Securing Downlink Non-Orthogonal Multiple Access Systems by Trusted Relays

A downlink single-input single-output non-orthogonal multiple access system is considered in which a base station (BS) is communicating with two legitimate users in the presence of an external eavesdropper. A group of trusted cooperative half-duplex relay nodes, powered by the BS, is employed to assist the BS's transmission. The goal is to design relaying schemes such that the legitimate users' secrecy rate region is maximized subject to a total power constraint on the BS and the relays' transmissions. Three relaying schemes are investigated: cooperative jamming, decode-and-forward, and amplify-and-forward. Depending on the scheme, secure beamforming signals are carefully designed for the relay nodes that either diminish the eavesdropper's rate without affecting that of the legitimate users, or increase the legitimate users' rates without increasing that of the eavesdropper. The results show that there is no relaying scheme that fits all conditions; the best relaying scheme depends on the system parameters, namely, the relays' and eavesdropper's distances from the BS, and the number of relays. They also show that the relatively simple cooperative jamming scheme outperforms other schemes when the relays are far from the BS and/or close to the eavesdropper.Comment: To appear in IEEE Globecom 201

Anonymous Networking amidst Eavesdroppers

The problem of security against timing based traffic analysis in wireless networks is considered in this work. An analytical measure of anonymity in eavesdropped networks is proposed using the information theoretic concept of equivocation. For a physical layer with orthogonal transmitter directed signaling, scheduling and relaying techniques are designed to maximize achievable network performance for any given level of anonymity. The network performance is measured by the achievable relay rates from the sources to destinations under latency and medium access constraints. In particular, analytical results are presented for two scenarios: For a two-hop network with maximum anonymity, achievable rate regions for a general m x 1 relay are characterized when nodes generate independent Poisson transmission schedules. The rate regions are presented for both strict and average delay constraints on traffic flow through the relay. For a multihop network with an arbitrary anonymity requirement, the problem of maximizing the sum-rate of flows (network throughput) is considered. A selective independent scheduling strategy is designed for this purpose, and using the analytical results for the two-hop network, the achievable throughput is characterized as a function of the anonymity level. The throughput-anonymity relation for the proposed strategy is shown to be equivalent to an information theoretic rate-distortion function

Securing UAV Communications Via Trajectory Optimization

Unmanned aerial vehicle (UAV) communications has drawn significant interest recently due to many advantages such as low cost, high mobility, and on-demand deployment. This paper addresses the issue of physical-layer security in a UAV communication system, where a UAV sends confidential information to a legitimate receiver in the presence of a potential eavesdropper which are both on the ground. We aim to maximize the secrecy rate of the system by jointly optimizing the UAV's trajectory and transmit power over a finite horizon. In contrast to the existing literature on wireless security with static nodes, we exploit the mobility of the UAV in this paper to enhance the secrecy rate via a new trajectory design. Although the formulated problem is non-convex and challenging to solve, we propose an iterative algorithm to solve the problem efficiently, based on the block coordinate descent and successive convex optimization methods. Specifically, the UAV's transmit power and trajectory are each optimized with the other fixed in an alternating manner until convergence. Numerical results show that the proposed algorithm significantly improves the secrecy rate of the UAV communication system, as compared to benchmark schemes without transmit power control or trajectory optimization.Comment: Accepted by IEEE GLOBECOM 201

Performance enhancement solutions in wireless communication networks

In this dissertation thesis, we study the new relaying protocols for different wireless network systems. We analyze and evaluate an efficiency of the transmission in terms of the outage probability over Rayleigh fading channels by mathematical analyses. The theoretical analyses are verified by performing Monte Carlo simulations. First, we study the cooperative relaying in the Two-Way Decode-and-Forward (DF) and multi-relay DF scheme for a secondary system to obtain spectrum access along with a primary system. In particular, we proposed the Two-Way DF scheme with Energy Harvesting, and the Two-Way DF Non-orthogonal Multiple Access (NOMA) scheme with digital network coding. Besides, we also investigate the wireless systems with multi-relay; the best relay selection is presented to optimize the effect of the proposed scheme. The transmission protocols of the proposed schemes EHAF (Energy Harvesting Amplify and Forward) and EHDF (Energy Harvesting Decode and Forward) are compared together in the same environment and in term of outage probability. Hence, with the obtained results, we conclude that the proposed schemes improve the performance of the wireless cooperative relaying systems, particularly their throughput. Second, we focus on investigating the NOMA technology and proposing the optimal solutions (protocols) to advance the data rate and to ensure the Quality of Service (QoS) for the users in the next generation of wireless communications. In this thesis, we propose a Two-Way DF NOMA scheme (called a TWNOMA protocol) in which an intermediate relay helps two source nodes to communicate with each other. Simulation and analysis results show that the proposed protocol TWNOMA is improving the data rate when comparing with a conventional Two-Way scheme using digital network coding (DNC) (called a TWDNC protocol), Two-Way scheme without using DNC (called a TWNDNC protocol) and Two-Way scheme in amplify-and-forward(AF) relay systems (called a TWANC protocol). Finally, we considered the combination of the NOMA and physical layer security (PLS) in the Underlay Cooperative Cognitive Network (UCCN). The best relay selection strategy is investigated, which uses the NOMA and considers the PLS to enhance the transmission efficiency and secrecy of the new generation wireless networks.V této dizertační práci je provedena studie nových přenosových protokolů pro různé bezdrátové síťové systémy. S využitím matematické analýzy jsme analyzovali a vyhodnotili efektivitu přenosu z hlediska pravděpodobnosti výpadku přes Rayleighův kanál. Teoretické analýzy jsou ověřeny provedenými simulacemi metodou Monte Carlo. Nejprve došlo ke studii kooperativního přenosu ve dvoucestném dekóduj-a-předej (Two-Way Decode-and-Forward–TWDF) a vícecestném DF schématu s větším počtem přenosových uzlů pro sekundární systém, kdy takto byl získán přístup ke spektru spolu s primárním systémem. Konkrétně jsme navrhli dvoucestné DF schéma se získáváním energie a dvoucestné DF neortogonální schéma s mnohonásobným přístupem (Non-orthogonal Multiple Access–NOMA) s digitálním síťovým kódováním. Kromě toho rovněž zkoumáme bezdrátové systémy s větším počtem přenosových uzlů, kde je přítomen výběr nejlepšího přenosového uzlu pro optimalizaci efektivnosti navrženého schématu. Přenosové protokoly navržených schémat EHAF (Energy Harvesting Amplify and Forward) a EHDF(Energy Harvesting Decode and Forward) jsou společně porovnány v identickém prostředí z pohledu pravděpodobnosti výpadku. Následně, na základě získaných výsledků, jsme dospěli k závěru, že navržená schémata vylepšují výkonnost bezdrátových kooperativních systémů, konkrétně jejich propustnost. Dále jsme se zaměřili na zkoumání NOMA technologie a navrhli optimální řešení (protokoly) pro urychlení datového přenosu a zajištění QoS v další generaci bezdrátových komunikací. V této práci jsme navrhli dvoucestné DF NOMA schéma (nazýváno jako TWNOMA protokol), ve kterém mezilehlý přenosový uzel napomáhá dvěma zdrojovým uzlům komunikovat mezi sebou. Výsledky simulace a analýzy ukazují, že navržený protokol TWNOMA vylepšuje dosaženou přenosovou rychlost v porovnání s konvenčním dvoucestným schématem používajícím DNC (TWDNC protokol), dvoucestným schématem bez použití DNC (TWNDNC protokol) a dvoucestným schématem v zesil-a-předej (amplify-and-forward) přenosových systémech (TWANC protokol). Nakonec jsme zvážili využití kombinace NOMA a zabezpečení fyzické vrstvy (Physical Layer Security–PLS) v podpůrné kooperativní kognitivní síti (Underlay Cooperative Cognitive Network–UCCN). Zde je zde zkoumán výběr nejlepšího přenosového uzlu, který užívá NOMA a bere v úvahu PLS pro efektivnější přenos a zabezpečení nové generace bezdrátových sítí.440 - Katedra telekomunikační technikyvyhově

Two-path succesive relaying schemes in the presence of inter-relay interference

Relaying is a promising technique to improve wireless network performance. A conventional relay transmits and receives signals in two orthogonal channels due to half duplex constraint of wireless network. This results in inefficient use of spectral resources. Two-Path Successive Relaying (TPSR) has been proposed to recover loss in spectral efficiency. However, the performance of TPSR is degraded by Inter-Relay Interference (IRI). This thesis investigates the performance of TPSR affected by IRI and proposes several schemes to improve relaying reliability, throughput and secrecy. Simulations revealed that the existing TPSR could perform worse than the conventional Half Duplex Relaying (HDR) scheme. Opportunistic TPSR schemes are proposed to improve the capacity performance. Several relay pair selection criteria are developed to ensure the selection of the best performing relay pair. Adaptive schemes which dynamically switch between TPSR and conventional HDR are proposed to further improve the performance. Simulation and analytical results show that the proposed schemes can achieve up to 45% ergodic capacity improvement and lower outage probability compared to baseline schemes, while achieving the maximum diversity and multiplexing tradeoff of the multi-input single-output channel. In addition, this thesis proposes secrecy TPSR schemes to protect secrecy of wireless transmission from eavesdropper. The use of two relays in the proposed schemes deliver more robust secrecy transmission while the use of scheduled jamming signals improves secrecy rate. Simulation and analytical results reveal that the proposed schemes can achieve up to 62% ergodic secrecy capacity improvement and quadratically lower intercept and secrecy outage probabilities if compared to existing schemes. Overall, this thesis demonstrates that the proposed TPSR schemes are able to deliver performance improvement in terms of throughput, reliability and secrecy in the presence of IRI