Distributed Content Replacement in Small Cell Networks using Continuous-Time Markov Chain

Content caching is a promising way to overcome backhaul limitations in small cell networks. However, in such type of networks, small base stations (SBSs) are always deployed with limited cache storages. Thus, it is necessary for SBSs to adjust their contents for better caching efficiency, so as to reduce backhaul traffic. In this paper, we study the content replacement problem to minimize the traffic flowing into the costly backhaul links. However, in small cell networks where SBSs make up backhaul mesh networks, the effectiveness of reducing backhaul traffic depends on the hop distance from the content location to the requesting user. On this basis, we formulate a hop minimization problem that is inherently combinatorial. Through log-sum-exp approximation, we can solve the problem and arrive at a close-form solution with guaranteed performance gap to the optimal solution. By exploiting the properties of continuous-time Markov chain (CTMC), the solution can be implemented by designing a CTMC that can instruct the content replacement process. As a consequence, a concise, efficient, and flexible content replacement strategy is proposed. Simulation results verify our analysis and show that our proposed strategy outperforms the conventional strategies.