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October 29th, 2006 
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Distributed Antenna Systems for Indoor Cellular Network
Some 70 - 80% of mobile traffic originates from inside buildings. This is especially true in urban environments where the focus of the mobile user is on high data rates. For mobile 3G network, only serving macro base station within a few hundred meters of the building can provide sufficient level of RF signal to support indoor voice/data services. In realty, only a few buildings will fall into this category. Soft handover in 3G network will further increase the traffic load on the network since each indoor mobile phone can be serviced by more than one macro cells (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 Systems (DAS).
DAS is used to distribute the RF signal evenly with sufficient strength inside a building to provide 3G voice and data services. DAS can be used to isolate the indoor network from the outdoor serving 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, 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 higher data rate for the HSPA service. To dominate the building with indoor coverage, directional antennas can be deployed at the edge and corners of the building and pointing towards the center of the building. The total indoor area is dominated by the indoor cell and at the same time minimizes leakage to the macro network.
DAS distributes a uniform dominant RF signal inside the building by splitting the signal from the indoor base station to multiple indoor antennas to provide coverage throughout the building. DAS can be classified as passive or active. Passive DAS uses passive components to distribute the RF signal. These passive components are coax cable, splitters, terminators, attenuators, circulators, couplers and filters (duplexer, diplexer or triplexer). Planning DAS includes calculating the maximum loss from base station to each antenna in the systems and does the link budget for the particular area that each antenna covers. The passive DAS design needs to adapt to the limitation of the building regarding the restriction to where and how the heavy coax cable can be installed. A detail site survey of the building needed to be done to make sure that there are cable routes to all antennas.
Active DAS has the ability to automatically compensate for the losses of the cables interconnecting the components in the system by using internal calibrating signals and amplifiers. It does not matter what the distance between the antenna and the base station, all 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 optical fiber up to 6 km in length. Each EU in turns connects to multiple remote units (RU) with thin coax 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 close to the antenna. A wideband active DAS can support multiple radio services, GSM, PCS, UMTS, EVDO, WiMax and WiFi.
Due to the loss and attenuation in the coax cable and passive components, passive DAS is only used in smaller building covers by a small number of indoor antennas to keep its degrading impact on HSPA performance to a minimum. For larger building, active DAS is used because it does not have cable and component loss and can boost the HSPA performance to the maximum. Troubleshoot in passive DAS is difficult and any fault in the systems will not raise an alarm at the base station because there is no surveillance of errors in the system. Active DAS monitors all units in the system and in the event of malfunction; it will send an alarm to the base station which enables the operator to pinpoint the source of the problem. Hence, active DAS is the preferred solution for large building with a lot of indoor antennas.
About the Author
Paul Ngai is an engineering consultant specialized in Telecommunications Network and Systems. He is also the principal of Network Systems Technologies LLC, a telecommunications consulting firm provides planning, analysis, design, testing and operation support services.
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