Optimization of the Downlink Spectral- and Energy-Efficiency of RIS-aided Multi-user URLLC MIMO Systems

TL;DR

This paper tackles the challenge of optimizing downlink spectral and energy efficiency in RIS-aided multi-user MIMO systems under ultra-reliable low-latency constraints, where finite-block-length (FBL) coding and the normal approximation govern achievable rates. It derives closed-form expressions for the channel dispersion in MU-MIMO with interference and develops a flexible optimization framework combining majorization-minimization, alternating optimization, and fractional programming to jointly optimize transmit covariances and RIS coefficients (including reflective and STAR-RIS) under realistic latency and reliability constraints. The proposed framework yields significant SE and EE gains, with RISs offering larger benefits for shorter packets and tighter BER requirements, and multiple data streams per user outperforming single-stream beamforming. The work establishes that RISs can be especially beneficial for URLLC scenarios and lays out future directions in robust design, interference-management integration, and exploration of emerging RIS variants.

Abstract

Modern wireless communication systems are expected to provide improved latency and reliability. To meet these expectations, a short packet length is needed, which makes the first-order Shannon rate an inaccurate performance metric for such communication systems. A more accurate approximation of the achievable rates of finite-block-length (FBL) coding regimes is known as the normal approximation (NA). It is therefore of substantial interest to study the optimization of the FBL rate in multi-user multiple-input multiple-output (MIMO) systems, in which each user may transmit and/or receive multiple data streams. Hence, we formulate a general optimization problem for improving the spectral and energy efficiency of multi-user MIMO-aided ultra-reliable low-latency communication (URLLC) systems, which are assisted by reconfigurable intelligent surfaces (RISs). We show that an RIS is capable of substantially improving the performance of multi-user MIMO-aided URLLC systems. Moreover, the benefits of RIS increase as the packet length and/or the tolerable bit error rate are reduced. This reveals that RISs can be even more beneficial in URLLC systems for improving the FBL rates than in conventional systems approaching Shannon rates.

Figures (13)

• Figure 1: A multi-cell BC assisted by RISs.
• Figure 2: Spectral efficiency metrics versus $P$ for $N_{{BS}}=4$, $N_{u}=4$, $K=2$, $L=2$, $M=2$, $n_t=256$, $\epsilon=10^{-5}$, and $N_{{RIS}}=20$.
• Figure 3: Average max-min rate and relative performance improvement by RIS versus $n_t$ for $P=10$ dB, $N_{{BS}}=4$, $N_{u}=3$, $K=2$, $L=2$, $M=2$, $\epsilon=10^{-5}$, and $N_{{RIS}}=20$.
• Figure 4: Average sum rate versus $n_t$ for $P=10$ dB, $L=2$, $M=2$, and $\epsilon=10^{-5}$.
• Figure 5: Average max-min rate and relative performance improvement by RIS versus $\epsilon$ for $P=10$ dB, $N_{{BS}}=4$, $N_{u}=3$, $K=2$, $L=2$, $M=2$, $n_t=256$ bits, and $N_{{RIS}}=20$.

Theorems & Definitions (11)

• Lemma 1: polyanskiy2010
• Lemma 2
• proof
• Remark 1
• Lemma 3
• proof
• Lemma 4: soleymani2022improper
• Lemma 5
• proof
• Remark 2