Digital Predistortion for Power Amplifier Gate Bias Transients
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Solution Overview
Problem
In wireless communication systems, particularly cellular base stations using time division duplexing (TDD), the transition time between uplink and downlink modes is hindered by the settling time of the gate bias voltage, leading to non-linear effects and interference, which existing technologies struggle to mitigate effectively.
Innovation Solution
A digital predistortion (DPD) system is employed to invert the power amplifier input signal based on a digital model of the time-varying gate bias voltage, using an error voltage model generated by an IIR filter and a time counter, to align predistortion with the gate bias transition, thereby compensating for the non-linear effects and allowing earlier transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the gate bias voltage is allowed to settle before transmission, then non-linear effects and interference are reduced, but transmission startup time increases
Solution Approach 1:
The system performs preliminary action by pre-calculating and storing compensation values in a lookup table that account for the gate bias voltage transient behavior. When transmission starts, these pre-computed compensation values are immediately applied to the input signal, eliminating the need to wait for the gate bias to settle naturally. This allows transmission to begin immediately while maintaining signal linearity through digital predistortion.
Solution Approach 2:
The system changes the parameter of the input signal by applying digital predistortion that dynamically adjusts the signal characteristics based on the gate bias voltage state. A time-varying compensation factor is applied to the input signal amplitude, transforming it to compensate for the expected non-linearities during the transient period. This parameter transformation allows the system to operate during the transient phase without suffering from non-linear effects.
2Loss of time
If digital predistortion is applied to compensate for gate bias transient effects, then transmission can start earlier, but system complexity increases
Solution Approach 1:
The system uses a lookup table that contains pre-stored compensation values representing the gate bias voltage transient behavior. Instead of implementing complex real-time mathematical models, the system copies pre-characterized compensation data into the lookup table during calibration or initialization. During operation, the system simply retrieves and applies the appropriate compensation value based on the current time or voltage state, dramatically reducing computational complexity while maintaining effective predistortion.
Solution Approach 2:
The system employs a time-counter based approach where the compensation value changes over time in a controlled manner. The predistortion function uses a simple time-varying parameter that evolves during the transient period and then stabilizes. This temporary, time-limited predistortion mechanism only operates during the startup transient phase, using simple time-based control rather than continuous complex feedback, thereby reducing overall system complexity.
Data Source
AI summary
A digital predistortion (DPD) system includes an input configured to receive an input signal. In some examples, a first signal path configured to generate a first signal based on the input signal. In some examples, an error model provider configured to generate an error model signal modeled after a gate bias error voltage associated with the DPD system. In some examples, a first combiner configured to combine the first signal and the error model signal to generate a first intermediate signal, and the DPD system generates an output signal based at least on the first intermediate signal.