BRIDGE: Block-Wise Speculative Coordination for Cloud-Edge Retrieval-Augmented Generation

Retrieval-augmented generation (RAG) improves factuality by conditioning LLMs on retrieved evidence, yet real-world knowledge is often split across tiers: cloud-based RAG can exploit large public corpora, whereas edge-based RAG is the natural place to access private, user-specific stores. This raises a key question: how can one jointly leverage cloud and edge knowledge without transferring sensitive edge data or incurring prohibitive key-value (KV) recomputation. A direct output-level aggregation requires per-token synchronization, causing severe stalls under heterogeneous decoding speeds, and token-wise speculation still suffers from frequent communication and rollback overhead, limiting efficiency and usability. To address these challenges, we present BRIDGE, a Block-wise speculative RAG framework for Inter-database Distributed GEneration. BRIDGE enables parallel cloud--edge retrieval and drafting, without exposing private data or recomputing KV states. It introduces block-wise transmission and verification to amortize communication cost and improve acceptance efficiency, thereby reducing rollback waste. BRIDGE further employs an adaptive block-length strategy to balance the benefits of larger blocks against their verification overhead. Across public-private RAG benchmarks, model families, and network regimes, BRIDGE significantly improves question-answer relevance and personalization, while reducing end-to-end latency by up to 79.1%.