Hybrid Full-Duplex and Alternate Multiple Relay Selection and Beamforming in AF Cooperative Networks

In this paper, multiple relay selection and beamforming techniques are applied to a dual-hop full-duplex (FD) amplify-and-forward relaying network. We show that our proposed techniques allow the selection to be adaptive to the residual self-interference (SI) level for each of the available relays in the network. The adaptivity of our selection schemes is manifested through a hybrid system that is based on FD relaying and switching based on the overall channel conditions and the statistics of the residual SI channel for each of the relays. In particular, different proposed techniques are shown to be able to adaptively decide on when and how often the used relays should be switched in the case of overwhelming residual SI. Our results show that allowing such a fusion considerably improves the overall performance of the considered relaying scheme in terms of bit error rate compared with state-of-the-art relay selection schemes.This work was supported by the Qatar National Research Fund (a member of the Qatar Foundation) through GSRA under Grant #2-1-0601-14011.Scopu

Sparsity-aware multiple relay selection in large multi-hop decode-and-forward relay networks

In this paper, we propose and investigate two novel techniques to perform multiple relay selection in large multi-hop decode-and-forward relay networks. The two proposed techniques exploit sparse signal recovery theory to select multiple relays using the orthogonal matching pursuit algorithm and outperform state-of-the-art techniques in terms of outage probability and computation complexity. To reduce the amount of collected channel state information (CSI), we propose a limited-feedback scheme where only a limited number of relays feedback their CSI. Furthermore, a detailed performance-complexity tradeoff investigation is conducted for the different studied techniques and verified by Monte Carlo simulations.NPRP grant 6-070-2-024 from the Qatar National Research Fund (a member of Qatar Foundation)Scopu

Mechanical recycling of polylactide, upgrading trends and combination of valorization techniques

The upcoming introduction of polylactides in the fractions of polymer waste encourages technologists to ascertain its valorization at the best quality conditions. Mechanical recycling of PLA represents one of the most cost-effective methodologies, but the recycled materials are usually directed to downgraded applications, due to the inherent thermomechanical degradation affecting its mechanical, thermal and rheological performance. In this review, the current state of mechanical recycling of PLA is reported, with special emphasis on a multi-scale comparison among different studies. Additionally, the applications of physical and chemical upgrading strategies, as well as the chances to blend and/ or composite recycled PLA are considered. Moreover, the different valorization techniques that can be combined to optimize the value of PLA goods along its life cycle are discussed. Finally, a list of different opportunities to nurture the background of the mechanical recycling of PLA is proposed, in order to contribute to the correct waste management of PLA wastes

Outage performance of OFDM-based relaying systems under IQ imbalance

In the presence of different IQ imbalance (IQI) levels in the different nodes of a cooperative system, it becomes necessary to investigate the effect of each imbalance parameter on the performance. In this contribution, we investigate the outage performance of OFDM based Decode and Forward (DF) and Amplify and Forward (AF) systems over Rayleigh channels under IQI. In particular, approximate outage probability expressions are derived and the effect of different IQI parameters is analyzed for different ranges of SNR. The diversity order is also derived depending on the IQI levels. Furthermore, we explain when and where it is more beneficial to invest in compensating the imbalance or in increasing the transmission power. Based on the outage performance, a comparative study between AF, DF and direct link transmission techniques is also discussed depending on the IQI parameters, the SNR range, the transmission rate and the relays position.Scopu

Outage performance of incremental relaying networks with OFDM subcarriers mapping schemes

The scarcity of radio resources coupled with the high data rate demands in the last few years, induced incremental relaying techniques one of the most promising technologies. This is simply because it allows reaping spatial diversity and saving the channel resources at the same time. In this contribution, we make use of the Decode and Forward incremental relaying technique in OFDM cooperative systems by proposing and analyzing two subcarrier mapping algorithms. Based on outage probability, diversity order and power gain analysis, we derive the optimal parameters for these algorithms to enhance the outage performance of DF cooperative system by reducing the effect of error propagation. Also, an incremental relay sharing scheme is proposed allowing multiple sources to share the same relay and results in an improved spectral efficiency, which is critical given the high cost and scarcity of RF spectrum driven by the growing big data.Scopu

Novel selective OFDM-based cooperative techniques for multi-hop systems

In multi-hop systems, the theoretical outage probabilities and diversity orders of novel Orthogonal Frequency Division Multiplexing (OFDM) based Decode and Forward (DF) cooperative techniques are derived for different methods that manipulate the subcarriers in order to improve the performance, while overcoming practical implementation issues and using limited number of relays. First, the outage probability of the Basic Selective OFDM technique is derived for a multi-hop system without the high SNR approximations, and the effect of joint selection in the last two hops is investigated. Then, Subcarrier Ordering technique (SO) is introduced and proved its efficiency in substituting the error correction at the intermediate hops while reducing the computation complexity. Furthermore, we propose alike efficient cooperative methods making use of subcarrier mapping to trade-off between outage performance and practical implementation problems. We also analyze the computation complexity of the different algorithms and we prove that, at high SNR, the proposed Improved Selective OFDMA reduces the complexity by up to N (Number of subcarriers) times without affecting the outage performance when compared to Selective OFDMA.Scopu

Outage performance of cooperative systems under IQ imbalance

In this contribution, we investigate the outage performance of OFDM-based Decode and Forward (DF), Amplify and Forward (AF) and Controlled DF (CDF) cooperative systems under IQ Imbalance (IQI). In particular, tractable and compact approximate outage probability expressions are derived and the effect of the different IQI parameters is analyzed for different SNR ranges. Furthermore, by localizing the error floor in terms of IQI and SNR for each technique, we demonstrate when it is more beneficial to invest in increasing the transmission power or in compensating the imbalance. Moreover, we prove that the IQI compensation should be concentrated in the relay for some techniques and in the destination for some others. A comparative study between AF, DF, CDF and direct link transmission techniques is also conducted for different IQI parameters, SNR ranges, transmission rates and relay's position. Hence, this work may create a paradigm for future studies of more effective adaptive IQI compensation techniques that concentrate the compensation on the right IQI, SNR ranges, transceivers depending on the used transmission technique.Scopu

Sparsity-Aware multiple relay selection in large dual-hop decode-And-forward broadband relay networks

In this paper, three novel techniques are proposed and investigated for multiple relay selection in dual hop OFDM networks. These techniques are based on the exploitation of sparse signal recovery theory and on carefully-designed groupings of the subcarriers depending on the channel quality. In particular, the proposed techniques use the Orthogonal Matching Pursuit algorithm which enables them to outperform existing techniques in terms of both outage probability and computation complexity. Furthermore, a detailed performance-complexity tradeoff investigation is presented for the different studied techniques and verified by Monte Carlo simulations. 2016 IEEE.Scopu

Secondary users selection and sparse narrow-band interference mitigation in cognitive radio networks

Spectrum scarcity is a critical problem that may reduce the effectiveness of wireless technologies and services. To address this problem, different spectrum management techniques have been proposed in the literature such as overlay cognitive radio (CR) where the unlicensed users can share the same spectrum with the licensed users. The main challenges in overlay CR networks are the identification and detection of the Primary User (PU) signals in a multi-source narrow-band interference (NBI) scenario. Therefore, in this paper, we investigate the performance of an orthogonal frequency division multiplexing (OFDM) overlay CR network with Secondary Users (SUs) and subcarriers selection schemes. Three approaches for SUs and subcarriers Selection named Direct, Distributed and Incremental selection techniques are proposed in this paper to increase the expected signal to interference and noise ratio based on full or partial knowledge of the channel state information (CSI). We also show that Distributed selection techniques provide all the SUs equal chances to be selected without affecting the selection diversity gain. General as well as simplified outage probability expressions are derived and extensive simulations are conducted to evaluate the performance of the proposed techniques and support the theoretical derivations. To accommodate more SUs, a new approach for asynchronous NBI estimation and mitigation in CR networks is investigated. Without any prior knowledge of the NBI characteristics and based on sparse signal recovery theory, the proposed approach allows the PU to exploit the sparsity of the SUs interference to recover it and approach the interference-free limit over practical ranges of NBI power levels.This publication was made possible by NPRP grant 6-070-2-024 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors.Scopu

Secondary users selection and sparse narrow-band interference mitigation in cognitive radio networks

International audienceSpectrum scarcity is a critical problem that may reduce the effectiveness of wireless technologies and services. To address this problem, different spectrum management techniques have been proposed in the literature such as overlay cognitive radio (CR) where the unlicensed users can share the same spectrum with the licensed users. The main challenges in overlay CR networks are the identification and detection of the Primary User (PU) signals in a multi-source narrow-band interference (NBI) scenario. Therefore, in this paper, we investigate the performance of an orthogonal frequency division multiplexing (OFDM) overlay CR network with Secondary Users (SUs) and subcarriers selection schemes. Three approaches for SUs and subcarriers Selection named Direct, Distributed and Incremental selection techniques are proposed in this paper to increase the expected signal to interference and noise ratio based on full or partial knowledge of the channel state information (CSI). We also show that Distributed selection techniques provide all the SUs equal chances to be selected without affecting the selection diversity gain. General as well as simplified outage probability expressions are derived and extensive simulations are conducted to evaluate the performance of the proposed techniques and support the theoretical derivations. To accommodate more SUs, a new approach for asynchronous NBI estimation and mitigation in CR networks is investigated. Without any prior knowledge of the NBI characteristics and based on sparse signal recovery theory, the proposed approach allows the PU to exploit the sparsity of the SUs interference to recover it and approach the interference-free limit over practical ranges of NBI power levels