In the last decades, the presence of locating systems has become increasingly established in several applications. This technology is indeed highly coupled in the usual day-life of the people. Just think about the Google maps system or packet tracking service provided by the shipping companies. So, an important requirement asked to many recent systems is the ability to locate, the person or the item using that specific technology. This feature has reached impressive improvement for the outdoor environments in the last decades. The solution adopted for these purposes is the Global Positioning System (GPS), which is a very reliable technology but, on the other hand, is not suitable for indoor applications.