SR(Scheduling Request)
Here's a technical breakdown of how SR works:
- Triggering SR: When a UE has uplink data to send but doesn’t have a scheduled resource allocation from the base station, it initiates a Scheduling Request. This might occur in scenarios like when the UE has just powered on, has data to send, or its allocated resources have been depleted.
- Contention-Based Process: SR is part of the contention-based random access procedure in LTE and 5G. The UE randomly selects a timing advance and transmits the SR in a specific resource known as the "Random Access Channel" (RACH).
- Timing Advance: The UE needs to adjust the timing of its transmission to ensure that it arrives within the specific time window assigned by the base station. The timing advance value assists in synchronizing the UE's transmission with the base station's reception window.
- SR Format: The SR message contains information about the UE's identity, the reason for the request, and sometimes additional information depending on the communication standard (LTE, 5G, etc.). This message is transmitted on the RACH.
- Contention Resolution: Multiple UEs might attempt to access the network simultaneously using the RACH. To resolve contention and avoid collisions, the base station uses contention resolution mechanisms like random backoff timers or multi-user detection algorithms to identify and allocate resources to the requesting UE.
- Granting Resources: Upon receiving the SR, if the base station successfully decodes the UE's request and determines it needs resources, it assigns a specific time-frequency resource to the UE for transmitting its uplink data. This assignment is often in the form of a scheduling grant.
- Transmission of Uplink Data: Once the UE receives the resource allocation, it uses the granted resource to transmit its uplink data to the base station.
- Acknowledgment: After receiving the uplink data, the base station sends an acknowledgment to the UE to confirm successful reception.