Digital Cancellation of the Remodulation Effect in IQ RFDAC Based LTE Direct Conversion Transmitters

Modern wireless transmitters supporting Long Term Evolution (LTE) face several issues when employing a radio-frequency-digital-to-analog-converter (RFDAC). One of these issues is the pulling effect between the RFDAC and the digitally controlled oscillator (DCO). This so-called remodulation occurs due to the unavoidable electromagnetic coupling between them. This results in spurious spectral emissions at the transmitter output and causes a severe degradation of the error vector magnitude (EVM). This paper introduces and studies the remodulation problem with a detailed modeling of the LTE transmitter, including an RFDAC and a phase locked loop. Also, the impact of the remodulation effect is analyzed with simulations for various LTE uplink scenarios. To mitigate the unwanted pulling effect, we propose a low-complex digital cancellation architecture that utilizes the inphase and quadrature samples of the transmit signal. Simulations show that the proposed architecture cancels the remodulation and improves the EVM of the transmitter significantly.