A channel allocation mechanism for cognitive radios (CRs) is proposed to enable maximum throughput and minimum forced termination probability for CR users (CUs) that are not capable of spectrum handover (SH). SH is an optimal collision-avoidance scheme used by CUs to evacuate from time-slots that will be used by primary users (PUs) based on time-slot assignment (TA) information obtained in advance. The proposed scheme has extensive applicability for CR-networks where SH is not possible owing to PU TA information not being obtainable in advance, which is very common. For CR-networks without SH capability, the proposed scheme enables remarkable throughput performance, equivalent to the level of when SH is used.Introduction: Owing to the rapidly increasing popularity of mobile devices, techniques that can maximise wireless channel utilisation are needed. In this Letter, an optimal cognitive radio (CR) channel allocation scheme is proposed. The objective of the proposed scheme is to minimise forced termination probability (P F ) and maximise throughput (r) of CR users (CUs). The channel is assumed to be time division multiple access (TDMA) based where primary users (PUs) have priority to access the M time-slots of the TDMA frame. The CUs can access the empty time-slots of the PUs, where each time-slot that a PU can use is divided into N CU sub-time-slots. Therefore, assuming that no PUs are using the TDMA frame, a total of NM time-slots can be used by CUs. When a PU accesses a time-slot, it will pre-empt the CUs that have been using that time-slot, which results in forced termination of CUs. When channel utilisation of the PUs is high, there will be very few vacant time-slots for CUs to access and therefore forced termination of CUs will occur unavoidably. Fig. 1 illustrates the PU and CU timeslots of a TDMA frame.
Irradiation of MoS 2 field-effect transistors (FETs) fabricated on Si/SiO 2 substrates with electron beams (e-beams) below 30 keV creates electron-hole pairs (EHP) in the SiO 2 , which increase the interface trap density (N it ) and change the current path in the channel, resulting in performance changes. In situ measurements of the electrical characteristics of the FET performed using a nano-probe system mounted inside a scanning electron microscope show that e-beam irradiation enables both multilayer and monolayer MoS 2 channels act as conductors. The e-beams mostly penetrate the channel owing to their large kinetic energy, while the EHPs formed in the SiO 2 layer can contribute to the conductance by flowing into the MoS 2 channel or inducing the gate bias effect. The analysis of the device parameters in the initial state and the vent-evacuation state after e-beam irradiation can clarify the effect of the interplay between the e-beam-induced EHPs and ambient adsorbates on the carrier behavior, which depends on the thickness of the MoS 2 layer. DC and low frequency noise analysis reveals that the e-beam-induced EHPs increase N it from 10 9 -10 10 to 10 11 cm −2 eV −1 in both monolayer and multilayer devices, while the interfacial Coulomb scattering parameter α SC increases by three times in the monolayer and decreases to one-tenth of its original value in the multilayer. In other words, an MoS 2 layer with a thickness of ~30 nm is less sensitive to adsorbates by surface screening. Thus, the carrier mobility in the monolayer device decreases from 45.7 to 40 cm 2 V −1 s −1 , while in the 30 nm-thick multilayer device, it increases from 4.9 to 5.6 cm 2 V −1 s −1 . This is further evidenced by simulations of the distribution of interface traps and channel carriers in the MoS 2 FET before and after e-beam irradiation, demonstrating that Coulomb scattering decreases as the effective channel moves away from the interface.
A feasible fabrication technique for nanomachines that are based on carbon nanotubes (CNTs) is demonstrated. Direct nanowiring of CNTs between micrometer islands is reported to have been achieved by a growth barrier technique, which prevents vertical growth of the CNTs, i.e., towards the substrate. The result is “straight” or “Y‐shaped” CNT bridges between predefined electrodes (see Figure).
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