How is ACPR improved?

Power back-off: operating the PA 6 to 10 dB below P1dB keeps it in the linear region but wastes DC power (efficiency drops from 50 percent to 5 to 10 percent). Digital predistortion (DPD): applies an inverse nonlinearity model before the PA, canceling the distortion. DPD improves ACPR by 15 to 25 dB, allowing the PA to operate closer to saturation. Doherty architecture: maintains high efficiency at 6 to 10 dB back-off by load modulation, achieving 40 to 50 percent average efficiency with OFDM signals. CFR (crest factor reduction): reduces the signal PAPR by 2 to 4 dB through peak clipping and filtering, allowing higher average power without compression.

What are 3GPP ACLR requirements?

LTE UE (user equipment): ACLR at plus or minus 1 channel bandwidth offset less than minus 30 dBc, at plus or minus 2 channel bandwidths less than minus 33 dBc. LTE eNB (base station): ACLR at first adjacent less than minus 45 dBc, at second adjacent less than minus 50 dBc. 5G NR FR1: similar requirements with carrier bandwidth-dependent measurement bandwidths. WiFi 802.11ac: adjacent channel rejection typically minus 25 to minus 40 dBc depending on data rate and channel bandwidth. The base station requirement is 15 dB stricter than the UE because the base station transmits at much higher power and must protect many users in adjacent channels.

Linearity Metric

ACPR / ACLR

Q: What causes poor ACPR?

PA nonlinearity generates intermodulation distortion products (IMD3, IMD5) that fall in adjacent channels, appearing as spectral regrowth. The severity depends on the PA's operating point relative to its compression point: operating closer to P1dB compresses the signal peaks and generates more IMD. High PAPR signals like OFDM (8 to 12 dB PAPR) are particularly vulnerable because the signal peaks drive the PA into compression while the average power is well below P1dB. Memory effects in the PA (thermal, bias network, trapping) create asymmetric spectral regrowth where upper and lower adjacent channels have different ACPR values.

/ay-see-pee-ahr/ — Adjacent Channel Power Ratio

Measures PA linearity: power leaked into adjacent channels vs. main channel (dBc). Caused by IMD3/5 spectral regrowth from PA nonlinearity. 3GPP LTE UE: −30 dBc @ ±1 BW, −33 dBc @ ±2 BW. eNB: −45 dBc. Improved by: power back-off (wastes efficiency), DPD (+15-25 dB improvement), Doherty (efficient at back-off), CFR (PAPR reduction). OFDM signals: 8-12 dB PAPR = worst case.

LTE UE: −30 dBc

eNB: −45 dBc

DPD gain: 15-25 dB

Understanding ACPR

ACPR (also called ACLR in 3GPP standards) is the key metric for transmitter linearity in modern wireless systems. Every power amplifier introduces some nonlinearity, generating intermodulation products that spread the transmitted signal's spectrum into adjacent frequency channels. This spectral regrowth interferes with other users operating in those adjacent channels, degrading their signal quality.

The challenge is particularly severe with modern wideband modulation schemes like OFDM (used in LTE, 5G NR, and WiFi). These signals have high peak-to-average power ratios (8-12 dB), meaning the signal peaks drive the PA into compression while the average power is well below the PA's maximum capability. The resulting nonlinear distortion manifests as spectral regrowth that must meet strict regulatory and standard-defined limits.

ACPR Equations

ACPR definition:
ACPR = ∫P_adj(f)df / ∫P_main(f)df
Expressed in dBc (always negative)

IMD3 spectral regrowth:
ACPR ≈ P_out − OIP3 − 10log(BW_adj/BW_main)
Higher OIP3 = better ACPR

Back-off vs. ACPR:
1 dB back-off ≈ 2 dB ACPR improvement
(for 3rd-order dominated systems)

DPD improvement:
ACPR_{with DPD} = ACPR_{no DPD} − 15 to 25 dB
Memory polynomial: 5-9 terms typical

ACPR Requirements by Standard

Standard	1st Adjacent	2nd Adjacent	Signal BW	PAPR
LTE UE	−30 dBc	−33 dBc	1.4-20 MHz	8-9 dB
LTE eNB	−45 dBc	−50 dBc	1.4-20 MHz	8-9 dB
5G NR FR1	−30 dBc	−33 dBc	5-100 MHz	10-12 dB
WiFi 802.11ax	−25 to −40	−40 to −50	20-160 MHz	10-12 dB
WCDMA UE	−33 dBc	−43 dBc	5 MHz	3-4 dB

Common Questions

Frequently Asked Questions

What causes poor ACPR?

PA nonlinearity: IMD3/IMD5 products fall in adjacent channels = spectral regrowth. Operating near P1dB compresses peaks. High-PAPR OFDM (8-12 dB) drives peaks into compression while average power is well below P1dB. Memory effects create asymmetric upper/lower ACPR.

How to improve?

Back-off: 6-10 dB below P1dB, linear but 5-10% efficiency. DPD: inverse model cancels distortion, +15-25 dB, allows near-saturation operation. Doherty: 40-50% efficiency at back-off via load modulation. CFR: reduce PAPR by 2-4 dB through peak clipping/filtering. Best: Doherty+DPD+CFR combined.

3GPP requirements?

LTE UE: −30 dBc 1st adj, −33 dBc 2nd. eNB: −45/−50 dBc (15 dB stricter, protects many users). 5G NR FR1: similar UE, up to 100 MHz BW. Base station stricter because higher TX power affects more adjacent-channel users. WiFi: −25 to −50 dBc depending on rate/BW.

Related Terms

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PA Linearization

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