The present disclosure relates to the optimization of user energy efficiency in Cell-Free Massive MIMO systems of the 6G mobile network. This is achieved by tracking the user's energy efficiency as a multi-variate time series and then utilizing an anomaly detection algorithm to detect if a user's energy efficiency drops to problematic levels. As a result of detecting an anomaly in the user's energy efficiency the algorithm can decide whether to support the user at a different frequency or access point, or that no remedial action is needed.