Interference Manegement Study For TDD Heterogeneous Networks

Heterogeneous networks (Het Nets) are an important network architecture in the LTE-Advanced systems. Through cell splitting with low power nodes from the macrocells, Het Nets may be able to improve the coverage of hotzones as well as to increase the total throughput of the system. The paper first introduces TDD technology, comparing its advantages and disadvantages. Time division duplex system, or TDD system differentiates its uplink signal from its downlink signal by means of subframe number. Thus, unlike the FDD system, TDD technology may utilize unpaired spectrum resources as well as uplink and downlink channel reciprocity. For all the benefits it gives, TDD technology has been received more attention among the standard organization of the next generation communication. TDD Het Nets are believed to be quite promising.The paper first summarizes typical research scenarios of Het Nets, and focuses on analyzing the interference of two common scenarios. Judging from the carrier frequency, indoor or outdoor deployments, and the existence of macrocells, the research scenarios can be categorized into three main types of settings. The paper analyzes the interference among macro-pico and macro-femto in detail. The interference in former settings comes from macrocell and affects low power picocells. In the latter scenario, the user equipment in macrocell may be suffered from the interference brought about by femtocells. As interference management is one of the key issues in Het Nets deployment, in solving this challenging problem, one can design more advanced signal processing techniques, as well as to design more optimal scheduling scheme in radio resource management layer, to coordinate or mitigate the interference. The thesis then studies three typical settings considering both approaches and its applications in TDD Het Nets interference management:1) For co-existence of macrocell and femtocells, co-tier and cross-tier interference emerge. By applying maximization of the weighted sum rate methods, the local optimal solution which maximizes the achievable rate is obtained in case of1user per cell, for either macrocell or picocells. Compared to MMSE, the data rate performance is improved, for the case of sum power constraint and individual power constraint. The detailed assessment of the complexity and backhaul headings is presented in the following context.2) In the case of the dense deployment of femtocells, the users in closed subscriber group may experience huge interference brought about by those closed access home eNodeBs. The paper proposes a joint precoding scheme which utilizes both codebook based and non-codebook based precoding scheme. In TDD system, the base stations can uses the precoding matrix indicator to process the preprocessing of the downlink transmission. They can also utilizes TDD reciprocity. In experiencing severe interference, the latter scheme brings the results no better than the first one. However, in high signal to noise ratio cases, the scheme which utilizes TDD reciprocity performs better. Therefore, in this paper, a switch paradigm is presented to help the system switch between codebook based scheme and non-codebook scheme. Simulation results show that the performance is improved by applying the switch scheme.3) In picocells splitting from macrocell scenario, cell range expansion technique is applied, which adds a specific bias to the original measured RSRP of picocells, for the purpose of serving more users in picocells. However, severe interference may be experienced by the picocell users, especially for those cell edge users. The almost blank subframes are scheduled to emerge in macrocell transmission. Picocell then schedules its transmission for cell edge users in the correspondent ABSs to avoid the interference from the macrocell. System level simulation results show that by applying these two techniques, the throughput of the system is improved and the performance of last5%user is guaranteed.In sum, Het Nets are one of the key techniques in LTE-Advanced system, and interference management is the key issue facing the deployment of Het Nets. The better interference management methods are, the more improvement of the performance of Het Nets as well as the user quality of service.