Random access
Random access procedure is performed prior to the attach process (registration for the UE to complete the network, the RPC establishment of a default bearer to the UE) for the UE and eNB establishing a radio link , Get / restore uplink synchronization. Because of the randomness, the complexity of the user's wireless environment determines the initiation of such access and use of resources is also randomness, which allows a network communication connection is established with the terminal becomes possible.
Positioned in architecture
As the "process" is a general term for a series of operations
UE and eNB is the process of establishing a wireless link
classification
Distinguished as prefixes way different
1. contention based random access
Based competition UE random access prefix conflicts in accordance with a certain algorithm based random from selecting a random prefix.
UE state is divided into two: RRC_idle and RRC_connect
After entering coonect, if no business UE within a period of time, then enter idle.
If the UE again to initiate business, UE needs to be done Service Request, Service Request simpler than Attach, no authentication process and query UE capabilities, includes only random access, RRC connection and establish a default bearer.
Step-out, non-synchronised: refers to the transmission loss of synchronization between the two bodies
Scenarios
. 1) RRCJDLE initial access in a state
2) Initial access after radio link error
3) UE has uplink data transmission, the uplink is detected but desynchronization
Process
DL-SCH (Uplink Shared CHannel): downlink shared channel, in the downlink transmission channel. TA (Timing Advance) is used to ensure the time synchronization of the eNB.
CCCH (Common Control Channel): the common control channel is a logical channel.
1) MSG1: UE transmits a random access preamble on the RACH, carrying a preamble; RACH (Random Access CHannel): a random access channel, is an uplink transport channel, the channel carrying the limited control information, and has a collision conflict characteristics. [Random access preamble]
2) MSG2: eNB after receiving side MSG1, the transmission on the DL-SCH generating a random access response (RAR) at the MAC layer, RAR response carries the TA adjustment commands as well as uplink grant and T-CRNTI (provisional the CRNTI); [ a random access response],
. 3) MSG3 (connection establishment request): UE after receiving MSG2 it is, determines whether the RAR message of their own (checked using the preamble ID), and sends the message MSG3, carrying the UE-ID. Generating RRC layer of the UE RRC Connection Request and transmitted on the CCCH logical channel mapped to the UL -SCH; [scheduling transmission]
. 4) MSG4 (RRC connection establishment): RRC Contention Resolution generated by the eNB RRC layer, and on the CCCH is mapped to the DL -SCH or DCCH (FFS) logical channel transmission, the UE correctly receives the contention resolution is completed MSG4. [Contention resolution]
1. In the random access procedure, message MSG1 and MSG2 are low-level, L3 layer is not visible, so the tracking signaling, the UE first signaling network is the MSG3 (RRC_CONN_REQ)
2.MSG2 the message by the eNB MAC layer produced by DL_SCH bearer MSG2 message may correspond to a plurality of UE random access request response time.
3.eNB PDCCH scheduling MSG2 is used, and is addressed by the RA-RNTI, RA-RNTI is determined by the position of the bearer resource PRACH MSG1 frequency;
4.MSG2 comprising an uplink transmission timing advance, uplink resource allocated to MSG3, a temporary C -RNTI the like;
5.UE after receiving MSG2, MSG3 transmission on its allocated uplink resource.
6 for different scenarios, different contents of the Msg3 comprising:
an initial access: carrying RRC layer RRC connection request generated, comprising S-TMSI UE or random number;
connection reestablishment: carrying RRC layer RRC connection reestablishment request is generated, C and the PCI -RNTI;
handover: transmitting an RRC RRC layer generating the handover complete message, and the C-RNTI UE;
uplink / downlink data arrival: transmission of the UE C-RNTI;
| Contention resolution |
Initial access and connection reestablishment scenes |
Handover, the uplink / downlink data arrival scene |
| Competition determination |
MSG3 message MSG4 carry copies of successful demodulation, the UE compares the level MSG3 identify itself in the transmission, it is determined that both are the same successfully compete |
If MSG4 UE scheduling commands received on the PDCCH, the competition success |
| Dispatch |
MSG4 using PDCCH scheduling the Temporary C-RNTI scrambled |
C-RNTI eNB using PDCCH scrambled scheduling MSG4 |
| C-RNTI |
MSG2 the issued temporary C-RNTI upgrade the C-RNTI for the UE after successful competition |
Assigned C-RNTI before the UE, it will also be in MSG3 pass eNB. After the contention resolution, the temporary C-RNTI been recovered, continue to use the original UE C-RNTI |
2. The non-contention based random access
Is a non-contention base station side by the downlink dedicated signaling to the UE assigned non-conflicting random access preamble.
Scenarios
1) UE has downlink data transmission, the uplink out of synchronization is detected:
2) after the handover, the new cell access: handover, the target eNB can tell by its serving eNB UE resources available
Process
1) MSG0: eNB to the UE through a downlink dedicated signaling prefix assigned to the non-collision of random access (non-contention Random Access Preamble), the prefix is not set in the broadcast on the BCH. BCH (Broadcast CHannel): a broadcast channel, in the downlink transmission channel.
2) MSG1: UE RACH transmission on the assigned random access preamble.
3) MSG2: ENB layer generates a MAC random access response, and transmits the DL-SCH. For the non-contention random access procedure, Preamble code assigned by the ENB, to the receiving end after correct RAR.
ENB UE according to the indication, the specified code at the specified Preamble PRACH initiate random access
MSG0: a random access indication message
for handover scenario, eNB notifies the UE through RRC signaling;
for downlink data arrival scene assisted positioning, by eNB PDCCH notifies UE;
MSG1: transmitting Preamble code
UE initiates a random access with the specified code in the Preamble eNB designated channel PRACH resources
MSG2: the random access response
format and content MSG2 with exactly the same competitive mechanism, in response to a plurality of UE can transmit the MSG1
Reference
