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Because fiber optics are less lossy than coaxial cables, the benefits of using technologies like fiber optics for routing include lower-cost cabling, easier routing paths through complex building structures, and lower power. Hybrid and passive DASs require amplification technology to increase the power levels in the sections preceding the lossy passive-cabling parts. Yet hybrid systems still suffer from interference issues, as the impact of the interference is directly related to the strength of the signals in the system.

DASs Bring Capacity Indoors, Outdoors, And Wherever You Need It, Fig. 4

A fully active DAS is a solution that distributes low-power points throughout the system and minimizes routing complexities. These DASs use optical routing that is similar to what is used by hybrid-DAS systems. But the fiber-optics and power infrastructure is routed directly to the point of antenna delivery (Fig. 4). In a large building, the wireless operators will generally have a room devoted to housing the operators’ various base stations, known as a base-station hotel.

In such a “hotel,” one or more main active DAS hubs will be connected to the operator’s base stations. To adjust the base-station signals to a nominal level and convert the signals from RF to optical signals, the necessary conversion electronics are contained within the main hubs. The signals carrying fiber optics are then routed to secondary hubs, which split and route the signals to the remote units. To save on high-powered electronic interconnects between the hubs and remote units, the remote units are individually powered. Some companies, such as Zinwave, use multimode fiber as interconnections between the base station and remote units. Beyond wideband capability, an additional benefit of this DAS typology is the potential to mitigate PIM throughout the system.

Having low power levels at the remote units and extremely short runs of RF transmission lines could decrease the opportunity for PIM generators, along with good installation practice. Andy Bell, Chief Technical Officer for Zinwave, explains, “Our active DAS uses a transmit antenna and a receive antenna that are separated by 3 to 4 m, which is about -40 dB. That provides you with -40 dB of PIM protection. The antennas are only putting out 100 mW. By the time the 100-mW signal reaches the receiver, it is a tenth of a millwatt. A normal system only has a filter. That doesn’t provide any PIM protection because there is no isolation at the downlink frequency.” This type of approach allows for much lower-power systems that benefit from a less lossy cabling structure and a much wider-bandwidth. Additional cost savings from requiring less PIM-stringent components also could be considered in the return on investment of such a system.  

DAS could bring connectivity to the bowels of a train tunnel system, a stadium full of photo-bombing fans, or a corporate complex that has merrily embraced bring-your-own-device (BYOD) mentality. Advances in DAS, including wideband multi-technology solutions, have gone a long way in expanding the scope of environments in which these systems can be found. Innately wideband interconnects within DAS enable more system flexibility while offering the potential to reduce failure modes. They could ensure future-proof platforms that can support the latest frequency bands and standards.

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