Power Efficient Cooperative Communication within IIoT Subnetworks: Relay or RIS?

The forthcoming sixth-generation (6G) industrial Internet-of-Things (IIoT) subnetworks are expected to support ultra-fast control communication cycles for numerous IoT devices. However, meeting the stringent requirements for low latency and high reliability poses significant challenges, particularly due to signal fading and physical obstructions. In this paper, we propose novel time division multiple access (TDMA) and frequency division multiple access (FDMA) communication protocols for cooperative transmission in IIoT subnetworks. These protocols leverage secondary access points (sAPs) as Decode-and-Forward (DF) and Amplify-and-Forward (AF) relays, enabling shorter cycle times while minimizing overall transmit power. A classification mechanism determines whether the highest-gain link for each IoT device is a single-hop or two-hop connection, and selects the corresponding sAP. We then formulate the problem of minimizing transmit power for DF/AF relaying while adhering to the delay and maximum power constraints. In the FDMA case, an additional constraint is introduced for bandwidth allocation to IoT devices during the first and second phases of cooperative transmission. To tackle the nonconvex problem, we employ the sequential parametric convex approximation (SPCA) method. We extend our analysis to a system model with reconfigurable intelligent surfaces (RISs), enabling transmission through direct and RIS-assisted channels, and optimizing for a multi-RIS scenario for comparative analysis. Simulation results show that our cooperative communication approach reduces the emitted power by up to 7.5 dB while maintaining an outage probability and a resource overflow rate below $10^{-6}$. While the RIS-based solution achieves greater power savings, the relay-based protocol outperforms RIS in terms of outage probability.

Secure SWIPT in STAR-RIS Aided Downlink MISO Rate-Splitting Multiple Access Networks

Recently, simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) have emerged as a novel technology that facilitates sustainable communication by providing 360 coverage and new degrees-of-freedom (DoF) for manipulating signal propagation as well as simultaneous wireless information and power transfer (SWIPT). Inspired by these applications, this paper presents a novel STAR-RIS-aided secure SWIPT system for downlink multiple input single output (MISO) Rate-Splitting multiple access (RSMA) networks. The transmitter concurrently communicates with the information receivers (IRs) and sends energy to untrusted energy receivers (UERs). UERs are also able to wiretap the IR streams. The paper assumes that the channel state information (CSI) of the IRs is known at the transmitter. However, only imperfect CSI (ICSI) for the UERs is available at the transmitter. The paper aims to maximize the achievable worst-case sum secrecy rate (WCSSR) of the IRs under a total transmit power constraint, a sum energy constraint for the UERs, and constraints on the transmission and reflection coefficients by jointly optimizing the precoders and the transmission and reflection beamforming at the STAR-RIS. The formulated problem is non-convex with intricately coupled variables, and to tackle this challenge a suboptimal two-step iterative algorithm based on the sequential parametric convex approximation (SPCA) method is proposed. Specifically, the precoders and the transmission and reflection beamforming vectors are optimized alternatingly. Simulations are conducted to show that the proposed RSMA-based algorithm in a STAR-RIS aided network can improve the secrecy of the confidential information and the overall spectral efficiency.