Adjacent Channel Leakage Ratio

ACLR is the ratio of transmitted power within an assigned channel to the power leaking into adjacent frequency channels. It is measured in dBc (decibels relative to carrier). A higher ACLR indicates a cleaner transmitter with less spectral regrowth. The measurement captures the integrated power in the adjacent channel bandwidth relative to the in-channel power, providing a single scalar value that aggregates multiple non-linear distortion mechanisms.

The primary source of ACLR variation is the power amplifier (PA). When driven near saturation for efficiency, the PA's transfer function becomes non-linear, generating intermodulation products and spectral regrowth. Key contributing factors include:

• AM-AM Distortion: Amplitude compression varying per device
• AM-PM Distortion: Amplitude-dependent phase shift unique to each PA
• Memory Effects: Thermal and electrical time constants causing history-dependent distortion These imperfections create a device-specific ACLR signature.

Even among identical transmitter models, ACLR values exhibit measurable variance due to the silicon lottery. Semiconductor process variations cause subtle differences in transistor threshold voltages, capacitances, and resistances within the PA. These microscopic manufacturing tolerances produce:

• Unique gain compression curves
• Distinct harmonic and intermodulation profiles
• Varying levels of spectral regrowth into adjacent channels This variance makes ACLR a viable physical-layer identifier.

Regulatory bodies like the FCC and ETSI mandate minimum ACLR thresholds to limit interference. However, fingerprinting exploits the precise value above this threshold, not just pass/fail compliance. A device may consistently operate at -45 dBc while another identical model operates at -47 dBc. This stable offset, driven by hardware imperfections, provides a distinguishing feature. The metric is already measured in standard test procedures, making it a zero-overhead fingerprinting source.

ACLR as a fingerprint exhibits strong short-term stability but drifts with temperature and aging. Key considerations:

• Temperature: PA gain and linearity shift with junction temperature, altering ACLR
• Aging: Semiconductor degradation over years can permanently shift the ACLR baseline
• Supply Voltage: Fluctuations affect PA bias points and linearity Robust fingerprinting systems must employ drift compensation algorithms to track these slow variations while maintaining device identity continuity.

ACLR is rarely used in isolation. It is combined with other hardware impairment metrics to form a device-unique fingerprint vector. Complementary features include:

• I/Q Imbalance: Gain and phase mismatch in the modulator
• Carrier Frequency Offset: Oscillator tolerance deviation
• Phase Noise Mask: Spectral spreading from local oscillator instabilities Together, these features create a high-dimensional signature resistant to channel conditions and spoofing attempts.