Distributed Antenna Systems - DAS
A Distributed Antenna System (DAS) is an antenna infrastructure used to improve the service of one or more Wireless Service Provider (WSP) within a defined coverage area. This coverage area can be either an indoor venue (e.g., office building, stadium, or airport) or an outdoor environment (e.g., college campus, metropolitan area). Unlike a traditional cell site which has antennas mounted on a single tower or building, a DAS uses several antennas spread across the target coverage area. This coverage area is determined by a WSP based on their need to improve signal quality or system capacity.
There are two key components to any DAS, the signal source and the distribution system. The selections made for each of these components determine the level of service provided.
DAS Signal Source
The first choice typically made by a WSP is what type of signal source is appropriate for the DAS. There are two types of signal sources which are used to connect the DAS to the WSP’s network:
- Bi-Directional Amplifier (BDA) – operates by amplifying and retransmitting the existing outdoor signal. A BDA uses an outdoor antenna, typically mounted on the rooftop, to bring the outdoor signal in to the building. This signal is amplified and fed into the distribution system.
- Base Transceiver Station (BTS) – this is the same radio that is used at a cell site and provides both coverage improvement and additional capacity to the venue. A BTS is typically installed in the main IT room of a building and connects to the WSP network via T1 or other Ethernet connection. The output of the BTS is routed into the distribution system.
DAS Distribution System
The DAS distribution system transports the signal from the source to the users located throughout the venue. The type of distribution used is determined by the size of the venue and the level of service required. There are two main types of distribution systems which are used in typical DAS systems:
- Passive DAS – is a network of coaxial cable, power dividers and antennas. Due to the lossy nature of coaxial cable, this type of DAS is typically used only in smaller facilities.
- Active DAS – converts the radio frequency (RF) signal to another format to transport throughout the venue. There are several components to an active DAS
- Main Hub – converts the RF signal to another format (e.g., fiber optic or Ethernet)
- Distribution Medium - fiber optic or Ethernet cable is used to transport the signal to the remote equipment which is typically located telco or IDF closets. The distribution medium can often use existing cable in the facility.
- Remote Equipment – converts the signal back to RF and connects to antennas via coaxial cable.
- Antennas – are located throughout the venue. The number of antennas required depends on the size of the venue and the quality of service required.
Which Solution is Best?
There is no direct answer to this question. The right DAS solution depends largely on the coverage requirements of the DAS and the unique characteristics of the venue to be covered. The answer can also be biased by the individual interests of the parties involved. Only an expert advisor can ensure that you get the right DAS for your building. If you think you need a DAS in your building or have been contacted by a WSP who would like to install one please don't hesitate to contact us.