Abstract
Cell-free (CF) massive multiple-input-multiple-output (MIMO) systems have a large number of individually controllable antennas distributed over a wide area for simultaneously serving a small number of user equipments (UEs). This solution has been considered as a promising next-generation technology due to its ability to offer a similar quality of service to all UEs despite its low-complexity signal processing. In this paper, we provide a comprehensive survey of CF massive MIMO systems. To be more specific, the benefit of the so-called channel hardening and the favorable propagation conditions are exploited. Furthermore, we quantify the advantages of CF massive MIMO systems in terms of their energy- and cost-efficiency. Additionally, the signal processing techniques invoked for reducing the fronthaul burden for joint channel estimation and for transmit precoding are analyzed. Finally, the open research challenges in both its deployment and network management are highlighted.
Highlights
The multiple-input multiple-output (MIMO) concept proposed in their respective seminal papers by Paulraj et al [1], Foschini [2] and Telatar [3] has been recognized as one of the most disruptive technologies of the recent decades
The approach is based on a simple detection scheme, under the assumption that the pilot sequences assigned to each user equipments (UEs) are mutually orthogonal
One of our important findings is that CF massive MIMO systems provide a better coverage than conventional collocated massive MIMO systems and uncoordinated small cells
Summary
The multiple-input multiple-output (MIMO) concept proposed in their respective seminal papers by Paulraj et al [1], Foschini [2] and Telatar [3] has been recognized as one of the most disruptive technologies of the recent decades. As a paradigm shift from MU-MIMO, massive MIMO uses a large number of antennas for simultaneously serving only a few UEs on each time-frequency resource slot by exploiting SDMA principle [7]–[9]. Distributed massive MIMO systems [11] rely on a large number of antennas geographically spread out over a cell, they achieve a high diversity gain against shadow fading by exploiting the independent fading of their signals to provide uniformly good service for all UEs. Distributed massive MIMO systems have been investigated in single-cell and multi-cell scenarios [12], [13]. A large number of distributed access points (APs) employing single or multiple antennas simultaneously serve all UEs by exploiting local CSI and performing joint transmission. The substantial benefits of CF massive MIMO systems are quantified and a range of challenging open issues is described
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