Dynamic AP Selection and Cluster Formation with Minimal Switching for Green Cell-Free Massive MIMO Networks

Aiming at the implementation of energy-efficient cell-free networks, several approaches have been proposed in the literature, which consider different access point (AP) switch ON/OFF (ASO) strategies for power minimization. Different from prior works, this paper focuses on additional factors that have an adverse effect not only on total power consumption but also on implementation complexity and operation cost. For instance, too frequent ON/OFF switching in an AP can lead to tapering off the potential power saving of ASO by incurring extra power consumption due to excessive switching. Indeed, frequent switching of APs might also result in thermal fatigue and severe lifetime degeneration. Moreover, time variations in the AP-UE (user equipment) clusters in favor of energy saving in a dynamic network bring additional signaling and implementation complexity. Thus, we propose a multi-objective optimization problem that aims to minimize the total power consumption together with AP switching and AP-UE clustering variations in comparison to the previous state of the network. The proposed problem is cast in mixed integer quadratic programming form and solved optimally. Our simulation results show that by limiting AP switching (node switching) and AP-UE cluster reformation switching (link switching), the total power consumption at the radio site only slightly increases, but the number of average switching drops significantly regardless of node or link switching. It achieves a good balance on the trade-off between radio power consumption and the side effects excessive switching will bring.