# Technology Trends for Massive MIMO towards 6G
## 2 Metasurface-Enabled Massive MIMO  
The burgeoning surge in wireless data traffic necessitates innovative communication paradigms supporting high data rates in the future 6G. Massive MIMO has attracted heated attention exploiting the implicit randomness of the wireless environment. However, traditional massive MIMO relies on the large-scale phased arrays, which induce high hardware cost and power consumption due to the energy-consuming phase shifters, especially when the number of antennas grows. This limits their scalability to support massive MIMO in practice. Recently, intelligent metasurfaces, as a new type of ultra-thin two-dimensional metamaterial inlaid with sub-wavelength scatters, has provided a novel technology to enable or realize massive MIMO in a cost-efficient way [6]. Capable of reflecting and/or refracting the incident signals simultaneously, the surface can actively shape uncontrollable wireless environments into a desirable form via flexible phase shift reconfiguration [7]. Since such a reconfiguration of each element is usually achieved via one or two PIN diodes controlled by the biased voltage, it only involves small hardware and power costs compared to the traditional phase arrays. Consequently, the metasurface can be easily extended to a large scale, providing a practical method for realizing massive MIMO. A general transmission model of one surface element is shown in Figure 1. After the incident signal arrives at the surface, part of it is reflected and the rest is refracted towards the other side. By defining the reflection-refraction ratio as \(\epsilon\), we have the reflected and refracted signals in Figure 1. Three different types of surfaces can then be classified below:  
* When \(\epsilon=0\), the surface only reflects the incident signal, leading to an intelligent reflecting surface (IRS). It can be attached to the wall, serving as a reflective relay for coverage [8]. * When \(\epsilon\rightarrow\infty\), the surface only refracts the incident signal, serving as a reconfigurable refractive surface (RRS). It can replace the antenna array at the base station for transmission and reception [9]. * When \(0<\epsilon<\infty\), the surface can reflect and refrac the incident signal simultaneously, which is defined as an intelligent omni-directional surface (IOS). Compared to IRS, it can achieve full-dimensional wireless communications despite users' locations with respect to the surface [10]. Due to their mature manufacturing techniques and low production costs, metasurfaces have been considered as an efficient method to realize massive MIMO. The metasurface can be deployed either at the transmitter as large-aperture MIMO antennas or in the channel as passive MIMO relays. Specifically, the use of a metasurface for the realization of massive MIMO offers several notable benefits:  
* Channel enhancement: The metasurface can optimize the wireless channel by reflecting or refracting the incident signals towards desired directions, thereby improving signal strength and reducing interference. * Beamforming and steering: By controlling the phase shift across the metasurface elements, it becomes possible to create focused beams and steer them toward specific \begin{table}
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**Technology Trends** & **Advantage** & **Challenge** & **Ref.** \\ \hline High-speed & Non-coherent & High mobility & [50; 51] \\ Applications & demodulation, & Non-convex & \\  & Multi-user & optimization & \\  & multiplexing & & \\ \hline Non-terrestrial & Ubiquitous connectivity & Enormous propagation & \\ \& Deep-space & \& deep space & Distributed \& \\  & Multi-planetary & edge computing & \\  & prosperity & & \\ \hline \hline \end{tabular}
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Table 2: _Cont._users or desired coverage areas. This enables spatially selective communications and enhances the system capacity. * Interference management: The intelligent metasurface can actively manipulate the propagation environment to mitigate interference from nearby cells or unwanted sources, improving overall system performance.