Spectral Mask

The primary regulatory metric for spectral regrowth. ACLR quantifies the ratio of transmitted power within an assigned channel to the power leaking into adjacent channels. 3GPP specifications define ACLR limits (typically -45 dBc for adjacent channel, -50 dBc for alternate channel) that transmitters must meet. ACLR is directly degraded by AM-AM distortion and AM-PM distortion in power amplifiers. Digital predistortion (DPD) is the dominant technique for restoring ACLR margins in modern base stations.

Nonlinear signal products generated at sum and difference frequencies when multiple signals pass through a nonlinear device. Third-order intermodulation (IMD3) products fall closest to the original carriers and are the most problematic for adjacent channel interference. IMD3 power increases at 3 dB per 1 dB of fundamental power increase, making it the dominant spectral regrowth mechanism. The third-order intercept point (IP3) is the theoretical figure of merit used to characterize this behavior.

A signal conditioning technique applied before the power amplifier to reduce the peak-to-average power ratio (PAPR) of the transmitted waveform. High PAPR signals like OFDM force amplifiers to operate with significant back-off to avoid clipping-induced spectral regrowth. CFR techniques include:

• Peak windowing: Smooth time-domain windowing for superior spectral containment
• Clipping and filtering: Hard limiting followed by frequency-domain filtering
• Pulse injection: Adding cancellation pulses at detected peaks Effective CFR enables higher average power operation while maintaining spectral mask compliance.

The two fundamental nonlinear mechanisms causing spectral regrowth. AM-AM distortion describes amplitude-to-amplitude nonlinearity where output amplitude deviates from the linear gain curve, causing gain compression. AM-PM distortion describes amplitude-to-phase conversion where the phase shift varies with instantaneous input envelope. AM-PM is particularly insidious because it causes spectral asymmetry in the regrowth profile. Both must be characterized and compensated in the predistortion model for effective spectral mask compliance.

A power amplifier phenomenon where the current output depends on past input states due to thermal dynamics, electrical biasing, and charge trapping in semiconductor materials. Memory effects cause frequency-dependent nonlinear behavior that complicates spectral regrowth cancellation. Short-term memory (electrical) affects wideband signal linearization, while long-term memory (thermal) causes slow envelope-dependent drift. Modern DPD models like memory polynomials and Volterra series explicitly account for these effects.

The deliberate reduction of a power amplifier's average operating power below its saturation or 1 dB compression point (P1dB) to improve linearity. Back-off directly reduces spectral regrowth but trades power efficiency for signal fidelity. Typical back-off values range from 6-12 dB for Class AB amplifiers without DPD. Digital predistortion enables operation with reduced back-off (3-6 dB), recovering significant efficiency while maintaining spectral mask compliance. This efficiency gain is the primary economic driver for DPD adoption.