31.1 PRINCIPLES OF CELLULAR MOBILE COMMUNICATIONS

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31.1 PRINCIPLES OF CELLULAR MOBILE COMMUNICATIONS

The growth of mobile communications during the last decade of the twentieth century has been exponential. Throughout the world, analog mobile networks slowly are being replaced by digital mobile networks. Though voice communication continues to be the main application in mobile systems, data services, particularly accessing the Internet, are catching up fast and, technology forecasters say that wireless Internet access devices will outnumber wired Internet access devices in the near future.

In cellular mobile communication systems, there are many standards, mainly because the systems evolved from analog systems in different countries. Cellular systems in the U.S., Japan, and Europe are based on different standards. In this section, we will study the general principles of cellular mobile communication systems and study the Global System for Mobile Communications (GSM), which has been the widely accepted standard in Europe, Asia, and Africa.

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Though the killer application on mobile communication systems continues to be voice communication, data applications are now catching up very fast.

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31.1.1 Single-Cell Systems

The first mobile communication systems were similar to TV broadcasting systems. A base station was located at the highest point in a service area. The base station had a very powerful transmitter. The base station catered to an area of about 50 km radius. The mobile terminals, consisting of antenna, radio transmitter, receiver, and associated control circuitry, were car mounted. For communication by one mobile terminal, one channel is used. A channel consists of two frequencies: one frequency for communication from the base station to the mobile terminal (called downlink) and one frequency for communication from the mobile terminal to the base station (called uplink). Each base station is assigned a number of channels, based on the subscriber density in the region.

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In single-cell systems, the entire service area is covered by one base station. The drawbacks of this approach are high-power transmitters are required, power consumption of the mobile phone will be high, and expansion of the system is difficult.

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These traditional mobile systems can be called single-cell systems because the entire coverage area is only one cell. The disadvantages of this type of system are:

  • Very powerful transmitters are required at the base station and the mobile terminals-high-power transmitters are costly.

  • The capacity of the system will be very low because only a fixed number of channels are available for a service area (because of the limited radio spectrum).

  • The number of subscribers who can make calls simultaneously also will be limited.

  • The size of the mobile terminals will be large because of the high-power transmitters.

  • Due to high-power radio devices on the mobile devices, the power consumption will be very high, calling for frequent recharging of the batteries of the mobile devices.

  • Expansion of the system to cater to a higher number of subscribers will be very difficult.

    To overcome these limitations, multicell systems have been developed.

31.1.2 Multicell Systems

Bell Laboratories introduced the concept of multicell systems in the early 1970s. The Nordic countries were the first to introduce commercial multicell mobile systems in 1981. In a multicell system, the service area is divided into cells as shown in Figure 31.1. Cell is the basic geographic unit in a mobile system. Each cell is represented by a hexagon. Each cell has a base station with a low-power transmitter. The size of the cell may vary, depending on the terrain: natural terrain such as mountains and lakes or manmade terrain such as buildings. Each cell is allocated some channels, and all the mobiles, when they are in that cell, use these channels for communication. The main attraction of this approach is the use of very low-power transmitters at the base stations as well as in the mobile phones.

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Figure 31.1: Multicell system with service area divided into cells (seven-cell cluster).

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In multicell systems, the service area is divided into small regions called cells. Each cell will have a base station with a low-power transmitter. Adjacent cells cannot use the same frequencies.

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In a multicell system, two adjacent cells cannot use the same channel because there will be interference. When a mobile subscriber moves from one cell to another cell while the call is in progress, there are two options: either the call has to be dropped or the mobile terminal has to switch to the channel used by the new cell. Since dropping a call is not acceptable, the other option is followed. When the mobile terminal is at the edge of one cell, the signal strength goes down, and the mobile terminal monitors the signal strengths of the channels in the adjacent cells and switches to the channel for which signal strength is high. The call will not be dropped, and conversation can continue using the new channel. This process is called handoff or handover. Certainly, handover introduces complexity in cellular mobile communication, but it has many advantages:

  • Because of the low power required at each base station and the mobile terminals, low-cost systems can be developed. The size of the mobile terminals also will be smaller.

  • Depending on the distance between the mobile terminal and the base station, variable power levels can be used for communication, reducing the power requirements and hence the battery requirements of the mobile terminals.

  • Based on the number of channels allocated for each cell, there will be a limit on the number of simultaneous calls. If the subscriber number or the traffic increases in a cell over a period of time, a cell can be split and new base stations can be installed.

  • The cell size is not fixed, so cells can be of different sizes. In urban areas with high subscriber density, cell size can be small, and in rural areas cell size can be large.

Note 

In a multicell system, when a mobile device moves from one cell to another cell, the frequency of operation will change. This process is called handover.

Frequency reuse: Every cellular service provider will be allocated a fixed number of channels for use in a service area. The service provider has to make best use of the channels to provide the maximum number of simultaneous calls. Though adjacent cells cannot use the same channels, the same channels can be reused in other cells, provided there is a minimum separation distance between the cells using the same channels. The concept of clusters is of importance here. A cluster is a group of cells, and no channels are reused within a cluster.

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The advantages of multicell systems are that only low-power transmitters are required at base stations and mobile terminals, frequencies can be reused, and expansion of the network is easy.

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In frequency reuse, each cell is assigned a group of radio channels. The same channels can be reused in another cluster of cells. In Figure 31.1, a 7-cell cluster is shown. Cells denoted by 1 in all three clusters can use the same set of channels.

Cell splitting: For economic reasons, at the beginning, the cellular service provider will not design the cellular system with cells of small size. The service provider may have large cells to start with and, as the subscriber load increases, the cells will be split and more base stations will be installed, and the frequency reuse pattern is reworked.

Note 

In multicell systems, each cell is assigned a group of radio channels. The same radio channels can be reused in another cell, provided a minimum distance is maintained between the two cells using the same radio channels.



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Principles of Digital Communication Systems and Computer Networks
Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)
ISBN: 1584503297
EAN: 2147483647
Year: 2003
Pages: 313
Authors: K V Prasad

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