In this paper, we present an interference model for cognitive radio (CR) networks employing power control, contention control or hybrid power/contention control schemes. For the first case, a power control scheme is proposed to govern the transmission power of a CR node. For the second one, a contention control scheme at the media access control (MAC) layer, based on carrier sense multiple access with collision avoidance (CSMA/CA), is proposed to coordinate the operation of CR nodes with transmission requests. The probability density functions of the interference received at a primary receiver from a CR network are first derived numerically for these two cases. For the hybrid case, where power and contention controls are jointly adopted by a CR node to govern its transmission, the interference is analyzed and compared with that of the first two schemes by simulations. Then, the interference distributions under the first two control schemes are fitted by log-normal distributions with greatly reduced complexity. Moreover, the effect of a hidden primary receiver on the interference experienced at the receiver is investigated. It is demonstrated that both power and contention controls are effective approaches to alleviate the interference caused by CR networks. Some in-depth analysis of the impact of key parameters on the interference of CR networks is given via numerical studies as well.
Considerable attention has been paid to the enantiomeric resolution and toxicity of some chiral organophosphorous pesticides (OPs) with one asymmetric center, but research concerning chiral OPs with two asymmetric centers is still limited. In the present study, the stereoisomeric separation and toxicity of fosthiazate, a chiral OP with two asymmetric centers on phosphorus and carbon atoms, was investigated. All four stereoisomers of fosthiazate were separated successfully with a Chiralpak(R) AD [amylase tris(3,5-dimethyl-phenyl carbamate)] column on high-performance liquid chromatography. The resolved isomers and the pairs of enantiomers were further distinguished using a circular dichroism detector and an optical rotation detector, designating the first (pk1) and third (pk3) eluted peaks as one pair of enantiomers and the second (pk2) and fourth (pk4) peaks as the other pair. The developed method was used to prepare microquantities of individual stereoisomers that were used for in vitro and in vivo bioassays. The inhibition potencies of the stereoisomers against acetylcholinesterase of Electrophorus electricus were slightly stereoselective, with a maximum difference of 1.4-fold among the isomers. A 3.1-fold difference, however, was observed in the acute toxicity of isomers to Daphnia magna. The 48-h toxicity of isomers to D. magna followed the order pk1 > pk2 > pk4 > racemate approximately pk3. The stereoselective toxicity to D. magna found in fosthiazate suggests that the environmental safety of fosthiazate should be evaluated on the basis of its individual isomers.
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