Multibeam Antenna Based on Butler Matrix for 3G/LTE/5G/B5G Base Station Applications

(1.School of Physics and Optoelectronic Engineering,Guangdong University of Technology,Guangzhou 510600,China；2.School of Electronic and Information Engineering,South China University of Technology,Guangzhou 510600,China.)

Abstract: With the rapid development of mobile communication technology and the explosion of data traffic,high capacity communication with high data transmission rate is urgently need⁃ed in densely populated areas.Since multibeam antennas are able to increase the communica⁃tion capacity and support a high data transmission rate,they have attracted a lot of research in⁃terest and have been actively investigated for base station applications.In addition,since multi-beam antennas based on Butler matrix (MABBMs) have the advantages of high gain,easy design and low profile,they are suitable for base station applications.The purposes of this paper is to provide an overview of the existing MABBMs.The specifications,principles of operation,design method and implementation of MABBMs are presented.The challenge of MABBMs for 3G/LTE/5G/B5G base station applications is discussed in the end.

Keywords: multi-beam antenna; base station application; Butler matrix; wideband antenna;multi-band antenna

1 Introduction

The high-capacity communication is urgently needed in densely populated areas with the fast growth of mobile communication technology and the development of da⁃ta traffic.In order to increase the channel capacity for mobile communications,two main typical methods are usually employed.One is to improve the frequency bandwidth by em⁃ploying wideband or multiband antennas[1–5],and the other is to divide a sector into multiple ones by using multibeam anten⁃nas[6–23].Moreover,both the two methods,namely wideband/multiband and multibeam operation,can be simultaneously used to further enhance the communication capacity.For ex⁃ample,to improve the capacity,a conventional sector base sta⁃tion antenna can be replaced by a wideband or dual-band mul⁃tibeam.In addition,as one of the key technologies of 5G com⁃munications,the multi-beam antenna technology,widely em⁃ployed for 3G/LTE/5G mobile communication and as a poten⁃tial technology for B5G mobile communication,is able to pro⁃vide high data transmission rate,improved signal-to-interfer⁃ence-plus-noise ratio,increased spectral and energetic effi⁃ciency,and versatile beam shaping[24].

To realize the design of a multibeam antenna,several typi⁃cal methods have been employed.One approach is to employ a reflector antenna.Multiple beams radiated at different an⁃gles can be easily obtained by placing multiple feeds at differ⁃ent positions in front of a reflector antenna[6–10].Another meth⁃od is to use a lens antenna[11–14].When a lens is excited by multiple feeds in different points,the propagation direction of the electromagnetic wave can be changed by the focusing or reflection function of the lens,thus generating multiple radia⁃tion beams[15–17].However,reflector-based and lens-based multibeam antennas suffer from large dimensions,which are generally suitable for millimeter wave frequencies,while not suitable for sub 6 GHz base station applications.Since multibeam antennas fed by Butler matrix have advantages of high gain,low profile and simple structure[18–24],they are expected to be an effective solution of multi-beam antenna for 3G/LTE/5G/B5G mobile communication systems.

In this paper,the multi-beam antennas based on Butler ma⁃trix (MABBM) technologies are reviewed.This paper is orga⁃nized as follows.In Section 2,the specifications for base sta⁃tion applications are discussed.In Section 3,the principles of operation and design approach of MABBMs are provided.Sec⁃tion 4 discusses the latest research progress of MABBMs for mobile communication systems.Section 5 presents the chal⁃lenges and Section 6 gives the conclusions.

2 Specifications for Base-Station Applications

For practical base station applications,several important specifications of an MABBM should be required.The first one is that the multiple beams need to exhibit stable 10 dB beam width of around 120° in the horizontal plane to realize good coverage.The second specification is that the cross level be⁃tween adjacent beams is required to be around -10 dB for good communication,as observed in Fig.1.If it is too high,the signals from two sectors will overlap,which will cause con⁃tinual handoff.On the contrary,the good coverage is not guar⁃anteed if the cross level is too low.The third specification is that the side lobe and grating lobe for each beam should be suppressed in a low level,to reduce signal interference with neighbor beams.Therefore,MABBMs with such performances over a multiple frequency or wide frequency band are in ur⁃gent need to meet the application requirements of base sta⁃tions in densely populated areas.

▲Figure 1.Tri-sector base station scenario.

3 Design Principle and Method of MABBMs

As a kind of passive multiport network,Butler matrix[25–27]has advantages of multiple phase differences,low loss,low profile and simple structure,which has been widely employed as antenna feeding network for multi-beam radiation.When anN×MButler matrix is connected to an antenna array withMelements,Nindependent beams with different directions can be produced as the input ports are excited simultaneously.The principles of operation and design method of multi-beam antennas based on Butler matrix are described in detail below.

3.1 Principles of Operation for MABBMs

The beam-scanning theory of the antenna array is used to analyze the working mechanism of an antenna array for gener⁃ating multi-beam radiation,which can be employed to guide the comprehensive design of MABBMs.According to the com⁃prehensive theory of antenna array[28],the beam-scanning an⁃gleθ0of a linear antenna array can be calculated as follows.

whereφanddrepresent the phase difference and spacing be⁃tween adjacent elements respectively,andλrepresents the wavelength associated with the working frequency in vacu⁃um,with the schematic diagram shown in Fig.2.According to Eq.(1),it is seen that the beam-scanning direction of the antenna array is determined by the wavelengthλcorrespond⁃ing to the operating frequency of the antenna,the phase dif⁃ferenceφand the spacingdbetween adjacent elements.When the working frequency and spacing are selected,the beam-scanning direction of the array is only determined by the phase difference of adjacent elements,and different phase differences produce different beam-scanning directions.Therefore,when multiple signals with different phase differ⁃ences excite an antenna array simultaneously,the antenna ar⁃ray can radiate multiple beams with different directions,realiz⁃ing multi-beam radiation.

▲Figure 2.Beam-scanning of a linear array.

Fig.3 shows the radiation pattern of a three-beam antenna array fed by a 3 × 5 Butler matrix at 2.2 GHz.The element is a half-wavelength electric dipole,and the spacing between ele⁃ments is 75 mm.The excitation of the array has equal ampli⁃tude and phase differences of -120°,0°,+120°.It can be seen that 3 beams with different directions have been successfully produced for the MABBM.

3.2 Design Method of MABBMs

▲Figure 3.Radiation pattern of three-beam antenna array.

In practical applications,the multi-beam radiation of the MABBMs includes two types:multiple beams in the horizontal plane or in the vertical plane[29–30],and 2D multiple beams in both horizontal and vertical planes[31–33].In order to simplify the analysis without loss of generality,this paper provides the design method of MABBMs with multiple beams in the hori⁃zontal plane.Another type of 2D MABBMs can be designed in a similar way.Generally,a 1D MABBM is mainly formed by anM×Larray,N×MButler matrices andL-way power divid⁃ers，as shown in Figs.4a，4b and 4c respectively.The design steps of the MABBM can be summarized as follows.

Step 1:ImplementingN×MButler matrix.Firstly,the num⁃bers (N) of radiation beams of an MABBM and input ports of Butler matrix are determined.The communication capacity is associated with the number of radiation beams,with greater number of radiation beams providing more capacity.The num⁃ber of radiation beams is obtained by the communication ca⁃pacity required,which is equal to the number of input ports of Butler matrix.Then the numbers (M) of the output ports of the Butler matrix,and the amplitude and phase difference of the Butler matrix are determined by the side lobe level required for each beam.On the basis above,theN×MButler matrix is designed，which will meet the bandwidth,amplitude and phase difference requirements.

Step 2:DesigningM×Larray.The number (M) of elements in the horizontal plane of the array is equal to the number of the output ports of the Butler matrix,and the number(L)of the ele⁃ments in the vertical plane is determined by the required gain.In addition,the spacing between adjacent elements in the hori⁃zontal plane plays an important role in the coverage area of the multiple beams and cross level between adjacent beams,which should be carefully selected.On this basis,the antenna element is designed，which will meet the requirement of the needed bandwidth and then the requiredM×Larray is implemnted.

Step 3:Implementing the MABBM.The output port of each power splitter is connected to the antenna element in the verti⁃cal plane through 50 Ω coaxial cables firstly,and then the in⁃put port of each power splitter is connected to the output port of the Butler matrix through 50 Ω coaxial cables for imple⁃menting the proposed multi-beam antenna.

4 Latest Research Progress of MABBMs for Base-Station Applications

▲Figure 4.Multi-beam antenna based on N×M Butler matrix with(a)M×L array;(b)N×M Butler matrix;(c)power divider.

Recently,various MABBMs have been proposed for mobile communication applications.In Ref.[34],a compact dualband two-beam 4 × 8 antenna array with dual polarizations for base station applications is proposed.It consists of two 4 × 4 subarrays operating at 3G (1 710–2 170 MHz) and long term evolution(LTE)(2 490–2 690 MHz)bands.For size miniatur⁃ization,the elements of the two 4 × 4 subarrays are inter⁃leaved with each other,as shown in Fig.5a.The mutual cou⁃pling between the elements operating at different bands is sup⁃pressed by using filtering antennas[35]with out-of-band radia⁃tion suppression.To obtain stable two-beam radiation patterns within the two operating bands,the beam-forming networks with little magnitude and phase imbalances are specially de⁃signed for each band.The configuration of the beam-forming network is illustrated in Fig.5b.It is composed of one 2 × 4 Butler matrix and four filtering power dividers (PDs).The array exhibits a stable 10 dB beam width around 120° in the azi⁃muth plane within the two entire bands,and the two-beam ra⁃diation patterns satisfy the coverage requirement of 120° in the azimuth plane for base station applications.Additionally,16.4 dBi/15.5 dBi peak gains and around -10 dB cross levels at the junction of two beams are achieved within the two oper⁃ating bands.

▲Figure 5.Dual-band two-beam antenna in Ref.[34].(a) Elements distribution of the inter⁃leaved configuration;(b)Beam forming network diagram.

In Ref.[36],a wideband dual-polarized 4 × 6 antenna array with two beams for base station applications is provided.It consists of three 4× 2 subarrays,with the configuration shown in Fig.6a.To obtain ±45° dual-polarized radiation,a wide⁃band crossed dipole is employed as a basic element.For each 4 × 2 subarray,the lower and upper 4 × 1 subarrays are mis⁃aligned in the horizontal plane.In this way,the 4×2 subarray is equivalent to an 8 × 1 subarray with a half of adjacent ele⁃ment spacing,resulting in good grating-lobe suppression.To achieve stable two-beam radiation with low side lobe over a wide frequency band,specific wideband beam-forming net⁃works with little magnitude and phase imbalances are de⁃signed.The diagram of the beam-forming network is plotted in Fig.6b.It consists of two 1-to-2 power dividers,two -45°phase shifters (PSs),two 2 × 4 Butler matrices (BMs) and eight 1-to-3 power dividers.Moreover,the adjacent element spacing is optimized to obtain a stable 10 dB beam width around 120°,thus satisfying the coverage requirement of 120° in the horizontal plane for base station applications.The array ex⁃hibits two beams with a stable 10 dB beam width around 120° in the horizontal plane and around-10 dB cross level between two beams.The impedance bandwidth is mea⁃sured to be 56.1% (1.64–2.92 GHz) for voltage standing wave ratio (VSWR)＜1.5 and horizontally side-lobe and grating-lobe levels of the two beams are measured to be better than 18 dB.

In Ref.[37],broad band three-beam an⁃tenna arrays based on Butler matrices are presented,which are employed to increase the capacity of 3G/LTE base stations.The essential part of the three-beam arrays is a wideband 3 × 3 Butler matrix,which is formed by quadrature couplers and fixed wideband phase shifters.Wideband quadra⁃ture and phase shifters are implemented by strip lines.To achieve the suitable beam width and the required crossed level be⁃tween adjacent beams,beam-forming net⁃works consisting of augmented 3 × 3 Butler matrices and power dividers are proposed to expand the number of output ports from three to five or six,as shown in Figs.7a and 7b respectively.Dual-polarized,three-beam antenna arrays with five and six elements covering 3G/LTE band are developed with good impedance matching,high isolation be⁃tween beams,and three-beam radiation in the horizontal plane over the wide frequen⁃cy band of 1.7–2.7 GHz.

▲Figure 6.Wideband two-beam antenna in Ref.[36].

▲Figure 7.Three-beam antennas with(a)five elements and(b)six elements in Ref.[37].

In Ref.[38],a compact four-beam slot antenna array fed by a dual-layer 4 × 8 Butler matrix with side lobe level suppres⁃sion by substrate integrated waveguide technology is pro⁃posed.To address the excessive crossovers in the classic 4×8 Butler matrix,a novel dual-layer configuration is proposed,which is formed by a 4×4 Butler matrix and an amplitude ta⁃per,as illustrated in Fig.8a.The 4×4 Butler matrix is em⁃ployed to provide four outputs with equal powers and desired phases,and the amplitude taper is used to convert the four out⁃puts with equal power divisions into eight outputs with un⁃equal power distributions for reducing side lobe level.The pro⁃posed topology of the 4 × 8 Butler matrix is employed to re⁃duce the required crossovers from original five sets to merely one set.Therefore,the 4×8 Butler matrix can be significantly simplified to achieve better compactness.Finally,a slot anten⁃na array with eight elements is fed by the proposed BM to pro⁃duce four-beam radiation with low side lobe level,with the simulated model and prototype as shown in Fig.8b.

▲Figure 8.Four-beam array in Ref.[38].

To further increase the communication capacity,a modified topology of a 2D multibeam antenna array[39]fed by a passive beamforming network is proposed by introducing two sets of vertical interconnections into the conventional array configura⁃tion.Different from the traditional design,the proposed array structure shown in Fig.9 can be integrated into multi-layered planar substrates conveniently,which has advantages of low loss characteristics,ease of realization,and low fabrication cost for millimeter wave applications.A 4 × 4 mul⁃tibeam antenna array that can generate 16 beams is then designed.The proposed ar⁃ray configuration provides a new means to implement the relatively large size 2D mul⁃tibeam antenna arrays with planar passive beamforming networks,which would be at⁃tractive for future millimeter wave wireless systems used for 5G/B5G communications.

5 Challenges of MABBMs

With the rapid development of mobile communication technology,mobile commu⁃nication systems are developing towards the trend of broad frequency band,multi⁃ple frequency bands,miniaturization,and low cost,which leads to the following chal⁃lenges for MABBMs.

(1)Design of Wideband or Multi-band MABBMs

In the 5G/B5G era,mobile communication systems such as 2G,3G,4G,5G and B5G will coexist for a long time in the fu⁃ture.In order to comply with the development trend of mobile communication,reduce the number of antennas,and improve the utilization of space resources and spectrum resources,an MABBM is required to cover multiple communication frequen⁃cy bands.Therefore,a broadband or multi-band MABBM with good impedance matching,high beam isolation,and excellent side lobe suppression is a major challenge.

Figure 9.Configuration of the planar 2D multibeam antenna array in Ref.[39].

(2)Miniaturization of MABBMs

Miniaturized MABBMs can not only reduce the spacing of mobile communication system,but also decrease the associat⁃ed cost.In order to achieve miniaturization of MABBMs,it is necessary to narrow the distance between antenna elements and reduce the size of the Butler matrix.This would introduce strong electromagnetic coupling and radiation interference,causing problems such as deterioration of beam solation and distortion of radiation pattern.Therefore,miniaturization of a MABBM with good electrical performance and radiation per⁃formance is another challenge.

6 Conclusions

In summary,the MABBM technologies have been reviewed in this paper.The specifications for base station applications,principles of operation,design and implementation of MABBMs are presented,and the latest research progress on broadband or multi-band MABBMs is analyzed.Even though a few related challenges remain to be solved,the full MABBM is regarded as a promising pathway towards the realization of high-perfor⁃mance 3G/LTE/5G/B5G mobile communication systems.