Quad Error (802.16 OFDM)

Quad Err (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.

Quadrature Skew Error for OFDM Orthogonal Frequency Division Multiplexing: OFDM employs multiple overlapping radio frequency carriers, each operating at a carefully chosen frequency that is Orthogonal to the others, to produce a transmission scheme that supports higher bit rates due to parallel channel operation. OFDM is an alternative tranmission scheme to DSSS and FHSS. 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 OFDM constellations are in the frequency domain. In an OFDM 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.