Distributed Antenna System (DAS) Basics
There are many cases, especially within buildings, tunnels, large public venues, stadiums, and etc. where the signals from a cell tower does not provide sufficient coverage, or the density of users is just too great for a typical cellular approach. In these circumstances DAS installation can be employed to enhance the coverage in these areas, or through a base transceiver station (BTS), NodeB, eNodeB, or small cell paring, offer enhanced service to a large number of users. Do to the diversity of DAS installations, there is not one single type of DAS, or DAS technology, and often multiple DAS techniques are used to enhance the wireless performance of cellular, and even WiFi, services in an area.
The simplest form of DAS, is the use of an off-air passive DAS installation, which couples wireless through a donor antenna to a cellular base station, and redirects that signal through coaxial transmission lines to targeted delivery antennas. The use of diplexers at the antennas allows for downlink and uplink, and this is typically the fastest, lowest, cost, and depending on the diplexers and antennas, can be used to serve multiple carriers, and even WiFi. In this case, the DAS performance is tied to the donor signal and RF design quality of the systems and isn’t capable of adding capacity.
Active DAS technologies also exist, where the RF signal is converted into a digital signal and sent to remote radio units (RRUs), which then convert the digital signal into an RF signal again for use with user equipment. This method reduces the amount of RF amplification and RF transmission lines necessary in passive DAS to distribute the signals. Routing complexity is also reduced as industry standard Ethernet or fiber optic cables can be used to transport the digitized RF signals, which greatly increased the length of cables that can be used, and reduces the cost and complexity of extending the system.
Active and passive DAS systems can also be combined, where certain areas of the distribution are either routed passively or actively. These hybrid systems are often used where cost considerations and routing constraints limit the use of an all Active DAS installation.
The latest developments in the DAS market are Digital DAS technologies, which use a baseband unit (BBU) to communicate digitally with a DAS master unit and RRUs without an RF interface. Though Digital DAS may be theoretically cheaper, less complex, and much more expandable that even Active DAS, it is likely that competing standards have yielding relatively little real world deployment.
DAS installations can be driven from either donor cellular sites, this is called off-air DAS, through a BTS/NodeB/eNodeB, or through a Small Cell. The difference between BTS/NodeB/eNodeB and Small Cell DAS, is that Small Cell DAS typically leverage the backhaul internet infrastructure to relay cellular, and sometimes WiFi, signals. BTS/NodeB/eNodeB-fed DAS operate much like traditional cellular base stations with distributed RF signal routing and antennas, instead of a tower, or roof-top, based broadcast style distribution. Also, due to limits with connection and capacity, a Small Cell DAS usually can’t offer the extent of capacity that is possible with BTS-fed DAS, which usually are fed by dedicated, carrier installed fiber connections to the carrier’s network.
A variety of RF, digital, and fiber optic technologies are employed in DAS systems, including repeaters, amplifiers, coaxial transmission lines, Ethernet cables, fiber optic cables, a wide variety of antennas, diplexers, RF to fiber converters, digital-to-analog converters (DACs), analog-to-digital converters (ADCs), digital signal processors (DSPs), mixers, filters, and all the associated RF and telecommunications test equipment and infrastructure to design, install, test, and maintain a DAS.