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Multimedia Wireless Systems

In the last years, the market of Mobile Telecommunications has known an unexpected growth, in particular with the Global System for Mobile communication (GSM), which, starting in Europe, has been acquired all over the world. The increasing demand of mobile services, starting from the traditional voice up to data, images and others, induces the evolution of systems such as GSM, that is defined as a Second Generation (2G) system; in this context, for instance, the GPRS (General Packet Radio Service) allows the introduction of the Packet Switching network in the GSM system. This evolution, named Second Plus (2+), is not enough to satisfy the market demand. Thus, some wideband solutions are going to be adopted in the next years in the world.
The ITU has named its proposal IMT2000, but in the world there are many proposals; in Europe, for instance, ETSI (the European Telecommunication Standard Institute) is standardizing the Universal Mobile Telecommunications System (UMTS). The objective of a Multimedia Wireless System is to provide a radio access to services, which should be comparable, in terms of quality, to those currently offered by the fixed infrastructure, and possibly better than those offered by the 2G systems; the result should be a seamless convergence of both fixed and mobile services. This network will supply different kind of services to the users (through virtual channels denoted by the term bearers), where each user can have active simultaneously one or more bearers. It has to support both circuitand packet switching, Real Time and Non-Real Time Traffic, with different characteristics, i.e.
Quality of Service (QoS), maximum Delay Constraint on the delivery, Constant and Variable Bit Rate (CBR and VBR, rispectively), speed of the Mobile Terminal (MT) up to 500 Km/h With Kbit/s and Mbit/s for quasi-stationary users (e.g. indoor); moreover it has to guarantee the mobility of the users, supplying Seamless Service, and the security of communications. Finally, it has to increase the capacity when compared to First (1G) and Second Generation. This variety of possibilities is defined Multimedia Services. In this context, many problems have to be solved. One of these is the Radio Resource Management (RRM); it can be treated at two levels the microscopic level, in which the data transmission through the common radio resource is concerned; at this level the system performance basically depends on the random distribution and motion, in a given environment, of the traffic sources (MTs) in the cell area, which cause different channel behavior experienced by the users, and the time-dependent activity of the bearers, which can request access to the radio resource in a random way; the macroscopic level, in which the management of the overall multi-cell system is concerned; at this level, due to the user mobility, a mechanism named “handover” (described below) is needed to maintain the continuity of connections, allowing a given QoS; then, the impact of the handover policies on the network design (from several point of views) is addressed since it can be a critical issue for the third generation multimedia systems. In this thesis both levels are examined; in this chapter we will briefly give a more detailed description of both aspects and the corresponding targets of the present work.

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1.1 Third Generation Cellular Networks 5 -100 -95 -90 -85 -80 -75 -70 -65 -60 -55 7000 7200 7400 7600 7800 8000 8200 8400 8600 8800 9000 G [ d B ] Time Slots user 0 user 1 user 2 Figure 1.2: Some examples of gain pro les of channels with multipath fading, for different users (6 Hz Doppler Band). -120 -110 -100 -90 -80 -70 -60 0 200 400 600 800 1000 1200 1400 G [ d B ] Time Slots 20 Hz 6 Hz 2 Hz Figure 1.3: Some gain pro les of channels with multipath fading, with 2, 6 and 20 Hz Doppler Band. affects the propagation behavior for each connection and, as a result, the power received at the BS. Therefore, since the MT changes its position during the connection, the serving BS could vary with the time; in fact, if the trajectory of the MT crosses one or more cells (in the electromagnetic sense), i.e. it is not restricted inside one only cell, the radio control has to pass from a BS to an other one. Then, a mechanism (named Handover) to maintain the continuity of connections with a given QoS, is needed; this procedure requires the coordination of the BSs involved to allow Seamless Service. Besides, the users are scattered all over the service area and access the network in a random manner; their spatial distribution is also random and can affect the system performance if the service area has not a homogeneous load. To optimally serve different mobility patterns, multiple-cell-layer architectures are taken into account, as picturesquely shown in Fig. 1.5. In this context, larger cells (e.g. macro- cells) serve faster low density traf c (for instance, voice users, traveling by train, in suburban scenario), whereas smaller cells (e.g. pico-cells) serve quasi-stationary high density traf c (for instance, data users, working at of ce, in urban scenario). The Handover mechanism consist of two sub-procedures [20, 21], as follows. Using UMTS terminology, Fig. 1.6 shows this concept in a a simple one-dimensional scenario. A change of connection control is executed in the access part of the Network at the ra-

Tesi di Dottorato

Dipartimento: DEIS - CSITE/CNR

Autore: Mirko Ferracioli Contatta »

Composta da 129 pagine.


Questa tesi ha raggiunto 1073 click dal 20/03/2004.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.