An RF front end is the set of chips between a phone's antenna and its modem. RF stands for radio frequency. When the phone receives, the front end filters out unwanted signals and strengthens the wanted one, before the modem decodes it. When the phone sends, the front end strengthens the modem's signal so that it can reach the tower.
Why is it needed? The air around a phone is full of radio signals: other operators' 4G and 5G, Wi-Fi, Bluetooth, FM radio and satellite navigation. The antenna picks up all of them. A signal from a tower a kilometre away can arrive at the antenna with a power of less than a millionth of a millionth of a watt. At the same moment, the phone's own transmitter may be sending at about 0.2 watts. Without the front end, the modem could not find the weak signal among all the others.
The front end has four main kinds of part. Switches connect the right antenna to the right path, because a phone has several antennas and supports dozens of frequency bands. Filters let through only one band and block everything else. For 5G in India, for example, one filter passes only frequencies between 3.3 and 3.8 GHz. A low-noise amplifier strengthens the weak received signal while adding as little electrical noise as possible. A power amplifier strengthens the outgoing signal. These are analog circuits, which handle smoothly varying signals rather than numbers. Some of them are made of special materials, such as gallium arsenide, instead of ordinary silicon, because those materials handle high frequencies and high power better.
Imagine a phone at a cricket match at a crowded stadium in Kolkata. Tens of thousands of phones nearby are sending signals at the same time. The phone's filter for its 5G band blocks the signals on all other bands. The low-noise amplifier then strengthens the phone's own signal enough for the cellular modem to decode it. When the phone sends a video clip, the power amplifier must keep its signal inside its own band, so that it does not disturb the phones around it. Far from a tower, the power amplifier must work harder. That is one reason a phone's battery drains faster where the signal is weak.
The front end serves Wi-Fi and Bluetooth as well, passing their signals to the Wi-Fi and Bluetooth chip. On the other side of the link is the tower's own radio unit, which does the same filtering and amplifying for hundreds of phones.
The front end is fitted and tuned for each new phone as part of smartphone design. Engineers place the antennas in a thin case, tune the front end for every band the phone must support, and measure the results with signal analysers and other test instruments. Front-end chips are designed by a few companies, some with engineers in Bengaluru, and are sold to phone makers, who pair them with the modem in the phone's system-on-chip.