Dear community friends, today I want to introduce LTE random access.
What is random access?
Random access is a mandatory process of establishing a radio link between the UE and the network. Data interoperation between the eNodeB and the UE can be performed normally only after the random access is complete.
Principles of Random Access
1. Random access is a process of uplink time-frequency synchronization between a UE and an E-UTRAN.
2. Before random access, the physical layer should receive the following information from the upper layer:
Random access channel PRACH parameters: PRACH configuration, frequency-domain position, and preamble format.
The cell uses the preamble root sequence and its cyclic shift parameters to demodulate random access preambles.
3. The random access process at the physical layer includes two steps:
Step 1 The UE sends a random access preamble.
Step 2 The E-UTRAN responds to random access.
Random Access Procedure
1. The higher layer requests to send a random access preamble, and then triggers a physical random access procedure.
2. The higher layer indicates, in the request, a preamble index, a preamble target receive power, a related RA-RNTI, and a resource status of a random access channel.
3. The UE determines that the transmit power of the random access channel is the target receive power of the preamble plus the path loss. The transmit power does not exceed the maximum transmit power of the UE, and the path loss is the value estimated by the UE through the downlink.
4. The preamble sequence is selected based on the preamble index.
5. The UE calculates the transmit power and uses the selected preamble sequence to transmit a single preamble in a specified random access channel resource.
6. Within the time window set by the higher layer, the UE attempts to detect the downlink control channel PDCCH identified by the RA-RNTI of the UE. If the PDCCH is detected, the corresponding downlink shared channel PDSCH is transmitted to the upper layer, and the upper layer parses the 20-bit response information from the shared channel.
Figure 1 Random Access Procedure
Successful random access means that the UE completes uplink synchronization.
That's all I've covered about random access, and I'll share MIMO technology in my next post.
You are welcome to read and comment.
