  Cloud Radio Access Network (C-RAN) is considered as one of the most promising
candidate for Fifth Generation (5G) wireless communication system. However,
C-RAN imposes stringent requirements on fronthaul in terms of capacity which in
turn becomes a bottleneck for deployment of cloud-RAN in dense networks.
Compressed CPRI and functional split are two primary solutions to overcome
this. However, this makes Remote Radio Unit (RRU) more complex and the
computational resources in RRU are localized and thus cannot be shared with
other RRUs when not in use. In this paper, we introduce the concept of Variable
Rate Fronthaul (VRF) for C-RAN. This scheme operates on a CPRI type of
interface (e.g., one that transmits I/Q data samples) with the novelty of
dynamically changing the cell bandwidth, and consequently the fronthaul data
rates, depending on the cell load, with the support of a Software Defined
Network (SDN) controller. This allows for a more efficient transport of C-RAN
cells data over a shared backhaul. We first propose a mathematical analysis of
the VRF performance using a queuing theory approach based on the Markov model.
We then provide the results of our simulation framework both for validation and
in support of the mathematical analysis. Our results show that the proposed VRF
scheme provides significantly lower blocking probability over a shared backhaul
than standard CPRI.
