Novel Synchronization Scheme for Cooperative ISAC Systems

TL;DR

This paper introduces a novel synchronization scheme for cooperative ISAC systems in cell-free mMIMO, addressing pilot contamination effects and optimizing synchronization performance and overhead through clustering and pilot sharing.

Contribution

It develops a new synchronization method combining maximum likelihood estimation and pilot sharing, with optimized cluster classification and pilot allocation strategies.

Findings

The proposed scheme outperforms benchmark schemes in synchronization accuracy.

Simulation results confirm the effectiveness of the joint optimization approach.

The method effectively mitigates pilot contamination impacts.

Abstract

Carrier frequency and timing synchronization play the fundamental roles in cooperative integrating communication and sensing (ISAC). To mitigate the effects of synchronization error, this paper develops a novel synchronization scheme in cell-free massive multiple-input multiple-output (mMIMO) systems. First, we characterize the impacts of pilot contamination on synchronization performance, i.e., Cramer-Rao bound (CRB). Furthermore, a maximum likelihood algorithm is presented to estimate the CFO and TO among the pairing APs. Then, to minimize the sum of CRBs, we devise a synchronization strategy based on a pilot-sharing scheme by jointly optimizing the cluster classification, synchronization overhead, and pilot-sharing scheme, while simultaneously considering the overhead and each AP's synchronization requirements. To solve this NP-hard problem, we simplify it into two sub-problems, namely cluster classification problem and the pilot sharing problem. To strike a balance between synchronization performance and overhead, we first classify the clusters by using the K-means algorithm, and propose a criteria to find a good set of master APs. Then, the pilot-sharing scheme is obtained by using the swap-matching operations. Simulation results validate the accuracy of our derivations and demonstrate the effectiveness of the proposed scheme over the benchmark schemes.