IQ Quad. Error (DOCSIS 4.0 Downstream)
( IQ Quadrature Skew Error) indicates the orthogonal error between the I and Q signals. Ideally, I and Q should be orthogonal (90 degrees apart). A quadrature skew error of 3 degrees means that I and Q are 93 degrees apart. A positive quadrature skew error corresponds to moving the Q axis counter-clockwise with respect to the I axis, resulting in an angle larger than 90 degrees in the upper right quadrant.
Understanding DOCSIS 4.0 Downstream IQ Quadrature Error
Quadrature Skew Error for DOCSIS 4.0 Downstream demodulation differs from Quadrature Skew Error for single subcarrier modulation in that quadrature skew error does not show up in the constellation diagram as an angled constellation. That's because quadrature skew error is in the time domain signal, but OFDM constellations are in the frequency domain. In an OFDM signal, quadrature skew error causes each subcarrier to interfere with the opposing subcarrier, which results in spreading each constellation point into a miniature copy of the entire constellation.
Interpreting a Zero IQ Gain Imbalance and IQ Quadrature Error Data Result
The VSA IQ Gain Imbalance and IQ Quadrature Skew Error algorithm require that a subcarrier and it's mirror subcarrier (for example, subcarrier +10 and subcarrier -10) to have non-zero power for two consecutive symbols. When this criteria is not satisfied for the measurement signal, the IQ Gain Imbalance and IQ Quadrature Error data result is shown as zero.
See Also
Available Error Summary Results
