Catálogo de publicaciones - libros

Mobile WiMAX is a promising wireless technology approaching market deployment. Much discussion concentrate on whether mobile WiMAX will reach a tipping point and become 4G or not. As a pre-mobile WiMAX system is being delivered, we decided to set up a real life field trial and perform the most important measurements over the system setup. The system was delivered with 4th order diversity. In this paper we analyze physical system performance based on field trial measurements, especially at locations with non line of sight conditions in urban areas. We investigate the gain with 2nd and 4th order base station diversity and derive analytical expressions. The system path loss is plotted and found to approach the Cost-231 Hata model for urban areas. Throughput is also measured and analyzed. Sub-channelization in the uplink points out to be an important feature for enhanced coverage.

IEEE 802.16 is yet a very recent technology and released hardware does frequently only support standards partially. The same applies to public available simulation tools, in particular for NS-2. As the latter is the de-facto standard in science and as we use it for our research in the context of the WEIRD project, we evaluate the of IEEE 802.16 support for NS-2. We present several general but also specific issues, which are important in order to carry out reliable research based on these tools. In particular, we show in much detail where modules deviate significantly and even fail totally.

The IEEE 802.16 technology defines the ARQ mechanism that enables a connection to resend data at the MAC level if an error is detected. In this paper, we analyze the key features and parameters of the ARQ mechanism. In particular, we consider a choice for the ARQ feedback type, a scheduling of the ARQ feedbacks and retransmissions, the ARQ block rearrangement, ARQ transmission window and ARQ block size. We run a number of simulation scenarios to study these parameters and how they impact a performance of application protocols. The simulation results reveal that the ARQ mechanism plays an important role in transmitting data over wireless channels in the IEEE 802.16 networks.

Optical packet switching (OPS) is a promising technology for future generation networks solutions. One of the great challenges to be addressed is differentiated quality of service (QoS) and a major concern in providing QoS is packet loss. In case of differentiated services, packet losses should be significantly less likely for high priority classes than for low priority classes. However, as packet losses are very unlikely they become rare events which poses serious challenges to conventional analysis methodologies. We present an importance sampling scheme for fast simulation of packet loss rates in this setting. The change of measure adapts according to ant colony optimization, a metaheuristic inspired by the foraging behavior of ants. Thereby, our simulation does not require intimate a priori knowledge of the system and overcomes a general drawback of importance sampling. Within very moderate simulation time, accurate results are provided for an OPS model with varying parameter settings.

Virtual Private Networks (VPNs) are replacing expensive leased lines since they use the public Internet as the communication channel. However, Internet by itself does not provide guarantees of security and QoS to send voice and video packets. In this paper we present a generic security and QoS model named GESEQ which is based on the DiffServ model that shows design considerations when delay-sensitive packets need to be protected by IPSec tunnels and also need to be treated preferentially to overcome the increase of packet latency, jitter and packet loss under network traffic congestion. We made and present a particular test scenario and its quantitative analysis to prove that GESEQ works efficiently when trying to send voice and video packages with security and QoS.

This study addresses the problems of the multicast traffic transmission in Metro Ethernet Networks. We developed an approach of the spanning tree construction which improves the QoS parameters for the multicast traffic.

In support of the on-going research and development activity in the design and performance assessment of radio access network integration schemes, we present novel resource sharing models for the efficient simulation of multi-access networks involving , and -based systems. The applicability of the proposed models in determining call and system level performance measures is demonstrated by means of a limited set of illustrative numerical experiments revealing the significant performance gains when integrating distinct access networks with a rather simple adio ccess selection scheme.

In cellular networks, repeated attempts occur as result of user behavior but also as automatic retries of blocked requests. Both phenomena play an important role in the system performance and therefore should not be ignored in its analysis. On the other hand, an exact Markovian model analysis of such systems has proven to be infeasible and resorting to approximate techniques is mandatory. We propose an approximate methodology which substantially improves the accuracy of existing methods while keeping computation time in a reasonable value. A numerical evaluation of the model is carried out to investigate the impact on performance of the parameters related to the retry phenomena. As a result, some useful guidelines for setting up the automatic retries are provided. Finally, we also show how our model can be used to obtain a tight performance approximation in the case where reattempts have a deterministic nature.

Orthogonal variable spreading factor (OVSF) codes are widely used to provide variable data rates for supporting different bandwidth requirements in wideband code division multiple access (WCDMA) systems. Many works in the literature have intensively investigated to find an optimal dynamic code assignment scheme for OVSF codes. Unlike earlier studies, which assign OVSF codes using conventional (CCA) or dynamic (DCA) code allocation schemes, in this paper, stochastic optimization methods which are genetic algorithm (GA) and simulated annealing (SA) were applied which population is adaptively constructed according to existing traffic density in the OVSF code-tree. Simulation results show that the GA and SA provide reduced code blocking probability and improved spectral efficiency when compared to the CCA and DCA schemes. It is also seen that, computational complexity of GA and SA are also more than CCA and DCA.

With the increase of wireless network and demand for the variety of services, the handoff resource reservation for the Quality of Service(QoS) will be significant issue in future wireless networks. But on the other hand, the efficiency of resource usage also needs to be considered for preventing waste of network resources due to the reservation. Therefore, this paper suggests an efficient resource reservation scheme to support QoS provision as well as the efficient resource utilization simultaneously. In the proposed scheme, handoff probability estimation with the filtering algorithm is used to calculate the optimal amount of resources to be reserved in the neighbor cells. So, the mobile units with the proposed scheme can predict the amount of resources required in the future and reserve resource effectively. The performance simulation shows that the proposed scheme is advantageous with the balance of QoS provision and resource utilization in the wireless network.