 Abstract—Present network deployment approaches are not designed to manage large numbers of base stations, key requirement of emerging mobile technologies is their self- organizing capabilities. This is even more crucial in case of small cells such as femtocells. This paper discuses an automatic cell identifier assignment procedure with the focus on collision events that can be viewed as cell overlapping problem. We investigate femtocell intersection probability when randomly placing femtocell access points within a macrocell. Various combinations of femtocell densities and femtocell radii are taken into account in our analysis. Our study and obtained results can be used to decide which cell identifier assignment methods should be applied in a given situation. Keywords—Cell id, Cell overlapping, Femtocells, Simulation, SON. I. INTRODUCTION N these days, the mobile data traffic is heavily increasing and in parallel the Average Revenue Per User (ARPU) is declining. It is expected that global mobile traffic will increase about 1000x in the following decade [1]. On the one hand, mobile operators try to find new opportunities and business models how to increase their ARPUs, but on the other hand, they have to deal with the present data traffic explosion. Latest studies of mobile traffic show that almost 2/3 of all traffic (including both voice and data) originate from customers that are located in their houses, offices, etc. [2]. The use of conventional macrocell approach to cover such indoor placed customers is quite expensive for mobile operators and sometimes even unfeasible. One of cost- effective solutions to deal with the indoor environment coverage and capacity requirements offers small cells, namely femtocell. Femtocells can be seen as small cells covered by inexpensive, low-power base stations, which work in operator's licensed spectrum. In this paper, the femtocell base station is denoted as Femto Access Point (FAP). In general, FAPs are under controlled and deployed by customers themselves. To mobile operator’s networks, they are connected by either wired or wireless backhaul; e.g. ADSLs, Manuscript received February 28, 2013. This work was supported by Grant Agency of the Czech Technical University in Prague, grant no. SGS13/199/OHK3/3T/13. M. Sedlacek is with the Dept. of Telecommunication Engineering, Czech Technical University in Prague, Prague Czech Republic (e-mail: sedlama9@fel.cvut.cz). R. Bestak is with Dept. of Telecommunication Engineering, Czech Technical University in Prague, Prague Czech Republic (e-mail: robert.bestak@fel.cvut.cz). optics, or nowadays even wireless or satellite links. To enable smooth, simple and mass deployment of femtocells, self-organizing concept has to be employed. Operators cannot expect their customers to have whatever knowledge how to install or configure FAPs, configuration (and optimization) has to be done automatically by FAPs themselves or with aid of network. As in case of a macrocell base station, a newly introduced FAP in the network has to be assigned a physical cell identifier. These cell identifiers are used to unambiguously identify cells within the given area. Notice that the cell identifier pool is shared by all types of cells in the network (femto, pico, metro, macro, etc.). When assigning cell identifiers, a cell identifier should be assigned by collision free manner, i.e. a base station is assigned a unique cell identifier in the area that the cell covers (see Fig. 1). Two automatic cell identifier assignment approaches can be generally distinguished: i) centralized [3]—[5], and distributed [6]—[7]. In the first case, a central network entity selects the appropriate cell identifier of a new base station whereas in the second case a newly introduced base station itself chooses its cell identifier. In view of the fact that FAPs are usually in possession of customers, a cell identifier assignment method should be automatic and be part of the self-organizing procedures. A problem of automatic centralized assignment methods lies in a current assumption that the geographic location of FAP is unvarying and known to the central network entity which can then select an appropriate cell identifier. To determine a FAP position, which is usually placed inside of buildings, is quite challenging task and in certain cases even impossible to find out; for example due to weak or even missing Global Positioning System (GPS) signal in buildings. In our study, Fig. 1. Example of collision event. Probability of Cell Overlapping in Femtocell Environment Marek Sedlacek and Robert Bestak I 244 978-1-4799-0404-4/13/$31.00 ©2013 IEEE TSP 2013