507 research outputs found
CR-MAC: A multichannel MAC protocol for cognitive radio ad hoc networks
This paper proposes a cross-layer based cognitive radio multichannel medium
access control (MAC) protocol with TDMA, which integrate the spectrum sensing
at physical (PHY) layer and the packet scheduling at MAC layer, for the ad hoc
wireless networks. The IEEE 802.11 standard allows for the use of multiple
channels available at the PHY layer, but its MAC protocol is designed only for
a single channel. A single channel MAC protocol does not work well in a
multichannel environment, because of the multichannel hidden terminal problem.
Our proposed protocol enables secondary users (SUs) to utilize multiple
channels by switching channels dynamically, thus increasing network throughput.
In our proposed protocol, each SU is equipped with only one spectrum agile
transceiver, but solves the multichannel hidden terminal problem using temporal
synchronization. The proposed cognitive radio MAC (CR-MAC) protocol allows SUs
to identify and use the unused frequency spectrum in a way that constrains the
level of interference to the primary users (PUs). Our scheme improves network
throughput significantly, especially when the network is highly congested. The
simulation results show that our proposed CR-MAC protocol successfully exploits
multiple channels and significantly improves network performance by using the
licensed spectrum band opportunistically and protects PUs from interference,
even in hidden terminal situations.Comment: 14 Pages, International Journa
Effect of hyperthermia on prognosis after acute ischemic stroke
<p><b>Background and Purpose:</b> Experimental studies have shown that hyperthermia is a determinant of poor outcome after ischemic stroke. Clinical studies evaluating the effect of temperature on poststroke outcome have, however, been limited by small sample sizes. We sought to evaluate the effect of temperature and timing of hyperthermia on outcome after ischemic stroke.</p>
<p><b>Methods:</b> Data of 5305 patients in acute stroke trials from the Virtual International Stroke Trials Archive (VISTA) data set were analyzed. Data for temperatures at baseline, eighth, 24th, 48th, and 72nd hours, and seventh day were assessed in relation to outcome (poor versus good) based on the modified Rankin Scale at 3 months. Hyperthermia was defined as temperature >37.2°C and poor outcome as 90-day modified Rankin Scale >2. Hazard ratios with 95% CIs were reported for hyperthermia in relation to the outcome. Logistic regression models, in relation to hyperthermia, were fitted for a set of preselected covariates at different time points to identify predictors/determinants of hyperthermia.</p>
<p><b>Results:</b> The average age of patients was 68.0±11.9 years, 2380 (44.9%) were females, and 42.3% (2233) received thrombolysis using recombinant tissue plasminogen activator. After adjustment, hyperthermia was a statistically significant predictor of poor outcome. The hazard ratios (95% CI) for poor outcome in relation to hyperthermia at different time points were: baseline 1.2 (1.0 to 1.4), eighth hour 1.7 (1.2 to 2.2), 24th hour 1.5 (1.2 to 1.9), 48th hour 2.0 (1.5 to 2.6), 72nd hour 2.2 (1.7 to 2.9), and seventh day 2.7 (2.0 to 3.8). Gender, stroke severity (National Institutes of Health Stroke Scale score >16), white blood cell count, and antibiotic use were significantly associated with hyperthermia (P≤0.01).</p>
<p><b>Conclusions:</b> Hyperthermia, in acute ischemic stroke, is associated with a poor clinical outcome. The later the hyperthermia occurs within the first week, the worse the prognosis. Severity of stroke and inflammation are important determinants of hyperthermia after ischemic stroke. In patients with acute ischemic stroke, aggressive measures to prevent and treat hyperthermia could improve the clinical outcomes.</p>
An Energy Efficient Multichannel MAC Protocol for Cognitive Radio Ad Hoc Networks
This paper presents a TDMA based energy efficient cognitive radio
multichannel medium access control (MAC) protocol called ECR-MAC for wireless
Ad Hoc Networks. ECR-MAC requires only a single half-duplex radio transceiver
on each node that integrates the spectrum sensing at physical (PHY) layer and
the packet scheduling at MAC layer. In addition to explicit frequency
negotiation which is adopted by conventional multichannel MAC protocols,
ECR-MAC introduces lightweight explicit time negotiation. This two-dimensional
negotiation enables ECR-MAC to exploit the advantage of both multiple channels
and TDMA, and achieve aggressive power savings by allowing nodes that are not
involved in communication to go into doze mode. The IEEE 802.11 standard allows
for the use of multiple channels available at the PHY layer, but its MAC
protocol is designed only for a single channel. A single channel MAC protocol
does not work well in a multichannel environment, because of the multichannel
hidden terminal problem. The proposed energy efficient ECR-MAC protocol allows
SUs to identify and use the unused frequency spectrum in a way that constrains
the level of interference to the primary users (PUs). Extensive simulation
results show that our proposed ECR-MAC protocol successfully exploits multiple
channels and significantly improves network performance by using the licensed
spectrum band opportunistically and protects QoS provisioning over cognitive
radio ad hoc networks.Comment: 8 Pages, International Journa
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