Massive MIMO Becomes Less Massive and More Open

The name “Massive MIMO” has been debated since its inception. Tom Marzetta introduced it ten years ago as one of several potential names for his envisioned MIMO technology with a very large number of antennas. Different researchers used different terminologies in their papers during the first years of research on the topic, but the community eventually converged to calling it Massive MIMO.

The apparent issue with that terminology is that the adjective “massive” can have different meanings. The first definition in the Merriam-Webster dictionary is “consisting of a large mass”, in the sense of being “bulky” and “heavy”. The second definition is “large in scope or degree”, in the sense of being “large in comparison to what is typical”.

It is probably the second definition that Marzetta had in mind when introducing the name “Massive MIMO”; that is, a MIMO technology with a number of antennas that is large in comparison to what was typically considered in the 4G era. Yet, there has been a perception in the industry that one cannot build a base station with many antennas without it also being bulky and heavy (i.e., the first definition).

Massive MIMO products are not heavy anymore

Ericsson and Huawei have recently proved that this perception is wrong. The Ericsson AIR 6419 that was announced in February (to be released later this year) contains 64 antenna-integrated radios in a box that is roughly 1 x 0.5 m, with a weight of only 20 kg. This can be compared with Ericsson’s first Massive MIMO product from 2018, which weighed 60 kg. The product is designed for the 3.5 GHz band, supports 200 MHz of bandwidth, and 320 W of output power. The box contains an application-specific integrated circuit (ASIC) that handles parts of the baseband processing. Huawei introduced a similar product in February that weighs 19 kg and supports 400 MHz of spectrum, but there are fewer details available regarding it.

The new Ericsson AIR 6419 only weighs 20 kg, thus it can be deployed by a single person. (Photo from Ericsson.)

These products seem very much in line with what Massive MIMO researchers like me have been imagining when writing scientific papers. It is impressive to see how quickly this vision has turned into reality, and how 5G has become synonymous with Massive MIMO deployments in sub-6 GHz bands, despite all the fuss about small cells with mmWave spectrum. While both technologies can be used to support higher traffic loads, it is clear that spatial multiplexing has now become the primary solution adopted by network operators in the 5G era.

Open RAN enabled Massive MIMO

While the new Ericsson and Huawei products demonstrate how a tight integration of antennas, radios, and baseband processing enables compact, low-weight Massive MIMO implementation, there is also an opposite trend. Mavenir and Xilinx have teamed up to build a Massive MIMO solution that builds on the Open RAN principle of decoupling hardware and software (so that the operator can buy these from different vendors). They claim that their first 64-antenna product, which combines Xilinx’s radio hardware with Mavenir’s cloud-computing platform, will be available by the end of this year. The drawback with the hardware-software decoupling is the higher energy consumption caused by increased fronthaul signaling (when all processing is done “in the cloud”) and the use of field-programmable gate arrays (FPGAs) instead of ASICs (since a higher level of flexibility is needed in the processing units when these are not co-designed with the radios).

Since the 5G technology is still in its infancy, it will be exciting to see how it evolves over the coming years. I believe we will see even larger antenna numbers in the 3.5 GHz band, new array form factors, products that integrate many frequency bands in the same box, digital beamforming in mmWave bands, and new types of distributed antenna deployments. The impact of Massive MIMO will be massive, even if the weight isn’t massive.

6 thoughts on “Massive MIMO Becomes Less Massive and More Open”

  1. This is where the fun begins

    a. Ericsson & Huawei, neither O-RAN I believe and they must be using ASIC (= lighter? 7nm/5nm?)
    b. O-RAN is FPGA based (heavier? More power hungry?)

    How can we meet Open with Light & less power hungry and still be able to decouple SW from GPP HW?

    1. I think the openness will always come at a price. One can take Mac versus PC as an example. Apple co-optimizes its hardware and software, to get better performance using the same type of components, or the same performance using simpler components as in a comparable PC. The benefit of the openness in the PC world is that one can buy off-the-shelf components and build a PC anyway you like.

  2. I like how Tom Marzetta described Massive MIMO in one conference: It is spatial multiplexing pushed to a pleasant extreme!
    The question that arises is: What is next after Massive MIMO?
    Is spatial multiplexing the end of the road or there might be some new technique to further enhance spectral and energy efficiency by many orders of magnitude, while being simple and practically feasible ..

    1. Since the data traffic increases by 50% per year, but the maximum bit rate per user is sufficiently for most applications, future networks will have to be evolved to multiplex more and more simultaneous transmissions.

      This can be achieved by using more spectrum, deploying more cells or use MIMO. 5G deployments are demonstrating that the spatial multiplexing achieved by MIMO is now key solution that operators are choosing (not more bandwidth in mmWave bands). Hence, I think we will see larger and larger MIMO deployments in the future, as well as cell-free MIMO deployments that can even out the rate variations over the coverage area.

    2. Great question! I and my students are working on “Physics-Based Wireless”, aka “Beyond Massive MIMO”. Massive MIMO is based on very simple notions for how antennas function and how radio wave propagate. A closer fusion of electromagnetic theory and communication theory may yield breakthroughs. All existing wireless schemes are operating far from any limitations imposed by nature. “simple and practically feasible” – not guaranteed!

      Tom Marzetta

  3. What’s the relationship between MIMO and O-RAN? In particular, why is O-RAN is suitable for FPGA so that Xilinx is investing in it? Maybe you can make a post on that too. I think that’d be interesting.

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