What is the difference between ACLR and ACPR?

They describe the exact same physical phenomenon: spectral regrowth caused by amplifier non-linearity. The difference is merely the terminology used by different standards bodies. ACPR (Adjacent Channel Power Ratio) was historically used in North American IS-95 CDMA and military standards. ACLR (Adjacent Channel Leakage Ratio) is the term adopted by 3GPP for WCDMA, LTE, and 5G NR. In practice, RF engineers use them interchangeably, though formal test reports must use the term specified by the governing standard (e.g., ACLR for a 5G base station test).

Why do power amplifiers cause ACLR degradation?

Modern waveforms like LTE and 5G OFDM consist of thousands of closely spaced subcarriers. When these subcarriers pass through a Power Amplifier (PA) operating near its compression point to maximize efficiency, the PA's non-linear transfer function creates third-order and fifth-order intermodulation products. Because the input consists of a broad band of frequencies, these intermodulation products appear as a continuous 'skirt' of noise spreading out on both sides of the main channel. This phenomenon is called spectral regrowth. If the PA is backed off (operated linearly), the ACLR improves, but the DC efficiency plummets. This forces designers to use linearization techniques like Digital Predistortion (DPD) to maintain good ACLR while running the PA in its efficient, non-linear region.

How is ACLR measured according to 3GPP standards?

ACLR is defined as the ratio of the filtered mean power centered on the assigned channel frequency to the filtered mean power centered on an adjacent channel frequency. For a 20 MHz LTE signal, the measurement integrates the power over a specific channel bandwidth (e.g., 18 MHz) using a Root Raised Cosine (RRC) filter. It integrates the main channel power, then shifts the measurement window exactly one channel spacing (20 MHz) above and below the carrier to measure the adjacent channels. It also measures the alternate channels (40 MHz away). 3GPP TS 38.104 for 5G base stations typically requires the first adjacent channel ACLR to be better than minus 45 dBc, ensuring the transmitter does not jam a neighboring operator's network.

System Design

Adjacent Channel Leakage Ratio (ACLR)

A 5G macro cell transmits 40 Watts of RF power into a 100 MHz channel. The operator who owns the adjacent spectrum paid billions of dollars for it, and they will not tolerate interference. If the base station's power amplifier is pushed too hard, intermodulation distortion causes the signal spectrum to spread outward, leaking energy into that neighboring band. Adjacent Channel Leakage Ratio (ACLR) quantifies this spectral regrowth. It is the ratio, in decibels, of the power in the assigned channel to the power spilling into the adjacent channel. Regulatory bodies and 3GPP standards strictly enforce ACLR limits (typically −45 dBc for base stations), forcing RF engineers to deploy advanced techniques like Digital Predistortion (DPD) to keep the spectrum clean while maintaining high amplifier efficiency.

Category: System Design

Alias: ACPR (Adjacent Channel Power Ratio)

Typical Limit: −45 dBc (3GPP Base Station)

ACLR Limits & Impact by Application

Application	Standard	Typical Target	Primary Consequence of Failure
5G Base Station (Macro)	3GPP TS 38.104	−45 dBc	Interference with adjacent operators, regulatory fines
LTE User Equipment (Phone)	3GPP TS 36.101	−30 to −33 dBc	Battery drain (if PA is backed off to pass) or failed certification
WiFi 6 Access Point	IEEE 802.11ax	−20 to −40 dBc (Spectral Mask)	Adjacent channel interference, reduced network throughput
Satellite Uplink	DVB-S2X	−25 to −30 dBc	Adjacent transponder jamming
Military Radios	MIL-STD	−50 to −60 dBc	Friendly-force communication denial (fratricide)

ACLR Calculation:
ACLR (dBc) = 10·log₁₀( P_adj / P_main )
Where P_main is the integrated power in the assigned channel, and P_adj is the integrated power in the adjacent channel, using specified measurement bandwidths.

Relationship to PA Linearity:
ACLR improves by roughly 2 to 3 dB for every 1 dB the amplifier is backed off from compression (for 3rd-order dominant distortion).

DPD Improvement:
A modern Digital Predistortion (DPD) system can improve ACLR by 15 to 20 dB, allowing the PA to operate much closer to saturation.

Common Questions

Frequently Asked Questions

ACLR vs. ACPR?

They measure the exact same physical phenomenon: spectral regrowth. ACPR (Adjacent Channel Power Ratio) is the traditional term used in North American IS-95 and military standards. ACLR is the specific terminology adopted by 3GPP for WCDMA, LTE, and 5G. They are used interchangeably in the lab.

What causes ACLR degradation?

Non-linearity in the power amplifier. Modern OFDM signals have high peak-to-average power ratios (PAPR). When signal peaks hit the PA's compression region, 3rd and 5th order intermodulation products are generated. Because the input is wideband, these IMD products manifest as a continuous "skirt" of noise spreading into adjacent channels.

How is it measured?

A spectrum analyzer integrates the RF power over the main channel bandwidth (using a specific filter shape, like RRC). It then shifts the integration window up by one channel spacing and measures the adjacent channel power. The difference in dB is the ACLR. 3GPP specifies exact integration bandwidths and filter roll-offs for every channel size.

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System Design

ACLR Compliance Tracker

Enter your PA output power, IMD3 levels, and target standard (LTE, 5G NR). Calculate expected ACLR performance and determine if DPD is required to meet regulatory emissions limits.

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