Quad Err (802.16 OFDMA)
Quad Err (deg), or IQ Quadrature Skew Error, is 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.
The output will show "***" when a valid data result is not possible. This is a function of the number of active subcarriers which are symmetric across DC.
When performing data burst analysis, the burst geometry and permutation algorithm define the active subcarriers for a given measurement. If there are not enough active symmetric subcarriers (i.e. subcarriers active on both sides of DC, like +10 and -10) to perform the IQ impairment estimation, "***" will be displayed on the summary table in place of a valid result. This may be more noticeable with UL Up Link (reverse link: from cell phone to base station)-OPUSC and UL-AMC Adaptive Modulation and Coding zone types due to the lack of subchannel rotation for those permutation types. Reconfiguring the geometry of the analyzed burst(s) will allow this result to be computed.
Quadrature Skew Error for OFDMA demodulation differs from Quadrature Skew Error for single carrier 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 OFDMA constellations are in the frequency domain. In an OFDMA signal, quadrature skew error causes each subcarrier to interfere with its opposing subcarrier, which results in spreading each constellation point into a miniature picture of the entire constellation.
Inaccurate IQ Measurement Results
The accuracy of the IQ Impairments, including IQ Gain Imbalance, IQ Quadrature Error and IQ Timing Skew, are dependent on the accurate detection of a reference signal. The accurate detection of a reference signal is dependent on the data modulation format, as well as the cumulative impairments observed during data subcarrier detection. If the incorrect data modulation format is used, IQ mismatch impairment estimates will be incorrect. This can be avoided by selecting Data Tone Modulation from the tab of the demodulation properties dialog box.
For more information and example measurement results, see the IQ Gain Imbalance topic.
See Also
IQ Gain Imbalance (802.16 OFDMA)
