Wideband DPD Signal Alignment With Farrow Fractional Delay
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Solution Overview
Problem
Current digital predistortion systems for power amplifiers in mobile communication systems face challenges in accurately aligning transmit and feedback signals with high frequency spacing, leading to significant delay alignment errors and suboptimal linearity due to the complexity of nonlinear amplifier characteristics.
Innovation Solution
A wideband digital predistortion system employing a Farrow-based fractional delay filter and an adaptive delay estimation method using a third-order Lagrange Farrow structure to accurately control the feedback path delay, enabling precise time alignment of signals with a minimum delay resolution of 0.1 ns, even in systems with instantaneous bandwidths exceeding 100 MHz.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional delay estimation methods are used in wideband digital predistortion systems, then the system complexity is reduced, but the delay alignment accuracy deteriorates significantly when carriers are widely frequency spaced
Solution Approach 1:
The delay estimation process is segmented into multiple stages: initial coarse delay estimation using correlation methods, followed by fine delay adjustment using Farrow-based fractional delay filters. This segmentation allows the system to achieve high precision without requiring a single complex estimation algorithm to handle the entire range of delay values.
Solution Approach 2:
The system employs dynamic delay adjustment mechanisms where the delay estimate is continuously refined based on feedback from timing error calculations. The Farrow filter structure allows dynamic interpolation between discrete delay samples, enabling the system to adapt to varying delay conditions while maintaining computational efficiency.
2Reliability
If high precision delay alignment is implemented using conventional methods, then the linearity of power amplifier is improved, but the computational complexity and processing time increase significantly
Solution Approach 1:
The system performs preliminary coarse delay estimation using efficient correlation-based methods before applying more computationally intensive Farrow filter-based fine adjustment. This preliminary action reduces the search space for subsequent precision alignment, significantly lowering overall computational complexity while maintaining high linearity performance.
Solution Approach 2:
The Farrow filter structure enables efficient parameter changes by using polynomial interpolation to generate fractional delay values from a fixed set of integer delay taps. This approach allows continuous delay adjustment without requiring recomputation of entire filter coefficients, reducing computational burden while achieving sub-sample delay precision for improved power amplifier linearity.
3Ease of manufacture
If analog predistortion is used to linearize power amplifier, then the implementation is simpler, but the predistortion accuracy and power efficiency deteriorate
Solution Approach 1:
The system replaces analog predistortion circuitry with digital signal processing implementations. The Farrow-based fractional delay filter and timing alignment algorithms are executed in the digital domain, providing superior precision and flexibility while maintaining implementation feasibility through software-defined approaches. This substitution eliminates the trade-off between analog simplicity and digital precision.
Data Source
AI summary
A system for time aligning widely frequency spaced signals includes a digital predistortion (DPD) processor and a power amplifier coupled to the DPD processor and operable to provide a transmit signal at a power amplifier output. The system also includes a feedback loop coupled to the power amplifier output. The feedback loop comprises an adaptive fractional delay filter, a delay estimator coupled to the adaptive fractional delay filter, and a DPD coefficient estimator coupled to the delay estimator.