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2. Random access
Overall reference: [36.300:10.1.5], [36.321:5.1]. The steps are as below:
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UE has read MIB/SIBs, DL sychronised, RACH parameters known.
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Random Access preamble
RACH
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UE (MAC/PHY) determines - sequence/power/PRACH resource/RA-RNTI, generates, and transmits random access preamble over RACH. Then UE monitors PDCCH (with RA-RNTI) for DL allocation. Ref: [36.213:6.1], [36.211:5.7.1]
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Random Access Response
PDSCH
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RAR is special MAC PDU. It contains Timing Advance (for UL synchronisation), UL grant (to send L3 message*), and temporary C-RNTI.
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RRC Connection Request
PUSCH
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From L3 point of view, the message goes over CCCH/SRB0. The ue-Identity included in this message is used for contention resolution at next step by MAC.
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RRC Connection Setup
PDSCH
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From L3 point of view, this message is meant for establishment of dedicated signaling/SRB1. At the same time, MAC performs contention resolution with this message.
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UL PRACH allocations are determined with steps mentioned in TS 36.211, based on parameters broadcast in SIBs. UL grant for RRCConnectionRequest is part of RAR. RAR itself and DL RRC message require DL allocations.
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* msg1 - RACH preamble
msg2 - RAR
msg3 - RRC Connection Request
msg4 - RRC Connection Setup
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