FPGAs and precision timing are two jobs that specialist chips do inside the radio equipment on mobile towers. An FPGA, or field-programmable gate array, is a chip whose circuits can be rewired by loading a new configuration file, even after the equipment has been installed. Precision timing is the work of keeping every tower's clock in step with every other tower's clock, to within a few millionths of a second.
Why does a tower need timing this exact? In 5G, most towers in India send and receive on the same frequencies, in the main band around 3.5 GHz. They take turns. For about 2 to 4 milliseconds, every tower sends to the phones. Then every tower listens to the phones. This is called time-division duplexing, or TDD. If one tower's clock drifts, it starts sending while its neighbours are still listening. Its strong signal then drowns out the weak signals from phones near the neighbouring towers.
Consider two 5G towers 800 metres apart in Bhubaneswar. The towers switch between sending and listening many times each second. The standards allow the towers' clocks to differ by only a few microseconds, a microsecond being a millionth of a second. One tower has a satellite navigation receiver on its roof, which gives it the exact time. That tower sends timing messages to the other tower over the fibre network. These messages use a standard called the precision time protocol, or PTP. An FPGA inside the second tower's radio unit reads each message, measures how long it took to arrive, and corrects its own clock. Both towers now switch at the same moment.
Why use an FPGA for this, and not an ordinary processor? An FPGA does fixed tasks, such as timestamping messages or processing radio samples, in hardware, very fast and at low power. And because it can be reconfigured, the equipment maker can fix a bug or support a new version of a standard by sending a new file to radios already on towers. A custom chip, by contrast, would have to be replaced.
FPGAs often handle tasks around a 5G baseband chip, and many makers of Open RAN radios use them. Before a new FPGA design ships, engineers usually test it on hardware emulation machines. An FPGA maker earns money through design wins: once its chip is chosen for a radio, it ships in every unit of that radio. Like most chip companies, FPGA makers are fabless: they design the chips but do not manufacture them.