Carrier Sense Random Packet CDMA Protocol in Dual-Channel Networks

Code resource wastage is caused by the reason that many hopping frequency (FH) sequences are unused, which occurs under the condition that the number of the actual subnets needed for the tactical network is far smaller than the networking capacity of code division net¬working. Dual-channel network (DCN), consisting of one single control channel and multiple data channels, can solve the code resource wastage effectively. To improve the anti-jamming capability of the control channel of DCN, code division multiple access (CDMA) technology was introduced, and a carrier sense random packet (CSRP) CDMA protocol based on random packet CDMA (RP-CDMA) was proposed. In CSRP-CDMA, we provide a carrier sensing random packet mechanism and a packet-segment acknowledgement policy. Furthermore, an analytical model was developed to evaluate the performance of CSRP-CDMA networks. In this model, the impacts of multi-access interference from both inter-clusters and intra-clusters were analyzed, and the mathematical expressions of packet transmission success probability, normalized network throughput and signal interference to noise ratio, were also derived. Analytical and simulation results demonstrate that the normalized network throughput of CSRP-CDMA outperforms traditional RP-CDMA by 10%, which can guarantee the resource utilization efficiency of the control channel in DCNs

Random Topologies and the emergence of cooperation: the role of short-cuts

We study in detail the role of short-cuts in promoting the emergence of cooperation in a network of agents playing the Prisoner's Dilemma Game (PDG). We introduce a model whose topology interpolates between the one-dimensional euclidean lattice (a ring) and the complete graph by changing the value of one parameter (the probability p to add a link between two nodes not already connected in the euclidean configuration). We show that there is a region of values of p in which cooperation is largely enhanced, whilst for smaller values of p only a few cooperators are present in the final state, and for p \rightarrow 1- cooperation is totally suppressed. We present analytical arguments that provide a very plausible interpretation of the simulation results, thus unveiling the mechanism by which short-cuts contribute to promote (or suppress) cooperation

Spatial oscillations in the spontaneous emission rate of an atom inside a metallic wedge

A method of images is applied to study the spontaneous emission of an atom inside a metallic wedge with an opening angle of $\pi/N$, where N is an arbitrary positive integer. We show the method of images gives a rate formula consistent with that from Quantum Electrodynamics. Using the method of images, we show the correspondence between the oscillations in the spontaneous emission rate and the closed-orbits of emitted photon going away and returning to the atom inside the wedge. The closed-orbits can be readily constructed using the method of images and they are also extracted from the spontaneous emission rate.Comment: 8 figure

Observation of a cyclotron harmonic spike in microwave-induced resistances in ultraclean GaAs/AlGaAs quantum wells

We report the observation of a colossal, narrow resistance peak that arises in ultraclean (mobility 3X10^7cm^2/Vs) GaAs/AlGaAs quantum wells (QWs) under millimeterwave irradiation and a weak magnetic field. Such a spike is superposed on the 2nd harmonic microwave-induced resistance oscillations (MIRO) but having an amplitude > 300% of the MIRO, and a typical FWHM ~50 mK, comparable with the Landau level width. Systematic studies show a correlation between the spike and a pronounced negative magnetoresistance in these QWs, suggesting a mechanism based on the interplay of strong scatterers and smooth disorder. Alternatively, the spike may be interpreted as a manifestation of quantum interference between the quadrupole resonance and the higher-order cyclotron transition in well-separated Landau levels.Comment: 4pages, 4figure

Wetenskap op weg na 'n nuwe identiteit en samewerking

A traditional approach in Calvinist philosophy is to start with an analysis of cosmic diversity in order to find the identity of a particular phenomenon, and then to end with an account of the inter-relatedness between the phenomenon and other things. Although Stoker's philosophy tries to keep a balance between diversity and coherence as an entrée to reality - his later publications reflected a remarkable shift hy emphasising the coherence and contextual meaning of things. Taking the coherence and contextual meaning of things as the point of departure, this article explores a converse version of the traditional approach by using coherence (and not identity) as a means to sound out some new meanings of science and scientific co-operation. An indication of transdisciplinary co-operation is rendered in terms of the preponderance of reality‘s call upon man to be known and cared for. The article ends with a few examples of unusual concepts that may serve as a revised index to identify world-friendly and humane scientific activities

Toekomskunde as morele verantwoordelikheid

Vir meeste van ons kortsigtiges het die jaar 2000 ’n veraf, ma- giese en newelagtige assosiasie. Ons praat baie makliker van ons er- varing van en herinnering aan die Smutsregering en die Tweede Wêreldoorlog wat 32 jaar gelede geëindig het. Maar ons voer vol- struispolitiek oor die jaar 2000 wat maar 23 jaar om die draai lê. Oor 23 jaar is die huidige geslag studente so oud soos baie van ons huidige personeel.

Grepe uit die eksistensialistiese etiek

In die jongste tyd is die moderne mens doodbang vir alle etiese programme, vir ’n Etiek waarin vir alle mense en tye voorgeskrewe reels, hoe ’n mens in elke situasie moet handel, gegee word

Piezoelectric mechanism of orientation of stripe structures in two-dimensional electron systems

A piezoelectric mechanism of orientation of stripes in two-dimensional quantum Hall systems in GaAs heterostructures is considered. The anisotropy of the elastic moduli and the boundary of the sample are taken into account. It is found that in the average the stripes line up with the [110] axis. In double layer systems the wave vector of the stripe structure rotates from the [110] to [100] axis if the period of density modulation becomes large than the interlayer distance. From the experimental point of view it means that in double layer systems anisotropic part of resistivity changes its sign under variation of the external magnetic field.Comment: 8 page