Joint caching and base station activation for green heterogeneous cellular networks

Heterogeneous cellular networks that overlay cache-endowed small-cell networks with macro-cell networks have emerged as a promising solution towards ultra-low latency, extra-high throughput, and sub-multiple energy consumption compared to the conventional cellular paradigm. A technique to further improve the energy efficiency of such multi-tier networks is to apply activation mechanisms, that dynamically power on/off a subset of the small-cells and macro-cells. In this work, we show that the activation policy should be jointly derived with the caching policy, that places popular content files at the base station caches. As a result content is fetched effectively closer to the mobile end-users and can be transported via energy-prudent links, while at the same time as many as possible of the base stations are powered off. We then formulate the energy-minimizing problem, which is NP-hard, and introduce a novel approximation framework for its efficient solution. Numerical results, that are based on system parameters driven from real trace datasets, show that our approach provides an excellent performance that is far better than the schemes that perform caching and base station activation in a disjoint manner.