Signal Synchronization in Digital Transmission Systems

B OTH wireless and wired communications are experiencing an unprecedented worldwide growth due to the ever-increasing demand of new services and to the recent deregulation of the field in many developed countries. This host of new applications relies on a number of sophisticated signaling formats: direct sequence spread-spectrum (DS/SS) signals to provide code division multiple access (CDMA), discrete multitone (DMT) modulation, and/or orthogonal frequency division multiplexing (OFDM) to combat selective fading, and turbo coding to yield a better quality of the link, just to mention a few samples of this trend. In order to function properly, a digital communication receiver must synchronize itself to the incoming signal. As is well known, there are various facets to the synchronization process. With bandpass modulation, the receiver must achieve carrier synchronization, meaning that its oscillator must acquire the same frequency and (for coherent detection) the same phase as the carrier signal at its input. Also, information is needed about the arrival times of the digital data symbols (symbol timing), so that the received waveform can be sampled at the right times with a minimum intersymbol interference. Finally, in DS/SS communications, it is necessary to achieve code synchronization , i.e., to align the locally generated code sequence with that of the desired user. Synchronization algorithms play a vital function in digital transmissions as their accuracy affects the system performance critically. Also, as they comprise a significant portion of the receiver's hardware and software, they have a considerable weight in the overall receiver cost and design effort. For these reasons it is not surprising that a vast literature has appeared in the recent past dealing with a host of applications, ranging from point-to-point transmissions (wired or wireless), audio and video digital broadcasting, to (radio) cellular systems. To overview the synchronization problems addressed in this Special Issue we consider first narrow-band point-to-point transmissions over the additive white Gaussian noise (AWGN) channel, which are typical of satellite links. By narrow-band we mean any single-carrier modulations with an RF bandwidth comparable to the information bit-rate of the link. They encompass conventional PSK and QAM and nonlinear schemes like CPM. In all these cases, synchronization amounts to estimating three scalar parameters: carrier frequency, carrier phase (coherent detection only), and symbol timing. At least conceptually, maximum likelihood (ML) estimation methods, or approximations thereof, offer a systematic guide to the derivation of good synchronizers. In this context the paper by Vanelli-Coralli …