digital predistortion

Digital predistortion, or DPD, is a real-time signal processing technique that deliberately distorts an input signal before it enters a power amplifier, in order to cancel out the amplifier's own nonlinear distortions on the output. The predistorted signal and the amplifier's response combine to produce a cleaner, more linear overall output than the amplifier alone could deliver. This is essential in modern wireless and broadband systems where spectral purity and efficiency must coexist.

Power amplifiers, especially at high output levels, compress gain at peak signal levels, generate harmonics, and cause intermodulation distortion between multiple frequency components. These nonlinearities cause signal regrowth outside the intended channel and degradation of modulation quality. Rather than backing off power output to stay linear, DPD applies an inverse transfer function to the baseband digital signal before the D/A converter and RF upconversion. When the amplifier distorts this already-distorted signal, the two effects partially cancel.

Implementation and feedback loops

DPD operates in a feedback loop. A coupler or directional element at the amplifier output captures the actual output signal, which is downconverted, digitized, and compared against the expected ideal output. A digital processor continuously measures the amplifier's actual input-to-output relationship and updates a lookup table or polynomial model. This model is inverted and applied to future input samples. The loop runs continuously because amplifier characteristics drift with temperature, load impedance, and aging.

Common DPD models use polynomial or memory polynomial representations, capturing both memoryless nonlinearity and dynamic memory effects caused by bias networks and device heating. Coefficients are typically updated every millisecond to several milliseconds. Modern implementations run on DSP cores or FPGAs embedded in the RF subsystem, keeping latency low enough that feedback remains valid.

DPD is standard in cellular base stations, satellite transponders, and long-term evolution (LTE) and 5G systems where power efficiency and spectral mask compliance are mandated. Without it, amplifiers must operate with 6 to 10 decibels of backoff from their saturation point, wasting power and heating. With DPD, amplifiers can run closer to saturation while maintaining adjacent channel leakage ratio (ACLR) within regulatory limits. The tradeoff is added complexity, latency, and computational load.