5G NR Random Access

5G NR Random Access

 

5G NR Random Access Procedure CBRA

Random Access: Random access required to do uplink synchronization between UE and eNB and gNB.

There are 2 types of Random Access Procedure in 5G Network.

 

Contention Based Random Access(CBRA): UE selects a random access preamble from a pool of preambles shared with other UE, so in CBRA UE taking the risk to select the same preamble as another UE.

 Contention Free Random Access: 5G Base station allocate a dedicated random access preamble. 

Contention Based Random Access

  • Contention Based Random access has four-step
  • MSG1 ( UE to gNB or MN-eNB)
  • MSG2 ( gNB to UE)
  • Msg3 (UE to gNB)
  • Ms4 (gNB to UE)

Message 1- UE–gNB or MN-eNB

  • Receive SSB( UE measure the RSRP on each received beams, selects the SSB and read the system information Block.
  • RMSI for a standalone scenario or RRC reconfiguration for NSA to determine PRACH configuration. Here we will check( RACH occasion: time-frequency resources, prach preamble format, Root sequence index(RSI)
  • Calculate RA-RNTI: The RA-RNTI associated with the PRACH in which the Random Access Preamble is transmitted if all condition for PRACH transmission is met.
  • Calculate PRACH TX power
  • Transmit the preambles

 

  1.  UE select the preamble which is available in CBRA access or Pool, here we have a parameter ( totalNumberOfRA-Preambles) , Contention based preambles with consecutive indexed associated with SS/PBCH block n, 0≤n≤N-1, per valid PRACH occasion start from the preamble index n.N total preamble/N where N total preamble.
  2. A UE is provided with a number N of SS/PBCH blocks associated with one PRACH occasion and a number R of contention-based preambles per SS/PBCH block per valid PRACH occasion by ssb-perRACH-OccasionAndCB PreamblesPerSSB.  means ssb-perRACH-OccasionAndCB PreamblesPerSSB allows the UE to quantify the number of contention-based PRACH preambles associated with each SSB beams.
  3. ssb-perRACH-Occasion means a number of SSB beams sharing the same PRACH occasion,64 preambles.
  • Now  SSB-perRACH-OccasionAndCB PreamblesPerSSB , If N= 1, SS/PBCH block is associated with 64 preambles 
  • Now  SSB-perRACH-OccasionAndCB PreamblesPerSSB , If N<1, SS/PBCH block is associated with 1/N , consecutive valid PRACH occasions and R contention-based preambles with consecutive indexes associated with the SS/PBCH block per valid PRACH occasion start from preamble index 0.
  • Now  SSB-perRACH-OccasionAndCB PreamblesPerSSB , If N≥1,  R contention-based preambles with consecutive indexes associated with SS/PBCH block n 0≤n≤N-1,