Uplink Scheduling in CDMA Packet-Data Systems

Uplink scheduling in wireless systems is gaining importance due to arising uplink intensive data services (ftp, image uploads etc.), which could be hampered by the currently in-built asymmetry in favor of the downlink. In this work, we propose and study algorithms for efficient uplink packet-data scheduling in a CDMA cell. The algorithms attempt to maximize system throughput under transmit power limitations on the mobiles assuming instantaneous knowledge of user queues and channels. However no channel statistics or traffic characterization is necessary. Apart from increasing throughput, the algorithms also improve fairness of service among users, hence reducing chances of buffer overflows for poorly located users. The major observation arising from our analysis is that it is advantageous on the uplink to schedule “strong” users one-at-a-time, and “weak” users in larger groups. This contrasts with the downlink where one-at-a-time transmission for all users has shown to be the preferred mode in much previous work. Based on the optimal schedules, we propose less complex and more practical approximate methods, both of which offer significant performance improvement compared to one-at-a-time transmission, and the widely acclaimed Proportional Fair (PF) algorithm, in simulations. When queue content cannot be fed back, we propose a simple modification of PF, Uplink PF (UPF), that offers similar improvement. Hereafter, we refer to users with low recieved power at the base even when transmitting at peak transmit power as “weak” users, and the strongly recieved users at the base as “strong” users. Krishnan Kumaran is currently a member of the Complex Systems Modeling section in the Corporate Strategic Research of ExxonMobil Corp., Clinton, NJ. Formerly, he was a Member of Technical Staff in the Mathematics of Networks and Systems Research Department at Bell Labs in Murray Hill, NJ, where his research interests were in modeling, analysis and simulation of design, resource management and scheduling issues in telecommunication networks. Lijun Qian is an assistant professor in the Department of Electrical Engineering at Prairie View A&M University. He received his B.S. from Tsinghua University in Beijing, M.S. from Technion-Israel Institute of Technology, and Ph.D. from WINLAB, Rutgers University, all in electrical engineering. Before joining PVAMU, he was a researcher at Networks and Systems Research Department of Bell Labs in Murray Hill, NJ. His major research interests are in wireless communications and networking technologies, especially in radio resource management, protocol design, TCP/RLP optimization and MPLS traffic engineering.     

Electrical characterization of carbon nanotube Y-junctions: a foundation for new nanoelectronics

A review on the syntheses and electrical characterization of Y-shaped multi-walled carbon nanotube morphologies is presented. Modified thermal CVD processes, using Ti precursors, are used to grow Y-junctions of different geometries and distribution of catalyst particles. It has been established that novel electrical switching behavior is feasible, where any one of the three branches of the Y-junction can be used for modulating the electrical current flow through the other two branches. Current blocking behavior, leading to perfect rectification, is seen which could be related to the interplay of the carrier lifetime and the transit time. The overall goal is to investigate the possibility of obtaining novel functionality at the nanoscale, which can lead to new device paradigms.     

Presentation and treatment of spontaneous aortocaval fistula

BACKGROUND: Spontaneous rupture of abdominal aortic aneurysm into the inferior vena cava is rare. The clinical presentation is highly variable, and the diagnosis can be difficult, often being made only at operation. The aortocaval fistula results in a large left-to-right shunt, which can cause cardiac failure. Once the diagnosis is made, treatment is by surgical closure of the fistula and repair of the aneurysm with a graft. METHODS: This is a retrospective review of a single surgeon's experience with aortocaval fistula complicating abdominal aortic aneurysms. RESULTS: Over a 15-year period, we had five patients with spontaneous aortocaval fistula who were treated operatively. Preoperative diagnosis was made in two, suspected in one, and not made in two, one of whom died (the only perioperative death in the series). CONCLUSIONS: Spontaneous aortocaval fistulas are uncommon, and their preoperative recognition is difficult. Hematuria in association with an abdominal aortic aneurysm should raise the suspicion of an aortocaval fistula. Surgical correction is possible, with survival rates comparable to those associated with rupture of aneurysms into the retroperitoneum. Early operative control of the fistula is important to optimize the preload to the heart.     

Foveation-Based Error Resilience and Unequal Error Protection over Mobile Networks

By exploiting new human-machine interface techniques, such as visual eyetrackers, it should be possible to develop more efficient visual multimedia services associated with low bandwidth, dynamic channel adaptation and robust visual data transmission. In this paper, we introduce foveation-based error resilience and unequal error protection techniques over highly error-prone mobile networks. Each frame is spatially divided into foveated and background layers according to perceptual importance. Perceptual importance is determined either through an eye tracker or by manually selecting a region of interest. We attempt to improve reconstructed visual quality by maintaining the high visual source throughput of the foveated layer using foveation-based error resilience and error correction using a combination of turbo codes and ARQ (automatic reQuest). In order to alleviate the degradation of visual quality, a foveation based bitstream partitioning is developed. In an effort to increase the source throughput of the foveated layer, we develop unequal delay-constrained ARQ (automatic reQuest) and rate compatible punctured turbo codes where the punctual pattern of RCPC (rate compatible punctured convolutional) codes in H.223 Annex C is used. In the simulation, the visual quality is significantly increased in the area of interest using foveation-based error resilience and unequal error protection; (as much as 3 dB FPSNR (foveal peak signal to noise ratio) improvement) at 40% packet error rate. Over real-fading statistics measured in the downtown area of Austin, Texas, the visual quality is increased up to 1.5 dB in PSNR and 1.8 dB in FPSNR at a channel SNR of 5 dB.     

Acoustic emission technique for leak detection in an end shield of a pressurised heavy water reactor

This paper discusses the successful application of the Acoustic Emission Technique (AET) for detection and location of leak paths present on the inaccessible side of an end shield of a Pressurised Heavy Water Reactor (PHWR). The methodology was based on the fact that air and water leak AE signals have different characteristic features. Baseline data was generated from a sound end-shield of a PHWR for characterizing the background noise. A mock up end-shield system with saw cut leak paths was used to verify the validity of the methodology. It was found that air leak signals under pressurisation (as low as 3 psi) could be detected by frequency domain analysis. Signals due to air leaks from various locations of a defective end-shield were acquired and analysed. It was possible to detect and locate leak paths. Presence of detected leak paths were further confirmed by alternate test.     

Literature reviews in student project reports

Writing project reports is an important part of the engineering curriculum at Singapore universities. One important section of the formal report is the literature review. Most universities around the world provide guidelines on writing reviews, emphasizing that plagiarism is unethical. However, these guidelines do not offer explicit training on how to avoid plagiarism. In order to write academically acceptable reviews while avoiding copying from source materials, students face a major challenge and resort to employing various strategies to cope with the task. In this study, we examined the literature review sections of final year project reports to find out how engineering undergraduates in a Singapore university cope with writing reviews and to suggest ways in which they can extend their skills to improve their literature reviews.     

Finite element crack growth algorithm for dynamic fracture

A new finite element crack growth algorithm has been developed to simulate dynamic fracture. In this algorithm, pseudo elements with very high initial density are placed below the crack plane and the density is reduced to zero in a gradual manner as the crack passes the element. A number of linear elastic and elasto-viscoplastic problems have been carried out to test the new algorithm. The results are compared with some of the existing crack growth models.     

Activation energy determination from low-temperature CV dispersion[semiconductor devices]

The response of semiconductor devices at low temperatures to changes in the voltage across the depletion region is limited by the dielectric relaxation time of the majority carriers in the bulk region. This results in a dispersion of the C-V curves at low temperatures. In this paper, we report a study of the dispersion seen in the accumulation and depletion regions of the C-V curve in n- and p-channel MOS transistors as well as in reverse biased one-sided abrupt junctions. From the admittance measured as a function of temperature and frequency, the dopant energy level is determined. The values of the activation energy measured using the diodes agree well with the corresponding values obtained using MOS devices     

Entropy and the timing capacity of discrete queues

Queueing systems which map Poisson input processes to Poisson output processes have been well-studied in classical queueing theory. This paper considers two discrete-time queues whose analogs in continuous-time possess the Poisson-in-Poisson-out property. It is shown that when packets arriving according to an arbitrary ergodic stationary arrival process are passed through these queueing systems, the corresponding departure process has an entropy rate no less (some times strictly more) than the entropy rate of the arrival process. Some useful by-products are discrete-time versions of: (i) a proof of the celebrated Burke's (1956) theorem, (ii) a proof of the uniqueness, amongst renewal inputs, of the Poisson process as a fixed point for exponential server queues proposed by Anantharam (1993), and (iii) connections with the timing capacity of queues described by Anantharam and Verdu (1996).     

Synthesis and characterisation of N-toluyl methacrylatoethyl carbamates,homopolymers and copolymers with methyl methacrylate

The paper describes the synthesis of N-2/4-toluyl methacrylatoethyl carbamates using 2/4-toluyl isocyanate and 2-hydroxyethyl methacrylate. Homopolymerisation and copolymerisation of these novel monomers with methyl methacrylate was carried out using benzoyl peroxide as an initiator and tetrahydrofuran as solvent. Photopolymerisation of N-4-toluyl methacrylatoethyl carbamate could be carried out without the use of photosensitiser. Structural characterisation of copolymers was done using ¹H-NMR. Thermal stability of copolymers was evaluated in a nitrogen atmosphere by dynamic thermogravimetry.