A comprehensive theoretical and experimental investigation is presented on the performances of cost-effective radio over fiber (RoF) links where the RoF Transmitter is directly modulated by signals complying the Universal Mobile Telecommunications Service (UMTS) and Worldwide Interoperability for Microwave Access (WiMAX) standards. A numerical model for calculating intermodulation related quantities like the adjacent channel leakage ratio has been developed for this purpose. Measurements have also been performed over real RoF Systems modulated by UMTS and WiMAX signals. A very good agreement has been found between measured and modelled results, and the feasibility of these connections over distances of tens of kilometers has been investigated.