Distributed antenna systems for indoor mobile networks

Posted by Mords1944 on November 22nd, 2020

70-80% of mobile traffic comes from buildings inside. This is especially true in urban environments where the mobile user's focus is on high data rates. For mobile 3G networks, only the operation of macro base stations within a few hundred meters of the building can provide sufficient RF level to support indoor voice / data services. In fact, only a few buildings fall into this category. Soft handover in 3G networks further increases the traffic load on the network as each indoor mobile phone can be operated 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 inside a building to provide 3G voice and data services. DAS can be used to isolate the indoor network from the outdoor operating macro cells to eliminate the soft handover of the indoor mobile phone. This reduces the traffic load and increases the speed of the 3G network. For high-speed HSPA data service, indoor DAS can also provide isolation between serving cells and non-serving cells in the outdoor network. This means less co-channel interference in the HSPA server cell and results in higher data rate for the HSPA service. To dominate the building with indoor coverage, directional cellular das installations can be placed in the edge and corners of the building and point towards the center of the building. The total indoor area is dominated by the indoor cell while minimizing leakage to the macro network.

DAS distributes a uniformly dominant RF signal inside the building by splitting the signal from the indoor base station into several 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 coaxial cable, splitters, terminators, attenuators, circulators, couplings and filters (duplexes, diplexes or triplexes). Planning DAS includes calculating the maximum loss from the base station to each antenna in the systems and making the link budget for the specific area that each antenna covers. The passive DAS design must be adapted to the limitation of the building with regard to the limitation of where and how the heavy coaxial cable can be installed. A detailed site survey of the building was to be carried out to ensure that there are cable routes to all antennas.

Active DAS has the ability to automatically compensate for the loss of the cables connecting the components of the system using internal calibration signals and amplifiers. It does not matter what the distance between the antenna and the base station is, all antennas in an active DAS will have the same performance (same noise and downlink power). Active DAS consists of a master unit (MU) connected to several expansion units (EU) with optical fiber up to 6 km in length. Each EU reconnects to several remote units (RU) with thin coax or CAT5 cable up to 400 m long. The MU controls and monitors the DAS performance. The EUs are distributed throughout the building and the RUs are installed close to the antenna. A broadband-enabled DAS can support multiple radio services, GSM, PCS, UMTS, EVDO, WiMax and Wi-Fi.

Due to the loss and attenuation of the coaxial cable and the passive components, passive DAS is used only in smaller building covers of a small number of indoor antennas to minimize the degrading impact on HSPA performance. For larger buildings, active DAS is used because it has no cable and component loss and can increase HSPA performance to the maximum. Troubleshooting in passive DAS is difficult and any fault in the systems does not give alarm at the base station because there is no monitoring of fault in the system. Active DAS monitors all devices in the system and in case of malfunction; it will send an alarm to the base station which enables the operator to locate the source of the problem. Therefore, active DAS is the preferred solution for large buildings with many indoor antennas.