  This paper studies the device activity detection problem in a multi-cell
massive multiple-input multiple-output (MIMO) system, in which the active
devices transmit signature sequences to multiple base-stations (BSs) that are
connected to a central unit (CU), and the BSs cooperate across multiple cells
to detect the active devices based on the sample covariance matrices at the
BSs. This paper demonstrates the importance of exploiting the knowledge of
channel large-scale fadings in this cooperative detection setting through a
phase transition analysis, which characterizes the length of signature
sequences needed for successful device activity detection in the massive MIMO
regime. It is shown that when the large-scale fadings are known, the phase
transition for the multi-cell scenario is approximately the same as that in a
single-cell system. In this case, the length of the signature sequences
required for reliable activity detection in the multi-cell system can be made
to be independent of the number of cells through cooperation, in contrast to
the case where the large-scale fadings are not known. Further, this paper
considers the case in which the fronthaul links between the BSs and the CU have
capacity constraints and proposes a novel cooperation scheme based on the
quantization of preliminary detection results at the BSs and the reconstruction
of the sample covariance matrices at the CU. Simulations show that the proposed
method significantly outperforms the scheme of directly quantizing the sample
covariance matrices.
