System and circuits investigation of wideband RF polar transmitters using envelope tracking for mobile WiMAX/WiBro applications

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2009-08

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High efficiency radio frequency (RF) power amplifiers (PAs) are critical in portable battery-operated wireless communication systems because they can dominate the power consumption. Linear power amplifier is required for applications using non-constant envelope modulated signals (e.g., WiMAX) to minimize distortions. Traditionally, linear power amplifiers are implemented by “backing-off†the output power of PA. However, for a spectrally-efficient high peak-to-average (PAR) signal, the average PA efficiency is much lower than the peak efficiency, which is very undesirable. This thesis focuses on Envelope Elimination and Restoration (EER) and Envelope Tracking (ET) efficiency enhancement techniques to improve the PA average efficiency for high PAR signals. A modified bias-dependent Cann’s model has been developed to study the behavior of our monolithic class-E SiGe PAs. The time-mismatch sensitivity between the amplitude and RF paths is compared for ET vs. EER systems based on our modified model and with measurement data. Besides, RF+Analog+Digital system simulations were performed to study the static and group delays in ET/EER based polar TX systems. Both simulations and measurement show that an ET-based polar system has higher resilience against timing mismatches between the amplitude and the RF/phase paths. Our laboratory measurement also validates the bias dependence of our modified Cann’s model. A high efficiency wideband envelope amplifier is also designed for EER/ET based polar transmitters for WiBro/WiMAX applications. Several simulation and measurement results were discussed in terms of optimized efficiency and system linearity. Finally, detailed system co-design simulations including RF circuits and digital DSP blocks show that our class-E PA can be linearized by the open-loop ET technique, and the entire ET-based polar TX system meets the stringent 802.16e TX mask with ~33% overall average efficiency at output power of 18.5dBm.

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