In two-tier networks, which consist of macrocells and femtocells, femtocells can offload the traffic from macrocells thereby improving indoor signal coverage. However, the dynamic deployment feature of femtocells may result in signal interference due to limited frequency spectrum. The tradeoff between broad signal coverage and increased signal interference deserves further exploration for practical network operation. In this paper, dynamic frequency resource management is proposed to avoid both co-tier and cross-tier Orthogonal Frequency Division Multiple Access downlink interference and increase frequency channel utilization under co-channel deployment. A graph-based non-conflict group discovery algorithm is proposed to discover the disjoint interference-free groups among femtocells in order to avoid the co-tier interference. A macrocell uses the femtocell gateway for frequency resource allocation among femtocells to avoid cross-tier interference. We formulate the optimized frequency resource assignment as a fractional knapsack problem and solve the problem by using a greedy method. The simulation results show that the average data transfer rate can be increased from 21% to 60%, whereas idle rate and blocking rate are decreased in the range of 15% Ï 22% and 60% Ï 82%, respectively, as compared with conventional graph coloring and graph-based dynamic frequency reuse schemes. Two-tier networks contain femtocells and macrocells. A macrocell deployment is significantly more costly than femtocell deployment. It requires acquiring, leasing, and determining a site for a base station installation, which may not be easy to accomplish. Femtocell technology offers plugand-play for configuration; low deployment cost and traffic offload from the macrocell. It improves the macrocell's capacity and signal coverage in a simple and economical manner [4,5]. A femtocell consists of a small cellular base station, operates on a licensed spectrum, and connects to a service provider's broadband system to offer wireless access service [6]. A sample deployment of a twotier LTE network consisting of several femtocell base stations (FBSs) and a macrocell base station (MBS) is depicted in Figure 1. The FBS provides access service for femto User Equipment (fUE), and the MBS does so for macro User Equipment (mUE). Femtocells provide a smaller radio signal and connect to the core network by a wired backhaul link. The femto gateway (FGW) is an intermediate node that controls and manages FBSs and performs traffic routing for the core network. In addition, the FGW also supports femto-specific functions such as admission control, handover control, and interference management.Because femtocells are usually installed as demanded by people, self-organization, and self-management functions, are very important for the operation of femtocell networks. Selforganization will allow femtocells to detect the installed environment in order to integrate themselves into the core network, where self-management will enable femtocells to tune parameters for netwo...

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The 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) defines a wireless network standard for high packet transmission rate and low packet latency provisions. Handover is one of the important features for helping user equipments (UEs) to roam between LTE networks. However, LTE networks adapt a make-before-break handover procedure, which may cause a brief disconnection, therefore results in the packet transmission delay and packet loss problems. In this paper, we propose a moving direction prediction-assisted handover scheme for LTE networks to lower the number of handovers. We first track the location of user equipments (UEs) to predict their moving direction. By referencing previous locations, the next moving direction of UEs is estimated with the cosine function in order to determine the candidate E-UTRAN NodeBs (eNBs) for handover. Then, a target eNB is selected from the candidate eNBs through an angle-based dynamic weight adjustment scheme. By selecting a proper target eNB for handover, thus the quality of network transmission can be enhanced. Simulation results demonstrate the ability of the proposed scheme in reducing 17% average handover times, compared with the standard handover procedure, thereby reducing 12% average number of packet loss and 5% average packet delay time.

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In heterogeneous wireless networks, the end users are expected to get more bandwidth from WLAN, other than from WMAN, for real time applications. However, adopting traditional Received Signal Strength (RSS) based approach could be inadequate for vertical handover. When WLAN is within the coverage of a WMAN Base Station, an associated Mobile Node (MN) can hardly connect to a WLAN Access Point (AP) due to the default high handover triggering threshold. On the other hand, without knowing the utilization information of scanned APs, once the MN handover to a busy AP by detecting a better signal, it may suffer from bandwidth contention. In this paper, we propose an efficient handover scheme, from WMAN to WLAN, to make use of the received signal to interference and noise ratio and the information about back-haul bandwidth for the criteria to trigger a handover. The time to start a handover is determined by considering the variations of data rate as the MN moves and the most suitable target AP is selected according to its maximum available bandwidth. The simulation results show that the MN consistently has a better throughput with the proposed scheme, as compared with the RSS-based vertical handover scheme. And among all candidates APs, the MN has the minimum packet delay with the selected AP association.