One of the main goals of the fourth generation (4G) wireless networks is to offer high data rates to all users anywhere within the coverage area. However, deliver- ing reliable services to users inside buildings remains a major challenge due to the significant attenuation of the buildings’ walls. An effective solution that has been recently adopted by several standards is to create a hierarchical cell structure where a macrocell is overlaid over a number of femtocells, which are devoted to provide coverage for indoor users. In such configuration, the end user will be connected to the femto base station (FBS) via a wireless link, but the FBS will be connected to service providers’ core network through a broadband connection such as the digital subscriber line (xDSL) or fiber to the x (FTTx). The FBS is known as ”femto BS” in WiMAX or ”Home NodeB (HNB)” in 3GPP terminology.
Depending on the spectrum availability, femtocells can be configured to func- tion in dedicated channels, or share the spectrum with other existing networks [2]. In the latter case, deploying femtocells within the coverage area of a macrocell may deteriorate the performance of the macrocell users due to the mutual interference be- tween the two systems. Although the FBS maximum transmit power can be adjusted
dynamically according to the macro user interference level, such process is not trivial and includes several trade-offs to consider. To achieve the desirable result, a receiver inside the building should get a high signal to interference and noise ratio (SINR) anywhere inside the building while the leakage of the FBS signal to the outside world should be kept to minimum. Achieving these two conflicting goals simultaneously is very challenging without considering the signal attenuation due to the propagation through walls, doors, windows and other building materials. Moreover, because of the complexity of the mixed indoor-outdoor environment, analyzing the interference in such environments using ray tracing [3] or finite-difference time-domain (FDTD) methods [4] would be highly complex, time-consuming, and it will be valid only for the case in study. Furthermore, it requires detailed information about the environment, such as the building’s floor plan, which is typically not accessible. Therefore, a simple model that captures the fundamental properties of such scenarios is indispensable.
In other words, buildings act as a shield that reduces the mutual interference between the macro and femto users. However, wall attenuation changes considerably in a broad range from 5 dB to 20 dB or more, based on the wall properties such as the type of material used and thickness. Furthermore, a signal transmitted through windows or doors is attenuated less than 3 dB.
In the literature, most of the works focused on the outdoor-to-indoor propa- gation models, aimed at extending the coverage of an outdoor transmitter to indoor receivers, and in fact a few papers considered the indoor-to-outdoor case. A com- prehensive study of the outdoor-indoor interface that includes the effects of different materials is reported in [5]. The final report of the European Co-Operation in the field of Scientific and Technical research, Action 231 (COST-231) [6] also includes a model for penetration of the signal inside buildings. The model which takes several factors such as frequency and distance into account, is calibrated empirically. Oestges et al. [7] proposed a similar model with the difference that the effect of the angle of incident was included. Several measurements were used to calibrate the model at 2.5 GHz. Another model which considers the distance and carrier frequency is explained in ITU-R M.1225 [8]. The models in [9] calculate the indoor path loss as a linear function of indoor distance. Extensive research was conducted to deterministically
methods [4, 10–12]. Such methods require a large amount of details including the to- pography and digitized 3D map of the city and their results are site-specific. In [13], the authors modeled the transmission of a signal from the rooms that have external walls or the rooms adjacent to them. The measurements have been taken in several frequencies from 0.9-3.5 GHz. The proposed frequency-dependent model also consid- ers the number of walls between the transmitter and outside. Although this work is unique, it does not model the transmitters farther from the external walls which limits its applicability.
This chapter presents a statistical model for indoor-to-outdoor path loss where the transmitter is a FBS inside a suburban house. In particular, the randomness due to building floor plans and FBS’s position is modeled. The proposed model is then used to analyze the femtocell signal interference on macro users. The chapter also investigates the issue of FBS’s placement to minimize the interference on macrocell users. The rest of the chapter is as follows. Section 3.2 describes the signal propa- gation into buildings. The proposed propagation model is discussed in Section 3.3. Note that the proposed model is based on a procedural building generation (PBG) algorithm, developed in Chapter 4. A very concise description of the algorithm is presented in this chapter for the sake of completeness. The algorithm is presented in details in the next chapter. Section 3.4 proposes the stochastic femtocell signal model. In Section 3.5, as an example of the propoed model’s applications, it is used to analyze the effects of FBS’s position on its interference on a macrocell user outside the building, and finally, Section 3.6 concludes the chapter and gives some ideas for further studies.