Ultra-Densification for Future Cellular Networks

The traffic volume in wireless communication has grown dramatically in the last decade. Recent predictions indicate such data storm will be even more violent in the short run. Potential solutions for accommodating the rapid traffic growth can be summed up into three categories: broadening the available bandwidth, improving the spectral efficiency, and densifying the infrastructure. In this thesis, we focus on the densification dimension which has been proven to be the most effective one in the past. The current gain of network densification mainly comes from cell splitting, thereby serving more user equipment (UE) simultaneously. This trend will decelerate as the base station (BS) density gets closer to or even surpasses the UE density which forms an ultra-dense network (UDN). Thus, it is crucial to understand the behavior and design operations of ultra-densification in future networks. An important question for future system design and operating strategy is which element is more effective than others. To this end, we start from comparing the effectiveness of densification with spectrum expansion and multi-antenna systems in terms of meeting certain traffic demand. Our findings show that deploying more BSs provides a substantial gain in sparse network but the gain decreases progressively in a UDN. Meanwhile, even with the same area throughput, different combinations of individual throughput and UE density lead to different requirements for resources. The diminishing gain appearing in UDNs makes us curious to know if there exists a terminal on the way of densification. Such uncertainty leads to the study on the asymptotic behavior of densification. By incorporating a sophisticated bounded dual-slope path loss model and practical UE densities in our analysis, we present the asymptotic behavior of ultra-densification: the coverage probability and area spectral efficiency (ASE) have non-zero convergences in asymptotic regions unless the UE density goes to infinity (full load). Our results suggest that network densification cannot always improve the UE performance or boost the network throughput. Next, we shift our focus to the operations of UDNs. We first study BS cooperations in two UDN scenarios: homogeneous and heterogeneous UDNs which are distinguished by BS types. In both cases, the cooperation rules become more complicated than those in traditional networks. Either channel state information (CSI) or extra delay information needs to be acquired in order to obtain cooperation gains. At last, we investigate the feasibility of applying random beamforming to initial access in millimeter-wave (mmWave) UDNs. To our surprise, the simple method can provide sufficient performance in both control and data plane, comparing with the existing schemes. Therefore, it may be unnecessary to develop complex algorithms for initial access in future dense mmWave networks. The findings indicate that UDN may complicate network operations while it may also facilitate the use of simple schemes. Our work provides insights into the understanding of the network densification and thus paves the way for the operational design of future UDNs.