When the UE has obtained system information, it has to request an RRCconnection. Like it has no dedicated resources, the UE requests the connection using the Random Access Procedure using common uplink resources.
- At the end of the procedure, the UE is RRC connected
- UE and eNodeB are able to exchange data using dedicated radio resources
This procedure is also used in case of:
- Initial access from RRC_IDLE
- RRC Connection Re-establishment procedure
- Handover
There are 4 steps to allow the UE to exchange signaling messages with the eNodeB
Random Access Response in LTE
- PDSCH signaled through PDCCH with RNTI = PRACH subframe number
- Contains the signature index of the detected RACH preamble (1rst contention resolution step)
- Contains the timing alignment command to apply (2 ways transmission time)
- Contains a temporary RNTI assigned to the UE
- Contains the resource assignment for next PUSCH (acts as DCI0)
The Physical Random Access Channel (PRACH):
- is made up of 6 RBs anywhere in the spectrum.
- occupies between 1 and 3 sub-frames per frame.
The preamble:
- is generated from the Zadoff-Chu sequence.
- is associated to an RA-RNTI.
There are 64 preamble sequences available per cell.
The response conveys:
- The RA-RNTI, to match the response with the preamble
- Timing alignment information
- The Temporary C-RNTI
- The initial UL grant, allocation of resources for the temporary C-RNTI.
The C-RNTI identifies an RRC Connection.
What is the Random Access Procedure in LTE?
The Random Access Procedure in LTE is the process used by a device (UE) to connect to the network for the first time or to re-establish a connection after it has been lost. This procedure allows the device to request access to the network in a way that minimizes the risk of collisions and interference with other devices.
Steps of Random Access Procedure
- Step 1: PRACH (Physical Random Access Channel) TransmissionThe device sends a random access preamble on the PRACH to indicate it wants to communicate with the network. The preamble is randomly chosen from a predefined set to reduce the chances of collision with other devices.
- Step 2: Random Access ResponseThe eNB (evolved NodeB) responds to the preamble with a Random Access Response (RAR) message. This response includes timing adjustments (T_ADJUST) to help the device synchronize with the network, and it also assigns a temporary identifier (RNTI) for communication.
- Step 3: RRC Connection RequestOnce the device receives the RAR, it sends an RRC (Radio Resource Control) Connection Request message to the eNB. This message includes the device’s identity and a request for resources to be allocated for data transmission.
- Step 4: RRC Connection SetupThe eNB responds with an RRC Connection Setup message, indicating the allocation of resources for the device to continue communication. This message also contains information for security settings and other configurations.
- Step 5: RRC Connection Setup CompleteThe device responds with an RRC Connection Setup Complete message, confirming that the connection setup is successful. The connection is now fully established, and data transmission can begin.
Why Random Access Procedure Matters
The random access procedure ensures that devices can efficiently and reliably request network resources when needed. It minimizes collisions between devices trying to connect and ensures that the network can handle connections from multiple devices without overloading.