What is a distributed antenna system (DAS)?
In a distributed antenna system (DAS), a signal source is connected to a group of antennas instead of a single antenna. A DAS is typically used to distribute cellular network coverage to heavily populated buildings, such as offices, high-rise apartments, shopping centers or sports stadiums.
What does a distributed antenna system do?
A distributed antenna system is used to obtain wireless signal coverage in inaccessible areas. It places many smaller, less powerful antennas in different locations instead of one big, powerful antenna. Cables connect small antennas back to a signal source.
An analogy can illustrate the usefulness of DAS. Imagine trying to play music for a large audience. When it’s all out, a powerful speaker is the most effective, cheapest way to play music. Now imagine a crowd of people distributed throughout an office building. A speaker in the middle of a building can be very loud in some places, very quiet in others, and reverberations can distort the sound. It is better to place small speakers evenly throughout the building, all playing the same music at a reasonable volume.
Cellular network service works similarly. Cellular providers place cell towers in high locations strategically throughout the outdoor area to provide even coverage. Like large speakers, these cell towers are powerful and work well outdoors. Inside a building, however, it is difficult for the receiver to detect the signal from outside. Any thick exterior wall can block, or weaken, the signal, an interior wall can cause the signal to bounce, and multiple people can all be competing for the same signal. To overcome these challenges, a DAS brings small antennas inside the building to distribute the signal more evenly.
DAS usually refers to cellphone signal coverage, not Wi-Fi connected to the internet. A large Wi-Fi system is more often called a Wi-Fi deployment or mesh network. Often, Wi-Fi can be deployed simultaneously or even on the same equipment as a DAS, causing some confusion. For example, the New York Metropolitan Transportation Authority uses DAS and Wi-Fi access points to provide both types of coverage in underground subway stations.
When designing a DAS, the main considerations are the signal source and the type of signal distribution.
Distributed signal sources in antenna system
DAS must get its wireless signal to be distributed from anywhere. It can be reused from another source outside the system, called off-air, or generated by the system itself, which can be small cell systems or larger base transceiver stations (BTS).
Off-air distributed antenna system
An off-air DAS uses an antenna in one location, connected to multiple antennas in another location, to repeat a signal. For example, a large antenna on the roof of a building picks up the signal from a nearby base station on the cell tower and retransmits it to smaller antennas connected throughout the building. It can be called a repeater because it does not create new wireless service but simply repeats whatever is already available. In the US, the Federal Communications Commission regulates these signal boosters.
An off-air DAS works best in locations with good cell phone coverage but can be interrupted by building construction. Office and apartment buildings in metropolitan areas are good candidates for off-air DAS.
Deploying off-air DAS is fast, simple and affordable. They require minimal cooperation with cellphone service providers and can work with any carrier. Because they only transmit signals, they reuse existing cellular infrastructure.
Off-air DAS may not be suitable for rural locations or particularly high-density buildings. The current signal strength or capacity may not be sufficient for the system.
Small-cell distributed antenna system
Small cell DAS — sometimes called microcell, picocell or femtocell — occurs when a cellular service company provides equipment that generates the signal and uses a normal internet connection to connect to the carrier’s network. Since these systems generate their own signal, they are ideal for locations with no coverage or poor cell coverage. This includes rural areas or service dead zones.
Small cell DAS are more complex and expensive to set up than off-air ones. Successful use requires agreements with cellular service providers and a high-bandwidth internet connection.
Distributed by BTS antenna system
BTS DAS is actually a whole cell tower base station with multiple antennas in one place. They may also be called NodeB, or 3G; ENodeB, or 4G-LTE; or GNodeB, or 5G-NR, stations. It is set up in partnership with cellular companies and often uses dedicated fiber optic backhaul links in the carrier’s network. As a large-scale infrastructure, BTS is used in situations where there are many people in dense areas, such as sports stadiums or skyscrapers.
Distributed antenna system signal distribution
DAS signal distribution modes are passive or active.
In passive DAS, radio signals are picked up at one location and transmitted via wire to another location. This happens with an amplifier and coaxial cables. Radio frequency (RF) couplers, splitters and taps divide the signal between different antennas.
Primarily using passive components, passive DAS can support all cell service providers simultaneously. Since they rely only on passive components, they may not work for large distributions with long wire runs. Additionally, working with RF signals can be complex and successful use requires specialized knowledge.
In an active DAS, the RF signal is converted to another type of signal for transmission to the second antenna and returned to the RF. Typically, fiber optic or ethernet is used for cable runs.
Active DAS components are more expensive to purchase and maintain. However, fiber or ethernet cable is often cheaper than the coaxial that a passive DAS requires. Cable runs can also be longer and easily expanded.
Digital DAS is a sub-type of active DAS that converts RF signals into digital packets. It can be used to integrate a DAS with an existing data network.
Hybrid DAS uses both active and passive distribution types. This improves system performance while lowering costs. For example, a hybrid system may use active fiber optics distribution systems to send the signal to each floor of a building and passive distribution to send the signal to multiple floor antennas.
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