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5G NR SR – Scheduling Request

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Let's delve into the technical details of the 5G New Radio (NR) Scheduling Request (SR).

1. Background:

In 5G NR, scheduling is a mechanism where the base station (gNB) assigns resources (like time and frequency) to User Equipment (UE) for transmitting data. Sometimes, the UE needs to send a scheduling request to the gNB when it has data to transmit but does not have resources allocated for it.

2. Scheduling Request (SR):

A Scheduling Request (SR) is a mechanism in 5G NR by which the UE can request the gNB for uplink resources to transmit data. SR is triggered when:

  • The UE has uplink data to send but doesn't have resources (like uplink grants) to send it.
  • The UE needs to notify the gNB that there's important data to send, and it needs uplink resources.

3. Procedure:

Here's a high-level procedure of how the SR works:

  1. SR Triggering: The UE monitors certain physical resources to detect the need for a scheduling request. For instance, if the UE has data to send but doesn't have an uplink grant, it might trigger an SR.
  2. SR Transmission: Upon detecting the need for a scheduling request, the UE sends a specific SR signal on the Physical Uplink Control Channel (PUCCH) or Physical Uplink Shared Channel (PUSCH). This signal is called the SR preamble.
  3. SR Preamble: The SR preamble is a predefined sequence that helps the gNB detect and decode the scheduling request from the UE.
  4. gNB Response: Once the gNB receives the SR, it responds by allocating uplink resources (like granting the UE an uplink resource block) so that the UE can send its data.

4. SR Configuration:

The SR procedure in 5G NR is configured by various parameters, such as:

  • SR periodicity: This defines how often the UE should send the SR. For example, the UE might be configured to send an SR every 20 milliseconds if it hasn't received an uplink grant.
  • SR resources: These are specific resources (like PUCCH or PUSCH) where the UE should send the SR.
  • SR configurations for different scenarios: Depending on the UE's mobility, location, or other factors, different SR configurations might be applied to optimize the performance.

5. Importance:

The SR mechanism is crucial for efficient resource utilization in 5G NR networks. By allowing UEs to request resources only when needed, it ensures that resources are not wasted. This dynamic allocation mechanism ensures that when UEs have data to send, they can quickly get the necessary resources, enhancing the overall efficiency and performance of the 5G network.