Humans can estimate the distance to objects using our eyesight. This perception of depth is enabled by the fact that our two eyes are separated to get different perspectives of the world. It is easier to determine the distance from our eyes to nearby objects than to things further away, because our visual inputs are then more different. The ability to estimate distances also varies between people, depending on the quality of their eyesight and ability to process the visual inputs in their brains.

An antenna array for wireless communications also has a depth perception; for example, we showed in a recent paper that if the receiver is at a distance from the transmitter that is closer than the Fraunhofer distance divided by 10, then the received signal is significantly different from one transmitted from further away. Hence, one can take D²/(5λ) as the maximum distance of the depth perception, where D is the aperture length of the array and λ is the wavelength. The derivation of this formula is based on evaluating when the spherical curvatures of the waves arriving from different distances are significantly different.

Our eyes are separated by roughly D=6 cm and the wavelength of visual light is roughly λ=600 nm. The formula above then says that the depth perception reaches up to 1200 m. In contrast, a typical 5G base station has an aperture length of D=1 m and operates at 3 GHz (λ=10 cm), which limits the depth perception to 2 m. This is why we seldom mention depth perception in wireless communications; the classical plane-wave approximation can be used when the depth variations are indistinguishable. However, the research community is now simultaneously considering the use of physically larger antenna arrays (larger D) and the utilization of higher frequency bands (smaller λ). For an array with length D=10 m and mmWave communications at λ=10 mm, the depth perception reaches up to 2000 m. I therefore believe that depth perception will become a standard feature in 6G and beyond.

There is a series of research papers that analyze this feature, often implicitly when mentioning terms such as the radiative near-field, finite-depth beamforming, extremely large aperture arrays, and holographic MIMO. If you are interested in learning more about this topic, I recommend our new book chapter “Near-Field Beamforming and Multiplexing Using Extremely Large Aperture Arrays“, authored by Parisa Ramezani and myself. We summarize the theory for how an antenna array can transmit a “beam” towards a nearby focal point so that the focusing effect vanishes both before and after that point. This feature is illustrated in the following figure:

This is not a physical curiosity but enables a large antenna array to simultaneously communicate with user devices located in the same direction but at different distances. The users just need to be at sufficiently different distances so that the focusing effect illustrated above can be applied to each user and result in roughly non-overlapping focal regions. We call this near-field multiplexing in the depth domain.

A less technical overview of this emerging topic can also be found in this video:

I would like to thank my collaborators Luca Sanguinetti, Özlem Tuğfe Demir, Andrea de Jesus Torres, and Parisa Ramezani for their contributions.

One question that I often receive from reviewers is: What is the difference between Cell-free Massive MIMO and the C-RAN technology? In this blog post, I will provide my usual answer to this question and then elaborate on why reviewers are not always satisfied with this answer.

In a nutshell, a base station consists of an antenna, a radio unit, and a baseband unit (or multiple antennas and radios, in the case of MIMO). These are traditionally co-located as follows: the antenna is deployed in a tower, the baseband unit is placed underneath, and the radio is located at one of these places.

Cloud Radio Access Network (C-RAN)

C-RAN is an alternative deployment architecture where the baseband units are deployed at other locations, “in the cloud” so to say. The C in C-RAN can also stand for “centralized”, which might be a preferable terminology since the word “cloud” is often associated with the use of general-purpose compute hardware. In C-RAN, the baseband processing tasks of many neighboring base stations are carried out in a single central processing unit (CPU), which might use specialized or general-purpose hardware. For latency reasons, the physical distance between a base station site and its CPU shouldn’t be more than a few kilometers. By pooling the processing resources in this way, it is possible to reduce the total hardware expenditure, particularly if some base stations often have low traffic load so the total hardware capability can be reduced by sharing. In other words, one can build a network that can handle high traffic anywhere, as long as it doesn’t happen everywhere at the same time. Another important benefit is that the neighboring base stations can more easily cooperate when their processing is anyway carried out at the same CPU.

In summary, C-RAN is a deployment architecture. Different air interfaces (e.g., 4G, 5G) can be implemented on top of the C-RAN architecture, making use of different physical layer techniques (e.g., Massive MIMO, coordinated multipoint).

Cell-free Massive MIMO

This is a new physical layer technology where the neighboring base stations (called access points in the related literature) are jointly serving the users in their vicinity. By carrying out coherent signal processing, the signal power can be boosted, the interference can be mitigated, and the cell boundaries are alleviated. We are essentially creating a wide-area network that is free from cells. There are different forms of Cell-Free Massive MIMO, characterized by where the baseband processing is carried out. It can either be fully centralized at the CPU or distributed as far as possible to pre-processing units located at each access point. The following video elaborates on these different implementation levels:

My answer

The simple answer to the question posed in the first paragraph is that C-RAN is a network architecture and Cell-free Massive MIMO is a physical-layer technology that can be deployed using either C-RAN or some other architecture. It is not a matter of selecting one or the other, but both can coexist and benefit from each other. My group is presenting a paper at ICC 2022 that exemplifies how to optimize the operation of Cell-free Massive MIMO when it is implemented in C-RAN.

The weak spot with my answer is that C-RAN was proposed already in 2011, based on the needs of the industry to consolidate their network assets, and a large amount of academic research has been carried out since then. Some of the papers on C-RAN have considered physical-layer techniques that resemble Cell-free Massive MIMO but without using that terminology. Some people might rightfully associate C-RAN with cell-free-like processing schemes, because they fit so well together. After all, Cell-free Massive MIMO is a revamp of Network MIMO that makes it more practical.