Closed-Loop Power Control
Understanding Closed-Loop Power Control
In any cellular system, the distance between a mobile device and the base station varies from tens of meters to several kilometers, creating a path loss variation of 60 to 80 dB at typical frequencies (700 MHz to 3.5 GHz). Without power control, a nearby user transmitting at full power would create massive interference for distant users, particularly devastating in CDMA systems where all users occupy the same bandwidth. Closed-loop power control ensures each user's signal arrives at the base station at just the right level to meet the quality target, no more and no less.
The CLPC mechanism operates in two nested loops. The inner loop (fast power control) runs at 1,500 Hz in WCDMA and up to 800 Hz in 5G NR, comparing the instantaneous received SIR against a target value. If SIR is above target, the base station sends a "down" TPC bit; if below, an "up" command. Each command adjusts the UE's transmit power by 1 dB (configurable to 2 dB in WCDMA). The outer loop adjusts the SIR target itself based on measured BLER: if block errors are below the 1% target, the SIR target is reduced by 0.1 to 0.5 dB per update, allowing lower transmit power. This dual-loop structure continuously minimizes transmit power while maintaining the required quality of service, maximizing both battery life and system capacity.
Power Control Equations
P = min(PCMAX, P0 + 10log10(M) + α·PL + δTF + f(TPC))
Received SIR:
SIR = Prx,signal / (Prx,interference + Pnoise)
Inner Loop TPC Update:
Ptx(n+1) = Ptx(n) + ΔTPC × sign(SIRtarget - SIRmeasured)
Where P0 = cell-specific power target (dBm), M = allocated resource blocks, α = path loss compensation factor (0.8 typical), PL = downlink path loss (dB), δTF = transport format offset, f(TPC) = accumulated TPC corrections, ΔTPC = step size (1 or 2 dB).
Power Control Standards Comparison
| Standard | TPC Rate | Step Size | Dynamic Range | Path Loss Compensation |
|---|---|---|---|---|
| WCDMA (3GPP R99) | 1,500 Hz | 1 or 2 dB | 80 dB | Full (α = 1) |
| CDMA2000 | 800 Hz | 0.5 or 1 dB | 73 dB | Full (α = 1) |
| LTE (4G) | 200 Hz (max) | 1 or 3 dB | 74 dB | Fractional (α = 0.8) |
| 5G NR FR1 | Up to 800 Hz | 1 or 2 dB | 73 dB | Fractional (α = 0.4 to 1.0) |
| 5G NR FR2 (mmWave) | Up to 800 Hz | 1 or 2 dB | 43 dB | Full (α = 1, beam-based) |
Frequently Asked Questions
How does CLPC solve the near-far problem?
In CDMA, all users share the same band, separated only by spreading codes. A close user could overwhelm distant users by 60 to 80 dB. CLPC equalizes received power by commanding nearby users to reduce and distant users to increase transmit power. At 1,500 commands/second, it tracks Rayleigh fading at pedestrian speeds, maintaining received SIR within 0.5 to 1 dB of target. CDMA capacity is directly proportional to power control accuracy.
What is the difference between inner and outer loop power control?
The inner loop (1,500 Hz WCDMA, 800 Hz 5G NR) compares instantaneous SIR against a target and sends 1-bit TPC commands (±1 dB). It tracks fast fading. The outer loop (10 to 100 Hz) adjusts the SIR target based on BLER: if BLER is below 1%, it lowers the target to reduce power. This dual structure minimizes transmit power while maintaining quality.
How does 5G NR power control differ from WCDMA?
5G NR uses fractional path loss compensation (α = 0.4 to 1.0, typically 0.8) instead of full compensation. Cell-edge users transmit slightly below full compensation, reducing intercell interference at the cost of marginally lower edge rates. This improves system spectral efficiency by 10 to 20%. NR also adds per-beam power control for mmWave and flexible TPC accumulation modes.