(LTE being US 4g)
5G and LTE networks share LTE’s frequencies because 5G is not wholly and immediately replacing LTE. A network operator can use frequency division duplex (FDD) and time division duplex (TDD) technologies to share spectrum. FDD is where different bands of frequency are used by users; one for uplink and the other for downlink. TDD is where one frequency band will be used for uplink and downlink, switching between the two from moment to moment. According to an approved United States patent assigned to AT&T, bands can be dynamically allocated between LTE and 5G in tens of milliseconds.