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Branch : Information Technology, VII SemesterCourse: Wireless & Mobile Computing

AntennaIn the 1890s, there were only a few antennas in the world. These rudimentary devices were primarlya part of experiments that demonstrated the transmission of electromagnetic waves. By World WarII, antennas had become so ubiquitous that their use had transformed the lives of the average personvia radio and television reception. The number of antennas in the United States was on the order ofone per household, representing growth rivaling the auto industry during the same period.By the early 21st century, thanks in large part to mobile phones, the average person now carries oneor more antennas on them wherever they go (cell phones can have multiple antennas, if GPS isused, for instance). This significant rate of growth is not likely to slow, as wireless communicationsystems become a larger part of everyday life. In addition, the strong growth in RFID devicessuggests that the number of antennas in use may increase to one antenna per object in the world(product, container, pet, banana, toy, cd, etc.). This number would dwarf the number of antennas inuse today. Hence, learning a little (or a large amount) about of antennas couldn't hurt, and will

contribute to one's overall understanding of the modern world.Classification of antenna

General classification

Antenna can be classified on various bases such as its geometrical shape and size, itsdirectivity, its radiation pattern, its application and its frequency and wavelength. Antennas are soclassified in order to have the proper selection of different type of antennas for various applicationsto meet the requirement. Here are some brief details

about different types of antenna.

Antenna can be classified on the basis of:

i) Geometrical shape & size

ii) Directivity

iii) Radiation pattern

iv) Application

i) Geometrical shape & size

We can classify antenna on the basis of its physical shape & size or its orientation. There are

various kinds of antennas falling in this category ofclassification.

a) Linear wire antennas

- Half-wavelength dipole (mono-pole) antenna, dipole antenna

b) Aperture antennas

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c) Array antennas

d) Microstrip antennas

e) Reflector antennas

f) Lens antennas

ii) Directivity

Antenna can also be classified on the basis of their effective direction, i.e. The direction inwhich the antenna can show its effect (radiation or reception). There are two types of antennasavailable falling in this category:

a) Directional antenna

b) Omni directional antenna

a) Directional antenna

This type of antenna receives or radiates electromagnetic waves more effectively in oneparticular direction than in other directions.

b) Omni directional antenna

This type of antenna radiates or receives electromagnetic waves in all direction except inazimuth plane. This type of antenna is non-directional in azimuth plane and directional in anyorthogonal plane or elevated plane. That means this antenna does not point only in one direction or

it has not the specific direction to radiate or receive electro-magnetic wave in any of orthogonalplane.

iii) Radiation pattern

Basically, there are three types of radiation pattern directional, omni directional and isotropicpattern. Among these, an isotropic antenna is a hypothetical lossless antenna having equal radiationin all directions.

iv) Application

On this basis, different antennas can be deployed into different application to meet ourrequirement. That is we have to choose the best antenna for the specific purpose. We can chooseantennas for mobile communication, for FM & AM broadcasting, for television broadcasting, forsatellite communication, RADARcommunication etc.

Antennas used in cellular mobile communication system

1) Mobile antenna

The requirement of mobile (motor vehicle mounted) antenna is an omni directional antenna,which can be located as high as possible from the point of reception. However the physical

limitation of antenna height on the vehicle restricts this requirement. Generally the antenna shouldat least clear the top of the vehicle.

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a. Roof mounted antenna

The antenna pattern of a roof mounted antenna is more or less uniformly distributed around themobile unit when measured at an antenna range in the 3 dB high gain antenna shows a 3 dB gainover the quarter wave antenna. However the gain of the antenna used at the mobile unit must belimited to 3 dB because the cell site antenna is arely as high as the broadcasting antenna and out ofsite conditions often prevail. The mobile antenna with a gain of more than 3 dB can receive only a

limited portion of total multipath signal in the elevation as measured under the out of site condition.

b. Glass mounted antenna

There are many kinds of glass-mounted antennas. Energy is coupled through the glass:therefore there is no need to drill a hole. However, some energy is dissipated on passage through theglass. The antenna gain range is 1 to 3 dB depending on the operating frequency. The position of theglass-mounted antenna is always lower than that of the roof-mounted antenna; generally there is a 3dB difference between these two type of antenna. Also glass-mounted antennas cannot be installedon the shaded glass found in some motor vehicles because this type of glass has a high metalcontent.

c. Mobile high gain antennas

A high gain antenna used on a mobile unit has been studied. This type of high gain antennashould be distinguished from the directional antenna. In the directional antenna, the antenna beampattern is suppressed horizontally; in the high gain antenna, the pattern is suppressed vertically. Toapply either a directional antenna or high gain antenna for reception in a radio environment, wemust know the origin of the signal. If we point the directional antenna opposite to the transmittersite, we would in theory receive nothing.

In a mobile radio environment, the scattered signals arrived at mobile unit from every directionwith equal probability. That is why an omni directional antenna is used the scattered signals alsoarrived from different elevation angles. Lee and Brandt used two types of antenna, one /4 whip antenna with an elevation coverage of 39o and one of 4 dB gain antenna (4 dB gain with respect tothe gain of a dipole) with an elevation coverage of 16 o, and measured the angle of signal arrival ionthe sub urban Keyport- Matawan area of new jersey. There are two type of test: a line of sight(LOS) condition and an out of sight (OOS) condition. In Lee and Brandt's study the transmitter was

located at an elevation of approximately of 100 m (300 ft) above sea level. The measured area wereabout 12 m (40 ft) above sea level and the path length about 3mi.The received signal from the 4 dBgain antenna was 4 dB stronger than that from the whip antenna under line of sight conditions. This

is what we would expect. However, the received signal from the 4 dB gain antenna was only about2 dB stronger than that from the whip antenna under OOS conditions.this is surprising. The regionfor the latter observation is that the scattered signals arriving under OOS conditions are spread overa wide elevation angle. A large portion of the signal outside the elevation angle of 16 o cannot bereceived by high gain antenna we may calculate the portion being received by the high gain antennafrom the measured beam width (the beam width can be roughly obtained from the equation:

d. Horizontally oriented space diversity antenna

A two-branch space diversity receiver mounted on a motor vehicle has the advantage of reducing

fading and thus can operate at a lower reception level. We must consider the following factor. Thetwo antennas can be mounted either in line with or perpendicular to the motion of the vehicle.Theoretical analysis and measured data indicate that the inline arrangement of the two antennas

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produces fewer level crossings that is less fading that the perpendicular arrangement does.

e. Vertically oriented space diversity antenna

The vertical separation between the two space diversity antennas can be determined from thecorrelation between their received signals. A set of measured data was obtained by using two

antennas vertically separated by 1.5 wavelengths.

2) Microwave antenna

Microwave antenna location

Sometimes the reception is poor after the microwave antenna has been mounted on the antennatower. A quick way to check the installation before making any other changes is to move themicrowave antenna around within a 2 to 4 ft radius of the previous position and check the receptionlevel. Surprisingly favorable results can be obtained immediately because multipart cancellation isavoided as a result of changing reflected paths at the receiving antenna. Also, at any fixedmicrowave antenna location, the received signal level over a 24-hr time period varies.

Characteristics of microwave antennas:

Microwave antennas can afford to concentrate their radiated power in a narrow beam becauseof the size of the antenna in comparison to the wavelength of the operating frequency; thus. Highantenna gain is obviously desirable. Some of the more significant characteristics are discussed inthe following paragraphs.

Propagation modes

Wireless transmissions propagate in three modes: ground-wave, sky-wave, and line-of-sight.Ground wave propagation follows the contour of the earth, while sky wave propagation usesreflection by both earth and ionosphere. Finally line of sight propagation requires the transmittingand receiving antennas to be within line of sight of each other. Which of these propagation modesdominates depends on the frequency of the underlying signal.

Examples of ground wave and sky wave communication are AM radio and international broadcasts

such as BBC. Above 30 MHz, neither ground wave nor sky wave propagation operates and thecommunication is through line of sight.

If h is the height of a transmitting (resp., receiving) antenna in meters, then the distance to thereceiver (resp., transmitter) for line-of-sight transmission should be at most

d = 3.57 h kms.

This can be proved using elementary geometry. Since microwaves are bent or refracted by the

atmosphere, the effective line of sight is, in fact, larger than the true line of sight. We introduce anadjustment factor K to capture the refraction effect and obtain

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d = 3.57 Kh kms.

If the two antenna have heights h1 and h2 meters, then the distance between them for LOS prop-

agation should be at most

3.57 K( h1 + h2 ) kms.

Fading

Fading refers to the variation of the signal strength with respect to time/distance and is widelyprevalent in wireless transmissions. The most common causes of fading in the wireless environment

are multipath propagation and mobility (of objects as well as the communicating devices).Multipath propagation. In wireless media, signals propagate using three principles: reflection,

scattering, and diffraction. Reflection occurs when the signal encounters a large solid surface,whose size is much larger than the wavelength of the signal, e.g., a solid wall. Diffraction occurswhen the signal encounters an edge or a corner, whose size is larger than the wavelength of thesignal, e.g., an edge of a wall. Finally, scattering occurs when the signal encounters small objects ofsize smaller than the wavelength of the signal. One consequence of multipath propagation is thatmultiple copies of a signal propagation along multiple different paths, and arrive at any point atdifferent times.So the signal received at a point is not only affected by the inherent noise, distortion, attenuation,

and dispersion in the channel but also the interaction of signals propagated along multiple paths.Delay spread. Suppose we transmit a probing pulse from a location and measure the received signalat the recepient location as a function of time. The signal power of the received signal spreads over

time due to multipath propagation. The delay spread is determined by the density function of theresulting spread of the delay over time. Average delay spread and root mean square delay pread aretwo parameters that can be calculated. Doppler spread. This is a measure of spectral broadeningcaused by the time rate of change of the mobile radio channel. It is caused by either relative motionbetween the mobile and base station or by movement of objects in the channel. When a puresinusoid of frequency f is transmitted the received signal spectrum, the Doppler spectrum, is therange [f fd , f |fd ], where fd is referred to as the Doppler spread, given by

v

fd = cos ,

where v is the relative velocity of the mobile and is the angle formed by the direction of themotion and that of the signal, and is the wavelength of the carrier.

3

The relative size of the delay spread, as compared to the symbol period, has an impact on whetherthe effect of fading is uniform over all frequencies or varies across frequencies in the spectrum ofthe channel. Flat fading occurs when the bandwidth of the signal is less than the bandwidth of thechannel, or delay spread is less than the symbol period. Frequency selective fading occurs when thebandwidth of the signal is greater than the bandwidth of the the channel, or the delay spread isgreater than symbol period.

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There are several fading models that capture different multipath propagation characteristics.Rayleigh fading models a worst-case scenario in which no path dominates (in fact, a LOS path maynot even exist). Rician fading assumes that one of the paths, usually LOS, dominates all the otherpaths. When the velocity of the mobile is high, the Doppler spread is high, and the the resultingchannel variations are faster than that of the baseband signal; this is referred to as fast fading. Whenchannel variations are slower than the baseband signal variations, then the resulting fading isreferred to as slow fading.

Channel access method

In telecommunications and computer networks, a channel access method or multiple access methodallows several terminals connected to the same multi-point transmission medium to transmit over itand to share its capacity. Examples of shared physical media are wireless networks, bus networks,ring networks, hub networks and half-duplex point-to-point links.

A channel-access scheme is based on a multiplexing method, that allows several data streams orsignals to share the same communication channel or physical medium. Multiplexing is in thiscontext provided by the physical layer. Note that multiplexing also may be used in full-duplexpoint-to-point communication between nodes in a switched network, which should not beconsidered as multiple access.

A channel-access scheme is also based on a multiple access protocol and control mechanism, alsoknown as media access control (MAC). This protocol deals with issues such as addressing,assigning multiplex channels to different users, and avoiding collisions. The MAC-layer is a sub-layer in Layer 2 (Data Link Layer) of the OSI model and a component of the Link Layer of theTCP/IP model.

Frequency Division Multiple Access (FDMA)

The frequency division multiple access (FDMA) channel-access scheme is based on the frequency-division multiplex (FDM) scheme, which provides different frequency bands to different data-streams. In the FDMA case, the data streams are allocated to different users or nodes. An exampleof FDMA systems were the first-generation (1G) cell-phone systems. A related technique is wave-length division multiple access (WDMA), based on wavelength division multiplex (WDM), wheredifferent users get different colors in fiber-optical communication.

Time division multiple access (TDMA)

The time division multiple access (TDMA) channel access scheme is based on the time divisionmultiplex (TDM) scheme, which provides different time-slots to different data-streams (in the

TDMA case to different transmitters) in a cyclically repetitive frame structure. For example, user 1may use time slot 1, user 2 time slot 2, etc. until the last user. Then it starts all over again,but sometimes user 1 may use time slot 1 in first frame and use another time slot in next frame.

Packet mode

Packet mode multiple-access is typically also based on time-domain multiplexing, but not in acyclically repetitive frame structure, and therefore it is not considered as TDM or TDMA. Due to itsrandom character it can be categorised as statistical multiplexing methods, making it possible toprovide dynamic bandwidth allocation.

Code division multiple access (CDMA)The code division multiple access (CDMA) scheme is based on spread spectrum. An example is the3G cell phone system.

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Demand Assigned Multiple Access (DAMA)

is a technology used to assign a channel to clients that don't need to use it constantly. DAMAsystems assign communication channels based on requests issued from user terminals to a networkcontrol system. When the circuit is no longer in use, the channels are then returned to the centralpool for reassignment to other users.

Channels are typically a pair of carrier frequencies (one for transmit and one for receive), but can be

other fixed bandwidth resources such as timeslots in a TDMA burst plan or even physical party linechannels. Once a channel is allocated to a given pair of nodes, it is not available to other users in thenetwork until their session is finished.

It allows utilizing of one channel (radio or baseband frequency, timeslot, etc.) by many userssequentially at different times. This technology is mainly useful with sparsely used networks oftransient clients, as opposed to PAMA (Permanently Assigned Multiple Access). By using DAMAtechnology the number of separate nodes that can use a limited pool of circuits can be greatlyincreased at the expense of no longer being able to provide simultaneous access for all possiblepairs of nodes. A five-channel DAMA network can only have five simultaneous conversations butcould have any number of nodes. A five-channel PAMA network permanently supports five

simultaneous conversations, with channel ownership remaining with their permanently assignednodes even when idle.

Packet-reservation multiple access (PRMA)Packet-reservation multiple access (PRMA) is viewed as a merger of slotted ALOHA and time-division multiple access (TDMA). Dispersed terminals transmit packets of speech information to acentral base station. When its speech activity detector indicates the beginning of a talkspurt, aterminal contends with other terminals for access to an available time slot. After the base stationdetects the first packet in the talkspurt, the terminal reserves future time slots for transmission ofsubsequent speech packets. The influence of several variables on PRMA efficiency, defined as thenumber of conversations per channel, is examined. The number of channels is the ratio of

transmission rate to speech coding rate. It is found that with 32-kb/s speech coding and 720-kb/stransmission (22.5 channels), PRMA supports up to 37 simultaneous conversations, or 1.64conservations per channel. The number of conversations per channel is at least 1.5 over a widerange of packet sizes (8 ms of speech per packet to 34 ms) and for all systems with 16 or morechannels (transmission rate ⩾512 kb/s, with 32-kb/s speech coding). Other factors studied arethe sensitivity of the speech activity detector, the retransmission probability of the contentionscheme, and the maximum time delay for the transmission of speech packets

Cell and Sector Terminology

With cellular radio we use a simple hexagon to represent a complex object: the geographical areacovered by cellular radio antennas. These areas are called cells. Using this shape let us picture thecellular idea, because on a map it only approximates the covered area. Why a hexagon and not acircle to represent cells?

When showing a cellular system we want to depict an area totally covered by radio, without anygaps. Any cellular system will have gaps in coverage, but the hexagonal shape lets us more neatlyvisualize, in theory, how the system is laid out. Notice how the circles below would leave gaps in

our layout. Still, why hexagons and not triangles or rhomboids? Read the text below and we'll cometo that discussion in just a bit.

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Notice the illustration below. The middle circles represent cell sites. This is where the base stationradio equipment and their antennas are located. A cell site gives radio coverage to a cell. Do youunderstand the difference between these two terms? The cell site is a location or a point, the cell is awide geographical area. Okay?

Most cells have been split into sectors or individual areas to make them more efficient and to letthem to carry more calls. Antennas transmit inward to each cell. That's very important to remember.They cover a portion or a sector of each cell, not the whole thing. Antennas from other cell sitescover the other portions. The covered area, if you look closely, resembles a sort of rhomboid, as

you'll see in the diagram after this one. The cell site equipment provides each sector with its own setof channels. In this example, just below , the cell site transmits and receives on three different setsof channels, one for each part or sector of the three cells it covers.

Is this discussion clear or still muddy? Skip ahead if you understand cells and sectors or come backif you get hung up on the terms at some later point. For most of us, let's go through this again, this

time from another point of view. Mark provides the diagram and makes some key points here:

"Most people see the cell as the blue hexagon, being defined by the tower in the center, with theantennae pointing in the directions indicated by the arrows. In reality, the cell is the red hexagon,with the towers at the corners, as you depict it above and I illustrate it below. The confusion comesfrom not realizing that a cell is a geographic area, not a point. We use the terms 'cell' (the coveragearea) and 'cell site' (the base station location) interchangeably, but they are not the same thing.

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Basic Theory and OperationCell phone theory is simple. Executing that theory is extremely complicated. Each cell site has abase station with a computerized 800 or 1900 megahertz transceiver and an antenna. This radioequipment provides coverage for an area that's usually two to ten miles in radius. Even smaller cellsites cover tunnels, subways and specific roadways. The area size depends on, among other things,topography, population, and traffic.

When you turn on your phone the mobile switch determines what cell will carry the call and assignsa vacant radio channel within that cell to take the conversation. It selects the cell to serve you bymeasuring signal strength, matching your mobile to the cell that has picked up the strongest signal.Managing handoffs or handovers, that is, moving from cell to cell, is handled in a similar manner.The base station serving your call sends a hand-off request to the mobile switch after your signaldrops below a handover threshold. The cell site makes several scans to confirm this and thenswitches your call to the next cell. You may drive fifty miles, use 8 different cells and never oncerealize that your call has been transferred. At least, that is the goal. Let's look at some details of thisamazing technology, starting with cellular's place in the radio spectrum and how it began.

The FCC allocates frequency space in the United States for commercial and amateur radio services.Some of these assignments may be coordinated with the International Telecommunications Unionbut many are not. Much debate and discussion over many years placed cellular frequencies in the800 megahertz band. By comparison, PCS or Personal Communication Services technology, still

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cellular radio, operates in the 1900 MHz band. The FCC also issues the necessary operating licensesto the different cellular providers.

Although the Bell System had trialed cellular in early 1978 in Chicago, and worldwide deploymentof AMPS began shortly thereafter, American commercial cellular development began in earnestonly after AT&T's breakup in 1984. The United States government decided to license two carriers ineach geographical area. One license went automatically to the local telephone companies, in

telecom parlance, the local exchange carriers or LECs. The other went to an individual, a companyor a group of investors who met a long list of requirements and who properly petitioned the FCC.And, perhaps most importantly, who won the cellular lottery. Since there were so many qualifiedapplicants, operating licenses were ultimately granted by the luck of a draw, not by a spectrumauction as they are today.

The local telephone companies were called the wireline carriers. The others were the non-wirelinecarriers. Each company in each area took half the spectrum available. What's called the "A Band"and the "B Band." The nonwireline carriers usually got the A Band and the wireline carriers got theB band. There's no real advantage to having either one. It's important to remember, though, thatdepending on the technology used, one carrier might provide more connections than a competitordoes with the same amount of spectrum. [See A Band, B Band

Channel Names and Functions

Certain channels carry only cellular system data. We call these control channels. This controlchannel is usually the first channel in each cell. It's responsible for call setup, in fact, many radioengineers prefer calling it the setup channel since that's what it does. Voice channels, bycomparison, are those paired frequencies which handle a call's traffic, be it voice or data, as well assignaling information about the call itself.

A cell or sector's first channel is always the control or setup channel for each cell. You have 21control channels if you have 21 cells. A call gets going, in other words, on the control channel first

and then drops out of the picture once the call gets assigned a voice channel. The voice channel thenhandles the conversation as well as further signaling between the mobile and the base station.

When discussing cell phone operation we call a base station's transmitting frequency the forwardpath. The cell phone's transmitting frequency, by comparison, is called the reverse path. Do notbecome confused. Both radio frequencies make up a channel as we've discussed before but we nowtreat them individually to discuss what direction information or traffic flows. Knowing whatdirection is important for later, when we discuss how calls are originated and how they are handled.

Once the MTSO or mobile telephone switch assigns a voice channel the two frequencies making upthe voice channel handle signaling during the actual conversation. You might note then that a calltwo channels: voice and data. Got it? Knowing this makes many things easier. A mobile's electronicserial number is only transmitted on the reverse control channel. A person tracking ESNs need onlymonitor one of 21 frequencies. They don't have to look through the entire band.

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So, we have two channels for every call with four frequencies involved. Clear? And a forward andreverse path for each frequency. Let's name them here. Again, a frequency is the medium uponwhich information travels. A path is the direction the information flows. Here you go:

--> Forward control path: Base station to mobile

Forward voice path: Base station to mobile

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one point on the Earth to another, or into various parts of the atmosphere. As a form ofelectromagnetic radiation, like light waves, radio waves are affected by the phenomena ofreflection, refraction, diffraction, absorption, polarization and scattering.

Radio propagation is affected by the daily changes of water vapor in the troposphere and ionizationin the upper atmosphere, due to the Sun. Understanding the effects of varying conditions on radiopropagation has many practical applications, from choosing frequencies for international shortwave

broadcasters, to designing reliable mobile telephone systems, to radio navigation, to operation ofradar systems.

Radio propagation is also affected by several other factors determined by its path from point topoint. This path can be a direct line of sight path or an over-the-horizon path aided by refraction inthe ionosphere, which is a region between approximately 60 and 600 km. Factors influencingionospheric radio signal propagation can include sporadic-E, spread-F, solar flares, geomagneticstorms, ionospheric layer tilts, and solar proton events.

Free space propagation

In free space, all electromagnetic waves (radio, light, X-rays, etc.) obey the inverse-square lawwhich states that the power density of an electromagnetic wave is proportional to the inverse of thesquare of the distance from a point source

Doubling the distance from a transmitter means that the power density of the radiated wave at thatnew location is reduced to one-quarter of its previous value.

The power density per surface unit is proportional to the product of the electric and magnetic field

strengths. Thus, doubling the propagation path distance from the transmitter reduces each of theirreceived field strengths over a free-space path by one-half.

Handoff in Wireless Mobile Networks

INTRODUCTION

Mobility is the most important feature of a wireless cellular communication system. Usu- ally,continuous service is achieved by supporting handoff (or handover) from one cell to another.Handoff is the process of changing the channel (frequency, time slot, spreadin code, or combinationof them) associated with the current connection while a call is in progress. It is often initiated either

by crossing a cell boundary or by a deterioration in quality of the signal in the current channel.Handoff is divided into two broad categories hard and soft handoffs. They are also characterized bybreak before make and make be- fore break. In hard handoffs, current resources are releasedbefore new resources are used; in soft handoffs, both existing and new resources are used during thehandoff process. Poorly designed handoff schemes tend to generate very heavy signaling traffic and,thereby, a dramatic decrease in quality of service (QoS). (In this chapter, a handoff is assumed tooccur only at the cell boundary.) The reason why handoffs are critical in cellu-lar communicationsystems is that neighboring cells are always using a disjoint subset of frequency bands, sonegotiations must take place between the mobile station (MS), the current serving base station (BS),and the next potential BS. Other related issues, such as decision making and priority strategiesduring overloading, might influence the overall performance. In the next section, we introducedifferent types of possible handoffs.

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TYPES OF HANDOFFS

Handoffs are broadly classified into two categorieshard and soft handoffs. Usually, the hardhandoff can be further divided into two different typesintra- and intercell handoffs. The softhandoff can also be divided into two different typesmultiway soft handoffs and softer handoffs.In this chapter, we focus primarily on the hard handoff.

Cell SectorizationOne way to increase to subscriber capacity of a cellular network is replace the omni-directionalantenna at each base station by three (or six) sector antennas of 120 (or 60) degrees opening. Eachsector can be considered as a new cell, with its own (set of) frequency channel(s).

The base station can either be located at

the center of the original (large) cell, or

the corners of the original (large) cell.

The use of directional sector antennas substantially reduces the interference among co-channelcells. This allows denser frequency reuse.

Sectorization is less expensive than cell-splitting, as it does not require the acquisition of new base

station sites.

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Cellular traffic

the mobile cellular network aspect of teletraffic measurements. Mobile radio networks have trafficissues that do not arise in connection with the fixed line PSTN. Important aspects of cellular trafficinclude: quality of service targets, traffic capacity and cell size, spectral efficiency and sectorization,traffic capacity versus coverage, and channel holding time analysis.

Teletraffic engineering in telecommunications network planning ensures that network costs are

minimised without compromising the quality of service (QoS) delivered to the user of the network.This field of engineering is based on probability theory and can be used to analyse mobile radionetworks, as well as other telecommunications networks.

A mobile handset which is moving in a cell will record a signal strength that varies. Signal strengthis subject to slow fading, fast fading and interference from other signals, resulting in degradation ofthe carrier-to-interference ratio (C/I). A high C/I ratio yields quality communication. A good C/Iratio is achieved in cellular systems by using optimum power levels through the power control ofmost links. When carrier power is too high, excessive interference is created, degrading the C/I ratiofor other traffic and reducing the traffic capacity of the radio subsystem. When carrier power is toolow, C/I is too low and QoS targets are not met.

Quality of Service targets

At the time that the cells of a radio subsystem are designed, Quality of Service (QoS) targets are set,for: traffic congestion and blocking, dominant coverage area, C/I, dropped call rate, handoverfailure rate, overall call success rate, ...

Traffic load and cell sizeThe more traffic generated, the more base stations will be needed to service the customers. Thenumber of base stations for a simple cellular network is equal to the number of cells. The trafficengineer can achieve the goal of satisfying the increasing population of customers by increasing the

number of cells in the area concerned, so this will also increases the number of base stations. Thismethod is called cell splitting (and combined with sectorization) is the only way of providingservices to a burgeoning population. This simply works by dividing the cells already present intosmaller sizes hence increasing the traffic capacity. Reduction of the cell radius enables the cell toaccommodate extra traffic. The cost of equipment can also be cut down by reducing the number ofbase stations through setting up three neighbouring cells, with the cells serving three 120 sectorswith different channel groups.

Poisson Arrival ProcessA commonly used model for random, mutually independent message arrivals is the Poisson process.

The Poisson distribution can be obtained by evaluating the following assumptions for arrivalsduring an infinitesimal short period of time delta t

The probability that one arrival occurs between tand t+delta tis t+ o( t), where is aconstant, independent of the time t, and independent of arrivals in earlier intervals. iscalled the arrival rate.

The number of arrivals in non-overlapping intervals are statistically independent. The probability of two or more arrivals happening during tis negligible compared to the

probability of zero or one arrival, i.e., it is of the order o( t).

Combining the first and third assumption, the probability of no arrivals during the interval t, t+delta tis found to be 1- t+ o( t).

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Arrival Rate

The arrival rate is expressed in the average number of arrivals during a unit of time.

Some Interesting Properties

The probability Pn of n packet arrivals in a time interval T becomes

( T)^nPn = ----- exp{- T}

n!

The distribution of the number of arrivals in a time interval of t,t+T is independent ofstarting time t.

The probability of n other arrivals, in addition to a given "test" arrival that is known to bepresent is exactly the same as the probability of n arrivals without any a priori assumptions.The test arrival has no influence on other arrivals. This property is used, for instance, in thecalculation of the throughput of random-access schemes, such as slotted ALOHA, in radionetworks with capture.

The probability of no arrivals during period of duration T isf(T) = exp{- T}

where f ( ) is the probability density function of the duration between two arrivals. Thus,interarrival times have a negative exponential distribution with mean 1/ .

Applications

The Poisson process is used to model

the arrival of new telephone calls

message arrivals in packet-data network