Low Complexity Precoding for OTFS-aided Cell-Free mMIMO ISAC System

This paper incorporates a low-complexity precoding scheme, termed partial zero-forcing (PZF), for an orthogonal frequency time space (OTFS) modulated cell-free massive multiple input multiple output (mMIMO) integrated sensing and communication (ISAC) system. This scheme assigns ZF precoding and maximal ratio transmission (MRT) to communication users based on their channel gain. This approach addresses the computational complexity of global ZF operations and the inability of MRT precoding to mitigate inter-user interference. In addition, we introduce the formulation of the mean-to-average-sidelobe-ratio (MASR) metric for sensing within this system. We derive novel closed-form spectral efficiency (SE) expressions and compute the MASR metric to evaluate sensing performance. The results demonstrate that PZF outperforms MRT and full-pilot ZF (FZF) in terms of SE and computational complexity, providing a scalable solution for high mobility beyond 5G applications. We also evaluate how downlink power allocation at the AP influences communication and sensing performance, using closed-form SE per user and MASR as metrics.