Leakage-Aware Sparse Subcarrier Allocation for AFDM-Based Integrated Sensing and Communications

Affine frequency division multiplexing (AFDM) provides a natural delay-Doppler (DD) representation for integrated sensing and communications (ISAC), but practical fractional delays and Doppler shifts destroy the ideal DD sparsity and induce structured leakage across the DD domain. This leakage degrades both the reliability of data detection and the sensing performance. This paper proposes a leakage-aware sparse subcarrier allocation strategy for AFDM-based ISAC systems. Instead of redesigning the waveform or suppressing leakage through complex processing, a deterministic on-off mask is applied in the subcarrier domain to activate only a subset of subcarriers for data transmission. Although allocation is performed in the subcarrier domain, it effectively reshapes the induced DD-domain coupling caused by fractional delay-Doppler effects. Simulation results show that the proposed allocation significantly improves bit error rate performance and enhances sensing robustness against leakage-induced sidelobes. The results further reveal an allocation-dependent communication-sensing trade-off, where sparser subcarrier activation suppresses leakage more effectively at the cost of reduced delay resolution.