EVM Minimization: IQ Offset (LTE)

Menu Path (uplink): MeasSetup > LTE Demod Properties... > Advanced tab > EVM Minimization > IQ Offset

When IQ Offset is selected, the demodulator adjusts the signal for any IQ offset found. When IQ Offset is cleared, the signal is not compensated for IQ offset.

This parameter is only available when Direction is set to Uplink and EVM Minimization is set to 3GPP. 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's IQ Offset correction replaces the IQ Offset Compensate parameter in earlier versions of the LTE Long Term Evolution demodulator. The difference is that IQ Offset Compensate applied a single correction to the entire frame, but the EVM Minimization IQ Offset parameter calculates IQ offset corrections using RS+Data and applies IQ offset corrections slot by slot.

In uplink, the subcarriers on either side of the carrier frequency are spaced apart by 7.5 KHz which is half the 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. subcarrier spacing. This means that instead of the OFDM subcarrier's zero crossings being at the carrier frequency, each subcarrier contributes something to the DC power level.

And, unlike downlink where any power found at DC can be assumed to come from an IQ offset, the value of the DC point in uplink needs to be calculated before the IQ offset can be calculated.

The demodulator compensates for the IQ offset in each slot by doing the following:

1. Subtracts the ideal time data from the measured time data for a slot (includes only the time data used for symbol FFT Fast Fourier Transform: A mathematical operation performed on a time-domain signal to yield the individual spectral components that constitute the signal. See Spectrum.)
2. Averages the difference to calculate an IQ offset value
3. Adjusts the measured time data for the slot by the IQ offset value

Compensating for IQ offset can have an adverse effect on EVM when some subcarriers points are misdetected. When subcarrier points are detected incorrectly, the DC level calculation will be incorrect and all subcarriers will be affected by the IQ offset correction which adds 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. power to compensate the signal. The power from the DC subcarrier is not orthogonal to the other subcarriers and will cause EVM to increase.

See Also

Half Subcarrier Shift

Format Tab

LTE Demod Properties Dialog Box