  We investigate the user-to-cell association (or user-clustering) and
beamforming design for Cloud Radio Access Networks (CRANs) and Fog Radio Access
Networks (FogRANs) for 5G. CRAN enables cloud centralized resource and power
allocation optimization over all the small cells served by multiple Access
Points (APs). However, the fronthaul links connecting each AP to the cloud
introduce delays and cause outdated Channel State Information (CSI). By
contrast, FogRAN enables lower latencies and better CSI qualities, at the cost
of local optimization. To alleviate these issues, we propose a hybrid algorithm
exploiting both the centralized feature of the cloud for globally-optimized
pre-scheduling using outdated CSIs and the distributed nature of FogRAN for
accurate beamforming with high quality CSIs. The centralized phase enables to
consider the interference patterns over the global network, while the
distributed phase allows for latency reduction. Simulation results show that
our hybrid algorithm for FogRAN outperforms the centralized algorithm under
imperfect CSI, both in terms of throughput and delays.
