The appetite for wireless high-data rate services is expected to continue for many years to come and drive the need for more capacity. Ultra-dense networks (UDNs) represent a paradigm shift where each base station (BS) serves only a few user equipments (UEs). By most accounts, most of the traffic will be generated indoor and operate in time-division duplex (TDD). This thesis considers dynamic TDD which has shown to perform well indoor for fluctuating traffic where the shorter communication range enables similar transmit powers to be used in uplink and downlink, but also generates potentially more harmful same-entity interference. Because of the sheer number of cells in UDN, the interference management needs to be both effective and scalable.

In the first part of the thesis, we compare static TDD with non-cooperative dynamic TDD and show that flexible time resource allocation is preferred for indoor UDNs. However, since it only provides a lower bound on performance, additional interference coordination is required. Unfortunately, existing schemes often consider either too few, too many, or simply the wrong interferers. We introduce a scheduling model that relates BS-to-BS interferences measured offline to individual BS activation probability taking into account traffic and propagation environment. Results show that the proposed scheme performs well when interference is high, and optimally when interference is low.

In the second part, we introduce cooperation to utilize the otherwise idle BSs and mitigate same- and other entity interference. Zero forcing (ZF) is employed in the downlink where not only downlink UEs but also uplink BSs are included in the precoding. Since downlink BSs do not know the information to be sent by uplink UEs beforehand, dummy symbols with zero power are transmitted. It shown that both uplink and downlink performance improves at low and medium load. Furthermore, it is possible to trade performance in the two directions at high load.

Meeting the seemingly never-ending increase in traffic over wireless networks presents a major challenge for future mobile network design. Given that much of the traffic is expected to be more time-varying and unpredictable, time division duplexing (TDD) is gaining increasing favorability in part thanks to its ability to better accommodate network-wide traffic variations. In order to account for traffic variations in individual cells on much shorter time scales, a more flexible variant called dynamic TDD has resurfaced as a promising technique to further improve resource utilization and performance. In dynamic TDD the traffic in each cell can be served immediately in either direction, but generates same-entity interference which is potentially more harmful. To avoid the much stronger downlink from saturating the uplink, this thesis considers dynamic TDD for dense and ultra-dense networks where transmission powers in the two directions are of comparable strength. Still, inter-cell interference remains an issue given the close proximity of some links. Because of the large number of cells comprising dense and ultra-dense networks, it is imperative that the interference management be both effective and scalable, which is the main focus of this thesis.

In the first part we focus on scalable radio resource management (RRM). We show that non-cooperative dynamic TDD is feasible for indoor ultra-dense deployment and highlight the benefit of employing beamsteering at both the base station (BS) and user equipment (UE) to mitigate interference distributively, especially at high load. Recognizing that beamsteering is better suited for higher frequencies and high data rate applications, we proceed to investigate the efficacy of receive-side interference management in the form of successive interference cancellation (SIC). Being that the interference distribution is different in dynamic TDD, we show that it suffices to cancel only strongest interferer at the UE side and the two strongest interferers at the BS. The combined benefit of SIC and dynamic TDD in reducing delay for low-rate traffic is also displayed. Next, we introduce limited inter-cell information exchange in order to leverage the resource allocation in the medium access control (MAC). To minimize the amount of information exchange and preserve scalability, a scheduling framework is proposed that relates real-time traffic to inter-BS interferences measured offline and mapped to the individual activation probability of each BS. The proposed scheme is shown to perform well with respect to comparable scalable schedulers when interference is high, and optimally when interference is low.

In ultra-dense networks it is expected that some BSs might not have a UE to serve. In the second part, we therefore introduce cooperation to utilize the otherwise idle BSs to improve network performance. To mitigate both same- and other-entity interference, zero forcing (ZF) precoding is employed where not only downlink UEs but also uplink BSs are included in the beamforming. Results show that both uplink and downlink performance improves at low and medium load, and that it is possible to trade performance in the two directions at high load.