No abstract

No abstract

Due to the increasingly complicated communication scenarios and network architectures as well as growing traffic demands for high speed connectivity, dynamic spectrum allocation in fifth generation (5G) networks becomes insufficient to guarantee the satisfaction of main network requirements in terms of spectrum efficiency (SE), scalability, delay, and energy efficiency (EE). Enormous multiple access schemes and cognitive radio (CR) network scenarios come to fulfill these requirements and enhance network functionalities. With multiple access schemes, users are able to transmit their data streams simultaneously under maximum capacity constraints. On the other hand, vacant spectrum holes are exploited in an opportunistic manner via CR and software defined radio. In order to exploit these spectrum holes as well as meeting different network requirements, several multiple access techniques have been presented that have been initiated through the adoption of orthogonal multiple access (OMA) scheme. Additionally, non-orthogonal multiple access (NOMA) and space division multiple access (SDMA) are presented to achieve a promising multiplexing gain as well as to address the inefficient spectrum utilization incurred with OMA schemes. However, such multiplexing gain is limited as it depend on the channel conditions. Accordingly, a generalized multiple access scheme has been presented recently, namely rate splitting multiple access (RSMA), to further enhance the SE. In this paper, we provide a comprehensive study regarding the key multiple access schemes presented for CRNs to further enhance the use of spectral resources, and additionally highlights the key implementation challenges and the enabling techniques addressed to overcome it. We have given a special attention to the enhances provided by RSMA as compared with OMA, SDMA, and NOMA techniques. Finally, some open issues are spotted to shed lights on the need for further studies and future research efforts.INDEX TERMS Multiple access, SDMA, NOMA, RSMA, cognitive radio (CR), 5G.

No abstract

Solvent effects on the kinetics of aquation of [Co(dien)(en)Cl] have been investigated within the temperature range (40 -60°C) in acetone-water and ethanol-water media of varying solvent compositions up to 60% by weight of the organic solvent component. The variation of the activation parameters (⌬G # , ⌬ H # , and ⌬S # ) with the mole fraction of the organic solvent component was analyzed and discussed. The isokinetic plots were linear in both solvent systems and revealed the existence of compensation effect due to a strong solute-solvent interactions. The correlation of log k with the ionizing power (Y) was found to be linear in both solvent mixtures. Moreover, the correlation of log k with was linear in case of ethanol-water medium while in case of acetone-water it was nonlinear.

Dicarboxylic Acids / Solvent Effects / Dissociation Constant / Thermodynamics / Relative PermittivityThe pK 1 and pK 2 values of malic, malonic, maleic, succinic, tartaric, and phthalic acids have been determined in ethane-1,2-diol-water media of varying solvent compositions (0-50 wt % ethane-1,2-diol) within the temperature range (25-65 • C). An extrathermodynamic analysis of the obtained results have been examined, in the light of ∆H • -∆S • relationship. The pK values have been correlated with the relative permittivity of the medium according to Born's electrostatic model.

No abstract

No abstract