LTE-TDD Random Access Process (6)-describe the process of competing access and non-competitive access by scene

This paper mainly describes the difference between competitive access and non-competitive access process under different scenarios.

In the article "LTE-TDD Random Access Process (1)-purpose and classification" has been mentioned that random access can be divided into two types: competitive access and non-competitive access.

Competitive scenarios include:

(1) Initial access of UE

(2) the reconstruction of UE

(3) The UE has uplink data sent, but the uplink out of step is detected

(4) UE has uplink data sent, but no SR resource

(5) switch

(6) ENB has downlink data sent, but upstream out of step is detected

Non-competitive scenarios include the positioning process in addition to the above (5) and (6). This article does not discuss the positioning process, only the scene description (1) ~ (6) The process of competing and non-competitive access.

1.UE initiating initial access

The primary process for the UE Initial initiator entry is shown.

UE Initial access, there is no crnti information, so both MSG3 and MSG4 need to use tcrnti scrambling, scrambling, and eventually to Crnti. Also, in Mac PDUs that are assembled with MSG3 and Mac layers, Crnti Mac control fields are not carried.

Because the different UE, may send the same preamble code in the same time frequency position, therefore some UE sends the MSG3 message, also received the ACK information from the network side, the network side also may not actually allow the UE to access. At this point the UE also need to decode MSG4 in the competition to resolve the identity (UE contention Resolution identity, through the Mac PDU encapsulation MSG3 code stream), to determine whether the network side confirmed the UE access. Because the MSG3 stream of different UE is different, the UE can compare the MSG3 stream (rrcconnectsetup message) previously sent with the competitor resolution cell in Mac PDU issued with MSG4, and see if it is identical. To determine whether the UE has completed the competitive access process.

2.UE reconstruction

There are many reasons for the UE rebuild, regardless of the reason, the UE has been out of step for the moment before rebuilding, so it is necessary to get TA by random access. The main flow of random access under this scenario is shown. Because the harq-ack of MSG3 and MSG4 is the same as the initial access scenario above, the two ACK messages are omitted from the interaction in order to streamline the process without affecting understanding.

Because the RRC is already in the rrc_connected State, the UE and ENB side has been saved with crnti information, so the MSG4 scrambling, scrambling to use Crnti, not tcrnti. The same reason, because the system already has the crnti, so after the random access is completed, Tcrnti will be discarded, will not overwrite the existing crnti, the subsequent process uses the old crnti to scrambling and scrambling.

The temporary c-rnti is promoted to C-rnti for a UE which detects RA success and does not already has a c-rnti; It is dropped by others. A UE which detects RA success and already have a c-rnti, resumes using its c-rnti.

Because the reconstruction request message Rrcconnectionreestablishmentrequest has carried the Crnti information, the ENB side is based on the Crnti and previously assigned Tcrnti, Can distinguish the competition process of the UE specific is which UE, so UE in the sending MSG3 mac PDU, is not required to send Crnti Mac CE control cell.

3.UE has upstream data sent, but upstream is out of sync or no SR resource

The main processes of competing access processes in both scenarios can be represented.

In particular, because in these two scenarios, the UE side of the RRC does not have uplink messages, can not carry the UE identity Crnti in the RRC layer message, so the UE needs to use the ul_grant assigned in the RAR, send a ENB CE to Crnti to identify the UE (that is, MSG3), of course, the subsequent MSG4 also need to encapsulate this Mac CE code stream into the UE contention Resolution Identity to complete the competitive solution process.

4. Toggle

The switching scenario takes precedence over the non-competitive access process, and if the non-competitive process is not performed, the competitive process is performed.

The main flowchart of the non-competitive process.

Because the non-competitive process, the source ENB will be in the rrcconnectionreconfiguration message to the UE using the preamble (the preamble value cannot be 0), as follows.

Competing access is required if non-competitive access is not possible. is the main flowchart of the competition process, do not carry a preamble or a preamble value of 0, indicating that the UE needs to perform competitive access.

Because it is a competing access, the source ENB sends a reconfiguration message without a preamble cell or with a preamble value of 0. In addition, because the Crnti before switching is present, MSG4 uses the existing crnti scrambling, scrambling, and not using Tcrnti.

5.eNB has downstream data, but upstream out of step is detected

This scenario also takes precedence over the non-competitive access process, and if the non-competitive process is not performed, the competitive process is executed.

The main flowchart of the non-competitive process.

In this scenario, the non-competitive preamble preamble used by the UE is carried in the PDCCH Order , that is enb by filling in the specific dci1a information to encode preamble information, the preamble value cannot be filled 0, Refer to the blog "LTE-TDD Random Access process (4)-riv analysis and selection of preamble resources ."

Competing access is required if non-competitive access is not possible. is the main flowchart of the competition process, you need to fill in the preamble value 0.

6. References

(1) 3GPP TS 36.300 V9.10.0 (2012-12) Overall description

(2) http://www.sharetechnote.com/

LTE-TDD Random Access Process (6)-describe the process of competing access and non-competitive access by scene