Abstract
In this paper, interference mitigation through downlinkpower control is considered for Macrocell Femtocell overlay.Specifically, the strong interference in the downlink from thehome base station to a nearby macrocell user should be properlycontrolled such that the quality-of-service of both the macrocelluser and the Femtocell users can be guaranteed. In this work,the fundamental capacity limitation of spatial spectrum sharingamong a macrocell user and a Femtocell user is identified. Adownlink power control problem is formulated to address theco-channel interference, as well as provide quality-of-service toboth the macrocell user and the Femtocell users. The feasibilitycondition of the problem is derived and both centralized anddistributed solutions are provided. Because the co-channel interferenceare from heterogeneous cells, a joint power control,channel management and admission control procedure is suggestedsuch that the priority of the macrocell users is alwaysensured. Simulation results demonstrate the effectiveness of theproposed schemes.
I. INTRODUCTION
Femtocells are cellular access points that connect to amobile operators network using residential DSL or cablebroadband connections. They have been developed to workwith a range of different cellular standards including CDMA,GSM and UMTS [1]. Studies show that more than 50%of all voice calls and more than 70% of all data trafficoriginate indoors [2]. A significant part of these indoor voiceand traffic originates from the homes of the subscribers. Byinstalling a home base station, a cellular subscriber may useany existing Internet access as a means to access the cellularbackbone infrastructure1, and only very short range wirelesscommunications from the user device to the home base stationneed to occur. Thus, the deployment of Femtocells withinexisting macrocells in a cellular network accommodates a largeportion of existing traffic, as well as facilitates the capacityrequirements of new applications and services. It is expectedto provide better coverage and capacity, as well as improvethe cost effectiveness of current cellular services.Femtocells are intended to enhance the coverage of amacrocell in the home environment. There are many technicalchallenges associated with the successful operations ofmacrocells and Femtocells when they coexist. Specifically,the radio interference between Femtocells and macrocells isa major concern and interference mitigation techniques are1the capacity of a Femtocell is indeed upper bounded by the capacity ofits corresponding Internet accessrequired. Interference mitigation techniques will impact thesystem performance, which will present the Femtocell withchallenges in managing its radio resources and maintainingQuality of Service (QoS) to its attached users.While uplink power control of Femtocells has been consideredin [3], downlink power control is a more critical issue [4].There are intensive ongoing research activities on interferencemitigation and power control of Femtocells in the standardbodies, such as the 3GPP TSG-RAN WR4 (please see [4] andthe references therein). However, most of the current worksfocus on determining the dynamic range of the downlinktransmission power of Femtocells, rather than an analyticalframework and detailed power control schemes.In this paper, the downlink interference scenario that existbetween a Femtocell and a macrocell is modeled and discussedin detail. Specifically, the fundamental limit of the achievableQoS of the macrocell users and the Femtocell users is given interms of the achievable sum signal-to-interference-plus-noiseratio (SINR) values of the users. Furthermore, a downlinkpower control problem for Femtocells is formulated and bothcentralized and distributed solutions are derived. Theoreticalanalysis and simulation results show that the proposed powercontrol mitigates the interference between the two overlaidsystems effectively.It worth noting that a mathematical framework for powercontrol in cognitive radio networks was developed by theauthors in their previous work [13]. It is interesting to findthat this framework (with modifications) also applies to spatialspectrum sharing in the context of macrocell Femtocelloverlay. In this work, we go one step further by providinginsights on relations between the achievable capacity of theoverlaid system and the geometric layout of the systems suchas the macrocell dead zone. Other contributions of this workinclude establishing a downlink power control frameworkfor the overlaid system, as well as proposing a hierarchicalarchitecture for joint power control and dynamic channel reallocation.The paper is organized as follows. Section II provides themodel of spatial spectrum sharing of a macrocell with a Femtocell,and the associated power control problem is formulated.The fundamental limit on achievable SINR of a macrocell userand a Femtocell user is given in Section III and the solutionof the power control problem is given for the case of a singleFemtocell.


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