Energy Efficient Caching in Backhaul-aware Ultra-dense Cellular Networks

Caching popular contents at base stations (BSs) has been regarded as an effective approach to alleviate the backhaul load and to improve the quality of service. To meet the explosive data traffic demand and to save energy consumption, energy efficiency (EE) has become an extremely important performance index for the 5th generation (5G) cellular networks. In general, there are two ways for improving the EE, i.e., improving the cache hit rate and optimizing the cache size. In this work, we investigate the energy efficient caching problem in backhaul-aware cellular networks jointly considering these two approaches. Note that the content popularity distribution changes with time slowly. To cache the most popular content, we consider a static content catalog, ranking from the most popular to the least popular based on the Zipf popularity distribution. Then we propose a distributed caching policy to improve the cache hit rate. Furthermore, we analyze the tradeoff between energy efficiency and cache capacity for which an optimization is formulated. We prove its convexity and derive a closed-form optimal cache capacity for maximizing the EE. Simulation results validate the proposed scheme and show that EE can be improved with appropriate choice of cache capacity.