# Technology Trends for Massive MIMO towards 6G
###### Abstract
\begin{table}
\begin{tabular}{c c c} \hline \hline
**KPI** & **5G** & **6G** \\ \hline Peak Data Rate & 20 Gb/s & \(\geq\)1 Tb/s \\ \hline Experienced Data Rate & 0.1 Gb/s & 1 Gb/s \\ \hline Peak spectral efficiency & 30 b/s/Hz & 60 b/s/Hz \\ \hline Experienced spectral efficiency & 0.3 b/s/Hz & 3 b/s/Hz \\ \hline Operating bandwidth & 400 MHz for sub-6 GHz & 400 MHz for sub-6 GHz \\  & 3.25 GHz for mmWave & 3.25 GHz for mmWave \\  & 10–100 GHz for THz bands \\ \hline Carrier bandwidth & 400 MHz & To be specified \\ \hline Latency & 1 ms & 100 ms \\ \hline Connection Density & \(10^{6}\) devices/km\({}^{2}\) & \(10^{7}\) devices/km\({}^{2}\) \\ \hline Area traffic capacity & 10 Mb/s/m\({}^{2}\) & 1 Gb/s/m\({}^{2}\) \\ \hline Mobility & 500 km/h & 1000 km/h \\ \hline Energy Efficiency & not specified & 1 Tb/J \\ \hline Jitter & not specified & 1 μs \\ \hline Reliability & 500 km/h & 1000 km/h \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of 5G and 6G KPIs [1; 2]. communications, localization, and sensing functions, thereby unlocking new use cases and bolstering wireless system performance in 6G. The initial two sections of this article are devoted to metasurface physical fundamentals for massive MIMO and metasurface-enabled massive MIMO for localization and sensing, respectively. Following that, we offer an exploration of ultra-massive MIMO at THz frequencies, as the adoption of small wavelengths and wide bandwidth introduces unique challenges across all aspects of wireless system design. We then investigate cell-free massive MIMO technology, renowned for improving spectral and energy efficiency. Subsequently, we survey and discuss the application of artificial intelligence (AI) for massive MIMO, followed by a review of massive MIMO-OFDM for high-speed applications. As the final future-oriented application of massive MIMO leading to 6G, non-terrestrial networks (NTNs) communications has been a focal point of standardization since 2017. Therefore, we present an in-depth review, discussion, and analysis of current and future non-terrestrial applications and architectures centered around massive MIMO before drawing conclusions in this article. We have compiled a summary of the technology trends and relevant references for 6G massive MIMO in Table 2, which presents the critical advantages and challenges of each technology trend. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Technology Trends** & **Advantage** & **Challenge** & **Ref.** \\ \hline \multirow{4}{*}{\begin{tabular}{c} Metasurface- \\ enabled \\ Massive MIMO \\ \end{tabular} } & Channel enhancement, & & \\  & Beamforming and & Type selection, & \\  & steering, & Coupling effect, & \\  & Interference alleviation, & Deployment, & \\  & Energy-efficient & Beamforming scheme & \\  & communication & & \\ \hline \multirow{4}{*}{\begin{tabular}{c} Localization \\ \& Sensing \\ \end{tabular} } & Customize channels, & & \begin{tabular}{c} Configuration, \\ Joint optimization, \\ Communication \\ integration, \\ Practical \\ implementation \\ \end{tabular} & \begin{tabular}{c} \\ \multirow{2}{*}{\begin{tabular}{c} 14-18 \\ \end{tabular} \\ \end{tabular} } \\  & & & & \\  & High capacity, & & \begin{tabular}{c} Intermittent and \\ unreliable links, \\ Large transceivers \\ and mobility, Phase \\ noise, Channel \\ sparsity, Spherical \\ wave and near- \\ field effects, \\ Beam squint \\ \end{tabular} & \
\begin{tabular}{c} \\ \multirow{2}{*}{[19]} \\ \end{tabular} \\  & & & & \\  & Improved spectral and & & \\  & energy efficiency, & & \\ Cell-Free & Uniform service & & \\ Massive MIMO & provision, & & \\ Cloudification and & & & \\ virtualization & & & \\ \hline \multirow{4}{*}{
\begin{tabular}{c} Artificial \\ Intelligence \\ \end{tabular} } & Reliable real-time & Integration of AI & \\  & transmission, & with wireless data, & \\ \cline{1-1}  & Improved channel & Segmented AI for & \\ \cline{1-1}  & estimation and prediction & specific applications & \\ \hline \hline \end{tabular}
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