Hybrid Self-Backhaul and Cache Assisted Millimeter Wave Two-Tier Heterogeneous Networks With MIMO Equipped Backhaul Access Points

This paper focuses on effective content delivering over the co-channel and full millimeter-wave (mm-Wave) heterogeneous networks (HetNets) by using hybrid self-backhaul and cache-assisted (HSBC-assisted) content delivering. Especially, the small base stations (SBSs) are equipped with a single antenna, a portion of which has the capacity-limited cache and the remaining of which has not to cache. The macro base stations (MBSs) are equipped with multiple antennas or even massive MIMO, which provide the wireless self-backhaul for the SBSs without cache. Instead of the oversimplified flat-top mm-Wave antenna pattern, a more actual and accurate one for the uniform linear array at MBSs is used. By modeling the locations of line-of-sight (LoS) and non-LoS base stations as independent Poisson point processes, this paper first presents the downlink content delivering model as well as the distributions of interferences. Then, to explore the impact of the specific MIMO self-backhaul on content delivering, a comprehensive investigation is conducted in terms of the coverage probability, achievable average area rate (AAR), average delay (AD), and energy effective (EE). The analytical and numerical results show that the utilization of multi-antenna MBSs not only improves the AAR and AD of self-backhaul transmission from MBS to SBS but also does the ones of cache and non-cache access links due to narrow beam and direction transmission decreasing interference. The results also show that in the proposed HSBC-assisted HetNets, under the scenario where the low-resolution antenna array is used and the cache capacity is large, the effect of hardware power consumption on the aggregate EE is negligible. This result shows a trade-off between AD and EE. With a projected EE, the joint utilization of cache and low-resolution antenna array allows more antennas equipped at MBSs so that the near-field propagation loss is compensated by the higher gain of directional antennas. Meanwhile, the results show that the achieved performance gain by the HSBC-assisted system over the traditional self-backhaul (TSB)-assisted greatly depends on the system parameters. With this observation, an adaptive TSB-HSBC-assisted system model is proposed. When the ratio factor of cache-assisted SBSs is less than the threshold, the sole TSB-assisted model is selected; otherwise, the HSBC-assisted model is used.