On the Performance of ISAC in Dynamic TDD Cell-Free Massive MIMO Systems

This paper studies integrated sensing and communication (ISAC) within the framework of dynamic time division duplex (DTDD) cell-free (CF) massive multiple-input multiple-output (mMIMO) systems. DTDD enables the CF-mMIMO system to cater to both uplink (UL) and downlink (DL) users with spatially separated half-duplex (HD) access points (APs) using the same time-frequency resources. Thus, in our work, the same set of UL APs is utilized for both UL data and target echo reception, while the DL APs transmit jointly precoded DL data streams and the target symbol. In UL, we present a generalized likelihood-ratio test (GLRT) at the central processing unit (CPU) for target detection and use signal-to-noise-plus-interference (SINR) optimal combiner for UL data processing. In DL, we propose two types of precoder: one “communication-centric” that nullifies the interference caused by the target signal to the DL users; and one “target-centric” based on the dominant eigenvector of the composite channel between the target and the APs. Finally, we numerically investigate the performance of the GLRT and also sum UL-DL spectral efficiency (SE) of the communication users and benchmark the results with conventional TDD-based systems. We observe the GLRT is robust to inter-AP interference, and DTDD almost doubles the 90%-likely sum UL-DL SE compared to traditional TDD-based CF-mMIMO ISAC systems.