In this paper, we investigate the optimal caching policy respectivelymaximizing the success probability and area spectral efficiency (ASE) in acache-enabled heterogeneous network (HetNet) where a tier of multi-antennamacro base stations (MBSs) is overlaid with a tier of helpers with caches.Under the probabilistic caching framework, we resort to stochastic geometrytheory to derive the success probability and ASE. After finding the optimalcaching policies, we analyze the impact of critical system parameters andcompare the ASE with traditional HetNet where the MBS tier is overlaid by atier of pico BSs (PBSs) with limited-capacity backhaul. Analytical andnumerical results show that the optimal caching probability is less skewedamong helpers to maximize the success probability when the ratios ofMBS-to-helper density, MBS-to-helper transmit power, user-to-helper density, orthe rate requirement are small, but is more skewed to maximize the ASE ingeneral. Compared with traditional HetNet, the helper density is much lowerthan the PBS density to achieve the same target ASE. The helper density can bereduced by increasing cache size. With given total cache size within an area,there exists an optimal helper node density that maximizes the ASE.
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