Hierarchical Scheduling for Suppression of Fronthaul Delay in C-RAN with Dynamic Functional Split

The cloud radio access network (C-RAN) can potentially reduce a network's deployment cost and energy consumption. However, a connection between a centralized baseband unit and distributed remote radio heads, known as a fronthaul, introduces an additional delay to both control and user planes. This delay is a serious limiting factor for radio resource management functionalities, such as scheduling, because the radio resources are assigned to users according to outdated channel quality information if the fronthaul delay is non-zero. This article provides an overview of existing scheduling approaches suitable for C-RAN and identifies their potential limitations. Based on these limitations, we outline a framework for hierarchical scheduling. The hierarchical scheduling mitigates a negative impact of the fronthaul delay on the throughput of non-cell-edge users and enables efficient retransmission of erroneous data. Besides, cell-edge users can still benefit from interference mitigation techniques requiring centralized control. We compare individual scheduling approaches and show that hierarchical scheduling increases the network throughput (by up to 26 percent) and reduces the number of retransmissions with respect to the existing solutions.