The passive mixer-first architecture has been known as the most promising solution for wideband, fully integrated software-defined radios. In this dissertation, a comprehensive model of N-phase passive mixer receivers is firstly introduced to analyze and design for optimized performance in this new type of receivers. After that, in order to suppression the out-of-band transmission problem inherent to passive mixer-first radios, a baseband technique based on DAC injection is presented. In order to meet the rising demand wireless spectrum, duplexing transicevers which is capable of receiving and transmitting simultaneously in the same or closely spaced frequency bands are required. Based on the passive mixer-first architecture, two distinct duplexing transceiver design are designed, which target different use scenarios.