Short Blocks, Fast Sensing: Finite Blocklength Tradeoffs in RIS-Assisted ISAC

TL;DR

This paper investigates the tradeoffs in RIS-assisted full-duplex ISAC systems under finite blocklength constraints, balancing rapid adaptation and sensing reliability for future 6G networks.

Contribution

It introduces an optimization framework that accounts for residual self-interference and finite blocklength effects, revealing the optimal balance between communication and sensing performance.

Findings

Short blocklengths enable fast adaptation but increase radar outage.

Longer blocklengths improve SINR and reduce outages but are more sensitive to motion.

A 'sweet spot' exists where blocklength and beamforming optimize both throughput and sensing.

Abstract

Integrated sensing and communication (ISAC) is a cornerstone for future sixth-generation (6G) networks, enabling simultaneous connectivity and environmental awareness. However, practical realization faces significant challenges, including residual self-interference (SI) in full-duplex systems and performance degradation of short-packet transmissions under finite blocklength (FBL) constraints. This work studies a reconfigurable intelligent surface (RIS)-assisted full-duplex ISAC system serving multiple downlink users while tracking a moving target, explicitly accounting for SI and FBL effects in both communication and sensing. We formulate an optimization framework to minimize service adaptation gaps while ensuring sensing reliability, solved via alternating optimization and successive convex approximation. Numerical results show that short blocklengths enable fast adaptation but raise radar outage from fewer pulses and motion sensitivity. Longer blocklengths improve signal-to-interference-plus-noise ratio (SINR) and reduce outages but allow motion to degrade sensing. A "sweet spot" arises where blocklength and beamformer allocation optimize throughput and sensing, seen as a local minimum in radar SINR variance. RIS-assisted optimization identifies this balance, achieving reliable communication and radar sensing jointly.