This paper studies the performance of cache-enabled dense small cell networksconsisting of multi-antenna sub-6 GHz and millimeter-wave base stations.Different from the existing works which only consider a single antenna at eachbase station, the optimal content placement is unknown when the base stationshave multiple antennas. We first derive the successful content deliveryprobability by accounting for the key channel features at sub-6 GHz and mmWavefrequencies. The maximization of the successful content delivery probability isa challenging problem. To tackle it, we first propose a constrainedcross-entropy algorithm which achieves the near-optimal solution with moderatecomplexity. We then develop another simple yet effective heuristicprobabilistic content placement scheme, termed two-stair algorithm, whichstrikes a balance between caching the most popular contents and achievingcontent diversity. Numerical results demonstrate the superior performance ofthe constrained cross-entropy method and that the two-stair algorithm yieldssignificantly better performance than only caching the most popular contents.The comparisons between the sub-6 GHz and mmWave systems reveal an interestingtradeoff between caching capacity and density for the mmWave system to achievesimilar performance as the sub-6 GHz system.
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