The Journey from mMIMO in 5G towards gMIMO in 6G

At VTC2025-Fall in Chengdu, Prof. Emil Björnson presented a concise and thought-provoking talk on how MIMO technology is likely to evolve as the industry moves towards 6G. The session focused on what he calls Gigantic MIMO, building directly on the success of Massive MIMO (mMIMO) in 5G while remaining grounded in what operators can realistically deploy in the early 2030s. For anyone following antenna evolution in 6G, this talk neatly connects theory, deployment reality and spectrum constraints.

Massive MIMO has already reshaped how cellular networks deliver capacity. Instead of separating users in time and frequency, modern systems rely on spatial separation through beamforming. By equipping base stations with far more antennas than the number of users being served, networks can multiplex many data streams simultaneously in the same time and frequency resources. This principle is no longer theoretical. It is widely deployed in 5G, particularly in the 3.5 GHz band, where base stations with around 64 antennas routinely serve multiple users with high spectral efficiency.

Emil revisited well known 5G field results to underline just how effective Massive MIMO already is. Even with multiple users active at the same time, the performance loss due to interference is relatively small. The key takeaway is that Massive MIMO works because the number of antennas remains comfortably larger than the number of spatial streams. That imbalance is what allows precise beamforming and interference suppression to do the heavy lifting.

Looking ahead to 6G, spectrum availability becomes a central constraint. Much of the discussion around 6G capacity assumes large amounts of new bandwidth, but Emil took a more cautious and realistic view. In the upper mid band, roughly between 7 and 24 GHz, there is certainly new spectrum, but not orders of magnitude more than what operators already aggregate across multiple bands today. From a per operator and per base station perspective, the step from 5G to 6G may mean moving from around 100 MHz to perhaps 200 MHz of bandwidth. That is useful, but it is not transformative on its own.

This is where antenna technology comes back into focus. Higher carrier frequencies allow many more antenna elements to be packed into the same physical aperture. A panel that accommodates just over a hundred elements at 3.5 GHz can support several hundred at around 7 or 8 GHz, and well over two thousand at 15 GHz. Without changing the physical size of the antenna panel, the array can become dramatically denser. The result is a natural evolution from today’s Massive MIMO systems to arrays with truly large numbers of antennas.

Emil refers to this step as Gigantic MIMO, with a practical definition centred on having at least 256 antenna ports while retaining fully digital beamforming and half wavelength spacing. The operating frequencies are firmly in the upper mid band, not in millimetre wave or terahertz bands, and the deployment philosophy remains familiar to operators. The base station form factor does not radically change, and the principle that the number of antennas should be much larger than the number of spatial streams is preserved.

The term Gigantic MIMO is not universally loved. In industry discussions, Giga MIMO is often used instead, and the shorthand gMIMO seems to be emerging as a convenient compromise. Interestingly, Emil himself used the gMIMO abbreviation during the talk. Regardless of terminology, the underlying idea is consistent. This is not about exotic surfaces, moving antennas or extreme near field operation everywhere. It is about scaling up what already works, using higher frequencies to increase antenna density while keeping deployments practical.

An important part of the presentation was what gMIMO does not require. Some of the more futuristic 6G concepts, such as ultra massive arrays at terahertz frequencies, holographic surfaces or fully cell free architectures, are fascinating but unlikely to be central to early 6G rollouts. Similarly, techniques like rate splitting multiple access or mechanically movable antennas add complexity without being essential to unlocking the main gains of large arrays in the upper mid band.

Near field effects were discussed in a pragmatic way. For a single large panel operating at typical cell site distances, most users will still be in the far field. True three dimensional beam focusing in both angle and range does not naturally appear just because the array has more elements. However, Emil showed that near field like behaviour can be achieved through multi panel deployments. By placing multiple panels a few metres apart but pointing in the same direction, the combined radiation patterns can enable more precise spatial focusing.

This multi panel approach is not far fetched. Existing urban sites already host multiple antenna panels for different generations, sectors and operators. Extending this model to 6G, with multiple gMIMO panels per sector, is entirely plausible. Beyond improved interference control, such configurations also open the door to integrated sensing capabilities, where one panel transmits while another receives, enabling the network to better understand its surrounding environment.

From a capacity perspective, the message was clear. The biggest gains in 6G are likely to come from scaling antenna numbers rather than relying on large jumps in bandwidth. Even modest increases in spectrum, combined with a significant rise in antenna ports, can deliver substantial improvements. Capacity scales with bandwidth multiplied by the number of antennas, and in the upper mid band the antenna term is where the real leverage lies.

For those following 6G research and standardisation, Emil’s talk provides a useful reality check. gMIMO is not a radical break from Massive MIMO but a logical and deployable evolution of it. It aligns well with operator expectations, spectrum realities and existing site practices. Whether one prefers the term Gigantic MIMO, Giga MIMO or simply gMIMO, the concept itself is likely to be a cornerstone of practical 6G radio access networks.

The slides of the talk are available here and the video is embedded below:

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