Performance evaluation of a C-RAN supporting a mixture of random and quasi-random traffic

Wireless Networks - In this paper we consider a cloud radio access network (C-RAN) where the remote radio heads (RRHs) are separated from the baseband signal processing servers, named baseband...     

Ultrasmall Nanoplatforms as Calcium‐Responsive Contrast Agents for Magnetic Resonance Imaging

The preparation of ultrasmall and rigid platforms (USRPs) that are covalently coupled to macrocycle‐based, calcium‐responsive/smart contrast agents (SCAs), and the initial in vitro and in vivo validation of the resulting nanosized probes (SCA‐USRPs) by means of magnetic resonance imaging (MRI) is reported. The synthetic procedure is robust, allowing preparation of the SCA‐USRPs on a multigram scale. The resulting platforms display the desired MRI activity—i.e., longitudinal relaxivity increases almost twice at 7 T magnetic field strength upon saturation with Ca²⁺. Cell viability is probed with the MTT assay using HEK‐293 cells, which show good tolerance for lower contrast agent concentrations over longer periods of time. On intravenous administration of SCA‐USRPs in living mice, MRI studies indicate their rapid accumulation in the renal pelvis and parenchyma. Importantly, the MRI signal increases in both kidney compartments when CaCl₂ is also administrated. Laser‐induced breakdown spectroscopy experiments confirm accumulation of SCA‐USRPs in the renal cortex. To the best of our knowledge, these are the first studies which demonstrate calcium‐sensitive MRI signal changes in vivo. Continuing contrast agent and MRI protocol optimizations should lead to wider application of these responsive probes and development of superior functional methods for monitoring calcium‐dependent physiological and pathological processes in a dynamic manner.     

Femtocells [Guest Editorial]

Momentum is building rapidly around femtocells, as several operators worldwide have commercially launched 3G femtocells. This will further accelerate during the course of 2010, with many more operators expected to deploy. As explained in the first part of this series, which appeared in the September 2009 issue of IEEE Communications Magazine, femtocells are small, inexpensive base stations operating in the licensed cellular bands; they are meant to be placed in individual homes or businesses, and backhauled onto the operator's network via wireline broadband access.     

Femtocells [Guest Editorial]

Femtocells are destined to transform the way mobile operators build their cellular networks and grow their coverage and capacity. Femtocells are small base stations operating in the licensed cellular bands. They are so small and inexpensive, and transmit at such low power, that they are meant to be placed in individual homes and backhauled onto the operator?s network via conventional digital subscriber line (DSL) or cable broadband access (or, when available, fiber).     

Performance behaviour of IEEE 802.11 distributed coordination function

The authors present an extensive investigation of the performance of the IEEE 802.11 medium access control (MAC) protocol, with respect to throughput and delay. For the protocol analysis, a new model, which describes the protocol's behaviour to a great extent by incorporating and extending the existing models, is proposed. The authors also present a detailed analysis of the end-to-end delay through the study of the MAC delay and the queueing delay. The authors use the Z-transform of backoff duration to obtain the mean value, the variance and the probability distribution of the MAC delay. For the queueing analysis, first the authors consider an M/G/l queue in order to provide a first look at the queueing delay. Second, the authors modify the input process of the queue so that the packet arrival process is described by an ON- OFF model, which expresses the bursty nature of traffic. In the investigations, data rates of 1, 5.5 and 11 Mbps are assumed to highlight the effect of the bit rate on network performance for both Basic and request-to-send/ clear-to-send access mechanisms. The throughput and delay analyses are validated by simulating the distributed coordination function, whereas the models are compared with the existing models based on their results. The accuracy of the analyses was found to be quite satisfactory.     

Parallel construction of SLR (1) and LALR (1) parsers

The generation of an LR parser consists of constructing a parse table, with one row per state (in a push-down automaton), and one column per terminal symbol. Traditionally, this is carried out row by row, with the computation of one row depending (potentially) on all the others. We present a technique for carrying out the lookahead computation of SLR (1) and LALR (1) parsers in a completely parallel fashion. Our technique performs the computation by column, rather than by row. We show that the computation is totally independent for each column, making it ideal for parallelization. The speedup factor of the technique is min (N, T), whereN is the number of processors andT is the number of terminal symbols in the user's grammar.     

Engset multi‐rate state‐dependent loss models with QoS guarantee

We consider a single link loss system of quasi‐random input, described by the Engset multirate loss model (EnMLM). Blocked calls may once reattempt to be connected to the system requiring less bandwidth; then the system is described by the single retry model for finite sources (f‐SRM). The EnMLM and the f‐SRM are extended with the single threshold finite source model (f‐STM), where calls may attempt to be connected to the system with less bandwidth requirements, according to the link occupancy, before blocking occurs. We focus on CBP equalization in the EnMLM, f‐SRM and f‐STM, under the bandwidth reservation (BR) policy. For this analysis, we apply two approximate methods, the Roberts' method and the method of reverse transition rates (RTR), which lead to a recursive CBP calculation. We evaluate the accuracy of the above models under the BR policy by comparing the analytical with simulation CBP results, based on the relative approximation errors (RAE). The results are highly satisfactory because they show that the proposed models (formulas) lead to small RAE. Copyright © 2005 John Wiley & Sons, Ltd.     

System Performance of Transmit Diversity Methods and a Two Fixed-Beam System in WCDMA

Downlink transmit diversity modes for WCDMA together with a two fixed-beam antenna array system are compared relative to the single antenna sectorized system in a radio network simulator. The transmit diversity methods investigated are: space-time transmit diversity and closed-loop mode I transmit diversity. Frequency selective (COST 259) and flat fading channels are considered and their impact to speech-only and data-only services is evaluated. A third service, which highlights the system performance of the various advanced antennas, is also investigated.The results in this investigation point out that the diversity gain in flat fading channels is substantial. In frequency-selective fading, the benefits of fixed beam systems is encouraging, whereas transmit diversity methods (especially Space-Time Transmit Diversity) is unsatisfactory.     

Single units and conscious vision

Figures that can be seen in more than one way are invaluable tools for the study of the neural basis of visual awareness, because such stimuli permit the dissociation of the neural responses that underlie what we perceive at any given time from those forming the sensory representation of a visual pattern. To study the former type of responses, monkeys were subjected to binocular rivalry, and the response of neurons in a number of different visual areas was studied while the animals reported their alternating percepts by pulling levers. Perception-related modulations of neural activity were found to occur to different extents in different cortical visual areas. The cells that were affected by suppression were almost exclusively binocular, and their proportion was found to increase in the higher processing stages of the visual system. The strongest correlations between neural activity and perception were observed in the visual areas of the temporal lobe. A strikingly large number of neurons in the early visual areas remained active during the perceptual suppression of the stimulus, a finding suggesting that conscious visual perception might be mediated by only a subset of the cells exhibiting stimulus selective responses. These physiological findings, together with a number of recent psychophysical studies, offer a new explanation of the phenomenon of binocular rivalry. Indeed, rivalry has long been considered to be closely linked with binocular fusion and stereopsis, and the sequences of dominance and suppression have been viewed as the result of competition between the two monocular channels. The physiological data presented here are incompatible with this interpretation. Rather than reflecting interocular competition, the rivalry is most probably between the two different central neural representations generated by the dichoptically presented stimuli. The mechanisms of rivalry are probably the same as, or very similar to, those underlying multistable perception in general, and further physiological studies might reveal much about the neural mechanisms of our perceptual organization.