D ESIGN OF M ICROSTRIP D UAL - BAND B ANDPASS F ILTER FOR S UB -6 GH Z 5G M OBILE C OMMUNICATIONS

In this paper, a microstrip bandpass filter for dual-band sub-6 GHz 5G mobile communications is designed. The two first resonance frequencies of the stepped-impedance resonator are used as the operating frequencies of the two passbands. Furthermore, the stepped-impedance resonator is folded to form an open-loop stepped-impedance resonator for compactness. This form of the resonator generates a second transmission zero in the upper stopband, which improves the out-of-band rejection. A 50 Ω tapped-line input/output is used to feed the filter. The proposed structure is designed, analyzed and manufactured and the measured results are found to be in good agreement with the simulation results. From the measured results, it is found that the proposed filter achieved a return loss of 21.5 dB and 28.3 dB, an insertion loss of 0.4 dB and 1.7 dB and a bandwidth of 12.5% and 10.81% at 3.61 GHz and 5.55 GHz, respectively. In addition, the proposed filter has a compact size, which makes it suitable for sub-6 GHz 5G mobile communications.


INTRODUCTION
The growing demand for mobile-phone users, connected devices and the proliferation of applications requiring high data rates have led to jump to the fifth generation (5G).Compared with 4G, the 5G will enable to deliver higher data rates: 1000 times faster than 4G [1].Many countries have started to deploy 5G networks and have awarded it frequency bands.In sub-6 GHz spectrum, the 3.6 GHz band (3400-3800 MHz) is allocated by many countries for 5G mobile communications, such as the European Union and the United Kingdom [2].Besides that, the unlicensed band (5150-5925 MHz) has also been added to the 5G spectrum to support the existing licensed 5G bands.Therefore, designing microwave components, such as bandpass filters (BPFs) with compact size and good electrical performance operating in these two bands, is necessary to meet the requirements of 5G.
In literature, numerous technologies have been used to design BPFs for sub-6 GHz applications [3]- [13].Among these, dual-band BPFs using substrate-integrated waveguides (SIWs) are reported in [3] and [4], the size of the filter in [3] is relatively large, while the filter in [4] provides a low return loss.A CPW BPF based on spiral-shaped DGSs is designed to operate at 3.54 GHz in [5] and its 3-dB bandwidth is ranging from 3.29 GHz to 3.79 GHz with an insertion loss less than 2 dB over the passband.In [6], a dual-band BPF consisted of folded open-circuited stubs with interdigital unit cells is proposed, but its circuit size is large.In [7], a dual-band BPF is designed using substrate-integrated suspended line (SISL) technology to operate at 3.45/4.9GHz.The two passbands are formed by using quarter-wavelength stepped-impedance resonators (QSIRs) and half-wavelength hairpin resonators (HWHRs).In [8], a dualband BPF using a dual-mode dielectric waveguide resonator operates at 2.1/3.53GHz with a fractional bandwidth of 2.27/1.67% is reported.A dual-band BPF with interlocked stepped-impedance resonators operating at 0.946/1.48GHz (SIRs) is presented in [9].The filter has a compact size, but the insertion loss is relatively high.A wideband bandpass spatial filter using a double square loop frequency-selective surface (DSLFSS) is proposed in [10].However, all these reported BPFs will work only for the 5G licensed bands and to the authors' knowledge, there is no dual-band BPF that covers both the licensed and unlicensed 5G bands in the literature.
In this paper, a low-cost microstrip dual-band BPF using two stepped-impedance resonators (SIRs) for 5G mobile communications is designed to cover the licensed 5G band 3600-3800 MHz and the unlicensed 5G band 5150-5925 MHz.The two first resonant mode frequencies of the SIR are used as the center frequencies of the first and second passbands of the filter.In addition, to further reduce the size of the circuit, SIRs are folded to form open-loop SIRs.By choosing the appropriate value of the gap between the edges of the high-impedance lines, a second transmission zero is generated in the upper stopband, which helps to improve the rejection level of the filter.The proposed dual-band BPF is designed, simulated and manufactured.The results of simulation and those of experiment are in good agreement.Moreover, the experiment results are compared with existing 5G filters.
The organization of the paper is as follows: In Section 2, resonance properties of the SIR and open-loop SIR are discussed based on the even-odd mode analysis and the design procedure of the proposed dualband BPF is presented.The experimental validation is shown in Section 3. Finally, conclusions drawn are given in Section 4.

MICROSTRIP DESIGN OF DUAL-BAND BPF 2.1 Analysis of the Typical SIR and Open-Loop SIR
In principle, the typical λ/2 SIR is composed of a λ/4 low-impedance line (Z1, 2θ1) and two-high λ/8 impedance lines at both ends (Z2, θ2), as depicted in Figure 1.Given that the structure is symmetrical, even-and odd-mode analysis can be utilized to describe the resonator.The even-and odd-mode equivalent circuits of the typical SIR are shown in the same figure.For simplicity, we choose θ = θ1= θ2.The input impedance can be expressed for the odd mode [13] as: where RZ is the ratio between the high-and low-impedance lines.By imposing zero input admittance of the odd mode (Y in,odd = 1/Z in,odd ), the first and the third resonant mode frequencies of the SIR can be written as: Similarly, for the even mode, the input impedance can be derived as [13]: The second and the fourth even-mode resonance frequencies can be deduced from Equation ( 4) by imposing zero input admittance of the even mode: (5) For miniaturization, the typical SIR shown in Figure 1 is folded to form an open-loop SIR, as depicted in Figure 2 with its even-and odd-mode equivalent circuits.Here, C models the electric coupling capacitance between the edges of the high-impedance lines.With the presence of this capacitance on the odd-mode equivalent circuit, the total electrical lengths at the odd-mode resonance frequencies increase, while in the even mode, these lengths are kept constant (the even-mode equivalent circuit for the openloop SIR is the same as that of the typical SIR).Therefore, the resonance frequencies of the open-loop SIR f1 and f3 shift down to the lower frequencies.To visualize the change of the magnitude S21 of the filter with different couplings, two simulations are displayed in Figure 4.It can be seen from the left part of the figure that, for a specific value of the gap between the edges of the high-impedance lines and besides the existing transmission zeros in the upper stopband, another transmission zero starts to appear and shifts to the upper frequencies when S1 increases.

BPF Design
In addition to that, the rejection level is enhanced in the upper stopband when the two transmission zeros are closer to each other.From the right part of the figure, it is clearly observed that the separation between resonators determines the bandwidth of the filter.As S2 decreases, the bandwidth of the filter increases and the rejection level in the lower and upper stopband decreases.It should be noted that the bandwidth of the filter is also controlled by the external quality factor of the filter which is determined by the position of the tapped input and output lines.Therefore, by choosing the appropriate position of the tapped lines and adjusting the separation between resonators, the desired bandwidth can be achieved.
After optimization, the dimensions of the BPF are determined, as listed in

IMPLEMENTATION AND RESULTS
The filter designed above has been fabricated on the same substrate and measured using VNA Master MS 2028C.Its fabricated prototype and measurement equipment are presented in Figure 7.   Performance comparisons of the presented dual-band BPF with filters designed in other previous works are listed in Table 2 showing that the proposed filter has good electrical performance in term of insertion and return losses, as well as compact size.

CONCLUSIONS
In this paper, open-loop SIRs with 50 Ω tapped input/output lines have been proposed to build dual-band BPF for licensed and unlicensed 5G bands in sub-6 GHz spectrum for mobile communications.Folding the two SIRs and choosing the appropriate value of the gap between the edges of the high-impedance lines help to create another transmission zero in the upper stopband.The proposed dual-band BPF is simulated and manufactured.The experimental results reveal 12.5% and 10.81% fractional bandwidths at 3.61 GHz and 5.55 GHz, respectively.The upper stopband extends more than 8 GHz with 19.8 dB rejection level.
Compared with some reported works, this filter has merits of covering the licensed and unlicensed 5G bands, low insertion loss, good return loss and compact size with a low-cost methodology, making it suitable for 5G mobile communications.

Figure 1 .
Figure 1.Structure of the traditional SIR with even-and odd-mode equivalent circuits.

Figure 2 .
Figure 2. Structure of the open-loop SIR with even-and odd-mode equivalent circuits.

Figure 3
Figure3depicts the proposed structure of the dual-band BPF using two open-loop SIRs with 50 Ω tapped line input/output on an RT/Duroid 5870 substrate of 0.79-mm thickness and with a dielectric constant of 2.33 and a loss tangent of 0.0012.As a first step, the center frequencies of the first and second passbands f1 and f2 of the filter are taken as 3.5 GHz and 5.5 GHz, respectively.The calculated impedance ratio Rz is 2.75 and the initial SIR dimensions are determined using the above equations.Then, the two SIRs are folded and used to design the dual-band BPF.A full-wave EM simulator (CST Microwave Studio) is used to simulate the filter.As expected, a deviation between the analytical and simulation results has been found.This deviation is caused by the coupling between the edges of the high-impedance line sections, C, mentioned above and the coupling between resonators since in the analysis one SIR has been considered.

Figure 5 .Figure 6 .
Figure 5. Simulation results of the proposed filter.The current distributions of the proposed BPF are displayed in Figure6.It is clearly observed that no current flows through port 1 to port 2 at the frequencies of the transmission zeros, while the current passes between the two ports at the center frequencies of the BPF, which indicates the BPF behavior of the structure.

Figure 7 .
Figure 7. (a) Photograph of the fabricated dual-band BPF.(b) Its measurement equipment.The group delay of the proposed dual-band BPF is displayed in Figure9.It can be seen that the filter has a flat response within the passbands.The group delay is below 1.5 ns in the first passband, while it is below 1.2 ns in the second passband.

Figure 8 .
Figure 8. Simulated and measured S-parameters of the Figure 9. Group delay of the dual-band BPF.dual-band BPF.

Table 1 .
The simulation results are shown in Figure5.It is seen that the working bands of the proposed dual-band BPF are 3380-3870 MHz and 5149-5840 MHz with insertion losses of 0.37 dB and 0.42 dB and return losses of 26.2 dB and 31.1 dB at 3.6 GHz and 5.55 GHz, respectively.Four transmission zeros are found at fz1 = 2.93 GHz, fz2= 4.39 GHz, fz3 = 6.29 GHz and fz4 = 6.86 GHz.The rejection level in the upper stopband is about 19.4 dB.

Table 1 .
Dimensions of the proposed filter.

Table 2 .
Comparison between this work and other previous 5G BPFs.