Symbol Time Adjustment (NB-IoT)

Default: Max of 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. Window Start / End

Range: Max of EVM Window Start / End
Min of EVM Window Start / End
EVM Window Start
EVM Window End
EVM Window Center
% of 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. Size:-3.125 to 0%

Symbol Time Adjustment determines where the FFT used for EVM and demodulation results is located within the symbol + cyclic prefix time data.

This parameter does not affect the calculation of the MIMO Info Table data results RSEVM and RSCTE, and any of the traces which show equalization information (Eq Chan Freq Resp, Eq Chan Freq Resp Diff, Eq Impulse Response, MIMO Eq Chan Freq Resp, MIMO Eq Chan Freq Resp Diff, MIMO Eq Impulse Response). All of these calculations are made using symbol FFTs taken from the center of the cyclic prefix.

Max or Min of EVM Window Start / End

When Max of EVM Window Start / End or Min of EVM Window Start / End is selected, the EVM for each subcarrier comes from the data set determined in the following manner:

For each 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. symbol, two FFTs are taken to determine the values of the subcarriers. The first FFT is taken starting at the beginning of the EVM Window. The second FFT is taken starting at the end of the EVM Window.
Two sets of EVMs are calculated for the subcarriers, one from each FFT.
Then an RMS average is taken over each set.
For Min of EVM Window Start / End, the set with the lower RMS average EVM is chosen as the set to use in EVM and demodulation results.

For Max of EVM Window Start / End, the set with the higher RMS average EVM is chosen as the set to use in EVM and demodulation results.

EVM Window Start, End, or Center

When one of these options is selected, the symbol FFT used for EVM calculations and demodulation results is taken starting from the Start, End, or Center of the EVM Window, depending on the option selected.

% of FFT Size

When % of FFT Size is selected, the symbol FFT used for EVM and demodulation results begins at the specified location. The minimum value of -3.125% begins the FFT at the beginning of the cyclic prefix. The maximum value of 0% begins the FFT at the end of the CP 1) Contention period, or 2) Cyclic prefix (beginning of the symbol). Setting the value to 0% will provide the maximum amount of time for all the paths in a multipath environment to arrive at the receiver before the symbol FFT is taken.

The % of FFT Size parameter is specified in percentage of the symbol length.

Concepts

A cyclic prefix (CP) is added to the beginning of each OFDM symbol. If there were no multipath and filters were perfect, the FFT could be done anywhere inside the time interval (CP+Symbol) and still get the same FFT data because the CP is simply an extension of the symbol in time. With imperfect filters, the CP allows the symbol FFT to avoid the edge effects at either end of the symbol. When there is multipath, as long as all the paths arrive at the receiver before the beginning of the symbol FFT, there would still be one full period of each path in the FFT (meaning only additions of phase delayed versions of the same frequency), and the channel equalization would be able to resolve the phase and amplitude characteristics of the channel.

When the CP is shorter than the longest delay path, some power from the previous OFDM symbol and the transition to the current symbol would be included in the symbol FFT, breaking subcarrier orthogonality, which results in higher EVM.