• frequency estimation
• MIMO-OFDM
• OFDM
• OFDM-SDMA

We are now facing a new information age with high demand of wireless communication systems. New services such as data and video require reliable high-speed transmissions over frequency-selective channels. Moreover, the radio spectrum, which is scarce and very expensive resource, must be allocated to many different applications and systems. Multicarrier communications based on orthogonal frequency division multiplexing (OFDM) have grown in popularity due to their ability to fulfill most of the requirements of such systems. However, among other problems, accurate frequency synchronization is a crucial issue for multicarrier systems as it is essential for reliable signal reception. This problem has received much attention in the last few
years and several methods which deal with the frequency synchronization problem are
available in literature. The aim of this thesis is to present novel advanced techniques for the frequency synchronization in multicarrier systems. In general terms, the main contribution of this dissertation is threefold. First, we address the problem of carrier frequency estimation for orthogonal frequency-division multiplexing (OFDM) systems in the presence of narrowband interference (NBI) with unknown power. This scenario is encountered in emerging spectrum sharing systems, where coexistence of different wireless services over the same frequency band may result into a remarkable co-channel interference. A second contribution comes from addressing the frequency
synchronization in multiple-input multiple-output (MIMO) OFDM systems. In such
systems accurate frequency synchronization in necessary in order to obtain the improvement in terms of capacity promised by the deployment of multiple antennas.
Finally, the frequency synchronization in a multiuser OFDM network with space division multiple access (SDMA) is investigated in the last part. In this scenario, the
main obstacle is the large number of parameters involved in the estimation process,
which makes the multiuser synchronization a rather challenging task.