Mobile coverage was historically built from tall masts covering wide areas. Capacity growth has pushed networks toward many small installations instead, and the change runs deeper than equipment size.
Capacity comes from reuse, not power
A given block of spectrum carries a fixed amount of data within one cell. Raising transmission power extends range without increasing that capacity.
The way to serve more users is to divide the area into more cells, each covering less ground, so the same spectrum is reused many times over.
This is why dense urban areas have far more sites than rural ones despite similar coverage on a map. The sites exist to add capacity rather than reach.
Higher frequencies force smaller cells
The wide channels that deliver high speeds sit at frequencies that travel poorly and are stopped by walls, vehicles and foliage.
Serving users at those frequencies requires being close to them, which means equipment on street furniture, building exteriors and inside venues.
The result is a network of many low-power units rather than fewer powerful ones, with coverage from each measured in hundreds of metres.
Every site needs power and a connection
A small cell is inexpensive as hardware and expensive as a deployment, because each one requires an electrical supply, a high-capacity link back to the network and permission to occupy the location.
That permission usually involves a municipal authority and a property owner, each with its own process and fee.
Rolling out thousands of such sites is therefore a legal and logistical exercise more than a technical one, which is why progress is uneven between cities.
Handover happens far more often
Smaller cells mean a moving user crosses boundaries frequently, and each crossing requires the network to transfer the connection without interrupting it.
Handover consumes signalling capacity and introduces opportunities for a dropped session, so a dense network must manage transitions much more actively than a sparse one.
Techniques that let a device maintain connections to more than one site simultaneously reduce the disruption, at the cost of additional complexity in the network.
Sharing infrastructure became normal
Because each site is costly to establish and cheap to equip, operators increasingly share physical locations while running their own radios.
Neutral host arrangements go further, with a third party operating equipment that several networks use, which is common in venues and transport hubs.
This is why coverage inside a large building is often uniform across networks while street-level coverage still differs markedly between them.