Between 70 and 80% of mobile traffic originates inside buildings. This is especially true in urban environments where the focus of the mobile user is on high data speeds. For the mobile 3G network, only the macro base station within a few hundred meters of the building can provide a sufficient level of RF signal to support indoor voice / data services. In reality, only a few buildings will fall into this category. Soft handover on the 3G network will further increase the traffic load on the network, as each indoor mobile phone can be serviced by more than one macro cell (base stations). In order to provide indoor high-speed mobile data services, such as HSPA (High speed Packet Access) or EVDO (Evolution – Data Optimized) services, the only solution is an indoor Distributed Antenna (DAS) system.
DAS is used to distribute the RF signal evenly strong enough within a building to provide 3G voice and data services. The DAS can be used to isolate the indoor network from the outdoor service macro cells to eliminate the soft handover of the indoor mobile phone. This will reduce the traffic load and increase the speed of the 3G network. For HSPA high-speed data service, the inner DAS can also provide isolation between serving and non-serving cells from the outer network. This means less co-channel interference in the HSPA serving cell and results in a higher data rate for the HSPA serving. To dominate the building with indoor coverage, directional antennas can be deployed at the edge and corners of the building and pointed towards the center of the building. The total interior area is dominated by the interior cell and at the same time minimizes leakage to the macro network.
DAS distributes a uniform dominant RF signal within the building by splitting the signal from the indoor base station to multiple indoor antennas to provide coverage throughout the building. The DAS can be classified as passive or active. Passive DAS uses passive components to distribute the RF signal. These passive components are coaxial cable, splitters, terminators, attenuators, circulators, couplers, and filters (duplexer, diplexer, or triplexer). DAS planning includes calculating the maximum loss from the base station to each antenna in the systems and balances the link for the particular area that each antenna covers. The design of the passive DAS must accommodate the limitation of the building with respect to the restriction of where and how heavy coaxial cable can be installed. A detailed building site survey is required to ensure that there are cable routes to all antennas.
Active DAS has the ability to automatically compensate for losses in cables that interconnect system components through the use of internal calibration signals and amplifiers. No matter what the distance is between the antenna and the base station, all antennas in an active DAS will have the same performance (same downlink power and noise figure). Active DAS consists of a master unit (MU) connected to multiple expansion units (UE) with fiber optics up to 6 km in length. Each UE, in turn, connects to multiple Remote Units (RUs) with a thin or CAT5 coaxial cable up to 400m in length. The MU controls and monitors the performance of the DAS. The UEs are distributed throughout the building and the RUs are installed near the antenna. An active broadband DAS can support multiple radio services, GSM, PCS, UMTS, EVDO, WiMax, and Wi-Fi.
Due to loss and attenuation in coaxial cable and passive components, passive DAS is only used on smaller building roofs by a small number of indoor antennas to keep its degrading impact on HSPA performance to a minimum. For larger buildings, Active DAS is used because it has no cable or component loss and can maximize HSPA performance. Troubleshooting in passive DAS is difficult and any failure in the systems will not generate an alarm at the base station because there is no error monitoring in the system. Active DAS monitors all units in the system and in case of failure; will send an alarm to the base station that allows the operator to identify the source of the problem. Therefore, Active DAS is the preferred solution for large buildings with many indoor antennas.