Last month, I wrote about how fiber was being used to expand bandwidth for cellular systems, connecting cell towers to the phone network and antennas on the tower to the base electronics. That helps our phone coverage when we’re driving or walking outside. But, as we all know, cell coverage can be poor inside buildings.


Indoor coverage is increasingly important as more users depend on their smartphones. AT&T recently reported that about 80 percent of all cellular calls originate indoors where outside antennas may not provide good coverage. In addition, when smartphone users get together in a public place, such as a sports stadium, they may all try to send messages, photos, videos or access websites. At one of the opening San Francisco 49ers games at Levi’s Stadium, fans reportedly used more than three terabytes of data.


Another issue for indoor users is local regulation that requires emergency services radio frequencies to be available indoors. This is especially important in tunnels for traffic or mass transit systems.


Getting cellular signals from outdoor antennas to indoor users is problematic because many building materials attenuate cellular signals. The only solution is to install low-power antennas indoors as part of a distributed antenna system (DAS).


There are two types of DASs: passive and active. A passive DAS has a central antenna on the building that receives and transmits cellular signals. The passive DAS amplifies and transmits the signal around the building over coaxial cable. Coaxial is used because the signals are transmitted as radio frequency (RF) a traditional purpose for coaxial. A passive DAS supports only a limited size building and, therefore, a limited number of users.


The active DAS is used in larger systems and gets its signals from a direct connection to cellular backhaul, which is the term used for the connection of cell systems into the phone systems. It is important for a DAS to include coverage for all major cellular providers (Verizon, AT&T, Sprint, T-Mobile, et al.).


An active DAS will digitize the cellular signals at a head end and distribute them over fiber optics throughout the facility to remote antenna units (RAUs). The RAUs connect one or more passive antennas over short coaxial cables. Small cell antennas, which combine RAUs and antennas in a small package, may be used instead.


We know that “wireless” communications systems are not completely wireless. Just like Wi-Fi access points are connected over cabling (fiber or unshielded twisted-pair copper), cell antennas must be connected into the phone system over cabling. In a typical DAS, the building also will have fiber from the service provider coming into a entrance facility and services distributed over fiber to the RAUs or small cells.


A DAS entrance facility is similar to an entrance facility for a local area network (LAN), but probably bigger to accommodate multiple service providers. It requires space for patch panels to connect service provider cables to equipment, space for head-end equipment for each service provider and then connections to the cabling for the building DAS.


DAS cabling is typically single-mode fiber chosen for its large bandwidth and distance capability and because almost all other telco equipment runs on single-­mode fiber. If a building has a LAN backbone or structured-cabling system with single-mode fiber in the backbone, that fiber is probably useful. Buildings with newer passive optical LANs (OLANs) will certainly be able to share fibers with the DAS.


Otherwise, new cables need to be installed, and the building owner should consider other future systems, like OLANs, that can be accommodated on the same cabling. Often Wi-Fi systems can share cabling with a DAS because the antenna location is similar.


Antenna location for a DAS system generally requires a site survey with appropriate cellular survey equipment. An office building may only need a couple of antennas on each floor. Sports facilities, it seems, are figured both on space and number of users with a maximum of about 100 users per antenna. For example, Levi’s Stadium seats approximately 70,000 people and has about 700 DAS antennas to serve them.


Of course, the remote antenna units or small cells require electrical power and a fiber connection. Depending on the equipment, the power may be alternating current (AC) or direct current (DC) with appropriate wiring and often a backup-power system.


Contractors familiar with LANs and structured cabling can easily handle a DAS as long as they are familiar with single-­mode fiber. Since single-mode is being used in many LANs, data centers and other systems requiring premises cabling, contractors are advised to get trained and outfitted for single-mode installation.


The Fiber Optic Association’s online self-study website, Fiber U (fiberu.org), has a free DAS self-study program that expands on this topic. There also is a YouTube video about DAS at https://youtu.be/VzPgqjomgy8.