A Novel 28 GHz Phased Array Antenna for 5G Mobile Communications

(1.Department of Electronic Engineering,Tsinghua University,Beijing 100084,China；2.Beijing Actenna Technology Co.,Ltd.,Beijing 100089,China)

Abstract: A novel phased array antenna consisting of 256 elements is presented and experi?mentally verified for 5G millimeter-wave wireless communications.The antenna integrated with a wave control circuit can perform real-time beam scanning by reconfiguring the phase of an antenna unit.The unit,designed at 28 GHz using a simple patch structure with one PIN diode,can be electronically controlled to generate 1 bit phase quantization.A prototype of the antenna is fabricated and measured to demonstrate the feasibility of this approach.The measurement results indicate that the antenna achieves high gain and fast beam-steer?ing,with the scan beams within ±60° range and the maximum gain up to 21.7 dBi.Further?more,it is also tested for wireless video transmission.In ZTE Shanghai,the antenna was used for the 5G New Radio (NR) test.The error vector magnitude (EVM) is less than 3%and the adjacent channel leakage ratio (ACLR) less than -35 dBc,which can meet 5G sys?tem requirements.Compared with the conventional phased array antenna,the proposed phased array has the advantages of low power consumption,low cost and conformal geome?try.Due to these characteristics,the antenna is promising for wide applications in 5G milli?meter-wave communication systems.

Keywords: 5G mobile communications;millimeter wave;phased array antenna

1 Introduction

5G wireless communication systems are actively tested and deployed worldwide now.They are supposed to enable extremely fast communication speed of up to 10 Gbit/s[1–2].Compared with 1G to 4G wireless com?munications,5G systems are mostly concerned with the channel capacity,power consumption,and system cost.They provide multiple application scenarios,such as en?hanced mobile broadband (eMBB),ultra-reliable and lowlatency communications (uRLLC),and massive machine type communications (mMTC).5G will affect every aspect of our lives,implementing the virtual reality (VR),Internet of Things (IoT),smart home,and so on.A lot of challenges need to be overcome to realize the above goals,including those in the transmission network.

Antennas are critical components in 5G wireless transmis?sion networks[3].While a lot of efforts have been devoted to an?tenna designs in mobile devices,here we focus on antennas on base stations.Gain and coverage are two critical requirements for base station antennas; however,they contradict each other:a higher gain results in a narrower beamwidth and hence a lim?ited coverage area.To solve this problem,multiple beams and beam scanning techniques,usually realized by phase array an?tennas,provide a feasible solution.

This paper presents a novel phased array antenna operat?ing at 28 GHz for 5G wireless communications.The phased array design method is introduced first,and the element and array configuration are discussed next.The measured re?sults of the antenna array and the communication test of 1 transmitter-1 reciever (1T-1R) and 1 transmitter-2 recievers(1T-2R) systems are presented to demonstrate the promising potential of the proposed mm-wave phased array for 5G sys?tems.In addition,the antenna was used for the 5G NR test of ZTE Shanghai,which shows good performance in the er?ror vector magnitude (EVM) and adjacent channel leakage ratio(ACLR).

2 Design Methods for Phased Arrays

To achieve the beam scan?ning capability in an anten?na array,the excitation phase of each element needs to be tunable in order to form a coherent phase front at the desired beam direc?tion.As shown in Fig.1,the phase control method is the key point in the design of phased arrays and two differ?ent methods can achieve phased arrays.

The major trend in phased array development is based on the microwave and milli?meter?wave integrated cir?cuit (MMIC) technology.The phase control function is re?alized from the antenna ele?ment“downward”and a rep?resentative device is a trans?mit/receive module connect?ed to each element.This cir?cuit approach has a lot of ad?vantages,such as excellent radiation performance and flexible radiation beam; how?ever,the power efficiency,antenna weight,and system cost are major concerns,es?pecially for those large-scale and high-frequency phased arrays.

A novel alternative ap?proach is to move the phase control function“upward”from antenna element and an example is a reconfigurable reflective surface.As shown in Fig.2,when electromagnetic wave im?pinges on this surface,an additional reflection phase is added upon reflection,which can be tuned by active components such as PIN diodes and varactor diodes.In this field ap?proach,both“radiation”and“phase control”functions are in?tegrated onto the surface.This new approach has attracted growing interests because of its high efficiency,conformal ge?ometry,feasibility for millimeter-wave and tera hertz opera?tion,and low system cost.

▲Figure 2.Structure of the novel phased array antenna.

The phased array using the field approach consists of three major components:a reconfigurable surface,a control module,and a feeding structure[4–6].An array of patch elements are arranged on the surface,and each element is integrat?ed with control devices,such as PIN diodes,varactor di?odes,MEMS switches and mechanic actuators.The sta?tuses of these devices are de?termined by a control mod?ule,usually a Micro-control?ler Unit (MCU) or a Field Programmable Gate Array(FPGA) board.To transmit/receive wave to/from the sur?face,a feed structure is nec?essary,which can be a horn in the far field[7],a passive ar?ray in the near field,or even a constrained feed network connected to the surface,as shown in Fig.3.

3 A Novel Mm-Wave Phased Array An?tenna Design and Measurement

3.1 Electromagnetic Sur?face Element

A phased electromagnetic surface antenna operating at 28 GHz for 5G millimeter?wave communications is de?signed and tested.The ele?ment geometry,array configu?ration,and measurement re?sults for both the antenna radi?ation and system throughput are presented in this section.

Fig.4 shows the geometry of a reconfigurable element,which consists of a patch layer,a ground layer,and a biasing line layer.A PIN diode is connected to the patch to control its reflection phase at 28 GHz.One side of the PIN diode is grounded directly,and the other side is connected to a DC bi?asing voltage through the patch and the biasing line.A fanshaped stub is used to isolate the DC and RF interference.The element dimensions are listed in the caption of Fig.4 and the element is carefully designed to obtain a 180°phase differ?ence between PIN On-Off statuses at 28 GHz.Moreover,the reflection loss of the element is less than 1 dB at 28 GHz,as shown in Fig.5.

▲Figure 3.Various structures of reconfigurable EM surface:(a) reflectarray with horn antenna; (b) transmi?tarray with horn antenna;(c)transmitarray with active module;(d)transmitarray with coupling network.

▲Figure 4.Geometry of a reconfigurable element designed at 28 GHz:p=5.35 mm;px=3.15 mm;py=3.025 mm;h1=0.508 mm;h2=0.500 mm.

3.2 Phased Array Antenna Design

Based on element design and simulation,a phased array consisting of 16× 16 patch elements is designed.In space-fed array designs,the reconfigurable reflective surface is usually placed in the far-field region of the feed.The magnitude of in?cident wave on the surface is related to the radiation pattern of feed,as well as the spatial distance between the feed and each element.

In this paper,a horn antenna is designed as the feed.The-10 dB beam width of the feed is ±30°,so the chosen dis?tance between the phase center of feed horn and the reflec?tive surface is 66 mm to balance spillover efficiency and illu?mination efficiency.

▲Figure 5.Simulated results of element:(a) phase result; (b) magni?tude result.

For a reconfigurable reflective surface,the required com?pensation phase φrmnfor the(m,n)th element is computed by

where φimnis the incident phase of the (m,n)th element,kis the free space wavenumber,is the unit vector in the main beam direction,is the position vector of the (m,n)th ele?ment,and Δφ is an additional optimized phase[8].If we use re?configurable elements with 360° full-phase coverage,the re?quired compensation phase for each element will be continu?ous.For a 1 bit phased array,there are only two-phase states for each element and they are controlled by PIN On or Off.Therefore,the required compensation phase should be quan?tized.As simulated in Fig.5 at 28 GHz,PIN Off means 130°and PIN ON means -50°.We use Eq.(2) to quantize the con?tinuous phase into 1 bit compensation phase.

The phase distribution on the electromagnetic reflective sur?face of the boresight beam is calculated,Fig.6a shows the continuous compensation phase and Fig.6b shows the quan?tized phase.

3.3 Phased Array Antenna Measurement

A phased array antenna prototype is built (Fig.7).It con?sists of 256 patch elements and each is individually controlled by an FPGA board behind the array surface.A horn antenna is designed as the feed for this antenna and the waveguide structure minimizes the feed loss.

▲Figure 6.Boresight beam phase distribution on the electromagnetic reflective surface:(a) continuous phase distribution; (b) 1 bit quantized phase distribution.

▲Figure 7.Photo of a 256-element phased array prototype for 5G mm-Wave communications.

The antenna prototype is measured in an anechoic chamber at Tsinghua University.Fig.8a shows the measured patterns at representative scanning angles:0°,15°,30°,45° and 60°,and Fig.8b shows the measured gains of different scanning beams.It is observed that by reconfiguring the elements’sta?tuses on the surface,the antenna beam can be scanned to the desired direction.In addition,the antenna is also measured from 27 GHz to 29 GHz,which shows the -3 dB gain-band?width of the phased array.

After measuring the phased array antenna,a wireless com?munication system is also built.A National Instrument (NI)millimeter-wave transceiver works at 28 GHz with a band?width of 800 MHz.We use 64 Quadrature Amplitude Modula?tion (64QAM)modulation cooperating with 7/8 turbo coding to build two test scenarios for video data transmission:1T-1R and 1T-2R.The phased array produces one switching beam for the 1T-1R test and dual beams for the 1T-2R test.The bit rate of both scenarios can achieve up to 2.87 bit/s when the distance between transmitter and reciever(s)is 6 m.

3.4 5G NR Measurement

In order to verify the communication performance of the phased array in mm-Wave band,5G NR measurement was conducted in the chamber of ZTE Shanghai.Fig.9 shows the connections of NR test system.

As depicted in Fig.9,the phased array acts as the transmit?ter antenna,and the horn antenna connected with the spec?trum analyzer acts as a receiver antenna.The EVM and ACLR were measured by this system.EVM is usually used as a mark to measure the linear performance of transmitter,as well as the transmitter antenna.ACLR indicates interference of trans?mission signal leakage to the same or similar communication system.After tested by the ZTE 5G NR system,the EVM and ACLR indices (Tables 1 and 2) show that the antenna can be used in 5G wireless communications,and it provides compara?ble performance to the existing products of ZTE.

▲Figure 8.Measured results of the 256-element phased array:(a) beam scanning measurement at 28 GHz;(b)gain measurement at 28 GHz.

▲Figure 9.5G New Radio(NR)test of the phased array.

4 Conclusions

The demand of phased array antennas for 5G millimeter-wave wireless communications has been increasing in recent years.This paper introduces a novel phased array approach using reconfigurable electromagnetic surface,which shows good performance in wireless fast data transmission.A 1 bit 256-element phased array operating at 28 GHz band is thor?oughly investigated with measurements.The measured beams can scan±60°and the maximum gain achieves up to 21.7 dBi.The array also achieves dual beams for 1T-2R wireless video transmission successfully.In addition,the array was tested for 5G NR system successfully and the EVM and ACLR indices also show good performance.Because the array only uses PIN diodes to control beam scanning,the power consumption is very low when it provides comparable performance to conven?tional phased arrays.The measured results show that the phased array is suitable for base stations and have a promising future for 5G millimeter-wave communication systems.

▼Table 1.EVM measurement results

▼Table 2.ACLR measurement results