Nr-prach: access scenario and access process

PRACH： Physical random access channel
effect ： Send in random access preamble Sequence , Base station calculation TA value , Get uplink synchronization , After completing the random access process , The terminal can carry out uplink communication with the base station ,prach Only in the process of access msg1.
There are two access methods ： Competitive access and non competitive access
1. Contention based random access process ：

2. Switching scenes （ Based on non competitive access ）

** Conditions for triggering random access ：（ Reference network ）

1. Initial access ：UE from RRC_IDLE State to RRC_CONNETTED state ;
2. RRC Connection reconstruction ： In order to UE Re establish the wireless connection after the wireless link fails ( During this period, the reconstruction of the community may be UE Cell with failed wireless link , Maybe not );
3. Switch ：UE be in RRC_CONNETED state , here UE A new cell needs to establish uplink synchronization ;
4. RRC_CONNETTED In state , When uplink or downlink data arrives , here UE The uplink is out of step ;
5. RRC_CONNETTED In state , Uplink data arrives , here UE Not used for SR Of PUCCH Resource time ;
6. SR Failure ： Re obtain... Through random access process PUCCH resources ;
7. RRC Request during synchronous reconfiguration ;
8. RRC_INACTIVE Access in state ：UE From RRC_INACTIVE State to RRC_CONNETTED state ;
9. stay SCell Establish time alignment when adding ;
10. Request other SI：UE be in RRC_IDLE State sum RRC_CONNETTED In the state of , Request other information through the random access process SI( Reference resources 38.300 Of 7.3.2);
11. Beam failure recovery ：UE When a failure is detected and a new beam is found , A new beam will be selected .
    therefore , Random access process 2 Different models ：

1. Contention based random access process ： Apply to the above /2)/3)/4)/5)/6)/8)/10)/11).
2. Based on non competitive random access process ： Apply to the above 3)/4)/7)/9)/10)/11).
   For the contention based random access process ,UE Only in PCell launch , And the random access process based on non competition ,UE That is to say, you can PCell It can also be initiated in SCell launch .