  Explosive data traffic growth leads to a continuous surge in capacity demands
across mobile networks. In order to provision high network capacity, small cell
base stations (SCBSs) are widely deployed. Owing to the close proximity to
mobile users, SCBSs can effectively enhance the network capacity and offloading
traffic load from macro BSs (MBSs). However, the cost-effective backhaul may
not be readily available for SCBSs, thus leading to backhaul constraints in
small cell networks (SCNs). Enabling cache in BSs may mitigate the backhaul
constraints in SCNs. Moreover, the dense deployment of SCBSs may incur
excessive energy consumption. To alleviate brown power consumption, renewable
energy will be explored to power BSs. In such a network, it is challenging to
dynamically balance traffic load among BSs to optimize the network utilities.
In this paper, we investigate the traffic load balancing in
backhaul-constrained cache-enabled small cell networks powered by hybrid energy
sources. We have proposed a network utility aware (NUA) traffic load balancing
scheme that optimizes user association to strike a tradeoff between the green
power utilization and the traffic delivery latency. On balancing the traffic
load, the proposed NUA traffic load balancing scheme considers the green power
utilization, the traffic delivery latency in both BSs and their backhaul, and
the cache hit ratio. The NUA traffic load balancing scheme allows dynamically
adjusting the tradeoff between the green power utilization and the traffic
delivery latency. We have proved the convergence and the optimality of the
proposed NUA traffic load balancing scheme. Through extensive simulations, we
have compared performance of the NUA traffic load balancing scheme with other
schemes and showed its advantages in backhaul-constrained cache-enabled small
cell networks with hybrid power supplies.
