Predistorter Lookup Table with Companding for Wide Dynamic Range
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Solution Overview
Problem
Existing predistortion lookup tables (LUTs) in wireless transmitters, particularly for WCDMA standards, face challenges in maintaining linearization under varying operational conditions and signal dynamic ranges, leading to inadequate performance due to uniform spacing and aging issues.
Innovation Solution
Implementing a predistorter with a non-uniformly spaced LUT and a compander to address nonlinearity, along with an interpolation offset calculation circuit for efficient signal processing, and using base-2 spaced entries with an address calculation block and interpolation circuit to improve mapping accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a uniformly spaced LUT is used, then the device complexity is reduced and ease of manufacture is improved, but the signal-to-noise ratio deteriorates at lower input signal levels
Solution Approach 1:
The patent applies local quality by using non-uniform spacing of LUT entries where the spacing is denser at lower input signal levels and sparser at higher levels. This allows the LUT to provide higher precision (better signal-to-noise ratio) where it is most needed (at low signal levels) while maintaining reasonable overall complexity. The compander transforms the input signal to achieve this non-uniform effective spacing.
Solution Approach 2:
The patent changes the parameter of LUT entry spacing from uniform to non-uniform. By modifying the spacing parameter based on signal level, the system achieves better performance across the full dynamic range. The compander parameter transformation enables this change in spacing characteristics without requiring a completely different LUT structure.
2Measurement precision
If a non-uniformly spaced LUT is used to improve mapping accuracy, then the signal-to-noise ratio is improved, but the device complexity increases
Solution Approach 1:
The patent introduces a compander as an intermediary device that transforms the input signal before it addresses the LUT. This compander mediates between the uniform LUT structure and the need for non-uniform effective spacing, allowing the simple uniform LUT to achieve non-uniform performance characteristics through the signal transformation provided by the compander.
Solution Approach 2:
The patent changes the parameter of the LUT addressing scheme from direct uniform spacing to compander-transformed non-uniform spacing. This parameter change in how the LUT is addressed (rather than changing the LUT physical structure) improves mapping accuracy while keeping the LUT itself relatively simple.
3Ease of manufacture
If factory calibration is used to create the predistortion LUT, then the manufacturing process is simplified, but the linearization performance deteriorates under varying operational conditions
Solution Approach 1:
The patent applies preliminary action by performing factory calibration to create the initial predistortion LUT, but also incorporates mechanisms to update and adapt this LUT during operation. The compander and non-uniform spacing provide a foundation that works well across varying conditions, and the system can be updated with measured data to maintain performance as the amplifier ages or conditions change.
Solution Approach 2:
The patent introduces dynamics by allowing the predistortion LUT to be updated and adapted during the amplifier's operational life. Rather than being completely static after factory calibration, the system can incorporate measured performance data to update the LUT entries, maintaining linearization performance as the amplifier characteristics drift due to aging or environmental changes.
Data Source
AI summary
A predistorters for use with a nonlinear element and methods of predistorting for a nonlinear element for use in a 3G, e.g., WCDMA transmitter. In one embodiment, the predistorter includes: (1) a lookup table having non-uniformly spaced entries therein, (2) a compander configured to compand an input signal based on a nonlinearity of the nonlinear element to address the entries and (3) an interpolation offset calculation circuit associated with the lookup table and configured to produce an output based on a value of the input signal and a linear interpolation involving at least two entries from the lookup table.