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

Adam Umra, Kevin Weinberger, Aymen Khaleel, Gerald Enzner, Aydin Sezgin

TL;DR

This paper addresses the joint design of RIS-assisted, full-duplex ISAC systems operating under residual self-interference and finite blocklength constraints. It formulates a nonconvex optimization problem to minimize the service adaptation gap while ensuring sensing reliability, incorporating both FBL rate penalties and radar QoS through a reformulation with slack variables and an alternating optimization/SCA solution. The key contributions are explicit modeling of residual SI, integration of FBL effects into the ISAC design, and demonstration of a mid-range blocklength sweet spot where simultaneous communication throughput and robust sensing are achieved via RIS beamforming. The results provide practical guidelines for 6G deployments, highlighting how RIS configuration and blocklength selection jointly affect reliability, responsiveness, and sensing stability in dynamic environments.

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.

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

TL;DR

This paper addresses the joint design of RIS-assisted, full-duplex ISAC systems operating under residual self-interference and finite blocklength constraints. It formulates a nonconvex optimization problem to minimize the service adaptation gap while ensuring sensing reliability, incorporating both FBL rate penalties and radar QoS through a reformulation with slack variables and an alternating optimization/SCA solution. The key contributions are explicit modeling of residual SI, integration of FBL effects into the ISAC design, and demonstration of a mid-range blocklength sweet spot where simultaneous communication throughput and robust sensing are achieved via RIS beamforming. The results provide practical guidelines for 6G deployments, highlighting how RIS configuration and blocklength selection jointly affect reliability, responsiveness, and sensing stability in dynamic environments.

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.

Paper Structure

This paper contains 10 sections, 22 equations, 3 figures, 1 algorithm.

Table of Contents

  1. Introduction
  2. System Model
  3. Communication Model
  4. Sensing Model
  5. Problem Formulation
  6. Numerical Results
  7. ISAC Communication Performance
  8. ISAC Sensing Performance
  9. Conclusion
  10. SCA-Based Alternating Optimization

Figures (3)

  • Figure 1: RIS-assisted multiuser ISAC system with a moving object.
  • Figure 2: Cumulative Throughput Shortfall (CTS) for the communication users in the observed coherence interval $T_\mathsf{coh}$.
  • Figure 3: Radar sensing performance: comparison of outage probability and radar echo SINR variance for two levels of SI cancellation. Left: ideal SI ($\rho=-120$ dB), right: higher residual SI ($\rho=-118$ dB).