5G networks have separated the data processing within the distributed unit (DU) from the signal-bearing radio unit (RU). Moving DUs to telco central offices at the edge of the access network reduces both the size and cost of base stations, which is critical given the significantly greater number of towers.

Most 3G/4G cell towers use microwave technology for connectivity. Antenna-sector data will run directly over fiber towards DUs.

The physical separation of the DU from the RU requires a much greater reliance on fiber to bridge between them. Moreover, that fiber must now accommodate much larger capacity to meet 5G bandwidth requirements, which can extend to as much 20 times the required capacity of 4G LTE. And these fiber connections must enable extremely low latency of up to 10 times less delay than LTE.

https://www.lightwaveonline.com/5g-mobile/article/14188094/the-case-for-using-pon-for-5g-fronthaul