Interference mitigation and interference avoidance for cellular OFDMA-TDD networks

In recent years, cellular systems based on orthogonal frequ ency division multiple access – time division duplex (OFDMA-TDD) have gained considerable popu larity. Two of the major reasons for this are, on the one hand, that OFDMA enables the receiver to ffectively cope with multipath propagation while keeping the complexity low. On the other hand, TDD offers efficient support for cell-specific uplink (UL)/downlink (DL) asymme try demands by allowing each cell to independently set its UL/DL switching point (SP). Howeve r, cell-independent SP gives rise to crossed slots . In particular, crossed slots arise when neighbouring cell s use the same slot in opposing link directions, resulting in base station (BS)-t o-BS interference and mobile station (MS)-to-MS interference. BS-to-BS interference, in parti cular, can be quite detrimental due to the exposed location of BSs, which leads to high probability of line-of-sight (LOS) conditions. The aim of this thesis is to address the BS-to-BS interferenc e problem in OFDMA-TDD cellular networks. A simulation-based approach is used to demonstra te the severity of BS-to-BS interference and a signal-to-interference-plus-noise ratio (S INR) equation for OFDMA is formulated to aid system performance analysis. The detrimental e ffects of crossed slot interference in OFDMA-TDD cellular networks are highlighted by comparing m ethods specifically targeting the crossed slots interference problem. In particular, the interference avoidance method fixed slot allocation (FSA) is compared against state of the art in terference mitigation approaches, viz: random time slot opposing (RTSO) and zone division (ZD). The comparison is done based on Monte Carlo simulations and the main comparison metric is spectral efficiency calculated using the SINR equation formulated in this thesis. The simul ation results demonstrate that when LOS conditions among BSs are present, both RTSO and ZD pe rform worse than FSA for all considered performance metrics. It is concluded from th e results that current interference mitigation techniques do not offer an effective solution to the BS-to-B S interference problem. Hence, new interference avoidancemethods, which unlike FSA, do not sacrifice the advantages of TDD are open research issues addressed in this thesis. The major contribution of this thesis is a novel cooperativeresource balancing technique that offers a solution to the crossed slot problem. The novel conc ept, termedasymmetry balancing , is targeted towards next-generation cellular systems, env isaged to havead hocand multi-hop capabilities. Asymmetry balancing completely avoids cros sed slots by keeping the TDD SPs synchronised among BSs. At the same time, the advantages of T DD are retained, which is enabled by introducing cooperation among the entities in th e network. If a cell faces resource shortage in one link direction, while having free resources in the opposite link direction, the free resources can be used to support the overloaded link dir ection. In particular, traffic can be offloaded to near-by mobile stations at neighbouring cell s that have available resources. To model the gains attained with asymmetry balancing, a mathem atical framework is developed which is verified by Monte Carlo simulations. In addition, as ymmetry balancing is compared against both ZD and FSA based on simulations and the results d emonstrate the superior performance of asymmetry balancing. It can be concluded that th e novel interference avoidance approach is a very promising candidate to solve the crossed s lot problem in next-generation OFDMA-TDD-based cellular networks. Declaration of originality I hereby declare that the research recorded in this thesis an d the thesis itself was composed and originated entirely by myself in the School of Engineering a nd Science at Jacobs University Bremen and in the School of Engineering at The University of E dinburgh. The software used to perform the simulations was written by m yself with the following exceptions: The channel model used in the study of the comparison between RTSO (random time slot opposing) and FSA (fixed slot allocation) was written by Van Duc Nguyen and Birendra Ghimire.