The key results of this measurement are Mean RMS Composite EVM, Max Peak Composite EVM, Mean Frequency Error, Mean IQ Origin Offset, Mean IQ Gain Imbalance, Mean IQ Quadrature Error, and In-Band Emission Margin. For more information on modulation accuracy measurements in LTE refer to the Transmit Signal Quality Measurements section of the Introduction to LTE Device Testing application note[1].

Auto Resource Block Detection Enabled

ModAcc measurement supports auto detection of carrier resource configurations. You can configure auto-detection by using the Auto RB Detection Enabled property, which defaults to True, where the modulation type, number of PUSCH clusters, PUSCH resource block offsets and PUSCH resource block sizes are automatically detected by the measurement.

Synchronization Mode

The frame and slot synchronization modes enable you to synchronize to the frame or slot boundaries. For more information on the frame structure of an LTE frame refer to LTE Frame Structure application note[1]. The ModAcc measurement is made over the Measurement Length (slots) property, starting at the Measurement Offset (slots) property from that boundary. The frame synchronization mode allows you to measure specific slots in a frame, while the slot mode starts the measurement at the first complete slot in the acquisition.

The marker synchronization mode is a unique mode that enables you to obtain the fastest modulation accuracy measurements. In the marker mode, you must provide a digital trigger at the start of the frame and use this trigger in ModAcc. The measurement internally uses the fact that the acquisition is triggered at the start of the frame and optimizes some processing to offer a faster measurement time.

The following images illustrate the slots that are measured in various synchronization modes, measurement offsets, and trigger configurations. The measurement length is 2 slots in all the examples.













ModAcc FFT Window Type, Length and Offset

An OFDM symbol consists of FFT length worth data samples (N) and cyclic prefix samples (CP). An FFT is taken only on N samples and hence the position in time for FFT can start at any instant within the cyclic prefix. The EVM requirements are met within a window, which has a length lesser than the CP length. The FFT start positions are relative to the center of the CP such that adjacent FFT start positions or FFT Window Length apart from each other.

Refer to annex E of 3GPP 36.521 specification for more details.

The samples used for FFT are determined by the FFT window length or the FFT window offset.

The following image illustrates the FFT window types and the flow of action.



If you set ModAcc FFT Window Type property to 3GPP, EVM calculation uses two reduced windows. You can specify the window's starting positions by setting the FFT Window Length property. The windows are determined as (ΔC – W/2) and (ΔC + W/2), where ΔC is the center of CP and the center of FFT Window, and W is the window length. For each OFDM symbol, the EVM is calculated for these reduced windows each having a length of the FFT size (N). These two timings leads to two different EVMs: EVMlow and EVMhigh. The averaging is done separately and the maximum of EVMlow and EVMhigh is returned as the final EVM.

All results related to EVM, phase error, magnitude error, and all other corresponding traces are affected by this selection and are reported with respect to the window resulting in maximum averaged EVM.

If you set FFT Window Length property to -1, RFmx automatically selects the FFT window length based on the Channel Bandwidth as given in below table. Refer to annex E.5.1 of the 3GPP 36.104 specification for more details.

Channel Bandwidth (MHz) FFT Size Normal CP Extended CP
EVM Window Length (W) W/(CP length) for Symbols 1-6 (%)* EVM Window Length (W) W/(CP length) for Symbols 1-6 (%)
1.4 128 5 55.6 28 87.5
3 256 12 66.7 58 90.6
5 512 32 88.9 124 96.9
10 1,024 66 91.7 250 97.7
15 1,536 102 94.4 378 98.4
20 2,048 136 94.4 504 98.4

* For normal CP, symbol 0 has a lower percentage value.

If you set the ModAcc FFT Window Type property to Custom, only one FFT window offset is used for EVM calculation. The start of the FFT window is relative to the start of OFDM symbol excluding the CP. You can specify the window offset by setting the FFT Window Offset property.

The following image illustrates the FFT Window Type, the FFT Window Length, and the FFT Window Offset compared with the window positions and offset.



References

[1] NI RF Academy. (2014, February 01). Introduction to LTE Device Testing. Retrieved March 10, 2015, from http://download.ni.com/evaluation/rf/Introduction_to_LTE_Device_Testing.pdf