Between 70 and 80% of mobile traffic originates inside buildings. This is especially true in urban environments where the mobile user’s focus is on high data rates. For 3G mobile network, only the macro serving base station within a few hundred meters of the building can provide a sufficient RF signal level 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, since each indoor mobile phone can be served by more than one macro cell (base stations). 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 System (DAS).
DAS is used to distribute the RF signal evenly with sufficient strength within a building to provide voice and 3G data services. DAS can be used to isolate the inner network from the outside serving macrocells to eliminate the soft handoff of the inside mobile phone. This will reduce the traffic load and increase the speed of the 3G network. For HSPA high-speed data service, indoor DAS can also provide isolation between serving and non-serving cells of the outdoor network. This means less co-channel interference in the HSPA serving cell and results in a higher data rate for the HSPA service. To dominate the building with indoor coverage, directional antennas can be deployed on the edge and corners of the building and point towards the center of the building. The total interior area is dominated by the interior cell while minimizing 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 (duplex, diplexer, or triplex). DAS planning includes calculating the maximum loss from the base station to each antenna in the systems and budgeting the link for the particular area each antenna covers. The design of the passive DAS must accommodate the constraints of the building in terms of restricting where and how heavy coax cable can be installed. A detailed site survey of the building needed to be done to make sure there are cable routes to all the antennas.
Active DAS has the ability to automatically compensate for cable losses interconnecting system components through the use of amplifiers and internal calibration signals. No matter what the distance between the antenna and the base station, all the antennas in an active DAS will have the same performance (same noise figure and downlink power). Active DAS consists of a Master Unit (MU) connected to multiple Expansion Units (EU) with fiber optics up to 6 km in length. Each UE in turn connects to multiple Remote Units (RUs) with a thin coaxial or CAT5 cable up to 400m in length. The MU controls and monitors the performance of the DAS. The EUs are distributed throughout the building and the RUs are installed near the antenna. An active broadband DAS can support multiple radio, GSM, PCS, UMTS, EVDO, WiMax and Wi-Fi services.
Due to loss and attenuation in the 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 drives in the system and in case of malfunctions; 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.
