A novel topology has been proposed for FDD+TDD base station array antennas. By employing this topology along with coaxial antennas, array radial and axial displacement techniques, multiple columns of 2G/3G/4G/5G base station antennas can be arranged on a relatively small power divider board while meeting the requirements for standing wave ratio (SWR) and gain. This approach significantly reduces the overall size of the FDD+TDD array antennas. Through a series of measurements, it has been observed that the base station antennas using this array topology (1400 mm × 490 mm × 105 mm) operate within the desired frequency bands with SWR values all below 1.4, and the half-power beamwidth remains within the range of 65±10 degrees.
A novel multi-antenna coexistence design for FDD+TDD base station antenna arrays is proposed, combining Low Band Antennas (LBAs) operating in the 0.69-0.96 GHz range and 1.4-2.7 GHz range, and Middle Band Antennas (MBAs) operating in the 1.4-2.7 GHz range. By using a compact topology and incorporating appropriate power dividers and phase shift circuits, the transmit and receive frequency bands can be controlled, and the array elements are organized into two columns, significantly reducing the overall size of the FDD+TDD antenna array [1-5]. Additionally, High Band Antennas (HBAs) operating in the 3.3-3.8 GHz range can form an N×M large-scale Multiple Input Multiple Output (MIMO) array when equipped with specialized power dividers, phase shift circuits, and connected to baseband and active circuits, allowing the uplink and downlink frequency bands to overlap in time segments, creating a Time Division Duplexing (TDD) base station antenna array suitable for 5G New Radio (NR) [6-8].
5G networks provide real-time seamless connectivity for Internet of Things (IoT) and Machine to Machine (M2M) communication, enabling the realization of the Internet of Everything. However, the deployment of 5G devices brings challenges to operators and equipment manufacturers, especially in terms of operational costs and base station architecture. For many specific environments, 5G devices cannot replace existing LTE networks. As a result, it is widely believed that 5G devices and LTE networks will coexist for a long time [9]. Based on this, the emergence of FDD+TDD base station antenna array topology is a natural development.
For FDD+TDD base station antennas, typically, FDD arrays are used in the 0.69-0.96 GHz and 1.4-2.7 GHz frequency bands, while TDD arrays are used in the 3.3-3.8 GHz frequency band. Furthermore, when HBAs and their controlling active circuits, power dividers, and phase shift circuits are integrated, and baseband networks are connected, it is possible to form a Time Division Duplexing Active Antenna Unit (AAU) base station antenna array.
To design the aforementioned FDD+TDD base station antenna array, the first step is to place LBAs, MBAs, and HBAs in the same antenna array while ensuring that they work effectively in their respective frequency bands without interfering with each other. However, in practice, when LBAs and MBAs are combined, the mutual coupling effect between them can be significant, especially when reducing the size of the array. Due to the height difference between LBAs and MBAs (LBAs are often taller than MBAs), when MBA operates, some electric fields scatter onto LBAs, causing antenna beam distortion, reduced gain, and degraded VSWR. To address this, researchers have explored various approaches, including the use of periodic or non-periodic filtering circuits mounted on the array radiating surface or balun and matching circuits to filter out cross-band interference; using periodic or non-periodic circuit boundaries on the ground plane beneath or near the array elements to absorb radiated waves; and applying special topologies to shape the array's beams to form a "0" pattern and adjust the gain by controlling beam width [10-15]. Additionally, there are special methods using metamaterials, metasurfaces, and acoustic surfaces to solve cross-band interference issues.
In the third category of research, when the array elements are densely distributed, any topology used will lead to various forms of distortion in the array antenna's radiation pattern due to interference between elements. Therefore, a combination of the above research results may become a major future direction. However, simultaneously using all these approaches in the array can result in high base station costs and complex component assembly, making it unsuitable for mass production. Therefore, for equipment manufacturers, the challenge lies in choosing the appropriate filtering circuits, frequency-selective surfaces, and topology, as well as how to integrate them to develop and produce the base station antenna.
For HBAs, beamforming technology is critical. The larger the number and scale of the antennas, the more significant the role beamforming technology can play. However, when the array size remains constant, the mutual coupling between antenna elements can degrade the performance of any individual HBA, resulting in suboptimal beamforming. Therefore, it is essential to design the topology of HBAs and set isolation bars between elements. Since HBAs adopt a multiple-input multiple-output structure, multiple input ports need to be set, which means that when adopting a new topology for HBAs, the power dividers, phase shift circuits, and cable routing all need to be redesigned, posing a considerable challenge for designers.
This paper proposes a multi-antenna coexistence design suitable for FDD+TDD base station antenna arrays, significantly reducing the overall size while preserving SWR, gain, and cross-polarization ratio. The design features five key aspects:
1. Coaxial technology is used to mount MBAs above LBAs with filtering circuits, creating Dual-band Dual-polarized Shared Aperture Antennas (DSA antennas) for both the 0.69-0.96 GHz and 1.7-2.7 GHz frequency bands. Different aperture directional couplers are placed above and below the DSA antennas, ensuring good SWR and gain while substantially reducing the FDD base station antenna array size.
2. Isolation bars are installed between LBA elements and between LBA and MBA elements, providing sufficient isolation even when the elements are closely spaced, minimizing coupling effects.
3. The DSA antenna elements at the axial end of the array are radially displaced while ensuring the cross-polarization ratio meets standards, reducing the Half-Power Beam Width (HPBW) of the radiated beam for the entire DSA antenna array.4. Isolation bars of varying heights and orientations are appropriately loaded in the off-axis or radial directions of each unit in the 5G array to maintain good isolation between HBAs.
5. The two columns containing DSA antennas and MBAs are placed on either side of the 5G array, ensuring that each column does not interfere with the other, thereby eliminating unnecessary costs associated with isolation bars for two columns of the same type of antennas.
The following sections provide a detailed explanation of this design.
Figure 1: Antenna model for the proposed FDD+TDD base station antenna array. The array is divided into two regions: the FDD antenna area and the TDD antenna area, with FDD antennas on both sides of the TDD antennas, symmetrically about the array's central axis. Each region contains a column of LBAs and a column of MBAs, except for DSA1 in the figure, all other LBAs are coaxial with MBAs. It should be noted that in this context, MBA refers to both MBA1 and MBA2 in Figure 1, collectively referred to as MBA in this paper.

The DSA antenna in Figure 1 is composed of an LBA and MBA1, where MBA1 is a dual-polarized dipole antenna used for the 1.7-2.7 GHz frequency range [16]. It is placed above the LBA, and the two share the same aperture, i.e., the line connecting the centers of LBA and MBA1 is parallel to the array's normal vector.
On each side of the DSA antenna, there is an MBA2, consisting of three layers of radiating elements and a balun, with a different shape from MBA1 but operating in the same 1.7-2.7 GHz frequency range [18]. When the array operates in the 1.7-2.7 GHz range, all MBA1 and MBA2 in the FDD antenna area are simultaneously excited, working as MBAs with a gain of up to 15 dBi.
Since the spacing between the DSA antenna elements and MBA2 elements in the FDD antenna area is small, filtering circuits are placed on the radiating surface of the LBA, and isolation bars are set between DSA antenna elements and MBA2 elements to prevent interference [19]. Using these two methods, the FDD antenna array can radiate normally with a reduced overall footprint.
In the TDD antenna area, there are five columns of HBAs, as shown in Figure 1, each column containing 12 elements, totaling 60 elements, arranged sequentially in the region. The HBA is a dual-polarized dipole antenna that uses efficient feeding methods and can be used in the 2.3-3.8 GHz frequency range. The antenna in [20] is modified by removing the Lange coupler and resizing the dual-line polarized radiating element for the 2.3-3.8 GHz frequency range. When equipped with efficient feeding circuits and phase shift circuits, the HBA in Figure 1 is formed. Below the elements, there are power dividers and phase shift circuits tailored to the input ports. Some circuit boards are longer and supply current to two HBAs, while others are shorter and supply current to only one HBA. Isolation bars and reflectors are placed around the elements, ensuring good isolation for all HBAs and maintaining the overall beamforming effect.
All elements, power dividers, phase shift circuits, and isolation bars of the base station antenna are covered with a 0.2 mm thick insulating pad made of FR4 material. This insulating pad effectively isolates the components above the ground plane from the ground plane, ensuring the proper third-order intermodulation of the base station antenna as a whole. Below the ground plane are the power dividers and phase shift circuits that control the overall current flow of the base station antenna and control the power dividers and phase shift circuits for the FDD antenna area and TDD antenna area. There are also coaxial cables connecting various power dividers. As these modules are unrelated to the proposed topology in this paper, they are not further discussed.
2.1 DSA Antenna Design Principles:
As shown in Figure 1, the DSA antenna is composed of an LBA and MBA1, with MBA1 positioned directly above the LBA, and their feed lines passing through the center of the other. The two do not have direct contact. The radiating surface of the LBA consists of two parts, one responsible for the 45° polarized electric field and the other for the -45° polarized electric field. The circuit shapes of these two radiating surface parts are identical and oriented orthogonally to each other. Each part has two sets of band-stop filters that filter out the waves radiated by MBA1, preventing significant scattering on the LBA and enabling the DSA antenna to work effectively in the 0.69-0.96 GHz and 1.7-2.7 GHz frequency ranges.
The radiating body of the LBA bends upward around the MBA1, serving three purposes: First, it reduces the cross-sectional area of the LBA while maintaining its equivalent aperture size; second, it houses the filtering structure that removes the waves radiated by MBA1; third, it forms a ring of reflection bands around MBA1. Because MBA1 is located above the LBA, the bent radiating body is placed around MBA1's balun, increasing MBA1's directionality and enhancing its energy in the direction of the intended angle. This ensures that the overall gain of the upward radiation beam from the MBA is not significantly reduced.
2.2 FDD Antenna Topology Principles:
When a single antenna transmits a signal, it experiences multipath fading during transmission. To reduce this effect, modern base stations use diversity techniques [21] and, in turn, use multipath fading to improve communication efficiency and quality. As time progresses from 2G to the current 5G, base station antennas have evolved from single-input single-output to multiple-input multiple-output. However, this transmission mode requires multiple columns of elements arranged in a small space, necessitating a special topology design to avoid interference between elements. Assuming the directional pattern function is F e (θ, ) and the antenna elements are arranged as shown in Figure 2, with the first element located at the origin of the coordinate axis, denoted as z 0, and the complex vector of the input current denoted as İ 0. Let the phase difference between the input currents of the first element and the second element be α 0, and the coordinate of the nth element on the Z-axis be z n-1, with its complex vector of input current denoted as İ n-1, and the phase difference between it and the previous element denoted as α n-1. The distance between the array and a distant point P is approximately r, and assuming α 0 is 0, under the ideal condition of no energy loss in the antenna's own path, the total field Ė of the line array at point P(r, θ, ) is given by:
where A is a scaling factor introduced due to the antenna element's actual form, accounting for the differences from the ideal point source due to the antenna's actual shape. Here, k can be understood as the propagation constant.
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If the spacing between elements is equal and denoted as d, and the relative phase difference of the feed currents between each element is α, then Equation (1) can be expressed as:
Equation (2) is obtained from Equation (1) with the assumption of equal element spacing and relative phase differences of the feed currents.
To obtain the radiation pattern of the FDD antenna area, the variation of the electric field with coordinates can be plotted based on the above equation.
For the FDD antenna area in Figure 1, the design of the DSA and MBA antennas can refer to the literature [7]. However, when they are distributed as shown in Figure 1, the coupling between the DSA and MBA antennas can be severe. To address this, modifications are made to the DSA antenna's radiating surface by bending the outer part of the LBA's radiating surface upward and creating a hollow structure that can form an equivalent band-stop filtering circuit with the circuits on the radiating surface. This structure allows the waves radiated by the MBA antenna to be absorbed by the DSA antenna, preventing any scattering.

Additionally, there are other impacts between the MBA and DSA antennas. When the MBA is in operation, the close proximity between MBA1 and MBA2 results in reduced isolation. The waves radiated by MBA2 in both the direction of its intended angle and the opposite direction will cause induction currents when they hit the ground plane. These induction currents will flow through the feed lines under the DSA antenna and partly reach the LBA, causing irregular radiation patterns. Another portion of the currents will reach MBA1, deteriorating the isolation between MBA1 and MBA2. To address this, isolation bars are placed between MBA2 and the DSA antenna (Isolation Bar 1 and Isolation Bar 2 in Figure 1) to isolate the currents between them, thereby improving the isolation between MBA2 and DSA.
2.3 TDD Antenna Topology Principles:
In Figure 1, it can be observed that the TDD antenna area employs a staggered arrangement of the HBA elements in adjacent columns. Assuming the distance between each element in a column is d1, they are shifted in the axial positive or negative direction by d1/2 relative to the adjacent column. For FDD antennas, each column works independently, so interference between columns must be completely eliminated. On the other hand, for MIMO antenna arrays, the distinction between columns and rows is not as crucial as in FDD antennas. MIMO treats all HBAs as a planar array and operates by changing phase and current amplitude. Therefore, for the topology design of the TDD antenna area in Figure 1, the spacing between each HBA is reasonably planned.
If the four columns of HBA in Figure 1, from left to right, are labeled as the 1st to the 4th column, and their respective total electric field vectors are Ė1 to Ė4, the total electric field radiated by the TDD antenna area can be represented as Ė. Then:
Equation (3) represents the total electric field radiated by the TDD antenna area, considering the contribution from each HBA.
To simplify the calculation, the entire array in the TDD antenna area can be represented as shown in Figure 3. The first column remains aligned with the z-axis, as in the line array in Figure 2. Each subsequent column is displaced in the y-direction by m, and the first element in each column (from left to right) is displaced in the z-direction by n. The equivalent distance from the 1st to the 4th column to the far-field point P is denoted as r1 to r4, and the input current complex vectors for each column are represented as I1 to I4.
Correct, by substituting the expressions in Equation (1), one can calculate the total electric field of the TDD antenna area even when some elements are displaced. As seen from this equation, as long as the amplitudes and phases of some HBAs are controlled, even with partial element displacement, the total field can still be made to be similar to that of a conventional topology. Moreover, the cross-polarization ratio will not exhibit significant deterioration. This property allows for effective control and optimization of the TDD antenna array while maintaining desired performance characteristics.

However, even with this topology, it is still not possible to completely eliminate the mutual coupling effects between adjacent HBAs. In order to ensure the miniaturization of the base station, this paper adds isolation bars between some elements. Based on measurements, isolation bars are only applied to adjacent ports with an isolation level greater than -20 dB. This prevents excessive loss in the spatial electric field due to the presence of too many isolation bars. As shown in Figure 1, within the TDD antenna area, there are isolation bars 1 extending along the array's radial direction to isolate the two ports with different polarizations. Additionally, there are isolation bars 2 extending along the 45° direction of the array to isolate the two ports with the same polarization. Furthermore, it is observed in Figure 1 that no isolation bars are placed between the HBAs sharing the same power distribution circuit board. This is because this design directly incorporates filtering circuits on the power distribution board. This approach effectively filters out currents from other antennas flowing into the respective ports, significantly reduces costs, and minimizes the deterioration of the cross-polarization ratio and gain due to the addition of excessive isolation bars between elements.
By distributing the 5G antenna array as shown in Figure 3 and adding the various isolation bars mentioned above, simulation results are shown in Figure 4. Figure 4(a) illustrates the vertical plane pattern of the TDD antenna at 3.5 GHz when the antenna beam points to the side direction of the base station with a 2° elevation angle, achieved by controlling the phase difference of the 5G element currents. Figure 4(b) depicts the vertical plane pattern of the TDD antenna at 3.5 GHz when the antenna beam points to the side direction of the base station with a 12° elevation angle, also achieved by controlling the phase difference of the 5G element currents.
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Absolutely, by substituting Equation (3) into the variation of Equation (4), it is indeed possible to calculate the electric field generated by the entire array when the terminal DSA1 antenna is radially displaced relative to other DSA elements. As a result of this design, the HPBW (Half-Power Beam Width) of the antenna narrows when operating in the frequency bands of 0.69-0.96 GHz and 1.7-2.7 GHz. This narrowing of HPBW indicates that the beam becomes more focused and directional, which can be advantageous for achieving higher gain and improved performance in certain applications.
The practical results of the assembled array, with integrated power division circuits and external components, were conducted to validate the effectiveness of the proposed design. The test results, as shown in Figure 5, indicate that the array operates well in all frequency bands. The Voltage Standing Wave Ratio (VSWR) is maintained below 1.4, ensuring efficient power transfer and minimal signal reflection. Additionally, the beam shows no distortion, and the Half-Power Beam Width (HPBW) remains within the desired range of 65±10 degrees. These results provide evidence that the proposed topology is reliable and performs as expected.

In order to verify the effectiveness of the isolation bars incorporated in the array design, a comparison was conducted by removing some isolation bars from the array and comparing the measured results with the complete array design, as shown in Figure 6. It can be observed that, when isolation bars 1 and 2 used for the FDD antenna area are removed, the isolation between MBA1 and MBA2 antennas at 1.7~2.0 GHz is greater than -20 dB, failing to meet the requirements for the base station antenna.
Upon adding isolation bar 1 alone, the isolation improves by approximately 7 dB within the 1.7~2.0 GHz frequency range but still falls short of the base station antenna requirements. However, when both isolation bar 1 and isolation bar 2 are installed simultaneously, the isolation remains below -28 dB across the entire operating frequency range, meeting the specifications for the base station antenna. These results confirm the significant impact of the isolation bars in enhancing the isolation between antennas and ensuring the performance criteria for the entire base station antenna.

Conclusion
In this study, we proposed a design for a base station antenna array that combines FDD and TDD functionalities. By using a co-axial configuration of DSA antennas with LBA and MBA, we reduced the overall footprint of LBA and MBA in the base station. To ensure the effective operation of MBA and DSA with reduced spacing, we employed various isolation bars to isolate different ports, enabling the FDD antenna area to work well in both 0.69-0.96 GHz and 1.7-2.7 GHz frequency bands.
To ensure independent operation of each FDD antenna array column in the base station, we placed two FDD antenna arrays on either side of the base station, ensuring no interference between the two columns. Additionally, we radially misaligned the end DSA1 antenna relative to other DSA antennas to reduce the beamwidth in the 0.69-0.96 GHz and 1.7-2.7 GHz frequency bands. In the TDD antenna area, we employed partial axial misalignment of HBA units and appropriately paired them with different types of isolation bars to optimize the isolation between each HBA unit, ensuring optimal performance of the HBA array.
Through practical measurements, we verified the reliability of this design and successfully achieved miniaturization of the FDD+TDD array. Future work will focus on further optimizing the array by introducing more efficient reflectors to achieve higher gain while maintaining its compact size.
This design significantly reduces the size of the FDD+TDD base station antenna array without compromising its performance, making it highly competitive in the future 5G and even 6G markets.
★Scan the QR code to access and download the full article on CNKI (China National Knowledge Infrastructure). ★Original article published in Mobile Communications, 2022, Issue 3. doi: 10.3969/j.issn.1006-1010.2022.03.010 Chinese Library Classification No.: TN929.5 Document Identification Code: A Article ID: 1006-1010(2022)03-0055-07
Author Profiles: Liu Mulin: Senior Engineer, holds a master's degree from Xidian University, currently serves as Vice President and Antenna Division Manager at Guangdong Tongyu Communication Co., Ltd., mainly engaged in antenna project development and research. Yue Cailong: Graduated with a bachelor's degree from the University of Electronic Science and Technology of China, currently serves as the Research and Development Manager at Guangdong Tongyu Communication Co., Ltd., mainly engaged in antenna research and development work, focusing on multi-frequency multi-system product design, ultra-wideband unit design, phase shift network design, and array topology design.