IQ Offset (LTE)
indicates the magnitude of carrier feedthrough (power at 0 Hz). When there is no carrier feedthrough, is zero (-infinity dB).
is calculated by RMS averaging the measured IQ offset for each symbol in the Measurement Interval and is expressed relative to the average signal power.
For a downlink signal, is calculated from the reference measurement channel.
For an ideal downlink signal, any IQ offset present would not affect the quality of the signal since the DC subcarrier In an orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) signal, the subcarrier whose frequency would be equal to the RF center frequency of the station. is orthogonal to the other subcarriers. However, when the signal is impaired so that the subcarriers are not orthogonal to DC (when Doppler shifted, for instance), any IQ offset on the DC subcarrier will affect the EVMs of other subcarriers.
For an uplink signal, even when the signal is an ideal signal, any IQ offset will adversely affect the subcarrier EVMs since the DC subcarrier is not orthogonal to the other subcarriers.
An IQ offset can be caused when the center (DC) carrier (which is supposed to be filtered out) leaks into the signal. IQ offset can also be caused when the baseband signal has a DC offset which then shows up as (DC) carrier power when the baseband signal is upconverted.
See IQ Offset (OFDMA) for more information about .
Also, see the EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format. Minimization IQ Offset parameter topic for information specific to LTE Long Term Evolution uplink signals.
See Also
