Which event of measurement is used for UMTS or LTE fallback in 5G?

Question

In LTE network, OFDMA is being used for downlink and S-FDMA is for uplink direction.
What about 5G radio access mechanism ? How radio resources would be accessed by 5G handsets ?

Answer 1

I think the event is not implemented yet which clearly specifies the event to fallback from 5G to LTE/WCDMA like B1,B2 in LTE.