Contributions to time-frequency synchronization in wireless systems

Time and frequency synchronization is an indispensable task for all wireless transceivers and systems. In modern wireless systems, such as 4G and future 5G systems, new wireless technologies set new challenges also to synchronization. In particular, new solutions for time and frequency synchronization are needed in multiantenna and cooperative systems. New research areas arise also in context of interference cancellation and cognitive radio systems where the transmission parameters of the signal of interest may not be known to the receiver. Similar applications can be found also in the military domain. Obviously, synchronization in such cases poses significant challenges. In this thesis, new methods for time and frequency synchronization in wireless systems are developed and analyzed. Both orthogonal frequency division multiplexing (OFDM) -based as well as direct sequence spread spectrum (DSSS) -based systems are considered. The thesis consists of seven publications and a summary which contains also an extensive literature review on the topic. Synchronization acquisition methods in DSSS-based systems without the knowledge of the spreading sequence are developed. A subspace -based method for asynchronous multi-user DSSS systems is proposed and shown to achieve nearly optimal processing gain corresponding to that of the conventional matched filter. The studied methods find applications in interference cancellation, cognitive radio as well as in military applications. Multiantenna methods for synchronization are analyzed. Probabilities of synchronization acquisition and false alarm are derived analytically for the multiantenna case where transmit, receive and time diversity schemes are used. The analytical results are verified by simulations. Also optimum synchronization sequence transmission schemes are derived. Extensive simulation results for various scenarios in terms of signal-to-noise ratio, spatial correlation and Doppler spread are provided. The results can be used to facilitate practical design of wireless multiantenna systems. The impacts of time and frequency offsets in cooperative OFDM wireless systems are analyzed and quantified and shown to be, in some cases, much more severe than in traditional OFDM systems. Solutions to estimation of multiple frequency offsets in cooperative OFDM systems are then studied. A method enabling low complexity algorithms to be used for the estimation is proposed and shown to achieve mean square error performance close to the Cramér-Rao bound.