Power Control
Power Control dynamically adjusts transmit power to maintain target signal quality while minimizing interference and conserving battery. Covers open-loop (path loss based), closed-loop (TPC commands), fractional power control (α), and uplink power formulas for 5G NR.
#1Why Power Control Matters
Without power control, near-far effects dominate — nearby UEs overpower distant ones, drowning their signals in interference:
Near-far problem — toggle power control to see the difference in received signal balance
| Problem | Without PC | With PC |
|---|---|---|
| Cell-edge UE | Unheard (drowned) | Received at target SINR |
| Interference | High (all at max) | Managed (only needed) |
| Battery | Drains rapidly | Conserved |
| Throughput | Low (IF-limited) | High (SINR-limited) |
#2Open-Loop Power Control (OLPC)
The UE estimates path loss from downlink RSRP and sets uplink power to compensate:
Fractional power control — drag α to see how received power varies with distance at the gNB
The α (Alpha) Parameter
| α Value | Behavior | Use Case |
|---|---|---|
| 1.0 (full) | All UEs arrive at same power | PUCCH, PRACH (reliability) |
| 0.6-0.8 | Partial compensation | PUSCH (optimal throughput) |
| 0.0 (none) | No path loss compensation | SRS (low interference) |
OLPC Formula
P_UL = P₀ + α × PL + f(Δ)
Path Loss = P_SSB − RSRP (measured from SSB). P₀ = target received power. α = fractional factor.
#3Closed-Loop Power Control (CLPC)
The gNB sends TPC (Transmit Power Control) commands to fine-tune power around the OLPC baseline:
Closed-loop tracking — TPC commands adjust power to track target SINR despite channel variations
TPC Modes
Accumulative (default)
f(i) = f(i-1) + δ(i)
Each command adds to accumulated offset. Range: ±22 dB. Default mode.
Absolute
f(i) = δ(i)
Each command sets absolute offset directly. No accumulation. For fast changes.
| DCI Field | Accumulative | Absolute |
|---|---|---|
| 00 | -1 dB | -4 dB |
| 01 | 0 dB | -1 dB |
| 10 | +1 dB | +1 dB |
| 11 | +3 dB | +4 dB |
#4Uplink Power Control — Channel by Channel
UL power breakdown — see how each component contributes to transmit power per channel
Channel Comparison
| Component | PUSCH | PUCCH | SRS | PRACH |
|---|---|---|---|---|
| α | 0.0–1.0 | 1.0 (fixed) | 0.0–0.5 | 1.0 (fixed) |
| Δ_TF (MCS) | Yes | Yes | No | No |
| CLPC f(i) | Yes | Yes (g(i)) | Yes (h(i)) | No |
| Preamble ramp | No | No | No | Yes |
#5Uplink Power Control — Unified View
P_channel = min { P_CMAX, P₀ + 10·log₁₀(M) + α·PL + Δ + f(i) }
P_CMAX = 23 dBm (Power Class 3, most UEs)
P₀ = target received power (RRC configured)
M = number of allocated RBs (bandwidth)
α = fractional path loss compensation (0 to 1)
PL = estimated path loss
Δ = MCS/format-dependent offset
f(i) = accumulated TPC corrections (closed-loop)
#6Downlink Power Control (Brief)
DL Power Allocation
DL power is scheduler-managed, not closed-loop like UL:
P_a— PDSCH-to-CSI-RS power ratio (0 dB typical)P_b— DMRS-to-PDSCH power ratio (0 dB typical)- Power boosting: gNB borrows from unused PRBs → up to +6 dB on active PRBs
#7Maximum Power Reduction (MPR)
UEs cannot always transmit at rated max power due to hardware and regulatory constraints:
P_CMAX = min { P_EMAX, P_PowerClass - MPR - A-MPR - ΔT_C }
| Modulation | RB Allocation | MPR (dB) | Why |
|---|---|---|---|
| QPSK | Edge | 0.0 | Low PAPR |
| 16QAM | Edge | 1.0 | Moderate PAPR |
| 64QAM | Edge | 2.0 | Higher PAPR |
| 256QAM | Edge | 2.5 | Highest PAPR |
| 256QAM | Interior | 3.5 | Worst case |
| Power Class | Max Power | Typical Use |
|---|---|---|
| PC1 | 31 dBm | Fixed wireless (CPE) |
| PC2 | 26 dBm | Premium smartphones |
| PC3 | 23 dBm | Standard (most common) |
| PC4 | 21 dBm | Low-cost IoT |
#8Power Headroom Reporting (PHR)
The UE reports how much additional power is available beyond current usage:
Power headroom — positive (margin), zero (at max), negative (power-limited)
| PHR Value | Meaning | gNB Action |
|---|---|---|
| Positive (> +5 dB) | UE has power margin | Can allocate more PRBs / higher MCS |
| Zero (≈ 0 dB) | At maximum power | Maintain current allocation |
| Negative (< 0 dB) | Power-limited (clipping) | Reduce PRBs or MCS urgently |
#9Practical Field Considerations
| Pitfall | Impact | Best Practice |
|---|---|---|
| α too high for PUSCH | Excessive inter-cell interference | Use α = 0.6–0.8 for data |
| P₀ too high | UEs near max everywhere | Start at -70 dBm, tune |
| P₀ too low | Cell-edge cannot be heard | Monitor edge BLER > 20% |
| MPR not accounted | UE clips lower than expected | Account 2–4 dB MPR margin |
| Stale PHR | Wrong scheduler decisions | PHR timer < 10 sec |
| No fractional PC for SRS | Excessive SRS interference | Use α = 0 for SRS |
Field Tuning Workflow
4-Step Process
- Configure: P₀=-70, α=0.8 (PUSCH), α=1.0 (PUCCH), α=0 (SRS)
- Measure: PHR distribution, UL SINR vs distance, edge BLER
- Analyze: Target: 60% PH > +5 dB, 20% PH = 0 dB
- Adjust: If too many at P_CMAX → decrease P₀; if edge BLER > 20% → increase P₀ or α
LTE vs 5G NR Power Control
| Aspect | LTE | 5G NR |
|---|---|---|
| α values | Discrete set (8 values) | Continuous 0.0–1.0 |
| CLPC loops | 2 (index 0,1) | 3 (index 0,1,2) |
| BWP-specific | No | Yes (P₀/α per BWP) |
| Power classes | PC3 (23 dBm) | PC1–PC4 (21–31 dBm) |
#10Further Reading
3GPP Specifications
- TS 38.213 — NR Physical Layer Procedures §7 (Uplink Power Control) — Primary reference
- TS 38.214 — NR Physical Layer Procedures for Data §6 (PUSCH power control)
- TS 38.101 — NR UE Radio Transmission (power classes, MPR, tolerances)
- TS 38.321 — NR MAC Protocol (PHR MAC CE format, triggering)
Companion Resources
Link Adaptation Page → Channel Coding Page → ☆ ISAC Scheduling → ☆ Energy-Saving MAC →