Leaky-Wave Antennas for 5G/B5G Mobile Communication Systems:A Survey

(1.College of Electronics and Information Engineering,Shenzhen University,Shenzhen 518060,China；2.Institute of Microelectronics,Tsinghua University,Beijing 100084,China)

Abstract: Since leaky-wave antennas (LWAs) have the advantages of high directivity,low loss and structural simplicity,LWAs are very suitable for designing millimeter-wave(mmW)antennas.The purpose of this paper is to review the latest research progress of LWAs for 5G/B5G mobile communication systems.Firstly,the conventional classification and design methods of LWAs are introduced and the effects of the phase constant and attenuation con?stant on the radiation characteristics are discussed.Then two types of new LWAs for 5G/B5G mobile communication systems including broadband fixed-beam LWAs and frequencyfixed beam-scanning LWAs are summarized.Finally,the challenges and future research di?rections of LWAs for 5G/B5G mobile communication systems are presented.

Keywords: 5G/B5G;beam-scanning;frequency-fixed;fixed-beam;millimeter-wave (mmW);leaky-wave antenna(LWA)

1 Introduction

Due to the rapid growth of mobile data services,the ex?isting spectrum resources can no longer meet the re?quirements of mobile communication systems.In or?der to increase the data transmission rate,a higher carrier frequency must be used to provide greater bandwidth.Since the millimeter-wave (mmW) frequency band has rich spectrum resources which can provide greater bandwidth,mmW antennas can be used in 5G/B5G mobile communica?tion systems.

Unlike traditional 4G antennas,mmW antennas must have the characteristics of low loss and high directivity to counter?act the greater path loss of mmW propagation in free space.The dimensions of mmW antennas are in the level of millime?ters,which poses a challenge to their process technology.Thus,the 5G/B5G mmW antennas should be designed as sim?ply as possible.In addition,5G/B5G mmW antennas should also have the characteristic of broadband to support 5G/B5G high data rate transmissions.

Generally,antenna arrays are used to generate highly direc?tional beams.However,in mmW systems,the feeding network of an array antenna significantly increases the system com?plexity and brings additional loss to the system.Phased?array antennas have the characteristics of electrically controlled beam-scanning and high directivity[1].In Ref.[2],the design considerations and solutions of mmW phased?array antennas applied to 5G smartphones are discussed in detail.Multibeam antennas[3–4]can simultaneously generate multiple beams with the high directivity,where each beam covers a pre?determined angular range.In Ref.[5],the applicability of multi-beam antennas for 5G mobile communications and relat?ed challenges are introduced.However,phased?array anten?nas and multi-beam antennas have complex feeding networks,which is not conducive to massive production.Since leakywave antennas (LWAs) have the advantages of high directivi?ty,low loss,and structural simplicity,they are expected to be an effective solution for 5G/B5G mmW antennas.

This paper is organized as follows.In Section 2,the classifi?cation and design methods of LWAs are introduced.Section 3 describes the latest research progress of LWAs for 5G/B5G mobile communication systems.Section 4 presents the chal?lenges and future research directions of LWAs for 5G/B5G mo?bile communication systems.Finally,we draw the conclusions in Section 5.

2 Classification and Design Methods of LWAs

LWAs are based on guided wave structures and can continu?ously radiate along their guided wave structures.Since LWAs have a large radiation aperture and do not require a complicat?ed feeding network,their structures are very simple and they have the characteristics of low loss and high directivity.

2.1 Classification of LWAs

Generally,LWAs can be categorized into three types,name?ly uniform LWAs,periodic LWAs and quasi-uniform LWAs[6].

A uniform LWA has a constant cross section along its guid?ed wave structure.Since its fundamental mode is a fast wave(β0＜k0),it can radiate when its guided wave structure is open.When a uniform LWA is fed,its main beam scans from end?fire direction to broadside direction as frequency increases.However,in most practical designs,the beam of a uniform LWA cannot scan to endfire or broadside direction.As shown in Fig.1[7],it is difficult for a slotted air-filled rectangular waveguide LWA to scan to broadside direction since the oper?ating frequency corresponds to the cut-off frequency of the waveguide.It is also difficult for a slotted air-filled rectangu?lar waveguide LWA to scan to endfire direction since the long slot of the rectangular waveguide is equivalent to a magnetic current source,and the endfire direction corresponds to the ra?diation zero point of the magnetic current source.

Compared with uniform LWAs,the fundamental mode of a periodic LWA is a slow wave.Thus,even if its guided wave structure is open,it cannot radiate.However,since a periodic LWA introduces periodic modulations along its guided wave structure,an infinite number of space harmonics[8–9]are gener?ated in the guided wave structure of the periodic LWA,where each space harmonic has a phase constant

where β0is the fundamental mode phase constant of the peri?odic LWA (slightly different from the fundamental mode phase constant when there is no periodic modulations),nis the harmonic order,andpis the period length.Although the fundamental mode of a periodic LWA is a slow wave,it is pos?sible to obtain a space harmonic (usually the first space har?monic)that is a fast wave by designing a certain period length.As shown in Fig.2,a rectangular dielectric rod with periodic metal strips is a classic example of periodic LWAs[10].Com?pared with the uniform LWA,the main advantage of a periodic LWA is that its phase constant can be negative (for example,β-1＜k0),thus allowing its beam scan from backward to for?ward.However,due to the open stop band of periodic struc?tures,the performance of a periodic LWA significantly deterio?rates when its beam is scanned close to the broadside direc?tion[8–9].The reason is that the period length is equal to the fundamental mode guided wavelength of the periodic LWA at the frequency corresponding to the open stop band.Thus,the reflections from all periodic elements are superposed in phase so that most of the energy is reflected back to the input port of the periodic LWA.In recent years,many articles have pro?posed various methods to suppress the open stop band of peri?odic LWAs[11–16].

▲Figure 1.A slotted air-filled rectangular waveguide.

▲Figure 2.A rectangular dielectric rod with periodic metal strips.

Similar to periodic LWAs,a quasi-uniform LWA also intro?duces periodic modulation along its guided wave structure.Thus,it can also generate infinite space harmonics.However,the fundamental mode of the quasi-uniform LWA is a fast wave like uniform LWAs,and its period length is selected to be small enough to ensure that radiation only comes from the fundamental mode.Thus,the period length of a uniform LWA does not directly determine the radiation mode.However,peri?odic modulations can be used to control the attenuation con?stant of leaky mode.A classic example of quasi-uniform LWAs is shown in Fig.3,where it radiates by a series of closely spaced periodic holes which are on the side wall of an air?filled rectangular waveguide[17].Compared with the slotted air-filled rectangular waveguide LWA[7],this design can pro?vide a smaller attenuation constant since periodic holes can?not completely cut the current lines on the side wall of the rectangular waveguides.However,the long slot of the side wall of the rectangular waveguides in Fig.1 cuts the current lines.

2.2 Design Methods of LWAs

LWAs can be designed by the microstrip line[18–23]and sub?strate integrated waveguide (SIW)[24–29].Their radiation char?acteristics are determined by the phase constants and attenua?tion constants where the phase constants determine their beam directions and the attenuation constants determine their beam widths.The beam direction of an LWA can be approxi?mated as

▲Figure 3.An air-filled rectangular waveguide with closely spaced pe?riodic holes.

whereθis the angle measured from the broadside direction,β is the phase constant of the LWA,andk0is the propagation constant of free space.The guided wave structure of an LWA is dispersive,which means that its phase constant does not change linearly with frequency,resulting in β/k0to change with frequency.Thus,the beam of an LWA will be scanned with frequency.We can obtain the required beam-scanning range by controlling the phase constant of an LWA.The beam width Δθof an LWA can be approximated as

where α is the attenuation constant.We can obtain a narrow beam by designing a small attenuation.The reason is that an LWA with small attenuation needs a long guided wave struc?ture to fully radiate energy,and the long guided wave struc?ture means that the effective radiation aperture is large.

Uniform LWAs use their fundamental mode to radiate.When designing a uniform LWA,we must first determine which waveguide will be used.The required operating band and phase constant of a uniform LWA can be obtained by se?lecting the geometry and material of its waveguide.As shown in Fig.1,the width and height of the rectangular are used to determine its operating band and phase constant.By adjusting the geometry of the slot,the attenuation constant can be con?trolled to obtain the required beam width.Quasi-uniform LWAs are similar to uniform LWAs,except that quasi-uniform LWAs introduce periodic disturbances such as the periodic holes.As shown in Fig.3,the attenuation constant of the rect?angular waveguide can be controlled by adjusting the geome?try and spacing of the holes to obtain the required beam width.

Since the fundamental mode of a periodic LWA is a slow wave,periodic modulation needs to be introduced to allow it to radiate.Periodic modulations will introduce infinite space har?monics and the phase constant of each space harmonic can be expressed as βn=β0+2πn/p.It can be seen that even if its fundamental mode is a slow wave,we can also obtain a space harmonic which is a fast wave.Usually the first space harmon?ic is used to radiate and its phase constant can be expressed as β-1=β0-2π/p.By properly selecting the period length,an operating band,where the first space harmonic wave is a fast wave and the rest of the harmonics waves are slow waves,can be obtained.Within this operating frequency band,con?trollable single beam radiation can be achieved.Fig.2 shows a classic periodic LWA.Its guided wave structure is a rectan?gular dielectric rod.The width and height of the dielectric rod areaandb,respectively,and the dielectric constant is?r.the fundamental mode phase constant β0of the dielectric rod can be determined by these parameters.By properly selecting the period length of the periodic metal strips,a specified operat?ing frequency band with the first harmonic being the fast wave and the remaining harmonics being the slow waves can be ob?tained,thereby obtaining the designated single beam-scanning range within the designated operating frequency band.By ad?justing the geometry of the metal strip,the attenuation con?stant can be controlled to obtain the specified beam width.

3 Latest Research Progress of LWAs for 5G/B5G Mobile Communication systems

3.1 Broadband Fixed Beam LWAs

The characteristic of frequency beam-scanning makes the pattern bandwidth (3 dB bandwidth of fixed direction radiated power density) of LWAs very narrow.The 5G/B5G antennas as devices for receiving and transmitting electromagnetic sig?nals should have the characteristic of broadband to obtain high data rate transmissions.In order to improve the pattern bandwidth of LWAs,various methods to reduce or completely cancel the dispersion characteristics of LWAs have been pro?posed[30–36].

In Ref.[30],an LWA loaded with a Huygens metasurface is proposed,which works in theX-band.As shown in Fig.4,by loading a Huygens matasurface above a coplanar waveguide(CPW) LWA,the frequency beam squint of the LWA can be effectively reduced.This reduction is due to the frequency de?pendence of the metasurface.In Ref.[31],an LWA loaded with a prism is proposed,which is integrated on a dielectric board.By loading a prism with complementary dispersion characteristics on the radiation aperture of the LWA,the fre?quency beam squint of the LWA can be eliminated.The oper?ating principle is illustrated in Fig.5.The LWA has a 20% 3 dB pattern bandwidth,and in the frequency range of 35 GHz to 40 GHz,the steering of the main radiation direction is only±0.5°.

In Ref.[32],a low-dispersion LWA based on groove gap waveguide[33]technology is proposed.Similar to Ref.[31],a prism with complementary dispersion characteristics is used to reduce frequency beam squint.The difference is that by us?ing low-loss groove gap waveguide technology,higher gain and radiation efficiency are obtained in the frequen?cy band of 11.4 GHz to 13.4 GHz.Fig.6 shows the structure of this LWA.In Ref.[34],a hybrid dispersion compen?sation method is proposed to reduce the frequency beam squints of LWAs.It can be seen as a combination of Refs.[30] and [32].Fig.7 illustrates the operating principles of the LWA when only the metasurface is loaded and when the complementary lens and metasurface are simultaneously load?ed.Compared with Ref.[32],by load?ing a gradient metasurface in front of the radiation aperture of the lens,the dimension of the compensation lens is reduced to a quarter of that in Ref.[32]and the beam direction can be customized.

▲Figure 4.A CPW LWA with a Huygens metasurface.

▲Figure 5.Operating principle of a prism to eliminate beam squinting of leaky-wave antennas.

▲Figure 6.Low-dispersion leaky-wave antenna(LWA)in groove-gap waveguide technology[32].

▲Figure 7.Operating principles of LWAs (a) loaded with metasurface and(b)loaded with both metasurface and compensation lens.

▲Figure 8.Low-dispersion leaky-wave antenna (LWA) using fully hol?ey groove gap waveguide technology[35].

In Ref.[35],a low-dispersion LWA operating in the 60 GHz band is proposed.Similar to Ref.[32],the frequency beam squint of the LWA is reduced by loading a lens with comple?mentary dispersion characteristics on the radiation aperture of the LWA.The difference is that the electromagnetic band gap structure,leakage structure,and lens structure are all imple?mented with metal periodic round holes,as shown in Fig.8.Thus,the solution in Ref.[35]has higher cost-efficiency and a more stable manufacturing process.Ref.[36] is based on the design ideas of Ref.[32].Compared with Ref.[32],it discuss?es how to simultaneously obtain low side lobes and low disper?sion characteristics.In addition,it is confirmed that an LWA loaded with a complementary dispersion lens can simultane?ously generate dual beams.In Ref.[36],two low-dispersion LWAs that operate in the 60 GHz band are designed.These two antennas use symmetric and asymmetric structures,re?spectively.The asymmetric structure can obtain the optimal side lobes and the symmetric structure can simultaneously ob?tain two stable beams.Both LWAs can obtain 20% bandwidth when the beam tilt angle is within ±0.5°.In the operating fre?quency band,the gains obtained by the asymmetric and sym?metric structures are stable at 17 dBi and 15 dBi,respective?ly.A comparison of the broadband fixed beam LWAs intro?duced above is given in Table 1.

3.2 Frequency?Fixed Beam?Scanning LWAs

5G mmW antennas should have high directivity to counter?act the greater path loss of mmW propagation in free space.However,the disadvantage of a highly directional beam is that its beam width becomes narrower,resulting in a smaller cover?age area.Using an LWA with frequency-fixed beam-scanning capability can improve beam coverage.

In Ref.[37],a frequency-fixed circularly polarized beamscanning LWA is proposed.The unit cells of the LWA are composed of a varactor diode and a subwavelength square patch.As shown in Fig.9,changing the capacitance of the varactor diode can control the dispersion behavior of the LWA.By adding different bias voltages,the capacitance of the varactor diode can be changed,thereby realizing electronical?ly controlled frequency-fixed beam-scanning.In Ref.[38],an LWA composed of corrugated microstrip lines with triangular modulation surface impedance is proposed (Fig.10).Due to the periodic modulation of the surface impedance,the guided waves propagating on the microstrip line can be effectively converted into leaky waves.By adding different bias voltages,the capacitance of the varactor diode can be changed,thereby controlling the surface impedance of the corrugated microstrip line.Different surface impedances correspond to different dis?persion behaviors,thus,the electronically controlled beamscanning of the LWA can be achieved by changing the bias voltage.This LWA can reach a beam-scanning range of 45° at each frequency from 5.5 to 5.8 GHz.

In Ref.[39],a frequency-fixed beam-scanning LWA based on a corrugated substrate integrated waveguide is proposed.Fig.11 shows its structure.The unit cells of the LWA consist of a rectangular ring slot and a metallic via connected to the ground.The rectangular ring slot is loaded by four varactor di?odes.By adjusting the direct current (DC)bias voltage,the se?ries and shunt capacitance of the varactor diodes can be con?trolled.Thus,the dispersion behaviors of the LWA can be con?trolled by the DC bias voltages to obtain electronically con?trolled frequency-fixed beam-scanning.In Ref.[40],a compos?ite right/left-hand (CRLH) microstrip LWA enabling frequen?cy-fixed electronically controlled beam-scanning in the fre?quency band of 4.75 GHz to 5.25 GHz is designed.Fig.12 shows its structure.By loading two sets of varactor diodes,the circuit parameters of the CRLH microstrip LWA can be controlled to change its dispersion behavior.Thus,by adding different amplitudes bias voltages to change the capacitance of the varactor diodes,the electronically controlled frequen?cy-fixed beam-scanning can be achieved.The measured re?sults show that it can achieve beam-scanning from -37° to 32°at 5 GHz and from-15°to 34°at 5.25 GHz.

▼Table 1.Comparison of the broadband fixed beam LWAs

▲Figure 9.Dispersion curves corresponding to different capacitances of the varactor diodes[37].

▲Figure 10.Corrugated microstrip leaky-wave antenna (LWA) loaded with varactor diodes[38].

▲Figure 11.Structure of the corrugated substrate integrated wave?guide leaky-wave antenna(LWA)[39].

In Ref.[41],a method is proposed to achieve frequencyfixed beam-scanning by controlling the period lengths of peri?odic LWAs.The designed antenna consists of a substrate inte?grated waveguide with an H-shaped slot and two rows of patch elements.Two symmetrical dumbbell-shaped slots are etched on the element.Fig.13 shows the structure and equivalent cir?cuit of the element.By controlling the operating state of each patch element through the diode on the parasitic strip,the LWA can obtain different effective period lengths at a frequen?cy-fixed; thus,the frequency-fixed electronically controlled beam-scanning capability can be obtained.The measured re?sults show that the antenna can achieve 125° beam-scanning range and the peak gain can reach 11.8 dBi.A comparison of the frequency-fixed beam-scanning LWAs introduced above is given in Table 2.

4 Challenges and Future Re?search Directions of LWAs

Leaky-wave antennas play a very important role in 5G/B5G mmW mo?bile communication systems,especial?ly affecting the performance of the en?tire mobile communication systems.A high-performance transceiver antenna can significantly improve the commu?nication quality.In order to obtain a high gain beam to counteract the great?er path loss of mmW propagation in free space,5G/B5G mmW antennas should have the characteristics of high directivity and low loss.In addition,in order to improve the data transmission rate,5G/B5G mmW antennas should also have a larger bandwidth.Based on these requirements,both fixedbeam and frequency-fixed beam-scan?ning LWAs are still facing the chal?lenges of low loss,simple structure and broadband.

Since LWAs have the characteris?tics of high directivity,low loss and structural simplicity,they are very suitable for designing mmW antennas.However,due to the dispersion charac?teristics of LWAs,their beams scan with frequency.Thus,the 3 dB pattern bandwidths of LWAs are very narrow,which is not conducive to high data rate trans?mission for 5G/B5G mobile communication systems.How to increase the pattern bandwidth of LWAs is a major challenge.Some recent articles have proposed several methods of using complementary metasurfaces or lenses to cancel the disper?sion characteristics of LWAs.Although these methods effec?tively improve the pattern bandwidth of LWAs,they make the antenna structure larger and increase the complexity and loss?es of LWAs.In addition,more complex structures are not con?ducive to cost efficiency.Thus,how to obtain a larger pattern bandwidth without significantly increasing the complexity of LWAs is an important research direction of LWAs.

▲Figure 12.Composite right/left-hand (CRLH)leaky-wave antenna (LWA)configuration[40]:(a)top view of the patch layer;(b)bottom view of the ground layer;(c)bottom view of the biasing layer.

▼Table 2.Comparison of the frequency?fixed beam?scanning LWAs

▲Figure 13.Configuration and equivalent circuit of the patch element proposed in [41]:(a) element structure;(b)equivalent circuit model.

The 5G/B5G mmW mobile communication system requires an antenna with high directivity that also means a smaller cov?erage area.Using LWA with frequency-fixed beam-scanning capability can improve beam coverage.Recent articles have introduced various methods to achieve frequency-fixed beamscanning,most of which use active circuits to control the dis?persion characteristics of LWAs to achieve frequency-fixed electronically controlled beam-scanning.Due to the introduc?tion of active circuits,the loss will increase and the structures become more complicated.How to overcome these problems is another important research direction of LWAs.In addition,most frequency-fixed beam-scanning LWAs can only be imple?mented at one frequency or a narrow band.How to achieve fre?quency-fixed beam-scanning in a broadband range is also a major challenge for LWAs.

5 Conclusions

This paper reviews the recent research on LWAs for 5G/B5G mobile communication systems.The structural character?istics and radiation characteristics of uniform LWAs,quasiuniform LWAs and periodic LWAs are first introduced in de?tail.The latest studies on broadband fixed-beam LWAs and frequency-fixed beam-scanning LWAs are then presented.The operating prin?ciples and applicability to 5G/B5G mo?bile communication systems of the broadband fixed-beam LWAs and fre?quency-fixed beam-scanning LWAs are discussed.Future research direc?tions on LWAs for 5G/B5G mobile communication systems should be how to obtain a larger pattern bandwidth without significantly increasing the complexity of LWAs and how to achieve frequency-fixed beam-scan?ning in a broadband range.