A Distributed antenna system (DAS) is a network of antennas, connected to a common source, distributed throughout a building or an area to improve network performance. The spacing between antennas is such that each antenna gives full coverage without overlapping with other antennas, hence reducing the number of antennas needed to cover the whole building. This network of antennas is also power efficient in comparison to a single, larger antenna covering a wide area.
A Distributed Antenna System can be designed for use indoors or outdoors and can be used to provide wireless coverage in hotels, subways, airports, hospitals, businesses, roadway tunnels etc. The wireless services typically provided by a DAS include PCS, cellular, Wi-Fi, police, fire, and emergency services.
A DAS has two basic components, a signal source, and a distribution system. The signal source is the input to the DAS network. It can be an on-site BTS (Base Transceiver Station), a small cell or an off-air system (via an antenna on the roof). The second part of a DAS network would be the distribution system. Once the signal is received by the signal source, it must be distributed throughout the building. There are four types of distribution systems: active (using fiber optic or ethernet cable), passive, hybrid, and digital.
Passive DAS: A passive DAS uses passive RF components such as coaxial cable, splitters, tapers and couplers to distribute signal inside a building. This system runs the wireless signals through “leaky” feeder cables that act as antennas all over the building; the signal leakage distributes the signals throughout a building.
Active DAS: An active DAS system converts the analog RF signal from the signal source to a digital signal for distribution. A master unit performs this analog-to-digital conversion. After conversion, this digital signal is transmitted through fiber optic or Ethernet cables to the antenna systems, which convert the signal back to analog and transmit it throughout the building.
Hybrid DAS: A hybrid DAS system uses both fiber optic cables and coaxial cables to distribute the signal throughout a building. In this system, the analog RF signal from the signal source is converted into a digital signal for distribution. This digital signal is transmitted through fiber optic or Ethernet cables to a Remote Radio Head (RRU) installed on each floor of a building. The RRU then converts the digital signal to an analog RF signal. This analog RF signal is then connected to multiple antennas on that floor with coaxial cables and other passive components.
Digital DAS: This system operates according to the Common Public Radio Interface (CPRI) specification, which allows a base band unit (BBU, which is a kind of BTS) to communicate directly with the DAS master unit and through to the remote units without any conversion to an analog RF interface.
Advantages of using DAS:
- Better defined coverage
- Fewer coverage holes
- Same coverage using a lower overall power
- Individual antennas do not need to be as high as a single antenna for the equivalent coverage
Disadvantages of using DAS:
- Higher cost as a result of additional infrastructure required
- Possible greater visual impact in some applications as a result of greater number of antennas, although they are likely to be much lower in height.