ET-DPD for Outphasing PAs

Unlike ideal isolating combiners, practical outphasing combiners (e.g., Chireix combiners) are non-isolating, meaning the two branch PAs see a load impedance that varies with the outphasing angle. This load modulation creates significant AM-AM and AM-PM distortion that is compounded when envelope tracking dynamically varies the supply voltage. ET-DPD must linearize a system where the PA's nonlinearity is a function of both instantaneous supply voltage and the time-varying load impedance presented by the combiner.

The outphasing transmitter decomposes a variable-envelope input signal into two constant-envelope, phase-modulated signals (S1 and S2). This signal component separator (SCS) creates a unique distortion mechanism: any bandwidth expansion or AM-PM distortion in the branch PAs is translated through the combiner into amplitude distortion at the output. ET-DPD must pre-distort the phase signals to account for how supply-dependent nonlinearities in each branch manifest after vector recombination.

Chireix outphasing combiners use shunt compensation reactances (+jX and -jX) to maximize efficiency at a specific back-off power level. These reactive elements create a frequency-dependent, non-linear impedance environment. When envelope tracking is applied, the varying drain capacitance of the PAs interacts with these compensation elements, shifting the optimal combiner tuning. ET-DPD models must capture this supply-dependent impedance detuning to maintain linearity across the ET voltage range.

Outphasing relies on perfect symmetry between the two amplifier branches. In practice, gain and phase mismatches between the two PAs, their supply modulators, and the RF paths create incomplete cancellation at the combiner, resulting in residual AM distortion. ET-DPD for outphasing must simultaneously correct for the individual nonlinearities of each branch while compensating for the differential errors that corrupt the vector summation at the output.

A complete ET-DPD model for outphasing requires a multi-input behavioral framework. The model must accept the two outphasing phase signals and the dynamic supply voltage as inputs to predict the combined output. This often necessitates a dual-input Volterra or neural network structure that captures the cross-coupling between the two RF paths and the supply modulation, accounting for the nonlinear interaction of all three variables at the combiner.

The primary advantage of combining ET with outphasing is the potential for very high efficiency across a wide dynamic range. However, the deepest efficiency enhancement often occurs at operating points where the combiner's nonlinearity is most severe. ET-DPD must be co-optimized with the shaping function and the outphasing angle mapping to find the Pareto-optimal front where the DPD can successfully linearize the system while preserving the efficiency gains from both techniques.

A transmitter technique where a variable-envelope signal is decomposed into two constant-envelope, phase-modulated signals. These signals drive two highly efficient, nonlinear PAs. The original amplitude modulation is restored by vector summing the PA outputs in a non-isolating power combiner. The combiner's reactive loading causes the PAs to see time-varying impedances, creating a unique nonlinear distortion profile that ET-DPD must correct.

A non-isolating combiner topology used in outphasing systems that includes shunt reactive compensation elements. These elements are tuned to maximize efficiency at a specific outphasing angle, creating an efficiency peak at back-off power levels. However, this compensation also introduces a strongly nonlinear, signal-dependent reactive load to the PAs, which is a primary source of the distortion requiring ET-DPD linearization.

The primary nonlinear mechanism in outphasing PAs. As the outphasing angle changes, the instantaneous impedance seen by each PA's drain varies dynamically. This load-pull effect modulates the PA's gain and phase response. When combined with envelope tracking, the supply voltage variation adds a second dimension to this load modulation, creating a complex, two-dimensional nonlinearity that requires a dual-input behavioral model.

An advanced outphasing architecture that combines envelope tracking with the LINC concept. Instead of using purely constant-envelope signals, the input is decomposed into signals with a discrete number of amplitude levels. The supply voltage is then switched between corresponding discrete levels. This reduces the required outphasing angle range, improving combiner efficiency and reducing reactive loading, but introduces supply-switching transients that DPD must handle.

A highly efficient variant where both the outphasing angle and the discrete supply voltages of the two PAs are controlled independently. This breaks the symmetry of traditional LINC, allowing for more degrees of freedom to maximize power-added efficiency (PAE) across the entire dynamic range. The resulting distortion surface is highly complex and non-separable, demanding a sophisticated ET-DPD joint model for linearization.

The digital signal processing block that decomposes the baseband input signal into the phase-modulated component signals for the two outphasing paths. The SCS computes the outphasing angle from the instantaneous signal amplitude. In ET-DPD for outphasing, the SCS must work in concert with the shaping function to coordinate the phase signals and the dynamic supply voltage, ensuring a coherent drive profile at the combiner.