Hybrid Beamforming for Massive MIMO: A Survey

Abstract

Hybrid multiple-antenna transceivers, which combine large-dimensional analog pre/postprocessing with lowerdimensional digital processing, are the most promising approach for reducing the hardware cost and training overhead in massive MIMO systems. This paper provides a comprehensive survey of the various incarnations of such structures that have been proposed in the literature. We provide a taxonomy in terms of the required channel state information (CSI), namely whether the processing adapts to the instantaneous or the average (second-order) CSI; while the former provides somewhat better signal-to-noise and interference ratio, the latter has much lower overhead for CSI acquisition. We furthermore distinguish hardware structures of different complexities. Finally, we point out the special design aspects for operation at millimeter-wave frequencies. Index TermsHybrid beamforming, 5G, Millimeter-wave.

Massive MIMO Performance—TDD Versus FDD: What Do Measurements Say?

“…In this environment, the UEs experience LOS propagation conditions to the array, along with a number of strong scattered components arising from interactions with the enclosing walls, outdoor furniture (to a large extent consisting of metallic items), and vegetation. Because of this, the Ricean Kfactor [22], [41], [42] is low compared to sites MS 1-MS 3 in campaign A; see Table II. In this work, we consider three propagation scenarios related to campaign B, summarized in Table I as scenarios 4, 5, and 6.…”

“…2. Other constraints on B leading to simplifications of the analog hardware are possible; the reader is referred to [22], [39] for a comprehensive survey of the field.…”

“…Such directions only have a well-defined operational meaning when the propagation environment offers strong direct or specular paths [22].…”

Hybrid beamforming for single-user MIMO with partially connected RF architecture

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support only one-stream transmission and is not viable for multi-stream case [6]. To exploit the full potential of both spatial multiplexing and beamforming in a MIMO system, fully digital beamforming architecture is leveraged [7], [8].

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“…Implementing a fully digital MIMO beamformer requires one radio frequency (RF) chain per antenna element. Due to the high number of power-hungry RF components, such as digital-to-analog converters (DACs), in conventional digital beamforming, the practical implementation of a massive MIMO system requires new approaches to be realized [2], [6]. As a solution, the combination of both digital and analog beamforming, using a limited number of RF chains, known as hybrid beamforming (HBF) has been considered.…”