ACLR

Adjacent channel leakage power ratio measures the fraction of a transmitter's output power that spills into frequency bands next to its assigned channel. It is expressed as a ratio, usually in decibels (dB), comparing the power in an adjacent channel to the power in the assigned channel itself. A transmitter with poor ACLR contaminates neighboring channels and risks interfering with other users' signals.

The measurement matters most in densely allocated spectrum. In cellular networks, where base stations and handsets operate on tightly spaced channels, ACLR limits are set by regulatory bodies and network operators. The 3GPP standards specify ACLR thresholds for LTE and 5G devices, typically in the range of 45 to 50 dB depending on the band and modulation scheme. Exceeding these limits can trigger automatic power reduction or service suspension.

Sources of adjacent channel leakage

Leakage occurs because real transmitters cannot produce perfectly rectangular frequency responses. The signal's spectral edges roll off rather than cut off sharply. Nonlinear amplifier behavior, phase noise, and imperfect filtering all broaden the transmitted spectrum. Modulation type affects ACLR significantly: wider-bandwidth schemes like wideband code division multiple access (WCDMA) are more prone to leakage than narrower formats.

Improving ACLR demands careful design of the power amplifier output stage, input filtering, and digital predistortion algorithms. High-performance devices use adaptive filters that suppress out-of-band energy in real time. Test and measurement equipment, spectrum analyzers and signal generators, quantify ACLR during device certification and field troubleshooting.

ACLR is distinct from out-of-band emissions, which measure total radiated power far from the assigned channel. Adjacent channel leakage is specifically the power that leaks into the immediate neighboring channels, making it the more stringent and operationally critical metric for shared spectrum.