A Synchronization Solution for Bi-static ISAC under NLOS with Rich Multipaths

Integrated sensing and communication (ISAC) is expected to play a prominent role in 6G. Avoiding full duplex transceivers, bi-static sensing is free from self-interference and able to leverage ubiquitous network devices, thus considered an indispensable scenario of ISAC. However, bi-static sensing must resolve the non-ideal synchronization of the transceiver nodes. Such timing offset (TO) remains a challenging issue, especially in the non-line-of-sight (NLOS) condition with rich multipaths. This article makes the best use of all multipaths, deriving the cyclic shift relation between the delay spectrums measured at the two sides of transceivers to estimate the TO. Based on this theoretical analysis, an algorithm called enhanced round-trip measurement (eRTM) is developed to mitigate the TO. Specifically, the cyclic cross-correlation between the amplitudes of the two normalized delay spectrums measured at the two sides of the transceivers respectively is conducted to estimate the TO, which is then used for TO mitigation. Extensive simulations have verified the advantages of the eRTM algorithm, especially in NLOS conditions and with low signal-to-noise ratios (SNRs). In addition, field tests conducted with a bi-static ISAC prototype system and eRTM algorithm, have achieved centimeter-level positioning accuracy even in NLOS conditions with rich multipaths, confirming the validities of our theoretical analysis as well as the proposed algorithm.