Stable Throughput of Cognitive Radios With and Without Relaying Capability

A scenario with two single-user links, one licensed to use the spectral resource (primary) and one unlicensed (secondary or cognitive), is considered. According to the cognitive radio principle, the activity of the secondary link is required not to interfere with the performance of the primary. Therefore, in this paper, it is assumed that the cognitive link accesses the channel only when sensed idle. Moreover, the analysis includes: (1) random packet arrivals; (2) sensing errors due to fading at the secondary link; (3) power allocation at the secondary transmitter based on long-term measurements. In this framework, the maximum stable throughput of the cognitive link (in packets/slot) is derived for a fixed throughput selected by the primary link. The model is modified so as to allow the secondary transmitter to act as a ldquotransparentrdquo relay for the primary link. In particular, packets that are not received correctly by the intended destination might be decoded successfully by the secondary transmitter. The latter can, then, queue and forward these packets to the intended receiver. A stable throughput of the secondary link with relaying is derived under the same conditions as before. Results show that benefits of relaying strongly depend on the topology (i.e., average channel powers) of the network.     

Avoiding Transient Loops During the Convergence of Link-State Routing Protocols

When using link-state protocols such as OSPF or IS-IS, forwarding loops can occur transiently when the routers adapt their forwarding tables as a response to a topological change. In this paper, we present a mechanism that lets the network converge to its optimal forwarding state without risking any transient loops and the related packet loss. The mechanism is based on an ordering of the updates of the forwarding tables of the routers. Our solution can be used in the case of a planned change in the state of a set of links and in the case of unpredictable changes when combined with a local protection scheme. The supported topology changes are link transitions from up to down, down to up, and updates of link metrics. Finally, we show by simulations that sub-second loop-free convergence is possible on a large Tier-1 ISP network.     

Optical and photoelectrical properties of AgCd<Subscript>2</Subscript>GaS<Subscript>4</Subscript> single-crystal compounds

The poorly known AgCd₂GaS₄ single-crystal compounds that crystallize in a rhombic structure (space group Pmn2₁) are studied. Deviations from the stoichiometric composition of the samples and random occupation of the cation sublattice sites by Ag and Ga ions result in violation of long-range order in the atomic arrangement and make the AgCd₂GaS₄ compounds structurally closer to disordered systems. In this case, it is found that the fundamental optical-absorption edge is smeared and shifted to longer wavelengths, and is adequately described by the Urbach rule. In addition, a broadening of the spectral peaks of photoconductivity and luminescence is observed. The concentration of charged point defects responsible for the smearing of the absorption edge is calculated. It is found to be 1.2×10²⁰ cm^?3. The AgCd₂GaS₄ single crystals are photosensitive semiconductors. From the position of the absorption edge, the optical band gap of the compound is estimated (E _g0 = 2.28 eV at T = 297 K). The photoluminescence spectra of the AgCd₂GaS₄ single crystals are similar to the spectra of defect-containing CdS single crystals; for the AgCd₂GaS₄ crystals, the emission peaks are shifted to longer wavelengths with respect to the peaks for CdS crystals by Δλ = 0.06?0.1 μm. From the analysis of the experimental data, some conclusions on the nature of photoactive centers in AgCd₂GaS₄ compounds are drawn.     

Ferromagnetic cobalt and iron top contacts on an organic semiconductor: Evidence for a reacted interface

The interface formation between the organic semiconductor copper-phthalocyanine and the ferromagnets Co and Fe has been investigated for the case of metal deposition onto the organic film using photoemission as well as X-ray absorption spectroscopy. Such interfaces might allow the injection of spin polarized currents into organic semiconductors. Our data reveal the formation of interfaces which are characterized by chemical reactions in the interfacial region.     

Joint optimization of delay and congestion in wavelength-routed optical networks using genetic algorithms

A new multipurpose genetic algorithm, based on Pareto optimality, is proposed to design logical topologies for wavelength-routed optical networks with the aim of minimizing both the congestion and the end-to-end delay. Simulation results show its efficiency when compared with other previously proposed algorithms, achieving in most cases optimal or near-optimal solutions, and in less time than other methods. Moreover, since the algorithm relies on Pareto optimality, not only does it obtain a single logical topology but a set of them, so that the network designer can easily select the most appropriate one according to the current network requirements.     

A behavioral-based multi-agent optimization algorithm for system level radio design

This paper introduces a multi-agent behavioral-based optimization algorithm for system level radio design. Making multi-standard wireless communication receivers that meet their specs while keeping the requirements of the individual blocks as relaxed as possible is the goal of this algorithm. In order to achieve this goal a “divide and conquer” approach is proposed. Different agents focus on different objectives that are pursued in parallel. Agents adopt different behaviors depending on the status of the environment and their interaction with other agents. Agents are cooperative by default as they try to meet their spec without making changes that affect other agents. However, more aggressive behaviors that lead to global changes can be adopted when needed. The interaction between these simple entities yields an emergent behavior able to deal smoothly with the complexity of the problem at hand.     

<Emphasis Type="Italic">f</Emphasis><Subscript>T</Subscript> × <Emphasis Type="Italic">BV</Emphasis><Subscript>cbo</Subscript> product modeling for SiGe:C HBTs

The specific aspects of SiGe:C HBT process and device simulation using TCAD are discussed. Cut-off frequency f _T and collector junction breakdown voltage BV _cbo dependences on carbon concentration in SiGe base area are investigated. The boron and carbon profiles in SiGe base are obtained to provide a trade-off between gain, cut-off frequency and break-down voltage. High values of f _T × BV _cbo product were achieved.     

Growth of 4H-polytype silicon carbide ingots on (10 \bar 1 0) seeds

The growth of 4 H-polytype silicon carbide ingots by the modified Lely method on (10 $ \bar 1 The growth of 4 H-polytype silicon carbide ingots by the modified Lely method on (100) seeds has been investigated. It is shown that this seed plane allows intense ingot outgrowth. Single-crystal ingots up to 60 mm in diameter can be obtained at growth rates below 0.6 mm/h. Investigation of the defect structure showed that the grown ingots are practically micropipe-free but contain stacking faults and carbon inclusions. Original Russian Text ? D.D. Avrov, A.V. Bulatov, S.I. Dorozhkin, A.O. Lebedev, Yu.M. Tairov, 2008, published in Fizika i Tekhnika Poluprovodnikov, 2008, Vol. 42, No. 12, pp. 1483–1487.     

Energy cost and error performance of range-aware, anchor-free localization algorithms

Many applications that use sensor data from a wireless sensor network (WSN) require corresponding node position information as well. Therefore, it is not surprising that a common figure of merit for localization algorithms is the accuracy of the position estimate produced. Similarly, the amount of communication required by a localization algorithm is often of paramount interest as well since it is common knowledge that communication expends the most energy in a WSN. However, localization algorithms seldom characterize their communication cost. Furthermore, when they do it is often merely qualitative and is typically described as “expensive”. For two types of range-aware, anchor-free localization algorithms we found the opposite to be true. Rather than being expensive, the communication costs were quite modest. So much so that we maintain range-aware, anchor-free localization algorithms should be chosen on the basis of the accuracy required by the intended application independent of the communication cost.In this paper, we examine the effect of node degree, node distribution, range error and network size on distance error and communication cost for both incremental and concurrent versions of range-aware, anchor-free algorithms. The concurrent algorithm is twice as accurate as the incremental, but less efficient. Furthermore, node degree influences the energy cost of the algorithms the most, but neither algorithm uses more than a surprisingly small 0.8% of a 560 mA h battery. This result indicates less energy efficient localization algorithms can be tolerated, especially if they provide better accuracy. Furthermore, if energy does need to be conserved, there is not much savings available within the localization algorithm and savings must be found in other areas such as the MAC protocol or routing algorithm.     

Enhanced efficiency and frequency assignment by optimizing the base stations location in a mobile radio network

Most mobile radio networks have been planned based on the classical cellular concept. However, alternative planning strategies that lead to more efficient network configurations are necessary due to the fact that the traffic density is generally far from constant throughout the service area, making necessary the relocation of base stations inside the traffic hotspots. If the traffic is characterized in a discrete way, the optimization of base stations location resembles vector quantization, a well-known problem in signal processing. In this paper, we use this analogy to propose a mobile radio network planning algorithm. Simulation results show that higher trunking efficiency as well as improved frequency assignment can be obtained if an existing mobile radio network is redesigned using the presented strategy. Raúl Chávez-Santiago was born in Oaxaca City, Mexico. He obtained the B.Sc. degree in communications and electronics engineering in 1997 from the School of Mechanical and Electrical Engineering (ESIME-IPN), and the M.Sc. degree in electrical engineering in 2001 from the Center of Research and Advanced Studies of the National Polytechnic Institute (CINVESTAV-IPN) in Mexico City. He has professional experience as Computer Networking Engineer, and Research and Teaching Assistant. Currently, he is completing his studies toward the Ph.D. degree at Ben-Gurion University (BGU) of the Negev, Israel. In 2002, he received the URSI Young Scientist Award. His main research interests are the optimal planning of radio communication networks, the electromagnetic compatibility of terrestrial and satellite radio systems, and the efficient use of the radio spectrum. He is an IEEE and IEICE student member, and a URSI radio scientist. Avi Raymond was born in Yavne, Israel. He received the B.Sc. and M.Sc. degrees in electrical engineering from Ben-Gurion University (BGU) of the Negev in 2000 and 2005, respectively. He worked for two years in a communication company in the field of Telephony and Broadband Services. He also worked as a Research Assistant in the communications laboratory at BGU. He currently works at Elta Electronic Industries Ltd. as System Engineer and pursues studies toward a second M.Sc. degree in systems engineering at the Technion Institute of Technology, Haifa, Israel. His research interests are the optimization algorithms for frequency assignment in cellular networks. Vladimir Lyandres was born in 1944, in Vologda, Russia. He received the M.Sc. degree in communications engineering in 1966 and the Ph.D. degree in communications theory in 1972 from the State University of Telecommunications (SUT), Saint Petersburg, Russia. He was with SUT until 1990 holding a position of Senior Scientific Associate and working on research and development of digital transmission systems, modeling of radio communication channels and algorithms of frequency planning for broadcasting and cellular systems. Since 1991 he holds a position of Researcher at the communications laboratory and Professor at the Department of Electrical and Computer Engineering, Ben-Gurion University (BGU) of the Negev, Israel. His research interests include synthesis of Markov models, combinatorial optimization, and adaptive power loading. He is a senior member of IEEE and member of IEICE.