In a wireless communication system, the RF signal integrity is often deteriorated by power amplifier (PA) nonlinearity and local oscillator (LO) pulling. This dissertation attempts to study power amplifier and local oscillator with the deliberate input distortion or interference for understanding, and hence improving, the resultant RF signal integrity issues. Furthermore, the scope of this study is extended to explore novel wireless applications. Based on the above thoughts, this dissertation includes three topics. The first topic is devoted to a baseband digital predistortion technique for enhancing the power amplifier linearity in a wireless RF transmitter. A digital predistorter has been designed to compensate the amplitude and phase distortion due to the nature of PAs, and the predistortion can enhance the linearity of linear PAs as well as switching-mode PAs. The second topic proceeds with a rigorous analysis of a local oscillator subject to injection signal. A phase-locked loop (PLL) under injection is analyzed in frequency domain to account for the inherent band-pass filtering on an injection signal. Such analysis can further predict the effect of co-frequency or co-channel interference on the PLL phase noise. A discrete-time analysis is also provided to predict output spectra of the LO pulled by a sinusoidal and modulated injection signal. The final topic presents a novel RF sensing circuit for a cognitive radio to sense spectral environment using injection locking and frequency demodulation techniques. The proposed RF sensing circuit can fast and reliably detect frequency and power for analog and digital modulation signals. In addition, the sensing principle and circuit architecture are delivered on theoretical basis developed in this dissertation. A discrete time approach is also investigated to compute the sensed output signal.