5G NR MAC Power Headroom Report (PHR)
Power Headroom Report is the MAC reporting mechanism used to expose uplink transmit power margin to the network. It becomes critical when uplink grants exist but the UE still cannot deliver the expected performance because usable transmit power is limited.
This page explains PHR as a practical MAC reference topic, not just a definition. In real troubleshooting, PHR helps separate backlog visibility problems from power-limited execution. Read it together with Scheduling Request, Buffer Status Report, HARQ, Timing Advance, and the 5G NR MAC Overview page when investigating weak or inconsistent uplink behavior.
| Technology | 5G NR |
|---|---|
| Protocol area | MAC uplink reporting |
| Main specification | 3GPP TS 38.321 |
| Related specifications | 3GPP TS 38.214 and 3GPP TS 38.331 |
| Release baseline | Release 18 |
| Main role | Expose remaining uplink power margin to the network |
| Why it matters | Explains when grants or backlog visibility are not the main uplink limitation |
| Best paired with | BSR, SR, HARQ, timing advance, uplink grants, and RF analysis |
Definition and purpose
PHR is the MAC-level report that informs the network about available uplink transmit power margin. It gives the scheduler context about whether the UE can realistically make full use of current or future uplink allocations.
Without PHR, uplink weakness can be misread as a grant problem, queue problem, or general radio problem. With PHR, engineers can see whether the UE is power-limited even when data is waiting and grants are present.
Where it fits
- PHR is part of the MAC reporting chain used for uplink resource interpretation.
- Scheduling Request indicates that uplink opportunity is needed.
- Buffer Status Report shows how much uplink data is waiting.
- PHR adds the missing question: can the UE use the granted uplink resources effectively under its current power conditions?
PHR types and reporting scope
| PHR form | Purpose | Why engineers care |
|---|---|---|
| Single-entry PHR | Reports power headroom for one serving context | Useful in simpler uplink cases where one reported margin explains the current limitation. |
| Multiple-entry PHR | Carries power headroom information for more than one serving context | Important in more advanced configurations where one uplink view is not enough. |
| PHR with PCMAX-related context | Pairs headroom information with power capability interpretation | Helps explain whether weak uplink is related to hard power limits rather than only scheduling behavior. |
Trigger context
| Trigger area | Why it matters |
|---|---|
| Configured reporting behavior | PHR only becomes visible when the configured MAC behavior allows or requires it. |
| Changing uplink power conditions | Power margin can change with path loss, cell-edge position, beam conditions, or serving-cell context. |
| Procedure timing | The usefulness of a PHR depends on when it is generated relative to grants, retransmissions, and queue buildup. |
| Reconfiguration events | RRC-driven configuration changes can alter how PHR should be interpreted in later traces. |
How PHR changes scheduler interpretation
| Observation | What PHR helps explain |
|---|---|
| Large uplink grant but weak user throughput | The UE may be power-limited and unable to exploit the allocation effectively. |
| Visible backlog and correct SR/BSR behavior but poor uplink progress | Demand signaling may be correct while power margin remains the real bottleneck. |
| Cell-edge or high-path-loss case with unstable uplink quality | PHR can confirm that transmit power headroom is tight rather than simply assuming generic coverage trouble. |
| Intermittent improvement after changing radio conditions | The power budget may have shifted even when the queue and grant pattern looked similar. |
What power-limited uplink looks like
A power-limited uplink usually appears as a mismatch between opportunity and outcome. Uplink resources may be granted, backlog may be known, and transmission may still fail to scale as expected because the UE has limited power margin under the current radio conditions.
This pattern often appears in cell-edge conditions, stressed RF conditions, or multi-factor uplink scenarios where scheduler logic alone does not explain the poor result. That is where PHR becomes one of the most useful MAC signals in the trace.
Why PHR should be read with HARQ, timing, and grants
- Low power headroom often becomes more meaningful when read together with HARQ behavior and retransmission growth.
- Timing Advance should be checked alongside PHR because weak uplink and poor alignment can appear together in difficult radio conditions.
- Grant analysis matters because PHR is most useful when you first confirm that the scheduler actually gave the UE an opportunity to transmit.
- PHY-side reading is often needed as well, especially for PUSCH quality and uplink link-budget interpretation.
Log-analysis notes
A useful PHR analysis should correlate queue state, grant timing, retransmission behavior, power reporting visibility, and radio conditions. Looking at PHR alone is not enough, but ignoring it often leads to the wrong root cause.
When uplink remains weak even after SR and BSR appear healthy, PHR often becomes the deciding clue. It is especially valuable when combined with MAC PDU decoding, HARQ, and RF-side evidence.
Troubleshooting
| Symptom | PHR area to inspect | Why |
|---|---|---|
| Uplink grant exists but throughput stays poor | Reported power margin and later HARQ behavior | The UE may have had opportunity but not enough usable power margin. |
| BSR shows demand but queue drain remains weak | PHR together with grants and RF context | Demand visibility may be correct while power remains the limiting factor. |
| Cell-edge uplink instability | PHR trend and radio-condition correlation | The trace may show that uplink degradation follows shrinking power headroom. |
| Intermittent uplink performance after reconfiguration | PHR timing relative to new configuration and serving conditions | The interpretation may have changed with configuration rather than with traffic alone. |
| Repeated retransmissions despite visible grants | PHR with HARQ and PUSCH quality context | Retransmissions may reflect power-limited transport quality rather than lack of scheduling. |
What to check when PHR becomes important
- Check whether the uplink problem still exists after confirming that SR and BSR behaved correctly.
- Check whether weak performance matches cell-edge, path-loss, beam, or RF-stress conditions.
- Check timing alignment and retransmission behavior at the same time.
- Check whether grants increased while useful throughput did not, which often makes PHR more important.
- Check whether the PHR interpretation changed after RRC reconfiguration or serving-cell changes.
Release 18 scope
The core MAC role of PHR remains stable in Release 18, but engineers should read it in the wider context of modern NR uplink behavior. Advanced configurations, richer MAC control signaling, and broader feature interaction make power reporting more important than in a very narrow early-NR reading.
Release 18 analysis is therefore not only about the existence of a PHR. It is about how power margin reporting interacts with scheduling, control elements, advanced UE behavior, and modern troubleshooting expectations across the NR MAC stack.
FAQ
What is Power Headroom Report in 5G NR MAC?
Power Headroom Report is the MAC mechanism used to inform the network about available uplink transmit power margin.
Why is PHR important for troubleshooting?
Because it helps explain why visible grants and visible backlog did not produce the expected uplink throughput or transport quality.
Should PHR be analyzed together with BSR?
Yes. BSR explains backlog visibility, while PHR helps explain whether that backlog can be served efficiently under current power conditions.
Is PHR only an RF topic?
No. PHR is a MAC reporting topic with strong RF relevance. It matters to protocol engineers, RAN teams, optimization engineers, and log analysts because it changes how uplink scheduler behavior should be interpreted.
Why can uplink still be weak when SR and BSR look normal?
Because uplink demand visibility may be correct while the UE remains power-limited, poorly aligned, or affected by retransmission-heavy radio conditions.
Does Release 18 MAC still treat PHR as important?
Yes. The core role is stable, and its practical value becomes stronger in modern NR analysis because uplink behavior is often more feature-rich and configuration-dependent.