Power Headroom Reporting (PHR)

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1. Why PHR Matters

The gNB doesn't know how much transmit power the UE is using vs. its maximum (P_CMAX, typically 23 dBm). Power Headroom = P_CMAX − P_PUSCH. Positive PH = remaining margin. Negative PH = power limited. The gNB uses PHR to decide grant size, MCS, and RB allocation.

⚡ Power Headroom Calculator

Adjust P_CMAX and PUSCH power to see the PH value, headroom bar, and what the gNB should do.

P_CMAX (dBm): 23 P_PUSCH (dBm): 12

PH > 10 dB

Cell center. Lots of headroom. gNB can increase RBs, use higher MCS, or add layers.

PH = 0-5 dB

Moderate margin. Current grant is fine. gNB maintains current allocation.

PH ≤ 0 dB

Power limited. gNB should reduce RBs, lower MCS, or trigger handover if persistent.

2. PHR Types

Type	Formula	When Used
Type 1	P_CMAX − P_PUSCH	Always (most common)
Type 2	P_CMAX − (P_PUSCH + P_PUCCH)	Simultaneous PUSCH + PUCCH
Type 3	P_CMAX − P_SRS	SRS-only (no PUSCH in slot)

Virtual PHR: When PHR is triggered but no actual PUSCH transmission exists (e.g., SCell just activated), UE calculates PH using a reference format (1 RB, reference MCS). The V bit in the MAC CE is set to 1 to indicate virtual PHR.

3. PHR MAC CE Format

📦 Multi-Cell PHR MAC CE — Byte-Level Structure

Interactive viewer showing the Ci bitmap, PCell PHR, and SCell PHR entries. Toggle cells to see the structure change.

Active SCells: 2 PH value (dB): 12

Field	Size	Purpose
Ci bitmap	1 byte	Which SCells are included (1 bit each)
PH (per cell)	6 bits	Power headroom: index 0-63 → PH = index − 32 dB
P	1 bit	Power backoff applied (MPR/AMPR)
V	1 bit	0 = real PH, 1 = virtual PH
P_CMAX,f,c	7 bits	UE max power for this cell/carrier

4. PHR Triggers

Trigger	Type	Description
periodicPHR-Timer	Periodic	Timer expiry (sf10–sf1000, infinity)
prohibitPHR-Timer + pathloss change	Event	Pathloss changed > threshold (dB1/3/6)
RRC configuration	Config	PHR function first configured
SCell activation	Event	New SCell needs PHR report
PSCell change	Event	DC cell group change
BWP switch	Event	Different power parameters on new BWP

⏱️ PHR Trigger Timeline

Animated timeline showing periodic and event-driven PHR triggers. Watch the timers and pathloss change detection.

periodicPHR-Timer:

5. Practical Field Considerations

Scenario	periodicTimer	prohibitTimer	txPowerChange
Dense urban	sf100	sf50	dB3
Rural (slow)	sf500	sf200	dB6
High mobility	sf50	sf20	dB1
IoT (power save)	sf1000	sf500	dB6
URLLC	sf20	sf10	dB1

Common pitfall: periodicPHR-Timer too long → gNB has stale power information → allocates too many RBs to power-limited UE → BLER spike. For high-mobility scenarios, use shorter timers.

LTE vs 5G NR PHR

Aspect	LTE	5G NR
PHR types	Type 1, 2	Type 1, 2, 3
Multi-cell	Up to 5 SCells	Up to 32 SCells
BWP awareness	None	Trigger on BWP switch
DC support	Dual PHR	phr-ModeOtherCG

6. 6G Evolution — AI-Predicted & Compressed PHR

AI-Predicted PHR

gNB AI predicts PH from historical reports, UE speed, beam patterns. Adjusts grants proactively. 50-70% fewer PHR reports needed.

2-Bit Compressed PHR

Instead of 6-bit value: send 2-bit category (high/moderate/limited/power-limited). Only full PHR when AI prediction error exceeds threshold.

Massive CA Support

64+ cells in 6G → compressed PHR essential to avoid 130+ byte MAC CEs. AI handles per-cell power optimization.

🚀 PHR Evolution — 4G to 6G

Compare PHR capabilities across generations.

Generation:

Dimension	4G (LTE)	5G NR	6G (Target)
PHR types	Type 1, 2	Type 1, 2, 3	+ AI-inferred
Multi-cell	Up to 5	Up to 32	64+
Report size	1-12 bytes	1-65 bytes	2-bit compressed
Prediction	None	None	AI-predicted

← MAC Overview Power Control → Scheduling → BSR → SR & RA → SPS & CG → DL Assignment → TA → DRX → ☆ Energy-Saving MAC