The concept of digital direct satellite broadcasting (D-DBS), which allows unprecedented flexibility by providing a large number of audio-visual services, is introduced. The concept assumes an information rate of about 40 Mb/s, which is compatible with practically all present-day transponders. After discussion of the general system concept, the optimization procedure is introduced, and results of the transmission system optimization are presented. Channel distortion and uplink/downlink interference effects are taken into account by means of a time domain system computer simulation approach. It is shown, by means of link budget analysis, how a medium-power direct-to-home TV satellite can provide multimedia services to users equipped with small (60-cm) dish antennas.>

Migration towards a full-digital implementation of modems is currently one of the main trends in transmission systems design. The authors describe a noncoherent all-digital delay lock loop (DDLL) suited for chip timing synchronization in band-limited direct sequence spread spectrum (DS/SS) systems, and they thoroughly analyze its performance. The key features of this novel scheme are represented by its low-complexity processing section together with its good tracking capability. Analytical expressions for the DDLL S-curve and steady-state timing jitter are derived and confirmed by a time-domain computer simulation. Furthermore, the Mean Time to Lose Lock (MTLL) of the loop is evaluated and some numerical results are reported. The proposed chip timing synchronization scheme reveals also an improved tracking performance when compared to the traditional analog DLL for rectangular chip DS/SS signals.>

Recent trends in digital communications are opening commercial applications to code division multiple access (CDMA). A novel access technique based on bandlimited quasi-synchronous CDMA (BLQS-CDMA) is described, showing all the advantages of synchronizing conventional direct sequence CDMA to drastically reduce the effect of self-noise. Bandlimitation is achieved with no detection loss by means of Nyquist chip shaping, leading to a simple all-digital demodulator structure. Detection losses due to imperfect carrier frequency and chip timing synchronization are analytically derived and numerical results are checked by computer simulations. Impairments due to satellite transponder distortions are evaluated. The full digital modem structure is presented, together with possible applications to mobile and very small aperture terminal (VSAT) satellite communications.>