PA Gain Feedback Control for RF Transmitter Linearity
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Solution Overview
Problem
Wireless communication units face challenges in maintaining optimal power control and linearity of RF power amplifiers due to variations in operating conditions, leading to issues such as signal-to-noise ratio problems, headroom issues, and failure to meet specifications like ACPR and EVM, especially under extreme temperatures or antenna mismatches.
Innovation Solution
A transmitter architecture that monitors PA gain from transmit slot to slot using an automatic output power control (AOC) loop system and Adaptive Predistortion (APD) system, performing gain adjustments in both digital and analogue domains to compensate for gain variations, and implementing a gain monitoring system to estimate and adjust gain offsets, ensuring optimal trade-offs between signal headroom and signal-to-noise ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If firmware programs variable gains with nominal values under nominal conditions, then the system operates correctly under normal conditions, but under extreme conditions (hot/cold temperature, poor antenna mismatch) the PA gain varies significantly causing non-optimal operation
Solution Approach 1:
The patent implements a gain monitoring system that performs preliminary measurements of the actual PA gain under current operating conditions before the transmitter begins operation. This preliminary action allows the system to determine appropriate gain values for digital domain logic elements and programmable analogue attenuators in advance, ensuring optimal performance even under extreme conditions before transmission starts.
Solution Approach 2:
The patent employs a feedback mechanism where the actual PA gain is measured and fed back to the control logic. This feedback loop enables the system to continuously monitor gain variations under different operating conditions and automatically adjust the programmed gain values of digital and analogue components to compensate for these variations, maintaining optimal operation across all conditions.
2Manufacturing precision
If linearisation techniques such as Adaptive Pre-distortion are used to improve linearity of efficient amplifiers, then linearity is improved, but the system becomes more complex and requires accurate gain knowledge
Solution Approach 1:
The gain monitoring system performs preliminary measurement and determination of optimal gain values before the linearisation techniques are applied. This preliminary action provides accurate gain knowledge to the Adaptive Pre-distortion system, enabling it to function effectively without requiring complex real-time gain estimation mechanisms, thereby reducing overall system complexity while maintaining high linearity performance.
3Reliability
If feedback is used to control PA input power based on output measurement, then power control regulation compensates for PA variations, but signal-to-noise ratio problems and headroom issues occur under extreme conditions
Solution Approach 1:
The patent implements preliminary measurement of actual PA gain under current operating conditions and uses this information to pre-calculate optimal gain settings for the transmit signal path. By determining appropriate gain values before transmission begins, the system avoids signal-to-noise ratio degradation and headroom issues that would otherwise occur during feedback-based power control adjustments, especially under extreme conditions.
Solution Approach 2:
The patent introduces an intermediary gain monitoring and calculation system that sits between the feedback power control loop and the actual signal path. This intermediary measures actual PA gain, calculates appropriate compensation values, and programs these into digital domain logic elements and programmable analogue attenuators, thereby mediating between the feedback control requirements and the actual signal transmission to maintain optimal signal quality.
Data Source
AI summary
A wireless communication unit comprises a transmitter having a forward path comprising a power amplifier, PA, and a feedback path operably coupled to the power amplifier, wherein the feedback path comprises a coupler arranged to feed back a portion of a signal to be transmitted and a controller logic module arranged to control a power control value of the power amplifier, such that the forward path and feedback path form a closed loop power control. The controller logic module is arranged to determine a gain variation in the transmitter and provide attenuation to a transmit signal passing therethrough on a transmit slot by transmit slot basis and instruct a power backoff where necessary.