Cell-Free Massive MIMO: New Concept

Conventional mobile networks (a.k.a. cellular wireless networks) are based on cellular topologies. With cellular topologies, a land area is divided into cells. Each cell is served by one base station. An interesting question is: shall the future mobile networks continue to have cells? My quick answer is no, cell-free networks should be the way to do in the future!

Future wireless networks have to manage at the same time billions of devices; each needs a high throughput to support many applications such as voice, real-time video, high quality movies, etc. Cellular networks could not handle such huge connections since user terminals at the cell boundary suffer from very high interference, and hence, perform badly. Furthermore, conventional cellular systems are designed mainly for human users. In future wireless networks, machine-type communications such as the Internet of Things, Internet of Everything, Smart X, etc. are expected to play an important role. The main challenge of machine-type communications is scalable and efficient connectivity for billions of devices. Centralized technology with cellular topologies does not seem to be working for such scenarios since each cell can cover a limited number of user terminals. So why not cell-free structures with decentralized technology? Of course, to serve many user terminals and to simplify the signal processing in a distributed manner, massive MIMO technology should be included. The combination between cell-free structure and massive MIMO technology yields the new concept: Cell-Free Massive MIMO.

What is Cell-Free Massive MIMO? Cell-Free Massive MIMO is a system where a massive number access points distributed over a large area coherently serve a massive number of user terminals in the same time/frequency band. Cell-Free Massive MIMO focuses on cellular frequencies. However, millimeter wave bands can be used as a combination with the cellular frequency bands. There are no concepts of cells or cell boundaries here. Of course, specific signal processing is used, see [1] for more details. Cell-Free Massive MIMO is a new concept. It is a new practical, useful, and scalable version of network MIMO (or cooperative multipoint joint processing) [2, 3]. To some extent, Massive MIMO technology based on the favorable propagation and channel hardening properties is used in Cell-Free Massive MIMO.

Cell-Free Massive MIMO is different from distributed Massive MIMO [4]. 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 figure below compares the structures of Cell-Free Massive MIMO and distributed Massive MIMO.

comami cellfree
Distributed Massive MIMO Cell-Free Massive MIMO

[1] H. Q. Ngo, A. Ashikhmin, H. Yang, E. G. Larsson, and T. L. Marzetta, “Cell-Free Massive MIMO versus Small Cells,” IEEE Trans. Wireless Commun., 2016 submitted for publication. Available: https://arxiv.org/abs/1602.08232

[2] G. Foschini, K. Karakayali, and R. A. Valenzuela, “Coordinating multiple antenna cellular networks to achieve enormous spectral efficiency,” IEE Proc. Commun. , vol. 152, pp. 548–555, Aug. 2006.

[3] E. Björnson, R. Zakhour, D. Gesbert, B. Ottersten, “Cooperative Multicell Precoding: Rate Region Characterization and Distributed Strategies with Instantaneous and Statistical CSI,” IEEE Trans. Signal Process., vol. 58, no. 8, pp. 4298-4310, Aug. 2010.

[4] K. T. Truong and R.W. Heath Jr., “The viability of distributed antennas for massive MIMO systems,” in Proc. Asilomar CSSC, 2013, pp. 1318–1323.

21 thoughts on “Cell-Free Massive MIMO: New Concept”

    1. Hi Thien,
      Good question! The density of APs depends on the density of the users of that area. I do not have a precise value of the ratio between the number of APs (M) and the number of users (K) now. This ratio depends on what you want. If you want a high spectral efficiency per user, then M/K should be large. But I believe M/K < 10, for most of the cases.

  1. how cell free Massive MIMO will be implemented in future, frequency re-use is core concept in the cellular technology.similarly the resources (spectrum and energy) are limited for cellular networks .how synchronization issues across APs are handled in cell-free communication

    1. As in collocated massive MIMO, cell-free massive MIMO offers favorable propagation channels, and hence, all users can share the same time-frequency resource with not very high inter-user interference. In cell-free massive MIMO, synchronization can be done locally. For example, at a given time slot, some APs are chosen as the level-1 APs, and the remaining APs are considered as level-2 APs. Each level-1 AP is connected to at least another level-1 AP. All level-2 APs get synchronized by their neighboring level-1 APs. This method is called the hierarchical method.

  2. Is there channel hardening in cell-free massive MIMO? with channel hardening meaning that the effective channel gain is practically deterministic, which makes downlink pilots redundant. If there is no channel hardening, is that a problem for cell-free massive MIMO?

    1. In cell-free massive MIMO, the channel still hardens, but is not as good as the channel in collocated massive MIMO (in term of the hardening property). In fact in [1], the signal processing at the users is done relying on the channel hardening property. The system performance is still very good. Furthermore, we can send downlink pilot via beamforming training scheme to improve the system performance. Note that the corresponding channel overhead for this downlink pilot scheme does not depend on the number of APs. So the channel hardening is not a problem for cell-free massive MIMO. More detail about this can be found here:
      – Giovanni Interdonato, Hien Quoc Ngo, Erik G. Larsson, and Pal Frenger, “How much do downlink pilots improve cell-free Massive MIMO?,” IEEE Global Communications Conference (GLOBECOM), 2016. Link: https://arxiv.org/abs/1607.04753

    1. In [1], we proposed to use beamforming at the APs in a distributed manner, not at the CPU. The CPU is used for exchanging the data and power control coefficients.

      1. Yes in [1] I mean what is the advantage of doing beamforming at AP? By doing so we send K times signal over the backhaul which is increasing the load of the backhaul, don’t we?

        1. You are right that by doing beamforming at the APs, the CPU has to send K signals to each AP over the backhaul links. However, its backhaul requirement is still lower than the backhaul requirement by doing beamforming at the CPU, when the number of APs (M) is large. More precisely, if beamforming scheme is performed at the CPU, then all APs have to send the channel estimates to the CPU. There are MK such coefficients for each coherence interval. So the backhaul requirements increase. More importantly, this makes the system unscalable (with respect to M).
          P/S: it is worth to compare the backhaul requirements for two cases (beamforming at the APs and beamforming at the CPU) when M is not very large.

  3. What is the difference between the cloud radio access network (C-RAN) and the cell-free massive MIMO system in this case? As both of them has the same structure, I mean multiple access points or RRHs connected to a CPU.

    1. C-RAN is an architecture that moves the baseband processing from the access points to “the cloud”. Cell-free massive MIMO means that many distributed access points are serving the users by coherent joint transmission. These two methods can be used together, but one can also use C-RAN with conventional base stations and/or cell-free without C-RAN.

  4. Hi. I study about cell-free massive MIMO. My thesis is about equipping each AP with more than one antenna. How can help us in cell-free Massive MIMO?

    I think more antennas per AP can help us to serve more user and it’s better for energy efficiency and spectrum efficiency.

    1. Hi! The pros and cons of having multiple antennas at each AP in cell-free Massive MIMO are discussed in the following paper: https://arxiv.org/abs/1710.00395

      You will basically get more channel hardening and having more antennas is always better from a spectral efficiency perspective. However, if you have to choose between 100 single-antenna APs and 25 four-antenna APs, the single-antenna alternative might be more beneficial since you get more macro diversity (shorter distances between a typical UE and its nearest APs). When it comes to energy efficiency, more antennas and/or APs are not always beneficial. The throughput gains must compensate for the increased energy consumption.

  5. Hi!

    I recently started reading a few papers to understand the basics of CF Massive MIMO. Is there any hot topic in research that hasn’t been addressed so far in CF Massive MIMO?

  6. As far as I understand, CF mMIMO is a large-scale version of nwMIMO / CoMP JT, which essentially can be viewed as a hybrid of conventional nwMIMO and mMIMO. That is, the cooperating BSs are divided into clusters via dynamic-cell clustering (a.k.a. user-centric clustering) and cooperation is restricted within each such cluster to facilitate cooperation / data sharing, as in conventional CoMP JT. The main difference is that the cooperation clusters in CF mMIMO are much larger, and this “massiveness” (in terms of the total number of transmit antennas – recall that CoMP JT is modeled mathematically as a composite MIMO BC) brings in the channel hardening and FP phenomena that we notice in the conventional mMIMO paradigm (at least, in the extent that this is possible by the fact that the antennas are not colocated but they are distributed instead). On the other hand, there are known tradeoffs from the CoMP literature in using larger vs. smaller cooperation clusters. Do these apply here as well?

    A somewhat related question: CoMP did not meet the expectations, but as far as I know this is because the LTE-A implementations utilized network-centric clustering which suffers from OCI and provides poor sum-SE, especially for heterogeneous user distributions. So, what would be the difference in performance and complexity between conventional CoMP JT with DCC and CF mMIMO (which, by definition, utilizes DCC as far as I understand – the “cell-free” terms implies that, I believe), i.e., what would be the difference between small clusters (conventional nwMIMO) and large clusters (CF mMIMO)?

    1. As you said, CF mMIMO is a particular form of CoMP JT with user-centric cooperation clusters and only data-sharing between the access points. This is not a new approach (I considered something similar in the papers “Cooperative Multicell Precoding: Rate Region Characterization and Distributed Strategies with Instantaneous and Statistical CSI” and “Optimality Properties, Distributed Strategies, and Measurement-Based Evaluation of Coordinated Multicell OFDMA Transmission” from 2010-2011), but it differs from the type of CoMP that was considered in LTE-A. For example, the “larger vs. smaller cooperation cluster” tradeoff is something that is created by the network-centric clustering approach in LTE-A. What CF mMIMO has contributed with is a renewed focus on CoMP JT with user-centric cooperation clusters, including how to deal with channel estimation and resource allocation.

      Many of these things are discussed in the overview paper “Ubiquitous Cell-Free Massive MIMO Communications” that is available on Arxiv: https://arxiv.org/abs/1804.03421

  7. Hello Sir,

    I have a couple of questions regarding joint transmission in CoMP, which is somehow related with cell-free massive MIMO, and I was hoping that you might clarify these things.

    1. Since in joint transmission each scheduled UE is served by two or more base stations, should these UEs have more than one antenna?
    2. Related with the previous question: Do these base stations that serve jointly a specific UE send the same data symbol to it or different data symbols? (In order to make this question more clear, let’s give a small example. Assume that there are two base stations, BS1 and BS2 and serve a user, let’s simply call her/him UE, by sending a single symbol each. Do they send a symbol s1 and a symbol s2, respectively, or they transmit the same symbol (let’s say, s)?)

    Thank you!

    1. 1. It is enough to have one antenna. Most papers on cell-free massive MIMO consider that case, but that is mainly for analytical tractability. Practical UEs will most certainly have more than one antenna (LTE phones have two receive antennas…)

      2. Both options are possible. Sending the same symbol is called coherent transmission and sending different symbols is called non coherent transmission. Coherent transmission is preferable but requires phase-coherency between the access points. If N access points transmit one phase-coherent symbol, the received power is proportional to N^2. If N access points transmit N different symbols the received signal power is proportional to N.

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