Although the acronym DAS - Distributed Antenna System – has been around for a while, and you must have come across it at least a dozen times, we feel there are very few sources that provide a comprehensive overview versus just technical aspect of DAS.
What is DAS?
Although concept of DAS has not changed that much, as wireless technologies evolved, so did DAS.
Originally DAS was motivated by signal degradation inside structures: buildings, tunnels, underground parking garages. Inside such structures cellular services users often experience choppy reception (or none at all), dropped calls and low download speeds. Certain building materials and distance from cell phone towers are the main reasons for poor transmission quality.
With advance of wireless technologies, coverage has become a secondary reason for DAS considerations. While, certainly, there are still many areas with just plain coverage issues (underground parking garages, on upper floors of high rise residential or commercial towers, many buildings and tunnels) capacity is becoming the number one reason wireless operators and venue owners choose to invest in DAS. We will cover reasons for DAS in more depth later, for now let’s discuss different types of signal enhancement solutions to help DAS and try to help you understand what will be applicable for one or another type of situation.
To better explain different types of DAS we will break down DAS into two main components: signal source and distribution network.
Distributed Antenna Systems consists of a signal source and RF Distribution network - a network of spatially separated antennas connected via interconnected passive and active components (we will explain passive or active later). The purpose of DAS is to enhance in-building wireless signal quality.
While we pride ourselves for attention to detail, we certainly do not intend to make this a thorough DAS technical textbook. The purpose of this guide is to introduce and explain a general concept of DAS. Some components of DAS are omitted on purpose not to make the text of this guide overloaded with technical explanations
RF signal source
Dedicated - wireless operator’s signal source – BTS (Base Transceiver Station) - is connected directly to DAS.
Off-air - BDAs (bi-directional amplifiers), repeaters.
RF Distribution Network
Remote Radio units
Fiber distribution network
Combiners, splitters, POIs (Points of Interface)
Antennas (donor and indoor/mobile)
Passive and Active DAS
An active component is a device that requires to be powered. Remote Radio units of DAS, many components of Head-End interface are active, hence the name for Active DAS
Components that operate without power source are passive components.
In Passive DAS signal source (be it repeater, BDA, or BTS) is connected directly to passive components (cable, splitters, antennas) while Active DAS will have active components between signal source and network of passive components.
Diagrams of Passive and Active DAS are shown in pictures 1 and 2 respectively.
Typical Passive DAS components
Typical Active DAS components
Note: So called “hybrid” solutions – some combination of Passive and Active DAS or designs that do not clearly fit a description of either Passive or Active DAS – also exist. If you are interested to learn more, we would be happy to explain or send you some additional materials.
Making a a choice between active and passive system if not always simple. While intuitively one may think that cost considerations will tip the scales in favour of a passive system – active components come at a cost – not everything is all that “black and white” as many erroneously think. Availability of space, negotiations with the landlord, neutrality and future friendliness of DAS and even cost – are just a few of the categories that will leave room for both Active and Passive DAS for years to come. We go into details providing arguments for one or another solution in FAQ section.
We invite you to contact us with any additional questions you might have or help you make the right decision about your particular case