Spectrum sharing is known as a key solution to accommodate the increasing number of users and the growing demand for throughput in wireless networks. While spectrum sharing improves the data rate in sparse networks, it suffers from interference of concurrent links in dense networks. In fact, interference is the primary barrier to enhance the overall throughput of the network, especially in the medium and high signal-to-noise ratios (SNR’s). Managing interference to overcome this barrier has emerged as a crucial step in developing efficient wireless networks. This thesis deals with optimum and sub-optimum interference management-cancelation in non-cooperative networks. Several techniques for interference management including novel strategies such as interference alignment and structural coding are investigated. These methods are applied to obtain optimum and sub-optimum coding strategies in such networks. It is shown that a single strategy is not able to achieve the maximum throughput in all possible scenarios and in fact a careful design is required to fully exploit all available resources in each realization of the system. This thesis begins with a complete investigation of the capacity region of the two-user Gaussian interference channel. This channel models the basic interaction between two users sharing the same spectrum for data communication. New outer bounds outperforming known bounds are derived using Genie-aided techniques. It is proved that these outer bounds meet the known inner bounds in some special cases, revealing the sum capacity of this channel over a certain range of parameters which has not been known in the past. A novel coding scheme applicable in networks with single antenna nodes is proposed next. This scheme converts a single antenna system to an equivalent Multiple Input Multiple Output (MIMO) system with fractional dimensions. Interference can be aligned along these dimensions and higher multiplexing gains can be achieved. Tools from the field of Diophantine approximation in number theory are used to show that the proposed coding scheme in fact mimics the traditional schemes used in MIMO systems where each data stream is sent along a direction and alignment happens when several streams are received along the same direction. Two types of constellation are proposed for the encoding part, namely the single layer constellation and the multi-layer constellation. Using single layer constellations, the coding scheme is applied to the two-user $X$ channel. It is proved that the total Degrees-of-Freedom (DOF), i.e. $\frac{4}{3}$, of the channel is achievable almost surely. This is the first example in which it is shown that a time invariant single antenna system does not fall short of achieving this known upper bound on the DOF. Using multi-layer constellations, the coding scheme is applied to the symmetric three-user GIC. Achievable DOFs are derived for all channel gains. It is observed that the DOF is everywhere discontinuous (as a function of the channel gain). In particular, it is proved that for the irrational channel gains the achievable DOF meets the upper bound of $\frac{3}{2}$. For the rational gains, the achievable DOF has a gap to the known upper bounds. By allowing carry over from multiple layers, however, it is shown that higher DOFs can be achieved for the latter. The $K$-user single-antenna Gaussian Interference Channel (GIC) is considered, where the channel coefficients are NOT necessarily time-variant or frequency selective. It is proved that the total DOF of this channel is $\frac{K}{2}$ almost surely, i.e. each user enjoys half of its maximum DOF. Indeed, we prove that the static time-invariant interference channels are rich enough to allow simultaneous interference alignment at all receivers. To derive this result, we show that single-antenna interference channels can be treated as \emph{pseudo multiple-antenna systems} with infinitely-many antennas. Such machinery enables us to prove that the real or complex $M \times M$ MIMO GIC achieves its total DOF, i.e., $\frac{MK}{2}$, $M \geq 1$. The pseudo multiple-antenna systems are developed based on a recent result in the field of Diophantine approximation which states that the convergence part of the Khintchine-Groshev theorem holds for points on non-degenerate manifolds. As a byproduct of the scheme, the total DOFs of the $K\times M$ $X$ channel and the uplink of cellular systems are derived. Interference alignment requires perfect knowledge of channel state information at all nodes. This requirement is sometimes infeasible and users invoke random coding to communicate with their corresponding receivers. Alternative interference management needs to be implemented and this problem is addressed in the last part of the thesis. A coding scheme for a single user communicating in a shared medium is proposed. Moreover, polynomial time algorithms are proposed to obtain best achievable rates in the system. Successive rate allocation for a $K$-user interference channel is performed using polynomial time algorithms.

Wireless networks are increasingly interference-limited, which motivates the development of sophisticated interference management techniques. One recently discovered approach is interference alignment, which attains the maximum sum rate scaling (with signal-to-noise ratio) in many network configurations. Interference alignment is not yet well understood from an engineering perspective. Such design considerations include (i) partial rather than complete knowledge of channel state information, (ii) correlated channels, (iii) bursty packet-based network traffic that requires the frequent setup and tear down of sessions, and (iv) the spatial distribution and interaction of transmit/receive pairs. This dissertation aims to establish the benefits and limitations of interference alignment under these four considerations. The first contribution of this dissertation considers an isolated group of transmit/receiver pairs (a cluster) cooperating through interference alignment and derives the signal-to-interference-plus-noise ratio distribution at each receiver for each stream. This distribution is used to compare interference alignment to beamforming and spatial multiplexing (as examples of common transmission techniques) in terms of sum rate to identify potential switching points between them. This dissertation identifies such switching points and provides design recommendations based on severity of the correlation or the channel state information uncertainty. The second contribution considers transmitters that are not associated with any interference alignment cooperating group but want to use the channel. The goal is to retain the benefits of interference alignment amid interference from the out-of-cluster transmitters. This dissertation shows that when the out-of-cluster transmitters have enough antennas, they can access the channel without changing the performance of the interference alignment receivers. Furthermore, optimum transmit filters maximizing the sum rate of the out-of-cluster transmit/receive pairs are derived. When insufficient antennas exist at the out-of-cluster transmitters, several transmit filters that trade off complexity and sum rate performance are presented. The last contribution, in contrast to the first two, takes into account the impact of large scale fading and the spatial distribution of the transmit/receive pairs on interference alignment by deriving the transmission capacity in a decentralized clustered interference alignment network. Channel state information uncertainty and feedback overhead are considered and the optimum training period is derived. Transmission capacity of interference alignment is compared to spatial multiplexing to highlight the tradeoff between channel estimation accuracy and the inter-cluster interference; the closer the nodes to each other, the higher the channel estimation accuracy and the inter-cluster interference. / text

During the past years wireless communications have been exhibiting an increased growth rendering them the most common way for communication. The continuously increasing demand for wireless services resulted in limited availability of the wireless spectrum. To this end, Cognitive Radio (CR) techniques have been proposed in literature during the past years. The concept of CR approach is to utilize advanced radio and signal-processing technology along with novel spectrum allocation policies to enable new unlicensed wireless users to operate in the existing occupied spectrum areas without degrading the performance of the existing licensed ones. Moreover, the broadcast and fading nature of the wireless channel results in severe degradation on the performance of wireless transmissions. A solution to the problem is the use of multiple-antenna systems so as to achieve spatial diversity. However, in many cases, the communication devices' nature permit the support of multiple antennas due to size, power consumption, and hardware limitations. To this end, cooperative communications provide an alternative way to achieve spatial diversity via virtual antenna arrays formed by single antenna nodes. It is noteworthy that cooperation has an important role within the CR literature as many techniques developed within its context exploiting the benefits of cooperation in order to achieve improved performance. Therefore, the aim of the present dissertation is to develop efficient and practical cognitive, cooperative and cognitive cooperative schemes. More specifically the contributions are the following ones. The first contribution is a novel CR communication scheme. During the past years numerous CR communication schemes have been presented in literature. To the best of our knowledge, the majority of them were developed assuming perfect Channel State Information (CSI) at the unlicensed user's side. There are several cases where the licensed users do not desire any interaction with the unlicensed ones. In such cases, the assumption that the unlicensed user can obtain CSI that concerns the licensed user channels is not valid and as a result the corresponding communication technique cannot be applied. Therefore, at first we propose an novel CR communication scheme that requires CSI that can be estimated in a completely blind manner. Then, the corresponding blind estimation scheme is developed. Another significant contribution is the theoretical results that have been derived for both the perfect CSI case and the imperfect CSI case (when the blind estimation scheme is employed for obtaining the corresponding CSI). Especially, the theoretical results that concern the imperfect CSI case are some of the first ones that appear in the relevant literature, to the best of our knowledge. The second contribution is a decentralized adaptive Eigenvalue-Based spectrum Sensing (EBSS) technique for multi-antenna CR Systems. Spectrum Sensing is a fundamental functionality in CR systems. In general, the unlicensed user employs a spectrum sensing technique in order to detect licensed user(s) activity in scenarios where the former user is permitted to establish a communication link only via spectrum areas that are temporarily free of the latter one's transmissions. EBSS techniques are known to achieve good performance and also to be applicable in a completely blind manner. In the literature so far, only batch and centralized cooperative EBSS techniques have been considered which, however, suffer from limitations that render them impractical in several cases such as, when time-varying channels are involved or continuous spectrum monitoring is required. Thus, the aim here is to develop practical cooperative adaptive versions of typical Eigenvalue-Based Spectrum Sensing (EBSS) techniques which could be applied in a completely decentralized manner and cope well in time-varying scenarios. To this end, at first, novel adaptive EBSS techniques are developed for the Maximum Eigenvalue Detector (MED), the Maximum-Minimum Eigenvalue Detector (MMED), and the Generalized Likelihood Ratio Test (GLRT) schemes, respectively, for a single-user (no cooperation) case. Then, a novel distributed subspace tracking method is proposed which enables the cooperating nodes to track the joint subspace of their received signals. Based on this method, cooperative decentralized versions of the adaptive EBSS techniques are subsequently developed that overcome the limitations of the existing batch centralized approaches. The third contribution is a new cooperative scheme for half-duplex uplink transmission. The technique is based on a virtual MIMO structure formed by the single antenna source and relays nodes along with the multi-antenna base station which is the destination node. The new technique aims at providing increased diversity and multiplexing gains, contrariwise to existing approaches where the proposed techniques achieve increased diversity gain at the cost of severe multiplexing gain loss. The theoretical outage probability and the corresponding Diversity Multiplexing Trade-off (DMT) curve of the proposed technique are also derived. The final contribution is two novel algorithms which enable the relay cooperation for the distributed computation of the beamforming weights in a blind and adaptive manner, without the need to forward the data to a fusion center. The proposed algorithms are constituent parts of two corresponding distributed beamforming schemes for relay networks that distribute the computational overhead equally among the relay nodes. In the first scheme, the beamforming vector is computed through minimization of the total transmit power subject to a receiver quality-of-service constraint (QoS). In the second scheme, the beamforming weights are obtained through maximization of the receiver signal-to-noise-ration (SNR) subject to a total transmit power constraint. The proposed approaches achieve close performance to the one of the optimal beamforming solutions derived assuming perfect channel state information at the relays' side. / Τα τελευταία χρόνια, οι ασύρματες επικοινωνίες γνωρίζουν ραγδαία ανάπτυξη και ουσιαστικά αποτελούν τον πιο κοινό τρόπο επικοινωνίας. Η συνεχώς αυξανόμενη ζήτηση για υπηρεσίες ασύρματων επικοινωνιών έχει ως αποτέλεσμα την περιορισμένη διαθεσιμότητα του ασύρματου φάσματος συχνοτήτων. Για να αντιμετωπισθεί το προηγούμενο πρόβλημα, προτάθηκαν στο πρόσφατο παρελθόν τεχνικές στα πλαίσια των συστημάτων γνωσιακών κόμβων (Cognitive Radio- CR). Η βασική ιδέα ενός συστήματος CR είναι να χρησιμοποιήσει προηγμένες τεχνικές επικοινωνιών και επεξεργασίας σήματος μαζί με νέες τεχνικές ανάθεσης συχνοτήτων φάσματος για να επιτρέψει σε νέους μη αδειοδοτημένους ασύρματους χρήστες να λειτουργήσουν σε συχνότητες φάσματος που έχουν ανατεθεί σε αδειοδοτημένους χρήστες χωρίς να επηρεάζουν την επίδοση των τελευταίων. Επιπλέον, η ισχυρή παρεμβολή στις ασύρματες μεταδόσεις και η επίσης ισχυρή εξασθένιση του σήματος ενός ασύρματου συστήματος επικοινωνιών έχουν ως αποτέλεσμα τη σημαντική υποβάθμιση της επίδοσης των ασυρμάτων επικοινωνιών. Μια λύση στο προηγούμενο πρόβλημα μπορεί να δοθεί με την χρήση πολλαπλών κεραιών για την επίτευξη χωρικής ποικιλομορφίας. Σε πολλές περιπτώσεις όμως, η φύση των τηλεπικοινωνιακών συσκευών δεν επιτρέπει την υποστήριξη των πολλαπλών κεραιών λόγω περιορισμών στο μέγεθος της συσκευής, στην κατανάλωση ισχύος και στο υλικό. Ένας εναλλακτικός τρόπος για την επίτευξη χωρικής ποικιλομορφίας είναι μέσω των συνεργατικών επικοινωνιών (Cooperative Communications) οι οποίες σχηματίζουν γενικευμένα συστήματα πολλαπλών κεραιών από συστήματα μιας κεραίας. Είναι αξιοσημείωτο ότι η συνεργασία παίζει σημαντικό ρόλο στη βιβλιογραφία των συστημάτων CR, βάσει της οποίας αναπτύσσονται τεχνικές μετάδοσης με βελτιωμένη επίδοση. Επομένως, ο στόχος της παρούσας διατριβής είναι η ανάπτυξη αποδοτικών και πρακτικών τεχνικών γνωσιακών, συνεργατικών και γνωσιακών-συνεργατικών τεχνικών επικοινωνίας. Πιο συγκεκριμένα, η συνεισφορά της διατριβής μπορεί να χωριστεί στα ακόλουθα σκέλη. Στο πρώτο σκέλος παρουσιάζουμε μια νέα τεχνική μετάδοσης για CR συστήματα. Στο πρόσφατο παρελθόν, μια πληθώρα τεχνικών μετάδοσης για CR συστήματα έχει παρουσιαστεί στη βιβλιογραφία. Η πληθώρα των τεχνικών αυτών έχει αναπτυχθεί υποθέτοντας τέλεια γνώση της κατάστασης των εμπλεκόμενων ασύρματων τεχνικών (Channel State Information – CSI) στην πλευρά των μη αδειοδοτημένων χρηστών. Υπάρχουν, όμως, αρκετές περιπτώσεις στις οποίες οι αδειοδοτημένοι χρήστες δεν επιθυμούν καμία αλληλεπίδραση με τους αντίστοιχους μη αδειοδοτημένους. Σε τέτοιες περιπτώσεις, η υπόθεση ότι ο μη αδειοδοτημένος χρήστης έχει πρόσβαση σε CSI που σχετίζεται με τα κανάλια του αντίστοιχου αδειοδοτημένου δεν είναι έγκυρη με αποτέλεσμα η σχετική τεχνική μετάδοσης να μην μπορεί να εφαρμοστεί. Συνεπώς, αρχικά προτείνουμε ένα καινοτόμο CR επικοινωνιακό σχήμα που απαιτεί CSI το οποίο μπορεί να υπολογισθεί με έναν εντελώς τυφλό τρόπο. Έπειτα η αντίστοιχη τυφλή τεχνική εκτίμησης αναπτύσσεται. Μια άλλη σημαντική συνεισφορά είναι τα θεωρητικά αποτελέσματα τα οποία έχουν εξαχθεί τόσο για την τέλεια CSI περίπτωση, όσο και για τη μη τέλεια CSI περίπτωση (όταν το τυφλό σχήμα εκτίμησης χρησιμοποιείται για την απόκτηση της αντίστοιχης CSI). Ειδικότερα, τα θεωρητικά αποτελέσματα τα οποία αφορούν τη μη τέλεια CSI περίπτωση είναι μερικά από τα πρώτα τα οποία εμφανίζονται στη σχετική βιβλιογραφία, από όσο γνωρίζουμε. Το δεύτερο σκέλος αφορά μια απόκεντρικοποιημένη προσαρμοστική τεχνική επισκόπησης φάσματος ιδιοτιμών (Eigenvalue-Based Spectrum Sensing – EBSS) για συστήματα CR πολλαπλών κεραιών. Γενικά, ο μη αδειοδοτημένος χρήστης χρησιμοποιεί μία τεχνική επισκόπησης φάσματος προκειμένου να ανιχνεύσει την δραστηριότητα των αδειοδοτημένων χρηστών σε σενάρια όπου ο πρώτος επιτρέπεται να μεταδώσει μόνο διαμέσου περιοχών φάσματος οι οποίες είναι προσωρινά ελέυθερες από μεταδόσεις του δεύτερου. Είναι γνωστό ότι οι τεχνικές EBSS επιτυγχάνουν καλή επίδοση και είναι εφαρμόσιμες με ένα εντελώς τυφλό τρόπο. Στην βιβλιογραφία μέχρι στιγμής, έχουν μελετηθεί μόνο κεντρικοποιημένες συνεργατικές τεχνικές και τεχνικές τύπου batch οι οποίες υποφέρουν από περιορισμούς που τις καθιστούν μη πρακτικές σε αρκετές περιπτώσεις όπως όταν εμπλέκονται χρονικά μεταβαλλόμενα κανάλια ή όταν απαιτείται συνεχής παρακολούθηση του φάσματος. Συνεπώς, ο σκοπός εδώ είναι η ανάπτυξη πρακτικών συνεργατικών και προσαρμοστικών εκδοχών τυπικών EBSS οι οποίες μπορούν να εφαρμοστούν με ένα εντελώς αποκεντρικοποιημένο τρόπο και αποδίδουν καλά σε σενάρια που εμπλέκουν χρονικά μεταβαλλόμενα κανάλια. Προς αυτή την κατεύθυνση αναπτύσσονται αρχικά, νέες προσαρμοστικές εκδοχές για τις τεχνικές Maximum Eigenvalue Detector (MED), Maximum-Minimum Eigenvalue Detector (MMED) και την Generalized Likelihood Ratio Test (GLRT) αντίστοιχα για την περίπτωση του ενός χρήστη. Έπειτα μια νέα μέθοδος κατανεμημένης παρακολούθησης υποχώρου προτείνεται, η οποία επιτρέπει στους συνεργαζόμενους κόμβους να παρακολουθούν τον κοινό υποχώρο των ληφθέντων σημάτων τους. Με βάση αυτή την μέθοδο, συνεργατικές και αποκεντρικοποιημένες εκδοχές των προσαρμοστικών EBSS τεχνικών αναπτύσσονται οι οποίες ξεπερνούν τους περιορισμούς των υπαρχόντων κεντρικοποιημένων τεχνικών τύπου batch. Το τρίτο σκέλος είναι μια νέα συνεργατική τεχνική για half-duplex uplink μετάδοση. Αυτή η τεχνική βασίζεται σε ένα γενικευμένο σύστημα πολλαπλών κεραιών σχηματιζόμενο από την μιας κεραίας πηγή και κόμβους συνεργατών μαζί με το σταθμό βάσης πολλαπλών κεραιών ο οποίος είναι και ο κόμβος προορισμός. Η νέα τεχνική στοχεύει στο να παρέχει αυξημένα κέρδη ποικιλομορφίας και πολυπλεξίας, σε αντίθεση με υπάρχουσες τεχνικές οι οποίες επιτυγχάνουν αυξημένα κέρδη ποικιλομορφίας σε βάρος του κέρδους ποικιλομορφίας. Η θεωρητική πιθανότητα διακοπής επικοινωνίας και η αντίστοιχη καμπύλη Diversity Multiplexing Tradeoff (DMT) της προτεινόμενης τεχνικής υπολογίζονται. Το τελευταίο σκέλος είναι δύο νέοι αλγόριθμοι, οι οποίοι επιτρέπουν την ενδο-συνεργασία των συνεργατικών κόμβων για τον κατανεμημένο υπολογισμό των βαρών της τεχνικής προσανατολισμένης επικοινωνίας με τυφλό και προσαρμοστικό τρόπο χωρίς την ανάγκη προώθησης των δεδομένων σε έναν κόμβο σταθμό. Οι προτεινόμενοι αλγόριθμοι είναι συστατικά στοιχεία δύο αντίστοιχων τεχνικών προσανατολισμένης επικοινωνίας για δίκτυα συνεργατών τα οποία κατανέμουν τον υπολογιστικό φόρτο ισότιμα στους συνεργαζόμενους κόμβους. Στο πρώτο σχήμα το διάνυσμα βαρών υπολογίζεται ώστε να ελαχιστοποιείται η συνολική ισχύς μετάδοσης υπό έναν περιορισμό στην ποιότητα μετάδοσης. Το δεύτερο σχήμα, το διάνυσμα βαρών υπολογίζεται ώστε να μεγιστοποιείται το SNR του υπό έναν περιορισμό στην επιτρεπόμενη συνολική ισχύς μετάδοσης. Οι προτεινόμενες τεχνικές επιτυγχάνουν επίδοση κοντινή σε εκείνη των βέλτιστων λύσεων, οι οποίες έχουν υπολογισθεί υποθέτοντας τέλειο CSI στους κόμβους συνεργάτες.

Dans cette thèse, nous étudions les algorithmes d’alignement d’interférence dans les réseaux hétérogènes basés sur la sélection des flux. Tout d’abord, nous considérons différents scénarios de déploiement des pico-cellules dans un contexte de connaissance parfaite des canaux de transmission au niveau des émetteurs. Deux algorithmes sont proposés respectivement pour les réseaux totalement et partiellement connectés. Afin d’assurer une équité entre les liens, les algorithmes garantissent qu’au moins un flux de chaque lien émetteur/récepteur soit sélectionné. La séquence des flux est choisie parmi un ensemble qui contient les séquences les plus souvent sélectionnées en effectuant une recherche exhaustive. Ces algorithmes sont significativement moins complexes que la recherche exhaustive tout en ayant une performance proche de celle-ci. Après la sélection d’un flux, les interférences entre ce flux et les flux qui n’ont pas encore été sélectionnées sont alignées par projections orthogonales. Dans une deuxième partie de la thèse, l’impact de la connaissance partielle des canaux de transmission sur les algorithmes proposés est analysé. Il est montré que les interférences entre flux causent alors une forte dégradation des performances en raison des erreurs de quantification. Pour réduire cette dégradation, un nouvel algorithme est développé pour ce contexte. Finalement, des schémas d’allocation adaptative des bits pour les voies de retour sont proposés afin d’augmenter les performances des algorithmes précédents. Les performances de ces schémas et de ces algorithmes sont évaluées en considérant différents scénarios avec différentes topologies des pico-cellules. Nous avons montré que les algorithmes proposés pour le cas des transmissions avec voie de retour sont significativement plus robustes et plus performants que les algorithmes d’alignement d’interférence classiques. / In this thesis, we study the stream selection based interference alignment (IA) algorithms, which can provide large multiplexing gain, to deal with the interference in the heterogeneous networks. Firstly, different deployment scenarios for the pico cells are investigated assuming perfect channel state information (CSI) at the transmitters.Two different stream selection IA algorithms are proposed for fully and partially connected interference networks and selecting at least one stream is guaranteed for each user. A stream sequence is selected among a predetermined set of sequences that mostly contribute to the sum-rate while performing an exhaustive search. In the proposed algorithms, the complexity of the exhaustive search is significantly decreased while keeping the performance relatively close. After selecting a stream, the interference generated between the selected and the unselected streams is aligned by orthogonal projections. Then, the influence of the imperfect CSI on the proposed algorithms is analyzed and it is observed that the intra-stream interference causes a significant degradation in the performance due to the quantization error. Therefore, we propose an algorithm for the limited feedback scheme. Finally, adaptive bit allocation schemes are presented to maximize the overall capacity for all the proposed algorithms. The performance evaluations are carried out considering different scenarios with different number and placements of pico cells. It is shown that the proposed algorithm for the limited feedback is more robust to channel imperfections compared to the existing IA algorithms.

No description available.

In wireless networks, reliable communication is a challenging issue due to many attenuation factors such as receiver noise, channel fading, interference and asynchronous delays. Lattice coding and decoding provide efficient solutions to many problems in wireless communications and multiuser information theory. The capability in achieving the fundamental limits, together with simple and efficient transmitter and receiver structures, make the lattice strategy a promising approach. This work deals with problems of lattice detection over fading channels and time asynchronism over the lattice-based compute-and-forward protocol. In multiple-input multiple-output (MIMO) systems, the use of lattice reduction significantly improves the performance of approximate detection techniques. In the first part of this thesis, by taking advantage of the temporal correlation of a Rayleigh fading channel, low complexity lattice reduction methods are investigated. We show that updating the reduced lattice basis adaptively with a careful use of previous channel realizations yields a significant saving in complexity with a minimal degradation in performance. Considering high data rate MIMO systems, we then investigate soft-output detection methods. Using the list sphere decoder (LSD) algorithm, an adaptive method is proposed to reduce the complexity of generating the list for evaluating the log-likelihood ratio (LLR) values. In the second part, by applying the lattice coding and decoding schemes over asynchronous networks, we study the impact of asynchronism on the compute-and-forward strategy. While the key idea in compute-and-forward is to decode a linear synchronous combination of transmitted codewords, the distributed relays receive random asynchronous versions of the combinations. Assuming different asynchronous models, we design the receiver structure prior to the decoder of compute-and-forward so that the achievable rates are maximized at any signal-to-noise-ratio (SNR). Finally, we consider symbol-asynchronous X networks with single antenna nodes over time-invariant channels. We exploit the asynchronism among the received signals in order to design the interference alignment scheme. It is shown that the asynchronism provides correlated channel variations which are proved to be sufficient to implement the vector interference alignment over the constant X network.

The number of secure data streams a relay-assisted interference channel can support has been an intriguing problem. The problem is not solved even for a fundamental scenario with a single antenna at each transmitter, receiver and relay. In this paper, we study the achievable secure degrees of freedom of instantaneous relay-assisted interference channels with real and complex coefficients. The study of secure degrees of freedom with complex coefficients is not a trivial multiuser extension of the scenarios with real channel coefficients as in the case for the degrees of freedom, due to secrecy constraints. We tackle this challenge by jointly designing the improper transmit signals and widely-linear relay processing strategies.

In wireless networks, reliable communication is a challenging issue due to many attenuation factors such as receiver noise, channel fading, interference and asynchronous delays. Lattice coding and decoding provide efficient solutions to many problems in wireless communications and multiuser information theory. The capability in achieving the fundamental limits, together with simple and efficient transmitter and receiver structures, make the lattice strategy a promising approach. This work deals with problems of lattice detection over fading channels and time asynchronism over the lattice-based compute-and-forward protocol. In multiple-input multiple-output (MIMO) systems, the use of lattice reduction significantly improves the performance of approximate detection techniques. In the first part of this thesis, by taking advantage of the temporal correlation of a Rayleigh fading channel, low complexity lattice reduction methods are investigated. We show that updating the reduced lattice basis adaptively with a careful use of previous channel realizations yields a significant saving in complexity with a minimal degradation in performance. Considering high data rate MIMO systems, we then investigate soft-output detection methods. Using the list sphere decoder (LSD) algorithm, an adaptive method is proposed to reduce the complexity of generating the list for evaluating the log-likelihood ratio (LLR) values. In the second part, by applying the lattice coding and decoding schemes over asynchronous networks, we study the impact of asynchronism on the compute-and-forward strategy. While the key idea in compute-and-forward is to decode a linear synchronous combination of transmitted codewords, the distributed relays receive random asynchronous versions of the combinations. Assuming different asynchronous models, we design the receiver structure prior to the decoder of compute-and-forward so that the achievable rates are maximized at any signal-to-noise-ratio (SNR). Finally, we consider symbol-asynchronous X networks with single antenna nodes over time-invariant channels. We exploit the asynchronism among the received signals in order to design the interference alignment scheme. It is shown that the asynchronism provides correlated channel variations which are proved to be sufficient to implement the vector interference alignment over the constant X network.

The number of secure data streams a relay-assisted interference channel can support has been an intriguing problem. The problem is not solved even for a fundamental scenario with a single antenna at each transmitter, receiver and relay. In this paper, we study the achievable secure degrees of freedom of instantaneous relay-assisted interference channels with real and complex coefficients. The study of secure degrees of freedom with complex coefficients is not a trivial multiuser extension of the scenarios with real channel coefficients as in the case for the degrees of freedom, due to secrecy constraints. We tackle this challenge by jointly designing the improper transmit signals and widely-linear relay processing strategies.