# Technology Trends for Massive MIMO towards 6G
###### Abstract  
At the dawn of the next-generation wireless systems and networks, massive multiple-input multiple-output (MIMO) in combination with leading-edge technologies, methodologies, and architectures are poised to be a cornerstone technology. Capitalizing on its successful integration and scalability within 5G and beyond, massive MIMO has proven its merits and adaptability. Notably, a series of evolutionary advancements and revolutionary trends have begun to materialize in recent years, envisioned to redefine the landscape of future 6G wireless systems and networks. In particular, the capabilities and performance of future massive MIMO systems will be amplified through the incorporation of cutting-edge technologies, structures and strategies. These include intelligent omni-surfaces (IOs)/intelligent reflecting surfaces (IRSs), artificial intelligence (AI), Terahertz (THz) communications, and cell-free architectures. In addition, an array of diverse applications built on the foundation of massive MIMO will continue to proliferate and thrive. These encompass wireless localization and sensing, vehicular communications, non-terrestrial communications, remote sensing, and inter-planetary communications, among others. 6G; massive MIMO; intelligent omni-surface (IOS); intelligent reflecting surface (IRS); cell free; artificial intelligence; vehicular communications; THz communications; non-terrestrial communications; remote sensing; inter-planetary communications +
Footnote †: journal: Sensors  
_Perspective_  
[PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] Technology Trends for Massive MIMO towards 6G
significant commercial demands, massive MIMO is poised to evolve further, profoundly transforming the landscape of telecommunications and related sectors in 6G. Intrinsic support for massive MIMO is embedded within 5G New Radio (NR) standards [3]. The initial 5G NR release, 3 GPP Release 15, encompasses fundamental features facilitating massive MIMO deployment across various scenarios. Key features include reciprocity-based operation for time division duplex (TDD) systems, high-precision channel state information (CSI) feedback for multi-user MIMO (MU-MIMO), and advanced beam management for high-frequency band operation with analog beamforming. Subsequent enhancements to massive MIMO were outlined in 3 GPP Release 16, including CSI feedback overhead reduction through spatial and frequency domain compression, beam management signaling overhead and latency reduction, and non-coherent joint transmission from multiple transmit and receive points (TRPs). 3 GPP Release 17 heralded further evolution of massive MIMO, with angle-delay reciprocity exploited for additional CSI feedback overhead reduction. A unified transmission configuration indicator (TCI) framework was introduced to bolster multi-beam operation, accompanied by inter-cell multi-TRP enhancements and multi-TRP-specific beam management features. 3 GPP Release 18 signifies the commencement of 5G Advanced, featuring further massive MIMO development. Potential areas under 3 GPP investigation include uplink MIMO enhancements (e.g., the use of eight transmission antennas in the uplink and multi-panel uplink transmission), the extension of the unified TCI framework to multi-TRP scenarios, a greater number of orthogonal demodulation reference signal (DMRS) ports for MU-MIMO, and CSI reporting enhancements for user equipment (UE) with medium to high velocities. Accelerated standardization and promising commercialization position massive MIMO as a cornerstone technology for 5G and beyond, anticipated to integrate with other enabling technologies and extend to novel verticals. This article delves into several technology trends shaping the evolution of massive MIMO en route to 6G. One such trend involves the surge of interest in intelligent surfaces [4; 5], which hold significant promise for energy and cost-efficient massive MIMO. Metasurface-enabled massive MIMO can facilitate joint