User Management in Cell-Free Massive MIMO Systems with Limited Fronthaul Capacity

In cell-free (CF) massive multiple-input multiple-output (mMIMO) architecture, large number of access points (APs) collaboratively serve much smaller number of users. Conventionally, it is assumed that each AP serves all users. In this work, however, to lessen the computational and hardware costs, instead of serving all users, each AP serves limited number of them. We propose an algorithm for assigning users to each AP. Besides, we assume each AP is connected to a central processing unit (CPU) via limited capacity fronthaul link. To efficiently address the fronthaul capacity constraints, three strategies for sending signals from APs to the CPU are employed; namely: compress-forward-estimate (CFE), estimate-compress-forward (ECF), and estimate-multiply compress-forward (EMCF). We drive the capacity of the aforementioned systems with user management constraint. It is shown that the proposed user management improves the performance of the ECF and EMCF from both sum spectral efficiency (SE) and per user SE aspects.