What is the difference between ACLR and ACPR?

ACLR and ACPR measure the same physical phenomenon (spectral regrowth into adjacent channels) but are defined differently. ACLR (Adjacent Channel Leakage Ratio) is the 3GPP term used in LTE and 5G NR specifications. It is defined as the ratio of the main channel power to the adjacent channel power, expressed as a positive number in dB (e.g., 45 dB). ACPR (Adjacent Channel Power Ratio) is the more general industry term, often defined as the ratio of adjacent channel power to main channel power, expressed as a negative number in dBc (e.g., minus 45 dBc). Numerically they are equivalent but with opposite sign conventions. Some vendors use ACPR and ACLR interchangeably, which can cause confusion. When reading specifications, always check whether the value is positive (ACLR convention) or negative (ACPR convention).

What causes poor ACLR?

ACLR degradation is caused by nonlinearity in the transmit chain, primarily the power amplifier. When a wideband modulated signal (OFDM, QAM) passes through a nonlinear PA, intermodulation products create spectral regrowth that spreads power beyond the allocated channel bandwidth into adjacent channels. Higher-order nonlinearities (AM-AM and AM-PM distortion) are the primary contributors. The ACLR worsens as the PA is driven closer to saturation because the compression increases the nonlinear distortion. Signals with high peak-to-average power ratio (PAPR) like OFDM are particularly susceptible because the signal peaks drive the PA into compression even when the average power is well below the P1dB point. Memory effects in the PA (thermal, electrical) create asymmetric spectral regrowth, where the upper and lower adjacent channels have different ACLR values.

How is ACLR improved?

Digital predistortion (DPD) is the most effective technique, improving ACLR by 10 to 20 dB by pre-compensating for the PA nonlinearity in the digital domain. The DPD algorithm models the PA inverse transfer function and applies it before the DAC. Output back-off (OBO) reduces the average power below P1dB, trading efficiency for linearity. Typically 6 to 10 dB back-off is needed without DPD. Crest factor reduction (CFR) clips the signal peaks to reduce PAPR by 2 to 4 dB, allowing the PA to operate closer to saturation. Doherty PA architecture provides 6 to 8 dB efficiency improvement at back-off, enabling better ACLR without the severe efficiency penalty of class-A back-off. Feedforward linearization cancels distortion products by subtracting an error signal, providing 20 to 30 dB improvement but at the cost of complexity and efficiency.

Transmitter Metric

ACLR

/ay-see-el-ahr/ — Adjacent Channel Leakage Ratio

ACLR = P_main/P_adjacent (dB). 3GPP-defined metric for spectral regrowth caused by PA nonlinearity. LTE E-UTRA: ≥45 dB, UTRA: ≥50 dB. 5G NR: ≥45 dB. DPD improves ACLR by 10-20 dB. Back-off trades efficiency for linearity. High PAPR signals (OFDM) most susceptible. ACLR = ACPR with opposite sign convention. Asymmetric regrowth from PA memory effects.

LTE: ≥45 dB

5G NR: ≥45 dB

DPD: +10-20 dB

Understanding ACLR

ACLR is the gatekeeper metric for cellular transmitter compliance. Every base station and handset must meet strict ACLR requirements to prevent interference with users on adjacent frequency channels. The challenge is that modern wideband signals (LTE: 20 MHz, 5G NR: 100 MHz) have high peak-to-average power ratios that drive the PA into compression, generating spectral regrowth that spills into neighboring channels.

The entire PA linearization industry exists because of ACLR. Without DPD, a typical Doherty PA achieves 30-35 dB ACLR at rated power. With DPD, the same PA achieves 50-55 dB ACLR, comfortably meeting 3GPP requirements while maintaining 40-50% drain efficiency. This single technology enables the economics of modern cellular infrastructure.

ACLR Equations

Definition:
ACLR = 10log(P_main/P_adj) dB
ACPR = 10log(P_adj/P_main) dBc
ACLR = −ACPR (sign convention)

3GPP measurement:
P_main: integrated over assigned BW
P_adj: integrated over adjacent BW
LTE: adj BW = assigned BW
Offset: center-to-center spacing

ACLR from OIP3:
ACLR ≈ 2(OIP3 − P_out) + 10log(BW_adj/BW_sig)
(3rd-order dominated approximation)

ACLR Requirements by Standard

Standard	ACLR 1st Adj	ACLR 2nd Adj	Channel BW	Offset
LTE (E-UTRA)	45 dB	N/A	5-20 MHz	BW + guard
LTE (UTRA adj)	50 dB	N/A	5 MHz WCDMA	BW/2 + 2.5
5G NR FR1	45 dB	N/A	5-100 MHz	BW + guard
5G NR FR2	28 dB	N/A	50-400 MHz	BW + guard
Wi-Fi 6E	25 dB	40 dB	20-160 MHz	Channel BW

Common Questions

Frequently Asked Questions

ACLR vs ACPR?

Same measurement, opposite sign. ACLR = P_main/P_adj (positive dB, 3GPP convention). ACPR = P_adj/P_main (negative dBc, industry convention). ACLR=45 dB = ACPR=−45 dBc. Always check sign convention in datasheets. Some vendors use interchangeably, causing confusion.

Root causes?

PA nonlinearity: AM-AM and AM-PM distortion create spectral regrowth. Worse near saturation. High-PAPR signals (OFDM: 8-12 dB PAPR) drive peaks into compression even at modest average power. Memory effects (thermal/electrical) cause asymmetric upper/lower adjacent ACLR. Mixer and DAC spurious also contribute.

How to improve?

DPD: +10-20 dB, models PA inverse, applied digitally. CFR: clips peaks, reduces PAPR 2-4 dB. Back-off: 6-10 dB OBO without DPD. Doherty PA: 6-8 dB efficiency improvement at back-off. Feedforward: +20-30 dB but complex. Modern BTS: DPD + Doherty = 50+ dB ACLR at 40-50% efficiency.

Related Terms

← ACPR Active Antenna →

PA Linearization

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