Réseaux idéaux sur des corps CM et des corps totalement réels

This thesis deals with the study of ideal lattices over number fields. Let K be a number field, which is assumed to be CM or totally real. An ideal lattice over K is a pair (I,b), where I is a fractional ideal of K and b : I × I → R is a symmetric positive definite bilinear form such that b(x, y) = Tr(αxy) for a totally positive α ∈ K ⊗ R. The ideal lattice defined previously will be denoted by (I,α). In the first part, we will focus on constructing modular lattices over number fields. In particular the case of cyclotomic fields will be treated more carefully. A modular lattice is a lattice which is similar to its dual lattice. H.-G. Quebbemann introduced this notion in 1995 and in 1997, and he also defined a notion of analytic extremality for these lattices. It seemed promising to look for ideal lattices which were modular. This investigation led to the introduction of Arakelov modular lattices. This notion has turned out to be very interesting. If K = Q(ζn ) is a cvclotomic field, then the set of levels ℓ for which there exists an Arakelov modular lattice of level ℓ over Q(ζn ) is explicitly given in this thesis. Moreover, assume that K is a CM field, and that we are given an Arakelov modular lattice (I,α) of level ℓ over K. Under an assumption on K (which is satisfied for cyclotomic fields), we describe a way to compute all the Arakelov modular lattices of level ℓ over K. In the second part of this thesis, a construction is introduced which enables us to associate a totally real field K' to a given CM-field K. This field K' has the following property : if (I,α) is an ideal lattice over K. where I is an ambiguous ideal, then there exists an ideal lattice over K' isometric (as a lattice) to (I,α). This leads to the construction of several classic lattices over totally real fields. It also enables us to bound the Euclidean minimum of the field K' with respect to that of the field K.

Algebraic methods for channel coding

Frédérique Oggier

This work is dedicated to developing algebraic methods for channel coding. Its goal is to show that in different contexts, namely single-antenna Rayleigh fading channels, coherent and non-coherent MIMO channels, algebraic techniques can provide useful tools for building efficient coding schemes. Rotated lattice signal constellations have been proposed as an alternative for transmission over the single-antenna Rayleigh fading channel. It has been shown that the performance of such modulation schemes essentially depends on two design parameters: the modulation diversity and the minimum product distance. Algebraic lattices, i.e., lattices constructed by the canonical embedding of an algebraic number field, or more precisely ideal lattices, provide an efficient tool for designing such codes, since the design criteria are related to properties of the underlying number field: the maximal diversity is guaranteed when using totally real number fields and the minimum product distance is optimized by considering fields with small discriminant. Furthermore, both shaping and labelling constraints are taken care of by constructing Zn-lattices. We present here the construction of several families of such n-dimensional lattices for any n, and compute their performance. We then give an upper bound on their minimal product distance, and show that with respect to this bound, existing lattice codes are optimal in the sense that no further significant coding gain could be reached. Cyclic division algebras have been introduced recently in the context of coherent Space-Time coding. These are non-commutative algebras which naturally yield families of invertible matrices, or in other words, linear codes that fulfill the rank criterion. In this work, we further exploit the algebraic structures of cyclic algebras to build Space-Time Block codes (STBCs) that satisfy the following properties: they have full rate, full diversity, non-vanishing constant minimum determinant for increasing spectral efficiency, uniform average transmitted energy per antenna and good shaping. We give algebraic constructions of such STBCs for 2, 3, 4 and 6 antennas and show that these are the only cases where they exist. We finally consider the problem of designing Space-Time codes in the noncoherent case. The goal is to construct maximal diversity Space-Time codewords, subject to a fixed constellation constraint. Using an interpretation of the noncoherent coding problem in terms of packing subspaces according to a given metric, we consider the construction of non-intersecting subspaces on finite alphabets. Techniques used here mainly derive from finite projective geometry.

EPFL2005

Arithmetic Bounds-Lenstra's Constant and Torsion of K-Groups

Julien Houriet

This thesis is concerned with computations of bounds for two different arithmetic invariants. In both cases it is done with the intention of proving some algebraic or arithmetic properties for number fields. The first part is devoted to computations of lower bounds for the Lenstra's constant. For a number field K the Lenstra's constant is denoted Λ(K) and defined as the length of the largest exceptional sequence in K. An exceptional sequence is a set of units in K such that for any two among them their difference is a unit as well. H.W. Lenstra showed that if Λ(K) is large enough – bigger than a constant depending on the degree and the discriminant of K – then the ring of integers of K is Euclidean with respect to the norm. Using computer software PARI/GP and some algorithms from graph theory we construct exceptional sequences in number fields having a small discriminant. These exceptional sequences yield lower bounds for Lenstra's constant which are large enough to prove the existence of 42 new Euclidean number fields of degree 8 to 12. The aim of the second part of this thesis is proving upper bounds for the torsion part of the K-groups of a number field ring of integers. A method due to C. Soulé yields bounds for the torsion of these K-groups depending on an invariant of hermitian lattices over number fields. Firstly we describe some properties of rank one hermitian lattices, especially of ideal lattices. Secondly we apply these properties to arbitrary rank hermitian lattices and this implies a significant improvement of the upper bounds for their invariants and accordingly for the torsion of K-groups. The progress mainly achieves much lower contributions of the number field attributes, particularly the degree and the absolute discriminant.

EPFL2010

Algebraic methods for channel coding

Frédérique Oggier

EPFL2005