MIMO Evolution Towards 6G: Modular Massive MIMO in Low-Frequency Bands

As the pace of global 5G network deployments accelerates, now is the moment for the cellular industry to realize the sixth generation (6G) cellular communication. In this article, the so-called modular massive MIMO (mmMIMO) is presented as one candidate technology for 6G. The 5G had relentlessly pushed the boundary of the cellular system’s operating frequency to millimeter wave bands and such a trend will be continued in the 6G era to further embrace the greenfield terahertz (THz) spectrum. Admittedly, however, the technical advances in 5G for low bands fall short, although low bands are crucial in serving a large number of users in a wide coverage area. Although, it would be ideal if massive MIMO could be utilized in low bands, it is less practical due to a large antenna form factor size. mmMIMO is a technology to distribute a large active antenna array with smaller standardized antenna modules, just like the LEGO blocks. Through this, the benefits of massive MIMO can be achieved in low bands, unconstrained from the spatial limitations. In this article, the concept of mmMIMO, its applicability, and needed research efforts to realize the technology are discussed. In addition, through the demonstration of a proof-of-concept system, it is shown that the technology will be within reach at the time of 6G massive commercialization around 2030. Lastly, the performance gain of mmMIMO is evidenced by system-level simulation.