Secure Content Delivery in Two-Tier Cache-Enabled mmWave Heterogeneous Networks

In this paper, we investigate secure content delivery in a two-tier cache-enabled millimeter wave (mmWave) heterogeneous network composed of a macro base station (MBS) and K small base stations (SBSs) with caching capabilities. We allocate finite cache units at the SBSs and MBS to pre-store files with high popularities, where the SBSs store the most popular files, and the MBS stores the less popular ones. To deliver the file requested by a legitimate user securely, two secure transmission schemes, namely, distributed beamforming and direct transmission, are employed at the SBSs and MBS, respectively. Moreover, artificial noise (AN) is combined with the above two transmission schemes to further improve transmission security. The connection outage probability, secrecy outage probability, and secrecy throughput for the proposed mmWave transmission schemes are obtained. Based on these results, we jointly design the transmission rates and the cache resource allocation between the SBSs and MBS to maximize the overall secrecy throughput. We also provide insights into how the overall secrecy throughput is influenced by various parameters, including transmission rates, power allocation ratio of the AN scheme, and cache allocation factor. Numerical results are eventually presented to validate our theoretical analysis and demonstrate the effectiveness of the proposed transmission schemes and cache resource allocation strategy.