The Road to 6G: Ten Physical Layer Challenges for Communications Engineers

Abstract

While the deployment of 5G cellular systems will continue well in to the next decade, much interest is already being generated towards technologies that will underlie its successor, 6G. Undeniably, 5G will have transformative impact on the way we live and communicate, yet, it is still far away from supporting the Internet-of-Everything (IoE), where upwards of a million devices per km 3 (both terrestrial and aerial) will require ubiquitous, reliable, low-latency connectivity. This article looks at some of the fundamental problems that pertain to key physical layer enablers for 6G. This includes highlighting challenges related to intelligent reflecting surfaces, cell-free massive MIMO and THz communications. Our analysis covers theoretical modeling challenges, hardware implementation issues and scalability among others. The article concludes by delineating the critical role of signal processing in the new era for wireless communications.

Channel Estimation and User Localization for IRS-Assisted MIMO-OFDM Systems

“…, y r,k,Ptr,Ttr ] ∈ C Mtr×PtrTtr . It can be equivalently regarded as a matricization of a thirdorder tensor as Y r,k = Matr(Y r,k ; 1, [3,2]), where Y r,k ∈ C Mtr×Ttr×Ptr fits the CP tensor model as [21], [22]…”

“…Millimeter wave (mmWave) (30-300 GHz) technologies have been identified as a promising candidate for tackling the data traffic deluge and frequency resource shortage in the fifth generation era [1]. Exploiting higher-frequency spectrum, e.g., terahertz (0.1-10 THz), has also been regarded as one of the potential development directions of the future sixth generation wireless communications [2]. The short signal wavelength enables miniaturized implementation of massive multiple-input multiple-output (MIMO) arrays, providing considerable directional beamforming gains [3].…”

“…where 2 The adjacent primary/secondary planes of each twin-IRS structure are expected to share identical propagation conditions, which can be equivalently regarded as a 2N I -element 3-D IRS. The proposed training designs can be directly applied to this 3-D topology.…”

“…where n h(v) ∈ I(N h(v) ). Now, the uplink channel matrices via the rth secondary IRS can be represented by (2) with the radiation patterns and steering vectors of (3), (4) being replaced by those of ( 5), ( 6), respectively. One can also generate (3), (4) as F (ϕ, θ; 0, 0) and a I (ϕ, θ; 0, 0) = a h (ϕ, θ; 0, 0) ⊗ a v (ϕ, θ; 0, 0), respectively.…”

Why do we need 6G?

“…The authors of [76] proposed some key physical-layer problems to be solved to move from 5G to 6G physical layer. In particular, 6G should improve 5G via the exploitation of higher frequency bands (mmWaves and especially Terahertz), of smart radio technologies such as Recon igurable Intelligent Surface (RIS), and the full realisation of a cell-less massive MIMO wireless system.…”