Outage Performance and Cooperative Diversity Under Amplify and Forward Relaying in Cognitive Radio Networks

This paper investigates the outage and diversity performance in a cognitive radio (CR) network, in which data are transmitted through amplify-and-forward protocol. A fixed CR node is selected as relay to combat fading effect. The CR user transmits or relays data when the result of cooperative spectrum sensing indicates that primary user (PU) channel is free and the relay node stops performing the protocol as soon as the PU is detected to be active. A closed-form expression of outage probability for the CR network is calculated based on which diversity order is derived. The results show that the proposed scheme suffers a loss in diversity order. To compensate for this loss, a repetition-based scheme is proposed in a delay-insensitive system at the cost of the challenge in implementation. In this case, we evaluate the outage performance and the average transmit duration and then compare them with the ones in the delay-sensitive system. Asymptotic analysis is also presented to show the outage performance advantage of using a relay over the one without using it. The numerical results presented in the paper verify our analysis.     

OMVPE growth of P-type GaN using solution Cp2Mg

Bis(cyclopentadienyl)magnesium (Cp₂Mg) is a common source for p-type doping in GaN and AlInGaP materials. It is a white crystalline solid with very low vapor pressure, leading to transport problems similar to solid trimethyindium (TMI). Some of these problems can be alleviated by a newly developed source-solution magnesocene, Cp₂Mg, dissolved in a solvent that is essentially nonvolatile. In this paper, we report the growth and comparative results of Mg-doped GaN grown by OMVPE using solid and solution Cp₂Mg. Using both sources, we optimized parameters to obtain high-quality GaN growth with hole concentrations up to 1 10¹⁸/cm³.     

Laser-Printed Plasmonic Metasurface Supporting Bound States in the Continuum Enhances and Shapes Infrared Spontaneous Emission of Coupled HgTe Quantum Dots

In order to advance the development of quantum emitter-based devices, it is essential to enhance light-matter interactions through coupling between semiconductor quantum dots with high quality factor resonators. Here, efficient tuning of the emission properties of HgTe quantum dots in the infrared spectral region is demonstrated by coupling them to a plasmonic metasurface that supports bound states in the continuum. The plasmonic metasurface, composed of an array of gold nanobumps, is fabricated using single-step direct laser printing, opening up new opportunities for creating exclusive 3D plasmonic nanostructures and advanced photonic devices in the infrared region. A 12-fold enhancement of the photoluminescence in the 900–1700 nm range is observed under optimal coupling conditions. By tuning the geometry of the plasmonic arrays, controllable shaping of the emission spectra is achieved, selectively enhancing specific wavelength ranges across the emission spectrum. The observed enhancement and shaping of the emission are attributed to the Purcell effect, as corroborated by systematic measurements of radiative lifetimes and optical simulations based on the numerical solution of Maxwell's equations. Moreover, coupling of the HgTe photoluminescence to high quality factor modes of the metasurface improves emission directivity, concentrating output within an ≈20° angle.     

Analysis of delay mean and variance of collision-free WDM rings with segment recirculation of blocked traffic

In Tunable-Transmitter Fixed-Receiver (TT-FR)-based Wavelength Division Multiplexed (WDM) ring topologies, each node is provided with a dedicated wavelength (home channel) for reception, which must be shared by the upstream nodes willing to communicate with it. Thus, to avoid channel collisions, it is necessary to define a Medium Access Control (MAC) mechanism that arbitrates access to a given destination wavelength. This work proposes and analyses a simple MAC mechanism that avoids channel collisions by recirculating traffic on the upstream ring segment where congestion was detected. Essentially, whenever a given node has got any traffic to transmit, it must first block access to in-transit traffic, which is reflected back to the upstream node over a second optical fibre. Such blocked traffic is given a second chance to pass through the congested node after a round segment delay, thus making use of the ring topology as buffering units. This work analyses the performance operation of such a MAC protocol under two policies applied to recirculated traffic: (1) recirculation bypass and (2) recirculation store-and-forward.     

Generalized correlation-delay-shift-keying scheme for noncoherent chaos-based communication systems

In this paper, we propose a generalized correlation-delay-shift-keying (GCDSK) scheme for noncoherent chaos-based communications. In the proposed scheme, several delayed versions of a chaotic signal are first produced. Some of them will be modulated by the binary data to be transmitted. The delayed signals will then be added to the original chaotic signal and transmitted. At the receiver, a simple correlator-type detector is employed to decode the binary symbols. The approximate bit error rate (BER) of the GCDSK scheme is derived analytically based on Gaussian approximation. Simulations are performed and compared with the noncoherent correlation-delay-shift-keying (CDSK) and differential chaos-shift-keying (DCSK) modulation schemes. The effects of the spreading factor, length of delay, and the number of delay units on the BER are fully studied. It is found that GCDSK can achieve better BER performance than DCSK under reasonable bit-energy-to-noise-power-spectral-density ratios.     

中国安防市场2009年料增长7.5%住宅系统增强市场前景

据iSuppli公司,由于中国政府投资于安防领域以及消费者青睐住宅安全系统,中国安防和监视设备市场有望经受住经济衰退的冲击,并在2009年及以后不断扩张. iSuppli公司预测,中国的安防产业将在2009年达到189亿美元,比2008年时的175亿美元增长7.5%.到2013年,该市场将扩大到265亿美元,2008-2013年的年复合增长率可达8.6%.相比之下,2008年全球安防市场的营业收入预计萎缩2%.     

Metamorphic AlInAs/GaInAs HEMTs on GaAs Substrates by MOCVD

Metamorphic AlInAs/GaInAs high-electron mobility transistors with very good device performance have been grown by metal-organic chemical vapor deposition (MOCVD), with the introduction of an effective multistage buffering scheme. Measured room-temperature Hall mobilities of the 2-DEG were over 8000 cm²/V ldr s with sheet carrier densities larger than 4 times 10¹² cm^-2. Transistors with 1-mum gate length exhibited transconductance up to 626 mS/mm. The unity current gain cutoff frequency f_T and the maximum oscillation frequency f_max were 39.1 and 71 GHz, respectively. These results are very encouraging toward the manufacturing of metamorphic devices on GaAs substrates by MOCVD.     

A Tree-Based Data Collecting Network Structure for Wireless Sensor Networks

In a sensor network with a large number of densely populated sensor nodes, a single target of interest may be detected by multiple sensor nodes simultaneously. Data collected from the sensor nodes are usually highly correlated, and hence energy saving using in-network data fusion becomes possible. A traditional data fusion scheme starts with dividing the network into clusters, followed by electing a sensor node as cluster head in each cluster. A cluster head is responsible for collecting data from all its cluster members, performing data fusion on these data and transmitting the fused data to the base station. Assuming that a sensor node is only capable of handling a single node-to-node transmission at a time and each transmission takes T time-slots, a cluster head with n cluster members will take at least nT time-slots to collect data from all its cluster members. In this paper, a tree-based network structure and its formation algorithms are proposed. Simulation results show that the proposed network structure can greatly reduce the delay in data collection.     

Analytical performance of M-ary time-hopping orthogonal PPM UWB systems under multiple access interference

In this paper, we extend the characteristic-function approach to derive an exact symbol error rate expression for the M-ary time-hopping orthogonal pulse position modulation (THPPM) ultra-wideband (UWB) system under the interference from multiple users. Monte Carlo simulations are then used to validate the analytical solutions. The results are found to be valid even when that the chip width is greater than the pulse duration. Moreover, the analytical results are compared to those found by Gaussian approximation and the Gaussian Quadrature Rule method. We further show that compared to a binary system, M-ary systems can produce better error rate performance.