A Study on the Deployment and Cooperative Operation of Ultra-Dense 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 proved to be the most effective in the past. The current gain of network densification mainly comes from cell splitting, thereby serving more user equipments (UEs) simultaneously. This trend will decelerate as base station (BS) density gets closer to or even surpass UE density which forms a ultra-dense network (UDN). Thus, it is crucial to understand the behavior of ultra-densification for future network provisioning. 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 multiantenna 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. Therefore, adding multiantennas on BSs or utilizing additional spectrum are more favorable than further densification in a UDN. The impact of traffic types is also discussed. Even with the same area throughput, different combinations of individual throughput and UE density lead to different requirements for resources. The diminishing gain appears in a UDN make 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 in the next part of the thesis. We incorporate a sophisticated bounded dual-slope path loss model and practical UE densities in our analysis. By using stochastic geometry, we derive the expressions of the coverage probability of a typical UE and network area spectral efficiency (ASE). In addition, we present the asymptotic behavior of ultra-densification: the coverage probability and ASE have nonzero convergence in asymptotic regions unless UE density goes to infinity (full load). Previous UDN studies focus on single BS association rule which results in a large portion of dormant BSs in the network. Thus, whether we can utilize these dormant BSs to improve system performance is an interesting question. To this end, we employ joint transmission (JT) techniques into a UDN. Two types of cooperation schemes are investigated: non-coherent JT and coherent JT depending on the availability of channel state information (CSI). Our results reveal that non-coherent JT is not beneficial in a UDN due to the simultaneous increase in both desired signal and interference. Coherent cooperative transmissions with CSI are able to increase UE spectral efficiency (SE) depending on the environmental characteristics captured by critical distance and path loss exponents.