EVM Minimization: IQ Offset (LTE)

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 Minimization's IQ Offset correction replaces the IQ Offset Compensate parameter in earlier versions of the LTE 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 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)
  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 power to compensate the signal. The power from the DC subcarrier is not orthogonal to the other subcarriers and will cause EVM to increase.

The IQ offset of the signal is shown in the Error Summary trace in the IQ Offset data result.

See Also

Half Subcarrier Shift

Format Tab

LTE Demod Properties Dialog Box