EVM measurement techniques for MUOS

Physical layer simulations and analysis techniques were used to develop the Error Vector Magnitude (EVM) metric specifying transmitter signal quality. These tools also proved to be very useful in specifying lower level hard-ware unit performance and predicting Mobile User Objective System (MUOS) satellite 8-PSK transmitter performance before the hardware was built. However, the verification of EVM compliance at Ka frequencies posed challenges. Initial measurements showed unacceptably high levels of EVM which exceeded specification. Attempts to remove the contribution of the test equipment distortion and isolate the device-under-test distortion using commercial oscilloscope VSA software were unsuccessful. In this paper, we describe methods used to develop an accurate EVM measurement. The transmitted modulated signal was first recorded using a digitizing scope. In-house system identification, equalization, demodulation and analysis algorithms were then used to remove signal distortion due to the test equipment. Results from EVM measurements on MUOS single-channel hard-ware are given and performance is shown to be consistent with estimates made three years earlier. The results reduce technical risk and verify transmitter design by demonstrating signal quality.