RF Front End Linearity Compensation Under Antenna Mismatch
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Solution Overview
Problem
Mobile device front end modules experience significant performance impairment due to degraded linearity, particularly in envelope tracking (ET) mode with wideband signals, caused by mismatch at the antenna and group delay in duplexers, leading to memory effects and AM-AM/AM-PM response variations across mismatch conditions.
Innovation Solution
A power amplifier system with a modulator, coupler, non-volatile memory, and processor that adjusts RF transmit signals based on VSWR measurements to compensate for memory effects, using an equalizer table and digital pre-distortion (DPD) to improve linearity, optionally with a programmable antenna tuner for impedance tuning.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If digital pre-distortion (DPD) is used to improve linearity, then linearity is improved, but performance degrades under mismatch conditions due to memory effects
Solution Approach 1:
The system dynamically adapts the DPD parameters based on detected VSWR conditions. When mismatch is detected, the system switches from a standard memory-less DPD approach to an adaptive DPD mode that accounts for memory effects, thereby maintaining linearity performance across varying impedance conditions.
Solution Approach 2:
The system changes the DPD parameters (such as delay taps, filter coefficients, and adaptation step sizes) based on the operating conditions including VSWR level and signal bandwidth. This allows the DPD to effectively compensate for memory effects that become prominent under mismatch conditions while maintaining efficiency under nominal conditions.
2Use of energy by moving object
If envelope tracking (ET) mode is used to improve power efficiency, then power efficiency is improved, but linearity degrades significantly under mismatch conditions
Solution Approach 1:
The system implements a feedback mechanism that continuously monitors VSWR conditions and signal characteristics. Based on this feedback, the system adjusts the DPD parameters in real-time to compensate for the interaction between ET mode operation and mismatch conditions, thereby maintaining linearity while preserving the power efficiency benefits of ET mode.
Solution Approach 2:
The system performs preliminary characterization of the power amplifier under various ET mode operating conditions and mismatch scenarios. This pre-characterization data is used to initialize the DPD parameters, enabling the system to quickly adapt to mismatch conditions without sacrificing the power efficiency advantages of ET mode operation.
3Device complexity
If standard DPD is used to simplify the system, then device complexity is reduced, but it becomes insufficient to address gain shape variation across the channel bandwidth
Solution Approach 1:
The system segments the channel bandwidth into multiple sub-bands and applies separate DPD compensation for each sub-band. This allows the system to address gain shape variations across the channel without requiring an excessively complex monolithic DPD structure, effectively managing the trade-off between complexity and performance.
Solution Approach 2:
The system extends the DPD approach from a single-dimensional memory-less model to a multi-dimensional model that incorporates time-delay elements and frequency-selective filtering. This dimensional extension enables the DPD to capture and compensate for memory effects and gain shape variations across the channel bandwidth while maintaining a practical implementation complexity.
Data Source
AI summary
A power amplifier system front end measures both forward and reverse power associated with an RF transmit signal. A processor is configured to use measurements derived from the measured forward and reverse power output to adjust the RF transmit signal in order to compensate for one or more memory effects of the power amplifier system.