Dissertation on Interference Management in Mobile Networks
On September 10, 2018, Rodrigo Alberto Justavino Castillo successfully defended his Ph.D. thesis Interference Management in Mobile Networks. The Ph.D. examination committee was formed by Prof. Alexander Schlaefer (Institute of Medical Technology) and the two reviewers Prof. Gerhard Bauch and Prof. Armin Wittneben (ETH Zürich).
This thesis deals with interference management in various types of wireless networks. Most modern mobile communication systems such as LTE allow the reuse of the same frequency bandwidth in all cells. In this way, the user throughput and the cell spectral efficiency are improved compared to previous systems such as UMTS and GSM. However, a consequence of this aggressive reuse is that users located in places where the coverage of several cells overlaps can experience inter-cell interference. This interference lowers the data rates of such cell-edge users and thus, the cell throughput. The number of interfered users increases in heterogeneous deployments, as small cells are deployed inside the coverage area of large cells; therefore, the overlapping coverage area of the cells increases. Moreover, because of the power differences between large and small cells, the interference levels can be higher compared to the homogeneous network case.
In this thesis, first, a summary of the current mechanisms available in LTE for interference management is given. Most of these mechanisms are based on the use of orthogonal resources and limited cooperation between the LTE base stations. For future releases of the LTE standard, a trend is observed towards increasing cooperation between base stations and network assisted interference cancellation.
After analyzing interference scenarios in a heterogeneous LTE network, two interference management schemes are proposed. The first one is based on interference avoidance and compatible with LTE release 10 and the second one is based on interference cancellation and it is proposed for future releases of the LTE standard.
The proposed interference avoidance scheme is an interference avoidance scheduler. This scheduler is compatible with the LTE standard and it is compared against a common fractional frequency reuse scheme combined with a proportional fair metric. Additionally, an opportunistic scheduler is also used as a reference. The results show that the proposed scheduler improves the data rate of cell-edge users at the expense of a small reduction of the data rate for the rest of the users compared to the opportunistic scheduler. In comparison, the fractional frequency reuse with proportional fair scheme offers a similar improvement for cell-edge users at the expense of a much larger reduction of the data rate for non-interfered users.
Before presenting the proposed interference cancellation scheme, an investigation into the use of Markov chain Monte Carlo techniques for MIMO joint detection in LTE is introduced. The motivation for this investigation is the reduction of complexity in joint detectors for interference cancellation. The results indicate that, compared to maximum likelihood detection, a significant complexity reduction can be observed for the Markov chain Monte Carlo based joint detector for the simultaneous detection of six or more parallel data streams.
After investigating Markov chain Monte Carlo techniques for MIMO joint detection with low complexity, a partial interference cancellation scheme based on the rate splitting scheme of Han and Kobayashi is presented. In this work, the rate splitting scheme presented by Han and Kobayashi is adapted to the case of an LTE system with MIMO transmission. The simulations show that the proposed scheme can improve the data rate of the interfered user in cases where a main interferer exists.
Finally, after analyzing interference management in terrestrial scenarios, the investigation moves towards scenarios which consider the special case of a mobile user flying on an airplane above a GSM or LTE network. These scenarios have become more common since airlines started to offer onboard users with mobile services. The objective of this study was to evaluate the possibility of a connection between an existing terrestrial network and an airborne mobile user. This study is important to evaluate the necessity on the airplane of a network control unit. The purpose of this device is to block the communication of onboard users with ground networks. It was investigated if such a connection is even possible without this device. The simulation results show that, under specific circumstances, the connection between an airborne mobile user and a GSM or LTE ground network is possible. In both cases, the limiting factor is inter-cell interference.