A distributed antenna system, or DAS, is a network of spatially separated antenna nodes connected to a common source via a transport medium that provides wireless service within a geographic area or structure. DAS antenna elevations are generally at or below the clutter level and node installations are compact.
The idea was described in a paper by Saleh et al.in 1987. These antennas have recently been employed by several service providers in many areas around the United States.
The idea works because less power is wasted in overcoming penetration and shadowing losses, and because a line-of-sightchannel is present more frequently, leading to reduced fade depths and reduced delay spread.
A distributed antenna system can be implemented using passive splitters and feeders, or active-repeateramplifiers can be included to overcome the feeder losses. In systems where equalization is applied, it may be desirable to introduce delays between the antenna elements. This artificially increases delay spread in areas of overlapped coverage, permitting quality improvements via time diversity.
If a given area is covered by many distributed antenna elements rather than a single antenna, then the total radiated power is reduced by approximately a factor N1–n/2 and the power per antenna is reduced by a factor Nn/2 where a simple power-law model with path-loss exponent n is assumed. As an alternative, the total area covered could be extended for a given limit of effective radiated power, which may be important to ensure compliance with safety limits on radiation into the human body.
Using a distributed antenna system to create an area of wireless coverage, it is possible to use this technique to propagate indoor WiFi for commercial uses. It is estimated that only about 5% of commercial WiFi use a distributed antenna system.