PhD Thesis

Title: Advanced Receiver Design for Distributed Cooperative Mobile Ad Hoc Networks

Academic Advisors: Charly POULLIAT, Marie-Laure BOUCHERET

Industrial Advisors: Antonio Maria CIPRIANO

Funding: This Ph.D. thesis has been carried out under the collaboration between “Signal & Communications" group of IRIT-INPT, a CNRS laboratory at the University of Toulouse, France, and “Waveform Design" team of Thales SIX GTS (former Thales Communications & Security), Gennevilliers, France, through a CIFRE funding.

Abstract --- Mobile ad hoc networks (MANETs) are rapidly deployable wireless communications systems, operating with minimal coordination to avoid spectral efficiency losses due to overhead. Cooperative transmission schemes are attractive for MANETs, but the distributed nature of protocols comes with increased interference, whose impact is further amplified by the need to push the limits of energy and spectral efficiency. These issues need to be mitigated through PHY layer signal processing algorithms with reasonable computational complexity.

Recent advances in iterative digital receiver design techniques exploit approximate Bayesian inference and derivative message passing techniques to improve the capabilities of well-established turbo detectors. In particular, expectation propagation (EP) is a flexible technique which offers attractive complexity-performance trade-offs in situations where conventional belief propagation is limited by computational complexity. Moreover, thanks to emerging techniques in deep learning, such iterative structures are cast into deep detection networks, where learning the algorithmic hyper-parameters further improves receiver performance.

In this thesis, EP-based finite-impulse response decision feedback equalizers are designed, and they achieve significant improvements over more conventional turbo-equalization techniques, especially in high spectral efficiency applications, with the advantage of being asymptotically predictable. A framework for designing frequency-domain EP-based receivers is proposed, in order to obtain detection architectures with low computational complexity. This framework is theoretically and numerically analysed with a focus on channel equalization, and then it is also extended to handle detection for time-varying channels and multiple-antenna systems. The design of multiple-user detectors and the impact of channel estimation are also explored to understand the capabilities and limits of this framework.

Finally, a finite-length performance prediction method is presented for carrying out link abstraction for the EP-based frequency domain equalizer, and the impact of accurate physical layer modelling is evaluated in the context of cooperative broadcasting in tactical MANETs, thanks to a flexible MAC-level simulator.
Keywords: Mobile ad-hoc networks (MANET), cooperative communications, cooperative broadcast, iterative detection, equalization, channel estimation, Bayesian inference, expectation propagation, message passing algorithms, deep unfolding, semi-analytic prediction.