Contents I. Introduction
Modern wireless communication systems deliver high data rate signals to provide multimedia data and broadband connectivity in the swift moving environment. The mobile world interoperability for microwave access (WiMAX) handles up-to 15 Mbps of data rate and 28 MHz of signal bandwidth with wireless coverage of four to six miles [1]. These signals have rapidly varying nonconstant envelopes to efficiently use the limited frequency resource and the peak-to-average power ratios (PAPRs) of the signals are very high. The power amplifiers for the systems should cover the wide bandwidth and have a high linearity to amplify the signals without distortions. Because of the large PAPRs, the power amplifiers have to operate at a large backed-off power, and the efficiency is rather poor. To enhance the efficiency in the large backed-off region, the most promising solution is the Doherty amplifier due to the simple circuit topology. Moreover, the Doherty amplifier improves the linearity through the third-order intermodulation cancellation (IM3) mechanism between the carrier and peaking amplifiers [2], [3]. From our early work in [4], we have seen that the IM3 cancellation of the Doherty amplifier can improve not only the out-of-band error such as adjacent channel leakage ratio, but also the in-band error such as relative constellation error (RCE).
