# A Review on 5G Sub-6 GHz Base Station Antenna Design Challenges
###### Abstract  
Modern wireless networks such as 5G require multiband MIMO-supported Base Station Antennas. As a result, antennas have multiple ports to support a range of frequency bands leading to multiple arrays within one compact antenna enclosure. The close proximity of the arrays results in significant scattering degrading pattern performance of each band while coupling between arrays leads to degradation in return loss and port-to-port isolations. Different design techniques are adopted in the literature to overcome such challenges. This paper provides a classification of challenges in BSA design and a cohesive list of design techniques adopted in the literature to overcome such challenges. base station antenna challenges; multiband antennas; multibeam antennas; antenna arrays +
Footnote †: journal: _electronics_  
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of the techniques used for capacity improvement is sectorization. A common sectorization technique used in 2G is to divide the previous omnidirectional cell into three sectors of 120\({}^{\circ}\) each. As a result, three antennas, each having a 10 dB beamwidth of 120\({}^{\circ}\), were used in the BTS. Another technique used in 2G networks to enhance capacity is the use of polarization diversity. The aim was to provide two orthogonal polarizations in the antenna array. The horizontal (H) and vertical (V) polarization were initially used, but \(\pm 45^{\circ}\), otherwise known as slant polarization, has been widely adopted in many BTS antennas since 2G. Figure 1a,b shows vertical and slant polarized dipole array configurations used in BSAs in 2G and following generations. The number of users increased rapidly, moving from 2G to 3G due to the introduction of mobile data services in 3G. As a result, operators had to explore further techniques to increase capacity. One solution was to further subdivide the sector into narrower sectors. This was done using narrow beam antennas with half-power beamwidth (HPBW) such as 65\({}^{\circ}\) or even 33\({}^{\circ}\). One of the disadvantages was increased antenna loading on the tower. A solution to antenna loading was achieved by introducing multibeam panel antennas. Such antennas are similar in appearance to conventional sector antennas but have multiple narrow beams. As a result, increased capacity is achieved without the need for additional antennas, as shown in Figure 2. The twin beams or multibeams are achieved by introducing hybrid couplers into the feed network. These multibeam antennas have been a prevalent choice among operators for mobile networks since then. An increased number of cells/sectors requires comprehensive network planning to reduce cell edge interference and the need for operators to adjust the cell/sector coverage. This is achieved in a BSA antenna by providing beam tilting in the elevation plane. Most of the BSA antennas from 3G networks and later were equipped with a Remote Electrical Tilt (RET) feature, allowing the operator to remotely configure the elevation beam tilt in a 0-10\({}^{\circ}\) range to optimize the network. The elevation beam tilt in the antenna is achieved using a phase shifter that provides a phase gradient to the elements in the antenna array. Therefore, most of the BSA antennas from 3G networks and beyond had the capability of beam tilting in the elevation plane. However, the elevation beam steering feature achieved using such phase shifters is only suitable for network optimization, not for beamforming in the elevation plane. With 2G and 3G technologies, an additional spectrum was introduced beyond previously used 800 MHz and 900 MHz bands. Primarily occupied  
Figure 1: (**a**) Vertical polarized dipole array. (**b**) Slant polarized dipole array. Figure 2: Patterns of a single-beam BSA, two narrow-beam BSAs, and a twin-beam BSA. higher frequency bands worldwide were in 1710-2100 MHz bands. With 4G LTE, additional spectra up to 2.6 GHz were used worldwide. In 5G, sub-6 GHz bands have an additional spectrum in 700 MHz bands and 3.4-3.6 GHz bands.