In this paper, we report both circuits design and system simulations using highly-efficient monolithic SiGe class-E power amplifier (PA) with an open-loop envelope tracking (ET) technique for mobile WiMAX/Wibro applications. The 1-stage and 2-stage class-E PAs were designed and fabricated in a 0.18μm BiCMOS SiGe technology. The 1-stage class-E PA achieved peak power added efficiency (PAE) of 62% at output power of 21dBm in single-tone measurement. The design of linear-assisted switching envelope amplifier is also discussed, which involves balancing the tradeoff between efficiency and signal fidelity. Detailed co-design system 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 transmit (TX) system meets the stringent 802.16e TX mask with ∼33% overall average efficiency at output power of 18.5dBm.