EVM Measurements-EVM vs subcarrier measurement,EVM vs symbol measurement

This page compares EVM vs subcarrier and EVM vs symbol and mentions difference between EVM vs subcarrier measurement and EVM vs symbol measurement with respect to WLAN/WiMAX system as per OFDM modulation.

As we know EVM is the short form of Error Vector Magnitude. It is also referred as RCE (Relative Constellation Error). It is obtained by computing vector from mapping position obtained for received symbols to ideal mapping position on the constellation diagram (IQ diagram). It displays rms value of EVM. This is shown in the figure-1.

EVM measurement

This EVM measurement is useful for performance analysis of modem under various impairments and imperfections e.g. Local Oscillator Stability, filter compression, DAC/ADC, symbol rates and interfering signals etc.

Moreover high PAPR requires highly linear PA (Power Amplifier). Due to this facts, it is advisable to have EVM measurement so that linearity and efficiency can be verified.

EVM formula

The figure-2 mentions EVM formula. In WLAN system, one OFDM symbol consists of total 64 subcarriers. This 64 subcarriers made of 48 data subcarriers, 4 pilot subcarriers, 1 DC, 6 left guard and 5 right guard subcarriers. Suppose there are 20 such symbols in a frame of duration 10ms with bandwidth of 20MHz.

Error Vector Spectrum-EVM vs subcarrier measurement

EVM vs subcarrier

The figure-3 depicts EVM vs subcarrier measurement. As shown X-axis is subcarriers and Y-axis is EVM. Here 52 total used subcarriers are plotted on X -axis and EVM for these respective 52 points are plotted on Y-axis. This type of plot is also known as Error Vector Spectrum.

Error Vector Time-EVM vs symbol measurement

EVM vs symbol

The figure-4 depicts EVM vs symbol measurement. As shown X-axis is symbols and Y-axis is EVM. Here 20 symbols are shown on X-axis and EVM for all the symbols i.e. 52 points per symbol which gives total 1040 points on the plot. Figure shows EVM measurement for all the subcarriers of symbol number-5. Above mentioned EVM formula will be used after individually determining EVM values of all 52 subcarriers in order to calculate aggregate EVM (rms) value for one OFDM symbol. This type of plot is also known as Error Vector Time.

Links to useful Measurements

Following are the links to other useful measurements available on Test and Measurement World website.
Difference between EVM vs subcarriers and EVM vs symbols
Current measurement vs Voltage measurement
Inductance measurement vs Capacitance measurement
Difference between Power and Energy measurements
EMC measurement vs EMI measurement
Electric and Magnetic Field Measurements

What is difference between or comparison between

Following links mention difference or comparison between various equipments:
comparison between VSG and VSA
DIfference between SNA ans VNA
Oscilloscope Bandwidth versus Rise time
Oscilloscope vs Logic Analyzer
Difference between Oscillscope types
Comparison between spectrum and network analyzer