and inter-element spacing of 0.16 λ 0 . • The proposed MIMO orientation helps in obtaining quad polarization, as compared to 10,12 , which suppress polarization mismatches in a highly scattering environment, thereby avoiding signal loss. • The unique arrangements of antenna elements help in attaining multiple polarization vectors and pattern diversity with uncorrelated beams in both azimuthal and elevation planes. • The antenna offers ECC < 0.08, TARC less than − 13 dB, and CCL < 0.25 bits/s/Hz, resulting in good diversity performance, as compared to 16,18,20 . • The antenna's radiation efficiency is maintained at ~ 70 to 80% as the diodes are integrated with the transmission line without disturbing the radiator, reducing radiation losses. • Thus, the proposed multiport antenna achieves spectrum efficiency and possesses better diversity characteristics to resolve connectivity issues in highly scattering environments.

Conclusions
A multifunctional twelve-port polarization diversity antenna is presented with high link reliability, better connectivity, and a high data rate in ultra-dense scattering environments. The antenna offers a wide impedance bandwidth and stable radiation characteristics in both UWB and multiband modes. The antenna also offers multiple polarization vectors to avoid fading and cross-polarization. The diversity performance of the MIMO antenna is validated by measuring the ECC, TARC, and CCL. The proposed antenna could be useful for indoor wireless network communication scenarios such as smart buildings, smart factories, airports, and shopping malls to obtain high-speed communication.

Data availability
The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.