in Figure 6.9, is the Dot by Ericsson. It is also a three-stage solution composed of a digital unit (DU), an indoor radio unit (IRU) and a RRH called Dot. The DU is the baseband part of a macro base station. The digital baseband signal (CPRI) is fed to IRU over an optical fibre. The IRU converts the digital optical baseband signal into an electrical analogue signal, which is then fed to the RRH over a LAN cable. The IRU also supports PoE functionality for powering the RRHs. One benefit of the design is that the RRH can be made small and lightweight. Optical fibre LAN cable Digital CPRI Analogue Figure 6.9 Dot architecture Small Cell Products Coaxial cable BTS 1 Optical fibre Coaxial cable BTS 2 Head end unit Remote unit Antenna BTS 3 Digital or analogue Analogue RF Analogue RF Figure 6.10 Active DAS architecture (e/o=electric to optical. o/e=optical to electric) 6.7 Distributed Antenna Systems In a DAS the RF signal of one or more base stations is distributed to a number of remote antennas in order to cover a pre-defined indoor or (less often) an outdoor area. There are two main categories of DAS: passive DAS and active DAS. In a passive DAS, a coaxial feeder cable is used for signal distribution along with passive RF components such as splitters and couplers. Being all passive, the system is reliable and has a moderate cost of materials. On the other hand, the use of coaxial cable brings two potential drawbacks: First, the system size is limited by the combined attenuation of the feeder itself and the required power splitting components. Second, installation of thick feeder is labour intensive and expensive. An active DAS combines optical and coaxial transport in order to reduce cabling cost and to extend the reach of the system. A schematic of an active DAS architecture is presented in Figure 6.10. The RF signal from one or more base stations (or other sources such as repeater) is fed to a head end unit which conditions the RF signals to an appropriate power level and combines them for transmission over optical