The use of base stations (BSs) and access points (APs) with a large number of antennas, called Massive MIMO (multiple-input multiple-output), is a key technology for increasing the capacity of 5G networks and beyond. While originally conceived for conventional sub-6 GHz frequencies, Massive MIMO (mMIMO) is ideal also for frequency bands in the range 30-300 GHz, known as millimeter wave (mmWave). Despite conceptual similarities, the way in which mMIMO can be exploited in these bands is radically different, due to their specific propagation behaviors and hardware characteristics. This paper reviews these differences and their implications, while dispelling common misunderstandings. Building on this foundation, we suggest appropriate signal processing schemes and use cases to efficiently exploit mMIMO in both frequency bands. * E. Björnson is with Linköping University, lower frequencies, which is achieved by mMIMO. There are, however, fundamental differences between how mMIMO technology can be designed, implemented, and exploited in sub-6 GHz and mmWave bands. In this paper, we provide a comparative overview, highlighting three main differences:1. The propagation channels build on the same physics, but basic phenomena such as diffraction, attenuation, and Fresnel zones are substantially different.2. The hardware implementation architecture changes with the increasing carrier frequency. More antennas can be integrated into a given area, but the insertion losses, intrinsic power-overhead in radio-frequency (RF) generation, and amplification result in diminishing gains.3. The signal processing algorithms depend on propagation and hardware. Channel estimation is resource-demanding at sub-6 GHz, while beamforming is straightforward. Conversely, mmWave channel estimation and beamforming are theoretically simpler since there are fewer propagation paths, but become challenging if hybrid beamforming is used.In the remainder of this article, we elaborate on these differences, including that they manifest how sub-6 GHz and mmWave bands are to be exploited to target different use-cases in 5G and beyond.
Difference I: The propagation channelAn understanding of the electromagnetic propagation is crucial when considering mMIMO systems and frequencies up to mmWave bands. The channels behave fundamentally different from what we are used to in cellular networks, which exposes weaknesses in the channel modeling simplifications conventionally made.