# A Review on 5G Sub-6 GHz Base Station Antenna Design Challenges
## 3 Challenges in BSA Design
### Port Measurements: Port to Port Isolation
Coupled resonator-based decoupling network is deployed in [41] to improve inter-band isolation from 8 to 10 dB. The currents induced by the coupled resonator helps to cancel the strong coupling between antennas operated in two frequency bands. Decoupling and matching network techniques implemented in [42] achieve inter-band isolation level over 10 dB in the 1.71 GHz t-1.76 GHz and 2.27-2.32 GHz bands. Figure 8: (**a**) Two-port single band antenna. (**b**) Four-port single band antenna. Figure 9: A dual-band antenna. As discussed in Section 3.4, the compactness of BSA design embedded scheme can cause low isolation. Different techniques were tried to improve the isolation in such schemes. In [1], a ring-shaped baffle is placed between the lower band and high band elements to decouple the two bands and achieve port-to-port isolation of 23 dB in the low band (0.77 to 0.98 GHz) and 17.5 dB in the high band (1.65 to 2.9 GHz). In [43], a similar configuration, in which a high band element nested inside a lower band, is used. Four arc-shaped baffle plates are used in this work, which results in port isolation greater than 27.3 dB for the lower band 704-960 MHz and 28.3 dB for the high band 1710-2690 MHz. The \(\pm 45^{\circ}\) dual-polarized antenna with the dielectric cavity achieves crosspolar isolation of 40 dB in [44] using carefully positioned symmetrical two shorting pins in the coax feedline. An orthogonal coupled sectorial loop-antennas with a cavity is used to achieve >30 dB intra-band isolation over a 1710-2170 MHz band in [35]. Decoupling networks such as bandgap structure [45], band stop decoupling unit [23], and filtering antenna elements [24] are also some commonly used techniques. A filtering technique with different configurations is employed to get better isolation. C-shaped filtering stubs as shown in Figure 10a are introduced in [15] for achieving port-to-port isolation >25. The purpose of the filtering stub is to control the current flow across the feeding line of the relevant port to act as a band stop for the specific band to achieve high isolation. Filtering response through parasitic elements as shown in Figure 10b is realized in [20], resulting in improved isolation of greater than 32 dB. The basic filtering structures include metasurface structure [46], slot [47], shorting vias [48], and parasitic elements [49], and defected ground structure [50] integrated with the radiator to achieve the filtering response for base station application. Although an extra filtering structure increases insertion loss, the filtering antenna as array elements without extra decoupling structure has been proposed in [24]. The radiating element realized the filtering response by adding the shorting pins and E-slot to achieve inter-band isolation of 35 dB. Balun design is modified to provide required filtering removing the need to have additional filters in [51]. Intra-band isolation >31 dB is achieved in [52] by carefully designing the dipoleto mutual coupling to complement the cross-polar isolation. A configuration named lower-band-ground-upper-band (L-G-U), where the high-band antennas are located above the lower-band antenna separated by a low pass surface, is presented in [53]. It demonstrates inter-band isolation better than 30 dB in both working bands. In [54], a frequency selective surface is introduced between the high band and low band elements in the L-G-U configuration to achieve inter-band isolation >25 dB. The frequency-selective surface is optimized to serve as top capacitive loading for low-band 0.69-0.96 GHz and act as a reflector for high-band 3.5-4.9 GHz. In [55], beam-to-beam isolation > 32 dB was achieved through Luneburg lens. This Luneburg lens antenna operates from 1710-2690 MHz, made up of a special periodic structure to become suitable for base station application. In the literature, a Luneburg lens with different materials was \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
\begin{tabular}{c} Cross polar \\ isolation (HB) \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}