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PHR (Power Headroom Report) in LTE

In the context of LTE (Long-Term Evolution), PHR stands for Power Headroom Reporting. It is a mechanism used by User Equipment (UE) to report its available transmit power to the base station (eNodeB) in the LTE network.

The purpose of PHR is to allow the eNodeB to effectively manage the resources and allocate appropriate power levels to UEs. By knowing the power headroom of each UE, the eNodeB can make informed decisions regarding resource allocation and power control, ensuring efficient and reliable communication.

Here’s how PHR works in LTE:

  • Power Headroom: Power headroom refers to the difference between the maximum transmit power of the UE and the actual power level it is currently using. It represents the UE’s capability to increase its transmit power without exceeding the limits.

Power Headroom = UE Max Transmission Power – PUSCH Power = Pmax – P_pusch

Power Headroom is Positive in Value (+) = means still have some bandwidth and can transmit more data if allowed.

power Headroom is negative in value (-) = Already transmitting more power than allowed.

  • Power Headroom Reporting (PHR): UEs periodically measure their available power headroom and report it to the eNodeB. This reporting is initiated by the eNodeB through specific signalling messages.
  • PHR Configuration: The eNodeB configures the PHR parameters for each UE, including the reporting periodicity and the measurement rules. The reporting periodicity determines how often the UE needs to send the PHR report, while the measurement rules define how the UE calculates its power headroom.
  • PHR Calculation: The UE calculates its power headroom by considering factors such as the maximum transmit power defined by the network, the current transmit power, and any power limitations due to neighbouring cells or interference. The calculation ensures that the UE reports an accurate representation of its available power.
  • PHR Report: The UE sends the PHR report to the eNodeB over the uplink channel. The report includes the power headroom value and any additional information specified by the PHR configuration.
  • Resource Allocation: Upon receiving the PHR reports from UEs, the eNodeB utilizes the information to make decisions on resource allocation and power control. It can adjust the assigned resources and power levels for each UE to maintain optimal system performance and avoid interference issues.

Power Headroom Report MAC Control Element

The Power Headroom Report (PHR) MAC control element can be recognized through a MAC PDU sub header containing a specific LCID, as indicated in table 6.2.1-2(3GPP TS 36.321). It is a compact entity with a fixed size and comprises a solitary octet, which is defined as follows (refer to figure 6.1.3.6-1) (3GPP TS 36.321):

Figure 6.1.3.6-1(3GPP TS 36.321): PHR MAC control element

Power Headroom levels for PHR

The power headroom reporting range is from -23 …+40 dB.

Power headroom report Mapping

On which Occasions UE will send the PHR

  • When down link pathloss threshold is reached (dl-PathlossChange).
  • periodicPHR-Timer is set.The reporting periodicity is configured by the eNodeB through Radio Resource Control (RRC) signalling. The UE calculates its Power Headroom value and sends periodic PHR messages to the eNodeB at the specified intervals. This type of PHR provides the eNodeB with information about the UE’s available transmit power over time, allowing for dynamic power control and resource allocation.

These can be configured in UE through RRC (RRC Connection Setup, RRC Connection Reconfiguration) in I.E phr-Config

PHR-PeriodicTimer: This parameter specifies the reporting periodicity for the UE’s PHR transmissions. It defines the interval at which the UE should send PHR reports to the eNodeB. The value of PHR-PeriodicTimer determines the duration between consecutive PHR reports.

  • Value in number of sub-frames.
  • Value sf10 corresponds to 10 subframes, sf30 corresponds to 30 subframes and so on.

PHR-ProhibitTimer: This parameter sets a duration during which the UE is prohibited from sending PHR reports after a specific event. It helps to prevent excessive reporting when certain events occur in quick succession. The PHR-ProhibitTimer ensures a controlled and efficient reporting mechanism.

  • Value in number of sub-frames.
  • Value sf0 corresponds to 0 subframe, sf10 corresponds to 10 subframes and so on.

dl-PathlossChange: DL Pathloss Change and the change of the required power backoff due to power management.

  • Value in dB.
  • Value dB1 corresponds to 1 dB, dB3 corresponds to 3 dB and so on

Extended Power Headroom Report MAC Control Element

The Extended Power Headroom Report (PHR) MAC control element is identified by a MAC PDU subheader with an LCID as specified in table 6.2.1-2.

  • It has a variable size and is defined in Figure 6.1.3.6a-2.
  • When Type 2 PH is reported, the octet containing the Type 2 PH field is included first after the octet indicating the presence of PH per SCell, followed by an octet containing the associated PCMAX,c field (if reported).
  • The PHR elements for the PCell and each SCell indicated in the bitmap are included in ascending order based on the ServCellIndex [8].
  • The Extended PHR MAC Control Element is defined by several fields:
  • Ci: This field indicates the presence of a PH field for the SCell with SCellIndex i. A value of “1” indicates a reported PH field for the corresponding SCell, while “0” indicates no reported PH field.
  • R: Reserved bit, set to “0”.
  • V: This field indicates if the PH value is based on a real transmission or a reference format. V=0 indicates real transmission on PUSCH (Type 1 PH) or PUCCH (Type 2 PH), while V=1 indicates the use of a reference format.
  • Power Headroom (PH): This field represents the power headroom level and has a length of 6 bits. The corresponding PH values and their power headroom levels can be found in Table 6.1.3.6-1.
  • P: This field indicates whether the MAC entity applies power backoff due to power management. P=1 signifies that the corresponding PCMAX,c field would have had a different value without power backoff due to power management.
  • PCMAX,c: If present, this field indicates the PCMAX,c or PCMAX ,c~ [2] used for calculating the preceding PH field. The reported PCMAX,c values and their corresponding nominal UE transmit power levels can be found in Table 6.1.3.6a-1.

Extended PHR MAC Control Element

Dual Connectivity Power Headroom Report MAC Control Element

The Dual Connectivity Power Headroom Report (PHR) MAC control element is identified by a MAC PDU subheader with a specific LCID. It can vary in size and is defined in Figure 6.1.3.6b-1.

When reporting Type 2 PH for the PCell, the Type 2 PH field is included first, followed by the associated PCMAX,c field (if reported). Similarly, when reporting Type 2 PH for the PSCell, the Type 2 PH field and its associated PCMAX,c field (if reported) are included. Then, in ascending order based on the ServCellIndex, the Type 1 PH field and its associated PCMAX,c field (if reported) are included for the PCell and all other serving cells of all MAC entities indicated in the bitmap.

The Dual Connectivity PHR MAC Control Element is defined as follows:

  • Ci: Indicates the presence of a PH field for a serving cell (except the PCell) with SCellIndex i. “1” means a reported PH field, while “0” means no reported PH field.
  • R: Reserved bit, always set to “0.”
  • V: Indicates if the PH value is based on a real transmission or a reference format. For Type 1 PH, V=0 means real transmission on PUSCH, and V=1 means a PUSCH reference format is used. For Type 2 PH, V=0 means real transmission on PUCCH, and V=1 means a PUCCH reference format is used. Also, V=0 indicates the presence of the associated PCMAX,c field, while V=1 means the associated PCMAX,c field is omitted.
  • Power Headroom (PH): Indicates the power headroom level with a field length of 6 bits. The reported PH and corresponding power headroom levels are specified in Table 6.1.3.6-1.
  • P: Indicates whether power backoff due to power management is applied. P=1 means power backoff has been applied, affecting the value of the corresponding PCMAX,c field if no power backoff were applied.
  • PCMAX,c: If present, indicates the PCMAX,c or PCMAX,c~ [2] used for calculating the preceding PH field. The reported PCMAX,c values and corresponding nominal UE transmit power levels can be found in Table 6.1.3.6a-1.

Fig: Dual Connectivity PHR MAC Control Element

References:

  • 36.133: E-UTRA Requirements for support of radio resource management
  • 36.321: E-UTRA Medium Access Control Protocol
  • 36.331: E-UTRA Radio Resource Control Protocol

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