Active DAS vs Passive DAS – Distributed Antenna System Topology

Active DAS vs Passive DAS

Distributed Antenna Systems (DAS) can be installed in various system topologies, some of which you’ve likely heard of, such as Passive DAS, Active DAS and even Hybrid DAS. It’s easy to get overwhelmed by the terminology and acronyms in the DAS industry. This article has been recently rewritten to align with MCF2022, as it has clarified the use of terminology and better aligned Australian terminology with the international market (now we just need to get everyone in the habit of using the terminology correct)

Fundamentally, every DAS system comprises two components, the RF Signal Source and the Distributed Antenna System (Signal Distribution throughout the building). Whilst they are both critical elements of the working in-building coverage solution, they can be considered separately to understand the role each plays easily.

Active DAS, Active DAS vs Passive DAS – Distributed Antenna System Topology

DAS Signal Sources

The signal source is a key consideration on determining the topology of the DAS. It also impacts the role of the operators, the overall capacity that can be delivered, the equipment space required and the complexity of installation. Without a signal source, a DAS won’t provide any benefit to the property, this is where the majority of projects fall over.

Other than legal “over the counter” licensed signal repeaters, all the below installations require engagement with the Australian mobile operators. It is not uncommon to have a Distributed Antenna System installed at a property without a carrier connection of any kind. This is typically a result of a lack of understanding where nobody has been tasked to engage the operators and guide their connection to the system.

Wireless Coverage Solutions provide turnkey DAS solutions through Design, Installation, and engagement to facilitate an Operator Connection. If you are not getting quality outcomes in your Distributed Antenna System Installations to date you need an end to end solution with accountability throughout the process.

Base-Transceiver Stations (BTS)

This is the most typical source of RF signal for a DAS connection from Telstra, Optus and Vodafone. It involves the installation of a multi rack-based solution in a central equipment location at the Point of Interconnection (POI) location for the DAS. In most cases, it will also include the installation of an operator’s UPS equipment.

These forms of installation require some planning in terms of electrical, cooling and space requirements, particularly when allowing space for all 3 operators to connect as required by MCF2018.

A Base-Transceiver Station (or commonly known as just a Base Station) will typically provide the most concise and robust deployment solution with all equipment centralised in one place, with backup batteries. It also provides for the deployment of several RF bands to maximise site performance and to provide for ample future upgrade options.

Active DAS, Active DAS vs Passive DAS – Distributed Antenna System Topology
Active DAS, Active DAS vs Passive DAS – Distributed Antenna System Topology

Distributed Base Stations (often called Remote Radio Units)

From time to time, there may be a good reason to distribute RF sources around the building whereby operators need to connect at multiple DAS POI locations. This presents some installation challenges as every location requires power, space and sometimes cooling. Each operator will require their own infrastructure in these locations, and often will require a separate unit for each RF band. This solution sounds complicated but in essence, it’s just multiple Base Station locations around the building.

Often this points-of-interconnection are in tight communications risers which might have had enough space for a multi-carrier Active DAS Remote unit (explained below), but have little chance of fitting in 3 operators worth of Remote Radio Units. Where possible the goal should be to centralise all this power-hungry equipment into a single location, however from time to time there is a justified reason to explore this topology with proper planning. Many times, this is done as an alternative to Active DAS to save money, however, it just moves the costs from the DAS to the Base Station deployment and adds unnecessary complexity.

Repeaters/Boosters (sometimes called Off-Air Repeaters)

Unlike the solutions above which produce onsite generated RF signal and bring with them a guaranteed amount of capacity (which is a must for any medium-large building), a repeater takes an outdoor signal, amplifies it, and sends it around a small area.

Repeaters are a common solution for smaller deployments such as residential, industrial, or small office space. They can still connect to a Distributed Antenna System, but normally you would only run a handful of antennas with a repeater as the signal source. The limitation in the solution is related to the output power of the repeater into the DAS being very small compared with the other types of signal sources.

The advantage of repeaters is they have a much lower cost of deployment than a full BTS/RRU operator deployment outlined above, and they can sometimes be deployed without consultation with the operators. Deployment is typically much faster, but only suited to smaller indoor areas. We have an entire in-depth article dedicated to signal boosters.

Active DAS, Active DAS vs Passive DAS – Distributed Antenna System Topology

Distributed Antenna System Topologies

Fundamentally, three common DAS topologies—Passive DAS, Hybrid DAS, and Active DAS—expand into variants depending on their exact implementation. The difference between them is a function of how the RF signal is distributed around the building.

Different markets define DAS terminology in different ways. The below is true for what we see as the most commonly accepted definitions in the Australian DAS market and is in line with MCF vocabulary. Don’t get hung up on the terminology; the important takeaways are the different methods of distribution signals and when to do one or the other.

Active DAS, Active DAS vs Passive DAS – Distributed Antenna System Topology

Passive DAS

A Passive DAS system has no active components, meaning the RF signal is distributed from the signal source around the building with devices that require no electricity, just combinations of coaxial cable, splitters, and antennas. In Australia, Passive DAS will typically involve a 4 Input 4 Output Multi-Network Combiner (MNC) which is used at as the Point of Interconnect for Telstra, Optus and Vodafone.

Pros

– Next to zero ongoing maintenance, passive DAS devices very rarely fail and consequently they are rather robust solutions.

– Easier to add on additional RF bands for future upgrades as passive DAS components can typically support the full range of frequencies used by cellular technologies typically
700-3800MHz

– Typically cost-effective to initially deploy and minimal maintenance required other than protecting the asset during renovations and the like.

Cons

– Coaxial cable distribution can be space-intensive on vertical risers which may not have enough capacity to support a Passive DAS. Coaxial cable can often be cumbersome to install.

– The distances a Passive DAS can provide coverage from the signal source location can be limited due to RF power loss through the system and may not be sufficient for very large buildings.

– A Passive DAS is essentially a ‘dumb’ solution when it comes to monitoring. If things do go wrong (normally cables being cut and the like) it can be harder to know where or even if something has happened until a customer complains

A note on Passive DAS Limitations

We see a lot of systems that should have been Hybrid DAS deployed as Passive DAS systems to reduce upfront costs. Often these result in inferior performance, increased connection costs or even outright refusals from operators to connect to them.

Passive DAS systems typically use a central equipment location. However, we sometimes see designs that require carrier connections at various riser locations, many of which barely have the space to support a single operator’s equipment, let alone all three. This often indicates poor design from the outset and that a site should have been deployed as an Active DAS.

This topology of several carrier connections can be viable. It is often referred to as D-RAN (Distributed RAN) solution and involves Remote Radio Heads (sometimes called RRUs) being deployed around the building. It requires a lot of careful planning to ensure there is enough space, power and cooling to handle all the required equipment for all three operators at each carrier connection point with room for future upgrades, whether it be additional sectors of future bands.

In the long run, these systems can cause a lot of heartache, require complex upgrades (where space often permits none), and incur expensive operator costs when they really should have been deployed with a different architecture from the outset. Be wary of vendors insisting they can deploy a site passively when you are hearing otherwise, as it’s likely to have long-term implications on all fronts and is worth the phone call to double-check. 

Active DAS

Within Active DAS there is often a bit of confusion, as it is a general term that essentially describes any DAS which has active elements (i.e. they require power) which could be at the antenna, on a floor-by-floor basis, or a larger segment of the building such as a cluster of floors.

All Active hardware should primarily be used to extend the coverage to places within the building that would not have been possible without the topology. There isn’t generally much of a performance benefit; it’s done out of necessity as it only adds cost, complexity, and maintenance.

Regardless of the flavour, all Active DAS will have a Master Unit that accepts all the signals from Telstra, Optus and Vodafone. The brain within this unit completes all the necessary processing and converts the RF to Fibre (or another structured cabling, but mostly fibre) for distribution around the property to their respective remote units.

It’s how those remote units function that determines whether we should strictly be calling to a Active DAS, or whether its indeed a Hybrid DAS

Active DAS, Active DAS vs Passive DAS – Distributed Antenna System Topology
Active DAS, Active DAS vs Passive DAS – Distributed Antenna System Topology

Traditional Active DAS Remotes (Also called High/Medium RF Power Remotes)

Traditional Active DAS involves using higher power remote units, which are often fibre-fed from the RF source to extend the coverage further away into a large building or into adjacent buildings. Strictly Speaking, this topology requires a combination of Active Remotes, which then feed Coaxial sections that come after these remotes, constituting a ‘Hybrid DAS’.

The traditional Hybrid DAS topology provides a ‘best of both worlds’ approach as it often limits the expensive Active hardware to easy to get to locations that can be accessed for future maintenance. It keeps the more robust coaxial cabling out onto the floors or areas where more delicate cabling would likely be damaged. It can also provide for a variety of upgrade paths

Pros

– Minimal ongoing maintenance, but when required the equipment locations are often accessible locations for straight-forward access.

– Can extend cellular coverage much further away from the RF Signal source in the Main Equipment location.

– Fibre requirements for vertical runs up risers are also much less space-intensive than equivalent coaxial for passive systems.

– The status of Active units themselves can be monitored remotely for any faults.

Cons

– Significant additional space required for deployment of remote units, typically within risers. Still a lower space requirement than a multi-carrier RRU solution.

– The cost of active deployment is significantly higher than a fully passive DAS deployment would be.

– Fibre runs between the Main Equipment location and the Remote Units need to be installed, adding an additional degree of complexity to the overall site design. Fibre although less cumbersome to install, has its own set of termination and testing requirements, requiring additional installation skillsets.

– Traditional Active systems still cannot detect issues further down the passive chain of DAS elements to the antennas.

‘Fully’ Active DAS Remotes (sometimes called Low Power Active DAS)

By comparison, and often where people’s brains go to when you mention Active DAS, is the idea of a ‘Fully’ Active DAS which will either be fibre (or in some cases Cat 6 cable) all the way to a low-powered remote unit, which also contains the antenna and as such there are no passive RF elements in the system. This essentially means that every single antenna is a remote unit, that requires its own power (which can get expensive fast). This type of DAS is rare in Australia, and typically, when someone in the Industry is referring to Active DAS they mean the former Traditional DAS topology, which still incorporates Passive components. But strictly speaking, a complete Active DAS would have no passive components in the system and be structured comms cabling all the way to the antennas themselves.

Active DAS, Active DAS vs Passive DAS – Distributed Antenna System Topology

These systems do often have a lot of whiz-bang features. They can allow for MIMO within the unit (more info on MIMO here). However, they do have some challenges – price, long term maintenance and access requirements. In fully active DAS upgrade paths often require additional hardware or complete swap outs which may have been more flexible in another configuration.

Pros

-Full fibre/CAT-6 Active DAS installations can be less labour-intensive depending on the environment than horizontal floor coaxial DAS installations. There is also a lower degree of complexity without coaxial terminating/testing etc.

-The overall project cost may be lower than other options when labour is considered, even if the cost of parts and materials is higher. The cost of providing power at each antenna location can negate this though.

-Fully active DAS’s can usually be monitored to the antenna remotely, allowing easier diagnosis of any issues as they arise.

-This architecture can best be suited to long sprawling cable pathways, for example, air port gates or retail centres.

 Cons

-Further consideration is required in protecting the fibre from damage, might require a significant amount of conduit to be installed, this is particularly hard on existing buildings. Likewise, these topologies are more susceptible to faults caused by disruptions such as tenancy re-fits, etc.

-As each antenna as in itself a low-powered remote unit, each antenna needs power and therefore, each antenna is a potential point of failure.

-Upgrade paths for such fully active DAS’s can be fairly involved if you require additional bands in the future if capacity wasn’t allowed from day 1

-When looked at side by side, often the site wide energy usage of these systems will be higher than their counterpart, which works against a lot of building energy-saving initiatives. Although hopefully this improves in time as more innovative RAN/Signal Source solutions are deployed by the operators

Active DAS, Active DAS vs Passive DAS – Distributed Antenna System Topology

Small Cells

You may have heard of Small Cells as a proposed coverage solution and they often get casually thrown into the active DAS category. Although they are a ‘signal source’ in their own right they also provide for the signal distribution so it’s a Source and System in one. They are essentially a ‘Fully Active’ remote that will typically have limited band support for a single operator. They can form part of a connectivity strategy and are particularly valuable in high dwell areas to add additional capacity (including 5G). Typically we recommend against a building wide deployment of small cells, as a property would require a triplicate deployment to get coverage from the three major operators which quickly becomes clunky and likely more expensive and less robust than a Multi Operator Active DAS would have been to begin with.

Digital DAS

This terminology was a recent addition in MCF2022 but is not overly abstract. Essentially this is an Active DAS (‘Fully’, ie that has fibre all the way to the antenna, and no coaxial at all). Distinctly not Hybrid DAS.

The key distinction has nothing to do with the architecture of the distribution of signal, and everything to do with how the operator connects their Signal Source. In every DAS distribution today (and likely for some time still) the operator generates and connects an RF signal into the DAS (only to convert it back to fibre later). In the future we hope to have an end state where RF signal doesn’t have to be generated until the endpoint, this will make DAS rooms require less space, power, cooling and cost. It would greatly simplify the connection process. To achieve this requires a lot of support of standards between equipment manufacturers, but the tide is slowly changing

So Passive or Hybrid or Active DAS – DEPENDS!

You may get told that Active DAS is just straight up better than Passive DAS, probably by a Distributed Antenna System installer trying to sell you an Active DAS which attracts a premium because of additional hardware requirements. The reality is Active DAS should be used out of necessity because of building size more so than the performance benefits. Beyond cost, Active DAS has extra considerations such as additional power requirements, more possible points of equipment failure, and more complicated upgrade paths requiring additional hardware or complete swap-outs.

Likewise, you may have one party adamant (or hopeful) that a site can be deployed passively when realistically this will mean compromises in system performance and often take the form of higher than usual operator costs in the form of distributed connection points discussed above.

All topologies are relevant today and are not going anywhere tomorrow. Each has its place and should be used appropriately. All Topologies provide for a 5G DAS upgrade path in the future.

Not all Active DAS is created equal, much like shopping for flatscreen TVs. Be sure to get expert advice when deploying Active DAS, and more than ever, engagement with the lead operator is required to ensure it is a system they support and is configured appropriately, or else risk having a system installed that nobody will connect to (it really does happen, and costs big $$$).

At the end of the day its a case of the right solution for the right problem. The layout of the building, the size, and the number of legitimate, viable DAS rooms will all determine the best topology. It’s not all about optimising upfront cost, performance, maintenance, cost of connection, sparing will all form part of the total cost of ownership

Are you considering deploying an Active DAS in your building? Wireless Coverage Solutions have deployed a variety of large venues with Active DAS (such as the MLC Centre). Do not hesitate to reach out to us for assistance with your next Distributed Antenna System installation.