Dual ‐ band ‐ independent tunable multiple ‐ input–multiple ‐ output antenna for 4G/5G new radio access network applications

Slot ‐ based dual ‐ band, low ‐ profile frequency reconfigurable (FR) multiple ‐ input–multiple ‐ output (MIMO) antenna design is presented here. Each element is composed of concentric annular slots that are loaded reactively using varactor diodes. Accurate placement of diodes and optimal biasing circuit configuration result in complete control on each of the tuned bands. The unique feature of the proposed work is the independent as well as concurrent tuning of each band over a wide frequency range from 1.7 to 3.8 GHz. Moreover, the competitive advantage of independent tuning and its narrow ‐ band (NB) operation may result in better power management for RF front ‐ end devices. The proposed antenna is fabricated using an RO ‐ 4350 substrate with board dimensions 60 � 120 � 0.76 mm 3 . The proposed dual ‐ band compact antenna design is suitable to be utilized in the current fourth generation (4G) wireless standards as well as for upcoming fifth generation (5G) new radio access networks (RANs) with sub 6 ‐


| INTRODUCTION
The fifth generation (5G) wireless technology will provide a very high data rate, low latency, enhanced energy and spectral efficiencies and reliable communication systems. The early stage of 5G commercialization for both sub-6 GHz along with mmwave bands will ensure a smooth transition to 5G by utilizing the existing standards. This will lead to the use of the sub 6-GHz bands to support and enhance the mobile broadband along with existing 4G sites that can be reused for 5G communications [1]. The new 5G radio access networks (RANs) will support multiple-input-multiple-output (MIMO) antenna systems and hence it is required to use multi-band antenna design with independent or concurrent tuning capabilities for frequency agile radio systems [2]. The most common challenges for sub-6 GHz antenna designs are the large bandwidth requirements, multi-band antenna designs and MIMO implementation with maximum antenna elements within the given space. Hence, the frequency reconfigurable (FR) antennas with concurrent and independent tuning along with multi-band operation with multiple connection and very wide sweep are highly desirable for 5G sub 6-GHz communication systems Various types of antenna designs were reported in the literature with continuous frequency sweep including dipoles [3][4][5], monopoles [6][7][8] and patch [9,10] antenna designs. Various slot-based antenna designs have been reported in the literature with continuous frequency and multi-band operation [11][12][13][14][15][16][17][18][19][20]. However, slot based antenna design is preferable because of the numerous advantages it offer. Slot-based antennas are usually compact planar designs, have the flexibility to be integrated with other system components, are potentially suitable for FR operation, capable to be tuned over a wideband.
In Ref. [11], a slot-based FR antenna was presented with dual band operation. Dual-resonances with reactive loading helped in bringing down the higher order bands to lower frequency range. Varactor diodes were used with capacitance values of 0.5-2.25 pF. The corresponding frequency sweep obtained was from 1.3 to 2.67. The overall antenna size was 110 � 150 � 0.5 mm 3 . In Ref. [12], a dual-band varactor loaded FR slot antenna was presented. The resonating bands obtained were 2.45 GHz and a wide band that covered GPS standards of 1.227/1.381/1.575 GHz. The antenna had a board dimension of 30 � 70 mm 2 . Another slot-based antenna with wide tuning was presented in Ref. [13]. A continuous frequency sweep was obtained from 0.42 to 1.48 GHz. A combination of PIN and varactor diodes was utilized to achieve FR operation. PIN diode was used for mode selection while continuous resonating bands were achieved using varactor diodes. A wide slot with footprint 1.5 � 20 mm 2 was etched out from the ground (GND) plane with its volume 35 � 20 � 0.5 mm 3 . The antenna presented in Ref. [11][12][13] are single element designs with either limited tuning capabilities or having antenna size with large board dimensions.
Slot-based MIMO antenna design with 4-antenna element was presented in Ref. [14,15] with wide tuning capabilities. In Ref. [14], 4-element slot antenna design was presented with dimensions 60 � 120 � 1.56 mm 3 . The triband antenna covered frequency bands: 1.32-1.49, 1.75-2.9 and 2.9-5.2 GHz. Similarly in Ref. [15], a single annular slot was used to obtain a frequency sweep from 1.8 to 2.5 GHz using varactor diodes. The 4-element was etched out from the GND plan dimensions of 60 � 120 mm 2 . Numerous antenna designs with independent tuning capabilities have been reported in the literature [16][17][18][19][20]. In these designs, an independent tuning of resonating bands was made possible using varactor diodes. The various important features of the proposed antenna designs are compared with the state of the art literature as given in Table-1. This includes single element dimensions, antenna size, frequency bands covered, number of diodes used per antenna element, independent and concurrent tuning capabilities along with MIMO operation. It can be seen that the proposed antenna performed better than the most relevant antenna designs being discussed in Ref [14][15][16][17][18][19][20][21].
This work presented a dual-band concentric annular slots antenna design with its flexibility to control both bands. The unique features included its independent and concurrent tuning capability across the dual-band operation, using a very simple biasing network. Thus, any combination of dual-band operation can be guaranteed over a frequency range from 1.7 to 3.8 GHz with a very compact size. The dual-band covered was from 1.7 to 2.4 GHz and 2.4 to 3.8 GHz, respectively. Thus, the complete control and flexibility of operation over the dual-band make it a suitable candidate for current 4G wireless networks and next generation 5G new RNA networks using cognitive radio (CR) techniques. Moreover, the competitive advantage of independent tuning capabilities with narrow-band (NB) operation ensure better power management in 5G communication along with stable antenna operation. The proposed antenna design was realized on board volume of 60 � 120 � 0.76 mm 3 with single element footprint of 11 � 11 mm 2 . To summarize, the proposed antenna design having independent and concurrent tuning with its planar structure is best suited for 5G new-RAN in sub-6 GHz band with very wide tuning capabilities.

| DESIGN DETAILS
This section describes in details the antenna's geometry, theory behind antenna operation with circuit model analysis and step by step antenna design procedure.

| Antenna geometry
The antenna geometry and circuit configuration of the 2-element concentric annular slot MIMO antenna design is shown in Figure 1(a,b). It consisted of two concentric annular slots-based FR MIMO antenna design on the ground (GND) plane. The antenna prototype was developed on RO4350 substrate board having ε r ¼ 3.48 and thickness of 0.76 mm using an LPKF S103 machine. Both inner and outer concentric slots were strategically placed on the outer edges of the GND plane with radii of 8.75 and 11.49 mm, respectively. The width of each slot was 0.5 mm. The dimensions and placement of both annular slots were optimized to obtain better MIMO performance, improved input impedance matching Z in over the given bands of interest. Both slots were excited using the same microstrip line that was placed on the opposite side of the slot structure. The top and bottom layers of the given antenna prototype are shown in Figure 2 (a,b) respectively. The top layer of the fabricated board has varactor diodes loading the slot structure, its biasing circuitry and microstrip feed-lines. The varactor diodes used were SMV 1231. In the given biasing circuitry, the varactor diode was placed in series with RF chokes (L 1 and L 2 ) and current limiting resistors (R 1 and R 2 ). The shorting posts were utilized to connect the circuit elements between top and bottom layers.

| Antenna operation
The slot antenna with short circuited structure at both ends can be modelled as λ/2 transmission line, corresponding to its fundamental resonance frequency [22]. The fundamental resonance frequency of the closed slot structure is given by: where c is the speed of light, ε r is the relative permittivity, f r is the fundamental resonance frequency of annular slot antenna and r 1 and r 2 are slots radii as shown in Figure 1(b). The term 0.5π(r 2 þ r 1 ) represents the mean circumference of circular slot-line structure [23]. Both annular slot structures were reactively loaded using varactor diodes. The given capacitance values increased the slot-line capacitance at a given point, thus resulting in bringing down the fundamental resonance frequency along with higher order resonance frequencies to lower bands. The reactive loading is a non-uniform operation that can be determined by using the varactor diodes location (L 1 ), its capacitance C v and impedance (Z o ) of the slot line structure. The transmission line equivalent circuit model of the slot antenna can be utilized to calculate the resonance frequency as given in Ref. [11]: where β is the propagation constant and it depends on the frequency of operation, C v and ω are the reverse biased varactor capacitance and angular frequency of operation, respectively. The resonance frequency of reactively loaded slot antenna can be determined numerically solving Equation (2). An equivalent circuit model of the concentric slots antenna was also investigated to understand the antenna's operation as shown in Figure 3. Figure 3(a) shows the series combination of a microstrip feed-line (series L f C f circuit) with a parallel combination of an RLC circuit representing an annular slot while that of concentric slots with feed-line are shown in Figure 3(b). The circuit diagram is helpful to understand the frequency agility, dual-band operation and input impedance matching at lower frequency bands. Such analysis is helpful to design other multi-bands antennas for desired frequency bands with the frequency reconfigurability.
The varactor diode can be modelled as a parallel combination an ideal diode D v and a capacitor C v , that is, series with L v as shown in Figure 3(c). The equivalent circuit model of complete antenna design is shown in Figure 3(d) [23]. The equivalent circuit model and its analysis are provided to understand the equivalent circuit parameters, antenna's multi-band operation, reactive loading of the slot and FR operation. The circuit element values of the equivalent design and its reactive loading parts can be extracted using ADS as given in Ref. [24].

| Antenna design procedure
The primary objective of this work is to design a compact size, dual-band antenna design with independent and 302 -HUSSAIN concurrent tuning capabilities for existing 4G standards as well as for 5G new RANs. Such antenna diversity is highly required for RF spectrum with 4G/5G integration. The proposed 5G radio access technology is suitable to be utilized for multiple concurrent connections using CR techniques.
The design procedure of this work started with a simple annular slot structure with arbitrary dimensions and fed with 50 Ω microstrip feed-line. The antenna with larger and smaller slots without any reactive loading, were operating at sub-4 GHz and sub-5 GHz, respectively. The width of each slot as well as the distance between them was optimized to tune the antenna to cover maximum bands of 4G and 5G wireless technologies. To bring down the resonating bands to lower frequencies as well as to obtain a continuous sweep of frequencies, two varactor diodes per antenna element were used.
The precise location and placement of the varactor diodes is crucial as it determines the resonating bands, better input impedance (Z in ) matching as well as frequency sweep across wide-band. Various locations of the varactor diode had different effects on the slot Z in matching. To obtain the optimum Z in matching, several parametric analyses were performed to carefully place the varactor diodes on the slot structure. The final optimized antenna's dimensions and diodes placement resulted in continuous frequency sweep from 1.7 to 3.8 GHz and thus covered several wireless bands of sub-6 GHz new RANs.

| SIMULATION AND MEASUREMENT RESULTS
The given antenna design was modelled and simulated using HFSS TM . The scattering parameters as well as the gain patterns and efficiency (%η) values were measured.

| MIMO antenna scattering parameters
The simulated and measured S-parameters of the proposed antenna design are given in Figures 4 and 5. From the S-curves of dual-band antenna design, the concurrent and independent tuning across any band was successfully demonstrated. The biasing circuit is optimized in such a way that same voltage can be applied at both þV terminal of the varactor diodes of each antenna element for concurrent operation, while different voltages were applied to get the independent tuning of the two bands .To demonstrate the independent tuning for each band, the capacitances (C in ) of varactor diodes (D 1 and D 3 ) are constant for inner slots, while the capacitances (C out ) for varactor diodes (D 2 and D 4 ) which varied from 0.466 to 2.35 pF with corresponding reverse bias voltage from 10 to 0 V. Figure 4(a,b) shows the simulated and measured S 11 curves for (C in ) of 0.466 pF (10 V). The upper band was forced to be tuned at 3.6 GHz, while the lower resonating band varied from 1.7 to 2.5 GHz. Similarly, Figure 5(a,b) shows the simulated and measured S 11 curves for (C in ¼ 0.970 and 2.35 pF) with corresponding reverse bias voltage of 3 and 0 V, respectively. A continuous frequency sweep of the upper band was observed with controlled reverse bias voltage from 2.4 to 3.8 GHz by varying the capacitance of varactor diodes. The minimum À 6 dB measured bandwidth (BW) of 125 MHz was observed over entire bands of operation.
F I G U R E 5 S-curves (a) simulated |s 11 | at C in ¼ 2.35 pF; (b) measured |s 11 | at V ¼ 0 V; (c) simulated |s 12 | at C in ¼ 0.97 pF; (d) measured |s 12 The mutual coupling S 12 between adjacent elements was also evaluated. The worst case values observed with and without DGS structure were 18 and 15.7 dB, respectively. Figure 5(c,d) shows the S 12 for a capacitance value of C in ¼ 0.97 pF and a reverse bias voltage of 3 V, respectively. Similar curves were also observed for other capacitance values.

| Current distribution
To analyse the mutual coupling between closely spaced antenna elements, the current density distribution of the given slot-line structure was analysed at the lowest operating band of 1.73 GHz as it is critical to consider. Two important parameters were investigated using this phenomenon. The length of the radiating slot structure can be mapped to determine the effective electrical length of the resonating bands as well as the mutual coupling between antenna elements. Figure 6 shows the current density for the capacitive loading, C out ¼ 2.35 pF. This resulted in antenna operation at a resonating band of f o ¼ 1.73 GHz. Figure 6(a,b) shows the current density distribution with and without the DGS structure. From Figure 6 (a), a considerable coupling has been observed between two ports. A high mutual coupling was observed at lower frequency bands due to the small electrical distance between the two antenna elements. Hence, it is important to analyse it at lower bands. However, insignificant mutual coupling was observed at higher frequency bands. This is the reason for the analysis to be performed at the lowest band. Figure 6(b) shows that using DGS structure had significantly reduced the coupling between two antenna elements. Such analysis can be performed to design an effective DGS and to reduce the coupling between the closely spaced antenna elements.

| Radiation characteristics
The proposed MIMO antenna design was also characterized for its radiation characteristics. The peak gain (PG) and efficiency (%η) values were evaluated at different frequency bands. For each measurement, a single antenna element was observed, while the second port was terminated with 50 Ω load. The two-dimensional (2D) gain patterns are shown in Figure 7 along with anechoic chamber measurement setup as shown in Figure 7(e). The total gain patterns for the given MIMO antenna element, Ant-1 and Ant-2, at 2.2 GHz are shown in Figure 7(a,b), respectively. The curves are θ-cut at ϕ ¼ 0°and ϕ ¼ 90°. Similarly, the 2D curves for Ant-1 and Ant-2 at 3.6 GHz are shown in Figure 7(c,d), respectively.
For the proposed design, a high co-pol to cross-pol ratio was observed and hence we had obtained good polarization purity across all the bands. All the co-pol and cross-pol patterns for the antenna elements (Ant-1 and Ant-2) at frequency bands of 2.2 and 3.6 GHz are shown in Figure 8 (a-d), respectively. It has been observed that all the antenna elements exhibit good polarization purity at the other bands as well.   Table 2. It has been observed that the ECC values are less than 0.5 over the entire bands of operation and hence good MIMO operation is obtained. The proposed design is working as a MIMO antenna and hence both the elements are operating at the same frequency band. The performance of MIMO is improved in a multi-path fading environment and hence it is highly desirable for signals to travel different paths before reaching the receiving antenna to enhance the MIMO diversity performance.

| Analysis of MIMO parameters
It is important to properly characterize the antenna for MIMO performance metrics. The most important parameter is the envelope correlation coefficient (ρ e ). It determines the field coupling between various correlated channels using radiation patterns. The ρ e value is required to be less than 0.5 for better MIMO operation [25]. For the given MIMO antenna design, ρ e is computed for both simulated and measured patterns. All the values with corresponding frequencies are given in Table 1. All the ρ e values over the entire band are well justified the proper MIMO operation of the proposed antenna design. Moreover, ECC, PG and efficiency (%η) values were also computed as shown in Table 2. It has been observed that the ECC values are less than 0.5 over the entire bands of operation and hence good MIMO operation is obtained.
Total active reflection coefficient (TARC) is another critical MIMO parameter for MIMO characterization [25] It is the measure of determine the effective BW MIMO antenna designs. For the given antenna design, the TARC curves were obtained by exciting first port at 1 j0 with zero phase. Various excitation phases were used for second port with variation from 0°, 30°, 60°, 90°, 150°and 180°for Cases 1-5, respectively. The combination of these input phases are given below: The TARC curves of the given antenna design are shown in Figure 9(a-d) show the TARC curves for C ¼ 0.84, 0.92, 2.09 and 5.08 pF, respectively. From the given TARC curves, it is evident that the operating BW of given MIMO antenna design is robust and variation of different excitation phases have minimal effect on it.

| CONCLUSIONS
In this work, a concentric annual slot-based 2-element dualband FR MIMO antenna is presented. The independent and concurrent tuning of each band over a wide frequency range The proposed antenna is suitable to be utilized in the current 4G wireless standards as well as for the upcoming 5G with sub 6-GHz bands of operation for new RANs using CR techniques.