Practical ISAC is constrained by static clutter and NLoS multipath, which obscure target-coupled echoes and induce spurious peaks for beam alignment. Existing receiver-side methods largely model targets as passive scatterers, limiting the structural separability of target echoes from the environment. This paper establishes a structural correspondence between these limitations and target-side Backscatter-MIMO responses: reflection modulation enables waveform-domain separation from unmodulated clutter, while retro-directional passive beamforming concentrates the tagged echo toward the BS-facing direction and suppresses NLoS-induced false-peak locking. To operationalize this correspondence, dual-end spatial locking is required to overcome cascaded backscatter loss and provide beam-domain angular information. We propose a downlink-triggered blind dual-end alignment protocol that jointly selects the BS and Backscatter-MIMO codeword indices from the tagged echo observed at the BS, without pilots, CSI feedback, or target synchronization. We further derive a clutter-aware remodulation waveform robust to fractional timing offsets and construct adjustable-width BS/Backscatter-MIMO codebooks via quadratic phase spoiling. For reliability characterization, we derive closed-form expressions for the coherence-averaged end-to-end success probability. The analysis shows that beam narrowing is not universally beneficial: in NLoS-dominated regimes, enlarging the array aperture may degrade alignment reliability. The optimal beamwidth is instead governed by cross-phase competition between discovery and alignment, yielding a nontrivial feasible region with an analytically characterized boundary. Simulations validate the analysis and demonstrate improved reliability-gated locked-link performance under strong clutter, severe NLoS multipath, and finite coherence time.

翻译：实际通信感知一体化系统受静态杂波和非视距多径影响，这些因素会掩盖目标耦合回波并引发波束对准的虚假峰值。现有接收端方法主要将目标建模为无源散射体，限制了目标回波与环境的结构可分离性。本文建立了这些限制与目标端反向散射MIMO响应之间的结构对应关系：反射调制可实现与未调杂波的波形域分离，而逆定向无源波束成形可将标记回波集中至基站方向并抑制非视距引起的虚假峰值锁定。为实现该对应关系的可操作性，需要双端空间锁定以克服级联反向散射损耗并提供波束域角度信息。我们提出一种下行触发盲双端对准协议，无需导频、信道状态信息反馈或目标同步，直接根据基站观测的标记回波联合选择基站和反向散射MIMO码字索引。进一步推导出对分数时偏鲁棒的杂波感知重调制波形，并通过二次相位扰动构造可调波束宽度的基站/反向散射MIMO码本。在可靠性表征方面，推导了相干平均端到端成功概率的闭式表达式。分析表明波束窄化并非普遍有益：在非视距主导场景中，增大阵列孔径可能降低对准可靠性。最优波束宽度由发现与对准之间的交叉相位竞争决定，形成具有解析特征边界的非平凡可行区域。仿真验证了分析结果，并展示了在强杂波、严重非视距多径和有限相干时间条件下，可靠性门控锁定链路性能的提升。