Sub-6 GHz band massive MIMO antenna system for variable deployment scenarios in 5G base stations

A compact massive multi input multi output (MIMO) antenna system with 1 × 4 (sector) subarray configuration operating at sub-6 GHz range for 5th generation (5G) base stations has been designed and analysed in various geometries (rectangular, triangular, and hexagonal). The capacity of the system can be increased by more than 10 times whereas the energy efficiency can be increased 100 times using the massive MIMO system (Larsson et al. in IEEE Commun Mag 52(2):186–195, 2014). A maximum of 5 sectors has been used, with each sector comprising of 1 × 4 subarray elements. Each sector consists of three layers, in which 1 × 4 patches are located on its top layer whereas it’s feeding network and ground plane are placed at the bottom layer and middle layer respectively. The entire system can operate in two modes, individual port operation and massive MIMO array operation with beam steering capabilities. The measured bandwidth of the system is 140 MHz that covers the frequencies from 3.36 to 3.50 GHz in sub-6 GHz band. The overall dimension of a unit subarray in terms of length, width and height is 280.5 × 56.1 × 2 mm3. The gain of an individual port is found to be 12.77 dBi and the overall gain of a single panel with 5 sectors arranged in rectangular form is 19.73 dBi. Mutual coupling among all the ports has been kept less than − 16 dB. The operating frequency range of the antenna array system is chosen in the range of 3.3–3.8 GHz, as this band has been allocated and targeted across the globe to enable 5G in Sub-6 GHz band.


Introduction
5G is expected to rule the world soon due to its ability to support ultrahigh speed broadband connection with minimum latencies in comparison to its old generations including 4G. With increase in demand for autonomous vehicles along with the requirement of smart cities and over the air internet, 5G will be the heart of the wireless communication environment (Larsson et al. 2014). Advanced coding techniques, millimetre wave (mm wave) communication, massive MIMO systems, and other 5G technologies can be used to meet the above requirements (Gao et al. 2015;Marzetta 2015;Bjornson et al. 2016). In this work, a massive MIMO antenna system is designed which is expected to have increased data rate and better efficiency in terms of spectrum, energy, and reduced interference due to its beam steering capabilities. Also, less expensive low power components can be preferred for massive MIMO systems which is an added advantage (Lu et al. 2014;Krishna and Sharma 2017;Ma et al. 2014). 5G spectrum bands are categorized into Low band, Mid band (Sub-6 GHz band) and High band (mm wave band), out of which Sub-6 GHz band is the most sorted region in the frequency spectrum due to its merits over mm wave in terms of distance coverage. Even though millimetre wave spectrum supports highest speeds, its limited coverage makes it impossible to deploy with immediate effect. But it can be deployed in selected locations like high traffic areas (Rajagopal et al. 2011;Ali and Sebak 2016;Garcia-Morales et al. 2020). As per the literature review, very few papers were found in the field of massive MIMO antenna system design. Most of those papers show design based on arrays operating in the frequency range of 28-38 GHz, which falls under millimetre wave communication. But in few works tested results based on fabricated prototype was left as future scope (Cai et al. 2020;Kabiri et al. 2019;Hong et al. 2017;Hong et al. 2017). The literature survey also shows that an antenna has been designed in low frequency (i.e., in ISM band) and simulation results have been displayed without any test results (Matalatala et al. 2020;Smith et al. 2020;Wang et al. 2020;Umer et al. 2020). Also, few research works shows the design of array antenna in either hexagonal or in triangular configuration (You et al. 2020). Research has been done with modified rectangular patch antenna loaded with multiple C slots for multiple applications and modified U-slot stacked microstrip patch antenna for ultra-wideband applications in S Band, C Band and X Band (Surana and Jain 2017;Jain and Surana 2016). Few other research works explain the effective use of reflectors and arrays in massive MIMO systems (Buzzi et al. 2020). In this work, we propose a massive MIMO antenna system design with multiple sectors, each sector comprising of 4 patches in 1 9 4 geometry. Different configurations like single sector, 3 sectors in triangular shape and 5 sectors in hexagonal shape has been designed and simulated. This work has been done with the assumption of maximum 5 sectors, each with 4 ports, which leads to a total of 20 ports. The number of ports was limited to 20 due to its increased fabrication cost and limited testing facilities. So, the system can be made with multiple number of ports if budget and testing facilities are not limited. The motivation of this research is to perform an analysis of various massive MIMO antenna geometries and to propose a compact antenna structure with beam steering ability.
The organization of this article is as follows. Section 1 gives a brief introduction and summarises the literature review which gives an insight about the proposed work in comparison with various other existing works. Section 2 explains the antenna system design. Section 3 comprises the simulation and fabrication details of the antenna and its result analysis for different antenna geometries. Also, the scattering parameters are analysed and compared. In Sect. 4, antenna testing and validation is explained. In Sect. 5, conclusion of present research work is made followed by acknowledgement in Sect. 6. Table 1 gives a clear picture about the different research works done by the researchers in the field of massive MIMO antenna system. The present work involves the antenna design in more than one geometry, so that the required configurations can be chosen and implemented in suitable deployment scenario. This gives the above work an edge over others where antenna array design is done in one geometry which limits its applications.

Single sector design
The patch antenna has been planned by ascertaining the significant boundaries like working frequency, dielectric constant, and height of the substrate by utilizing the plan The length, width, and height of the conventional patch antenna for 3.3-3.8 GHz is found to be 20.81 9 20.81 9 1 mm 3 . A solitary sector includes a 1 9 4 patch array made of a three-layer Printed Circuit Board (PCB). The circularly polarized patch antenna is arranged on the top layer of the PCB. The top layer patches are dealt with signals from the bottommost layer through via (hole) along the middle layer. Feed line is engraved in the base layer however focus layer is believed to be the ground plane which will be the reference for both the patch layer and the feed network layer. The substrate used in this arrangement is FR4 with a thickness of 1 mm and relative permittivity of 4.05. Figure 1 shows the portrayals of a single sector with four patches. Figure 2 depicts the 3D view of the 5-sector antenna array structure coordinated in hexagonal geometry.

Feed system
The reduction of false radiation from the feed system can be made possible by attaching the corporate feed to the base of the antenna structure, so that it will be suitable for base station installation purposes without pre-level undesirable associations. Each element of the sector was connected to the feeding structure below with the help of a via through the ground plane. Instead of using any other impedance matching network, impedance can be matched by varying the pin-inset distance from the patch edge with relevant changes in the corporate feed under the structure. Among the number of methods available to feed a 4 by 1 micro strip patch array structure, this design incorporates all the co-polarized antenna fed from a single point beneath. Quarter wave transformer has been connected to permit some degree of impedance matching. Efficiency of the antenna array may be affected due to the ohmic and dielectric losses in the resulting feed network. But the use of low loss tangent dielectric can overcome few of the loss mechanisms.

MIMO antenna array system
The antenna system works at 3.4 GHz frequency and has a deliberate transmission capacity of 140 MHz. Beam steering can be achieved when more than one segment array is implemented in one sector. The total dimension of a single sector is 280.5 9 56.1 9 2 mm. 3 Figure 3 shows the pictorial representation of the relative multitude of three configurations in rectangular, triangular, and hexagonal geometries.

Simulation and measured resultsdiscussion
Simulation of the antenna has been performed using 3D EM simulation software CST Studio suite. Tested results versus simulated results will give a better picture on the performance of the antenna system in different configurations. So, a suitable configuration can be proposed for varying deployment scenarios. Figures 4 and 5 shows the simulation model as well as the fabricated prototype model for measurement. The above results show that the maximum total gain of 12.95 dBi can be achieved for each unit sector, whereas the total gain of the panel comprising of all 5 sectors is found to be 19.73 dBi. Figure 6a, b shows the measurement setup using Vector Network Analyzer (VNA). The simulated and measured reflection coefficient of a unit sector is shown in Fig. 7a, b depicts the reflection coefficient of the unit port in real VNA format.
The frequency band supported by the designed massive MIMO array is from 3.36 to 3.50 GHz, which clearly fits into the sub-6 GHz band and is the targeted and allocated band for 5G wireless communication across the world for immediate deployment. Figure 7c shows the reflection coefficients of all the five sectors in log magnitude scale when measured with vector network analyser. All the five sectors seem to be in close vicinity in terms of performance. Envelope Correlation Coefficient (ECC) (Blanch et al. 2003) is calculated using the mathematical formula given below. ECC will let us know how independent the antenna is without any interference.
where ! F1 h; u ð Þ is the radiation pattern of the antenna system when port 'i' is excited. Computation of correlation coefficient using theabove-mentioned formula requires the simulated or measured radiation patterns of the antenna. Moreover, this approach is time consuming as well as hard to measure.
Instead, it will be appropriate to use the S-parameter characterization for the calculation of correlation between antenna elements in an array antenna. This is a much simpler method since it does not require the antenna radiation pattern of the arrays. The main advantage of this method is the fast analysis and broadband analysis of correlation results. However, this method will be under the assumption that all antennas are lossless along with uniformly distributed incoming waves. Figure 8 shows the simulated and measured coupling coefficients between port 1 and the remaining ports in rectangular array configuration. Fig. 9 shows the simulated and measured coupling coefficients between various ports in the all-important hexagonal geometry. Table 2 shows the comparison between three different configurations of the array (rectangular, triangular, and hexagonal). Maximum coupling between ports has been found to be -12.3 dB for rectangular array with 5 sectors (subarray) and -14.2 dB for triangular array with 3 sectors. The maximum coupling between the sectors is found to be -13.9 dB for hexagonal array geometry in the desired frequency band. Figures 10 and 11 show measured and simulated results of directivity of 5 sector (hexagonal configuration) and 3 sector (triangular configuration) respectively at phi = 0 and at phi = 90°. Table 3 shows the coupling factor between sectors in hexagonal configuration with various point of tendency (h) between the sectors as addressed in Fig. 2. It has been observed from the obtained results that the shared coupling addressed in dB diminishes with increment in the point of tendency between the areas. It should be perceived that the previously mentioned values in Table 3 is for corner ports only, as the change of angle (h) will have an impact with corner ports only. Any remaining contiguous ports will stay consistent as -12.3 dB irrespective of various geometries. The benefit of varying the angle (h) is that the angle of the beam steered towards the user can be adjusted with reduced coupling. Indeed, even without varying angle (h), the coupling between the various areas is discovered to be good.
With results showing reduced mutual coupling between sectors, the performance of the massive MIMO can be further improved by reduced interference due to its  Figures 12 and 13 shows the results of the beam steering abilities of unit sector and three sector structures at 0°, 90°and 120°r espectively. Here the beam steering process is accomplished by varying the phase of the input signals towards different elements (Younus et al. 2021). This process of phase shifting allows the beam to be targeted towards the desired user.
It has been understood from the obtained results that, with change in phase of the input signals to the ports, the proposed system attains beam steering property. Results also justify that beam steering will be more effective with conformal array configuration than with linear array due to its omnidirectional coverage. Table 4 gives us the comparison between beam steering capacities of rectangular and triangular configurations. Results show that the triangular configuration is delivering better performance with reduced angular width but with reduced main lobe gain. So, it is understood that massive MIMO array configurations with beam steering capabilities can be utilized in different application areas based on coverage requirement and geometric locations every geometry is giving convincing outcome in their own way. Rectangular configuration can be preferred over application areas where coverage is in single direction with multiple users. But conformal antenna structure like triangular and hexagonal geometries can be preferred in areas like city centre where omnidirectional coverage is required to Fig. 4 a 3D realized gain pattern of a unit sector, b 3D gain pattern of a 5-sector in one panel, c 3D gain pattern of a 3-sector unit, d 3D gain pattern of a 5-sector unit serve multiple users in any direction. With the beam steering capability of the proposed design, it will be apt to incorporate triangular or hexagonal geometry for uniformly populated areas. Research has also been done on usage of massive MIMO for underwater IOT and industry 4.0 environments (Lee 2021;Arnold et al. 2021).

Testing and validation
The proposed multi sector massive MIMO exhibit model is created by Zeta Printed Circuit Services fabrication unit, Chennai, India. A triple layer PCB has been utilized for the design of the array antenna with FR4 substrate. Design and fabrication were completed with the use of Gerber files obtained from the simulation software. Here, patches lie on the top layer while ground layer and feed network lie on the centre layer and base layer respectively. Testing of the fabricated antenna array system was done using Key sight vector network analyser, model 9916A located at the Keysight Centre of Excellence Laboratory, Department of Electronics and Communication Engineering, Kumaraguru College of Technology, Coimbatore, India. It is learned from the present work that, if the angle of inclination is  increased from 0 to 15°, the coupling between the sectors decreases drastically and also the beam steering angle of the arrays from the base station tower towards the user in ground level will be much improved.

Conclusion
The 5G Massive MIMO single base station antenna systems for sub 6 GHz band operation are well designed, simulated, fabricated, and measured. A 5-port antenna system is designed for the 1 9 4 antenna subarray. In this work, a massive MIMO antenna array system is designed and analysed in three different configurations (rectangular, triangular, and hexagonal). The maximum gain that can be achieved through a single port subarray is 12.95 dBi. Also, the bandwidth obtained in the system is 140 MHz. Above all, the analysis of the hexagonal geometry with all sectors inclined at 5-15°shows that the angle of radiation can be varied based on demand. It has been found from the obtained results that the performance of the designed antenna array in all the three configurations are in Fig. 7 a Simulated and tested reflection coefficient of the unit port. b Reflection coefficient of the unit port in real VNA format. c Measured reflection coefficients of all five sectors in log magnitude scale acceptable range which can be used for different deployment scenarios. Finally, it is a simple design which is more flexible where faulty antennas can be easily replaced by a spare one without interfering the operations of the entire array system.