Spectral and Energy Efficiency Maximization of MISO STAR-RIS-assisted URLLC Systems

TL;DR

This paper introduces a flexible optimization framework for enhancing spectral and energy efficiency in URLLC systems aided by STAR-RIS, demonstrating significant improvements through optimized reflection strategies and various transmission schemes.

Contribution

The paper develops a general optimization framework applicable to interference-limited URLLC systems with STAR-RIS, including realistic RIS models and multiple transmission strategies, advancing system performance.

Findings

RIS significantly boosts spectral and energy efficiency when optimized.

STAR-RIS can outperform regular RIS in coverage and efficiency.

Energy splitting scheme outperforms mode and time switching schemes.

Abstract

This paper proposes a general optimization framework to improve the spectral and energy efficiency (EE) of ultra-reliable low-latency communication (URLLC) simultaneous-transfer-and-receive (STAR) reconfigurable intelligent surface (RIS)-assisted interference-limited systems with finite block length (FBL). This framework can solve a large variety of optimization problems in which the objective and/or constraints are linear functions of the rates and/or EE of users. Additionally, the framework can be applied to any interference-limited system with treating interference as noise as the decoding strategy at receivers. We consider a multi-cell broadcast channel as an example and show how this framework can be specialized to solve the minimum-weighted rate, weighted sum rate, global EE and weighted EE of the system. We make realistic assumptions regarding the (STAR-)RIS by considering three different feasibility sets for the components of either regular RIS or STAR-RIS. Our results show that RIS can substantially increase the spectral and EE of URLLC systems if the reflecting coefficients are properly optimized. Moreover, we consider three different transmission strategies for STAR-RIS as energy splitting (ES), mode switching (MS), and time switching (TS). We show that STAR-RIS can outperform a regular RIS when the regular RIS cannot cover all the users. Furthermore, it is shown that the ES scheme outperforms the MS and TS schemes.