Small cells are small, low-power mobile base stations that cover a short distance, such as one street, one railway platform or one floor of a building. A base station is the radio equipment that sends and receives the signal for phones. A normal tower, called a macro cell, covers an area up to a few kilometres across. A small cell covers from a few tens of metres up to a few hundred metres. Small cells are a part of the wider family of radio units and antennas.
Why are they needed? A tower can serve only a limited number of phones at full speed, because all of them share the same radio spectrum. When thousands of people stand in one small place, each phone gets a smaller share. Building another tall tower in a crowded city is slow and expensive. A small cell fits on a lamp post or a wall, and takes the nearby phones off the tower.
Here is a typical case. A railway station in Vijayawada has about 4,000 passengers on its platforms during the evening rush. A tower 700 metres away covers the whole station, but its sector is overloaded and data on phones slows to a crawl. The operator fixes small cells on the roofs above the platforms. Each one transmits at a few watts, while a tower's radio transmits at tens of watts. Each small cell covers about 150 metres of platform. Phones near a small cell now connect to it, and the tower serves fewer phones.
A small cell needs the same functions as a big tower, in a box the size of a suitcase or smaller. Inside, a 5G baseband chip does the signal processing on a single chip. The cell is controlled by RAN software, like any other radio. Each small cell also needs power and a link back to the network, usually a fibre cable. Getting permission and power for thousands of poles is often the hardest part of a small cell rollout.
How does a small cell differ from a distributed antenna system (DAS)? A small cell is a complete base station, usually for one operator. A DAS has no base station of its own. It only spreads the signals of several operators through many antennas inside a building. Small cells are also central to Open RAN, because many new companies build them.