Date Published: March 22, 2017
Publisher: Public Library of Science
Author(s): Nak Woon Sung, Ngoc-Thai Pham, Thong Huynh, Won-Joo Hwang, Ilsun You, Kim-Kwang Raymond Choo, Wen-Bo Du.
The deployment of large number of femtocell base stations allows us to extend the coverage and efficiently utilize resources in a low cost manner. However, the small cell size of femtocell networks can result in frequent handovers to the mobile user, and consequently throughput degradation. Thus, in this paper, we propose predictive association control schemes to improve the system’s effective throughput. Our design focuses on reducing handover frequency without impacting on throughput. The proposed schemes determine handover decisions that contribute most to the network throughput and are proper for distributed implementations. The simulation results show significant gains compared with existing methods in terms of handover frequency and network throughput perspective.
The small femtocell can be inexpensively deployed to improve the quality of service (QoS) in wireless networks . Installation of low-cost femtocells is generally indoors or in hotspots (to facilitate communications with cellular networks via a wired backhaul or separate radio frequency backhaul channels) as these femtocells can significantly improve indoor coverage and capacity. This is due to their capability to cover corners where radio signal from macro base station (MBS) may be weak, and short-distance communications enable a higher signal-to-noise ratio (SINR). Massive femtocell base stations (FBSs) within a range of a MBS may extend the coverage to wider areas such as large office buildings. In such a scenario, a mobile station (MS) may move to the new position, then associating with a new FBS to maintain connectivity. In another scenario, an active MS associates with a FBS to start a data session, which will be terminated when it is finished. In both scenarios, frequent associations or re-associations, or handovers are necessary to provide seamless service to the MS [2, 3]. Thus, the MSs need to make appropriate and informed association decisions.
Studies on association control for network-wide performance have been extensively investigated under different environments, e.g., wireless cellular network [9, 10], WiFi 802.11 [4, 11, 12], with different names such as association control [9, 13] or handoff decision [4, 14], or load balancing [10, 11]. In [9, 13, 14], the authors formulate the handover problem as the dynamic control problem and solve it by using Markov Decision Process framework, in order to obtain better network-wide utility performance. However, solving these algorithms requires centralized computation and is NP-hard.
In this research, we consider a downlink system of femtocell networks, comprising M BSs and I MSs (Fig 1). We denote by m ∈ M the index of BSs and i ∈ I that of MSs, respectively. Both FBSs and MBSs use the same technology such as 3GPP LTE or IEEE 802.16m/WiMAX and only differ from each other in signal strength and cell sizes. In our system model neighboring MBSs are assumed to be included in M BSs for simplicity. And we consider the downlink performance in our association control policy.
In this section, we construct association decision as an optimal control problem in use of the dynamic programming formulation. This formulation provides an insight into association decision and formal derivation of online policy for some specific and practical cases.
In this section the proposed online policy and the existing methods are evaluated and compared each other by varying performance criteria.
Dense femtocell networks provide better throughput and extend coverage areas for mobile users. However, the deployment of the large number of femtocell base stations results in more frequent handovers to moving MSs which have an effect on the throughput of the MSs. In this paper we have presented the formal formulation of association control problem regarding to achievable throughput, handover frequency, and mobility prediction of MSs in dense femtocell networks. The association control problem is considered as the dynamic programming problem. In these formulations the term ‘femtocell’ is extended to larger cell sizes encompassing picocells and microcells.