Cell-free Massive MIMO and Radio Stripes

I have recorded a popular science video that explains how a cell-free network architecture can provide major performance improvements over 5G cellular networks, and why radio stripes is a promising way to implement it:

If you want more technical details, I recommend our recent survey paper “Ubiquitous Cell-Free Massive MIMO Communications“. One of the authors, Dr. Hien Quoc Ngo at Queen’s University Belfast, has created a blog about Cell-free Massive MIMO. In particular, it contains a list of papers on the topic and links to the programming code of some of them.

17 thoughts on “Cell-free Massive MIMO and Radio Stripes”

  1. Hi Emil,

    Thanks for sharing.
    Any comments on where the PAs sit for these antennas? What type of processing can be done in these black boxes?


    1. The PAs are placed next to each antenna. I’m imagining an output power per antenna of around 0.1 W, similar to a mobile phone.

      As much processing as possible should be done in the black boxes, so that the complexity of the tasks that must be done centrally becomes independent of the number of antennas – this gives a kind of scalability for systems with massive number of distributed antennas. In the uplink, it could, for example, be MR or LMMSE processing using the locally available CSI. The softly decoded data signals are then accumulated along the stripe so that the central processing unit only gets an accumulated version that is used for final hard decoding.

    1. This will of course be taken into account in the design. Suppose the stripe is using a design similar to an Ethernet cable with a power over Ethernet protocol. Then we could use at least 100 W per stripe, which would be enough for 100 antennas (0.1 W transmit power per antenna and the rest for antenna processing).

      1. Prof. Emil

        Thank you so much for the reply.
        I noticed that the antennna should be located along some straight line, as shown in your paper, so is it because the blended antenna’s performance will be degraded and disabled, including gain and directivity diagram?

        1. The paper only contains illustrations and are not conclusive when it comes to deployment strategies. One of the good properties with radio stripes is that they can be bended and deployed in many different ways. If the antennas are roughly omni-directional, there is no need to think about directivity when deploying the stripe. If the antenna elements have clear directivity, one probably needs to have antennas that point in different directions or small arrays with beamforming capability to ensure coverage in all directions.

  2. Hi Emil,

    I’m interested in the 5g vs cell free massive MIMO SINR diagram at 20:00 mins. In 5g SINR color diagram you have given different cones with less yellow color (good SINR part). Each cone represent a beam (I mean ssb indicated beam) in 5g? (Question 1). If so good SINR part is less, so you mean even though we do hybrid beamforming for a UE, within the beam there will be cell center, middle and edge kind of SINR variations? (Question 2). My basic question is in 5g the hybrid beamforming done for a cell center vs cell middle vs cell edge UE varies? I mean SINR seen by the UE will varies? Please let me know.

    1. Question 1: No, each cone in the cellular case represents a cell and the data rate that a user can get at different places in the cell.

      Question 2: Since the base stations are well separated in a cellular network, there are large variations in signal strength between different parts of the cell (center, middle, edge). It can very by 1 million times! Beamforming improves the signal quality for all UEs but it does not reduce the signal strength variations.

      PS. I’m not sure why you are asking about _hybrid_ beamforming. It is digital beamforming that the system should preferably use.

  3. Hi,
    Thanks for sharing. I have one question, what is the stochastic channel model for cell free massive MIMO? In other words what are the differences between channel model in cellular massive MIMO and cell free massive MIMO?
    Thanks in advance

    1. There are no major differences. Correlated Rayleigh fading models are often used in both cases. 3GPP has propagation models (pathloss + shadow fading) that are designed for indoor and microcell usage. Those models are appropriate also for cell-free massive MIMO. A cell-free system might be deployed in the same way as a cellular network with small cells. The major difference is that users are served by many access points, instead of only one.

  4. Hello Professor Bjornson.
    I have a question.
    In cell-free systems which channel is more preferable channel with correlated Rayleigh fading or channel with uncorrelated Rayleigh fading? Is not it depended on the number of APs antennas?

    1. I don’t think there is a clear answer to this yet, but my impression is that uncorrelated fading is preferable since every AP has so few antennas that it cannot make efficient use of the spatial correlation.

  5. Hello professor Bjornson
    Thank you for your useful answers, as always.
    As I know in cellular massive MIMO systems the pilot signals are transmitted in UL for channel estimation and that is a principal for the system.
    I want to know is there such a principal in the cell-free systems?
    Can not we achieve a better performance if we transmit the pilot signal in downlink in cell-free systems?

  6. Hello Professor Bjornson.
    I have two question about one of your paper “Multiple antenna for beyond 5G”
    1) You say in somewhere of the paper that in cell-free systems all APs only need to know the CSI between themselves and all the users and do not require to share the CSI. I want to know whether this statement is always true for any of receive combining vectors?
    2) According to the figure 3 in the paper, to some extent you think power control is essential in cell-free systems? Is not it benificial for all users to transmitt with full power?

    1. 1) Well, you can certainly create combining schemes where the statement is true. But there are normally two categories: Distributed schemes where each AP processes the received signals using its local CSI, Centralized schemes where each AP sends it local CSI to the CPU which performs the processing. In both cases, each AP only has local CSI.

      2) Power control is always important, but my impression is that it is harder to perform downlink power allocation than uplink power control. Transmitting with full power in the uplink seems to be close to the sum-rate maximization solution.

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