Handover management in high-dense femtocellular networks

Mostafa Zaman Chowdhury 0 Yeong Min Jang 0 0 Department of Electronics Engineering, Kookmin University , Seoul 136-702, Korea Femtocell technology is envisioned to be widely deployed in subscribers' homes to provide high data rate communications with quality of service. Dense deployment of femtocells will offload large amounts of traffic from the macrocellular network to the femtocellular network by the successful integration of macrocellular and femtocellular networks. Efficient handling of handover calls is the key for successful femtocell/macrocell integration. For dense femtocells, intelligent integrated femtocell/macrocell network architecture, a neighbor cell list with a minimum number of femtocells, effective call admission control (CAC), and handover processes with proper signaling are the open research issues. An appropriate traffic model for the integrated femtocell/macrocell network is also not yet developed. In this article, we present the major issues of mobility management for the integrated femtocell/macrocell network. We propose a novel algorithm to create a neighbor cell list with a minimum, but appropriate, number of cells for handover. We also propose detailed handover procedures and a novel traffic model for the integrated femtocell/macrocell network. The proposed CAC effectively handles various calls. The numerical and simulation results show the importance of the integrated femtocell/macrocell network and the performance improvement of the proposed schemes. Our proposed schemes for dense femtocells will be very effective for those in research and industry to implement. 1. Introduction Future wireless networks will necessitate high data rates with improved quality of service (QoS) and low cost. A femtocellular network [1-9] is one of the most promising technologies to meet the tremendous demand of increasing wireless capacity by various wireless applications for future wireless communications. Femtocells operate in the spectrum licensed for cellular service providers. The key feature of the femtocell technology is that users require no new equipment (UE). The deployment cost of the femtocell is very low while providing a high data rate. Thus, the deployment of femtocells at a large scale [5,6] is the ultimate objective of this technology. Indeed, a well-designed femtocell/macrocell-integrated network can divert huge amounts of traffic from congested and expensive macrocellular networks to femtocellular networks. From the wireless operator point of view, the ability to offload a large amount of traffic from macrocellular networks to femtocellular networks is the most important advantage of the femtocell/macrocell-integrated network architecture. This will not only reduce the investment capital, the maintenance expenses, and the operational costs, but will also improve the reliability of the cellular networks [5]. Figure 1 shows an example of femtocellular network deployment. The femtocells are deployed under the macrocellular network coverage or in a separate non-macrocellular coverage area. In the overlaid macrocell coverage area, femtocell-to-femtocell, femtocell-to-macrocell, and macrocellto-femtocell handovers occur owing to the deployment of femtocells. The frequency of these handovers increases as the density of femtocells is increased. Thus, effective handover mechanisms are essential to support these handovers. The efficient femtocell-to-femtocell and femtocellto-macrocell handovers result in seamless movement of femtocell users. Even though the macrocell-to-femtocell handover is not essential for seamless movement, efficient handling of this handover type can reduce huge traffic loads Figure 1 Example of a dense femtocellular network deployment scenario. of macrocellular networks by transferring the calls to femtocells. The large- and dense-scale deployment of femtocells suffers from several challenges [2-5]. Handover is one challenging issue among several issues. For efficient handover management, four factors, namely, intelligent network support, signal flow control for the handovers, reduced neighbor cell list, and an effective call admission control (CAC) policy, are essential. To the best of the authors knowledge, complete research results regarding these issues are still unpublished. However, a few research groups (e.g., [10,11]) have partially discussed some ideas regarding handover issues in femtocellular networks. Bai et al. [10] proposed a handover mechanism based on the decision made by an entity connected with a femtocell access point (FAP). This entity considers the user type, access mode of the FAP, and current load of the FAP to make a decision about the target femtocell. However, their scheme does not consider the creation of a neighbor cell list. Zhang et al. [11] presented a handover optimization algorithm based on the UEs mobility state. They also presented an analytical model for the handover signaling cost analysis. Here, we propose some novel approaches to solve the mobility management issues for densely deployed femtocellular networks. We suggest self-organizing network (SON) features to support the dense femtocellular networks, detail handover call flows for different handovers, an algorithm to create an appropriate neighbor cell list (including the neighbor femtocell list and the neighbor macrocell list), and an efficient CAC to handle various calls. We also propose a novel traffic model for the integrated femtocell/macrocell scenario. When the number of femtocells increases, the system architectures must support the efficient management of a large number of FAPs and a huge number of handover calls. The SON features [5,12,13] can support the coordination among the FAPs as well as among the FAPs and macrocellular BS to execute smooth handover. The ability to seamlessly move between the macrocellular network and the femtocellular networks is a key driver for femtocell network deployment. Moreover, handover between two networks should be performed with minimum signaling. Owing to some modifications of the existing network and protocol architecture for integrated femtocell/macrocell networks, the proposed signal flows for handover procedures are slightly different as compared to the macrocellular case. In a dense femtocellular network deployment, thousands of femtocells can be deployed within a small coverage area. As a result, this may present huge interference effects. Whenever a mobile station (MS) realizes that the received signal from the serving FAP is going down, the MS may receive multiple signals from several of the neighbor FAPs for handover. Thus, the neighbor cell list based on the received signal only will contain a large number of femtocells. In addition, a hidden FAP problem may arise. The hidden FAP problem arises when a neighbor FAP is very close to the MS but the MS cannot receive the signal owing to some barrier (e.g., a wall) between the MS and that FAP. Thus, the hidden FAPs will be out o (...truncated)