Scalable Cell-Free Massive MIMO

Cell-free massive MIMO is likely one of the technologies that will form the backbone of any xG with x>5. What distinguishes cell-free massive MIMO from distributed MIMO, network MIMO or cooperative multi-point (CoMP)? The short answer is that cell-free massive MIMO works, it can deliver uniformly good service throughout the coverage area, and it requires no prior knowledge of short-term CSI (just like regular cellular massive MIMO). A longer answer is here. The price to pay for this superiority, no shock, is the lack of scalability: for canonical cell-free massive MIMO there is a practical limit on how large the system can be, and this scalability concerns both the power control, the signal processing, and the organization of the backhaul.

At ICC this year we presented this approach towards scalable cell-free massive MIMO. A key insight is that power control is extremely vital for performance, and a scalable cell-free massive MIMO solution requires a scalable power control policy. No surprise, some performance must be sacrificed relative to canonical cell-free massive MIMO. Coincidentally, another paper in the same session (WC-26) also devised a power control policy with similar qualities!

Take-away point? There are only three things that matter for the design of cell-free massive MIMO signal processing algorithms and power control policies: scalability, scalability and scalability…

6 thoughts on “Scalable Cell-Free Massive MIMO”

  1. Why do we apply power control in cell-free Massive MIMO?
    As I know, power control is used to control the power of pilot in uplink to harness pilot contamination. What are the other reasons of using power control?

    1. What you describe is one recent use case for power control, but the same thing is normally done also in the transmission of uplink data. The classical reason to use power control is to avoid near-far effects, where the received signals from cell-center users are much stronger than the signals from cell-edge users. In Massive MIMO, we can optimize the power control towards a performance metric, such as maximum sum rate or max-min fairness. You can find some references to papers in the following blog post: https://ma-mimo.ellintech.se/2018/10/02/are-pilots-and-data-transmitted-at-the-same-power/

      Section 7.1 in my book “Massive MIMO networks” is also discussing these issues.

  2. I have two question about this blog:
    1. Why cell-free MIMO has less channel hardening as mentioned in the research ‘Ubiquitous Cell-Free Massive MIMO Communications’?
    2. Another thing you mentioned that cell-free MIMO can provide uniformly good service. But in my opinion, whether it can provide fair service is depended on the power policy you adopt. Which means that even a celluar MIMO with adopt max-min power allocation policy can provide fair service. Isn’t it?

    1. 1. Channel hardening appear when you combine the signals from many antennas that have similar average SNRs. In cell-free massive MIMO, you will have many antennas but they are spread out and therefore have different SNRs. This is investigated in the following paper: https://arxiv.org/pdf/1710.00395

      2. Yes, in both cases you need to utilize a max-min power allocation policy, but the rate that every user gets under such a policy will be different depending on the network architecture. I think what Erik means is that the rate will be larger in a cell-free system than a cellular system since the antennas are spread out and therefore there is a lower risk that a user has a bad channel to all its neighboring access points.

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