Adjacent Channel Power Ratio

ACPR is the de facto standard for spectral emission masks in wireless standards including 3GPP, IEEE 802.11, and ETSI specifications. Regulatory bodies mandate specific ACPR limits—typically -45 dBc to -60 dBc depending on the standard—to prevent interference between adjacent carriers. Failure to meet these limits results in certification rejection, making ACPR a pass/fail metric for transmitter design. The measurement directly quantifies the out-of-band emissions that digital predistortion (DPD) systems are designed to suppress.

ACPR is a direct consequence of AM-AM and AM-PM distortion in power amplifiers. When a band-limited signal passes through a nonlinear device, spectral regrowth occurs—the signal bandwidth expands into adjacent channels. The third-order intercept point (IP3) and 1 dB compression point (P1dB) are strongly correlated with ACPR performance. Higher-order nonlinearities (5th, 7th order) contribute to far-out ACPR, while 3rd-order products dominate the immediate adjacent channel. Memory effects cause asymmetry in the upper and lower ACPR measurements.

ACPR is measured using a spectrum analyzer or vector signal analyzer with specific resolution bandwidth (RBW) settings defined by the wireless standard. The measurement integrates power across the assigned channel bandwidth and compares it to the integrated power in offset channels at specified frequency offsets. Key parameters include:

• Channel bandwidth (e.g., 5 MHz for WCDMA, 20 MHz for LTE)
• Offset frequency (e.g., ±5 MHz, ±10 MHz from carrier center)
• Measurement bandwidth (typically equal to channel bandwidth)
• Detector type (RMS averaging is standard)

Adjacent Channel Power Ratio (ACPR) and Adjacent Channel Leakage Ratio (ACLR) are often used interchangeably but have subtle distinctions. ACPR typically refers to the ratio of power in the adjacent channel to power in the main channel, while ACLR emphasizes the leakage aspect—power that has leaked from the main channel into adjacent spectrum. In practice, 3GPP specifications use ACLR, while North American standards and general RF engineering literature frequently use ACPR. Both metrics serve the identical purpose of quantifying spectral containment.

Digital predistortion systems are evaluated by their ACPR improvement—the reduction in adjacent channel power achieved through linearization. A well-designed DPD system typically achieves 15-25 dB of ACPR improvement, bringing a raw PA from -30 dBc to better than -50 dBc. The improvement is limited by:

• Model fidelity (NMSE of the behavioral model)
• Bandwidth of the feedback path (must capture 3-5x the signal bandwidth)
• DAC/ADC resolution in the transmit and observation paths
• Time alignment accuracy between reference and feedback signals

Modern 5G NR signals with 100 MHz to 400 MHz instantaneous bandwidth present extreme ACPR challenges. Wideband signals excite frequency-dependent memory effects across the entire modulation bandwidth, causing asymmetric spectral regrowth that cannot be corrected by memoryless predistortion. The feedback receiver must digitize 3-5x the signal bandwidth to capture 3rd and 5th order distortion products, requiring multi-GSPS ADCs. Carrier aggregation compounds this by creating inter-band distortion products that fall into protected spectrum far from the carrier frequencies.

The appearance of unwanted frequency components in adjacent channels caused by intermodulation distortion when a band-limited signal passes through a nonlinear power amplifier. This phenomenon directly creates the interference that ACPR measures.

• Caused by odd-order nonlinearities mixing signal components
• Regrowth spectrum follows the shape of the original signal convolved with itself
• Primary target of digital predistortion linearization techniques

Often used interchangeably with ACPR, ACLR specifically measures the ratio of filtered mean power centered on the assigned channel frequency to the filtered mean power centered on an adjacent channel frequency.

• Defined in 3GPP specifications for LTE and 5G NR compliance
• Typically measured with specified measurement bandwidths and channel offsets
• Regulatory limits vary by band and deployment scenario

EVM measures in-band signal quality by calculating the magnitude of the vector difference between the ideal reference constellation points and the actual transmitted symbols.

• While ACPR quantifies out-of-band distortion, EVM quantifies in-band impairment
• Both metrics degrade simultaneously under nonlinear amplifier operation
• DPD systems must balance EVM improvement against ACPR reduction targets

ACEPR is a model validation metric that specifically measures the prediction error power in adjacent channels when comparing a behavioral model's output to measured amplifier data.

• Assesses a model's ability to predict out-of-band distortion accurately
• Critical for validating DPD models before hardware implementation
• Complements NMSE by focusing on spectral regions most relevant to regulatory compliance

PAPR is the ratio of instantaneous peak power to average power in a communication signal. High PAPR signals force power amplifiers to operate with significant back-off from compression, reducing efficiency.

• OFDM signals exhibit PAPR of 10-12 dB without reduction techniques
• Crest factor reduction is often applied before DPD to improve overall system efficiency
• Higher PAPR signals generate more severe spectral regrowth when clipped by amplifier saturation

IMD is the fundamental nonlinear mechanism that generates spectral regrowth. When multiple frequency components pass through a nonlinear device, they produce sum and difference frequency products.

• Third-order IMD products fall closest to the original carriers and are hardest to filter
• Fifth and seventh-order products contribute to far-out ACPR degradation
• Memory effects cause IMD asymmetry between upper and lower adjacent channels