ogies and even by multiple operators. If the DAS has been designed for GSM and HSPA, it is typically simple to add LTE just by plugging in LTE base station as long as the antennas and splitters support the new LTE bands. It is also possible to share the DAS system by multiple operators which is an attractive solution for underground systems, shopping malls and other public premises. LTE Small Cell Optimization Indoor antennas Splitters Coaxial cables Macro base station Figure 5.8 Distributed antenna system (DAS) overview The typical DAS installation has only single coaxial cable which makes the usage of Multiple Input Multiple Output (MIMO) difficult. If MIMO is required in DAS, then two parallel cables are needed in the installation phase. The lack of MIMO is not considered as a major problem since DAS installation provides good signal levels which enables reasonably high data rates even with single-stream transmission. The capacity upgrade in DAS can be challenging. The number of sectors in DAS must be defined in the installation phase. Adding more frequencies is typically simple in DAS but adding more sectors can be difficult. Sector splitting can lead to major redesign requirements in the cable installation. Distributed Antenna System (DAS) solutions can be classified as passive, active or hybrid systems. Passive DAS: In passive systems, the wireless signal from the RF source is distributed to the antennas for transmission without any amplification through a series of passive components. Active DAS: In active DAS, the RF signal from a source is converted to a digital signal for transmission over fibre optics or cable. It is fed to multiple remote units that convert the signal back to an RF signal for transmission through an antenna. Hybrid DAS: A hybrid DAS system is a combination of passive and active systems. In a hybrid DAS system, fibre optic or CAT5 cable is still used to connect the head end (master unit) to the remote units. However, passive DAS is used for distributing the RF to the antennas from the 