## 2 Fundamental Background
### Defected Ground Structure Layouts  
Historically, the DGS was designed as a unit dumbbell-shaped defect placed underneath a microstrip line to provide stop-band characteristics [89]. This configuration contributes to preventing electromagnetic waves from traveling down the microstrip line over a range of frequencies. Given that the mutual coupling effect mainly originates from surface waves that exist when MIMO elements share a common ground plane, the DGS can change the surface current distribution of the antennas to enhance isolation [23]. In other words, introducing the DGS in MIMO antennas results in the transformation of effective capacitance and inductance of the transmission line to operate as a filter [90]. Over the years, numerous DGS shapes have been obtained and reported in the literature for different applications [91; 92; 93; 94]. These shapes have been deployed with the goal of suppressing the surface waves, attaining compact geometry, rejecting harmonics, and realizing the ease of fabrication. Table 2 provides a concise review of DGS techniques while highlighting their advantages and disadvantages [91; 92; 93; 94]. Descriptively, DGS represents a low-cost method for realizing wave suppression and compact design in contrast to other techniques like EBG structures, circulators, and hybrid couplers. The SRR-based configurations have demonstrated substantial improvements in several antenna performance metrics within the wide spectrum of EBG/DGS designs [95; 96; 29]. Generally, SRR comprises of a pair of concentric metals with splits at the opposite opening direction on a substrate, as shown in Figure 5a. Their resonance characteristic stems from the regulated inductive-capacitive phenomenon, leading to an electric current flow through the metallic rings as well as gaps. The CSRR structure constitutes a negative \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Feed Methods** & \multicolumn{2}{c}{**Advantages**} & \multicolumn{2}{c}{**Disadvantages**} \\ \hline \multirow{4}{*}{Microstrip Line} & • Easy fabrication and modeling. & • Limited bandwidth. \\  & • Easy integration with other RF components on the & • Spurious feed radiation may occur from surface wave. \\  & • Simplicity for modeling impedance matching and low-profile design. & • Transmission line may cause interference in designs. \\ \hline \multirow{4}{*}{Coaxial Probe} & • Easy fabrication. & • Suitable for high permittivity and thick substrates. & • Limited bandwidth. \\  & • Direct connection to external circuitry & • Direct connection to external circuitry & • Difficult modeling. \\  & • (e.g., transceivers). & • Integration complexity due to the need for vertical integration. \\  & • Low spurious feed radiation. & • Higher antenna RE. & • Difficultly in modeling impedance matching. \\ \hline \multirow{4}{*}{Aperture Coupled} & • Easy modeling. & • • Difficultly in modeling impedance matching. \\  & • Enhancement of gain and BW. & • • Complex fabrication. & • Backward radiation due to aperture. \\ \cline{1-1}  & • Low spurious feed radiation. & • Difficultly in modeling impedance matching. \\ \hline \multirow{2}{*}{Proximity-Coupled} & • Easy modeling. & • Difficultly in modeling impedance matching. \\ \cline{1-1}  & • Enhanced of gain and BW. & • Complex fabrication. \\ \cline{1-1}  & • Low spurious feed radiation. & • Complex fabrication. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Advantages and disadvantages of different antenna feed networks. \begin{table}
\begin{tabular}{c c} \hline \hline
**Advantages** & **Disadvantages** \\ \hline Suppression of surface wave & Backward radiation \\ \hline Harmonic suppression & Difficulty in analysis \\ \hline Enhancement of bandwidth & Difficulty in design \\ \hline Easy fabrication and cost effective & Radiation pattern distortion \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Advantages and disadvantages of defected ground structure-based antennas [91; 92; 93; 94; 95; 96]. variant of SRR, as illustrated in Figure 5b.