On the Performance of Dynamic TDD in Ultra-Dense Wireless Access Networks

Sammanfattning: 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.