Wide-band systems seem set to become the next significant generation in wireless communications. They include both wireless local area networks and cellular systems with a large coverage area and will provide a higher data rate and access to Internet and video services, for example. Although most of the data processing is performed digitally, the requirements and possibilities to implement the analogue part of the radio receiver will be different compared to the second-generation narrow-band receivers. Direct conversion architecture is a distinct candidate for wide-band systems because some non-idealities involved in baseband signal processing are significantly relaxed. This work analyzes the requirements and feasibility of direct conversion in wide-band systems. The main emphasis is on cellular systems based on direct sequence code division multiple access, but the same principles are generally valid in all receivers for different applications. The basic principles and design methods involved in receiver design are overviewed as well as the most common radio architectures. Also, in a detailed analysis, the fundamental limitations of the direct conversion architecture are analyzed in wide-band signal processing.