Physical layer security in cognitive radio networks using improper gaussian signaling

Orientador: Prof. Dr. Evelio Martin Garcia FernandezCoorientador: Prof. Dr. Samuel Baraldi MafraTese (doutorado) - Universidade Federal do Paraná, Setor de Tecnologia, Programa de Pós-Graduação em Engenharia Elétrica. Defesa : Curitiba, 25/10/2018Inclui referências: p.55-60Resumo: Em redes de comunicação sem fio que possuem restrições de interferência, a adoção de sinais assimétricos ou impróprios pode atingir taxas de transmissão mais altas do que as obtidas com sinais próprios, devido a maior entropia diferencial destes. Portanto, uma vez que o desempenho de segurança de uma rede está diretamente relacionado a taxa de transmissão de seus usuários, esta tese propõe o emprego de sinais impróprios para melhorar o desempenho do sigilo em redes de Radio Cognitivo. Ate onde sabemos, este e o primeiro trabalho que aborda a Segurança da Camada Física deste tipo de sistema usando sinais assimétricos. Os resultados foram obtidos para dois cenários diferentes em um mesmo modelo de sistema: uma rede cognitiva underlay com uma ligacao direta entre o transmissor secundário e seu receptor, cuja comunicação está sendo espionada. Usuários primários e secundários causam interferência entre si. Em ambos os cenários, apenas a informação estatística do estado do canal foi considerada disponível para os usuários cognitivos. Para o primeiro cenário, em que a localização dos nós do sistema foi definida arbitrariamente, derivamos uma expressao analótica para a Probabilidade de Falha de Sigilo, a principal métrica de desempenho analisada, e foi mostrado que a adoção de sinalização impropria pode ser benéfica tanto para os usuários que causam quanto para os que recebem interferência. Em um segundo cenário, em que a localização dos nos foi distribuída uniformemente sobre uma célula circular, encontramos valores ótimos ou sub-ótimos para a potencia de transmissão e grau de impropriedade dos sinais dos usuários secundários simultaneamente, a fim de otimizar o desempenho de segurança da rede. A otimização foi feita com o auxílio de Algoritmos Genéticos. Em seguida, os benefícios do esquema de transmissão em termos da probabilidade de falha de sigilo e da vazão de dados segura do sistema, bem como o custo de eficiência energética foram avaliados. Os resultados indicam que, para sistemas limitados por interferência, ao buscar por baixas probabilidades de falha de sigilo, e sempre uma estratégia melhor para os usuários secundários adotar algum grau de impropriedade em suas transmissões. Além disso, a adoção de sinais impróprios também pode melhorar as taxas seguras atingíveis no lado dos usuários cognitivos em redes underlay. No entanto, em termos de eficiência energética do sistema, otimizar apenas a potencia de transmissão secundaria e adotar sinais próprios obtém o melhor desempenho. Os resultados apresentados nesta pesquisa são promissores, uma vez que em muitas redes sem fio, inclusive cognitivas, existem restrições de interferência e sinais assimétricos poderiam alcançar um desempenho melhor do que os próprios, o paradigma atual. Palavras-chave: Radio Cognitivo, Segurança na Camada Física, SinaisAbstract: In interference constrained wireless communication networks, adopting asymmetric or improper signals may attain higher transmission rates than those achieved by proper ones, due to the higher differential entropy of the latter. Therefore, since the secrecy performance of a network is directly related to the transmission rate of its users, this thesis proposes employing improper signals in order to enhance the secrecy performance of Cognitive Radio networks. As far as we know, this is the first work that addresses the Physical Layer Security of these type of system by using asymmetric signals. The results were obtained for two different scenarios in the same system model: an underlay cognitive network with a direct link between secondary transmitter and receiver, whose communication is being eavesdropped. Both primary and secondary users cause interference at each other. In both scenarios only Statistical Channel State Information was considered available at the cognitive users. For the first scenario, in which nodes locations were defined arbitrarily, we derived an analytical expression for the Secrecy Outage Probability, the main performance metric analyzed, and it was shown that adopting improper signaling can be beneficial for users either causing or receiving interference. In a second scenario, in which nodes locations were uniformly distributed over a circular cell, we found optimal or suboptimal values of the secondary users transmit power and degree of impropriety, concurrently, in order to optimize the secrecy performance, with the aid of Genetic Algorithms. Then, the benefits of the transmission scheme in terms of the Secrecy Outage Probability and the Secure Throughput of the system, as well as the Secure Energy Efficiency cost were assessed. Results indicate that, for systems with interference constraints, when searching for lower Secrecy Outage Probabilities, it is always a better strategy for the Secondary Users to adopt some degree of impropriety in their transmissions. In addition, adopting improper signals can also improve the achievable secure rates at the cognitive users side in underlay networks. However, in terms of the energy efficiency of the system, optimizing only the secondary transmit power while employing proper signals achieves the best performance. The results presented in this research are promising, since in many wireless channels, including Cognitive Networks, there are interference constraints and asymmetric signals could attain better performance than proper ones, the current paradigm. Keywords: Cognitive Radio Networks, Physical Layer Security, Improper Gaussian Signaling, Secrecy Outage Probability

Improper Gaussian signaling for multiple-access channels in underlay cognitive radio

This paper considers an unlicensed multiple-access channel (MAC) that coexists with a licensed point-to-point user, following the underlay cognitive radio paradigm. We assume that every transceiver except the secondary base station has one antenna and that the primary user (PU) is protected by a minimum rate constraint. In contrast to the conventional assumption of proper Gaussian signaling, we allow the secondary users to transmit improper Gaussian signals, which are correlated with their complex conjugate. When the secondary base station performs zero-forcing, we show that improper signaling is optimal if the sum of the interference channel gains (in an equivalent canonical model) is above a certain threshold. Additionally, we derive an efficient algorithm to compute the transmission parameters that attain the rate region boundary for this scenario. The proposed algorithm exploits a single-user representation of the secondary MAC along with new results on the optimality of improper signaling in the single-user case when the PU is corrupted by an improper noise.The work of C. Lameiro and P. J. Schreier was supported by the German Research Foundation (DFG) under grants SCHR 1384/6-1 and LA 4107/1-1. The work of I. Santamaría was supported by the Ministerio de Economía y Competitividad (MINECO) and AEI/FEDER funds of the UE, Spain, under projects RACHEL (TEC2013-47141-C4-3-R) and CARMEN (TEC2016-75067-C4-4-R)

Information-theoretic analysis of a family of improper discrete constellations

Non-circular or improper Gaussian signaling has proven beneficial in several interference-limited wireless networks. However, all implementable coding schemes are based on finite discrete constellations rather than Gaussian signals. In this paper, we propose a new family of improper constellations generated by widely linear processing of a square M-QAM (quadrature amplitude modulation) signal. This family of discrete constellations is parameterized by κ, the circularity coefficient and a phase ϕ. For uncoded communication systems, this phase should be optimized as ϕ∗(κ) to maximize the minimum Euclidean distance between points of the improper constellation, therefore minimizing the bit error rate (BER). For the more relevant case of coded communications, where the coded symbols are constrained to be in this family of improper constellations using ϕ∗(κ), it is shown theoretically and further corroborated by simulations that, except for a shaping loss of 1.53 dB encountered at a high signal-to-noise ratio (snr), there is no rate loss with respect to the improper Gaussian capacity. In this sense, the proposed family of constellations can be viewed as the improper counterpart of the standard proper M-QAM constellations widely used in coded communication systems.The work of Pedro M. Crespo and Ignacio Santamaria has been partially supported by the Ministerio de Economía y Competitividad (MINECO) of Spain and Agencia Estatal de Investigación (AEI)/(European Fund for Economic and Regional Development) FEDER funds of the E.U., under Grants TEC2013-47141-C4-R (RACHEL), TEC2016-75067-C4-4-R (CARMEN). The work of Christian Lameiro and Peter J. Schreier was supported by the German Research Foundation (DFG) under Grants LA 4107/1-1 and SCHR 1384/6-1