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This paper provides an overview of the advanced S-UMTS testbed (ATB; S-UMTS = satellite universal mobile telecommunication system) project that is funded by the European Space Agency, which was instrumental to define, validate, and demonstrate the adaptations of third-generation (3G) mobile technologies based on wideband code-division multiple access (W-CDMA) for supporting via-satellite services. Such services are often generically referred to as S-UMTS. One of the main project targets was to develop and demonstrate, in the laboratory and over the air, a set of technical solutions for the efficient provision of point-to-point (PTP, i.e., interactive) and point-to-multipoint (PTM, i.e., multicasting/broadcasting) services to best exploit the precious satellite communications resources. With respect to previous work, which was mostly devoted to the connection-oriented mode (i.e., circuit basis), particular emphasis was dedicated to optimizing solutions for the connectionless modes (i.e., packet basis and reliable multicasting/broadcasting, both real time and nonreal time). As a matter of fact, the wide-area delivery of multimedia services to mobile users is expected to represent the most important commercial opportunity for S-UMTS systems, as witnessed by recent initiatives taking place in the United States, Korea, and Japan. After dwelling on the aims of the ATB project, this paper presents the key required adaptations and extensions, for satellite applications, to the W-CDMA scheme as standardized by the Third Generation Partnership Project for terrestrial applications. This paper then continues by describing the architecture of the real-time end-to-end testbed (taking the name ATB after the project name), which was developed with the main objective of supporting laboratory and over-the-air trials, and its main constituting elements. In a companion paper, the architecture of the overall via-satellite demonstrator, which is largely based on the ATB, is addressed, to- - gether with the key results of the laboratory and over-the-air trials.