Research On Radio Resource Management In LTE And Two-tier Macrocell/Femtocell Networks

Advanced radio resource management (RRM) technologies in Long Term Evolution (LTE) and two-tier Macrocell/Femtocell networks are researched in this dissertation. The main contents include two parts: research on RRM algorithms in LTE networks and RRM in two-tier Macrocell/Femtocell Networks. For the LTE networks, the RRM algorithms research focus on the following aspects:study on imperfect adaptive beam-forming and scheduling schemes in LTE Time Division Duplexing (TDD) system under mixed services scenario; proposing a dynamic resource allocation scheme considering inter-cell interference (ICI) based on game theory in LTE system; proposing an effective distributed resource allocation algorithm which combines interference limited power control scheme and soft frequency reuse (SFR) in LTE uplink. For the two-tier Macrocell/Femtocell networks, firstly, attention is paid on analysis of the cross-tier interference and research on distributed resource allocation. Based on the game theory, an iterative algorithm between the sub-channel allocation and power allocation is proposed. Secondly, the vertical handover in the two-tier network is investigated and a novel optimizing scheme is produced considering the Femtocell access type, UE’s velocity, the current network condition and the operation cost of the network. The evolution of LTE and the two-tier Macrocell/Femtocell networks not only needs the upgrade in system fundamental architecture, but also the innovation and optimization of the RRM algorithms. RRM is a key factor relating to the performance of whole network. In this dissertation, the basic knowledge of LTE is introduced firstly including Orthogonal Frequency Division Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO). Further, the LTE system frame structure and features of wireless resource allocation are introduced consisting of ICI, dynamic sub-channel allocation and distributed network architecture. Next, the key issues of Femtocell are introduced comprising of the background, the basic network architecture and access type. Then the system level simulation methodology of wireless communication network is clarified. The selection of scenarios, the correlative simulation modules and simulation flowcharts are designed. Also the simulator takes the differences of the LTE system and Macrocell/Femtocell two-tier systems into account. At last, the RRM is investigated and the performance for the LTE systems and two-tier Macrocell/Femtocell networks are evaluated respectively. Some crucial and hot issues are well solved in this part, such as, the ICI and resource allocation in LTE system, cross-tier interference and vertical handover in the two-tier Macrocell/Femtocell networks.Our research work can be composed of following6parts:Firstly, the LTE system is based on Orthogonal Frequency Division Multiple Access (OFDMA). Compared to the CDMA (Code Division Multiple Access) system, the LTE system has an extra frequency resource dimension. So a time-frequency domain cross-layer scheduler is proposed to achieve the diversity gain by making use of frequency resource flexibly. Secondly, the categories of services are increasing to satisfy the demand of the users. The wireless communications system has to guarantee the different Quality of Service (QoS) for different services. A new adaptive emergency threshold scheduler is designed in a mixed traffics scenario of LTE systems which consists of File Transfer Protocol (FTP) and video stream services. Simulation results show that the proposed AET scheduler provides a significant improvement from the throughput point without degrading the QoS of video stream service.Thirdly, How to allocate the resource (power, sub-channel, etc.) effectively to mitigate the ICI in LTE is a hot discussing issue. Solving this problem well can get improved system performance and good user experience. In this part, based on the game theory, the distributed resource allocation algorithms are given to both LTE uplink and downlink system. The inter-cell interference coordination (ICIC) is molded as non-cooperative game. The existence of Nash equilibrium proves the convergence of the proposed scheme and indicates how Nash equilibrium can be reached. Two timing schemes including sequential and parallel are discussed. Our proposed scheme combines SFR and takes the user’s location information into account to achieve the improved system performance.Fourthly, a distributed resource allocation scheme combines SFR and uplink interference limited power control is considered to use the frequency, power resources effectively and avoid severe ICI. This scheme is suitable to any load unbalanced scenarios no matter in a certain cell or across cells. The interference constraints can be dynamically modified according to the load variations. This adaptive interference power constrains scheme only requires minor coordination between cells. Finally, the proposed scheme decomposes the multi-cell resource optimization problem into a distributed single-cell problem, which makes it more suitable for practical use.Fifthly, the introduction of Femtocell solves the indoor coverage and capacity demand. However, it also brings challenges, such as, the cross-tier interference, the unplanned deployment and large number of Femtocells. To solve these problems, how to allocate the resources (e.g. sub-channels, power) is a critical issue to mitigate the cross-layer interference, avoid large amount of exchange signaling and satisfy the user’s QoS. In this part, the interference situation in two tier networks s analyzed firstly and an iterative process between power allocation and sub-channel allocation process is proposed to get a good stable state based on the game theory. Finally, a macrocell link quality protection scheme is proposed to guarantee the macrocell UE’s QoS at the presence of severe cross-tier interference from Femtocells when the system is crowded. The simulation results show that the new distributed algorithm can solve the resource allocation problem in two-hierarchy Macrocell/Femtocell networks well with low complexity and get notable performance gain.Finally, the mobility management is studied in two-tier networks and the Macrocell/Femtocell networks. Considering many factors comprehensively, the vertical handover problem can be formulated as follows:given a network of base stations (BSs), Femtocells in hybrid access mode, and UEs (subscriber and non-subscriber), how do the networks decide that which UEs should initiate handover requests; how to find appropriate attachment points for these UEs (via vertical or horizontal handover) through optimizing a well-defined objective function? Herein, a new vertical handover scheme is proposed to solve the problems above in two steps. Firstly, pick the handover candidates who should initiate handover requests regarding the practical constraints, such as, RSS, velocity and UE type to avoid unnecessary handovers. Secondly, optimize the objective function to facilitate the operator management of the whole network. Specially, the objective function developed comprises multiple factors cared by the network operator. In this paper, the equitable distribution of traffic load across available attachment points and service cost minimization are as the combined optimization target. The network operators can flexibly put different degree of emphasis on the factors they most concern. This combined objective function has good expansibility and portability. Results based on the detailed performance evaluation demonstrate promoted efficacy of the proposed scheme compared to the traditional ones.