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Fiber DAS (Distributed Antenna Systems) & Small Cell Systems

Sep 09, 2013

What Is a DAS?

We know that DAS technology has been available for many years, but what exactly is a Fiber DAS system and why is DAS becoming so important?

DAS stands for Distributed Antenna System. In the past, indoor cellular coverage became unreliable due to issues with bringing RF signals inside of large buildings. A Distributed Antenna System has become an efficient method of dealing with isolated patches of limited or no coverage inside large structures. This is done by installing a network of small antennas throughout a building, which serve as repeaters.

Fiber Optic Distributed Antenna System DAS

With demand for wireless services constantly growing, it has become an absolute necessity to ensure that people within all areas of buildings receive a reliable wireless service. This is especially true from public safety perspective, for ensuring first responders can connect via wireless connectivity tools such as wireless phones and radios to help perform their task of saving lives. Subscribers or consumers are demanding more coverage from their providers as well. Whether it be an emergency situation, or a work or domestic situation, voice, data, and multimedia needs must be met - especially if there's an emergency. Today, DAS has proven to be the most reliable means for both wireless operators and building owners to provide reliable cellular wireless coverage within large buildings.

The Drive To Do More With Fewer Assets.

Innovation in DAS and optical networks is now being driven by the need to do more with fewer assets. Cellular service providers are required to support even more subscribers who are increasingly using more data-intensive applications like video, so it has become essential for small-cell networks to provide the required capacity and coverage. Today, service providers are looking more towards DAS as part of their outdoor network plans in order to create the small cells they require, and mobile operators are now seeking to employ the latest DAS and fiber technologies to generate leaner and more cost-effective operations.

How a "Distributed Antenna System" (DAS) Works.

With a typical in-building DAS, a cellular source like a wireless base-station uses a wired landline connection or an antenna to provide a reliable wireless signal to a building. An antenna is connected to a central controller, which is connected to the base station of the wireless carrier's network. Within the building and connected through coaxial and fiber cabling, a DAS consists of bi-directional amplifiers, head-end equipment, in-building antenna access points, and remote units for fiber distribution. With DAS, a wireless signal is distributed via a system of remote antennas and managed hubs to a sequence of connected outdoor or indoor multi-technology, multi-band radio heads.

Passive and Active DAS.

DAS can be either passive or active: With a passive DAS, cell-phone signals are sourced from antennas located on the roof and run via leaky feeder cables throughout a building, meaning that the signal is distributed via signal leakage. Alternatively, RF over fiber DAS system is an active DAS because the signal is passed via fiber cables from roof antennas as well as underground Carrier feed(s) in most cases, enabling the system to boost and amplify signals as required without as much loss as Passive DAS.

DAS operates on the RF band licensed to specific wireless carriers, which means that no company can undertake its own DAS deployment without the involvement of at least one carrier.

The Head-End of a DAS.

The head-end of a DAS is where the host unit connects to mobile operator base stations. Head-ends require power, space, and cooling. At the head-end, base stations are typically located by service providers to deliver the cellular signal. The signal is taken by the main hub, digitized, and distributed by way of a fiber optic network that is high-bandwidth to other hubs and radio heads. The radio converts the signals at the antenna from RF to digital, and from digital to RF.

In direct contrast to older analog systems where RF signals were transported over coaxial cabling and the performance weakened in proportion to the distance of the remote antenna from the main hub, digitizing the signal on the fiber means that the signal from the mobile can be transported, without compromising strength by DAS, to any connected remote antenna, regardless how far from the base station and the main hub it may be.

Where is DAS Used?

Distributed Antenna Systems are typically used in office buildings, airports, stadiums, suburbs, urban canyons, and other venues where service providers are required to enhance their network capacity and/or coverage. By using remote antennas to focus a base-station's signal on a specific area, a DAS has higher capacity and is capable of delivering consistent coverage over the designated area. In fact, some DAS schemes support thousands of subscribers and extend for miles. In order to protect the mobile network and keep it out-of-sight, DAS remote antennas are positioned on street cabinets, lamp posts, telephone poles, and other street fixtures.

DAS versus Small-Cell Technology.

Often we see Distributed Antenna Systems being evaluated against picocells and microcells which are small-cell technologies that providers use to tackle capacity or coverage problems. However, while picocells and microcells may be cheap and easy to install, they typically offer low capacity because they only support one service provider's frequency band. And, because a picocell only supports a few simultaneous subscribers, it has limited capacity and serves a comparatively small area. In addition, costs increase because each microcell or picocell needs its own backhaul connection to the network.

A DAS is capable of supporting multiple services and frequency bands with just one set of antennas and one backhaul connection in areas where subscribers demand coverage for all major service providers.

Single Carrier Small Cell System Design Sample.

Small Cell System For Single Carrier

Fiber Efficiency Solutions.

Being able to access the required fiber to make the backhaul connections is probably one of the key challenges when rolling out small-cell architecture. A service provider would potentially need to build a whole new fiber network to support a small cell deployment or a DAS. However, a DAS has various ways of making more effective use of fiber than microcells or picocells.

To begin with, only one backhaul connection is required by a DAS for an entire DAS network. Whereas each and every microcell or picocell requires its own backhaul connection. With DAS, thousands of subscribers can be served by a fiber backhaul connection, whereas only a handful of subscribers would be served by a microcell or picocell's fiber connection.

Next, a built-in network aggregation point is provided by a DAS. Out of the main hub, today's modern DAS can do 8:1 aggregation, gathering up to eight different frequency bands over just one fiber pair; whereas service providers would need to use separate microcells or picocells to support each frequency band. Various fiber saving technologies, like DAS head-ends, can also be used. DAS head-ends are capable of combining the capabilities of two or more base-stations, allowing the mobile operator to improve capabilities by adding another base-station. The bonus is that no additional antennas, head-ends, or radio heads are required.

Optional fiber-saving technologies can be achieved using advanced fiber solutions:

  • Capacity Aggregation.

    Today's DAS resolutions can broadcast the capacity from a connected base-station to all radio heads or remote antennas in the system. Capacity aggregation simplifies network design and management and saves on base-stations and head-end units.

  • All-Digital Transport.

    With digital DAS solutions, RF signals are transported from the head-end in digital format to the remote antenna unit or the radio head, which means that there's no attenuation of the signal between the head end and antenna. Thus, the digital signal can be simulcast from the head-end to all remote antennas in the system.

  • Fiber-Saving Technologies.

    There're a few fiber-saving technologies that are capable of minimizing amount of fiber to be pulled and spliced. DWDM (Dense Wave Division Multiplexing) or CWDM (Course Wave Division Multiplexing) can be used to increase the data-carrying capacity of both individual fibers and fiber pairs by multiplexing 80 DWDM or eight CWDM wavelengths on a single fiber. Cost-effective 10-Gbit transport solutions are another option, which greatly decrease the amount of fiber required in a DAS network. Over one single fiber pair, up to 225 MHz of spectrum can be supported by a 10-Gbit transport; over three times the capacity of most baseband transport options. Coupled with DWDM or CWDM, this offers substantial capacity delivery over a single fiber.

A Distributed Antenna System is an effective and efficient way of distributing RF spectrum from a shared RF source, where BTS (Base Transceiver Station) signals are connected to a host and the BTS's signals are disbursed to numerous remote antenna positions via a fiber network.

How DAS Uses Existing Fiber-to-the-X (FTTx) Installation.

Service providers can avoid having to create a whole new fiber network for a DAS by partnering with wireline carriers and piggybacking DAS traffic on existing FTTx (Fiber-to-the-x) installations. In return, wireline carriers are participating by offering certain services to wireless service providers, like providing physical leased space for the network gear and infrastructure, or alternatively, providing access to fiber and offering optical efficiencies.

DAS systems overlay very effectively onto FTTx networks. Typically, the fiber in an FTTx network is hosted in a serving office with easy access to the required facilities to host BTS resources. The wireline operator leases the space and provides access to fiber to the remote nodes, HVAC, backhaul, and electrical, which means that the wireless operator is not required to create a new site to locate BTS resources. This results in a situation where the wireline operator at the serving office has the fiber running deep into the network, effectively serving its FTTx investment.

The wireline carriers, by either offering wavelength services from the FTTx plant or utilizing spare dark fibers, is able to achieve regular income by leasing fibers to wireless operators for their use. The wireless operators in turn are able to supply the BTS capacity to remote DAS nodes. This scenario offers the wireline carrier further opportunities to monetise their investments, whilst the wireless operator achieves both a time-to-market efficient result and a cost-effective solution to delivering wireless services.

Overlaying DAS on an FTTx network allows a wireless carrier to:

  • Monetize available dark or spare fibers that may be reserved for either spares or expanding the wireline network.
  • Optical splitters linking the base-station wireless signal to the FTT network can be used for distributing to remote units.
  • Where dark fibers are not available, wavelength services can be offered.
  • Minimize fiber usage by using DWDM or CWDM to split out wavelengths for use in a DAS.
  • Use existing fiber assets to speed time-to-market, thus taking advantage of already approved zoning applications.
  • House wireless carrier base stations in existing offices, basements, or other enclosures, to provide signals for the DAS.
  • Minimize environmental impact and cost by utilising common HVAC, backhaul, and power.
  • By using easy-to-zone, aesthetically acceptable solutions overlaid on existing real estate and infrastructure, time-to-service is minimized.

Common Public Radio Interface (CPRI) and DAS.

Trying to match Distributed Antenna Systems with a mobile base-station has been an ongoing challenge. Using RF as a means of interfacing only adds additional cost and complexity to the deployment. Up until this point in time, DAS equipment has been unable to use the CPRI defined for base stations, but we're now seeing DAS equipment that does use CPRI, and it is solving some very important issues.

Between the radio equipment control (base-station or REC) and the radio equipment (radio head or RE), the publicly available specification for the key internal interface of radio base stations is defined by CPRI. A number of companies are now cooperating to classify the CPRI specifications, and these include Nokia Siemens Networks, NEC, Ericsson, Huawei, and Alcatel-Lucent. Several revisions of the CPRI have been made, and currently we're at version 5.0.

The original idea of CPRI was to come up with an open benchmark for interfacing base stations with radio heads. But because CPRI is neither public nor common, it can't truly be an open benchmark. What has happened, though, is that each manufacturer has created its own version of CPRI that is only effective when interfacing its own base stations with its own radio heads - similar to what occurred with the ISDN (Integrated Services Digital Network) for (PBXs) Public Branch Exchanges.

DAS equipment is not produced by major manufacturers of base stations, and because each BTS manufacturer's CPRI interface is unique, DAS systems distributed by third-party OEMs in the digital domain have, until now, been unable to directly interface DAS head-end equipment with base stations through CPRI.

Therefore, it is through the RF signal that DAS head-end interfaces with base stations, and this has been the case for more than 20 years, or since the inception of DAS. The problem is that there's a huge mismatch of power that must be accommodated between base stations and DAS head-ends to allow this interface to work. With a typical base station putting out approximately 40 W and a DAS head-end taking in approximately 0.25 W, feeding 40 W into a DAS would destroy the head-end. Therefore, before a base-station can interface with a DAS its power must be dramatically reduced.

Challenges faced when reducing the output of base-station power include:

  • Heat.

    Because RF attenuators produce a lot of heat, more money needs to be spent on air-conditioning in DAS deployment areas.

  • Complexity.

    Attenuators are racks of passive equipment which reduce base station power. External plumbing, like combiners, splitters, circulators, and so on, between the base station and the DAS adds to the cost, complexity, and size of the deployment.

  • Cost.

    All this extra plumbing required for attenuators and the extra manpower resources needed in both designing and deploying the RF plumbing adds additional operating expenditure to the overall deployment, thus making the DAS business case for mobile operators even worse.

  • Space.

    A DAS deployment becomes much larger than it necessarily needs to be due to the racks of attenuators taking up unnecessary floorspace. It often occurs that intended facility has insufficient floorspace to accommodate the entire deployment, requiring a separate off-site facility to be built. Many mobile operators find these additional expenses enough to kill the deal.

  • Inefficiency.

    One of the major cost drivers in a base station are the amplifiers. For their base stations, mobile operators must invest in large, power-hungry amplifiers, then in actual deployment the power sustainability is dramatically reduced.

The need for all this power, plumbing, saving space, and cooling costs involved in DAS deployment can be eliminated when interfacing directly with a base station via CPRI instead of RF. It is critical to consider all these essential elements when evaluating both the financial and practical viability of DAS employment. Ability of DAS manufacturers to use CPRI instead of traditional RF will dramatically improve both the deployment time and the business case for mobile operators, thus increasing the market reach of DAS. Direct cooperation will be required from manufacturers of base stations, because they will need to develop custom CPRI interfaces to work with each manufacturer of base stations.

Installation, Inspection, Approval, Certification, and Maintenance.

Distributed Antenna Systems (DAS) and Small Cell Systems for signal enhancement require initial approvals from carriers and fire marshal's office/department. Installed DAS & Small Cell systems also require regular annual re-inspections and re-certifications to stay in compliance with fire codes. In addition to initial installation, inspection, and certification for approval by mobile service provider(s), we provide regular annual re-inspections and re-certifications of pre-installed signal enhancement systems nationwide. This is required by fire marshal's fire code enforcement team to meet respective city's fire codes.


It can be seen that deploying DAS with fiber networks does provide many advantages. Mobile operators will continue looking for reliable and robust services for their subscribers, and it seems that fiber and DAS will be playing major roles in service rollouts. Whether coverage is required in government buildings, hospitality, healthcare, public venues, higher education campuses, or to improve public safety coverage, one thing is for certain - there is a DAS solution for you. Please submit location details for quote of Small Cell / DAS installation service or Re-Inspection / Re-Certification service to get started.

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  • Thorough explanation of distributed antenna systems. I have a question, how many new stadiums and arenas have DAS installed in them? I’ve read a lot about DAS and it seems like they’re made for structures like that. With public safety boosters becoming mandatory in some areas, it would make sense to put them and DAS in as well.

    Tyler Fitzgerald on
  • I wanted to see if I can answer Paul’s question, “How far underground do you get cell phone reception?” It depends on the material blocking the cell phone signal. As you’ve probably experienced when you’re in an elevator, cell phone signals can get weak or even blocked when you’re surrounded by materials blocking the signal being sent from a distant tower to your phone. The same applies for underground. There’s no set distance to block a signal, but rather, how much materials are there interfering with or blocking the signal. You might get service in a basement or even a sub-basement, but chances are the signal will be very weak because of the materials weakening or blocking your signal. There are ways to get around this as seen in this blog—DAS. These systems boost the signal throughout the building so you can get a good signal wherever you are, regardless of the materials surrounding you.

    Mike Brooks on
  • You certainly know your stuff so maybe you can answer me this: How far underground do you get cell phone reception?

    Paul Cooper on
  • It sounds like a DAS setup is a powerful way to increase your wireless network’s power. It also sounds like DAS design and construction is the perfect fit for certain areas (such as some of the ones mentioned like stadiums and airports). Are these becoming fairly common because I know wireless service is expected by people in these areas. I mean, who wants to try using a cell phone in a stadium and get zero bars?

    M.J. Pritcherd on
  • This was something I haven’t seen a lot of; a discussion of how DAS works and the situations where it’s advantageous. I’ve read a lot about cell phone boosters and how they can help people in homes and businesses with improved call clarity, etc. I’m glad I read this because I’ve had friends ask me about DAS and I wasn’t sure. Thanks for posting this.

    Wally G. Newton on

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