When we talk about ‘In-Building Wireless’ (IBW), what do we actually mean?

Though ‘IBW” may sound like one single solution, that term encompasses an entire class of diverse, specialized solutions and technologies that enterprise owners, architects, and engineers must understand in order to implement the right IBW solution for their venue that balance the costs and benefits of each solution.

Historically, when considering Cellular IBW (as opposed to Wi-Fi, radio, Zigbee or other protocols), the technologies being considered would fall into two more significant categories: either distributed antenna systems (DAS) or small cells. The landscape has gotten more complicated and this dichotomy is no longer as pronounced as it once was. This is good for consumers because there’s more choice and costs have come down – but more choice involves more customer education and decision time. 

There’s no such thing as a ‘one size fits all’ solution in the world of Cellular DAS.  The size of the venue, layout, building materials, user counts, number of required carriers (AT&T, Verizon, T-Mobile or Sprint), signal protocol (3G/4G, SISO, MIMO) are just a few factors that influence some of the technology choices.


A Distributed Antenna System (DAS) distributes signal, but it generally doesn’t generate the signal itself. A DAS needs to be fed signal from another source. Generally, this source is either off-air (via an antenna on the roof), an on-site Base Transceiver Station (BTS), or small cells.

DAS is a scalable solution, meaning it can be expanded to cover larger spaces and unusually shaped areas. For example, when cellular service is added to a subway tunnel, arena or in an area such as a tunneled mountain road, it’s likely being provided with DAS.

Traditionally, a BTS-fed DAS was the de facto means of delivering a uniformly high quality of service (QoS) cellular system into an area with capacity issues or challenging coverage areas such as:

  • Large indoor areas such as high-rise office buildings, hotels and large hospitals
  • A contiguous outdoor area, such as a stadium or sports arena
  • A combination of indoor and outdoor environments, such as a college campus

Active DAS

Active DAS simply means the components require a power source to operate. Building owners, property managers, and enterprise customers have a number of options when there’s a cellular coverage problem. Traditionally, Active Distributed Antenna Systems (DAS) were synonymous with BTS-fed systems and were the option for many customers. These systems are typically expensive ($2 to $4 per square foot) and take months to deploy, although for customers with capacity issues (think football stadium or airport) it is the preferred solution. These systems usually were fed via a BTS and customers needed to sign retransmission agreements which each carrier whose signal they wanted to boost.

Because of the large areas typically covered with these systems, Active DAS systems were usually implemented with fiber, since it does not have the distance limitations between active components that coax and CAT5/6 has.

Leading players in this category include Commscope and Corning, with other smaller, niche players including Solid, Cobham, Comba, JMA Wireless, Zinwave and G-Way Microwave. For many end-user customers it is difficult to differentiate these products on technical grounds. Price and customer support are as important differentiators. 

Off-Air DAS

Off-Air, or Passive DAS, uses a Donor Antenna on the exterior of a building that connects to the closest cell site and re-amplifies signal in a building (using a device called a Bi-Directional Amplifier or BDA). This is essentially a glorified antenna system. This means that Passive DAS heavily rely on adequate signal from a nearby cell tower. So, if you have good signal at the Donor Antenna this is a cost-effective solution for many venues that simply have a coverage (not a capacity) problem. 

A passive DAS uses passive RF components such as coaxial cable, splitters, taps and couplers to distribute signal inside a building. The farther the antenna is from the signal source and any amplifiers, the more attenuation (loss) there will be in the power broadcast from that antenna.

There are a number of manufacturers of Passive DAS systems using BDAs: Wilson Electronics, SureCall and Nextivity e.g. Nextivity’s Cel-Fi Quatra product is an excellent choice is you have existing CAT5/6 in your building (most products utilize coax).

These solutions had been frowned upon the carriers prior to FCC approval of the technology in 2013, but that has ended and for many customers it is the solution of choice. 

Hybrid DAS

A hybrid DAS combines functionalities of both passive and active systems. So-called Remote Radio Units (RRUs) are separate from the antennas, allowing the system to use both fiber optic cable and coaxial cable to distribute the signal through a building. These systems can combine signal sources from small-cells and off-air antennas and utilize much of the existing cable infrastructure.


While an Active Fiber DAS fed by a BTS is the appropriate solution for many customers, it is not always economical for smaller enterprises. That’s when small cells come into play. They provide a flexible, self-contained and more cost-effective solution than a BTS fed systems.

Small cells are essentially what the name implies – small versions of macro cell sites, including base station, radio and antennas, usually combined into a single physical unit. There are innovative companies like RSRF (www.rsrf.com) that are using small cells from multiple carriers and combining the signals to run over the same infrastructure wiring, thus eliminating one of the major drawbacks to the technology (single carrier). As a BTS replacement, small cells can provide a very cost-effective signal source to a DAS.

They are often used as an interim solution in getting a DAS operational quickly, as the time required to get carrier approval and installation of a BTS can take months.

If you’re considering a small cell deployment but expect to significantly expand your IBW solution in future, bear in mind that small cells have an upper limit on coverage. They can reliably support 50 or so simultaneous users at a time, which for many small to mid-sized enterprise environments will be more than adequate. You should also consider that conventional small cells cannot dynamically share capacity between access points, meaning that large groups of users in small spaces (such as cafeterias) can create bottlenecks.


This article has attempted to characterize the DAS landscape and is by no means definitive or technically sophisticated. There are nuances to each DAS type (active components can exist in an off-air system, e.g.) that complicate the environment. 

Choosing the right technology for your building is contingent on a number of factors we’ve outlined here. At the end of the day, customers want their phones to simply work. It’s incumbent on the integrator to determine the best technology for the specific building. Sometimes that’s an Off-Air Passive DAS, sometimes a Small-Cell fed DAS, sometimes a BTS Fed Active, Fiber DAS.  There are also hybrid implementation that may mix and match the various technologies.

IBW solutions are always evolving and improving; leading to more economic, greater performing IBW and giving owners, managers and architects – as well as installers – a greater array of options over time. For owners, managers and architects this means more choices when it comes to IBW. But more choices involve finding an integrator who has experience in recommending and deploying various types of solutions and can pick the one right for you. 

For additional information, please contact In-Building Wireless Solutions  for both cellular and Emergency Responder Radio help at  https://in-buildingwireless.com/contact/.