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.

Concept

As illustrated in the figure, the idea is to split the transmitted power among several antenna

Antenna (radio)

An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

 elements, separated in space so as to provide coverage over the same area as a single antenna but with reduced total power and improved reliability. A single antenna radiating at high power (a) is replaced by a group of low-power antennas to cover the same area (b). 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. It should be noted that these antenna must be installed at 200 m from any public building

The idea works because less power is wasted in overcoming penetration and shadowing losses, and because a line-of-sight

Line-of-sight propagation

Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line...

 channel is present more frequently, leading to reduced fade depths

Fading

In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...

 and reduced delay spread.

A distributed antenna system can be implemented using passive splitter

DSL filter

A DSL filter is an analog low-pass filter installed between analog devices and a plain old telephone service telephone line, in order to prevent interference between such devices and a digital subscriber line service operating on the same line...

s and feeders, or active repeater

Repeater

A repeater is an electronic device that receives asignal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances.-Description:...

 amplifier

Amplifier

Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...

s 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 N^1–n/2 and the power per antenna is reduced by a factor N^n/2 where a simple power law

Power law

A power law is a special kind of mathematical relationship between two quantities. When the frequency of an event varies as a power of some attribute of that event , the frequency is said to follow a power law. For instance, the number of cities having a certain population size is found to vary...

 path loss 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

Radiation

In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

into the human body.

Use in Wifi Networks

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.

Regulatory Challenges

There are several regulatory challenges that arise out of DAS deployments. DAS is used in scenarios where alternate technologies are infeasible due to - terrain, zoning challenges for cell towers, infeasible cell tower placements, etc. Regulatory challenges arise at the federal, state and municipal levels. Recent academic works address this. Other industry resources like The DAS Forum also address regulatory issues in their conferences. More details can be found on their website.

DAS Installers

Being that Distributed Antenna Systems are still relatively new, finding a quality installation company can be a difficult task. The reason for this is because large carrier companies tend to hire the “cream of the crop” and consistently train these companies giving them more knowledge and expertise than companies that are trying to break into the industry. This being the case, there are many design and build companies that have a hold on the DAS market in their sector of the US and will only continue to grow, as wireless technology continues to become more and more part of our everyday life.