Terminology » Ultra Massive MIMO in 6G: Benefits and Limitations

Ultra Massive MIMO in 6G: Benefits and Limitations

Ultra-Massive MIMO (UM-MIMO) is the next evolutionary step of the Massive MIMO technology that was a cornerstone of 5G. In Ultra-massive mimo based wireless communication system, base stations are equipped with an extremely large number of antenna elements, potentially scaling into the thousands. It is sometimes referred to as “Giga-MIMO”.

MIMO (Multiple Input Multiple Output): The basic concept of using multiple antennas at both the transmitter and receiver to improve communication performance.
Massive MIMO (5G): Scaled this up to a “large” number of antennas (e.g., 64, 128, 256) at the base station. This allowed for sharp, steerable beams of data to be sent to multiple users simultaneously.
Ultra-Massive MIMO (6G): Pushes this to an even greater extreme. This massive increase in antenna count is made physically possible by another key 6G enabler: the move to higher frequencies (i.e. THz).

6G will operate in much higher (sub-)Terahertz (THz) frequency bands. Radio waves at these frequencies have extremely short wavelengths. This means that individual antenna elements can be made incredibly small, allowing thousands of them to be packed into the same physical area (aperture) that a 5G Massive MIMO array occupies today.

Advantages of Ultra-Massive MIMO

Following are some of the benefits of Ultra-Massive MIMO (UM-MIMO).

With the help of spatial multiplexing technique, UM-MIMO based antennas can increase the overall data throughput (capacity) of the network. Hence it can be used for applications requiring more bandwidth such as holographic communication and immersive XR.
Offers higher energy efficiency as it directs energy using focused beams to intended user and very little power is wasted in other directions. Hence lowers overall power required to transmit each bit of data reliably.
Due to transmission of focused beam of energy to intended user, it reduces interference to other users as well.
Having a massive number of antennas creates a highly diverse radio environment with many potential signal paths. This makes the communication link more resilient to blockages and fading, leading to the ultra reliable connections required for mission critical applications.
UM-MIMO enables high precision sensing which is foundational technology for Joint Communications and Sensing (JCAS) which can be employed in autonomous driving and augmented reality.

Disadvantages of Ultra-Massive MIMO

Following are some of the Limitations and Challenges of Ultra-Massive MIMO (UM-MIMO).

Hardware complexity : Antenna elements used in this huge quantity requires their own RF chain which leads to extremely complex and expensive hardware.
Heat dissipation : Due to multiple rf chains nearby, heat dissipation management is a concern.

These issues (hardware complexity and heat disssipation) can be addressed using new types of antennas, such as the futuristic plasmonic antennas.

Complex signal processing algorithms are required to manage thousands of beams in real time. The algorithms include calculation of channel state information (CSI), precoding (i.e. beamforming calculations) and user scheduling management.
As we know, for small antenna arrays, users are usually located in the far field where radio waves are flat. For UM-MIMO, users are located in the near field where waves are curved or spherical in shape. This requires modified mathematical models for channel estimation and beamforming.
For the base station to steer beams accurately, it needs to know the state of the wireless channel for each user. Obtaining this information from thousands of users without creating overwhelming feedback traffic (overhead) is a major bottleneck that researchers are working to solve, likely using advanced AI/ML techniques.

Conclusion: In essence, UM MIMO offers a transformative opportunity to unlock the full potential of 6G, enabling ultra high capacity and fine grained spatial control. The transition from massive to ultra massive arrays will require innovative solutions across signal processing, antenna design and network architecture if 6G’s lofty goals are to be realised.