Basics of Cell-Free Massive MIMO

Cell-free massive multiple-input multiple-output (MIMO) is a system comprising a very large number of access points and a relatively smaller number of active users distributed over a large area. The access points use directly measured channel characteristics to jointly serve all users in the same time-frequency band. There are no cells or cell boundaries.

Cell-free massive MIMO is a very new topic, so there are still many misunderstandings. Below are my point-to-point responses to common questions I have received recently about cell-free massive MIMO.

1. Why cell-free massive MIMO?

- We have two options: cellular or not cellular (cell-free). But we know that cellular networks are hitting the physical limit. Whatever the techniques we use, we cannot overcome the inherent intercell interference of cellular networks. Thus, cell-free massive MIMO is the only way to ensure connectivity for future generation wireless networks.

2. From first papers on cell-free massive MIMO [1] and [2],  can we understand that cell-free massive MIMO is a system where conjugate beamforming and matched filtering are used for downlink and uplink transmissions, respectively?

- No. These papers propose the concept of cell-free massive MIMO. And the conjugate beamforming/matched filtering technique is just a specific transmission scheme used to show the potentials of cell-free massive MIMO. Basically, cell-free massive MIMO system is a system which uses massive MIMO technologies to remove the cells. We can use any techniques as long as they are feasible for practical implementation.  

A completed system design for cell-free massive MIMO is still a big open problem.

3. In cell-free massive MIMO, each access point and each user have a single-antenna?

- No. In general, they can be equipped with multiple antennas.

4. In cell-free massive MIMO, will all access points in the network cooperate to serve a given user?

- No. Each user will be served by a group of access points.

5. So cell-free massive MIMO is a user-centric system?

User-centric is a suitable approach used to implement cell-free massive MIMO systems. We can say that cell-free massive MIMO is a combination of user-centric and massive MIMO. But again, a completed system design for cell-free massive MIMO is still a big open problem.

6. Are cell-free massive MIMO and distributed massive MIMO the same?

No, they are different. Cell-free massive MIMO is different from distributed Massive MIMO. Both systems use many service antennas in a distributed way to serve many user terminals, but they are not entirely the same. With distributed Massive MIMO, the base station antennas are distributed within each cell, and these antennas only serve user terminals within that cell. By contrast, in cell-free massive MIMO there are no cells. All service antennas coherently serve all user terminals. The figures below compares the structures of Cell-Free Massive MIMO and distributed Massive MIMO.

Distributed Massive MIMO  

                                                            

Cell-Free Massive MIMO

                                                            

7. In cell-free massive MIMO, we do not have favorable propagation and channel hardening?

- No. It depends on the assumptions we made.
Similarly to cellular (colocated) massive MIMO, in rich scattering environments (e.g. Rayleigh fading), we still have favorable propagation and channel hardening, but the level of favorable propagation and channel hardening in cell-free massive MIMO is less than that in colocated massive MIMO. In addition, this depends a lot on the access point/user density, power control approaches, and number of antennas per access point.
In line-of-sight propagation environments, cell-free massive MIMO with multiple-antenna users can offer favorable propagation, but colocated massive MIMO may not.

8. What is the difference between cell-free massive MIMO and network MIMO/CoMP-JT?

- Cell-free massive MIMO is scalable and useful version of network MIMO/ CoMP-JT. (The same as multiuser MIMO and massive MIMO). 

9. What is the difference between cell-free massive MIMO and Fog massive MIMO?

- Fog massive MIMO is an example of cell-free massive MIMO in which a specific access point selection scheme based on on-fly-pilot contamination control mechanism is used.