5G beam-steering 2×2 butler matrix with slotted waveguide antenna array

In this research paper, substrate integrated waveguide (SIW) was proposed as a technique by realizing bilateral edge walls to produce a compact 5G beam-steering antenna at 24 GHz. The beam forming network is produced using SIW directional coupler perform as 2×2 Butler Matrix (BM) fed with SIW slotted waveguide antenna array. The output signal is steered from -29 degrees and +29 degrees when the signal is fed to the respective input ports. If one of the input ports is fed, the signal is evenly distributed between the adjacent output ports with 90 degree constant phase shift. The compact size of directional coupler was designed by longitude slots on the surface of SIW substrate with bandwith of 16.85% at the operating frequency. The proposed antenna produce gain of 6.34 dB at operating frequency and the promising outcome of the beam steering make proposed design suitable for 5G communications especially with tracking capabilities.

antenna array has two slot elements that are longitudinally staggered with respect to one another which consecutively fed by 2×2 Butler Matrix without having to utilize phase shifter and crossover. Using the proposed approach design, the 2×2 Butler matrix beamforming antenna network simulation presented 90±5° output phase difference thus exhibit two beam direction steerable radiations at 30±1°.

Beam Steering Antenna 2.1. SIW Design
Substrate integrated waveguide is made of a rectangular waveguide with arrays of hole vias and has ability to create bilateral side walls due to transition within structures. These vias act as walls of the waveguide supporting current flow, thus allowing for waveguide mode propagation. Substrate integrated waveguide is preferred in high frequency design due to high density integration applications with low loss. Figure 1 illustrates the SIW topology with via hole of diameter, d, horizontal spacing between two holes pitch, p and vertical spacing between holes, a. In order to provide vertical current paths, via hole is shorted to both planes. The propagating modes of SIW can be analogous as in rectangular waveguide [16][17][18] since the vertical metal walls are replaced by via holes. With the optimized dimensions, an equivalent waveguide of dimension equal to a can provide a promising operational bandwidth at 24 GHz.

Figure 1. SIW topology
In (1-3), resonance frequency, fr is determined and the size of SIW cavity is optimized in order to support TE10 mode as discussed in [19]. In analysis, weff and leff represent width and length of the SIW cavity dimesion. In order to minimize the leakage loss between nearby hole, pitch, p needs to be kept small based on (4)-(5).

SIW Directional Coupler
Directional coupler is a four port microwave network which delivers power from any port to the other three output port. SIW directional coupler support TE10 mode where the signal fed to the coupler transfer power from input port to the other output ports. Due to quarter wavelength transmission, the reference signal line lead phase shift of the coupler signal with 90° phase. The design of proposed coupler is shown in Figure 2 (a) with two perpendicular rectangular waveguide while Figure 2  controls the coupling value to be ±3 dB. 90° directional coupler is realized by two pairs of 50 Ω and 35.35 Ω quarter wavelength microstrip line. When the signal fed at Port 1, the signal equally distributed to port 2 and port 3, while port 4 is isolated since there is no power reaching it.
Line impedance between these outputs port is matched at 50 Ω to maximize the power transfer and minimize the reflection from the load. With all ports matched, power entering port 1 is evenly divided between ports 2 and 3, with a 90° phase shift between these outputs [20]. This scattering matrix indicates that the 90 degree hybrid has the property of delivering two output signals with same magnitude and implies that it has a high degree of symmetry. The signal arrive at Port 4 is out of phase, thus no power is coupled to this port. The coupling factor can be determined from (6) where the Waperture dimension, optimized to be 4 mm, while pitch, p is given as 1 mm and diameter, d of air hole is 0.4 mm.

2×2 Butler Matrix using Directional Coupler
The Butler Matrix has N inputs×N outputs (N=2 n ) of where n is the matrix order. The switched beam Butler Matrix system has the ability to increase the channel capacity limited by interference. As shown in Figure 3, 2×2 Butler matrix input beam ports equals to the number of output elements ports. When the signal is fed in the input port, two output signals with the same magnitude but a phase shift of ± 90° will be generated. When RF signal excites each of the input ports, signal is distributed equally with a constant phase between them. The proposed system has the element of transmitting signals at output ports when the signal is fed at the input ports. The separation between the SIW antenna arrays is controlled to achieve the desired beams and reduced Sidelobe Level (SL). N x N Butler Matrix can generate output signals with equal power and a section phase shift between adjacent output ports, δi [21]. where i=± 1/2, ± 3/2, ± 5/2... ± (N-1) /2. The phase shifts between two different adjacent ports, δi of the SIW directional coupler can be obtained according to (7) with N represent the number of input ports. As clearly tabulated, phase difference between outputs port are ± 90º as listed in Table 1.

SIW Slotted Array Antenna with SIW Directional Coupler
Slotted waveguide series fed antenna array is used as the radiating patch to provide beamforming networks to enable communication and tracking functions. This proposed antenna array with λ0 /2 spacing at 24 GHz was integrated with the SIW Butler Matrix using coupling fed technique. The two by two SIW symmetrical slotted array antennas with slot dimension of coupling aperture 1 mm×2.7 mm is illustrated in Figure 4 and detailed dimension is listed in Table 2. The SIW directional coupler Butler Matrix integrated with series connected patches antenna and fed with aperture couple is designed to produce narrow E-plane beamwidth with maximum gain and low mutual coupling when input signal is fed in input port, Port 1 and Port 2.  The amplitude and phase excitation of each elements of phase array antenna is individually controlled to form a radiated beam of desired shape. Hence, beam scanning is operated with the antenna aperture remaining fixed in space without involvement of mechanical motion in the scanning process. Array factor generally is a function of the number of elements, geometrical arrangement, relative magnitude, relative spacing and relative phase. The corresponding phase shift across element is given by (8) and beam direction of Butler Matrix is given by (9) [22], where λ represents the wavelength and d is the antenna element spacing. From the calculation, two beams generated with ± 30° beam direction respectively.

SIW Directional Coupler
A steerable SIW directional coupler has been designed and simulated using CST Microwave Studio on the 0.508 mm thickness Rogers RO4350 substrate with relative permittivity of 3.48. The dimension of the directional coupler is optimized to 15 mm×15 mm and connected to the serially patch antenna array. Figure 5 and Figure 6 illustrated the results of the directional coupler in terms of return loss, isolation, coupling and phase difference between the coupled ports. The simulation shows the return loss, S11 and isolation value S41 less than -10dB, which are -15.22 dB and -15.71 dB, indicated promising return loss value. The coupling factor shows average of -3±2 dB of transmission coupling power indicated the output signal is distributed equally between output ports. From the results, it is shown that S21 and S31, which are -4.15 dB and -4.06 dB at 24 GHz while phase difference of 92° between two outputs ports. Detailed simulation results are tabulated in Table 3. Directional coupler impedance is designed to be matched to the transmission line delivering the energy for antenna radiating. From the simulated result showed that Voltage Standing Wave Ratio, VSWR shows compromising value of 1.11, indicates that the coupler has matched impedance to the transmission line.

Steerable Antenna
The input return loss and isolation simulation results are shown in Figure 7. The return loss, S11 simulation value shown as -24.68 dB while isolation, S21 shows the value of -11 dB with fractional bandwidth of 1 GHz (f1=23.7 GHz, f2=24.7 GHz). The array generates two beams at different angles in its H-plane due to the phase progression in its individual elements. The H-plane radiation patterns are shown in Figure 8 which showed that two steered beams are formed when each of the two ports are fed individually with maximum gain of 6.34 dB. The beams are mirror images of each other where when the signal is fed to Port 1, the main lobe beam will direct to 29° while the signal will direct to -29° when the signal is fed to Port 2. Base on theoretical calculation, the beam angles are 30 ±1° from the perpendicular. The array exhibit angular width 3 dB of 27° when fed with Port 1 and Port 2 respectively as shown in Figure 9.  Comparison between the proposed approach is been performed with previous literature review. From Table 4, it can be seen that the performance of the proposed method of SIW with slotted antenna 2×2 beamformer is comparable with the result obtained in those reported in the previous work. The SIW technique by having via hole which create bilateral waveguide wall made it possible for development of high performance millimeter wave and sub-millimeter wave components in 5G wireless communication at 24 GHz without compromising the performance of the proposed beamforming performance.

Conclusion
In this paper, a SIW 2×2 directional coupler as Butler Matrix integrated with slotted waveguide antenna array for future 5G communications is designed and simulated. At 24 GHz, proposed design achieves promising results of return loss, transmission, coupling, isolation and