Equalizer Training (LTE-Advanced)

Default: RS
Moving Avg. Filter selected: 19 RS
Equalization method (uplink): ZF

Range: Off, RS, RS+Data
Moving Avg. Filter: 1 to 399 RS
Equalization method (uplink): ZF or LS

Equalizer Training tells the demodulator whether or not to equalize the signal (compensate for the measured channel frequency response).

Channel equalization only applies to phase and amplitude. For information about frame-level timing correction, see Sync Type.

Small-scale deviations (slot-by-slot or symbol-by-symbol) from the equalization channel frequency response are compensated by EVM Minimization.

Uplink

Channel frequency responses are computed and equalization is applied on a slot-by-slot basis. These per-slot channel frequency responses are shown in the Per-slot Eq Ch Freq Resp trace. The Eq Chan Freq Resp trace however shows a single set of channel frequency response coefficients computed from the time data in the Search Time trace (capture length determined by Result Length).

When RS+Data is selected for uplink, the LTE Long Term Evolution-Advanced demodulator calculates the equalizer channel frequency response according to the standard using the DM-RS DeModulation Reference Signal (LTE) subcarriers and the DFT discrete Fourier transform-spread (SC-FDMA Single Carrier - Frequency Division Multiple Access) subcarriers (PUSCH Physical Uplink Shared Channel). The LTE-Advanced standard specifies that an RS+Data equalization should be performed for uplink signals.

When RS is selected, the signal will be equalized using the channel frequency response calculated using the DM directed mesh: The realizations of a physical mesh using substantially directional antennas. See also: mesh-RS subcarriers in the signal.

When Off is selected, the channel frequency response will still be calculated from the DM-RS subcarriers but will not be applied to the signal.

PRACH Physical Random Access Channel equalization is done differently from the other uplink channels' equalization. First, the channel frequency response is calculated for a PRACH transmission by comparing the received preamble sequence to the reference preamble sequence. Then, the channel frequency response is averaged to a single correction value and this correction is applied to all subcarriers in the PRACH preamble. Each PRACH transmission is equalized separately from the other PRACH transmissions.

PRACH equalization is done this way because if each PRACH subcarrier were corrected individually, the equalization would simply remove the error from the PRACH transmission (resulting in near zero 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.) since the channel frequency response would be calculated from the same subcarriers that were being equalized.

Equalizer Mode

The Equalizer Training section of the Advanced tab contains two selections for the equalizer to use. Select ZF to use the zero-forcing equalizer or select LS to use the least squares equalizer.

Downlink

The channel frequency response is computed over the entire Result Length, and the resulting coefficients are shown in the Eq Chan Freq Resp trace.

When Off is selected, no equalization will be applied to the signal.

When RS is selected, equalization will be performed using the frequency response calculated from the cell-specific reference signal for the reference C-RS/Rx path. The channel frequency response for subcarriers between reference signals will be linearly interpolated.

For downlink, the standard only specifies using the reference signal for equalization. However, the LTE-Advanced demodulator can apply a RS+Data equalization for single-channel downlink signals.

When RS+Data is selected for downlink, equalization will be performed using the frequency response calculated using the cell-specific reference signal and the data subcarriers. RS+Data equalization is not supported for multi-antenna downlink signals (when number of measurement channels is greater than 1).

When including data (PDSCH Physical Downlink Shared Channel) subcarriers in equalizer calculations:

The demodulator equalizes the signal using the cell-specific reference signal and demodulates the data subcarrier values.
Using the demodulated signal, the demodulator calculates a reference LTE component carrier signal (shown in IQ Ref)
Then the demodulator calculates another equalizer channel frequency response by comparing all the measured PDSCH and C-RS Cell-specific RS subcarrier values with the corresponding reference subcarrier values
Finally, the channel frequency response including PDSCH is applied to the signal, the signal is demodulated, and the results of the demodulation are shown on the traces

A moving average can be applied to the C-RS subcarriers in frequency. For more information, see Moving Avg. Filter.

To see the measured channel frequency response for the reference C-RS/Rx path, use the Eq Chan Freq Resp trace.

To see the measured channel frequency responses for all C-RS/Rx paths, use the MIMO Eq Chan Freq Resp trace.

The Equalizer Training setting determines what subcarriers are used when the Tracking method of EVM Minimization is selected. See the EVM Minimization topic for more information.