Author:

Keywords:

SISTA

Abstract:

The number of mobile devices and the amount of traffic generated by them has grown at a tremendous pace in the last years and it is expected to continue growing. This growth contrasts with the limited bandwidth that needs to be shared among users. Network densification has been proposed as a promising technique to satisfy the previous demands over a shared bandwidth. This is realized by increasing the density of base stations deployed. Although network densification can improve the signal-to-interference-plus-noise ratio (SINR) of the users located close to the serving base station, it can also increase the inter-cell interference received by other users. In current cellular networks, base stations deal with inter-cell interference by splitting the bandwidth in two parts. The first one is assigned to users with low interference (typically in the cell center) and it is reused in each cell. The second one is assigned to users with high interference (typically in the cell edge) and it is orthogonalized between users connected to different base stations. This cooperative allocation of bandwidth resources requires sharing channel state information (CSI) through a backhaul link. However, when orthogonalizing resources, the amount of resources that each user receives decreases with the number of users. Furthermore, the operating costs of maintaining a backhaul link for CSI sharing and the amount of CSI that needs to be exchanged between base stations and users makes such interference management techniques infeasible to be implemented in dense networks. These constraints urgently call for cooperative techniques that can reuse resources, while keeping the exchange of CSI between base stations to a minimum, which is the goal of the strategies presented in this thesis. The common denominator of the strategies presented in this thesis is the overhearing capabilities of the network. In dense networks, the proximity of base stations and users results in finding strong links between nodes that have no intentional communication. This could be between users and interfering base stations (first case) or between different base stations (second case). As for the first case, base stations can use CSI transmitted by users from other base stations to allocate resources efficiently. To this end, we propose a neighbor-friendly power control strategy that allows neighboring base stations to reuse resources while minimizing inter-cell interference. The proposed power control strategy avoids any CSI exchange between base stations and can be tuned to increase the data rate of users served by the base station performing the power control strategy, while protecting the data rate of users from a neighboring cell. In high interference conditions, the proposed approach can achieve a data rate increase of the cell edge users by a factor of 3.5 compared to IWF, 15 compared to soft frequency reuse (SFR) and 60 compared to equal power allocation (EPA). Also, users in dense networks constantly receive transmitted data from other base stations intended to users in their proximity. To this end, we develop a transmission strategy based on aligned frequency reuse (AFR) that allows cell edge users to relay overheard signals to the intended users through D2D communication during the time-slots in which users are not receiving their intended signal. We show that our approach increases the diversity and spectral efficiency of all the users in the network without decreasing the achievable degrees of freedom (DoF). Furthermore, we develop it for two cellular array configurations and we extend it to the case with multiple cell edge users. As for the second case, base stations in dense networks can act as relays by transmitting overheard data to users from other base stations, introducing spatial diversity. For this purpose, we propose a multiple-relay communication protocol (MRCP) for achieving fairness in dense networks. MRCP is applicable to an arbitrary number of base stations and users, while keeping a small transmission time compared to traditional space-time network coding (STNC) techniques. We show that our approach achieves the highest max-min fairness among users and almost full diversity when compared to other communication schemes. MRCP requires all the base stations and users in the network (or in a cluster of base stations) to overhear each other, while typically base stations can only overhear the closest neighboring base stations. For this purpose, we exploit a basic knowledge of the network topology using the Wyner cellular model. This is done by allowing simultaneous transmissions of those BSs that do not overhear each other during the transmission phase and the relaying phase. We then reduce the number of time-slots by allowing simultaneous transmissions of all the BSs during the relaying phase. Our result shows that our scheme is able to improve the spectral efficiency and bit error rate with unequal transmit power and unequal average channel gains. In order to implement the previous approaches, base stations must devote some of their resources to relay the overheard signals. Evidently, this has a cost on the consumed power and on the amount of resources that could be otherwise used for their own users. For this purpose, we analyse two approaches that exploit the overhearing capabilities of the system in terms of spectral efficiency, energy efficiency, and success rate and compare them with non-overhearing approaches. We show that when at least one indirect link is stronger than the direct links, exploiting the overhearing capabilities of the transmitting nodes provides the highest performance. Finally, we propose a sub-optimal power control strategy for the two overhearing approaches that with a simple comparison and closed-form formulas can achieve an energy efficiency close to the optimal.