To meet requirements, facility managers are turning to DAS, a solution that boosts signals within the building to meet public safety network requirements but can also accommodate other frequencies and technologies at the same time. A DAS can meet public safety network (PSN) requirements, provide coverage to multiple wireless service providers and in some cases even incorporate Wi-Fi in the same infrastructure. But it's a complex design that isn't yet broadly understood by the architectural/engineering/construction community, requirements can vary from city to city, and DAS can be costly depending on the specific solution needs.
As defined by Distributed Antenna Systems (DAS) Forum, a distributed antenna system is a network spatially separated by antenna nodes connected to a common source via transport medium that provides wireless service within a geographic area or structure.
The types of in-building DAS design are passive, active and hybrid design.
Passive DAS design uses coaxial cable, antenna, splitters and BTS (base transceiver station) or BDA (Bi-directional amplifier) equipment. These systems are called passive because the antenna uses no electronic components (no amplification) between the antenna and BTS equipment; it is a straight coax connection. The coaxial cable is capable of supporting multiple carrier frequencies. Due to no amplification between the antenna and BTS equipment, the antennas located farther from the BTS will encounter higher signal loss, thus exhibiting a much lower output power in the downlink and higher noise in the uplink compared with antennas that are closer to the BTS. For this reason, passive DAS design is typically used for smaller coverage areas (less than 200,000 square feet).
Active DAS design doesn't solely rely on the coaxial cabling from RF source to the antennas. Instead, these systems distribute the signal using managed hubs, remote access units (RAUs) and standard cabling. An active system uses single-mode or multi-mode fiber from a main hub (MHub) to various expansion hubs (EHubs), and then uses standard category 6 cable to connect to each expansion hub to its RAUs and antennas. The optical link allows the EHubs to be located up to 6 km (approx. 3.73 miles) away from the MHub, which is why active DAS design is most commonly used when large coverage/footprints are required (greater than 200,000 square feet). Consequently, active systems are more expensive than passive DAS design due to the active electronics used throughout.
Hybrid DAS design is a combination of passive design (coax) with vertical optical links (fiber) for distributing the signals along the vertical risers of the building. Although the hybrid system has lower signal loss than pure passive systems, the loss still incurs since the antennas are connected via coaxial cable.
What should a proposal for a DAS solution include?
The need to prevent "dead-zones" in new buildings is becoming a public safety requirement. Currently this issue is often coming to the surface during the final stages of construction, when efforts are underway to obtain certificates of occupancy. This late surprise may result in change orders, potentially delaying overall construction and breaking budgets. A DAS consultant can help navigate and select the proper technology early and save significant headaches later, and maybe a life as well.
Gislene D. Weig, electrical engineer, RCDD, is a senior consultant at PlanNet Consulting, where she is involved with large-scale projects that include voice/data, wired and wireless communication systems, and data network design. She can be reached at gweig@plannet.net.
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