An accurate two dimensional Markov chain model for IEEE 802.11n DCF

According to the amendment 5 of the IEEE 802.11 standard, 802.11n still uses the distributed coordination function (DCF) access method as mandatory function in access points and wireless stations (essentially to assure compatibility with previous 802.11 versions). This article provides an accurate two dimensional Markov chain model to investigate the throughput performance of IEEE 802.11n networks when frame aggregation and block acknowledgements (Block-ACK) schemes are adopted. Our proposed model considered packet loss either from collisions or channel errors. Further, it took anomalous slots and the freezing of backoff counter into account. The contribution of this work was the analysis of the DCF performance under error-prone channels considering both 802.11n MAC schemes and the anomalous slot in the backoff process. To validate the accuracy of our proposed model, we compared its mathematical simulation results with those obtained using the 802.11n DCF in the network simulator (NS-2) and with other analytical models investigating the performance of 802.11n DCF. Simulation results proved the accuracy of our model.     

Improvement of downlink LTE system performances using nonlinear equalization methods based on SVM and Wiener–Hammerstein

In this paper, we present versatile nonlinear equalizer based on support vector machine for a standard downlink LTE link. The nonlinear effects are introduced through a solid state power amplifier. The equalizer is compared with a linear RLS and nonlinear Wiener–Hammerstein (W–H) for 16QAM–MIMO–OFDM transmission, revealing improved performance at 1 Mb/s, by 3 and 2 dB in SNR, respectively, when targeting a EVM (dB) of ?60 dB. SVM equalization also outperforms the Wiener–Hammerstein by 1 dB in SNR.     

Adaptive power control algorithm with stabilization zone for third generation mobile networks

In this paper, we propose an evolution of adaptive power control algorithms for Universal Mobile Telecommunications System (Umts). The proposed mechanism uses an adaptive power step to carry out with high fluctuations in the radio interface. Furthermore, we propose the integration of a stabilization zone where the transmitted power is kept constant to reduce oscillations around the target QoS level. In this algorithm, only the interpretation of power control feedback commands is ameliorated by adding some intelligence to mobiles and base stations. Moreover, the standardized radio interface protocol is used without modification. The algorithm can be used in both link directions of the Frequency Division Duplex (Fdd) mode and in the downlink of the Time Division Duplex (Tdd) mode, where closed-loop power control algorithms are used. Simulation results show that the proposed algorithm decreases the outage probability and reduces drastically the transmitted power, which are the main objectives of power control algorithms.     

Robust geocast routing protocols for safety and comfort applications in VANETs

Routing in cooperative vehicular networks is a challenging task because of high mobility of vehicles and difficulty of localization. In this paper, we study the geocast routing problem in Vehicular Ad‐hoc NETworks (VANETs), which aims at delivering data to a specific group of mobile vehicles identified by their geographical location. Although many geocast routing protocols have been proposed, only partial inherent constraints of VANETs (such as mobility, internal network fragmentation problem, external network fragmentation problem, and overload) are taken into account. Therefore, we propose two novel and robust geocast routing protocols: the first one, called Robust Geocast Routing Protocol for Safety Applications (RGRP‐SA), is dedicated to road safety applications, while the second, namely, Robust Geocast Routing Protocol for Comfort Applications (RGRP‐CA), is designed for comfort applications. Simulations conducted in NS‐2 demonstrate that our safety‐oriented RGRP‐SA protocol outperforms Inter‐Vehicle Geocast protocol and Mobicast Routing Protocol in VANETs by sending up to 25% more packets, cutting the end‐to‐end delay in half, and solving the internal network fragmentation problem. Besides, it is also shown that our comfort‐oriented RGRP‐CA protocol serves well comfort applications with only light overhead by solving internal and external network fragmentation problems and providing more reliable data delivery (with a 25% higher packet delivery ratio) and higher network throughput utilization in comparison with Mobicast with Carry‐and‐Forward protocol. Copyright © 2015 John Wiley & Sons, Ltd.     

New amphoteric surfactants containing the 2-perfluoroalkyl 2-hydroxy ethyl group and an amino acid residue

Amphoteric perfluoroalkylated surfactants containing a hydroxyl group were prepared by the addition of 2-perfluoroalkyl-1,2-epoxy ethane to a starting (L,D or L) amino acid (glycine, alanine, β-alanine, serine, 2-amino butyric acid, norvaline, norleucine, methionine, sarcosine, aspartic acid, glutamic acid).     

Activity concentrations and mean annual effective dose from gamma-emitting radionuclides in the Lebanese diet

Since the primary factor contributing to the internal effective dose in the human organism is contaminated food, the control of radionuclides in food represents the most important means of protection. This study was conducted to determine the levels of the dietary exposure of the Lebanese population to gamma-emitting radioisotopes. The activity concentrations of gamma-emitting radioisotopes have been measured in food samples that represent the market basket of an adult urban population in Lebanon. The artificial radionuclide (137)Cs was measured above detection limits in only fish, meat and milk-based deserts. The most abundant natural radionuclide was 40K (31-121 Bq kg(-1)), with the highest content in fish and meat samples. The annual mean effective dose contributed by 40K in the reference typical diet was estimated equal to 186 microSv y(-1), a value reasonably consistent with findings reported by several other countries.     

Alliance-based clustering scheme for group key management in mobile ad hoc networks

Several protocols have been proposed to deal with the group key management problem in mobile ad hoc networks (MANETs). Most of these protocols organize the network into clusters to reduce the cost of key refresh or rekeying. Rekeying constitutes a challenging issue in group key management because it must be launched whenever the constitution of the group is altered following a leave or a join operation. However, cluster maintenance may also generate significative communication overhead. So, the clustering algorithm is an important factor in the performance of any key management solution. A clustering algorithm that ensures stable clusters in spite of mobility is very appreciable in mobile ad hoc networks. In fact, all the overhead due to the traffic generated by cluster adjustments and the related rekeying procedures will be saved. As far as we know, no existing clustering algorithm takes into account self-stabilization while relying on the mobility resilience of graph alliances. In this paper, we propose a fully distributed and self-stabilizing clustering algorithm for key management in MANETs where each cluster is an alliance.