Rethinking Passive RIS: Finite Blocklength Reliability Analysis Under Thermal Noise

TL;DR

This paper analyzes the impact of thermal noise in RIS-assisted short-packet communications within the finite blocklength regime, revealing that ignoring thermal noise overestimates reliability and that larger RIS does not always enhance performance.

Contribution

It introduces a unified analytical framework for RIS with thermal noise in the finite blocklength regime, highlighting limitations of idealized models and the importance of thermal noise considerations.

Findings

Ignoring RIS thermal noise overestimates reliability.

Increasing RIS size may not improve performance in low power regimes.

Thermal noise significantly affects system reliability and goodput.

Abstract

Short-packet communication alters the fundamental performance limits of reconfigurable intelligent surface (RIS)-assisted systems, making conventional analyses based on the infinite blocklength regime insufficient. This work investigates RIS-assisted transmission in the finite blocklength (FBL) regime while explicitly incorporating thermal noise generated by passive RIS elements, an effect commonly neglected in existing models. A unified analytical framework is developed to characterize the block-error rate (BLER), its asymptotic behavior, and the resulting goodput under both uniform and non-uniform RIS reflection coefficients. Our results show that ignoring RIS thermal noise leads to a pronounced overestimation of reliability with the mismatch increasing as the number of reflecting elements grows. Furthermore, increasing the RIS size does not always improve performance, particularly in the low transmit power regime where accumulated noise becomes dominant. Overall, the results highlight fundamental limitations of idealized RIS models and demonstrate the need for incorporating thermal noise for accurate system evaluation.