Reconfigurable Intelligent Surfaces in Upper Mid-Band 6G Networks: Gain or Pain?

TL;DR

The conditions under which RIS can provide major benefits and optimal strategies for deploying RIS to enhance the performance of 6G upper mid-band communication systems are identified.

Abstract

Reconfigurable intelligent surfaces (RISs) have emerged as one of the most studied topics in recent years, hailed as a transformative technology with the potential to revolutionize future wireless systems. While RISs are recognized for their ability to enhance spectral efficiency, coverage, and the reliability of wireless channels, several challenges remain. Notably, convincing and profitable use cases must be developed before widespread commercial deployment can be realized. The first sixth-generation (6G) networks will most likely utilize upper mid-band frequencies (i.e., 7-24\,GHz). This is regarded as the \textit{golden band} since it combines good coverage, much new spectrum, and enables many antennas in compact form factors. There has been much prior work on channel modeling, coexistence, and possible implementation scenarios for these bands. There are significant frequency-specific challenges related to RIS deployment, use cases, number of required elements, channel estimation, and control. These are previously unaddressed for the upper mid-band. In this paper, we aim to bridge this gap by exploring various use cases, including RIS-assisted fixed wireless access (FWA), enhanced capacity in mobile communications, and increased reliability at the cell edge. We identify the conditions under which RIS can provide major benefits and optimal strategies for deploying RIS to enhance the performance of 6G upper mid-band communication systems.

Figures (5)

• Figure 1: SE performance comparisons for RIS-assisted communications. Node locations: transmitter (0,0) m, receiver (0, 100) m, RIS (25, 50) m. In the legends, pairs refer to transmitter-RIS and RIS-receiver links. The transmitter-receiver link is considered to be nLoS. Accordingly, the first beneficial scenario is Use Case 1: RIS-Assisted Fixed Wireless Access (FWA) when both Tx-RIS and RIS-Rx links have LoS.
• Figure 2: SE performance comparisons for RIS-assisted communications vs RIS location at $f_c=7.8$ GHz with a realistic/probabilistic LoS channel model (e.g., 3GPP UMiETSI). Node locations ($x,y,z$): transmitter (0,0,10) m, receiver (0, 200, 2) m, RIS ($\mathrm{RIS}_x$, $\mathrm{RIS}_y$, 5) m. According to the $y$-axis location of RIS ($\mathrm{RIS}_y$), we always pick an $x$-axis location ($\mathrm{RIS}_x$) to have 30$^{\circ}$ azimuth angle between transmitter-RIS or RIS-receiver, whichever RIS is closed to. Transmit power is $T_X=30$ dBm. Accordingly, there are two possible use cases to enhance mobile capacity. Use Case 2: Enhanced Capacity For Distributed Users within the Cell when RIS is in the LoS region of the transmitter. Use Case 3: Enhanced Capacity for a Region of Interest (ROI) when RIS has LoS path for all users in ROI.
• Figure 3: The CDF of the SE at $T_X=30$ dBm. The left-hand side figure shows Use Case 2 where the SE is achieved at different 2000 random user locations within the cell and RIS is located in the LoS region of the transmitter. The right-hand-side figure shows Use Case 3 where the SE is achieved at different 200 random user locations within the region of interest where RIS has LoS links for all users in this region.
• Figure 4: Use Case 4: Increased reliability at the cell edge. The figures at the right-hand-side present coverage probability (reliability) performance comparisons of the cell-edge user when a RIS is deployed nearby (LoS region) the cell-edge.
• Figure 5: Performance comparisons of all use cases with respect to number of elements ($N$). Transmit power is $T_X=30$ dBm. SE performance is given on the left $y$-axis for the first three use cases, and coverage probability is given for Use Case 4 on the right $y$-axis as a performance metric. $\acute{R}=2$ bit/s/Hz is used for the QoS requirement in Use Case 4. For all use cases, the point-to-point communication benchmark performance points and the required number of elements for RIS-only communication to reach these benchmarks are illustrated with the markers.