A mobile signal booster — also called a mobile network booster or cellular repeater system — is a set of equipment that captures existing outdoor mobile signal, amplifies it, and rebroadcasts it inside a building. It doesn't generate new signal from nothing; it works with whatever usable signal already reaches the outside of your property.
The three core components
Every booster system, regardless of size, is built from three parts: an outdoor antenna that picks up signal from nearby mobile towers, an amplifier (or booster unit) that strengthens that signal, and one or more indoor antennas that broadcast the amplified signal inside the building. Cabling connects all three, routed to minimise loss along the way.
When a booster helps — and when it doesn't
Boosters are effective when there is reasonable outdoor signal but poor indoor reception, which is the most common scenario in Mumbai's concrete and glass-heavy buildings. They are far less effective in areas where outdoor signal itself is very weak or absent, since there's nothing meaningful to amplify. This is exactly why a site survey — measuring actual outdoor and indoor signal levels — comes before any recommendation.