Transmitter Characterization Using Offset Preamble Subcarriers
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Solution Overview
Problem
Existing methods for characterizing transmitter non-linearity in wireless communication devices, such as observing in-band error vector magnitude (EVM) and out-of-band emissions, are not feasible for orthogonal frequency division multiple access (OFDMA) or code division multiple access (CDMA) systems due to time domain symbol quality issues and require wide bandwidth and large dynamic range feedback paths, which become problematic with fluctuating transmitter output power levels.
Innovation Solution
The technique involves shifting at least one subcarrier in the preamble of a baseband signal frame to offset its frequency, allowing inter-modulation distortion to be easily extracted using Fast Fourier Transform computations, and appending multiple frame portions for better measurement resolution, enabling accurate pre-distortion parameter calculation and improved digital pre-distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If in-band EVM is used to observe transmitter non-linearity, then the non-linear characteristics can be reflected in EVM degradation, but the method is not feasible for OFDMA or CDMA systems where symbol quality is not readily observed in the time domain
Solution Approach 1:
The patent transforms the measurement approach from time domain to frequency domain by using Fast Fourier Transform (FFT) to analyze the transmitted signal. Instead of computing EVM in the time domain, the method extracts subcarrier amplitudes and phases from the frequency domain representation, enabling non-linearity characterization in OFDMA/CDMA systems where time domain symbol quality is not readily observable.
2Measurement precision
If out-of-band emissions are used to observe transmitter non-linearity, then spectrum growth can be detected, but the bandwidth of the feedback path needs to be many times wider than the bandwidth of the transmitter output signal
Solution Approach 1:
The patent extracts the necessary measurement information directly from the in-band signal by analyzing subcarrier components in the frequency domain. Instead of requiring a wide bandwidth feedback path to capture out-of-band emissions, the method extracts amplitude and phase information from specific subcarriers within the normal signal bandwidth, eliminating the need for excessive feedback path bandwidth.
Solution Approach 2:
The patent introduces frequency domain analysis (FFT) as an intermediary technique that enables non-linearity measurement without requiring wide bandwidth feedback paths. By transforming the time domain signal to frequency domain and analyzing subcarrier characteristics, the method mediates between the need for accurate non-linearity measurement and the constraint of limited feedback path bandwidth.
3Measurement precision
If out-of-band emissions are used to observe transmitter non-linearity, then spectrum growth can be detected, but the dynamic range of the feedback path needs to be large enough to accurately observe higher order inter-modulation distortion
Solution Approach 1:
The patent extracts inter-modulation distortion characteristics by analyzing the amplitude and phase of specific subcarriers in the frequency domain. By measuring the deviation of actual subcarrier amplitudes and phases from expected values, the method observes higher order inter-modulation distortion without requiring a feedback path with excessive dynamic range, as the measurements are derived from in-band signal components rather than weak out-of-band emissions.
4Measurement precision
If conventional subcarrier frequencies are used, then inter-modulation distortion falls on top of desired subcarriers and cannot be easily extracted, but frequency shifting adds complexity to the signal processing
Solution Approach 1:
The patent introduces asymmetric frequency shifting where specific subcarriers (particularly the DC subcarrier and potentially other reference subcarriers) are intentionally offset from their nominal frequencies by a fixed amount. This asymmetric shift moves inter-modulation distortion products away from overlapping with desired subcarriers, enabling easy extraction of distortion measurements through simple frequency domain analysis without requiring complex signal processing techniques.
Data Source
AI summary
Techniques are provided to pre-distort a signal that is transmitted by a transmitter of a wireless communication device, e.g., a device configured for wireless radio frequency communication. The transmitter of the device inherently introduces distortion to the baseband signal to be transmitted. In at least one frame of a baseband signal to be transmitted, at least one subcarrier in a preamble of the frame is shifted in frequency such that the at least one subcarrier is offset from a normal subcarrier frequency position. The at least one frame of the baseband signal is supplied to the transmitter that is configured to produce a transmit signal for transmission. The transmit signal at an output of the transmitter is sampled or detected and inter-modulation distortion in the transmit signal is determined at one or more frequencies as a result of shifting of the frequency of the at least one subcarrier in the preamble of the at least one frame. The baseband signal is pre-distorted based on the inter-modulation distortion prior to coupling to the transmitter for transmission.