Correcting error vector magnitude measurements

1. A method for use with symbols from encoding that is based on an amplitude and a phase of a carrier signal, the method comprising:

generating a mapping function that relates first error vector magnitudes (EVMs) for first symbols to second EVMs for the first symbols, where the first EVMs are corrupted by radio frequency (RF) noise and the second EVMs are corrupted by both RF noise and symbol decoding errors;

obtaining a third EVM for a second symbol, the third EVM being corrupted by both RF noise and a symbol decoding error; and

adjusting the third EVM using the mapping function to correct the symbol decoding error in the third EVM.

2. The method of claim 1 , wherein the mapping function comprises a look-up table (LUT) relating the first EVMs to the second EVMs for a type of encoding; and

wherein adjusting the third EVM comprises selecting the LUT, selecting a second EVM in the LUT based on the third EVM, and replacing the third EVM with the first EVM corresponding to the selected second EVM.

3. The method of claim 2 , wherein the type of encoding comprises quadrature amplitude modulation.

4. The method of claim 1 , wherein the mapping function is based on a simulation configured to perform operations comprising:

determining first values corresponding to the first EVMs based on versions of the first symbols that are uncorrupted by RF noise and versions of the first symbols that are corrupted by the RF noise;

averaging sets of the first values to produce each first EVM;

determining second values corresponding to the second EVMs based on the versions of the first symbols that are corrupted by the RF noise and versions of the first symbols that are corrupted by both the RF noise and the symbol decoding error; and

averaging sets of the second values to produce each second EVM.

5. The method of claim 4 , wherein determining each first value comprises:

adding RF noise to a select first symbol; and

determining a first value based on the select first symbol and the select first symbol with RF noise added, the first value being an EVM for the select first symbol that would occur without a symbol decoding error.

6. The method of claim 5 , wherein adding the RF noise comprises:

selecting random noise from a Gaussian distribution of noise; and

combining the random noise with the select first symbol.

7. The method of claim 5 , wherein determining each second value comprises:

adding a symbol decoding error to the select first symbol with RF noise added; and

determining a second value based on the select first symbol with RF noise added and the select first symbol with both RF noise added and symbol decoding error, the second value being an EVM for the select first symbol that would be occur with the symbol decoding error.

8. The method of claim 7 , wherein the symbol decoding error comprises false error that is based on one or more rules applied by a constellation decoder configured to decode symbols.

9. The method of claim 7 , wherein at least one of the rules requires selection of a shortest error vector between an ideal symbol location on a constellation and the select first symbol with RF noise added.

10. The method of claim 1 , wherein the first symbols are obtained by randomly sampling a data set from a normal distribution.

11. The method of claim 1 , further comprising:

storing the mapping function in a constellation decoder where the third EVM is adjusted.

12. A method of generating a mapping function that relates first error vector magnitudes (EVMs) for first symbols to second EVMs for the first symbols, where the first EVMs are corrupted by radio frequency (RF) noise and the second EVMs are corrupted by both RF noise and symbol decoding errors, the method comprising:

determining first values corresponding to the first EVMs based on versions of the first symbols that are uncorrupted by RF noise and versions of the first symbols that are corrupted by the RF noise;

averaging sets of the first values to produce each first EVM;

determining second values corresponding to the second EVMs based on the versions of the first symbols that are corrupted by the RF noise and versions of the first symbols that are corrupted by both the RF noise and the symbol decoding error;

averaging sets of the second values to produce each second EVM; and

relating the first EVMs and the second EVMs.

13. The method of claim 12 , wherein the first symbols are obtained by randomly sampling a data set from a normal distribution; and

wherein the method comprises determining the versions of the first symbols that are uncorrupted by the RF noise by adding the RF noise to the first symbols.

14. The method of claim 13 , wherein adding the RF noise comprises:

selecting random noise from a Gaussian distribution of noise; and

combining the random noise with each first symbol.

15. One or more non-transitory machine readable media storing instructions that are executable by one or more processing devices to implement a method for use with symbols from encoding that is based on an amplitude and a phase of a carrier signal, the instructions being executable to perform operations comprising:

generating a mapping function that relates first error vector magnitudes (EVMs) for first symbols to second EVMs for the first symbols, where the first EVMs are corrupted by radio frequency (RF) noise and the second EVMs are corrupted by both RF noise and symbol decoding errors;

obtaining a third EVM for a second symbol, the third EVM being corrupted by both RF noise and a symbol decoding error; and

adjusting the third EVM using the mapping function to correct the symbol decoding error in the third EVM.

16. The one or more non-transitory machine readable media of claim 15 , wherein the mapping function comprises a look-up table (LUT) relating the first EVMs to the second EVMs for a type of encoding; and

wherein adjusting the third EVM comprises selecting the LUT, selecting a second EVM in the LUT based on the third EVM, and replacing the third EVM with the first EVM corresponding to the selected second EVM.

17. The one or more non-transitory machine readable media of claim 16 , wherein the type of encoding comprises quadrature amplitude modulation.

18. The one or more non-transitory machine readable media of claim 15 , wherein the mapping function is based on a simulation configured to perform operations comprising:

determining first values corresponding to the first EVMs based on versions of the first symbols that are uncorrupted by RF noise and versions of the first symbols that are corrupted by the RF noise;

averaging sets of the first values to produce each first EVM;

determining second values corresponding to the second EVMs based on the versions of the first symbols that are corrupted by the RF noise and versions of the first symbols that are corrupted by both the RF noise and the symbol decoding error; and

averaging sets of the second values to produce each second EVM.

19. The one or more non-transitory machine readable media of claim 18 , wherein determining each first value comprises:

adding RF noise to a select first symbol; and

determining a first value based on the select first symbol and the select first symbol with RF noise added, the first value being an EVM for the select first symbol that would occur without a symbol decoding error.

20. The one or more non-transitory machine readable media of claim 19 , wherein adding the RF noise comprises:

selecting random noise from a Gaussian distribution of noise; and

combining the random noise with the select first symbol.

21. The one or more non-transitory machine readable media of claim 19 , wherein determining each second value comprises:

adding a symbol decoding error to the select first symbol with RF noise added; and

determining a second value based on the select first symbol with RF noise added and the select first symbol with both RF noise added and symbol decoding error, the second value being an EVM for the select first symbol that would be occur with the symbol decoding error.

22. The one or more non-transitory machine readable media of claim 21 , wherein the symbol decoding error comprises false error that is based on one or more rules applied by a constellation decoder configured to decode symbols.

23. The one or more non-transitory machine readable media of claim 21 , wherein at least one of the rules requires selection of a shortest error vector between an ideal symbol location on a constellation and the select first symbol with RF noise added.

24. The one or more non-transitory machine readable media of claim 15 , wherein the first symbols are obtained by randomly sampling a data set from a normal distribution.

25. The one or more non-transitory machine readable media of claim 15 , wherein the operations comprise:

storing the mapping function in a constellation decoder where the third EVM is adjusted.

26. One or more non-transitory machine readable media storing instructions that are executable by one or more processing devices to implement a method of generating a mapping function that relates first error vector magnitudes (EVMs) for first symbols to second EVMs for the first symbols, where the first EVMs are corrupted by radio frequency (RF) noise and the second EVMs are corrupted by both RF noise and symbol decoding errors, the instructions being executable to perform operations comprising:

determining first values corresponding to the first EVMs based on versions of the first symbols that are uncorrupted by RF noise and versions of the first symbols that are corrupted by the RF noise;

averaging sets of the first values to produce each first EVM;

determining second values corresponding to the second EVMs based on the versions of the first symbols that are corrupted by the RF noise and versions of the first symbols that are corrupted by both the RF noise and the symbol decoding error;

averaging sets of the second values to produce each second EVM; and

relating the first EVMs and the second EVMs.

27. The one or more non-transitory machine readable media of claim 26 , wherein the first symbols are obtained by randomly sampling a data set from a normal distribution; and

wherein the operations comprise determining the versions of the first symbols that are corrupted by the RF noise by adding the RF noise to the first symbols.

28. The one or more non-transitory machine readable media of claim 27 , wherein adding the RF noise comprises:

selecting random noise from a Gaussian distribution of noise; and

combining the random noise with each first symbol.

29. A test system comprising:

memory storing (i) instructions that are executable, and (ii) a mapping function that relates first error vector magnitudes (EVMs) for first symbols to second EVMs for the first symbols, where the first EVMs are corrupted by radio frequency (RF) noise and the second EVMs are corrupted by both RF noise and symbol decoding errors;

a decoder to receive a signal from a device under test, the decoder to obtain a third EVM for a second symbol that is based on the signal, the third EVM being corrupted by both RF noise and a symbol decoding error; and

one or more processing devices to execute the instructions to adjust the third EVM using the mapping function to correct the symbol decoding error in the third EVM.

30. The test system of claim 29 , wherein the third EVM is based on a symbol decoding error added by the decoder, the symbol decoding error being based on at least one predefined rule implemented by the decoder.