THE EFFECT OF RADIO FREQUENCY INTERFACE IMPERFECTIONS ON THE ERROR VECTOR MAGNITUDE (EVM) IN LTE AND 5G SYSTEMS
Authors: Pyatin Ilya , Boiko Juliy 
Published: December 2024

The development of telecommunications proceeds by increasing the speed of information transmission, improving the quality of communication, and providing new services. Modern mobile phones have a transmitter and a receiver located in the same housing. Double frequency conversion is used to transfer the spectrum of signals from the main frequency band to the carrier frequency. The radio frequency interface (RF Front End) plays an important role in ensuring high-quality communication and data transmission. It is located between the antenna switch and the mixer and ensures matching with the antenna and converting the signals to a form suitable for further processing or radiation into free space. The quality of the connection can be estimated by the value of the error vector (EVM). EVM is a measure of the difference between the ideal transmitter symbols and the measured receiver symbols after equalization. Reducing EVM allows to improve transmission speed, reduce packet loss. The work is devoted to the study of the influence of the imperfection of the radio frequency interface on the level of EVM in modern telecommunications. The EVM level is affected by in-phase and quadrature imbalance, phase noise, power amplifier nonlinearities, as well as the presence of noise in the communication channel and the immunity of the signal-code design. An I/Q imbalance results from a gain mismatch or phase shift between parallel sections of signal conversions in the transmitter.A variable gain amplifier (VGA) changes the signal level at the input of a power amplifier (HPA), reducing or increasing its power. Thus, the HPA works in saturation mode or with a certain reserve (back-off). The larger the IBO (input back-off), the lower the input power of the signal to the HPA and the less distortion, but the efficiency of the amplifier is reduced. The lower the VGA gain, the greater the IBO margin. As the VGA gain increases, the input power to the HPA increases, which reduces the IBO and leads to an increased risk of nonlinear distortion due to saturation of the HPA. The phase noise of the local oscillator expands the spectral power density of the signal, leads to a deviation of the phase of each subcarrier, which violates their orthogonality. The Simulink models of the transmitter and receiver of the digital communication system with additionally introduced impairments of the radio frequency units were studied, the EVM levels, the power in the channel were estimated. An example of a 4G and 5G radio frequency interface is the QM78207 chip, which includes a high-frequency amplifier, filters, a switch and other components. With the provision of new services, the radio frequency interface becomes more complicated, which imposes higher requirements on the quality of its components. The maximum EVM for a 64-QAM constellation is 8% (-22 dB). When the amplitude imbalance IQ changes from 0 to 3 dB, the value of EVM increases from -32 dB to -15 dB for SNR=50 dB. When changing the phase imbalance IQ from 0 to 15°, the EVM value increases from -32 dB to -17 dB for SNR=50 dB.