Impact of dispersion caused by bandwidth limitation on the linearity of multilevel LINC transmitters

This paper studies the impact of bandwidth limitation on the linearity of multilevel LINC (ML-LINC) transmitter. By using the signals with discrete levels for characterisation, both amplitude and phase dispersions around levels can be observed, which result in the degradation of system linearity. It is also shown that improvement on both adjacent channel leakage ratio (ACLR) and error vector magnitude (EVM) can be obtained by either increasing the number of levels or system bandwidth in each amplification path. A measurement setup including two highly efficient power amplifiers (PAs) in GaN technology, which delivers 41 dBm output power with drain efficiency of 69% at 2.05 GHz, is built to verify the analysis. With the configuration of 16 levels and 80 MHz system bandwidth, the built ML-LINC transmitter gives 36 dBm average output power with 54% drain efficiency after calibration for the standard 5 MHz LTE downlink signal with a PAPR of 7.5 dB. The measured ACLR is -50.5 dBc and EVM is 1.6% for QPSK and 3.1% for 64-QAM with enough margin left according to the standard.