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Title: Dense small cell networks for next generation wireless systems
Author: Jafari, Amir Hossein
ISNI:       0000 0004 7428 1538
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2017
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Today's heterogeneous networks comprised of mostly macrocells and indoor small cells will not be able to meet the upcoming traffic demands. Indeed, it is forecasted that in comparison to today's network capacity, at least a 100x network capacity increase will be required to meet the traffic demands in 2020. As a result, researchers are now looking at using every tool at hand to improve network capacity. In this epic campaign, three main directions are noteworthy, i.e., enhance spatial reuse through network densification, use of larger bandwidths by exploiting higher spectrum frequencies both in licensed and unlicensed spectrum and enhance spectral efficiency through multi-antenna transmissions and cooperative communications. In this thesis, special attention is paid to network densification and its implications when transiting to ultra-dense small cell networks. First a simulation based analysis of ultra-dense small cell networks is presented. In this analysis, it is aimed to investigate how each of the three discussed capacity enhancement paradigms contributes to achieve the required 1 Gbps data rate. It is shown that network densification with an average inter-site distance (ISD) of 35 m can increase the average user equipment (UE) throughput by 7.56x, while the use of the 10 GHz band with a 500 MHz bandwidth can further increase the network capacity up to 5x, resulting in an average of 1.27 Gbps per UE. It is further shown that the use of beamforming with up to 4 antennas per small cell base station (BS) lacks behind with average throughput gain of up to 1.45x. It is also examined how idle mode feature (the capability to switch off small cell BSs with no active UEs) plays a key role in the performance of ultra-dense small cell networks. Next, the impact of network densification on multi-user diversity is explored and it is demonstrated that as a result of densification the proportional fair alike schedulers can lose their advantages over round robin schedulers. It is shown that at low ISDs, the proportional fair scheduling gain over the round robin one in terms of cell throughput is only around 10% and, therefore, it is suggested that due to significantly lower complexity of round robin schedulers, they could be an alternative to proportional fair ones in ultra-dense small cell networks. Furthermore, the energy efficiency of ultra-dense small cell networks is analysed and it is elaborated that in order to make the deployment of ultra-dense small cell networks more energy-efficient, it is important to develop advanced idle mode algorithms than cause the small cell BSs to consume zero power in idle mode. It is further explained that future small cell networks should benefit from energy harvesting technologies to reap their required energy from environmental resources. Next, it is expressed that in order to cost-effectively deploy dense small cells networks, it is necessary to acquire an in-depth theoretical understanding of the implications brought by dense small cell networks. Due to proximity of UEs to small cell BSs in ultra-dense small cell networks, there is a high probability of line-of-sight (LOS) communication which causes a transition from non-lineof- sight (NLOS) to line-of-sight (LOS) for many communication links. It is explained that the simplistic single slope path loss models that only consider a single path loss exponent and do not differentiate among LOS and NLOS transmissions are not applicable in ultra-dense small cell networks and, therefore, a distance based piecewise path loss model featuring piecewise path loss functions that consider probabilistic LOS and NLOS transmissions is proposed. In addition to the proposed path loss model, distance-dependent Rician fading with a variant Rician K factor is also considered to assess the performance of ultra-dense small cell networks. The analysis demonstrates that the network coverage probability first increases with the increase in small cell BS density, but as network becomes denser the coverage probability decreases and the impact of multi-path fading is almost negligible. This implies that in contrary to previous conclusion, in dense small cell networks, the small cell BS density does matter. Finally, it is proved that spatial multiplexing (SM) gains in multiple input multiple output (MIMO) cellular networks are limited when used in combination with ultra-dense small cell networks. In ultra-sense small cell networks, due to dominant LOS communication between UE and BS and insufficient spacing between antenna elements at both UE and BS, spatial channel correlation is considerably enhanced and, therefore, spatial multiplexing gain is suffered. To overcome this challenge, a new transmission technique entitled diversity pulse shaped transmission (DPST) is proposed. In DPST, transmit signals at adjacent antenna elements are shaped with distinct interpolating filters where each antenna transmits its own data stream with a relative time offset with respect to its adjacent antenna. The time offset which must be a fraction of symbol period generates deterministic inter-symbol-interference (ISI) into the channel, allowing to increase the diversity among different channel pairs. At the receiver, a fractionally spaced equalizer (FSE) is exploited and it is shown that the combined effects of DPST and FSE enable the receiver to sense a less correlated channel. Simulation results demonstrate that in 2x2 and 4x4 MIMO systems, DPST can enhance the UE throughput by almost 2x and 4x, respectively.
Supervisor: Zhang, Jie Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available