Equalizer Training (NB-IoT)
RS
Moving Avg. Filter selected: 3 RS
Off, RS, RS+Data
Moving Avg. Filter: 1 to 3 RS
Equalization Mode (uplink): ZF or LS
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. 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 is selected for uplink, the NB-IoT 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 Frequency Division Multiple Access: Method of allowing multiple users to share the radio frequency spectrum by assigning each active user an individual frequency channel. In this practice, users are dynamically allocated a group of frequencies so that the apparent availability is greater than the number of channels.) subcarriers (NPUSCH). The NB-IoT standard specifies that an equalization should be performed for uplink signals.
When 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 is selected, the channel frequency response will still be calculated from the DM-RS subcarriers but will not be applied to the signal.
NPRACH equalization is done differently from NPUSCH equalization. First, the channel frequency response is calculated for a NPRACH transmission by comparing the received preamble sequence to the reference preamble sequence symbol group by symbol group (Preamble has 5 symbol groups and each symbol group has a single subcarrier in frequency domain). Then, the channel frequency response is averaged to a single correction value and this correction is applied to the subcarrier in the NPRACH symbol group. Each NPRACH transmission is equalized separately from the other NPRACH transmissions.
NPRACH equalization is done this way because if each NPRACH subcarrier were corrected individually, the equalization would simply remove the error from the NPRACH 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 section of the NB-IoT Demod Properties Advanced tab contains two selections for the equalizer to use. Select to use the zero-forcing equalizer or select 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 is selected, no equalization will be applied to the signal.
When is selected, equalization will be performed using the frequency response calculated from the NRS and NPSS. 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 NB-IoT demodulator can apply a equalization for single-channel downlink signals.
When is selected for downlink, equalization will be performed using the frequency response calculated using the narrowband reference signal and the data subcarriers.
When including data (NPDSCH) subcarriers in equalizer calculations:
- The demodulator equalizes the signal using the narrowband reference signal and demodulates the data subcarrier values.
- Using the demodulated signal, the demodulator calculates a reference NB-IoT signal (shown in IQ Ref)
- Then the demodulator calculates another equalizer channel frequency response by comparing all the measured NPDSCH and NRS subcarrier values with the corresponding reference subcarrier values
- Finally, the channel frequency response including NPDSCH 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 RS subcarriers in frequency. For more information, see Moving Avg. Filter.
See Also
