IQ Skew (802.16 OFDMA)

IQ Skew (sec), or I/Q Timing Skew, is the deviation in path length between the I&Q branches.

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.

IQ impairment estimation is important for determining problems in the IQ upconverter. The IQ impairment results are the IQ Gain Imbalance (amplitude deviation between I&Q paths), the IQ Quadrature Error (deviation from 90 degrees between I&Q paths), the IQ Offset (baseband DC level usually originating from LO feedthru) and the IQ Timing Skew (deviation in path length between I&Q branches).

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 Manual Data Tone Modulation from the Advanced tab of the demodulation properties dialog box.

For more information and example measurement results, see the IQ Gain Imbalance topic.