Time-Domain EVM Computation for OFDM Signal Testing
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Solution Overview
Problem
Conventional methods for computing error vector magnitude (EVM) for OFDM signals are inefficient, requiring transformation to the frequency domain and increasing testing time for devices under test, limiting the number of evaluations per unit time.
Innovation Solution
A method for computing EVM in the time domain using a stored reference signal, eliminating the need for frequency domain transformation, allowing for faster computation and matching standard-defined composite EVM with a scalar multiple.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional frequency domain transformation methods are used to compute EVM for OFDM signals, then measurement precision is maintained, but productivity decreases due to increased computation time
Solution Approach 1:
The patent changes the computational domain parameter from frequency domain to time domain. By computing EVM directly in the time domain using the formula EVM_TD = sqrt(mean(|x[n] - y[n]|^2)) where x[n] and y[n] are time-domain samples, the method eliminates the need for FFT transformations while maintaining measurement accuracy through proper time-domain signal alignment and synchronization.
2Measurement precision
If frequency domain transformation is applied to compute EVM, then standard-defined composite EVM is obtained, but loss of time increases due to additional processing steps
Solution Approach 1:
The patent extracts and eliminates the unnecessary FFT transformation step from the conventional EVM computation process. By directly comparing time-domain samples of the transmitted signal with received signal samples, the method removes the intermediate frequency domain transformation step while still obtaining the composite EVM metric needed for signal quality assessment.
Solution Approach 2:
The patent performs preliminary signal synchronization and alignment in the time domain before EVM computation. By pre-aligning the time-domain samples of transmitted and received signals using synchronization pilots or known signal structures, the method ensures accurate sample-by-sample comparison without requiring subsequent frequency domain transformations.
3Measurement precision
If conventional EVM computation methods are used, then measurement accuracy is maintained, but device complexity increases due to additional transformation operations
Solution Approach 1:
The patent inverts the conventional approach by computing EVM in the time domain instead of transforming to the frequency domain. This inversion simplifies the computational process from requiring FFT operations and frequency domain comparisons to simple time-domain sample subtraction and averaging, reducing device complexity while maintaining accuracy.
Data Source
AI summary
A mechanism for determining an error vector magnitude EVMTD for a signal transmitted by a device under test (DUT). A receiver (typically an RF signal analyzer) produces a baseband signal in response to the signal transmission. An OFDM input signal (derived from the baseband signal) is accessed from memory. The OFDM input signal includes a sequence of time-domain OFDM input symbols. A reference signal is accessed from the memory. The reference signal includes a sequence of time-domain OFDM reference symbols. EVMTD is computed in the time domain based on a time-domain difference signal, i.e., a time-domain difference between the sequence of time-domain OFDM input symbols and the sequence of time-domain OFDM reference symbols. The error vector magnitude EVMTD is determined without transforming the sequence of time-domain OFDM input symbols to the frequency domain. The error vector magnitude EVMTD is related to a standard-defined composite EVM by a scalar multiple.