The Rise of 'giga MIMO' or 'gigantic MIMO' (gMIMO)

'giga MIMO' or 'gigantic MIMO' (shortened to gMIMO) has now come to attention thanks to a paper shared by Prof. Emil Björnson. To avoid confusion and help people not involved in this area of technology, let us quickly understand the different MIMO terminology:

Multiple-Input and Multiple-Output (MIMO): MIMO is used when more than one antenna is used for transmission and/or reception of the radio signals. All commercial 4G/LTE networks and most devices use at least 2 antennas for transmission and reception, which is referred to as 2x2 MIMO. In simple terms, more antennas mean either more data rates or much more reliable reception. With 5G, it's common to have 2x2, 4x4 and 8x8 MIMO (using the NxM convention for FDD) or 2T2R, 4T4R or 8T8R (using the N transmit and M receive convention for TDD) MIMO antennas. 

massive MIMO (mMIMO): mMIMO is the same as MIMO but for 16 antennas or above. mMIMO is generally used for higher frequencies, with TDD bands, though I have seen some prototypes for FDD bands as well. 16T16R is not that popular but 32T32R (also shortened to 32TR) and 64T64R (64TR) are very popular. Note that these numbers refer to the number of antennas but internally each antennas can generally have more than one element. So 64TR is often available with 128 elements but some new high-end antennas even have 192 elements (3 elements per antenna). Massive doesn't mean massive size, just the 'massive' number of antennas. A typical active mMIMO antenna is smaller in size compared to low band passive FDD antennas.

ultra-massive MIMO (umMIMO): umMIMO has a very large number of antenna elements and is also huge in size. This is more in a concept phase and I haven't seen (m)any prototypes. You can see a conceptual picture here. China mobile had a fantastic picture from early 5G days which is shared in this post. 

giga MIMO or gigantic MIMO (gMIMO): While mMIMO was designed with mid-band and high-band (mmWave) in mind and umMIMO was conceptualised with mmWave and sub-THz in mind, gMIMO has been visualised for cmWave bands. A prototype of Giga MIMO was seen at MWC 2024 which can be seen in the tweet above. gMIMO will have more antennas than mMIMO and it would be a bit larger, but not as huge as umMIMO.

Prof. Emil Björnson shared a research paper on Gigantic MIMO on his LinkedIn post here. He also wrote a blog post explaining the journey from Massive MIMO to Gigantic MIMO in 6G here. There are some good discussions on this LinkedIn post here as well. 

Prof. Björnson also made a very nice explainer video on this topic which is embedded below. He always kindly shares his slides as well which are available here.

While I don't have any issues with gMIMO being a 'giga' or 'gigantic' MIMO, I think in the long run it might be better to use 'giga' to avoid any confusion. Here 'giga' refers to the gigabit data rates that 6G is expected to deliver. 

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