Load Modulation DPD

Unlike static DPD, load modulation DPD continuously tracks the active impedance presented to each power amplifier as beamforming weights change. The system models the load-pull contours of the PA and adapts the predistorter coefficients in real-time to maintain linearity across all steering angles.

• Compensates for VSWR variations caused by mutual coupling
• Maintains ACLR performance during beam scanning
• Requires real-time impedance estimation or pre-characterized lookup tables

The predistorter incorporates cross-coupled Volterra kernels that model not only the PA's intrinsic nonlinearity but also the interaction between adjacent elements. This captures how the distortion from one branch modulates the load seen by its neighbors.

• Extends memory polynomial models with mutual coupling terms
• Accounts for intermodulation products generated by crosstalk
• Uses sparse estimation to prune insignificant coupling paths

To reduce computational complexity, load modulation DPD often pre-computes coefficient sets for discrete beam angles and interpolates between them during continuous steering. This beam-indexed approach avoids solving the full inverse model at every symbol period.

• Stores predistorter coefficients in a 2D lookup table indexed by azimuth and elevation
• Interpolates using bilinear or spline methods for smooth transitions
• Reduces real-time computation by orders of magnitude

Load modulation DPD optimizes linearization across the entire array manifold rather than per-element. The objective function minimizes the error vector magnitude (EVM) in the far-field beam direction, not just at individual PA outputs.

• Formulates DPD as a spatial optimization problem
• Uses over-the-air feedback to capture combined radiated distortion
• Compensates for beam-squint effects in wideband systems

The impedance modulation caused by beam steering interacts with thermal memory effects in the PA. Load modulation DPD models this coupled electro-thermal behavior to prevent linearity degradation during rapid beam switching or sustained operation at extreme steering angles.

• Incorporates long-term thermal time constants into the behavioral model
• Compensates for gain and phase drift due to die temperature changes
• Critical for GaN-based Doherty PAs with strong thermal sensitivity

In massive MIMO arrays with hundreds of elements, per-element load modulation DPD is impractical. Sub-array clustering groups elements with similar impedance trajectories and applies a shared predistorter to each cluster, balancing linearity performance against implementation cost.

• Groups elements based on mutual coupling similarity metrics
• Uses principal component analysis to identify dominant impedance modes
• Achieves near-optimal linearization with significantly reduced coefficient storage