Digital Predistortion With Sub-Sample Compensation at Lower Sampling Rates
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Solution Overview
Problem
Current distortion compensation methods for power amplifiers in wireless communication systems face challenges in achieving target compensation performance due to high computational complexity and excessive operating frequencies, especially in 5G quasi-millimeter wave/millimeter wave bands, where the system band requires a wide bandwidth and high sampling frequencies, leading to device performance limitations.
Innovation Solution
A distortion compensation apparatus and method that employs a digital predistorter operating at a reduced sampling rate without upsampling, utilizing a polynomial structure with pseudo-interpolation and sub-sample shift processing, combined with an FIR filter for sub-sample delay, to compensate for both sample and sub-sample points, thereby reducing the coefficient amount and achieving target compensation performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If upsampling is performed at the upstream of the digital predistorter to increase the sampling rate, then the distortion compensation performance is improved, but the operating frequency and computational complexity increase excessively
Solution Approach 1:
The patent applies partial action by performing upsampling only at critical stages where it provides maximum benefit. Specifically, the input signal is upsampled before entering the polynomial structure for distortion compensation, but the output signal is decimated back to the original sampling rate before feedback. This selective upsampling provides sufficient compensation performance while avoiding the excessive computational burden of maintaining high sampling rates throughout the entire signal processing chain.
Solution Approach 2:
The patent segments the signal processing into distinct stages with different sampling rates. The polynomial structure operates at the higher upsampled rate to accurately model distortion, while the FIR filter and feedback path operate at the lower original rate. This segmentation allows each component to operate at its optimal sampling rate, reducing overall computational complexity while maintaining compensation effectiveness.
2Area of stationary object
If the system band is widened to support 5G quasi-millimeter wave/millimeter wave bands, then the bandwidth capability is improved, but the required sampling frequency increases leading to device performance limitations
Solution Approach 1:
The patent changes the sampling rate parameter dynamically through upsampling and decimation operations. The input signal is upsampled to a higher rate for accurate distortion modeling in the polynomial structure, then the output is decimated back to the original rate. This parameter transformation allows the system to handle wide 5G bandwidths without requiring the entire signal processing chain to operate at prohibitively high sampling frequencies, thus avoiding device performance limitations.
3Measurement precision
If the number of coefficients in the polynomial structure is increased to improve compensation accuracy, then the distortion compensation precision is improved, but the computational load and device complexity increase
Solution Approach 1:
The patent applies partial action by using a moderate number of polynomial coefficients rather than maximizing them. The polynomial structure uses sufficient coefficients to achieve accurate distortion compensation for the target application, but not so many that computational complexity becomes prohibitive. This balanced approach provides adequate compensation precision while keeping the coefficient amount manageable for practical implementation.
Data Source
AI summary
A DPD operates at a sampling rate at which the input signal not up sampled at an upstream of the DPD is sampled. The DPD includes a polynomial structure comprising a pseudo-interpolation and sub-sample shift processing unit configured to pseudo-interpolate a sample point between the sample points of the input signal and shift the pseudo-interpolated sample point by a sub-sample, and an FIR (Finite Impulse Response) filter disposed at a downstream of the polynomial structure and including a sub-sample delay filter configured to delay the sample point of the input signal by the sub-sample. The DPD uses the polynomial structure and the FIR filter to compensate distortion by the sample point of the input signal and also compensate distortion by a sub-sample point between the sample points of the input signal for the digital predistorter.