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University of Malaya

Dr.HarikrishnanDepartment of Electrical Engineering

e-mail: hrkhari@um.edu.my

KXGT 6106Wireless Communication

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Course Outline

Course work (mini project) 20 %

Test 20 %Final Examination 60 %

University of Malaya KXGT 6106 HRK 2/18

Reference

T. S. Rappaport Wireless Communications : Principles and Practices Pren.Hall, 2002

J. Schiller Mobile Communications Addison Wesley, 2005

S. Haykin and M. Moher Modern Wireless Communications Prentice Hall, 2005

W. C. Y. Lee Wireless & Cellular Telecommunications 3 rd Edition, McGraw Hill, 2006

IEEE Transactions on Wireless Communications.

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Wireless Communication Channel

Wireless communication channel : includes all of the antenna and propagation effects within it.

An understanding of the wireless channel is an essential part of the understanding of theoperation, design and analysis of any wireless system, whether it be for cellular mobile phones,

for radio paging or for mobile satellite systems.

Figure 1

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The classic architecture of a generic communication system is illustrated in Figure 2.

An information source (e.g. a person speaking, a video camera, a computer sending data)attempts to send information to a destination (a person listening, a video monitor, a computer

receiving data).

Wireless Communication Channel (contd)

The data is converted into a signal suitable for sending by the transmitter and is then sentthrough the channel.

The channel itself modifies the signal in ways which may be more or less unpredictable to thereceiver, so the receiver must be designed to overcome these modifications and hence todeliver the information to its final destination with as few errors or distortions as possible.

Figure 2

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The representation in Figure 2 applies to all types of system, whether wireless or otherwise.

In the wireless channel specifically, the noise sources can be subdivided into multiplicative andadditive effects, as illustrated in Figure 3.

Wireless Communication Channel (contd)

The additive noise arises from noise generated within the receiver itself, such as thermal andshot noise in passive and active devices and also from external sources such as atmosphericeffects, cosmic radiation and interference from other transmitters and electrical appliances.

The multiplicative noise arises from the various processes encountered by transmitted waveson their way from the transmitter antenna to the receiver antenna :

Figure 3

a. The directional characteristics of both the transmitter and receiver antennas

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Wireless Communication Channel (contd)

It is conventional to subdivide the multiplicative processes in the channel into three types offading : path loss, shadowing ( or slow fading ) and fast fading ( or multipath fading ), whichappear as in Figure 4.

b. Reflection (from the smooth surfaces of walls and hills).

c. Absorption (by walls, trees and by the atmosphere)

d. Scattering (from rough surfaces such as the sea, rough ground and the leaves andbranches of trees)

e. Refraction (due to atmospheric layers and layered or graded material)

All of these processes vary as the relative positions of the transmitter and receiver change andas any contributing objects between the antennas are moved.

Figure 4

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An example of the three fading processes is illustrated in Figure 5: it shows the signal receivedby a mobile receiver moving away from a transmitting base station.

Wireless Communication Channel (contd)

Figure 5University of Malaya KXGT 6106 HRK 7/18

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The path loss is an overall decrease in field strength as the distance between the transmitterand the receiver increases. The physical processes which cause it are the outward spreading ofwaves from the transmit antenna and the obstructing affects of trees and buildings.

Superimposed on the path loss is the shadowing, which changes more rapidly, with significantvariations over distances of hundreds of metres and generally involving variations up to 20 dB.Shadowing arises due to the varying nature of the particular obstructions between the base andthe mobile, such as particular tall buildings or dense woods.

Wireless Communication Channel (contd)

Fast fading involves variations on the scale of a half-wavelength (50 cm at 300 MHz, 17 cm at900 MHz) and frequently introduces variations as large as 35 to 40 dB. It results from theconstructive and destructive interference between multiple waves reaching the mobile from thebase station

The Electromagnetic Spectrum

Practical radio communication takes place at frequencies from around 3 kHz to 300 GHz, whichcorresponds to wavelengths in free space from 100 km to 1 mm.

The basic resource exploited in wireless communication systems is the electromagneticspectrum, illustrated in Figure 6.

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The Electromagnetic Spectrum (contd)

The frequencies chosen for new systems tended to increase over the years as the demand forwireless communication has increased : this is because enormous bandwidths are available atthe higher frequencies.

This shift has created challenges in the technology needed to support reliable communications,but it does have the advantage that antenna structures can be smaller in absolute size tosupport a given level of performance.

As the size of obstructions relative to a wavelength increases, their obstructing effects also tendto increase reducing the range for systems operated at increasing frequencies.

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System Types

Figure 7 shows the six types of wireless communication system which are specifically treated.

Satellite fixed links : These are typically created between fixed earth stations with large dishantennas and geostationary earth-orbiting satellites. The propagation effects are largely due tothe Earths atmosphere, including meteorological effects such as rain. Usually operated in the

SHF and EHF bands.

Figure 7

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Terrestrial fixed links : Used for creating high data rate links between points on the earth, forservices such as telephone and data networks, plus interconnections between base stations incellular systems. Also used for covering wide areas in urban and suburban environments fortelephone and data services to residential and commercial buildings. Meteorological effects areagain significant, together with the obstructing effects of hills, trees and buildings. Frequenciesfrom VHF through to EHF are common.

Megacells : These are provided by satellite systems (or by high altitude platforms such as

System Types (contd)

,

low user densities. A single satellite in a low earth orbit would typically cover a region of 1000km in diameter. The propagation effects are dominated by objects close to the user, butatmospheric effects also play a role at higher frequencies. Most systems in the near future willoperate at L and S bands to provide voice and low rate data services, but systems operating ashigh as Ka band will soon be deployed to provide internet access at high rates over limited

areas.

Macrocells : Designed to provide mobile services (including both voice and paging), particularlyoutdoors, to rural, suburban and urban environments with medium traffic densities. Base stationantenna heights are greater than the surrounding buildings, providing a cell radius from around

1 km to many tens of kilometres. Mostly operated at VHF and UHF.University of Malaya KXGT 6106 HRK 11/18

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System Types (contd)

Microcells : Designed for high traffic densities in urban and suburban areas to users bothoutdoors and within buildings. Base station antennas are lower than nearby building rooftops,so coverage area is defined by street layout. Cell length up to around 500 m. Again mostlyoperated at VHF and UHF, but services as high as 60 GHz have been proposed.

Picocells : Very high traffic density or high data rate applications, indoor environments. Usersmay be both mobile and fixed : fixed users are exemplified by wireless local area networksbetween computers. Coverage is defined by the shape and characteristics of rooms, and

.

Coverage and mobility : The system must be available at all locations where users wish to useit. In early development of a new system, this implies outdoor coverage over a wide area. Asthe system is developed and users become more demanding, the depth of coverage must be

extended to include indoor locations. In order to operate with a single device between differentsystems, the systems must provide mobility with respect to the allocation of resources andsupport of interworking between different standards.

Capacity : As the number of users in a mobile system grow, the demands placed on the

resources available from the allocated spectrum grow proportionally. These demands are

Aims of Cellular Systems

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Aims of Cellular Systems (contd)

exacerbated by increasing use of high data rate services. This necessitates dense reuse ofchannels between cells in order to minimize problems with blocked or dropped calls. If a call isblocked, users are refused access to the network because there are no available channels. If acall is dropped, it may be interrupted because the user moves into a cell with no free channels.

Dropped calls can also arise from inadequate coverage.

Quality : In a mature network, the emphasis is on ensuring that the services provided to theusers are of high quality this includes the perceived speech quality in a voice system and thebit error rate (BER) in a data system.

Cellular Networks Figure 8 shows the key elements of a standard cellular network.

The central hub of the network is the mobile switching centre (MSC), often simply called theswitch. This provides connection between the cellular network and the public switched

telephone network (PSTN) and also between cellular subscribers.

Details of the subscribers for whom this network is the home network are held on a databasecalled the home location register (HLR), while the details of subscribers who have entered thenetwork from elsewhere are on the visitor location register (VLR). This details includesauthentication and billing details.

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Cellular Networks (contd)

The coverage area of the network is handled by a large number of base stations. The basestation subsystem (BSS) is composed of a base station controller (BSC) which handles thelogical functionality, plus one or several base transceiver stations (BTS) which contain air

interface (AI) with the mobile stations (MS).The air interface includes all of the channel effects as well as the modulation, demodulation andchannel allocation procedures within the MS and BTS. A single BSS may handle 50 calls andan MSC may handle some 100 BSSs.

Figure 8

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Cellular Concept

Each BTS generically known as base station (BS), must be designed to cover as completely aspossible, a designated area or cell as illustrated in Figure 9.

The power loss involved in transmission between the base and the mobile is the path loss anddepends particularly on antenna height, carrier frequency and distance. An approximate modelof the path loss is given by :

Figure 9

2R m b

4T c

P 1 h hk

P L r= =

(1)

where P R is the power received at the mobile input terminals, P T is the base station transmit

power, h m and h b are the mobile and base station antenna height respectively, r is the distanceUniversity of Malaya KXGT 6106 HRK 15/18

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Cellular Concept (contd)

The quantity L is the path loss and depends mainly on the characteristics of the path betweenthe base station and the mobile rather than on equipment in the system. The precisedependencies are functions of the environment type (urban, rural, etc .)

At higher frequencies the range for a given path loss is reduced, so more cells are required tocover a given area. To increase the cell radius for a given transmit power, the key variable

between the base station and the mobile, f c is the carrier frequency and k is some constant ofproportionality.

surrounding clutter (trees, buildings, etc. ), but no so high as to cause excessive interference todistant co-channel cells. It must be chosen with due regard for environment and local planningregulations. Natural terrain features and buildings can be used to increase the effective antennaheight to increase coverage, or to control the limits of coverage by acting as shieldingobstructions.

When multiple cells and multiple users are served by a system, the system designer mustallocate the available channels ( in frequency, time and space ) to the cells in such a way as tominimize the interaction between cells. One approach would be to allocate completely distinctchannels to very cell, but this would limit the total number of cells possible in a systemaccording to the spectrum which the designer has available.

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Cellular Concept (contd)

Instead, the key idea of cellular systems is that it is possible to serve an unlimited number ofsubscribers, distributed over an unlimited area, using only a limited number of channels, byefficient channel reuse.

A set of cells, each of which operates on a different channel (or group of channels) is groupedtogether to form a cluster. The cluster is then repeated as many times as necessary to cover avery wide area.

Figure 10

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Cellular Concept (contd)

Figure 10 illustrates the use of a seven cell cluster. The use of hexagonal areas to representthe cells is highly idealized, but it is still commonly when path loss is treated as a function ofdistance only, within a uniform environment.

The smaller the cluster size, therefore, the more efficiently the available channels are used. Theallowable cluster size, and hence the spectral efficiency of the system is limited by the level ofinterference the system can stand for acceptable quality.

This level is determined by the ratio between the wanted and interfering signals which can betolerated for reasonable quality communications in the system. These levels depend on thetypes of modulation, coding and synchronization schemes employed within the base station andmobile.

The ratio is called the threshold carrier-to-interference power ratio (C/I).

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