TempoCross: instance-aware sparse representation for multimodal temporal fusion in 3D detection

With rising performance demands for autonomous driving perception systems, bird's-eye-view (BEV) perception that combines heterogeneous sensor data and temporal information has become a key research focus. Recent studies have advanced either multimodal temporal detection through BEV-level feature aggregation or sparse object-centric temporal modeling. However, integrating these two directions while preserving modality-specific temporal states remains less explored for LiDAR-camera detection. In this work, we present TempoCross, a 3D detection method based on instance-aware sparse representations for multimodal temporal fusion. TempoCross encodes features from different timestamps and sensor modalities in a unified instance space, enabling adaptive extraction of critical target information from modality- and time-specific states. Initially, both branches perform a preliminary cross-modal fusion to generate queries for the current frame. In the motion compensation module, a hybrid motion modeling strategy reduces alignment discrepancies between historical and current instances caused by complex object motion. This strategy combines explicit rigid-body transformations with implicit learnable deformation residuals, improving both accuracy and robustness in cross-frame instance association. Next, temporal-aware enhancement integrates the initial queries and current-frame features with motion-compensated historical instances. Finally, a lightweight cross-attention fuses current and historical instances from both branches. This formulation reduces the reliance on repeatedly propagating full-scene fused BEV features and concentrates temporal interaction on target-related instance states. On the nuScenes test set, TempoCross achieves 74.1% mAP and 75.7% NDS, outperforming mainstream baseline detectors. The results support the effectiveness of combining LiDAR-camera fusion with instance-aware sparse temporal modeling.