Active DAS vs Passive DAS – Distributed Antenna System Topology

Active DAS vs Passive DAS

Distributed Antenna Systems (DAS) can be deployed in various system topologies, some of which you’ve likely heard of such as Passive DAS and Active DAS. It’s easy to get overwhelmed by the terminology and acronyms in the DAS industry.

Fundamentally, every DAS system is made up of 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 thought of separately in order to easily understand the role each plays.

DAS Signal Sources

The signal source is a key consideration on determining topology of the DAS. It also impacts the role of the operators, the overall capacity that can 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 deployments 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 deployments 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 room at the Point of Interconnection (POI) location for the DAS. In most cases, it will also include the deployment 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 the main equipment room, 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 there are two DAS topologies, Passive DAS and Active DAS, which then expand into variants depending on their exact implementation. The difference between them is a function of the way in which 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 in line with MCF2018 vocabulary. Don’t get hung up on the terminology, the important takeaways are the different methods of distribution signal 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 the 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 which should have been Active DAS, deployed as Passive DAS systems to save money. Often these result in inferior performance or outright refusals from operators to connect to them.

Passive DAS systems typically will utilise a central equipment location, however, at times we see designs where they require carrier-POI’s at a variety of riser locations, many of which barely have the space to support a single operator’s equipment let alone all three. This is often an indicator of poor design from the outset and that a site should have been deployed as an Active DAS.

This topology of several carrier-POI’s can sometimes work. It is often referred to as D-RAN (Distributed RAN) solution and involves Remote Radio Heads (sometimes called RRUs) to be 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 POI.

In the long run, these systems can end up causing a lot of heartaches, require complex upgrades (where space often permits none), and 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 its 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 DAS regardless of the flavour will have a Master Unit which accepts all the various signals from Telstra, Optus and Vodafone. The brain within this unit completes all the necessary processing and converts the RF to Fibre (mostly) for distribution around the property to their respective remote units.

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 Power Remotes)

Traditional Active DAS involves the use of higher power remote units which are often fibre-fed from the RF source (although there are a couple of legacy variants still out there) to extend the coverage further away either into a large building or into adjacent buildings.

Where the confusion around Active DAS often arises is that in a traditional Active DAS the remote unit then feeds into a smaller ‘passive’ network of device elements to distribute the signal, consequently, this can be called Hybrid in some contexts (it’s neither here nor there)  The use of Active hardware should primarily be used just to extend the coverage to places within the building that would not have been possible without the topology.

The traditional active DAS topology provides a ‘best of both worlds’ approach as it often limits the active hardware to easy to get to locations which 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 inaccessible locations for straight-forward access.

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

– 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 or nearby electrical switch rooms etc. 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 Room 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 (Hybrid) DAS topology

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 are far less labour intensive than more cumbersome passive 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 remotely all the way to the antenna, allowing easier diagnosis of any issues as they arise.

 Cons

-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 expensive as these will usually involve the replacement or extra installation of active equipment at each antenna location.

-Fibre runs throughout ceilings to antennas are more fragile and sensitive than rigid passive coaxial cabling, making fully active DAS systems far more susceptible to faults caused by disruptions such as tenancy re-fits, etc.

What is Hybrid DAS?

People get really excited by this one, as it sounds awesome! But it’s just a DAS system that has some Passive sections and some Active sections. Common locations close to the operator central equipment room, such as a car park and the lower floors may be covered as a Passive DAS, while some locations further away could be covered with Active DAS. Realistically most Active DAS solutions are more strictly speaking Hybrid systems. It is unusual to have a site complete covered by Active DAS without at least a portion of the site being covered by a Passive DAS (there are exceptions when the equipment room is very far away, like in train tunnels).

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.

So Passive or Active DAS – BOTH!

You may get told that Active DAS is just straight up better than Passive DAS, probably by someone 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

Both Active and Passive topologies are relevant today and are not going anywhere tomorrow. Each has its place and should be used appropriately. Both 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 else risk having a system installed that nobody will connect to.

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 project.