Dual-port circularly polarized dielectric resonator–based antenna with reconfigurable integrated multifunctional filter for 5G cognitive radio applications

Abstract

In this paper, the design of a circularly polarized (CP) multiple-input–multiple-output (MIMO) antenna system is presented, utilizing a dielectric resonator (DR). This presented antenna system is subsequently integrated with a multifunctional filter, all meticulously structured on a single substrate. The multifunctional filter operates in three modes: reconfigurable band-pass and band-reject filter as well as all-pass filter. The overall structure works as a tunable filtenna. The designed filtenna is expanded into a two-port MIMO system on a unified substrate, providing strong port isolation below −28.5 dB. The overall dimension of proposed radiator is 180 × 180 × 1.6 mm³. The value of peak gain is 5.19 dBic. By switching the states of PIN diodes, the designed filtenna operates as a sensing and communicating antenna for interweave and underlay cognitive radios (CRs). The proposed antenna supports the CP waves within the working band, i.e., 3.6–4.5 GHz. The simulated results are validated by comparing them with the measured results showing less variation among them. MIMO parameters, including the envelope correlation coefficient and diversity gain, have been calculated for the proposed filtenna, representing its suitability for 5G-CR applications.

Introduction

The two frequency bands that can be extensively used in the implementation of 5G communication systems are the sub-6 GHz region and the millimeter-wave band [1]. In the era of 5G systems, fundamental technologies such as multiple-input–multiple-output (MIMO), beamforming, cognitive radio (CR), and edge computing play a crucial role [2]. The usage of MIMO technology instead of conventional single-input–single-output systems is an effective choice to achieve high data rates [3]. In the past, various works have been done on MIMO antennas for achieving enhanced gain, impedance bandwidth as well as isolation among the different ports of the MIMO [4–6]. The CR technology provides better spectrum utilization [7]. In any CR system, there are typically two types of users: primary users (PUs) who hold a license and secondary users (SUs) who do not possess a license. The operations performed by CR are subcategorized as (i) spectrum interweave CR and (ii) spectrum underlay CR. For the interweave CR system, an ultrawideband (UWB) or wideband (WB) antenna acts as a sensing antenna that senses or scans the whole frequency band of interest and identifies the unused portion of the spectrum by the PU. To facilitate communication with the SU and grant them access to utilize the unused spectrum, a narrowband communication antenna is necessary. In the underlay system, both SU and PU can simultaneously access the spectrum, but the signal-to-interference plus noise ratio value of the SU must be below the allowed standard value. The SU must operate in a manner that does not interfere with the PU. If interference occurs, it is imperative to stop the communication of the SU promptly. Therefore, for underlay systems, the UWB or WB sensing antenna should have reconfigurable band-notch characteristics for the communication of SU [8]. In the literature, very few designs were available that work in all the modes of CR, i.e., interweave CR and underlay CR. The design presented in paper [9] works for the interweave CR only and no discussion on underlay operation was given. In paper [10], all the CR operations can be performed simultaneously by all ports but, isolation between the ports was enhanced by using reflectors, so due to extra reflectors overall physical size of the design was increased. The problem of low isolation was improved in paper [11] by properly orienting the ports and placing them at an appropriate distance from each other. But at the same time, the tunability range was reduced for interweave and underlay CR systems. To minimize the abovementioned isolation problem, in paper [12], the authors added two separate ports which further improved the tuning range of the communicating antennas.

All the abovementioned designs were based on microstrip radiators integrated with CR-MIMO functionalities. The main drawback of using a microstrip radiator was low isolation among the ports in specified impedance bandwidth and lower gain after integrating lumped elements like PIN diodes, varactor diodes, etc. Additionally, it is crucial to note that in all the aforementioned designs, circularly polarized (CP) behavior was not explored. CP should be thoroughly investigated and considered in the design process for a comprehensive approach to wireless communication. Therefore, in this research, the proposed antenna is founded on a dielectric resonator (DR) as the primary radiator. The drawbacks listed above could be removed using a dielectric resonator antenna (DRA) integrated with MIMO-CR functionality. In the proposed design, appropriate orientation, and spacing of the antenna elements have been chosen which improved the isolation. To obtain wide impedance as well as CP (<3 dB) bandwidth, a modified C-shaped aperture is taken. Finally, the antenna is integrated with a reconfigurable multipurpose filter and this same proposed structure works for both interweave and underlay showing gain in the range from 3.13to 5.19 dBic.

The workflow of this paper is discussed as follows: The “Two-port WB CP-MIMO antenna design” section describes the design of a single-port-modified C-shaped aperture-fed antenna supporting a frequency range of 3.8–5.05 GHz. Single-port antenna is then extended to a two-port MIMO antenna with 90° rotated elements for better isolation among ports and then its WB CP behavioral analysis is discussed; In the “Design of multifunctional filter” section, a multipurpose reconfigurable filter showing all-pass, band-pass, and band-reject filtering characteristics for CR applications is presented; Further, in the “CP-MIMO antenna and multifunctional filter integration” section, integration of the CP-MIMO antenna with multipurpose reconfigurable filter is depicted; the simulated results are then compared with the measured ones as well as comparison of the proposed work with the other state-of-the-art designs are presented in the “Measurement outcomes” section; finally, the “Conclusion” section concludes the paper.

Two-port WB CP-MIMO antenna design

In this section, the various design steps of a two-port WB CP-MIMO antenna, covering the frequency range from 3.8 to 5.05 GHz within the sub-6 GHz band, are discussed. Figure 1 shows the single-element, dual-port MIMO antenna and proposed reconfigurable CP-MIMO antenna. The presented structure is simulated and analyzed in Ansys HFSS EM simulation software. Initially, a single-port antenna is designed, which involves three steps as shown in Fig. 2. The evaluation of the single-port antenna is discussed in Fig. 2. The optimized dimension is mentioned in Table 1. It shows that Antenna-I is a simple ring slot aperture-coupled fed DRA. In Antenna II, the shape of the aperture remains the same, but the feeding line is altered by incorporating a circular ring at one of the edges of the feedline. Finally, in Antenna III, the circular ring-shaped aperture is further modified while keeping the rest of the antenna structure unchanged, as shown in Fig. 2(c). When we feed the DR-based antenna with a modified C-shaped aperture, then HEM_12δ mode is generated inside the ceramic at 5 GHz. This is because the C-shaped aperture, i.e., feeding structure behaves as a magnetic dipole [13]. The resonant frequency associated with the HEM_12δ mode can be predicted through HEM_11δ mode. The empirical formula of HEM_11δ mode can be calculated using the following equation [14]:

(1)\begin{equation}{{\text{f}}_{r,HEM11\delta }} = \frac{{18.972 \times {{10}^8}}}{{2{{\pi D}}\sqrt {{\varepsilon _{alumina}}} }}\left\{ {0.27 + 0.36\left( {\frac{{\text{D}}}{{2{\text{h}}}}} \right) + 0.02{{\left( {\frac{{\text{D}}}{{2{\text{h}}}}} \right)}^2}} \right\}{\text{ }}\end{equation}

(2)\begin{equation}{{\text{f}}_{r,HEM12\delta }} = 1.3 \times {{\text{f}}_{r,HEM11\delta }}\,\,\,\,\,\,\,\,\,\,\,\,\end{equation}

Figure 1. Geometrical layout of designed MIMO antenna. (a) Top view of single element CP DRA, (b) Perspective view of single-element CP DRA, (c) 2-port CP-MIMO DRA, and (d) Proposed reconfigurable CP-MIMO DRA.

Figure 2. Schematic of different antenna configurations. (a) Antenna I, (b) Antenna II, and (c) Antenna III.

Table 1. Optimized parameters of the proposed filtenna (unit in mm)

The S-parameters and axial ratio curves for all three antenna configurations are depicted in Fig. 3(a) and (b). From Fig. 3(a), it can be said that the maximum impedance bandwidth is achieved in the case of Antenna-III, i.e., 3.8–5.05 GHz. From Fig. 3(b), it can be observed that Antenna-I shows linear polarization characteristics. For the Antenna II configuration, the addition of a circular ring in the feedline serves to further decrease the axial ratio value, though it remains above the 3 dB standard. Therefore, the aperture slot undergoes more modifications, resulting in the Antenna III configuration. By applying modifications in the aperture, orthogonal field components are generated with a phase difference among them creating a wide circular polarization bandwidth. This can be visualized by the electric field (E-field) distribution over the DR. Figure 4(a) and (b) shows the field distribution at ωt = 0° and 90°. It is confirmed from the field distribution that the E-field vector is moving orthogonally at 4.9 GHz. It confirms the CP behavior of the proposed antenna. The modal field variation in the DRA is shown in Fig. 4(c). It shows that the proposed antenna works in HEM_12δ mode. Finally, the two-port MIMO structure is formed by placing the elements anti-parallelly (180° rotated) and giving appropriate spacing between them. This antenna arrangement is shown in Fig. 1(c). The S-parameters of the presented structure are displayed in Fig. 5. In this two-port MIMO design, the ground planes of the two elements are interconnected using a thin metal strip, an essential requirement for a MIMO structure [15]. The two-port MIMO configuration produces good impedance matching giving WB response from 3.8 to 5.05 GHz, which is the same as single-port antenna. The presented structure demonstrates excellent isolation performance, showing values below 28.5 dB. In the MIMO configuration, an overlapping axial ratio bandwidth of 40% is achieved. The optimized parameters of the proposed structure are depicted in Table 1.

Figure 3. Results of Antenna-I, -II, and -III. (a) Reflection coefficient and (b) Axial ratio.

Figure 4. E-field distribution at 4.9 GHz for (a) ωt = 0°, (b) ωt = 90°, and (c) HEM_12δ mode.

Figure 5. S-parameter variation of the dual-port antenna.

Design of multifunctional filter

As it is known, integrating CR capabilities with the MIMO antenna system requires achieving both sensing and communicating functions from a single port. This can be done by introducing a reconfigurable multifunctional filter within the feed line of the antenna elements. If a simple microstrip line is designed then it serves as an all-pass filter [11]. From various literature, it was observed that split ring resonator (SRR) structures work as bandpass filters [16]. Figure 6(a) and (b) shows the circular SRR structure added to the feed line and its S-parameter variation. From Fig. 6, it can be seen that the feed line integrated circular SRR structure shows bandpass behavior within the sensing range of 3.8–5.05 GHz. For designing the SRR, the following equation is used [17]:

(3)\begin{align}{f_r} & =\nonumber \\ & \,\, \frac{1}{{2\pi \sqrt {0.0002\,L\,\left( {2.303\,lo{g_{10}}\frac{{4L}}{{{h_s}}} - 2.451} \right)\left[ {\,\frac{{\left( {3\,{R_{avg}} - g} \right){C_{Pul}}}}{2} + \,\frac{{{\varepsilon _s}{h_s}d}}{{2g}}} \right]} }}\end{align}

Figure 6. Filter design and its results. (a) Layout of multipurpose filter; (b) S-parameter of circular SRR, reflection coefficient in case of (c) all-pass filter and (d) band-reject filter.

The parameters described in the equation are as follows: R _avg, which shows the average value of the two designed split rings; g, indicating the spacing or gap; C _pul, showing the capacitance per unit length; L, calculated as L = 6 * n – g, where n is a given parameter; and d represents the spacing between the inner and outer ring. Figure 6(c) shows the reflection coefficient of the multifunctional filter after applying the five PIN diodes (D₁, D₂, D₃, D₄, D₅). In the proposed filter design, when all the diodes are ON, then it behaves as all pass filter. In case, when all the diodes are OFF, then the filter acts as a band pass filter. In case, when D1, D4, and D5 are ON, and D2 and D3 are OFF, it behaves as a band notch filter, whose S-parameters are depicted in Fig. 6(d).

CP-MIMO antenna and multifunctional filter integration

In this section, the discussion revolves around a multifunctional reconfigurable filter that is integrated with the two-port CP-MIMO DR-based WB antenna. This seamless integration is effortlessly achieved with minimal matching losses, primarily because both the antenna and filter feedlines are maintained at the same dimensions. The overall structure of the proposed two-port CP-MIMO antenna with integrated CR is depicted in Fig. 1(c). Figure 7 shows the reflection coefficient curve of the proposed MIMO antenna when all the diodes are OFF. From Fig. 7, it can be said that the designed antenna works between 3.8 and 5.05 GHz, in case of all diodes are OFF. The radiator is used for sensing purposes. Only a single port is analyzed in this context, given that the design is symmetrical and the same antenna elements are used for the two-port structure. Therefore, similar results will be obtained for both ports. Figure 8(a) and (b) shows the input reflection coefficient variation with different values of reverse bias voltages of the varactor diode in two different scenarios. In case I, all the PIN diodes are OFF, while in case II, D1, D4, and D5 are ON, and D2 and D3 are OFF. From Fig. 8(a), the passband is reconfigured to a higher frequency with an increase in reverse bias voltage. From Fig. 8(b) it can be observed that increasing the reverse bias voltage results in the reconfiguration of band rejection to a higher frequency. In Case I, the antenna operates for CR interweave, whereas in Case II, it works as CR underlay.

Figure 7. Reflection coefficient curve of MIMO antenna when all the PIN diode OFF (sensing case).

Figure 8. Reflection coefficient plot with alteration in reverse bias voltage across varactor diode. (a) Interweave case and (b) Underlay case.

Measurement outcomes

The fabricated prototype of the proposed two-port CP-MIMO antenna with CR feature is shown in Fig. 9. To validate the simulated outcomes, measurements were conducted on the fabricated antenna, and a comparison was made between the simulated and measured results. The S-parameters of the proposed antenna is measured using Agilent VNA E5071C. The simulated and measured S-parameter curve for all three operations of CR are shown in Fig. 10(a–d). It is visible from the graph that there is a slight variation in the measured and simulated outcomes. This could be possibly due to the adhesive used between the ceramic resonator and the FR4 substrate. The use of biasing lines could also be the reason for the same. The proposed antenna can efficiently perform sensing operations in the spectrum 3.8–5.05 GHz. Through appropriate biasing of lumped elements, specifically varactor diodes and PIN diodes, tunability is realized for interweave operation ranging from 3.8 to 4.3 GHz. Conversely, for the underlay mode, the tunable range covers from 3.8 to 4.5 GHz.

Figure 9. Pictures of fabricated antenna. (a) Top view and (b) Bottom view.

Figure 10. S-parameter variation of CP-MIMO DRA (a) |S₁₁| in sensing mode (b) |S₂₁| in sensing mode (c) Interweave mode when V _r = 3 V (d) Underlay mode when V _r = 3 V.

The axial ratio and gain of the presented geometry are displayed in Fig. 11. The axial ratio spans from 3.6 to 4.5 GHz. Also, the measured gain is above 4.0 dBic at the operating frequency band. The calculated radiation efficiency is displayed in Fig. 11(b). It shows more than 80% of radiation efficiency at the working frequency band. Figure 12 measured and simulated left-handed circular polarization (LHPC) and right-hand circular polarization (RHCP) radiation patterns at 4.0 GHz with port-1 and port-2, respectively. From Fig. 12, it is observed that the radiation pattern is broadsided with both the antenna ports, which confirms the HEM_12δ mode inside the cylindrical ceramic. The designed radiator shows LHCP features because LHCP is more dominant as compared to RHCP. Table 2 compares the performance of the designed CP dual-port MIMO DRA for CR applications with other existing MIMO antennas for CR applications. From Table 2, it can be observed that the designed structure has two major advantages over the other designed one: (i) WB CP feature is achieved in sensing mode, which has not been discussed till now for CR applications and (ii) gain value is high as compare to other existing printed MIMO antenna for CR application.

Figure 11. Far-field parameters of the proposed radiator. (a) Gain, (b) Radiation efficiency, and (c) Axial ratio.

Figure 12. Measured and simulated LHCP and RHCP pattern at 4.0 GHz. (a) Port-1 and (b) Port-2.

Table 2. Performance comparison of proposed MIMO radiator with other existing MIMO radiators for CR applications

Further, to analyze the MIMO behavior of the proposed antenna, the envelope correlation coefficient (ECC) and diversity gain (DG) have been listed in Table 3. By observing the values of ECC and DG, it can be said that the ECC value is well below the standard acceptable limit of 0.5 in the whole sensing band which signifies better MIMO behavior. Further, the DG of the proposed antenna is around 10 dB within the sensing range again validating better MIMO performance.

Table 3. Calculated values of ECC and DG of designed MIMO antenna

Conclusion

In this work, a two-port CP-MIMO DRA for 5G-CR applications is designed and analyzed. DR is used as a radiator, for getting two important advantages: (i) isolation improvement (>25 dB) without using any separate structure; and (ii) improved gain (>5.0 dBic) performance in the presence of active elements like diodes. The proposed antenna performs sensing from 3.8 to 5.05 GHz. For communication under interweave CR mode, the bandpass reconfigurability spans from 3.2 to 4.3 GHz. For spectrum underlay, the band reject operation demonstrates tunability ranging from 3.1 to 4.5 GHz. The proposed two-port antenna shows an overlapping axial ratio BW of 40% in the desired frequency range. The proposed two ports MIMO antenna can work efficiently for the midband 5G-CR systems.