NR Power Headroom Report (2/3): What Does PHR Tell the gNB?
How NR calculates power headroom, distinguishes Type 1, Type 2, and Type 3 PH, and reports real or reference transmissions through a PHR MAC CE.
NR Uplink Power Control and PHR Series
- Power Budget, PSD, and
P_CMAX- What Does a Power Headroom Report Tell the gNB?
- How a gNB Scheduler Uses PHR — Coming soon
In the previous article, we summarized PUSCH transmit power with the following structure:
\[P_{\text{actual}} = \min \{ P_{\text{CMAX}}, P_{\text{required}} \}\]The gNB cannot directly observe the UE’s internal power budget. Instead, the UE uses a Power Headroom Report (PHR) to indicate how much margin remains before it reaches its maximum power for a real or reference uplink transmission.
1. What Does PH Mean?
An intuitive expression for Type 1 power headroom is:
\[PH = P_{\text{CMAX}} - P_{\text{required}}\]PH > 0: The UE has margin above the power required by the power-control equation.PH ≈ 0: The UE is near its maximum power.PH < 0: The required power exceeds the maximum power.
For example, suppose:
\[P_{\text{CMAX}} = 23\text{ dBm}\]and:
\[P_{\text{required}} = 18\text{ dBm}\]The power headroom is 5 dB:
\[PH = 23 - 18 = 5\text{ dB}\]If the required power were 26 dBm instead, the PH would be -3 dB. This does not mean that the UE actually transmitted at 26 dBm. Its actual transmit power may be capped by P_CMAX; PH represents the difference between the nominal maximum power and the estimated required power.
2. Type 1 PH
Type 1 PH is the power headroom for UL-SCH/PUSCH transmission on an activated serving cell.
A simplified expression based on a real PUSCH transmission is:
\[PH_{\text{type1,real}} = P_{\text{CMAX}} - \left\{ P_O + 10\log_{10}(2^\mu M_{\text{RB}}) + \alpha PL + \Delta_{TF} + f \right\}\]Here, M_RB, Δ_TF, and the power-control adjustment reflect the conditions of the real PUSCH transmission.
A real Type 1 PH therefore answers a question close to:
Given this PUSCH grant, how much power headroom does the UE have left?
Type 1 is the PH type most directly related to PUSCH scheduling.
3. Type 2 and Type 3 PH
The three PH types are not simply generic categories for every uplink channel. Each type needs to be understood together with the deployment and transmission for which it is defined.
| Type | Basis | Meaning |
|---|---|---|
| Type 1 | UL-SCH/PUSCH | PUSCH power headroom for an activated serving cell |
| Type 2 | SpCell of the other MAC entity | Power headroom related to UL-SCH and PUCCH transmission on the other MAC entity’s SpCell in EN-DC, NE-DC, and NGEN-DC |
| Type 3 | SRS | SRS power headroom for an activated serving cell |
It would be too broad to describe Type 2 simply as “PH when PUSCH and PUCCH are transmitted simultaneously” on an ordinary NR serving cell. Following the definition in TS 38.321, it is more accurate to identify Type 2 as the PH type associated with the SpCell of the other MAC entity in dual-connectivity scenarios.
Because Type 3 is based on SRS transmission, it can help characterize uplink sounding and the associated power conditions.
4. Real Transmissions and Reference Formats
A PHR may be required even when a serving cell has no real PUSCH or SRS transmission at that moment. In that case, the UE can calculate PH using a reference format defined by the specifications.
| PH type | Real-transmission basis | Reference basis |
|---|---|---|
| Type 1 | Real PUSCH | PUSCH reference format |
| Type 2 | Real PUCCH | PUCCH reference format |
| Type 3 | Real SRS | SRS reference format |
Conceptually, Type 1 reference PH can be written as:
\[PH_{\text{type1,reference}} = \widetilde{P}_{\text{CMAX}} - \left\{ P_O + \alpha PL + f \right\}\]Because this calculation does not use a real PUSCH allocation, it is not based on the M_RB and Δ_TF of an actual grant. Reference PH is therefore best understood as:
A headroom estimate calculated under specification-defined reference PUSCH conditions
The term reference PH is preferable here to the potentially ambiguous expression “virtual PH.”
5. phr-ModeOtherCG = virtual Is a Different Concept
Dual connectivity introduces a separate setting called phr-ModeOtherCG.
When this setting is virtual, the UE calculates PH for one cell group under the assumption that no PUSCH or PUCCH is transmitted in the other cell group.
This virtual condition is distinct from using a reference format because a serving cell has no real transmission.
| Situation | Meaning |
|---|---|
| No real transmission | Calculate PH using the reference format for that PH type |
phr-ModeOtherCG = virtual | Calculate PH assuming no uplink transmission in the other cell group |
Calling both cases “virtual PHR” obscures which assumption the scheduler should apply.
6. When Is a PHR Triggered?
PHR is not continuous, per-slot telemetry. RRC configures the reporting behavior, and MAC transmits a PHR MAC CE when a trigger condition is met and an uplink grant is available.
Representative triggers include:
- Expiry of
phr-PeriodicTimer - A path-loss change greater than the configured threshold when the
phr-ProhibitTimercondition is satisfied - Configuration or reconfiguration of PHR functionality
- Activation of an SCell with configured uplink resources
- Certain multi-connectivity events, such as SCG activation or PSCell addition
The PH received by the gNB therefore reflects the conditions under which the PHR was calculated and transmitted. If path loss, beam, RB allocation, power-control state, or P_CMAX has changed since then, the report may no longer represent the current state.
7. What Does the PHR MAC CE Carry?
The PH field in a MAC CE does not carry a continuous floating-point value. It carries a quantized power-headroom level defined by the specifications.
In Multiple Entry PHR MAC CEs, the V field identifies whether PH is based on a real transmission or a reference format:
V = 0: Based on a real transmissionV = 1: Based on a reference format
For Type 1, Type 2, and Type 3 PH in these MAC CEs, V = 0 also indicates that the octet containing the associated P_CMAX field is present, whereas V = 1 indicates that it is omitted. A scheduler must therefore not assume that every reported PH always arrives with a corresponding P_CMAX.
Depending on carrier aggregation, dual connectivity, and the reporting configuration, the UE may use a Single Entry or Multiple Entry PHR MAC CE. An implementation should therefore track all of the following:
- Which serving cell does the PH belong to?
- Is it Type 1, Type 2, or Type 3?
- Is it based on a real transmission or a reference format?
- Is the associated
P_CMAXincluded? - When was the report triggered, and when was it transmitted?
8. What PHR Does Not Tell You
PH is not CQI or SINR.
What PHR tells the gNB is:
How much margin remains before the UE reaches its maximum power when performing a real or reference uplink transmission according to the power-control equation?
PHR alone does not directly reveal:
- Whether PUSCH will be decoded at the target BLER
- How strong the current interference is
- Which MCS is appropriate
- How much data is actually waiting in the UE’s buffer
The scheduler must combine PHR with information such as PUSCH DMRS measurements, HARQ results, OLLA state, and BSR.
9. Preparing for Scheduler Use
Before using PHR in a scheduler, keep at least these four distinctions in mind:
- PH is a power margin, not a channel-quality indicator.
- Real-transmission PH and reference PH cannot be interpreted in exactly the same way.
- PHR is event- or timer-driven, so it may be stale.
- The associated
P_CMAXis not present in every report.
The next article will examine how a gNB uses this information to evaluate candidate RB allocations and combine PHR with SINR, BLER, and BSR when making scheduling decisions.