## 5 Massive MIMO
### Open Challenges of mMIMO
The reported result was a 10 dB mutual coupling reduction over 3.3-4.5 GHz and demonstrates the advantages of the wide-band frequency spectrum, embed-able, radiation pattern distortion alleviation, and dual-polarized capabilities for MIMO applications. Another, mutual coupling reduction for large antenna arrays is proposed in [185], using \(\varepsilon\)-negative meta-surface superstrate. This decoupling meta-surface structure is very useful to restore the radiation patterns, maximum mutual coupling reduction of 25 dB, broaden the bandwidth of the array, and minimize the active voltage standing wave ratio. The work published in [186] also represents low mutual coupling, good decoupling of the radiators, and impedance matching. In [187], a dual-band MIMO antenna is proposed over a frequency range of 2.7-3.1 GHz and 4.3-4.6 GHz showing improved isolation of higher than 21 dB and efficiency of 80% and good MIMO diversity performances. The mutual coupling was reduced by using a split ring resonator structure on the radiator. Another hybrid decoupling technique to reduce mutual coupling is presented in [152] using the defected ground structure (DGS) and circular ring parasites. The DGS reduced the effect of coupling between antenna elements whereas, the circular ring \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref.** & **Year** & **Solution Suggested** & **Frequency** & **Outcome Type** \\  & & & **GHz** & \\ \hline
[179] & 2018 & Metal meandering strips & 3.2–5 & Simulated \\
[180] & 2018 & Pattern multiplicity & 2.5–2.64 & Simulated and tested \\
[181] & 2019 & Transmission- line decoupling & 2.45 & Simulated and tested \\
[182] & 2019 & Molecule-shaped structure & 2.4–10.6 & Simulated and tested \\
[183] & 2019 & Frequency-selective surface & 3.5–4.9 & Simulated and tested \\
[184] & 2020 & Decoupling network & 3.3–4.5 & Simulated and tested \\
[185] & 2020 & Decoupling super-strate & 3.3–4.5 & Simulated and tested \\
[186] & 2021 & Sub-Miniaturization & 0.61–0.96 & Simulated and tested \\  & & & \(-\)1.7–5 & \\
[187] & 2021 & Split Ring Resonators & 2.73–3.12 & Simulated and tested \\  & & & \(-\)4.33–4.68 & \\
[152] & 2022 & Defected Ground Structure & 5.82–5.94 & Simulated and tested \\
[188] & 2022 & SICL Feeding Structure & 24.5–26.5 & Simulated and tested \\
[189] & 2023 & Defective Ground Structure & 2.2–2.64 & Simulated and tested \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Futuristic suggested solutions for MIMO antenna to reduce mutual coupling. parasites reduced the mutual coupling between perpendicular antenna elements. A novel 64-element dual-polarized mMIMO antenna operating in 24.5-26.5 GHz frequency band is presented in [188] and mutual coupling is reduced to \(-\)23 dB for the entire bandwidth by using substrate-integrated coaxial line feeding structure. As the Substrate-integrated Coaxial Line Feeding (SICL) transmission contains a microstrip line, shielded by two rolls of metallic vias on both sides and two ground planes above and below, aiming for an extremely low coupling feature. In [189], L-shaped stubs and DGS are used to reduce mutual coupling effects. The L-shaped stubs are placed on one side of the two patches, which generate a new coupling current for antenna excitation and create an additional coupling path. The huge amount of channel state information in beamforming will surely be questionable, particularly for the downlink. Therefore, mMIMO can only be operational in the time division duplexing because of feedback schemes, restrictive cost of channel estimation, and channel reciprocity. Thus, it will be unsuitable for frequency division duplexing but pilot contamination in time division duplexing is also a huge challenge in mMIMO. So, new means of channel estimation, feedback measures, and solutions need to be proposed for 5G and B5G. Moreover, if the transmission power is excessive in an amount that is typically 3-5\(\times\), the mMIMO will experience thermal noise and pilot contamination from other cells which require passive cooling.