Contention Resolution is a mechanism by which gNB can resolve the contention by the same PRACH preambles transmitted by multiple UEs. This is performed by msg2 and  msg4 of the RACH process.

As multiple UEs may select the same preamble for initial access, contention resolution ensures the cell validates which UE should keep the initially assigned C-RNTI after the Random Access Response.

The UE Contention Resolution Identity MAC CE is identified by MAC subheader with LCID as specified in Table 6.2.1-1. It has a fixed 48-bit size and consists of a single field defined as follows. UE Contention Resolution Identity contains the UL CCCH SDU. If the UL CCCH SDU is longer than 48 bits, this field contains the first 48 bits of the UL CCCH SDU.

<  38.321-Figure 6.1.3.3-1: UE Contention Resolution Identity MAC CE >

Reference

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