Dimitrios I. Axiotis

Services in interworking 3G and WLAN environments

Interworking WLANs and 3G mobile networks are expected to provide ubiquitous wireless communications at high data rates and a large variety of services with variable bandwidth and QoS requirements, across a wide range of propagation environments and mobility conditions, using dual mode terminals. The interworking of the two networks is a major step toward a new generation of wireless networks in which other radio technologies are also be integrated. In this article we present possible architectures that enable the interworking of 3G and WLAN networks. We then address the capabilities of various terminal types and describe future services in the interworking environment. Finally, we present market forecasts on terminal and service demand growth.

Interoperability criteria, mechanisms, and evaluation of system performance for transparently interoperating WLAN and UMTS-HSDPA networks

The main challenge in the development of future wireless communication systems is to provide users with a wide range of services across different radio access technologies through a single mobile terminal, while maintaining the minimum QoS requirements, and ideally with no limits on the coverage area, mobility or radio conditions. Thus, the need for seamless interworking between heterogeneous wireless communication systems consisting of multiple radio access technologies and overlapping networks emerges. In this article we address the main issues that arise while implementing the interoperability mechanisms between two different radio access networks, with emphasis on UMTS-HSDPA and WLAN (HIPERLAN/2). Two interoperability mechanisms are introduced and described in detail: initial user assignment (optimal network selection) and intersystem handover. Both mechanisms are activated via the optimization of a suitably defined cost function which takes into account all the appropriate system level parameters that trigger the interoperability process. Finally, we investigate the overall performance of the proposed mechanisms by means of a software simulation platform. A number of simulations have been carried out in order to demonstrate the performance enhancements achieved by the proposed mechanisms in terms of unsatisfied users, dropped handovers, and system throughput.

An empirical model for predicting building penetration loss at 2 GHz for high elevation angles

We present an empirical model that predicts the building penetration loss at the 2 GHz band for high elevation angles involved in mobile communications served by high altitude platform stations. The model takes into account the elevation angle of the specific building with respect to the position of the platform. The penetration loss is found to be an increasing function of the elevation angle for a building with concrete walls and normal window size. The predicted values are compared with a ray-tracing simulation at 2 GHz and a published measurement campaign at 5 GHz; good agreement is reported.

The effect of platform instability on the system level performance of HAPS UMTS

High Altitude Platform Stations (HAPS) are a new, promising means of providing Third Generation (3G) mobile services. One of the potential problems of the system is the platform positional instability due to stratospheric winds. In this letter, we present a study of the effect of positional instability on the system level performance of a Universal Mobile Telecommunications System (UMTS) served by HAPS. A platform instability model obtained through measurements of stratospheric winds gives input into a UMTS system level dynamic simulator and the effect of instability on the system capacity and call blocking probability is examined. Finally, it is shown that the positional instability can be counterbalanced by a simple propulsion mechanism, thus reducing the performance degradation.

2 GHz outdoor to indoor propagation at high elevation angles

The case of outdoor to indoor propagation prediction at high elevation angles has scarcely been addressed and no direct relationship between elevation angles and building penetration loss has been reported. High altitude platform stations (HAPS) - a new means of providing international mobile telephony (IMT-2000) - will be operating at high elevation angles. This paper examines the impact of high elevation angles in the propagation mechanism and penetration-into-buildings loss. A 3D ray-tracing software tool was developed in order to simulate a specific environment of 11 buildings and take into account parameters, such as diffraction from building edges and rooftops. This approach concludes that the penetration loss is an increasing function of the elevation angle.

ToD: An Intelligent Location-Based Transport Management System

In this paper we present a novel taxi on demand (ToD) management system that implements a location based and traffic-sensing approach and incorporates automated taxi booking, dispatching, and monitoring application. ToD solution effectively addresses current taxi sector needs, and could be potentially expanded to emerging car-pooling/sharing models, allowing integration between modes of transport in urban/densely populated environments.

Impact of Voice Traffic on the Performance of Packet Data Transmission in TETRA Networks

This paper studies the impact of voice traffic on the performance of packet data (PD) transmission in terrestrial trunked radio (TETRA) networks. Particular emphasis is given on the performance of applications characterized by occasionally transmitting small messages between a number of radio terminals and a fixed server while these terminals are also conducting group voice calls. Such applications are constantly increasing in TETRA networks, and so does the need to dimension and configure these networks to meet their reliability, delay, and loss requirements. However, the impact of group voice calls on PD transmission has not been sufficiently studied, and this constitutes the main motivation of our performance analysis. We present simulation results for a variety of practical scenarios, and we discuss how key performance indicators such as PD delay and loss rate are affected by the group call activity of the radio terminals. Our results can help in dimensioning and planning TETRA networks, particularly in determining the number of PD terminals that can be supported in a cell for a given traffic profile and for quality-of-service requirements.

System level evaluation of reconfigurable MIMO techniques enhancements for HSDPA

In this paper the system level performance of an HSDPA (high speed downlink packet access) MIMO network, which combats the effects of antenna correlation by applying novel reconfigurable techniques is investigated. Reconfigurability is achieved at the link level by applying a linear precoding scheme at the transmitter space-time block encoder. An appropriate link to system interface has been developed, taking into account the linear precoding technique. The system level performance metrics of interest are the average system throughput and the number of satisfied users. Simulation results are presented in order to demonstrate the performance enhancements achieved by the application of reconfigurable techniques compared to the conventional ones.

UDP Performance Measurements Over TETRA IP

This paper presents the results of an extensive measurement survey on the performance of the user datagram protocol (UDP) over the packet data channel (PDCH) of terrestrial trunked radio (TETRA) networks. Measurements were taken over a client-server communication for UDP datagram sizes varying from 50 to 600 bytes with an increment of 50 bytes. For each datagram size, a constant bit rate UDP traffic generator was used and a 1800s measurement was taken with network performance monitoring software. Results obtained include throughput and delay jitter (mean values and standard deviations), percentage of lost datagrams and percentage of datagrams received out-of-order versus the datagram sizes.

On the Effects of Short Data Service-Transport Layer Message Length in QoS Metrics of Tetra Networks

This paper presents the results of a measurement survey on the effects of short data service-transport layer (SDS- TL) message length in QoS metrics of TETRA networks. We developed a novel software measurement platform that runs over the TETRA peripheral equipment interface (PEI), generates SDS-TL traffic, performs measurements and presents statistics of QoS metrics. Measurements were taken for SDS-TL sizes varying from 10 to 190 bytes (excluding the TL header) with an increment of 5 bytes. For each SDS-TL size, 1500 messages were exchanged through the measurement platform between TETRA mobile terminals positioned in different cells. Results obtained include the end- to-end transmission delay (mean and standard deviation) and the percentage of lost SDS-TL versus the message size.