Caching at base stations (BSs) is a promising way to offload traffic andeliminate backhaul bottleneck in heterogeneous networks (HetNets). In thispaper, we investigate the optimal content placement maximizing the successfuloffloading probability in a cache-enabled HetNet where a tier of multi-antennamacro BSs (MBSs) is overlaid with a tier of helpers with caches. Based onprobabilistic caching framework, we resort to stochastic geometry theory toderive the closed-form successful offloading probability and formulate thecaching probability optimization problem, which is not concave in general. Intwo extreme cases with high and low user-to-helper density ratios, we obtainthe optimal caching probability and analyze the impacts of BS density andtransmit power of the two tiers and the signal-to-interference-plus-noise ratio(SINR) threshold. In general case, we obtain the optimal caching probabilitythat maximizes the lower bound of successful offloading probability and analyzethe impact of user density. Simulation and numerical results show that when theratios of MBS-to-helper density, MBS-to-helper transmit power anduser-to-helper density, and the SINR threshold are large, the optimal cachingpolicy tends to cache the most popular files everywhere.
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