Contention-based vs contention-free RACH

Two Flavors of Random Access

Contention-Based Random Access allows any UE to attempt access by selecting a preamble randomly from a shared pool of 64 sequences. The randomness means multiple UEs can collide on the same preamble, requiring the full four-message exchange with contention resolution. Contention-Free Random Access eliminates collision entirely because the gNB assigns a specific dedicated preamble to a particular UE through RRC signaling before the access attempt. Since only that one UE will transmit the assigned preamble, no contention resolution is needed and the procedure completes with just Msg1 and Msg2.