Study On Interference Management In Two-Tier Heterogeneous Networks

Data explosion drives the evolution of the architecture of mobile networks. Shrinking cell size can meet the dramatically increasing demand for the mobile traffic due to the decreasing cell load and short transmission path. Together with macro cell which is used to provide wide coverage, they form heterogeneous networks (HetNets). Femtocell reduces the capital expenditure as there is no need dedicated wire line deployment between base stations (BSs), and at the same time it is more flexible to be deployed by users. Due to these merits, we focus on femtocells in this dissertation.HetNets bring new interference issues. Due to the two-tier architecture, the 3rd Generation Partnership Project (3GPP) technical report categorizes HetNet interference issues in two types:cross-tier interference (interference between macro cell and femtocells) and co-tier interference (interference among femtocells). A typical solution is partial channel deployment where orthogonal channels are allocated to femtocells and macro cell (to mitigate cross-tier interference) or interfering femtocells (to mitigate co-tier interference), which leads to the inefficient spectrum utility compared with co-channel deployment.To improve the spectrum utility, some fine-grained interference management schemes are necessary. However, for cross-tier interference issue, femtocell base station (FBS) directly plugs into an IP-based wire line backhaul network where delay cannot be predicted readily-making the real-time information exchange impossible. For the co-tier interference scenario, femtocells occupy small geographical regions in unplanned manners and serve small number of users, which makes the traditional statistic approach that treats the interference based on the law of large numbers less effective. As a matter of fact, we cannot directly apply the inter-cell interference coordination (ICIC) scheme popularly used in macro cells to femtocell networks.Concerning the limitations on the cross-tier scenario and co-tier scenario, this dissertation proposes a series of interference management schemes. First, a two-dimensional (2D) resource allocation scheme is designed to adapt channel variations and dynamic traffic in the cross-tier interference scenario. Second, a convex optimization based algorithm is proposed to further improve the performance of a popularly used co-tier interference management scheme. Third, a graph based solution is developed to join admission control and resource allocation in the co-tier interference scenario. At last, we investigate the uplink co-tier interference management with successive group decoding.The research works are supported by the National Natural Science Foundation of China (No.61231008); State Key Laboratory Foundation (ISN1002005, ISN090305); the 111 Project (No.B08038); Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0852). The main achievements and results of this dissertation are listed as follows:1.2D resource pattern optimization approach for interference avoidance in HetNets is proposed. By optimizing the resource pattern based on variable channel conditions, plus the introduction of group lasso term, our scheme exploits channel variations in both time and frequency domains and in addition avoids the interference. By grouping the resources in our optimization model, our scheme is flexible and robust against heavy burst traffic. It is also lightweight in terms of coordination overhead and completely distributed, making it suitable for the Long Term Evolution (LTE) protocol. Extensive simulations investigate the effect of coordination period and group number on different performance metrics and the effectiveness of our proposed method compared with sole frequency domain and time domain resource allocation approaches.2. Convex optimization based joint channel and power allocation scheme for multi-user orthogonal frequency division multiplexing (OFDM) networks is proposed. The investigated problem is the key problem in a distributed co-tier interference management solution, which is generally NP-hard. We reorganize the problem with only power variables and design a novel relaxation scheme to make it convex. The interior-point method is employed to solve the relaxation problem in the polynomial time, and the theoretical complexity is further presented. Simulation results show that our scheme can provide high energy efficiency compared with the existing methods and low computational complexity.3. Graph based efficient resource allocation scheme to maximize the number of users with Quality of Service (QoS) demands is proposed. Traffic class leads to higher demand for network designers on QoS provision. In the interference limited co-tier femtocell scenario, users’ diverse requirements may not be guaranteed at the same time. On the other side, femtocells are developed to reuse more resources. Concerning these two objectives, we join admission control and resource allocation and formulate a multi-objective optimization problem. The typical solution is solving the relaxation problem iteratively to approach the optimal solution but will be computationally prohibitive when the network scales up. We further develop a heuristic method based on the chordal graph, which shows to perform well in the case of large-scale networks.4. Uplink co-tier interference management in femtocell networks with successive group decoding is studied. We propose a new scheme to mitigate the uplink co-tier interference in dense femtocell networks, assuming that advanced receivers are employed by the FBSs. We exploit resource reuse by taking advantage of the successive group decoder (SGD) such that users can opportunistically access the entitled resources of nearby cells. The SGD decodes some interference signals in order to best decode the useful signal. Multi-cell uplink resource allocation with SGDs is formulated as a joint channel, rate and decoding group (CRG) allocation problem to maximize the weighted sum rates of the variable bit rate (VBR) users while meeting the rate requirements of the guaranteed bit rate (GBR) users. Due to the NP-hardness of the problem, we propose a greedy algorithm where the idea is to let GBR users opportunistically transmit on the channels of nearby cells and release more interference-free channels for high-rate transmission of VBR users. A semi-analytical framework is developed to provide a rough estimate of the potential throughput gain by the proposed technique. Simulation results show that the throughput gain over the conventional orthogonal resource allocation ranges from 10%-100% with different traffic proportion, number of users and rate requirement.