Dynamic spectrum sharing (DSS) played an important role in the 4G-to-5G transition by allowing 5G new radio (NR) to enter valuable legacy spectrum without immediate static refarming. Yet practical deployments also exposed the cost of coexistence of NR with long-term evolution (LTE), including overheads, control-channel bottlenecks, neighbor-cell interference, etc. As 6G begins to take shape, spectrum scarcity below 7 GHz is again making 5G-6G spectrum sharing a migration tool of interest. Multi radio access technology spectrum sharing (MRSS) is being considered by the 3rd generation partnership project (3GPP) as a key mechanism for 5G-6G coexistence. This article reviews the lessons learned from LTE-NR DSS and examines how those lessons should shape MRSS design. The main challenge is no longer basic coexistence feasibility, but coexistence efficiency which determines whether MRSS will become a broadly usable framework for 5G-to-6G spectrum migration.

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