Equalization of CPM signals over doubly-selective aeronautical channels

Communication technologies have always been one of the fundamental milestones of the aeronautical environment. Despite the growing demand for high performances, the aviation context is reluctant to move towards new technologies. Common communication strategies are not suitable to transmit at very high data rates over time- and/or frequency-dispersive (i.e., doubly-selective) air-ground channels, therefore, new requirements have to be fulfilled by an incremental approach, that is, by updating some parts of the legacy systems. This thesis deals with receiver synthesis for aeronautical communication data-links employing continuous-phase modulated (CPM) signals over doubly-selective wireless communication channels. The goal is to design efficient and low-complexity time-varying equalizers, by exploiting all of the CPM signal features, in order to compensate for the effects due to the rapidly time-varying aeronautical channels. The application of the basis expansion model (BEM) to a typical aeronautical communication channel is considered and validated by computer simulations. The second-order statistical characterization of the pseudo-symbols arising from Laurent representation of CPM signals is introduced and discussed. Both linear time-varying (LTV) and widely-linear time-varying (WLTV) zero forcing (ZF) and minimum mean square error (MMSE) receiver structures for CPM signals operating over doubly-selective channels are proposed and implemented by using the BEM model for the channel. Monte Carlo simulation results, carried out in typical aeronautical scenarios, show that the proposed approaches are able to work satisfactorily also over rapidly time-varying channels

MIMO designs for filter bank multicarrier and multiantenna systems based on OQAM

From the perspective of increasingly data rate requirements in mobile communications, it is deemed necessary to do further research so that the future goals can be reached. To that end, the radio-based communications are resorting to multicarrier modulations and spatial diversity. Until today, the orthogonal frequency division multiplexing (OFDM) modulation is regarded as the dominant technology. On one hand, the OFDM modulation is able to accommodate multiantenna configurations in a very straightforward manner. On the other hand, the poor stopband attenuation exhibited by the OFDM modulation, highlights that a definitely tight synchronization is required. In addition, the cyclic prefix (CP) has to be sufficiently long to avoid inter-block interference, which may substantially reduce the spectral efficiency. In order to overcome the OFDM drawbacks, the filter bank multicarrier modulation based on OQAM (FBMC/OQAM) is introduced. This modulation does not need any CP and benefits from pulse shaping techniques. This aspect becomes crucial in cognitive radio networks and communication systems where nodes are unlikely to be synchronized. In principle, the poor frequency confinement exhibited by OFDM should tip the balance towards FBMC/OQAM. However, the perfect reconstruction property of FBMC/OQAM systems does not hold in presence of multipath fading. This means that the FBMC/OQAM modulation is affected by inter-symbol and inter-carrier interference, unless the channel is equalized to some extent. This observation highlights that the FBMC/OQAM extension to MIMO architectures becomes a big challenge due to the need to cope with both modulation- and multiantenna-induced interference. The goal of this thesis is to study how the FBMC/OQAM modulation scheme can benefit from the degrees of freedom provided by the spatial dimension. In this regard, the first attempt to put the research on track is based on designing signal processing techniques at reception. In this case the emphasis is on single-input-multiple-output (SIMO) architectures. Next, the possibility of pre-equalizing the channel at transmission is investigated. It is considered that multiple antennas are placed at the transmit side giving rise to a multiple-input-single-output (MISO) configuration. In this scenario, the research is not only focused on counteracting the channel but also on distributing the power among subcarriers. Finally, the joint transmitter and receiver design in multiple-input-multiple-output (MIMO) communication systems is covered. From the theory developed in this thesis, it is possible to conclude that the techniques originally devised in the OFDM context can be easily adapted to FBMC/OQAM systems if the channel frequency response is flat within the subchannels. However, metrics such as the peak to average power ratio or the sensitivity to the carrier frequency offset constraint the number of subcarriers, so that the frequency selectivity may be appreciable at the subcarrier level. Then, the flat fading assumption is not satisfied and the specificities of FBMC/OQAM systems have to be considered. In this situation, the proposed techniques allow FBMC/OQAM to remain competitive with OFDM. In addition, for some multiantenna configurations and propagation conditions FBMC/OQAM turns out to be the best choice. The simulation-based results together with the theoretical analysis conducted in this thesis contribute to make progress towards the application of FBMC/OQAM to MIMO channels. The signal processing techniques that are described in this dissertation allow designers to exploit the potentials of FBMC/OQAM and MIMO to improve the link reliability as well as the spectral efficiency