Finite-Blocklength ISAC Multiple Access

Joint communication and radar sensing, also known as integrated sensing and communication (ISAC), is evolving current networks into multifunctional systems with significant potential. This work investigates the ISAC multiple access problem in the finite blocklength (FBL) regime, which has been largely overlooked in existing studies. A dual-functional ISAC receiver is considered, which simultaneously decodes information from multiple users and performs channel state sensing in uplink transmission. Overall, fundamental limits on energy efficiency and communication-sensing tradeoffs are analyzed for dual-functional ISAC multiple access systems. Specifically, achievability bounds on energy efficiency and the corresponding performance floors of decoding error and sensing accuracy are characterized under fixed blocklength and energy constraints. The FBL energy efficiency of representative ISAC multiple access schemes, including TDMA, ALOHA, and power domain and code domain nonorthogonal multiple access (PD/CD-NOMA), is compared under different user densities, demonstrating that CD-NOMA exhibits the most robust energy efficiency against access density, while the others perform favorably only at low densities. The tightness of the derived bounds is further examined and proved by relating to the derived exact pairwise error probability. Additionally, the communication-sensing tradeoffs for dual-functional ISAC multiple access in FBL regime are revealed. Moreover, we derive a universal Cramér–Rao bound (CRB) for generic sensing parameters to demonstrate the effectiveness of treating channel estimation as a high-level sensing objective, and further validate this approach through an illustrative example based on 3GPP-oriented channel modeling. Numerical results corroborate the energy efficiency analysis, bound tightness, and communication-sensing tradeoffs, providing insights for dualfunctional ISAC multiple access systems.