Adjacent Channel Power Ratio (ACPR)

The physical mechanism that causes spectral regrowth and degrades ACPR. When multiple frequency components pass through a nonlinear power amplifier, they mix to generate sum and difference products at frequencies not present in the original signal.

• Third-order IMD (IMD3) products fall closest to the main channel and are the dominant contributor to adjacent channel leakage.
• Fifth and seventh-order products affect alternate channels further from the carrier.
• The relationship between IMD and ACPR is direct: reducing IMD through linearization directly improves the measured ACPR.

The engineering discipline focused on reducing out-of-band emissions caused by power amplifier nonlinearity. Spectral regrowth is the broadening of a signal's bandwidth as it passes through a nonlinear device, spilling power into adjacent channels.

• Digital predistortion (DPD) is the primary mitigation technique, applying an inverse nonlinearity before the amplifier.
• Envelope tracking reduces regrowth by dynamically adjusting the PA supply voltage to match instantaneous signal envelope.
• Regulatory bodies like the FCC and ETSI set strict spectral emission masks that define maximum allowable ACPR levels.

A complementary linearity metric that measures in-band distortion, whereas ACPR measures out-of-band distortion. EVM quantifies the deviation of actual transmitted constellation points from their ideal reference positions.

• Expressed as a percentage or in dB, with lower values indicating better modulation accuracy.
• Trade-off with ACPR: Aggressive DPD can improve ACPR at the expense of EVM if the predistorter introduces in-band noise.
• Modern 5G NR specifications require EVM below 3.5% for 256-QAM and below 1% for 1024-QAM modulation schemes.

A signal conditioning technique that works in tandem with DPD to improve ACPR performance. CFR reduces the peak-to-average power ratio (PAPR) of the transmitted signal before it reaches the power amplifier.

• Clipping and filtering is the most common CFR method, deliberately limiting signal peaks to prevent PA saturation.
• Peak windowing applies smooth attenuation around detected peaks to minimize spectral regrowth from the CFR process itself.
• Coordinated CFR-DPD design ensures that CFR does not introduce distortion that the DPD cannot subsequently correct, maximizing overall ACPR improvement.

A term used interchangeably with ACPR in many contexts, though subtle distinctions exist. ACLR is defined as the ratio of the total power in the adjacent channel to the total power in the assigned channel.

• 3GPP specifications use ACLR as the standard metric for transmitter conformance testing.
• ACLR requirements for base stations are typically 45 dB or better for the first adjacent channel.
• The measurement bandwidth and channel spacing are defined by the specific radio access technology (e.g., 5 MHz offset for WCDMA, configurable for 5G NR).

An alternative linearity figure of merit that uses a notched noise signal to characterize distortion. A broadband noise signal with a narrow spectral notch is applied to the amplifier, and the depth of the notch at the output indicates linearity.

• NPR directly measures the distortion floor that fills in the notch, analogous to how nonlinearity fills adjacent channels.
• Particularly useful for multi-carrier and wideband systems where traditional two-tone IMD testing is insufficient.
• NPR values above 20 dB are typically required for acceptable performance in cable and satellite communication systems.