Federated Learning Over Device-Centric Cell-Free Networks: A Long-Term Perspective

Federated learning (FL) is a promising distributed machine learning approach with enhanced data privacy protection. However, wireless communication remains a key bottleneck, directly affecting the efficiency and performance of FL. In this paper, we introduce a device-centric cell-free network to mitigate the negative effects of random fading and limited radio resources on FL. The convergence gap, representing the difference between the FL model’s performance and that of the optimal model, is analyzed to evaluate the impact of communication and computation factors, including inter-device interference, on FL performance. Then, access point (AP)-device association, transmission power, and computation frequency are jointly optimized to minimize the convergence gap. Lyapunov techniques are employed to decouple the long-term optimization into a series of online solvable problems. A deep reinforcement learning-based scheme is proposed to optimize the AP association and transmission power for devices, reducing the computational complexity from a prohibitive level to a real-time feasible quadratic level. Additionally, a closed-form solution for the optimal device computation frequency is derived. Simulation results show that the proposed scheme significantly outperforms the traditional cell-free FL and cellular FL schemes in both model training performance and energy efficiency.