What are the four types of Distributed Antenna System?
The four types of DAS signal distribution systems are passive DAS, active DAS, hybrid DAS, and digital DAS.
1. Passive DAS
Passive distributed antenna systems have a repeater that sends signals through coaxial cables, splitters, couplers, and taps to distribute passive radio frequency throughout a site or building. Passive DAS is ideal when you only need to extend coverage in a small area and your facility has thick masonry, concrete and metal building components.
Passive DAS is the simplest of all four distributed antenna systems, and also the most affordable. It doesn't require too many components to run, so it's also low maintenance (except for long coaxial cables, which tend to suffer from attenuation or signal dropout and need to be replaced in time).
2. Active DAS
Active distributed antenna systems are more complex than passive DAS because the signal they broadcast is converted. Active DAS consists of a main unit and a remote radio unit (RRU). The former accepts analog radio frequency (RF) transmissions from a signal source. It converts them to digital signals, then transmits them via Ethernet or fiber optic cabling to RRUs throughout the building. The RRU converts the digital signal back to a radio frequency signal so it can be recognized by cell phones and other connected devices.
Due to the use of fiber optics, active DAS is ideal for large venues with many structural obstacles, such as thick concrete walls or steel partitions, that impede signal distribution. The system converts the source signal into light, transmits it through fiber optics, and then converts it back into a recognizable radio frequency signal once it reaches its intended destination.
3. Hybrid DAS
As the name implies, a hybrid distributed antenna system combines active and passive DAS technologies. They use coax, fiber optics, and RRUs.
Hybrid DAS uses fiber optic cables instead of coaxial cables (an upgrade from passive DAS) to capture the signal from the source and send it to RRUs installed on each building floor. The RRU converts the optical signal to a radio frequency signal and broadcasts it using an antenna connected to the RRU via a coaxial cable.
Hybrid DAS is priced between passive DAS and active DAS. It is also more flexible: it can accommodate scenarios where only passive or active DAS can solve the problem. By combining the two, IT technicians have more control over transmission intensity and location.
4. Digital DAS
Digital distributed antenna systems are still in their infancy and have not yet been widely adopted. However, it has a simple principle of operation: it does away with the conversion of the radio frequency to digital signal and distributes the digital signal along the way.
Digital DAS uses a baseband unit (BBU) connected to a main unit that distributes digital signals over fiber optic or Ethernet cables. Since it does not use radio frequency, it is less susceptible to interference. IT technicians can direct fixed capacity to specific areas within the venue, such as conference halls, cafeterias, or atriums.
Digital DAS is promising but expensive and only works if the source and DAS master unit support the Common Public Radio Interface (CPRI) specification.
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