30-07-2012, 11:20 AM
A Preemptive Chanel Allocation for Multimedia in Wireless Mobile Network
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INTRODUCTION
The introduction of next-generation cellular technology, such as W-CDMA , has brought in a rapid growth of multimedia traffic. In 3G wireless networks, various types of multimedia traffic may come to reality . Among these traffic, some have low bandwidth and stringent delay requirements, such as conversational traffic; some have high bandwidth requirement initially but a certain level of packet loss and bandwidth degradation are tolerable in case of congestion, such as streaming video; while the others are delay insensitive, such as interactive and background traffic. Therefore, it is very crucial that an adequate channel allocation scheme should be developed to fulfill individual QOS requirements in the mobile wireless networks.
Preemption policies subject to a certain set of bandwidth constraints have been recently proposed for differentiated-service-aware traffic in wired networks. In cellular networks, mobile users may easily crossover the cell boundary that generates handoff calls. Thus, call requests to a cellular system are usually composed of two types of calls, new calls and handoff calls. From a users perspective, the dropping of a handoff call gives more serious impact than the blocking of a new call. Therefore, preemptive channel assignment schemes become very important in reducing handoff-call dropping probabilities. For instance, the sub-rating channel assignment strategy, proposed by Lin et al., would accept a handoff call by ‘‘sub-rating’’ an existing connection when there are no free channels in the system. However, Lins strategy simply considered one type of traffic, voice.
Generations of Mobile Networks
The first radiotelephone service was introduced in the US at the end of the 1940s, and was meant to connect mobile users in cars to the public fixed network. In the 1960s, a new system launched by Bell Systems, called Improved Mobile Telephone Service” (IMTS), brought many improvements like direct dialing and higher bandwidth. The first analog cellular systems were based on IMTS and developed in the late 1960s and early 1970s. The systems were “cellular” because coverage areas were split into smaller areas or “cells”, each of which is served by a low power transmitter and receiver.
The Basics of Cellular Technology
Mobile operators use radio spectrum to provide their services. Spectrum is generally considered a scarce resource, and has been allocated as such. It has traditionally been shared by a number of industries, including broadcasting, mobile communications and the military. At the World Radio Conference (WRC) in 1993, spectrum allocations for 2G mobile were agreed based on expected demand growth at the time. At WRC 2000, the resolutions of the WRC expanded significantly the spectrum capacity to be used for 3G, by allowing the use of current 2G spectrum blocks for 3G capacity Cellular technology, was enhanced through a division of frequencies, and the resulting addition of available channels. However, this reduced the total bandwidth available to each user, affecting the quality of service. Cellular technology allowed for the division of geographical areas, rather than frequencies, leading to a more efficient use of the radio spectrum. This geographical re-use of radio channels is known as “frequency reuse”. In a cellular network (Fig. 1.1), cells are generally organized in groups of seven to form a cluster. There is a “cell site” or “base station” at the centre of each cell, which houses the transmitter/receiver antennae and switching equipment. The size of a cell depends on the density of subscribers in an area: for instance, in a densely populated area, the capacity of the network can be improved by reducing the size of a cell or by adding more overlapping cells. This increases the number of channels available without increasing the actual number of frequencies being used.