Buffer Status Report 5G NR(MAC)

Article by Abhijeet Kumar

Buffer Status Report (BSR) Overview:

The Buffer Status Report (BSR) is a mechanism used in the Medium Access Control (MAC) layer to inform the serving gNB (gNodeB) about the amount of uplink data available in a User Equipment's (UE) transmission buffer. This information is critical for the gNB to allocate uplink resources efficiently.

Let's Understand about MAC Structure and LCG

• Logical Channels Overview:

In the context of the MAC (Medium Access Control) layer of the 5G NR (New Radio) protocol, logical channels are used to differentiate between various types of data that the MAC layer handles. These logical channels are abstract representations of the data transfer services provided by the MAC layer to the RLC (Radio Link Control) layer above it.

Types of Logical Channels:

Control Channels:

BCCH (Broadcast Control Channel): Used for broadcasting system control information.

PCCH (Paging Control Channel): Used for paging information when the network needs to contact a UE (User Equipment).

CCCH (Common Control Channel): Used for transmitting control information during connection setup.

DCCH (Dedicated Control Channel): Used for transmitting dedicated control information to a specific UE.

MCCH (Multicast Control Channel): Used for control information for multicast/broadcast services.

Traffic Channels:

DTCH (Dedicated Traffic Channel): Used for user data that is dedicated to a specific UE.

MTCH (Multicast Traffic Channel): Used for transmitting multicast traffic data.

Mapping of Logical Channels to Transport Channels:

Uplink: Logical channels like CCCH, DCCH, and DTCH are mapped to the UL-SCH (Uplink Shared Channel).

Downlink: Logical channels like BCCH, PCCH, CCCH, DCCH, and DTCH are mapped to DL-SCH (Downlink Shared Channel).

Location of BSR (Buffer Status Report) in the MAC Layer:

The BSR is part of the MAC Control Elements (CEs) as detailed in the provided PDF document. Specifically, it is detailed in Section 6.1.3.1, which describes the BSR MAC CEs in the 5G NR MAC protocol.

BSR Mechanism:

• Purpose: The BSR informs the network about the amount of data that is waiting in the UE's buffer to be transmitted. This helps the network in scheduling uplink resources more efficiently.
• Triggers for BSR:
• Regular BSR:
• Triggered when UL data for a logical channel belonging to an LCG becomes available to the MAC entity.
• This happens if:
• The UL data belongs to a logical channel with a higher priority than any other logical channel containing available UL data in any LCG.
• logical channels in any LCG contain available UL data.
• Padding BSR:

Triggered when UL resources are allocated, and the number of padding bits is equal to or larger than the size of the Buffer Status Report MAC CE plus its subheader.

• Retransmission BSR (RetxBSR):

Triggered when the retxBSR-Timer expires, and at least one logical channel in an LCG still contains UL data.

• Periodic BSR:

Triggered when the periodicBSR-Timer expires, ensuring that the gNB regularly receives updates on the buffer status..

BSR Formats:

• Short BSR: Used when data is reported for one logical channel group (LCG).
• Long BSR: Used when data is reported for multiple LCGs.
• Truncated BSR: Used when there is not enough space in the MAC PDU to include a full BSR.

Logical Channel Prioritization:

• The MAC layer uses logical channel prioritization to determine the order in which data from different logical channels is sent. This ensures that higher-priority data is transmitted first.

Relation to MAC Control:

• The MAC layer controls the flow of data between logical channels and transport channels. It ensures that data is multiplexed correctly, based on the scheduling and prioritization decisions.

BSR Format Structure:

The BSR MAC CE can be divided into specific fields based on the type of BSR:

1. LCG ID: Identifies the LCG for which the buffer status is being reported.
2. Buffer Size: Indicates the amount of data in the buffer, typically encoded in a 6-bit or 8-bit field depending on the BSR type.
3. Subheader: A MAC subheader precedes the BSR CE and indicates the type and length of the BSR being reported

Short BSR

Short BSR:

• Use Case: Triggered when there is data for only one LCG or when padding allows for reporting a single LCG.
• Structure: Contains the LCG ID and a 6-bit buffer size field.

• this information.

Structure:

The Short BSR MAC CE consists of the following fields:

1. LCG ID (2 bits):This field identifies the Logical Channel Group (LCG) for which the buffer status is being reported.There are typically 4 LCGs (LCG0 to LCG3), hence the 2-bit field can represent values from 0 to 3.
2. Buffer Size (6 bits):This field indicates the amount of data waiting in the buffer for the specific LCG.The buffer size is encoded in a 6-bit field, which provides a range of possible values. The actual value represents the size of the data in the buffer in bytes.

1. Data Generation:

• Action: The UE generates a new video frame.
• Data Size: 3000 bytes
• Buffer Status Before New Data: 2000 bytes already in LCG0.
• Buffer Status After New Data: 2000 bytes (existing) + 3000 bytes (new) = 5000 bytes

2. Triggering the Short BSR:

• Trigger Condition: New data arrives in the buffer, requiring the UE to report the updated buffer status to the gNB.
• LCG ID: The buffer status will be reported for LCG0.
• Buffer Size in BSR: The BSR reports the current buffer size for LCG0: 5000 bytes.

3. Transmission of the Short BSR:

• MAC PDU Construction:
• Short BSR MAC Control Element (CE): Includes the LCG ID (LCG0) and buffer size (5000 bytes).
• MAC PDU Size: Let's assume the MAC PDU (including headers) is 50 bytes.
• Transmission:The MAC PDU is sent to the gNB over the PUSCH.

4. Reception and Processing at the gNB:

• gNB MAC Layer:
• Receives the MAC PDU.
• Extracts the Short BSR CE.
• LCG ID: Identifies LCG0.
• Buffer Size: Extracts the buffer size: 5000 bytes.
• Uplink Resource Scheduling:
• The gNB calculates the required uplink resources to accommodate the reported buffer size.
• Required Resource Blocks: If one resource block can carry 1000 bytes:
• Required?Resource?Blocks=5000?bytes/1000?bytes?per?block=5?blocks

5. Data Transmission:

• DCI Message:
• The gNB sends a DCI message to the UE, indicating the allocation of 5 uplink resource blocks.
• UE Uses Allocated Resources:
• The UE transmits the video data (5000 bytes) using the allocated 5 uplink resource blocks.

6. Post-Transmission:

• Buffer Status Update:
• After the transmission, the buffer status for LCG0 is updated.
• Buffer Status After Transmission: 5000 bytes - 5000 bytes (transmitted) = 0 bytes
• BSR Trigger for New Data:
• If new video data arrives or if the buffer still contains data, a new BSR (Regular, Periodic, or Padding BSR) may be triggered.

Usage Scenario:

• Example: Consider a scenario where the UE is engaged in a video call, which uses LCG0. If the UE only has data in the buffer for LCG0 and there is enough space in the MAC PDU, it will use a Short BSR to report the buffer status of LCG0. This report helps the gNB to allocate the necessary uplink resources for the UE.

Advantages:

• Efficiency: The Short BSR allows the UE to efficiently report the status of a single LCG, reducing the signaling overhead and making the best use of the available space in the MAC PDU.

Long BSR

This figure illustrates the structure of the Long BSR, Long Truncated BSR, and Pre-emptive BSR MAC Control Elements (CEs) in the 5G NR MAC layer. These BSR types are used by the UE to report the buffer status for multiple Logical Channel Groups (LCGs) simultaneously.

1. Long BSR MAC CE

Purpose:

• The Long BSR is used when the UE needs to report the buffer status for all or multiple active Logical Channel Groups (LCGs). This BSR format allows the UE to convey comprehensive information about the amount of data waiting to be transmitted in each LCG.

Structure:

• LCG IDs (LCG0 to LCG7):
• These fields indicate the logical channel groups for which the buffer status is being reported. The figure shows eight possible LCGs (LCG0 to LCG7), each corresponding to a different logical channel or group of channels.
• Buffer Sizes:
• For each LCG, a corresponding buffer size is reported. The buffer size fields indicate the amount of data (in bytes) waiting to be transmitted for each LCG.
• In the figure, Buffer Size 1 corresponds to LCG0, Buffer Size 2 corresponds to LCG1, and so on, up to Buffer Size m for LCG7.
• Octets:
• The buffer sizes are transmitted in a sequence of octets (bytes). For example, Octet 1 may contain the buffer size for LCG0, Octet 2 for LCG1, and so on.

Usage Scenario:

• Example: If a UE is simultaneously handling multiple applications, such as streaming video (LCG0), downloading a file (LCG1), and engaging in voice communication (LCG2), the UE might need to report the buffer status for all these LCGs to ensure the gNB allocates sufficient resources for each type of data.

2. Long Truncated BSR MAC CE

Purpose:

• The Long Truncated BSR is used when there isn't enough space in the MAC PDU to report the buffer status for all LCGs. In this case, the UE reports the buffer status for as many LCGs as possible, prioritizing those with the most critical or highest priority data.

Structure:

• The structure is similar to the Long BSR, with buffer sizes for each LCG. However, due to space constraints, not all LCGs may be reported. Only the buffer sizes that fit within the available space are included.

Usage Scenario:

• Example: If the UE is running out of space in the MAC PDU but still needs to report the buffer status, it will use a Long Truncated BSR to report as much of the buffer status as possible. For instance, if space allows for only four LCGs, the UE might report buffer sizes for LCG0, LCG1, LCG2, and LCG3, leaving out the rest.

1. Long BSR Example

Scenario: A User Equipment (UE) is simultaneously handling multiple applications:

• LCG0: Streaming video (e.g., 5,000 bytes waiting in the buffer).
• LCG1: Downloading a large file (e.g., 15,000 bytes waiting in the buffer).
• LCG2: Uploading photos to a cloud service (e.g., 2,000 bytes waiting in the buffer).
• LCG3: Sending messages through a messaging app (e.g., 500 bytes waiting in the buffer).

Use of Long BSR:

• The UE needs to inform the gNB (base station) about all the data waiting in its buffer so that the gNB can allocate appropriate uplink resources.
• The UE will send a Long BSR that includes buffer sizes for LCG0, LCG1, LCG2, and LCG3.
• Example Long BSR MAC CE:LCG0 Buffer Size: 5,000 bytes (e.g., Octet 1).LCG1 Buffer Size: 15,000 bytes (e.g., Octet 2).LCG2 Buffer Size: 2,000 bytes (e.g., Octet 3).LCG3 Buffer Size: 500 bytes (e.g., Octet 4).

Outcome:

• The gNB receives the Long BSR and schedules uplink resources accordingly. For example, it may allocate more resources to LCG1 due to its larger buffer size, ensuring that the large file download proceeds smoothly while also maintaining the video stream and other activities.