Analyzing LINC Systems

Abstract

In this article, an analysis of LINC PA efficiency and linearity is observed. The use of two highly efficient PAs operated with constant envelope signals in the LINC transmitter does not in and of itself guarantee that the overall efficiency of the LINC amplifier will be high. It's shown that the use of matched combiner gives good linearity but the efficiency is decreased when the peak-to-average of signal increases. The same isolation between the two amplifiers is what gives the high linearity at the output. To avoid the efficiency losses in the matched combiner, the lossless Chireix-outphasing combiner has been presented. The nonisolating combiner presents time-varying impedances to the output of RF amplifier of each branch as the phase difference between the branches increases, thus significantly improving the LINC average efficiency. Impedance mismatching between the amplifiers and outphasing combiner have been explicitly taken into account. Closed-form expressions for the voltages at the output of the combiner, the instantaneous impedance seen by each amplifier, as well as the instantaneous efficiencyof the combiner, have been developed and presented. It has been shown that the new voltage expressions are consistent with the nonlinearity behavior of the Chireix combiner/thus demonstrating that the combining structure is a source of nonlinearity. To validate this approach many types of signals are considered as CDMA WCDMA and OFDMA (QPSKA, 16-QAM, 64-QAM) signals. Other sources of distortion such as imbalances between two branches are presented and how to compensate this imbalance is proposed. In light of the results obtained, the use of the Chireixoutphasing combiner with saturated nonlinear amplifiers to reach the objective of both high-efficiency and high-linearity amplification does not seem possible. The potential use of phase predistortion to reach this objective remains to be investigated.