18 Juin – Thesis defense - Titouan Petitpied
10 h Amphi J.P. Dom - laboratory IMS / Building A31 (Talence campus) - partial videoconferencing
FTN communication systems: from single-carrier to multi-carrier links.
This thesis is a multi-criteria study of the Faster-Than-Nyquist (FTN) technique. We first consider in the context of single-carrier communications without propagation channel. We assume two types of FTN signal shaping: the linear FTN classically studied, and the circular FTN which has the advantage of simplifying the filtering process that can be performed in the frequency domain. Under these assumptions, we define different criteria of interest: spectral efficiency, energy efficiency through Peak-to-Average Power Ratio (PAPR), computational complexity of the reception algorithms. We then show that the potential of FTN is higher than the one of Nyquist communications because it allows the PAPR to be optimized. However, FTN signaling is not currently used because of the computational complexity required by the reception algorithms for Inter-Symbol Interference (ISI) processing. Indeed, Minimum Mean-Square-Error (MMSE) filtering approaches offer substantial gains at low spectral efficiency, but there is a need to switch to Maximum A Posteriori (MAP) detectors, which are more efficient in terms of error rate when the ISI is too powerful. However, these MAP approaches require computational complexity that is prohibitively high for rich constellations. Thus, we are interested in passing message algorithms to reduce the computational cost of interference processing. To do so, we propose to use Expectation Propagation (EP) which aims at filtering the ISI just like MMSE algorithms, while constraining the constellation of symbols. This constraint is imposed by a processing block called "Constellation Matcher" which is responsible for realigning the MMSE estimate of the symbols with the known constellation. Hence, an iterative symbol processing appears between a filter and this Constellation Matcher block. In order to extend the capabilities of the EP to FTN signals, we propose different distributions families in which EP messages are exchanged. We study three families that give rise to the time-domain EP processing for the most classical family, the frequency-domain EP processing subject to the use of the circular shaping of the FTN, and the Widely Linear (WL) approach which processes separately the I and Q parts of the received signal. These different EP receivers for FTN are then used in the multi-criteria study and we show SNR gains up to 8 dB compared to Nyquist communications, while limiting the computational complexity. The different types of EP receivers then allow a trade-off between computational complexity and error rate. The last application framework of this study is the multi-carrier context of the FTN signaling. Given the benefits of EP approaches for the multi-criteria study, we adapt these algorithms to the multi-carrier case and show significant gains compared to the literature, but also compared to Nyquist communications.