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Increasingly, distributed antenna systems (DASs) have been providing much-needed wireless service in locations ranging from sports stadiums to private yachts. The term “DAS” describes a network of antennas and interconnects that route telecommunications signals throughout a localized environment. The goal of a DAS is to bring wireless connectivity to areas where macrocells cannot adequately meet demand. There are three main types of distributed-antenna systems. Yet all of them encounter challenges when it comes to installation. Several companies have intriguing methods of solving those challenges, such as the need to implement high-capacity data links alongside often necessary public-safety bands.

DASs are becoming more commonplace in corporate, public-transportation, and entertainment/shopping facilities (Fig. 1). When asked what is influencing this trend, Colin Abrey, Chief Marketing Officer of Zinwave, replies, “The biggest one at the moment is the proliferation of mobile data. To generate the best performance, you need to put the user close to the antenna. There is also an increasing demand for public-safety services to be available inside a building.” Wireless network operators have an incentive to provide coverage wherever their customers require network access. This demand for coverage precipitates a need for solutions that can maintain connectivity indoors, outdoors, and on the go (Fig. 2).

DASs Bring Capacity Indoors, Outdoors, And Wherever You Need It, Fig. 1

Because macrocells are designed for coverage over large outdoor areas, the communications link between a user’s devices inside a building and the tower may not always have a strong signal. A weak or sporadic signal would induce a much lower-capacity link and put less data at the fingertips of the user. DASs and small cells have been promoted as solutions for these problems. These approaches differ by the scenarios in which they provide the best solution. According to Abrey, “DASs would not typically be found in very small buildings or homes. Smaller environments that are single service would not typically use DASs; they would use small cells. A medium to large building, about 50,000 square feet and beyond would be the realm of DASs, as well as multi-service environments like 2G, 3G, and 4G-LTE.”

DASs Bring Capacity Indoors, Outdoors, And Wherever You Need It, Fig. 2

The oldest style of the distributed antenna system is fully passive. These systems are designed with a repeater antenna, which is mounted on roofs to relay the cell-tower signals to distributed antennas within the building. Passive DASs often require long runs of coaxial cables and repeaters, which are necessary for signal integrity considering the losses of the cable. Such systems may not be suited for all environments when considering costs and in-building routing.

Though wideband, this style of fully passive DAS is limited to the capacity of the repeater channel. It is therefore not suited to high-capacity links. The challenges of passive DAS include coexistence, passive intermodulation (PIM), and other types of interference. Among the solutions that can help mitigate these problems are quasi-omnidirectional antennas and antenna test sweeps with PIM equipment throughout the location. As most of these performance issues are derived from signal-routing problems, fiber-optic and unshielded-twisted-pair (UTP) cabling solutions have found a place in the market.

Hybrid DASs rely on a fusion of passive and active systems. The passive portion of the hybrid DAS includes the last few meters of coaxial cabling and the electronics that convert optical signals into RF signals. The active portion of the system is usually a fiber-optic signal path that connects directly with a base station and is routed to hubs distributed throughout a building.

DASs Bring Capacity Indoors, Outdoors, And Wherever You Need It, Fig. 3

The goal of all DASs is to offer operation on all frequencies that benefit the end user. This includes common cellular bands as well as public-safety and even WiFi bands. Mike Williamson, Director of DAS Product Line Management for CommScope, shares, “Distributed antenna systems should be designed to support any technology today and in the future. There may be some technical challenges with the systems when new frequency bands are released.” Williamson notes that DASs often have frequency-selective circuitry. Using amplifiers and filters as an example, a system will not be able to provide service if the amplifier or filters used in the DAS do not operate across the frequencies of interest. Such an approach would require adding new components to the system—potentially increasing cost and complexity (Fig. 3)

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