A Target Detection Analysis in Cell-Free Massive MIMO Joint Communication and Radar Systems

This paper considers the cell-free (CF) massive MIMO architecture from a joint communication and radar point of view. We propose a protocol for communication and sensing, where the network allocates a set of access-points (APs) to participate in the uplink together with the users (UEs) to serve the network with a radar signal. This realizes the radar system as a distributed bistatic radar system, where the objective is to recover the radar echoes from the multiuser interference. The imposed cost on the communication system is the loss of one AP and the need to schedule one additional virtual UE per allocated AP. We present two modes of radar sensing, occurring in either the uplink training segment or the data payload segment of the communication frame. A subspace signal model and its corresponding generalized likelihood ratio test is developed in order to evaluate the detection performance. We present expressions for the probably of detection and false alarm, and demonstrate the system numerically. Our main message is that coordinating the network to transmit in the uplink together with the UEs serves as an interesting approach in enabling radar sensing in CF massive MIMO systems.