Catálogo de publicaciones - libros

To realize high data rate wireless communication systems, much attention is being payed to multi-user diversity due to large bandwidth availability. Multi-user diversity based opportunistic scheduling is a modern view communication over fading wireless channels, whereby, unlike rate adaptation based schemes, channel variations are exploited rather than mitigated. This paper proposes a multi-user diversity scheme for IEEE 802.11 infrastructure wireless LAN to enhance the throughput. Numerical investigations show the throughput superiority of the scheme over IEEE 802.11 standard and other method.

This paper investigates the performance of two direct sequence ultra wideband (DS-UWB) communication systems over multipath fading channels. The first system is the classic DS-UWB system and the second is obtained by adding a forward error control code (FEC), convolutional code. For the two systems, bipolar pulse amplitude modulation (BPAM) is applied. In the receiver end, the selective diversity combining receiver-SRake receiver is used as a feasible receiver. In the proposed coded scheme, we use soft output Viterbi algorithm (SOVA). The analysis shows that the coded DS-UWB system outperforms the conventional DS-UWB system significantly with the increase of the SRake fingers. The effective order of processing gains achieved by the coded scheme is the product of the number of SRake branches and the free distance of the code applied.

In this paper, we propose an efficient video data transmission protocol using the cross-layer approach in ad hoc networks. Due to node movement, the MANET is frequently changing path and each path has different transmission rate so that it shows low performance when transmitters select a constant transmission late at the encoding time regardless path condition. So, we need an effective video transmission method that considers physical layer channel statistics, node’s energy status, and network topology changes at the same time unlike the OSI layered protocol in that each layer is independent and hard to control adaptively video transmission according to the network conditions. In the proposed CVTP protocol, a source node selects an optimal path using multilayer information such as node’s residual energy, channel condition and hop counts to increase path life time and throughput. And a video source can determine an adequate video coding rate to achieve high PSNR and loss packet loss ratio.

This paper proposes a dynamic IP address configuration (DAC) scheme for mSCTP handover, which exploits the information from the link layer to support SCTP handover between heterogeneous access networks. The proposed DAC scheme enables a mobile terminal to automatically add a new IP address in the newly visited region, change the primary path, and delete the old IP address so as to support the mSCTP handover. For experimental analysis of mSCTP handover, we consider the handover scenarios for 3G-BWA and 3G-WLAN over Linux platforms. From experimental results, it is shown that the throughput and handover latency of the mSCTP handover would be affected by the timing of primary-change and the overlapping period.

In recent years, there have been many researches about Mobile Ad-hoc Networks (MANETs) which is available to communicate freely between mobile devices by using multi-hop without any support of relay base or access point. TCP that used the most widely transport protocol in the Internet repeats packet loss and retransmission because it increases congestion window size by using reactive congestion control until packet loss occurs. As a result of this, the energy of mobile device is wasted unnecessarily.

In this paper, we propose TCP-New Veno in order to improve the energy efficiency of mobile device. According to the network state, the scheme adjusts appropriate size of congestion window. Therefore, the energy efficiency of mobile device and utilization of bandwidth are improved by the scheme. From the simulation by using ns-2, we could see more improved energy efficiency with TCP-New Veno than those with TCP in MANETs.

This paper addresses the problem of scheduling time sensitive messages on dual channel wireless sensor networks. Besides the bandwidth expansion, partitioning evenly the transmission time makes it possible to inherit the optimality of EDF scheduling scheme on the dual channels based on fixed-size time slots synchronized across the two channels. Slot rearrangement also maximizes the number of switchable pairs, so the allocation can be switched dynamically between the channels according to the current channel status, enhancing the reliability of timely message delivery. Simulation results show that the rearrangement scheme can generate 70 % of switchable pairs even when the utilization reaches the saturation point, improving the ratio of successful transmission by up to 18 % when the packet error rate exceeds 0.4, compared with global EDF or NCASP.

This paper presents our Linux/RTOS hybrid operating environment constructed upon Gandalf VMM. Gandalf can host multiple RTOSes along with Linux, and RTOSes and Linux execute within their own isolated protection domains; thus, they can be spatially and temporally protected from each other. We design Gandalf from scratch as a simple and efficient VMM in order to minimize overheads incurred by virtualization. The simplicity and efficiency are achieved by the hybrid of para- and nearly full-virtualization approaches. The implementation of the presented hybrid operating environment is on the PC/AT compatible platform with the Intel IA-32 processor with ITRON as an RTOS. From the measurement results, we make clear that the benefits of using a VMM to construct a hybrid environment exceed shortcomings by showing the impact on performance is limited.

This paper presents an efficient technique for code compression. In our work, a sequence of instructions that occurs repeatedly in an application will be compressed to reduce its code size. During compression, each instruction is first divided into the operation part and the register part, and then only the operation part is compressed. For reducing the run-time overhead, we propose an instruction prefetching mechanism to speed the decompression. Moreover, we devise some optimization techniques to improve the code size reduction and the performance, and show their impacts. The experimental results show that our work can achieve a code size reduction of 33% on average and a low overhead in the process of decompression at run time for these benchmarks.

Recent research in real-time systems has much focused on new task models for flexible scheduling and fault-tolerant real-time systems. In this paper, we propose a novel task model for the purpose of flexible scheduling in reliable real-time systems. In the proposed dual-mode -reliable task model, a task periodically releases fast mode jobs or reliable mode jobs with the constraint that reliable mode jobs must be executed at least once for any consecutive periods to guarantee the reliability of task. We also propose scheduling algorithms and compare performance through simulation results.

Ubiquitous wireless Internet access networks (UWIANs) integrate mobile ad hoc networks into the Internet to achieve ubiquitous Internet connectivity. The Internet connectivity is provided by Internet gateways in the UWIANs. Most of the Internet connectivity research has not considered a malicious environment. However, UWIANs will not be able to succeed without an effective security solution due to wireless links and energy constraints. Thus, security is a critical factor to the success of ubiquitous Internet connectivity. In this paper, we propose a secure Internet gateway discovery protocol in order to provide the secure Internet connectivity. A registration mechanism is also proposed to secure a foreign network and an ad hoc mobile node when it connects within the network. The efficiency of the protocol is shown via simulation.