Multiple PRACH Transmissions with Same Beams:
-
Multiple PRACH transmissions with the same beams is a feature introduced in 5G Release 18 by 3GPP.
-
In 5G, the PRACH is used by the UE to initiate the random access procedure, which is the first step in connecting to the network. By allowing multiple PRACH transmissions using the same beam, the network improves the likelihood of successful detection and synchronization, especially in scenarios with high path loss or signal attenuation.
Scenario:
- For example, a UE located at the edge of a cell where the signal strength is weak. In such a scenario, a single PRACH transmission might fail due to poor signal conditions. However, by allowing the UE to transmit multiple PRACH preambles using the same beam, the chances of at least one of these transmissions being detected by the base station are significantly increased.
Call Flow:
Initial Random Access Request:
- Step 1: UE selects a random access preamble from a predefined set and transmits it using the selected beam.
- Step 2: UE repeats the transmission of the same preamble using the same beam configuration multiple times to increase the probability of detection.
- Reception and Processing by gNB:
- Step 3: gNB (base station) monitors the PRACH resources and detects the repeated preambles. The repetition improves the chances of correct detection and timing estimation.
- Step 4: Upon detecting the PRACH preamble, the gNB sends a Random Access Response (RAR) message, including timing advance information and initial uplink grant.
- Random Access Response:
- Step 5: UE receives the RAR, which confirms the successful detection of its PRACH preamble. The RAR provides the UE with the necessary timing advance and uplink resources to continue the connection setup.
- Step 6: UE uses the provided uplink grant to send the RRC (Radio Resource Control) connection request message.
- RRC Connection Setup:
- Step 7: gNB processes the RRC connection request and responds with an RRC connection setup message.
- Step 8: UE completes the RRC connection setup by sending an RRC connection setup complete message to the gNB.
LinkedIn: 