# A Review on 5G Sub-6 GHz Base Station Antenna Design Challenges
## 3 Challenges in BSA Design
### Port Measurements: Impedance Bandwidth  
One of the key challenges for the modern base station antennas is the bandwidth. While the spectrum is harmonized for certain mobile telecommunication bands, different countries tend to use different portions of the spectrum based on the license given to operators. It is desirable to design base station antennas to cover the entire allocated spectrum despite operators not having access to full bandwidth as it allows one antenna design to be used globally instead of variants for different geographic markets. However, it is difficult to design a single base radiating element that operates in all the frequency bands listed in Table 2. Hence, it is often the practice to use multiple radiating elements to cover multiple bands.  
Impedance bandwidth is an important measure in BSAs as it provides the frequency band in which a minimum return loss level is achieved. Typically this is about 14 dB for BSAs [34]. It can be challenging to achieve the return loss requirements over a wide band. This wideband matching needs to be achieved both for radiating elements as well as the feed network to meet the requirements. Generally, the techniques used to widen the impedance bandwidth can be categorized into three parts. (a) Use of a wideband Balun (b) modifications to radiator, and (c) use of parasitic elements.  
\begin{table}
\begin{tabular}{c c c} \hline
**Design Challenge** & **Solutions/Techniques** & **References** \\ \hline \multirow{4}{*}{Achieving wide impedance bandwidth} & Wideband balun design & [9; 10; 11; 12] \\  & Modification to radiator shape & [13; 14; 15] \\  & Use of parasitic element/s to widen bandwidth & [16; 17; 18; 19; 20] \\ \hline \multirow{2}{*}{Achieving high isolation levels} & Use of differential feed structure & [16; 21; 22] \\  & Use of decoupling network & [15; 23; 24] \\ \hline \multirow{2}{*}{Stable HPBW in bandwidth of operation} & Cavity shape reflector & [16] \\  & Convex shaped reflector & [19] \\ \hline \multirow{2}{*}{Achieving high front-to-back ratio} & Modifications to the radiator-Downward & [25] \\  & sloping dipoles & \\ \hline \multirow{2}{*}{Minimizing Beam Squint} & Enforce symmetric current distribution on the & [26; 27; 28] \\  & radiating element & [26; 28; 27] \\ \hline \multirow{2}{*}{Achieving high Cross polarization discrimination} & Modifications to the radiator shape & [29; 30] \\ \hline \multirow{2}{*}{Improving the gain} & Modifications to the radiator-addition of notch & \\  & metal wall & [30] \\ \hline \multirow{2}{*}{Achieving beam steering} & Butler matrix & [7] \\  & Luneburg lens & [8] \\  & Digital beamforming-integrated RF transceiver & [31] \\ \hline \multirow{2}{*}{Achieving compact size designs/cost} & Multiband compact radiating element design & [2; 4; 32; 33] \\ \hline \end{tabular}
\end{table}
Table 1: A summary of available solutions for design challenges in Base Station Antenna design.  
\begin{table}
\begin{tabular}{c c c} \hline
**No.** & **Systems** & **Bands (MHz)** \\ \hline
1 & FDD 1700 & 1690–1710 \\
2 & DCS & 1710–1880 \\
3 & PCS & 1850–1990 \\
4 & UMTS & 1920–2170 \\
5 & LTE & 2300–2400 \\
6 & LTE+ & 2570–2700 \\
7 & GSM850 & 880–915 \\
8 & GSM900 & 925–960 \\
9 & Sub-6 & 3300–3600 \\
10 & mm-wave & 24,000–28,000 (Non-contiguous) \\ \hline \end{tabular}
\end{table}
Table 2: Widely adapted frequency bands for mobile communication systems.