Performance analysis of an improved power-saving medium access protocol for IEEE 802.11 point coordination function WLAN

Wireless enabled portable devices must operate with the highest possible energy efficiency while still maintaining a minimum level and quality of service to meet the user's expectations. The authors analyse the performance of a new pointer-based medium access control protocol that was designed to significantly improve the energy efficiency of user terminals in wireless local area networks. The new protocol, pointer controlled slot allocation and resynchronisation protocol (PCSAR), is based on the existing IEEE 802.11 point coordination function (PCF) standard. PCSAR reduces energy consumption by removing the need for power saving stations to remain awake and listen to the channel. Using OPNET, simulations were performed under symmetric channel loading conditions to compare the performance of PCSAR with the infrastructure power saving mode of IEEE 802.11, PCF-PS. The simulation results demonstrate a significant improvement in energy efficiency without significant reduction in performance when using PCSAR. For a wireless network consisting of an access point and 8 stations in power saving mode, the energy saving was up to 31% while using PCSAR instead of PCF-PS, depending upon frame error rate and load. The results also show that PCSAR offers significantly reduced uplink access delay over PCF-PS while modestly improving uplink throughput.

Implementation of a novel credit based SCFQ scheduler for Broadband Wireless Access

A novel tag computation circuit for a credit based Self-Clocked Fair Queuing (SCFQ) Scheduler is presented. The scheduler combines Weighted Fair Queuing (WFQ) with a credit based bandwidth reallocation scheme. The proposed architecture is able to reallocate bandwidth on the fly if particular links suffer from channel quality degradation .The hardware architecture is parallel and pipelined enabling an aggregated throughput rate of 180 million tag computations per second. The throughput performance is ideal for Broadband Wireless Access applications, allowing room for relatively complex computations in QoS aware adaptive scheduling. The high-level system break-down is described and synthesis results for Altera Stratix II FPGA technology are presented.

Toward Self-Assembled Ferroelectric Random Access Memories: Hard-Wired Switching Capacitor Arrays with Almost Tb/in.2 Densities

We report on the successful fabrication of arrays of switchable nanocapacitors made by harnessing the self-assembly of materials. The structures are composed of arrays of 20-40 nm diameter Pt nanowires, spaced 50-100 nm apart, electrodeposited through nanoporous alumina onto a thin film lower electrode on a silicon wafer. A thin film ferroelectric (both barium titanate (BTO) and lead zirconium titanate (PZT)) has been deposited on top of the nanowire array, followed by the deposition of thin film upper electrodes. The PZT nanocapacitors exhibit hysteresis loops with substantial remnant polarizations, while although the switching performance was inferior, the low-field characteristics of the BTO nanocapacitors show dielectric behavior comparable to conventional thin film heterostructures. While registration is not sufficient for commercial RAM production, this is nevertheless an embryonic form of the highest density hard-wired FRAM capacitor array reported to date and compares favorably with atomic force microscopy read-write densities.

Use and access to ICT in students of public secondary schools in Chihuahua City, Mexico: inclusion in teaching and digital literacy

This paper addresses the issue of the digital divide in students of public secondary schools at Chihuahua City, Mexico. It seeks to identify potential inequality of opportunities with regards to subjects’ access to information, knowledge and education through the ICT (internet, mobile telephony, broadband and television). The study takes three schools as investigative stage, using the survey as a data collection instrument, identifying patterns of behavior regarding: general knowledge of them, access to computer equipment and internet, and characterization of their use. Other aspects of analysis are the identification of the educational level of parents and access to technology resources available for academic and non-academic purposes in various application areas (home, school and social environment). The proposal concludes, that it is through the recollection of alternatives suggested by the teachers themselves to incorporate ICT for teaching purposes in a systematic and planned fashion, whose greatest reflection manifests in better digital literacy indicators.

On the Diversity-Multiplexing Tradefoff for Wireless Cooperative Multiple Access Systems

A spectrally efficient strategy is proposed for cooperative multiple access (CMA) channels in a centralized communication environment with $N$ users. By applying superposition coding, each user will transmit a mixture containing its own information as well as the other users', which means that each user shares parts of its power with the others. The use of superposition coding in cooperative networks was first proposed in , which will be generalized to a multiple-user scenario in this paper. Since the proposed CMA system can be seen as a precoded point-to-point multiple-antenna system, its performance can be best evaluated using the diversity-multiplexing tradeoff. By carefully categorizing the outage events, the diversity-multiplexing tradeoff can be obtained, which shows that the proposed cooperative strategy can achieve larger diversity/multiplexing gain than the compared transmission schemes at any diversity/multiplexing gain. Furthermore, it is demonstrated that the proposed strategy can achieve optimal tradeoff for multiplexing gains $0leq r leq 1$ whereas the compared cooperative scheme is only optimal for $0leq r leq ({1}/{N})$. As discussed in the paper, such superiority of the proposed CMA system is due to the fact that the relaying transmission does not consume extra channel use and, hence, the deteriorating effect of cooperative communication on the data rate is effectively limited.

Access Point Association Coordination in Densely Deployed 802.11 Wireless Networks

Dense deployment of wireless local area network (WLAN) access points (APs) is an important part of the next generation Wi-Fi and standardization (802.11ax) efforts are underway. Increasing demand for WLAN connectivity motivates such dense deployments, especially in geographical areas with large numbers of users, such as stadiums, large enterprises, multi-tenant buildings, and urban cities. Although densification of WLAN APs guarantees coverage, it is susceptible to increased interference and uncoordinated association of stations (STAs) to APs, which degrade network throughput. Therefore, to improve network throughput, algorithms are proposed in this thesis to optimally coordinate AP associations in the presence of interference. In essence, coordination of APs in dense WLANs (DWLANs) is achieved through coordination of STAs' associations with APs. While existing approaches suggest tuning of APs' beacon powers or using transmit power control (TPC) for association control, here, the signal-to-interference-plus-noise ratio (SINRs) of STAs and the clear channel assessment (CCA) threshold of the 802.11 MAC protocol are employed. The proposed algorithms in this thesis enhance throughput and minimize coverage holes inherent in cell breathing and TPC techniques by not altering the transmit powers of APs, which determine cell coverage. Besides uncoordinated AP associations, unnecessary frequent transmission deferment is envisaged as another problem in DWLANs due to the clear channel assessment aspect of the carrier sensing multiple access collision avoidance (CSMA/CA) scheme in 802.11 standards and the short spatial reuse distance between co-channel APs. To address this problem in addition to AP association coordination, an algorithm is proposed for CCA threshold adjustment in each AP cell, such that CCA threshold used in one cell mitigates transmission deferment in neighboring cells. Performance evaluation reveals that the proposed association optimization algorithms achieve significant gain in throughput when compared with the default strongest signal first (SSF) association scheme in the current 802.11 standard. Also, further gain in throughput is observed when the CCA threshold adjustment is combined with the optimized association. Results show that when STA-AP association is optimized and CCA threshold is adjusted in each cell, throughput improves. Finally, transmission delay and the number of packet re-transmissions due to collision and contention significantly decrease.

Improved power-saving medium access protocol for IEEE 802.11e QoS-enabled wireless networks

The performance of a new pointer-based medium-access control protocol that was designed to significantly improve the energy efficiency of user terminals in quality-of-service-enabled wireless local area networks was analysed. The new protocol, pointer-controlled slot allocation and resynchronisation protocol (PCSARe), is based on the hybrid coordination function-controlled channel access mode of the IEEE 802.11e standard. PCSARe reduces energy consumption by removing the need for power-saving stations to remain awake for channel listening. Discrete event network simulations were performed to compare the performance of PCSARe with the non-automatic power save delivery (APSD) and scheduled-APSD power-saving modes of IEEE 802.11e. The simulation results show a demonstrable improvement in energy efficiency without significant reduction in performance when using PCSARe. For a wireless network consisting of an access point and eight stations in power-saving mode, the energy saving was up to 39% when using PCSARe instead of IEEE 802.11e non-APSD. The results also show that PCSARe offers significantly reduced uplink access delay over IEEE 802.11e non-APSD, while modestly improving the uplink throughput. Furthermore, although both had the same energy consumption, PCSARe gave a 25% reduction in downlink access delay compared with IEEE 802.11e S-APSD.

Perceived and reported access to the general practitioner: an international comparison of universal access and mixed private/public systems.

Timely and convenient access to primary healthcare is essential for the health of the population as delays can incur additional health and financial costs. Access to health care is under increasing scrutiny as part of the drive to contain escalating costs, while attempting to maintain equity in service provision. The objective was to compare primary care services in Republic of Ireland and Northern Ireland, and to report on perceived and reported access to GP services in universal access and mixed private/public systems. A questionnaire study was performed in Northern Ireland (NI) and the Republic of Ireland (ROI). Patients of 20 practices in the ROI and NI were contacted (n = 22,796). Main outcome measures were overall satisfaction and the access to GP services. Individual responses and scale scores were derived using the General Practice Assessment Questionnaire (G-PAQ). The response rate was 52% (n = 11,870). Overall satisfaction with GP practices was higher in ROI than in NI (84.2% and 80.9% respectively). Access scores were higher in ROI than in NI (69.2% and 57.0% respectively) Less than 1 in 10 patients in ROI waited two or more working days to see a doctor of choice (8.1%) compared to almost half (45.0%) in NI. In NI overall satisfaction decreased as practice size increased; 82.8%, 80.4%, and 75.8%. In both systems, in large practices, accessibility is reduced when compared to smaller practices. The faster access to GP services in ROI may be due to the deterrent effect of the consultation charge freeing up services although, as it is the poorest and sickest who are deterred by the charge this improved accessibility may come at a significant cost in terms of equity. The underlying concern for policy makers centres around provision of equitable services.

Direct Sensing in Liquids Using Whispering-Gallery-Mode Droplet Resonators

Liquid droplets suspended by the tip of a thin wire, a glass capillary, or a needle form high-Q optical resonators, thanks to surface tension. Under gravity equilibrium conditions, the maximum drop diameter is approximately 1.5 mm for paraffin oil (volume ∼ 0.5 μL) using, for instance, a silica fiber with 250 μm thickness. Whispering gallery modes are excited by a free-space near-infrared laser that is frequency locked to the cavity resonance. The droplet cavity serves as a miniature laboratory for sensing of chemical species and particles.