Radio units and antennas are the equipment on a mobile tower that sends the wireless signal to your phone and receives the signal from it. The antenna is the panel that sends and catches radio waves. The radio unit is the box beside it, or built into it, that turns digital data into radio waves and radio waves back into digital data. Together they are the visible part of the radio access network (RAN).
A typical tower site has three antenna panels. Each panel points in a different direction and covers one third of the circle around the tower. This one third is called a sector. Each sector has its own radio units, often one for each frequency band that the operator uses.
Consider a rooftop site in Kanpur that an operator upgrades for 5G. The site already has 4G radios. The operator adds three new 5G radio units, one for each sector, using the band around 3.5 GHz. Each new unit has 64 small antennas built into a single panel. This design is called massive MIMO, short for multiple input, multiple output. With 64 antennas, the radio unit can shape a narrow beam towards each group of phones, instead of spreading the signal in all directions. So several phones in one sector can use the same frequency at the same time without disturbing each other.
The radio unit does only part of the work. It converts between radio waves and digital samples, and strengthens weak signals. The heavier signal processing, such as decoding what each phone sent, happens in the RAN software, which often runs on a server at the base of the tower or in a nearby building. The two are joined by a fibre cable called fronthaul. Inside the radio unit, chips such as FPGAs do the fast, fixed calculations. In some equipment, the heavier processing runs on 5G baseband chips instead of a server. Smaller versions of the same equipment, called small cells, are fixed to poles and walls.
After processing, the tower's traffic leaves over microwave backhaul or fibre. Before any of this, the site itself is chosen with network planning software.
Traditionally, an operator bought the radio unit and the software that controls it as one package from one company. With Open RAN, the radio unit follows a public standard. So an operator can buy the radio from one company and the software from another.