A distributed antenna system (DAS) is a network of small antennas spread through a building, such as a mall, an office tower or an airport, that carries the mobile signals of every operator to places a tower cannot reach. It is also called in-building coverage or in-building solutions (IBS). It is the reason your phone works in a mall's basement car park.
Radio signals from a tower lose strength when they pass through concrete, steel and glass. Deep inside a large building, or two floors underground, the signal from outside is often too weak to use. A DAS brings the signal inside instead.
A DAS has three pieces. The first is a central room, called the head end. Each operator places its base station there, or feeds its signal there from outside. The second is a set of cables, usually fibre and coaxial cable, that runs from the head end to every floor. The third is a large number of small antennas on the ceilings, often one every 20 or 30 metres. Each antenna sends and receives the signals of all the operators at once.
Imagine a mall in Lucknow with three shopping floors and two basement levels of parking. Shoppers on the lower basement have no signal at all, so they cannot pay for parking by UPI. The mall owner does not want each of four operators to drill its own cables through the building. So the mall allows one company to install a single DAS. All four operators connect their base stations to the head end. The 200 ceiling antennas then carry all four operators' signals to every shop and both parking levels.
A company that installs and runs such a shared system is called a neutral host. It owns the equipment, and the operators pay it to carry their signals. Sharing one system among several customers is a form of multi-tenancy. Each operator's RAN software still controls its own base station.
How does a DAS differ from small cells? A small cell is a complete base station, usually for one operator. A DAS has no base station of its own. It only carries the operators' signals through the building. Newer DAS designs carry 5G as well, though the higher frequencies of 5G need antennas placed closer together. A building that needs coverage for one company's own machines only may choose private 5G instead.