Adjacent-Channel Leakage Ratio

Adjacent-channel leakage ratio, usually written ACLR, measures how much transmitter power leaks into a neighboring channel compared with the power in the assigned channel. It is a practical RF compliance and coexistence metric because a transmitter can meet its wanted-link budget while still polluting adjacent spectrum.

ACLR is controlled by modulation bandwidth, pulse shaping, power-amplifier linearity, digital predistortion, filtering, oscillator quality, clipping, gain compression and measurement bandwidth. It is not a receiver sensitivity number. It describes what the transmitter emits outside its intended channel.

Basic Formula

For a main channel and one adjacent channel measured over specified bandwidths:

where:

• P_{main} is integrated transmitted power in the assigned channel;
• P_{adj} is integrated power in the adjacent measurement channel;
• both powers are normally expressed in dBm over the stated bandwidth and offset.

If upper and lower adjacent channels are measured separately:

and:

The limiting result is usually the smaller value:

Mask Margin

If a requirement states a minimum ACLR:

then the adjacent-channel margin is:

Equivalently, the allowed adjacent-channel power is:

This form is useful during spectrum-mask review because it compares a measured adjacent-channel integrated power with the allowed limit.

Worked Example

A transmitter has measured main-channel power:

The upper adjacent channel contains:

and the lower adjacent channel contains:

The measured leakage ratios are:

and:

If the requirement is:

the margins are:

and:

The transmitter fails the adjacent-channel leakage requirement even though the wanted channel power may be correct.

Backoff and Filtering

Suppose amplifier backoff and filtering reduce the main channel to:

and reduce adjacent powers to:

and:

The new ratios are:

and:

The ACLR problem is fixed, but the wanted channel lost 2 dB of transmit power. That loss must be checked against EIRP, link margin and coverage requirements.

Difference From Adjacent-Channel Rejection

ACLR is a transmitter-side emission metric. Adjacent-channel rejection is a receiver-side selectivity metric. A bad transmitter can create excessive adjacent-channel leakage; a weak receiver can fail to reject a legal adjacent transmitter. In a coexistence problem, both sides may need review.

ACLR also differs from EIRP. EIRP describes radiated power in a direction relative to an isotropic reference. ACLR describes how that transmitted power is distributed between the intended channel and neighboring channels.

Engineering Use

Engineers use ACLR for transmitter compliance, RF power-amplifier backoff, digital predistortion release, occupied-bandwidth review, channel planning, private wireless coexistence, base-station acceptance, telemetry radios, radar-like emitters and lab-to-field troubleshooting.

Poor ACLR can come from power amplifier compression, clipping, insufficient filtering, wrong modulation settings, DAC images, oscillator phase noise, impedance mismatch, thermal drift or firmware configuration errors. Increasing transmit power often worsens ACLR because the power amplifier moves closer to nonlinear operation.

Validation Evidence

A defensible ACLR measurement states center frequency, channel bandwidth, adjacent-channel offset, measurement bandwidth, detector mode, averaging, resolution bandwidth, waveform, modulation order, symbol rate, output power, amplifier backoff, filter configuration, antenna or conducted test boundary, calibration state and uncertainty.

For regulatory or coexistence decisions, conducted data may not be enough. Radiated tests, installed cable losses, antenna gain, duty cycle, enclosure leakage and site occupancy can change the practical interference risk.

Common Mistakes

Common mistakes include comparing ACLR values measured with different bandwidths, quoting only the better adjacent side, ignoring measurement detector settings, treating a spectrum peak as integrated adjacent power, fixing ACLR by reducing power without checking link margin, and assuming a legal transmitter cannot still harm a weak adjacent receiver.

The practical rule is to define the measurement bandwidth and offset, integrate the main and adjacent powers consistently, compare both adjacent sides with the requirement, then check the effect of any fix on EIRP and service margin.