MAC CE Power Header Room(PHR)

Article by Abhijeet Kumar

What is a MAC Control Element?

A MAC Control Element (CE) is a specialized data structure within the MAC Protocol Data Unit (PDU) used for conveying control information. Unlike MAC Service Data Units (SDUs), which carry user data, MAC CEs carry control information necessary for the operation of the MAC layer and overall network coordination.

Types of MAC Control Elements

MAC Control Elements come in various types, each serving a specific function in the 5G NR system. Some common types include:

1. Power Headroom Report (PHR) CE: Provides information about the UE's power status, indicating how much additional power is available for transmission.
2. Buffer Status Report (BSR) CE: Communicates the status of the UE’s uplink data buffers, helping the network manage resource allocation efficiently.
3. Scheduling Request (SR) CE: Used by the UE to request uplink resources from the network.
4. Beam Failure Recovery (BFR) CE: Helps in indicating the need for beam recovery procedures due to signal degradation.
5. CRNTI CE: Communicates the temporary identity of the UE for certain procedures.
6. DRX Command CE: Manages Discontinuous Reception (DRX) cycles to save battery life.

Role of MAC Control Elements

The primary role of MAC Control Elements is to facilitate control communication between the UE and the network, ensuring effective and efficient management of radio resources. Here are some key roles:

1. Resource Allocation and Scheduling:BSR and SR: By reporting buffer statuses and requesting scheduling, MAC CEs enable dynamic and efficient allocation of uplink resources, ensuring that the network can accommodate varying traffic loads.
2. Power Management:PHR: By providing power headroom information, MAC CEs assist the network in managing power control, optimizing the balance between power efficiency and transmission quality.
3. Mobility and Beam Management:BFR: Essential for maintaining robust communication links, especially in high-mobility scenarios or environments with significant signal obstructions.
4. Identity Management:CRNTI: Crucial for identifying and managing UEs during various network procedures, including handovers and resource allocation.
5. Energy Efficiency:DRX Command: Facilitates energy-saving strategies by managing DRX cycles, allowing UEs to enter low-power states when data transmission is not required.

Structure of MAC Control Elements

MAC CEs are embedded within MAC PDUs and have a specific structure, typically comprising a header and a payload. The header indicates the type of CE and its length, while the payload carries the actual control information. The precise format and structure can vary depending on the type of CE.

Transmission of MAC Control Elements

MAC Control Elements can be transmitted in two ways:

1. Standalone MAC CEs: Sent independently within a MAC PDU when no user data is available.
2. Piggybacked MAC CEs: Included alongside user data within a MAC PDU, optimizing resource utilization by combining control information with data transmission.

Why We Need MAC CEs

Without MAC CEs, the communication process would be chaotic and inefficient. Devices would constantly compete for access to the radio channel, leading to collisions and dropped connections. Data packets might get lost or corrupted without a way to request retransmissions. Power usage would be uncontrolled, draining batteries quickly.

MAC CEs bring order and optimization to the 5G communication process, ensuring that data flows smoothly, reliably, and efficiently between your device and the network. They are the unsung heroes that enable the high-speed, low-latency, and power-efficient communication that 5G promises.

Role of Power Headroom Reporting (PHR) in 5G NR

Power Headroom Reporting (PHR) is essential in the 5G NR (New Radio) system for efficient power management and resource allocation in uplink transmissions. PHR informs the network about the difference between the maximum transmission power and the current transmission power of the User Equipment (UE). This information helps the network optimize power control and scheduling decisions, ensuring that the UE can maintain a reliable uplink connection without exceeding its power capabilities.

Single Entry PHR MAC CE (6.1.3.8)

The Single Entry PHR MAC CE is used to report the power headroom for a single serving cell. This control element is identified by a specific Logical Channel ID (LCID) in the MAC subheader.

Figure 6.1.3.8-1: Single Entry PHR MAC CE

The structure of the Single Entry PHR MAC CE includes the following fields:

• P (Power Management Indicator): This 1-bit field indicates whether power backoff due to power management is applied when the UE operates in FR2.
• R (Reserved): This 1-bit field is reserved and set to 0.
• PH (Power Headroom): This 6-bit field indicates the power headroom level for the primary cell (PCell).
• MPE or R (Measured Power Reduction or Reserved): This field is used to indicate the effective power reduction for Maximum Permissible Exposure (MPE) or is reserved.
• PCMAX,f,c: This 6-bit field indicates the nominal UE transmit power level.

Table 6.1.3.8-1: Power Headroom Levels for PHR

This table maps the 6-bit Power Headroom (PH) field to specific power headroom levels. Each PH value (from 0 to 63) corresponds to a different power headroom level (POWER_HEADROOM_0 to POWER_HEADROOM_63). The power headroom level represents the difference between the current transmission power and the maximum allowable transmission power.

Table 6.1.3.8-2: Nominal UE Transmit Power Level for PHR

This table maps the 6-bit PCMAX,f,c field to specific nominal UE transmit power levels (PCMAX_C_00 to PCMAX_C_63). Each value of PCMAX,f,c corresponds to a different nominal transmit power level.

Table 6.1.3.8-3: Effective Power Reduction for MPE P-MPR

This table maps the 2-bit MPE field to specific measured Power Management Reduction (P-MPR) values. Each MPE value (from 0 to 3) corresponds to a different P-MPR value (P-MPR_00 to P-MPR_03).

How It Works

Power Headroom Reporting Process

1. PH Calculation:
2. Transmit Power Level Reporting:
3. Power Management Indicator:
4. Effective Power Reduction Reporting:

Example of VoLTE Service:

Scenario Details

• Service: VoLTE
• Current Transmission Power: 15 dBm
• Maximum Allowable Transmission Power: 23 dBm
• Frequency Range: FR1 (Frequency Range 1)

Step-by-Step Process

Step 1: Calculate Power Headroom (PH)

The power headroom is calculated as: Power?Headroom=Maximum?Allowable?Transmission?Power?Current?Transmission?Power

Power?Headroom=23dBm?15dBm=8dBm

Step 2: Map Power Headroom to PH Field

Using Table 6.1.3.8-1, the corresponding PH value for a power headroom of 8 dBm is: POWER_HEADROOM=8

Thus, the PH field in the MAC CE will be set to 8.

Step 3: Determine Nominal UE Transmit Power Level

Using Table 6.1.3.8-2, the corresponding value for the current transmit power of 15 dBm is: PCMAX_C=15

Therefore, the PCMAX,f,c field will be set to 15.

Step 4: Set the Power Management Indicator (P)

Since the UE is operating in FR1 and not FR2, there is no power backoff due to power management applied. Hence, the P field is set to 0.

Step 5: Construct the Single Entry PHR MAC CE

The Single Entry PHR MAC CE is constructed with the following fields:

• P (Power Management Indicator): 0
• R (Reserved): 0
• PH (Power Headroom): 8
• MPE or R: Not applicable, set to 0
• PCMAX,f,c: 15

Multiple Entry PHR MAC CE (6.1.3.9)

The Multiple Entry PHR MAC CE reports power headroom for multiple serving cells, making it suitable for scenarios with carrier aggregation or dual connectivity.

Structure

The Multiple Entry PHR MAC CE includes:

• Bitmap: Indicates which serving cells' power headroom values are reported.
• Type 2 PH Field and PCMAX,f,c Field (if reported): For the SpCell (Secondary Cell) of the other MAC entity.
• Type 1 PH Field and PCMAX,f,c Field (if reported): For the PCell (Primary Cell).
• Type X PH Fields and PCMAX,f,c Fields (if reported): For other serving cells indicated in the bitmap, in ascending order based on ServCellIndex.

A configuration parameter determines the presence of Type 2 PH field for SpCell (phr-Type2OtherCell).

Structure Breakdown

The structure consists of several fields for different serving cells, each field indicating specific information:

1. C7, C6, ..., C1, R: These are reserved bits.
2. P (Power Management Indicator): This 1-bit field indicates whether power backoff due to power management is applied.
3. V (Validity Indicator): This 1-bit field indicates whether the corresponding PHR and PCMAX,f,c fields are valid.
4. PH (Power Headroom): This 6-bit field indicates the power headroom level for the serving cell.
5. PCMAX,f,c: This field indicates the nominal UE transmit power level.

Detailed Fields Explanation:

1. PH (Type 2, SpCell of the other MAC entity):
2. PH (Type 1, PCell):
3. PH (Type X, Serving Cell 1):
4. PH (Type X, Serving Cell n):

How It Works:

Reporting Process:

1. PH Calculation for Each Cell:
2. Nominal Transmit Power Levels:
3. Constructing the MAC CE:

Example:

Consider a UE connected to three cells:

• PCell: Current transmit power = 15 dBm, Max transmit power = 23 dBm, PH = 8 dBm
• SCell1: Current transmit power = 17 dBm, Max transmit power = 23 dBm, PH = 6 dBm
• SCell2: Current transmit power = 18 dBm, Max transmit power = 23 dBm, PH = 5 dBm

Why Single and Multiple Entry PHR are Needed

• Single Entry PHR: This is simpler and used when the UE is connected to only one serving cell or when only the primary cell's power headroom needs to be reported. It reduces signaling overhead in scenarios where multi-cell reporting is unnecessary.
• Multiple Entry PHR: This is essential for more complex scenarios involving carrier aggregation or dual connectivity, where the UE is connected to multiple serving cells. It provides a comprehensive view of the UE's power capabilities across all active cells, enabling the network to optimize power control and resource allocation more effectively.

Detailed Explanation with Examples

Scenario 1: Single Serving Cell

• Setup: A UE is connected to a single cell, Cell A.
• Current Power: The UE is transmitting at 18 dBm.
• Maximum Power: The maximum allowable transmission power is 23 dBm.
• Power Headroom: The power headroom is 5 dBm.
• PHR Reporting: The UE reports this power headroom using the Single Entry PHR MAC CE. The PH field will indicate the level corresponding to 5 dBm. If power management adjustments (e.g., due to FR2) are applied, the P field will reflect this.

Scenario 2: Multiple Serving Cells

• Setup: A UE is connected to three cells, Cell A (PCell), Cell B (SCell1), and Cell C (SCell2).
• Current Power Levels: The UE transmits at 18 dBm on Cell A, 19 dBm on Cell B, and 20 dBm on Cell C.
• Maximum Power Levels: The maximum allowable transmission power is 23 dBm on all cells.
• Power Headroom: The power headroom is 5 dBm for Cell A, 4 dBm for Cell B, and 3 dBm for Cell C.
• PHR Reporting: The UE uses the Multiple Entry PHR MAC CE. The bitmap indicates that power headroom for all three cells is reported. The PH fields will indicate the levels corresponding to 5 dBm, 4 dBm, and 3 dBm, respectively. The PCMAX,f,c fields will provide additional information if configured.

RRC Parameters for PHR.

The Radio Resource Control (RRC) layer in 5G NR configures various parameters to control the Power Headroom Reporting (PHR). These parameters are crucial for ensuring efficient uplink power management and resource allocation. The key RRC parameters related to PHR include:

1. phr-PeriodicTimer: This timer controls the periodicity of PHR reporting. When this timer expires, a PHR is triggered.
2. phr-ProhibitTimer: This timer prevents the UE from reporting PHR too frequently. If it is running, PHR reporting is prohibited.
3. phr-Tx-PowerFactorChange: This parameter defines the threshold for the change in the UE's transmit power, which can trigger a PHR if the change exceeds the configured value.
4. phr-Type2OtherCell: Indicates if the UE should report Type 2 PHR for the SpCell (Secondary Cell) of the other MAC entity.
5. phr-ModeOtherCG: Indicates the mode (real or virtual) used for PHR of activated cells that are part of the other Cell Group (MCG or SCG).
6. multiplePHR: Indicates if the UE should use the Multiple Entry PHR MAC CE or the Single Entry PHR MAC CE. It is configured to true for scenarios involving multi-Radio Dual Connectivity (MR-DC) and uplink carrier aggregation.
7. mpe-Reporting-FR2: Indicates whether the UE should report MPE P-MPR in the PHR MAC control element when operating in Frequency Range 2 (FR2).
8. mpe-ProhibitTimer: Similar to the phr-ProhibitTimer but specific to MPE reporting.
9. mpe-Threshold: Defines the P-MPR threshold in dB for reporting MPE P-MPR when FR2 is configured.
10. numberOfN: Indicates the number of reported P-MPR values in a PHR MAC CE.
11. phr-AssumedPUSCH-Reporting: Indicates if PHR with assumed PUSCH (Physical Uplink Shared Channel) is reported.
12. twoPHRMode: Indicates if power headroom should be reported as two PHRs, each associated with an SRS (Sounding Reference Signal) resource set.

PHR Algorithm

The PHR algorithm in 5G NR involves several steps to determine when and how PHR should be reported. Here’s a detailed breakdown of the PHR algorithm:

1. Trigger Conditions: PHR can be triggered under various conditions:

• Expiry of phr-PeriodicTimer.
• Expiry of phr-ProhibitTimer, provided there is a change in path loss greater than phr-Tx-PowerFactorChange dB.
• Upon configuration or reconfiguration of PHR functionality by upper layers.
• Activation of an SCell with configured uplink.
• Activation of an SCG.
• Addition of a PSCell, except if the SCG is deactivated.
• Canceling PHR: PHRs can be canceled under certain conditions, such as when the uplink grant can accommodate all pending data and the PHR MAC CE plus its subheader.
• Generating PHR:When triggered, the MAC entity checks if it has uplink resources for new transmission.
• For each activated serving cell, it obtains the necessary values (e.g., PCMAX,f,c) from the physical layer.
• It generates the appropriate PHR MAC CE (Single Entry or Multiple Entry) based on the configuration.
• Reporting Multiple PHRs: In scenarios involving multiple serving cells:The algorithm uses a bitmap to indicate the presence of PHR fields for each serving cell.
• For each serving cell, it reports the power headroom and the associated PCMAX,f,c.
• If configured with twoPHRMode, it reports two power headrooms for cells configured with multiple TRP PUSCH repetition.

Real-Time Example

Scenario: A UE connected to a 5G NR network with carrier aggregation involving one Primary Cell (PCell) and two Secondary Cells (SCells).

1. Initial Setup:
2. PHR Triggering:
3. Reporting:
4. PHR MAC CE Structure: