OFDM Communication with Cooperative Relays

Signal fading due to multi-path propagation is one of the major impairments to meet the demands of next generation wireless networks for high data rate services. To mitigate the fading effects, time, frequency, and spatial diversity techniques or their hybrid can be used. Among different types of diversity techniques, spatial diversity is of special interest as is does not incur system losses in terms of delay and bandwidth efficiency.TelecommunicationsElectrical Engineering, Mathematics and Computer Scienc

Relaying Techniques for Multi Hop Differential Transmitted Reference IR-UWB Systems

This thesis develops novel relaying techniques to overcome the limited coverage of Impulse Radio Ultra Wideband (IR-UWB) systems based on Differential Transmitted Reference (DTR). Firstly, we describe a cooperative approach for two hop Amplify-and-Forward (A&F) relaying that exploits both the signal forwarded by the relay and the one directly transmitted by the source. After deriving the log-likelihood ratio based decision rule, we propose a semi-analytical power allocation strategy by evaluating a closed form expression for the effective Signal to Noise Ratio (SNR) at the destination, which is maximized by exhaustive search. Successively, we present a Joint Power Allocation and Path Selection (JPAPS) method for multi hop Decode-and-Forward (D&F) relaying. Starting from the heuristic consideration that the overall Bit Error Rate (BER) of the system is essentially driven by the quality of the path with the best performance, the proposed technique associates to each possible route a metric given by an approximation of the minimum BER which can be achieved as the power allocation coefficients vary and then takes into account only the path minimizing that metric. Specifically, we employ an equal SNR power allocation strategy that yields a closed form expression for the power allocation coefficients and we describe a path selection algorithm with polynomial complexity. Simulation results show the remarkable SNR gains obtained by the proposed schemes with respect to direct transmission and existing relaying techniques. Lo scopo di questa tesi è elaborare nuove tecniche di relaying per risolvere il problema della copertura limitata in sistemi radio ad impulsi a banda ultra larga (Impulse-Radio Ultra-Wideband, IR-UWB) basati su Differential Transmitted Reference (DTR). Innanzi tutto, si descrive un approccio cooperativo per singolo relay Amplify-and-Forward (A&F) che sfrutta sia il segnale inoltrato dal relay sia quello trasmesso direttamente dalla sorgente. Dopo aver introdotto una regola di decisione basata sul logaritmo del rapporto di verosimiglianza, si propone una strategia di allocazione di potenza semi-analitica valutando un'espressione in forma chiusa per il rapporto segnale rumore (SNR) effettivo al nodo destinazione, che viene massimizzato per mezzo di una ricerca esaustiva. Successivamente, si presenta un metodo congiunto di allocazione di potenza e scelta del cammino ottimo (Joint Power Allocation and Path Selection, JPAPS) per relay Decode-and-Forward (D&F) multipli. Partendo dalla considerazione euristica che la probabilità d'errore complessiva del sistema dipende essenzialmente dalla qualità del cammino migliore, la tecnica proposta associa ad ogni possibile percorso una metrica data da un'approssimazione della minima probabilità d'errore ottenibile al variare dei coefficienti di allocazione di potenza e poi prende in considerazione soltanto il cammino che minimizza tale metrica. Specificatamente, si adopera una strategia di allocazione di potenza in cui si impone l'uguaglianza degli SNR dei singoli link (equal SNR power allocation strategy), ottenendo un'espressione in forma chiusa per i coefficienti di allocazione di potenza. Inoltre, si descrive un algoritmo di scelta del cammino ottimo con complessità polinomiale. I risultati delle simulazioni mostrano i notevoli guadagni in termini di SNR ottenuti dagli schemi proposti rispetto alla trasmissione diretta e alle altre tecniche di relaying esistenti

Receiver Design and Security for Low Power Wireless Communications Systems

This dissertation focuses on two important areas in wireless communications: receiver design and security. In the first part of this dissertation we consider low data rate receiver design for ultra-wideband (UWB), a wideband radio technology that promises to help solve the frequency allocation problem that often inhibits narrowband systems. Reference-based receivers are promising candidates in the UWB regime, because the conventional rake receiver designs suffers from complexity limitations and inaccuracies in channel estimation. Many reference-based systems have arisen as viable solutions for receivers. We unify these systems as well as other systems into the general framework for performance analysis to suggest the optimal system for varying constraints. We improve the performance of frequency-shifted reference (FSR-UWB) for an average power constraint by halving the frequency offset and employing a sample-and-hold approach across the frame period. Also, we introduce a novel peak mitigation technique; tone reservation, for the multi-differential (MD) version of FSR-UWB, to reduce the high peak-to-average power ratio observed as the data carriers increase. The next part of this dissertation is about wireless security which is ubiquitous in modern news. Cryptography is widely use for security but it assumes limited computational abilities of an eavesdropper, is based on the unproven hardness of the underlying primitives, and allows for the message to be recorded and decrypted later. In this dissertation we consider an information-theoretic security approach to guaranteeing everlasting secrecy. We contribute a new secrecy rate pair outage formulation, where an outage event is based on the instantaneous rates of the destination and the eavesdropper being below and above desired thresholds, respectively. In our new secrecy rate pair outage formulation, two new unaccounted outage events emerge: secrecy breach, where the eavesdropper is above the targeted threshold; unreliable, where the destination is unable to successfully decode the message. The former case must be carefully avoided, while for the latter case we can exploit automatic retransmissions (ARQ) while maintaining the eavesdropper intercept probability below the target threshold. We look at both ``simple\u27\u27 receivers and also complex receivers that use a buffer to store previous messages to maximally combine signal-to-noise ratio (SNR). Then we extend these results to the two-hop case where we maximize the end-to-end secure throughput by optimizing the intercept probability at each eavesdropper given a total end-to-end intercept constraint. Lastly, we consider the difficult case in information-theoretic security: the near eavesdropper case, where we contribute an optimal power allocation algorithm that leverages nearby chatter nodes to generate noise to reduce the probability of intercept by the eavesdropper while minimally impeding the source-to-destination communication. As shown in both one-hop and two-hop cases, allowing a slight outage at the destination for cases of when the eavesdropper is above a specific threshold greatly improves secrecy performance

Timing and Carrier Synchronization in Wireless Communication Systems: A Survey and Classification of Research in the Last 5 Years

Timing and carrier synchronization is a fundamental requirement for any wireless communication system to work properly. Timing synchronization is the process by which a receiver node determines the correct instants of time at which to sample the incoming signal. Carrier synchronization is the process by which a receiver adapts the frequency and phase of its local carrier oscillator with those of the received signal. In this paper, we survey the literature over the last 5 years (2010–2014) and present a comprehensive literature review and classification of the recent research progress in achieving timing and carrier synchronization in single-input single-output (SISO), multiple-input multiple-output (MIMO), cooperative relaying, and multiuser/multicell interference networks. Considering both single-carrier and multi-carrier communication systems, we survey and categorize the timing and carrier synchronization techniques proposed for the different communication systems focusing on the system model assumptions for synchronization, the synchronization challenges, and the state-of-the-art synchronization solutions and their limitations. Finally, we envision some future research directions

Code-Multiplexing-Based One-Way Detect-and-Forward Relaying Schemes for Multiuser UWB MIMO Systems

In this paper, we consider decode-and-forward (DF) one-way relaying schemes for multiuser impulse-radio ultrawideband (UWB) communications. We assume low-complexity terminals with limited processing capabilities and a central transceiver unit (i.e., the relay) with a higher computational capacity. All nodes have a single antenna differently from the relay in which multiple antennas may be installed. In order to keep the complexity as low as possible, we concentrate on noncoherent transceiver architectures based on multiuser code-multiplexing transmitted-reference schemes. We propose various relaying systems with different computational complexity and different levels of required channel knowledge. The proposed schemes largely outperform systems without relay in terms of both bit error rate (BER) performance and coverage

Cooperative communications in wireless networks : novel approaches in the mac layer

Master'sMASTER OF ENGINEERIN

Radio Communications

In the last decades the restless evolution of information and communication technologies (ICT) brought to a deep transformation of our habits. The growth of the Internet and the advances in hardware and software implementations modified our way to communicate and to share information. In this book, an overview of the major issues faced today by researchers in the field of radio communications is given through 35 high quality chapters written by specialists working in universities and research centers all over the world. Various aspects will be deeply discussed: channel modeling, beamforming, multiple antennas, cooperative networks, opportunistic scheduling, advanced admission control, handover management, systems performance assessment, routing issues in mobility conditions, localization, web security. Advanced techniques for the radio resource management will be discussed both in single and multiple radio technologies; either in infrastructure, mesh or ad hoc networks

Ultra-Wideband Relay Communication Systems

Impulse-radio ultra-wide-band (IR-UWB) signaling is a promising technique for high-speed, short-range relay communications networks. Depending on how the relay node retransmits the signal, there are two main relay schemes: conventional one-directional (one-way) relay model, and bi-directional (two-way) relay model. In bi-directional relay communications, wireless network coding (WNC), also called physical-layer network coding (PNC), could be applied to overcome the spectral efficiency limitation of the conventional one-way relay. In the first part of this work, we propose asynchronous, differential, and bidirectional decode and forward (ADBDF) and asynchronous, differential, and bidirectional denoise and forward (ADBDNF) UWB relay methods, where the relay node (RN) does not need to be synchronized with the end nodes (ENs). The proposed schemes are attractive for networks in which stringent/complicated synchronization between the RN and the ENs may not be feasible. The second part of this work focuses on UWB channel classification. We propose a 2-dimensional (2-D) LOS/NLOS classification scheme that uses skewness of the channel impulse/pulse response. The proposed channel classification decreases the complexity of existing channel classification methods and can be used in a variety of areas such as localization, relay communications, and cooperative communications. The final part of this work deals with compressive sensing (CS) algorithms that employ sub-Nyquist sampling for UWB communications. We develop coarse graining (CG) for the proposed CS sub-Nyquist sampling technique, which leads to: (1) reduced sampling rate at the receiver, and hence reduced use of analog-to-digital converters (ADCs) resources; and (2) low-complexity channel estimation