EVM Minimization (LTE)

Default: 3GPP, all types of corrections selected

Range: Off, 3GPP, Tracking
Types: Amplitude, Timing, Frequency/Phase, IQ Offset

EVM Minimization uses the reference signal (C-RS Cell-specific RS for downlink or DM-RS DeModulation Reference Signal (LTE) for uplink) to correct the signal. There are four types of corrections that can be applied to the signal to minimize 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.. They are Amplitude, Frequency/Phase, Timing, and IQ Offset.

There are four EVM minimization choices:

Off – EVM minimization corrections are not applied to the signal.
3GPP – the demodulator calculates timing, frequency/phase and IQ offset corrections using the reference signal and the data subcarriers as defined in Section F.3.1 of 36.141 for DL Down Link (forward link: from base station to cell phone) and Section E.3.1 of 36.521 for UL Up Link (reverse link: from cell phone to base station). For downlink, the data subcarriers are from PDSCH Physical Downlink Shared Channel, and for uplink the data subcarriers are from PUSCH Physical Uplink Shared Channel and PUCCH Physical Uplink Control Channel.

The demodulator applies the corrections on a slot-by-slot basis for uplink, or on a subframe-by-subframe basis for downlink, as defined by the LTE Long Term Evolution standard.

3GPP EVM Minimization is not supported for multi-antenna downlink signals (when number of measurement channels is greater than 1).

Tracking – the demodulator applies corrections on a symbol-by-symbol basis and the Equalizer Training parameter determines whether data subcarriers are included in calculating corrections. When Equalizer Training is set to RS+Data, EVM Minimization Tracking is performed using the reference signal and the PDSCH data subcarriers. When Equalizer Training is set to RS or Off, EVM Minimization Tracking is performed using only the reference signal.
Low SNR – (Uplink only) EVM Minimization is based on 3GPP non-compliant pilot tracking procedure, using synchronization algorithms optimized for low SNR Signal-to-Noise Ratio (Signal to Noise Ratio) test scenarios.

The use of this feature is recommended for very low SNR receiver sensitivity BER Bit Error Ratio - A ratio of the number of errors to data bits received on a digital circuit. (Bit Error Rate) or BLER Block Error Rate - A ratio of the number of erroneous blocks to the total number of blocks received on a digital circuit. Block error rate (BLER) is used for W-CDMA performance requirements tests (demodulation tests in multipath conditions, etc). BLER is measured after channel de-interleaving and decoding by evaluating the Cyclic Redundancy Check (CRC) on each transport block. (Block Error Rate) test scenarios where the signal being measured can be very noisy (e.g. where SNR levels can be around 0dB).

To achieve the optimal BER or BLER measurement performance for these very low SNR receiver test scenarios, consider selecting the following Advanced GUI tab parameter choices:
- Select EVM Minimization as Low SNR. Recommended in order to apply synchronization algorithms optimized for very low SNR test scenarios.
- Select Equalizer Mode as ZF (supported for Uplink test scenarios). Recommended for very low SNR test scenarios to achieve better equalization results.
- Consider selecting Equalizer Training as RS+Data for lower order Data modulation format scenarios (e.g. QPSK Quadrature phase shift keying), otherwise select RS for higher order Data modulation format scenarios.
- Enable Moving Avg. Filter. Also note increasing the length (number) of RS pilots may potentially achieve better equalization results.
- Also consider disabling EVM Minimization Tracking options Amplitude, Timing, Frequency and IQ Offset, which may potentially achieve better results.
  
  While Low SNR Evm Minimization is selected, the user-selected Symbol Time Adjustment mode will be ignored, and instead the reported EVM results will be measured using EVM Window Center.

Reference signal subcarriers are transmitted periodically in time and frequency. The demodulator compares the reference signals with the expected data sequence and computes an error, or correction value, that can be used to track phase, amplitude, and timing at the symbol level when Tracking is selected, and at the slot or subframe level when 3GPP is selected. For subcarriers that do not have a corresponding reference subcarrier to compare to, the correction value is calculated by linearly interpolating between RS (and data subcarriers, when Equalizer Training is set to RS+Data) subcarrier corrections .

When corrections are averaged and applied to a slot or subframe, the same correction is applied to each symbol in the slot or subframe.

These are the four types of EVM Minimization corrections available. Click on the name of a correction type for more information.

Amplitude - When selected, the average reference signal amplitude error will be used to correct the amplitudes of the subcarriers.
Frequency/Phase - When selected, the average reference signal phase difference will be used to adjust subcarrier phase.
Timing - When selected, the average slope (average rate of change) of the reference signal's phase in the frequency domain is used to correct the timing.
IQ Offset (uplink, 3GPP EVM Minimization only) - When selected, any IQ offset present is corrected on a slot-by-slot basis. This type of EVM minimization is only available EVM Minimization is set to 3GPP and the direction is uplink.

For uplink, both equalization and 3GPP EVM Minimization occur on a slot-by-slot basis, while for downlink, equalization occurs over the entire Measurement Interval and 3GPP EVM Minimization occurs on a subframe-by-subframe basis.