Energy-efficient design of two-tier femtocell networks

Wang et al. EURASIP Journal on Wireless Communications and Networking Energy-efficient design of two-tier femtocell networks Ying Wang 0 Yuan Zhang 0 Yongce Chen 0 Rong Wei 0 0 Equal contributors State Key Laboratory of Networking and Switching Technology, Beijing University of Posts and Telecommunications , 100876 Beijing , China With the exponential increase in mobile internet traffic, future cellular networks face great challenges to satisfy the demand of network capacity. At the same time, high data rate transmission and rapid increasing number of users also seriously burden the power consumption and the cost of cellular networks. Base station (BS) is the main part of power consumption, so reducing energy consumption of the BS can obviously reduce the total energy consumption. This paper investigates the effect on energy efficiency by adopting activity-aware sleeping strategies both in macrocell base station (MBS) and femtocell access point (FAP) in a two-tier femtocell network. By using stochastic geometry, we develop a trade-off between energy saving and coverage extension, regarded as the FAP additional connections by neighboring femtocell user equipment (FUE) or macrocell user equipment (MUE). Specifically, we derive users' coverage probabilities, which is defined as the probability of user connecting to BS, in closed forms with different sleeping strategies and access policies. Moreover, we formulate power consumption minimization and energy efficiency problem and determine the optimal joint MBS-FAP operating regimes. Numerical results show that sleeping scheme and femtocell access mode both have effects on coverage probability and energy efficiency, and the effect of femtocell access mode on coverage probability is greater than sleeping scheme. Two-tier femtocell network; Sleeping strategy; Stochastic geometry; Coverage probability; Energy efficiency 1 Introduction Looking ahead to the year 2020 and beyond, there will be explosive growth in mobile data traffic. It is estimated that hotspot traffic may grow up to 1,000 times [1]. Small cells seem to be a reasonable way to cater to the rapid increasing data traffic and provide higher spectral efficiency and energy efficiency. Generally speaking, small cells such as femtocells, which provide improved radio coverage and throughput by wireless access, operate in three access modes: open access, closed access, and hybrid access. When femtocells work in closed access mode, only registered femtocell user equipments (FUEs) can connect to the femtocell access point (FAP). In open access mode, all kinds of users will be allowed to connect to the FAP. In hybrid access mode, registered users and non-registered macro users that satisfy certain constraints can connect to the FAP. Although there already have some researches on femtocells with partially open channels [2], for simplicity, we consider completely closed access mode and completely open access mode in this paper. Besides the deployment of small cells, there are several aspects for energy conservation: base stations (BSs), user terminals, and the exploitation of renewable energy. BSs consume nearly 60% to 80% of the total energy in cellular networks, thus saving the energy consumption of BSs can implement green network effectively [3]. Macrocell base stations (MBSs) are usually powered on, while traffic demands are different in various time and areas [4], so these unnecessary MBSs waste energy seriously. In addition, in a two-tier femtocell network, to cope with the fierce increase of mobile traffic demand, network operators deploy massive and uncoordinated FAPs, which leads to a considerable increase in energy consumption. Adopting sleeping mode in these unnecessary BSs is the most direct and effective way for energy conservation, so we employ the method in heterogeneous networks, while the coverage problem caused by such operation cannot be ignored. In this paper, we use stochastic geometry [5,6] to study the trade-off between coverage extension (i.e., FAP additional connections by neighboring FUE or MUE) and energy saving in two-tier femtocell networks. MBSs and FAPs are set to work in a coordinated sleeping mode, where certain MBSs will be shut off while other active MBSs and FAPs would extend their coverage to avoid coverage hole. Specifically, an activity-aware sleeping strategy for the two-tier femtocell network is proposed. Then, the probabilities of users successfully communicate with BSs, termed users coverage probabilities, are derived in closed forms with different sleeping strategies and access modes. In addition, energy consumption optimization and energy efficiency problems are proposed. Numerical results prove that adopting sleeping strategy in both MBSs and FAPs can effectively improve energy efficiency, and the gain depends on the strategy and femtocell access mode. The main contributions of this paper are listed as follows: Using stochastic geometry, we analyze the problems of total power consumption and energy efficiency through deploying the activity-aware sleeping strategy in cognitive MBSs and FAPs. A concrete analysis on the coverage extension problem is proposed. In particular, expressions for the coverage probabilities with different sleeping policies and access modes are derived in closed forms. On the basis of uncertainties such as the activity probabilities of MBS and FAP, the probabilities of MBS and FAP remain in operation and the minimum total power consumption are derived. Finally, we explore how the activity probabilities of MBSs and FAPs affect energy efficiency. The effects of noise and power control used to maintain network coverage are also investigated. The remainder of this paper is organized as follows: Section 2 discusses related works and Section 3 presents system model. In Section 4, activity-aware sleeping strategy is proposed and the other sleeping strategies are introduced for comparison. In Section 5, coverage probability is derived firstly with different sleeping strategies and femtocell access policies, then the total power consumption is optimized, and energy efficiency is analyzed lastly. Section 6 validates our analysis through simulation results and numerical results, and concluding remarks are given in Section 7. 2 Related work There are mainly two types of methods to decrease the total energy consumption of BSs in heterogeneous networks: Turning small cell access points (SAPs) into sleeping mode when they are not serving any users. Adopting sleeping mode in MBSs. For turning SAPs into sleeping mode, [7] introduces energy-efficient sleeping mode algorithms for small cell BSs and discusses three different strategies for algorithm control, e.g., small cell controlled strategy, core network controlled strategy, and user equipment controlled strategy. A trade-off strategy between traffic offloading from the macrocell and the energy consumption of the small cell is proposed in [8], who provides (...truncated)