Review on filtering antenna technology for 5G mobile system

In recent years, people’s increased demand for wireless communication technology fuels the development of 5G communication technology growth. Electronic equipment in the current wireless communication system is developing in the direction of low power consumption, multi-function, miniaturization, and integration, which also puts higher technical standards for RF front-end. In recent years, the wireless communication industry is building lighter, thinner, and smaller products with multifunction. The integration of filter and antenna is one of the important ways for the miniaturization of electronic systems. The antenna and filter are integrated to form a filtering antenna, which can effectively reduce the size and weight of the RF front end while realizing the function of radiation and filtering. This paper first introduces the research background and significance of this topic according to the background of mobile communication technology, and the main challenges faced by 5G communication. Then, the working principle of filtering antenna and multiple-input multiple-output technology (MIMO) are introduced, and finally the development and research status of 5G filtering antenna are summarized, and some relevant personal views are put forward.


Introduction
The continuous development of mobile communication technology has prompted people to pursue more efficient communication methods.Since the 21st century, the development of science and technology is changing with each passing day, which makes mobile phones, computers and other wireless communication equipment become indispensable in people's daily life.People's demand for wireless communication technology is increasingly extensive.The emergence of 5G technology marks that the development of wireless communication technology has entered a new stage.
Nowadays, wireless communication systems are developing rapidly in the direction of miniaturization and integration.However, this also puts higher technical standards for the RF front ends, especially for filters and antennas.Antennas, as the core equipment for receiving and transmitting electromagnetic waves in communication systems, plays an indispensable role in electronic systems such as radio communication, broadcasting, radar, and navigation.In RF communication systems, the spectrum is a very valuable and crowded resource.The function of the filter is like a sieve, which eliminates the interference factors in the transmission, based on the premise of ensuring information integrity, and obtains accurate and reliable information as much as possible.In traditional design approaches, researchers build antennas and filters independently and then cascade them through matching networks.This approach reduces design complexity, but inevitably increases RF front-end size, complexity, and additional losses.
In order to solve this problem, a method of integrated design of filter and antenna is proposed, and a new microwave multifunctional device -filtering antenna is developed [1].The filtering antenna can filter out some specific frequency bands, mainly by adding some resonant structures to the antenna.Therefore, the impedance matching of the original antenna at the frequency point can be destroyed to achieve filtering characteristics.The filtering antenna, owing the performance of both the filter and antenna, has a smaller size compared with the separate design, which can realize the miniaturization of the system [2] [3].Therefore, the filtering antenna has a good application prospect in 5G technology.
At present, 5G technology has gradually entered the commercial process.The use of 5G provides more possibilities for the development of technology, but also puts forward the requirements for the versatility of the corresponding communication equipment.China's 5G commercial applications are now mainly concentrated in the Sub 6G band, which is like some mainstream technology bands (such as 4G, Bluetooth, WLAN).For mobile terminal products, it is necessary to cover a variety of compatible functions [4].5G, as a new generation of mobile communication technology, 5G will bring us greater communication capacity, faster communication speed and higher connection reliability compared with 4G.However, this also puts higher requirements for 5G antennas.First, the limited design area and the exponentially increased number of devices require smaller and more integrated devices.Secondly, the widespread application of MIMO technology requires low correlation between antenna elements.Massive MIMO technology can realize space division multiplexing, which can effectively improve the spectral efficiency.Based on these requirements, this paper focuses on the research of filtering antenna.

The introduction of MIMO technology
In the process of human exploration of mobile communication, from 1G (analog), 2G (GSM), 3G (LTE), to 4G (TD-LTE), the use of frequency bands from GSM800, GSM1800, to 3G CDMA2000, and then to 4G TDD/FDD.Bandwidth is growing simultaneously with the continuous improvement of frequency bands.People's sense of use has also promoted by leaps and bounds.For 5G, large-scale MIMO technology is used to make the transmission rate faster than 4G more than 100 times.Therefore, MIMO technology is introduced into 5G mobile communication system.By Diversity technology, deploying more antennas on the transmitting side and the receiving side, the communication quality and transmission reliability are improved because of its unique multipath propagation characteristics.Meanwhile, multiplexing technology is used to improve the system capacity and transmission rate through multiplex technique.Therefore, the channel capacity and communication quality can be improved exponentially on the limited space resources and spectrum resources.For MIMO multiantenna systems, smaller filtering antennas can greatly reduce the size and cost of RF front-end deployments [5].

Principle of filtering antenna integration
Filtering antenna, as the integration of filter and antenna, can undoubtedly meet the requirements of small volume antennas in 5G technology.Figure 1 shows the integration process of filtering antenna.The filtering antenna has the functions of frequency selection and radiation at the same time, and effectively reduce the size of the RF front-end system and the processing cost, realizing the requirements of miniaturization.In addition, it saves the matching network required to cascade the two in traditional RF front-end systems, reducing system size and losses.Therefore, filtering antennas have important reference value in modern 5G communication technology [6].

Description of several typical filters
The specific application of filtering antenna is inseparable from the appropriate selection of filter.Filter, a very important passive microwave device, is widely used in the communication system due to its frequency selection function.The filter can ensure the normal passage of the allowed signal components and block other interference components.Different filters allow different signal components to pass through.Based on this, filters can be divided into the following five categories: low-pass, high-pass, band-pass, band-stop and all-pass filters.Figure 2 illustrates the amplitude-versus-frequency curve of several filters mentioned above.According to the pole-zero distribution of the filter, it can be divided into Butterworth filter, Chebyshev filter and elliptic filter.The Butterworth filter has a monotonic response curve, and its in-band attenuation is flat.Chebyshev filter has equal ripple passband and the monotonous attenuation band, which makes its edge relatively steep.Elliptic filters have equally corrugated passband and stopband, with steepest edges at the expense of out-of-band attenuation away from the passband [8].
In the hybrid design scheme of the filtering antenna, the filtering response can be generated by introducing radiation zero.The filtering characteristics obtained can be regarded as the quasi-elliptic filtering response.The introduced radiation zero is regarded as the transmission zero of the elliptic filter.The passband attenuation and insertion loss brought by the parasitic structure inflect the ripples in the band.Generally, the insertion loss of the filtering antenna is much smaller than the way of cascading filter, so it can be ignored.

The working principle and characteristic parameters of dipole
The dipole is a classic and the most widely used antenna.The working principle of dipole is closely related to the filtering antenna.A dipole consists of two straight wires of equal diameter and length.The constant amplitude inverting voltage excites the middle two ends.The length of each wire is , which is called the arm length of the symmetric oscillator;  is the wave number and  is the angle between the line connecting the observation point to the source point and the axis where the straight wire is located.
The radiation field is calculated as follows [9]: When 1 ≤ , Sinusoidal currents are distributed throughout the antenna and remain in phase, so there is no sidelobe in the pattern of dipole generation, and the maximum radiation direction is always  = 90 °.
When 1 > , the antiphase current appears in the dipole, and the radiation field generated by this part of the current will not strengthen the radiation field of other parts, but will weaken them.With the increase of the electrical size of the antenna, the sidelobe appears and gradually increases.When  exceeds 1.4 the maximum radiation direction begins to deviate from  = 90 °.
Therefore, the radiation pattern of symmetric oscillators with different electric lengths is different.The directivity functions of dipoles corresponding to several typical values are given below:

Introduction of Yagi filtering antenna
Yagi antenna is a common type of antenna.In recent years, there are some related on designing filtering antennas based on Yagi antenna [10].
Yagi antenna is composed of a driving dipole and several passive parasitic parallel elements.The center of the driver element is the feed point, the midpoint of the other oscillator is directly fixed on the antenna metal support rod.The metal rod is perpendicular to the oscillator and does not excite the current along the rod.It has little effect on the radiation pattern and antenna impedance, and only has a mechanical supporting function.Hidetsugu Yagi and Shintaro Uda, professor from Tohoku University, first proposed the structure of Yagi antenna.Its planform is shown in Figure 3. Due to Yagi antenna's ability of providing high directional gain, it is widely used in radar and TV receiving systems because of the simple structure and low cost.However, it also has the disadvantage of the narrow working band.
The amplitude and phase position of the generated parasitic currents can be changed by adjusting the dimensions and distances of these parasitic parallel cells properly.The direction pattern of the driving dipole and the direction pattern of the parasitic radiation generated by the induced current can be superimposed in the direction of the end beam, thus forming a high-gain end beam.Yagi antenna is widely used in communication system because of its directional radiation characteristics.However, Yagi antenna, as a high-gain antenna of directional radiation, has been faced with the problem of narrow bandwidth.The relative bandwidth of traditional Yagi antenna is only about 5%, and the bandwidth of general printed Yagi antenna is only 10-20%, which is difficult to adapt to the requirements of large bandwidth in 5G technology.For 5G emerging frequency bands, the relative bandwidth needs to be above 66% in order to fully cover them.In addition, due to the existence of multiple parasitic structures, Yagi antenna is prone to produce various higher modes, so it needs a suitable stopband designed to suppress these modes.The wideband performance of stopband is vital for multi-standard systems due to its capacity of effectively eliminating out-of-band interference from adjacent wireless systems or internal nonlinear front-end modules [11].
In response to the above discussion, the antenna can obtain more resonance in the desired frequency band by reconstructing the dipole feed and introducing appropriate parasitic elements, and introduce appropriate radiation zero at the edge of the working frequency band to form a filtering response.In this way, the wideband filtering quasi-Yagi antenna with ultra-wideband external suppression is generated.Therefore, the antenna can effectively extend the working band and achieve better frequency selection characteristics without affecting the radiation performance.In addition, by embedding a stopband structure in the feeder, ultra-wide out-of-band suppression is achieved [12].Figure 4 illustrates the structure diagram of the filtering quasi-Yagi antenna designed in this method.

The research process
Plebanski, chief engineer at Marconi in Poland, first proposed the concept of Filtering antenna in 1929.Until the early 21st century, academic reports on filtering antenna gradually increased.In recent years, many scholars have paid attention to the filtering antenna [13].
At present, there are three design methods of filtering antenna.The first is to cascade the filter and the antenna directly, and take the antenna radiator as the last level resonator of the filter circuit [14].For example, Queudet et al. proposed the integrated design of patch antenna and the bandpass filter at the European Microwave Conference in 2002 [15].In the same year, Abbaspour-Tamijani also published a paper combining antenna and filter design [16].In this way, the integrated design based on the filter avoids the impedance mismatch problem in the cascaded design of two devices independently.It is simple and easy to operate.Based on the existing research foundation of filter and antenna, the cascaded design of the two devices is designed on a circuit board or stacked up and down through the multi-layer structure, and then adjusted and matched as a whole [17].
The second method is to integrate the filter circuit into the antenna feed network.The second method overcomes the disadvantages of the first method.Since the antenna is equivalent to the series or parallel RLC circuit, the second method take their structural characteristics in to consideration, and design the resonant mode of filter and radiation antenna.In this design method, the antenna has a dual identity.It has the radiation function of the traditional antenna, and works as the final stage resonator of the filter.Undoubtedly, this design method reduces the additional size created by the cascade of filters and antennas.However, this design method is usually used in the field of the narrowband antenna, and it is not easy to implement filtering in broadband.Due to the shortcomings of the second design method, the design method of the filtering antenna is still being explored.
The third method is to construct a non-radiating resonant mode outside the antenna band or introduce a parasitic structure on the radiator to generate the radiating zero.The third type of design method is different from the filtering principle adopted by the first two types [18].It can realize the high selectivity of the filtering antenna without adding the filter structure and can be applied in more situations, but the design size will increase.At the same time, compared the previous two design methods, the loss will be reduced because the cascade of the filter and antenna is not used, and the filtering antenna will also play an active role in ensuring high selectivity [19].The characteristic of filtering frequency selection can be achieved by adding the gain radiation zero, meanwhile, the impedance bandwidth can also be expanded.According to the development requirements of 5G communication, it is particularly important to realize a filtering antenna with higher performance in limited size.
Each of the three methods has its own advantages and disadvantages.In practical engineering application, different design methods can be selected according to different requirements.Here are a few typical design approaches.Haitao Cheng, Yazid Yusuf et al. designed the integrated three-pole vertical cavity filter and slot antenna.The gap antenna is used to replace one of the coaxial ports of the filter.Theoretically, the transition loss between the filter and the antenna is close to zero, so the total efficiency of the filtering antenna system is improved.The filtering antenna is not only small, but also can achieve good radiation characteristics.This integrated approach can be applied to large-scale phased array antennas, which has the advantages of improving efficiency and reducing co-address interference.However, it has the disadvantage of the complicated manufacturing process, which is likely to cause errors and difficult to achieve the theoretical value [20].
Jianhong Zuo, Xinwei Chen et al. jointly designed the filter and antenna through the way of layering.The microstrip patch antenna and filter structure were placed at the top and bottom, respectively, and the upper and lower medium bases shared the same ground plane.Punching holes connected the antenna and the filter.The method of optimizing the impedance between the filter and the antenna is used to improve the performance of the filtering antenna.This filtering antenna has better bandwidth, larger gain, and smaller size, but the filtering characteristics of the antenna is not good enough [21].
Different design approaches can improve the antenna performance, but they also create new problems.In current research, it is difficult to find a truly ideal antenna.Therefore, the antenna design method that integrates the performance of different aspects needs further theoretical research and practical breakthroughs.

Conclusion
The transmission of information is constantly improving with the continuous development of communication industry.From the early face-to-face communication to the popularization of modern radiocommunication technology, radiocommunication services is dispensable for communication activities in people's lives.The main trend in the development of wireless communication systems is multi-functional, large-capacity, and ultra-wideband systems.
At present, electronic equipment and communication technology are becoming multifunctional.In order to reach better performance in aspects of using area, access speed, coverage area, etc. it is undoubted that the design method requires multiple technologies to complement each other.Consequently, the problem of reducing the volume of electronic equipment has risen.To coordinate the conflict between multifunction and small volume, electronic equipment needs to be highly integrated.For antennas, while meeting the standards of impedance matching, broadband, low cross-polarization level, and high isolation, it needs to be equipped with high integration, strong anti-interference, miniaturization, and other characteristics.But at the same time, the number of integrated antennas in a single system gradually increases, resulting in a decrease in antennas' performance.In this complex communication environment, the interference between signals is more intense, which puts forward higher standards for filtering antennas.
In addition, improving transmission capacity and transmission speed is the goal of wireless communication system development.In 5G practical applications, MIMO antennas can greatly improve the communication channel capacity and data transmission rate while without losing frequency bandwidth and transmission power.Filtering antenna can effectively reduce the size and cost of the RF front end of the MIMO antenna, which has great practical value in applications.However, when apply filtering antenna in the practical situation, the size and cost must be taken into consideration, which can be seen as the practical requirement for filtering antenna applications.
At present, due to the requirements of large capacity and high speed, the bandwidth used is much greater than that of the 2G/3G/4G era.This further increases the research demand for new antennas.As a transmitting and receiving device for signals, antenna is a very important part of signal transmission in radio communication systems.The antenna structure directly affects the performance of the antenna, furthermore, it affects the communication quality of the entire system.The design of the antenna is essential for the entire communication system.The Yagi antenna mentioned in this paper has been facing the problem of narrow bandwidth, and it is difficult to adapt to the new large bandwidth demand.Therefore, how to improve the impedance bandwidth of the antenna is necessary.The main methods are: reducing the Q value, adopting multiple resonance modes, parasitic structure loading technology, etc. [12].
This paper takes the Yagi filtering antenna as an example to study the 5G filtering antenna, summarizes the development process of the filtering antenna, and finally compares three design methods of filtering antennas.In the process of research, it is found that the design scheme of various filtering antennas strengths certain performance of filtering antenna.At the same time, it will also generate problems such as poor isolation, unsatisfactory coupling coefficient and difficulty in achieving the manufacturing process as well.Although there is no such thing as a perfect antenna in the current design scheme, filtering antenna can be carefully designed under the demand of required parameters to better suit the current situation.So far, the second and third design methods mentioned in the paper do not have a perfect design flow.This has great potential in future development.It is conceivable that the antenna design with small size and high integration will be more adaptable to the current communication system, and the filtering antenna with crossed multiple technologies needs to be further studied.

Figure 2 .
Figure 2. The schematic diagram of filters.