Joint AP-user association and caching strategy for delivery delay minimization in cell-free massive MIMO systems

Edge caching at access points (APs) is a promising approach to alleviate the fronthaul burden and reduce user-perceived delay. However, the edge caching placement is still challenging considering the coupling between caching and AP-user association, limited fronthaul capacity, and multi-AP deployment in the cell-free (CF) massive MIMO systems. To this end, the authors establish a framework for the joint problem of AP-user association and caching to minimize the content delivery delay which considers both cooperation delivery delay and radio access delay. It is an integer nonlinear programming problem and NP-hard. The optimization problem is first decomposed into an AP-user association sub-problem and a caching placement sub-problem to address this problem. A two-stage matching algorithm is further proposed to achieve AP-user association and a modified genetic algorithm to determine caching placement. A computationally efficient iterative algorithm is developed to solve the joint optimization problem. Finally, the global convergence and computational complexity of the proposed strategy are analyzed theoretically. Simulation results reveal that the proposed strategy can achieve better delivery delay performance than benchmark schemes.