Distributed Antenna System

DAS diagram

A typical DAS node for the Videotron 3G network in Montreal, Quebec

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 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.^[1] 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 channel 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 repeater amplifiers 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 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 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.^[2]

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. ^[3] Other industry resources like The DAS Forum also address regulatory issues in their conferences. More details can be found on their website. ^[4]

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.

References

1. ^ A. A. M. Saleh, A. J. Rustako and R. S. Roman, Distributed Antennas for Indoor Radio Communications, IEEE Transactions on Commun., vol. 35, pp. 1245-1251, Dec. 1987
2. ^ Estimate made by LComm Global http://lcommglobal.com/Distrubuted_Antenna.php
3. ^ Establishing a Regulatory Framework for Distributed Antenna Systems Available at SSRN: http://ssrn.com/abstract=1161209
4. ^ http://TheDASForum.org