JungKyo Sohn

Exergy-based performance analysis of the heavy-duty gas turbine in part-load operating conditions

Abstract The present study describes details of exergy-based performance characteristics of a heavy-duty gas turbine, 150MW-class GE 7F model. Results have shown that a chemical reaction in the combustor of which the exergy destruction ratio is 28.3% at full-load is one of the major sources of exergy destructions in the gas turbine. It was found that, in spite of its usefulness to the performance enhancement of the combined cycle plant in part-load operations, the variable inlet guide vane located in front of the multi-stage compressor caused the increase of exergy destruction in the first stage (about 10 times lager than that of other stages below 80% load) and decreased the overall compressor efficiency. Also, it was discovered that the magnitude of exergy destruction by the cooling air in turbine stages is large enough to influence the overall turbine efficiency. The exergy destruction by the cooling air is more than half of the total exergy destruction of each cooled turbine stage.

Utility-based load balancing in WLAN/UMTS internetworking systems

The complementary characteristics of 3 G cellular networks (such as UMTS) and IEEEs 802.11 wireless LANs (WLANs) have stimulated intensive research efforts to integrate UMTS and WLAN networks. In such integrated networks, a mobile station is able to switch between UMTS and WLAN networks based on various network parameters such as network throughput, packet delay and other network or user quality criteria. In this paper, we develop a utility-based access selection algorithm targeted to achieve load balancing between UMTS and WLAN networks. The performance of our algorithm is evaluated using OPNET with dual-mode mobile stations.

Scheduling Algorithms For Policy Driven QoS Support in HSDPA Networks

Increasing demand for high data-rate multimedia services has led to the emergence of high-speed data transfer features such as high-speed downlink packet access (HSDPA) for WCDMA networks. The role of the MAC-hs scheduler is vital in HSDPA in improving overall system performance. Using opportunistic scheduling to exploit multi-user diversity for efficient transmission of best effort services or considering user fairness constraints has been the main focus of most scheduling algorithms. However the need for strict QoS support for services such as streaming, gaming, and VoIP is growing. Service operators can enforce their own policies in meeting these QoS requirements. In this paper we introduce two QoS-aware policy driven scheduling algorithms. We developed an HSDPA system in OPNET, and implemented our scheduling algorithms along with other well-known algorithms. OPNET simulations show that unlike other schedulers, our strict and loose policy scheduling (SPS and LPS) algorithms comply with the policy constraints if allowed by radio conditions and cell capacity.

Digital watermarking of polygonal meshes with linear operators of scale functions

Digital watermarking is already used to establish the copyright of graphics, audio and text, and is now increasingly important for the protection of geometric data as well. Watermarking polygonal models in the spectral domain gives protection against similarity transformation, mesh smoothing, and additive random noise attacks. However, drawbacks exist in analyzing the eigenspace of Laplacian matrices. In this paper we generalize an existing spectral decomposition and propose a new spatial watermarking technique based on this generalization. While inserting the watermark, we avoid the cost of finding the eigenvalues and eigenvectors of a Laplacian matrix in spectral decomposition; instead we use linear operators derived from scaling functions that are generated from Chebyshev polynomials. Experimental results show how the cost of inserting and detecting watermarks can be traded off against robustness under attacks like additive random noise and affine transformation.

Common radio resource management for access selection in multi-access networks

In this paper, we address the problem of access selection in multi-access networks. As admission and handover process durations are in the order of seconds and decentralized decisions are made over multiple cells spread over the network, such decisions have to be made quickly and in a distributed manner while keeping the network stable and its load balanced. We propose a utility-based approach to access selection. In this model, policies governing the network operations are embodied in the design of marginal utility functions and decisions are made using minimal computations. Developed algorithms are then evaluated using a Java-based multi-access simulator called MANSim that we developed.

Integrating admission control and packet scheduling for quality controlled streaming services in HSDPA networks

High-Speed Downlink Packet Access (HSDPA) uses a shared forward link packet data channel that can achieve peak data rates up to 14.4 Mbps. The newly introduced features that are key to reaching such high rates are link adaptation, Hybrid ARQ (HARQ) and fast scheduling. The decision to admit new users is still taken by the Radio Network Controller (RNC); however, the scheduling role has been moved to the base station (Node-B) for fast adaptability. Although in high speed cellular networks, opportunistic scheduling can exploit multi-user diversity to maximize throughput, with the growing need for strict QoS guarantees for services such as streaming, gaming, and VoIP, a balance should be struck between maximizing throughput and providing user satisfaction. Service operators can enforce their own policies in providing user satisfaction. In this paper, we consider a QoS-aware policy driven scheduling algorithm and its interaction with the admission control mechanism at the RNC. We developed a dynamic HSDPA network simulator in OPNET with link adaptation, HARQ, fast scheduling and quality based admission control. Our simulations show that even with the presence of an admission controller at RNC, the well-known proportional fairness (PF) algorithm dismally fails to provide user satisfaction for streaming services, whereas our Strict Policy Scheduling (SPS) algorithm substantially increases user satisfaction as intended by the admission controller while providing a significant cell capacity gain.

Analysis of Uplink Traffic Characteristics and Impact on Performance in Mobile Data Networks

User-generated content (UGC) also known as user- created content (UCC) refers to various kinds of media contents that are produced by end users. A recent trend in the use of the Internet exhibits that data traffic from UGC is rapidly growing with the potential to create a huge amount of uplink traffic for wireless operators. As the reality of UGCs scope and power is becoming crystalized, modeling and analysis of uplink traffic has just begun to receive growing attention in the wireless community. Consequently, we have also analyzed live uplink traffic traces obtained by monitoring 3G networks of a mobile data service provider (SK Telecom in Korea). Our statistical analysis shows the self-similarity in this traffic trace. In order to evaluate the impact of this characteristics on mobile data networks, we use the WiMAX module available in OPNET software. Our trace-driven simulation results indicate burstiness in the aggregated traffic received at a base station (BS) as well as in the traffic generated at each subscriber station (SS). The impact of the data burst at SSs is shown to be negligible when the overall traffic load is relatively small but become significant when a large volume of delay-sensitive traffic is generated in many SSs even if the overall network load is less than the network capacity.

Cost-based approach to access selection and vertical handover decision in multi-access networks

In multi access network environments, mobile stations may encounter multiple choices for selecting an access network. Carefully designed access selection schemes can provide not only mobile users with better services but also network operators with better resource utilizations. It is also envisioned that further improvements can be achieved by redistributing mobile stations from one access network to anther (i.e., vertical handovers). Such decisions should be made by following carefully designed, yet simple to implement, protocols. In this paper, we present a cost-based scheme for access selection and vertical handover decision algorithms. The proposed algorithm was implemented in a Java-based simulator called MANSim (Multiple Access Network Simulator) that we developed. Our simulation results show that during the congested periods, the network throughput is significantly improved with greatly reduced call drop rate.

Policy Driven Scheduling to Provide Differentiated QoS for Delay Sensitive Services in HSDPA

Increasing demand for high data-rate multimedia real time services has led to the use of asynchronous time shared channels in the forward link for real-time services in 3G wireless networks, such as HSDPA, EV-DO and EV-DV. While network service providers would like to impose operational policies on the packet schedulers based on QoS requirements by real-time services to maximize profit, existing schedulers are not capable to deal with such challenge due to time varying channel conditions and constraints on the total forward link transmit power. In this paper, we consider the problem of scheduling users on the forward link in a multiuser system where multiple users can be scheduled during each interval. We introduce a delay aware policy driven scheduling algorithm SPS-Delay (Strict Policy Scheduling for Delay) to support differentiated QoS requirements in the form of tolerable latency specified by the user applications. We developed an HSDPA system in OPNET, and implemented our scheduling algorithm along with other well- known algorithms. Our simulations show that SPS-Delay reduces packet drop rate by as much as 80% and increases the number of satisfied users by as much as 32% when compared to PF.