5G Fixed Beam Switching on Microstrip Patch Antenna

Received Jan 16, 2017 Revised Mar 18, 2017 Accepted Mar 30, 2017 5G technology is using millimeter-wave band to improve the wireless communication system. However, narrow transmitter and receiver beams have caused the beam coverage area to be limited. Due to propagation limitations of mm wave band, beam forming technology with multi-beam based communication system, has been focused to overcome the problem. In this letter, a fixed beam switching method is introduced. By changing the switches, four different configurations of patch array antennas are designed to investigate their performances in terms of radiation patterns, beam forming angle, gain, half-power bandwidth and impedance bandwidth at 28 GHz operating frequency for 5G application. Mircostrip antenna is preferred due to its low profile, easy in feeding and array configurations. Three different beam directions had been formed at -15°, 0°, and 15° with half-power bandwidth of range 45 ̊ to 50 ̊. Keyword:


INTRODUCTION
Wireless communication has undergone a tremendous evolution to meet the demand of high traffic capacity due to the drastic increasing usage of smartphones and mobile electronic devices. Begin from 0G technology since 1970s, followed by 1G,2G, and 3G, until today 4G technology, they show a great development on mobile technology from Radio Common Carrier (0G-RCC) to Long Term Evolution Advance (4G-LTEA). In order to meet the fast growing wireless data capacity demands due to increasing users of smartphones, high growth in web and streaming, 5G technology now are highly given attention and undergo a huge research. The 5G technology has huge data capabilities to support multi-Gbps data rates and has ability to gather unrestricted call volumes as well as infinite data broadcast within latest mobile technology [1]. Therefore millimeter-wave communication systems are required.
Millimeter-wave communication systems using narrow beams at the transmitter and receiver, which suppress the interference of neighboring beams. The narrow beam also strongly reduces the angular spread of the incoming waves and the multipath components of millimeter waves to be limited. Therefore, by having the beam forming, beam-forming weights can be adjusted to a desired area or location [2]. There are two types of beam forming technology. The first one is fixed beam forming and another is adaptive beam forming [3]. Adaptive beam forming forms effective beams by adapting beam width and beam direction depending on the surrounding state of radio channels. It requires high hardware complexity and needs extra feedback mechanism for beam forming. While in fixed beam forming, beams with a fixed direction and width are generated. Therefore, switched beam forming is introduced as it is simpler and lesser operation system than adaptive beam forming. The concepts of beam steering or beam shifting are developed for scanning array antennas. Scanning array antennas were developed initially for aircraft, maritime and aeronautical applications. In terms of practical implementation, various technologies such as GPS, radiofrequency identification (RFID), tracking, traffic control and collision avoidance radars and wireless local area network (W-LAN), have been employed in developing the indoor localization and scanning systems [4].
Modern wireless communication systems require a low profile, lightweight, high gain and simple structure antennas to ensure reliability, mobility, and high efficiency [5]. Therefore, mircostrip antenna is highly preferred due to its low profile, easy to fabricate and feed, and easy to use in the array or incorporate with other microstrip circuit elements [6].
Patch antennas are used as simple and highly preferred in many applications. Circular polarizations, dual characteristics, dual frequency operation, frequency agility, broad bandwidth, feed line flexibility and beam scanning can be easily obtained from these patch antennas [7]. The patch can take any shape with rectangular and circular configurations are the most famous. The radiation pattern of a patch array antenna is fixed. However, by controlling the progressive phase difference between the elements, the maximum radiation can be shifted in any desired direction to form a scanning array [8]. This type of antenna is called a phase array antenna.
In this letter, by controlling the switches, four different configurations of patch array antennas which operate at 28 GHz are designed and presented. The antennas are designed with beam shifted at +15˚ and -15˚ by consider the overall performances of the antenna in terms of gain, bandwidth, return loss, VSWR and side lobe level as well as beam shifted angle. Figure 1(a), (b), (c) and (d) show the proposed antennas with four different configurations to compare on their performances. Each antenna basically makes up of three layers. The lower layer is a fully ground plane with copper that covers the rectangular shaped substrate. The middle substrate is Rogers 5880 with dielectric constant, ε_r=2.2 and dielectric loss tangent, tan δ =0.0009, and a height of substrate, h=0. 254mm. The thickness of the copper used is 0.017mm. The upper layer is the patch antenna. The rectangular patch has a size of 4.52 × 3.52mm (width × length). Distance between two patches is almost equivalent to λ/2 (calculate from center-to-center of the patches). Quarter wave transformer with power division method is used for matching patch by a microstrip line with 50Ω input impedance. The operating frequency of all designed antennas is about 28 GHz, which is suitable for 5G applications.

RESEARCH METHOD
Substitute =3.73 mm, ΔL=0.13 mm; L= 3.47 mm. Calculation of the insert feed length, : where is width of inset, is 50 Ω transmission lines, is characteristic impedance and is input impedance at the edge of the patch antenna. The parameters of the antenna had been optimized so that the antenna can perform optimally as shown in Table 1.   Figure 3, it can be seen that there are four locations of switches. By manipulating the switches, four different configurations of patch array antennas were designed as shown in Figure 1. In this letter, two radiating elements of patch array antennas are presented. The different configurations of designed antennas are comparing their performance in terms of gain, bandwidth and beam shift angle.

RESULTS AND DISCUSSION
The patch array antennas operate at 28 GHz is designed and optimized using software CST Microwave Studio. Simulated results of S-parameter designed antennas are compared with measured results given by the performance network analyzer. Simulated far-field radiation patterns for each different configuration of designing antennas are shown as well. Figure 4 presents the measured and simulated reflection coefficient results of the fabricated antennas. Two different configurations of antennas designed (b) and (c) have an almost similar curve of due to both designed antennas are only opposite in the shape of the patch. The difference in the measured and simulated results is mainly caused by the shift in the resonant frequencies [9]. This frequency shift is due to fabrication tolerance on the insertion loss of Cu microstrip during etching and gravure processes. The reflection coefficients for all designed antennas are about -20 dB with some frequency shifting except for Antenna 4 which has the value of -12 dB. This is due to the structure of Antenna 4 causes significant mutual coupling between two radiating patches, influencing the return loss of the radiating elements [10].  Figure 5 shows simulated far-field for four different designed antennas. It can be seen that there is no beam shifting in Antenna (a) and Antenna (d) as there is no phase shift difference between two radiating patches. However, the main beam of the Antenna (b) and Antenna (c) have shifted by 15˚. This is because the two radiating patch elements radiate at different rate due to the existence of phase shift difference. As a summary, manipulating the electrical length of the feeders can provide a phase difference between two patches of antennas.   Table 2 shows the summary on the performances of the four units design antennas. Antenna 1 has the highest gain of 10.9 dB while Antenna 4 has the lowest gain of 9.54 dB. Antenna 2 and Antenna 3 have same gain of 10.4 dB as both structures are just opposite to each other. The simulated bandwidth (BW) and measured bandwidth show a good agreement as there is only slightly different in the taken reading. However, Antenna 4 has very low bandwidth with a simulated bandwidth of 0.51 dB and measured bandwidth of 0.60 dB. The low bandwidth is due to the strong mutual coupling between the elements that degrade the overall performance of the antenna array [11]. All the designed antennas have wide half power bandwidth of range 45˚ to 50˚.

CONCLUSION
In this letter, the beam switching method with one input port to connect with two radiating elements phased arrays antenna were designed, fabricated, and analysed. The beam of the linear antenna array can be steered only in one plane. The simulated results showed that both Antenna 2 and 3 provide switched range from -15°to 15° in the H-plane, while Antenna 1 and 4 project the main beam at the broadside in the H-plane.