Interference Management In Heterogeneous Cellular Network

With the rapid development of the mobile Internet and explosive growth of emerging services and mobile applications, mobile users put forward a higher requirement for the data rates. By deploying small cells on the conventional macrocell network, heterogeneous cellular network (HCN) can effectively improve the cellular capacity, which makes it a promising technology for future mobile communication systems. In order to maximize the use of spectrum resources,3GPP (The3rd Generation Partnership Project) LTE (Long Term Evolution) tends to share the same frequency between the macrocell and small cells. The cross-layer interference has become a important factor in restricting the performance of cell networks. Therefore, it is still a challenge to manage the interference in HCN to improve the spectral efficiency (SE). Three-pronged approaches, including access control, power control and time-domain resource scheduling, are investigated to solve the problem. The main contributions in this dissertation include:1This dissertation first studies the access control of femtocell in HCNs. Closed access allows authorized home users to monopolize the femtocell backhaul, which leads to the severe cross-layer interference between the femtocell and the surrounding macrocell users. The corss-layer interference can be reduced if the femtocells are opening for the macrocell users. The open access improves the interference enviroment of femtocell, at the expense of sacrificing parts of spectrum resources. With introduce of interference factor, the average ergodic rates of the femtocell and macrocell are both obtained under closed access and open access. Then, the impact of open radius of femtocell on the performance of femtocell owner is investigated. To improve the performance of femtocell (macrocell), this dissertation also proposes the algorithms, to schedule the handoff macrocell users according to the variety of interference environment.2. This dissertation studies the uplink power control of small cells in HCNs. Using the same spectrum, the uplink transmission of the small base stations cause interference to the macro base station, especially for the edge users of small cells. With more and more small cells overlaid on the macrocell, the cumulative interference is negligible. By introducing the interference price, the problem of power control in HCNs can be transformed into the economic problem, where macro base stations tax the small cells for the uplink cross-layer interference. Then, the problem of interference management in HCN can be turned into two convex optimization problems. At the side of small cells, the problem is to obtain the closed-form formula of optimal power allocation under fixed interference price; while the problem at the side of macro base station is to obtain the optimal interference price for the small cells. Furthermore, the uniform interference price is investigated and then a distributed power control algorithm is proposed. The distributed algorithm based on the uniform interference price reduces the overhead on the control information, which makes the power control mechanism more feasible in the actual system.3. This dissertation studies the time-domain interference coordination technology in HCNs. When cell range expansion (CRE) is taken in a HCN, the users in the expanded region will suffer severe downlink interference from the macro base stations.3GPP R10proposes a time-domain interference technology to anticipate the interference. In this part, the distribution of the base station is characterized by poisson point process (PPP), which is helpful to obtain the theoretical analytical results. Based on the PPP model, this dissertation obtains the formula of the outage probability and average ergodic rate of users. Then, the impact of bias and time-domain resource partitions on the network performance, such as network fairness and spectral efficiency is investigated. The work provides the reference value for the configuration of the bias of small cells and ratio of almost blank subframe (ABS) in the actual system.