Basics to Survey EWC Notes

8/6/2019 Basics to Survey EWC Notes

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Introduction

Telecommunication:

Telecommunications is a general term for a vast array of technologies that

send information over distances. Mobile phones, land lines, satellite phones

and voice over Internet protocol (VoIP) are all telephony technologies -- just

one field of telecommunications. Radio, television and networks are a few

more examples of telecommunication.

While most people associate telecommunications with modern technologies,

the strict definition of the term encompasses primitive and even ancient forms

of telecommunication. Among these is the use of smoke signals as a kind of

visual telegraph. Puffs of smoke were time-released by smothering a fire with

a blanket, then quickly removing and replacing the blanket. Widely used by

the American Indians, smoke signals could communicate short messages

over long distances, assuming a clear line of sight.

Other forms of early telecommunications include relay fires or beacons. Used

foremostly in warfare, relay fires required a handful of men posted along a

range of hilltops, with the last man closest to the area where troop movement

was expected. When armies were spotted in the distance, he would light a

bonfire. The fire could be seen from a good distance away by the next man in

the relay, who would in turn light his own bonfire, and so the fires were lit in

succession along the range, creating an effective telecommunications signal

that traveled back over several miles in a relatively short period of time.

Finally, the last man in the relay would light a beacon to signal his army below

that the opponent was en-route.

The arrangement of a ship's flags and semaphores were other forms oftelecommunications. A semaphore was a mechanical device atop a tower with

paddle-like blades or flags. The device would be set in a specific position to

communicate information.

Throughout the 19th century, telecommunications devices became more

sophisticated with the advent of electricity, leading to the telegraph, Morse

code, and signal lamps. A signal lamp, the optical version of the telegraph, is

a powerful lamp with shutters that block the light in long or short durations to


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translate to the dots and dashes of Morse code. A heliograph is another

optical telegraph -- a mirror used to reflect light to mimic a signal lamp.

In the 20th century, telecommunications reached beyond our planet. In June

1969, the world watched and listened as astronauts walked on the moon.

Twenty years later, in August 1989, we would see pictures of Neptune arrive

back from the Voyager 2 spacecraft, riding radio waves that traveled over

roughly three billion miles (4.8 billion km) to reach us in a matter of a few

hours.

Strides in telecommunications have changed the world immeasurably. While

pockets of humankind were once isolated from each other, people now have

multiple ways to see and hear what is occurring on the other side of the world

in real time. Satellite technology, television, the Internet and telephony keep

the globe connected in a humming buzz of interactive voices and pictures. In

short, telecommunications has come a long way from smoke signals.

In modern telecommunication, a communications system is a collection of

individual communications networks, transmission systems, relay stations,

tributary stations, and data terminal equipment (DTE) usually capable of

interconnection and interoperation to form an integrated whole. The

components of a communications system serve a common purpose, are

technically compatible, use common procedures, respond to controls, and

operate in unison. Telecommunications is a method of communication (e.g.,

for sports broadcasting, mass media, journalism, etc.).

Basic Telecommunication System

A communications subsystem is a functional unit or operational assembly that

is smaller than the larger assembly under consideration. Examples of


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communications subsystems in the Defense Communications System (DCS)

are (a) a satellite link with one Earth terminal in CONUS and one in Europe,

(b) the interconnect facilities at each Earth terminal of the satellite link, and (c)

an optical fiber cable with its driver and receiver in either of the interconnect

facilities. Communication subsystem (b) basically consists of a receiver,

frequency translator and a transmitter. It also contains transponders and other

transponders in it and communication satellite communication system

receives signals from the antenna subsystem.

Wireless Communication:

Wireless communication is the transfer of information over a distance without

the use of electrical conductors or "wires". The distances involved may be

short (a few meters as in television remote control) or very long (thousands or

even millions of kilometers for radio communications). When the context is

clear the term is often simply shortened to "wireless". Wireless

communications is generally considered to be a branch of

telecommunications. Wireless Communication is upgrading quite rapidly day

by day.

Wireless devices are various types of fixed, mobile, portable two way radios,

cellular telephones, wireless internet, personal digital assistants (PDAs), and

wireless networking. Other examples of wireless technology include GPS

units, garage door openers and or garage doors, wireless computer mice and

keyboards, satellite television and cordless telephones.

Wireless operations permits services, such as long range communications,

that are impossible or impractical to implement with the use of wires. The term

is commonly used in the telecommunications industry to refer to

telecommunications systems (e.g. radio transmitters and receivers, remote

controls, computer networks, network terminals, etc.) which use some form of

energy (e.g. radio frequency (RF), infrared light, laser light, visible light,

acoustic energy, etc.) to transfer information without the use of wires.

Information is transferred in this manner over both short and long distances.


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HISTORY OF WIRELESS COMMUNICATION:

Photophone: The world's first wireless telephone conversation occurred in

1880, when Alexander Graham Bell and Charles Sumner Tainter invented

and patented the photophone, a telephone that conducted audio

conversations wirelessly over modulated light beams (which are narrow

projections of electromagnetic waves). In that distant era when utilities did not

yet exist to provide electricity, and lasers had not even been conceived of in

science fiction, there were no practical applications for their invention, which

was highly limited by the availability of both sunlight and good weather.

Similar to free space optical communication, the photophone also required a

clear line of sight between its transmitter and its receiver. It would be several

decades before the photophone's principals found their first practical

applications in military communications and later in fiber-optic

communications.

Radio: The term "wireless" came into public use to refer to a radio receiver or

transceiver (a dual purpose receiver and transmitter device), establishing its

usage in the field of wireless telegraphy early on; now the term is used to

describe modern wireless connections such as in cellular networks andwireless broadband Internet. It is also used in a general sense to refer to any

type of operation that is implemented without the use of wires, such as

"wireless remote control" or "wireless energy transfer", regardless of the

specific technology (e.g. radio, infrared, ultrasonic) that is used to accomplish

the operation. While Guglielmo Marconi and Karl Ferdinand Braun were

awarded the 1909 Nobel Prize for Physics for their contribution to wireless

telegraphy, it has only been of recent years that Nikola Tesla has beenformally recognized as the true father and inventor of radio.

Early wireless work:

David E. Hughes, eight years before Hertz's experiments, transmitted radio

signals over a few hundred yards by means of a clockwork keyed transmitter.

As this was before Maxwell work was understood, Hughes' contemporaries

dismissed his achievement as mere "Induction". In 1885, T. A. Edison used a

vibrator magnet for induction transmission. In 1888, Edison deploys a system


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of signaling on the Lehigh Valley Railroad. In 1891, Edison obtained the

wireless patent for this method using inductance.

In the history of wireless technology, the demonstration of the theory of

electromagnetic waves by Heinrich Hertz in 1888 was important. The theory

of electromagnetic waves were predicted from the research of James Clerk

Maxwell and Michael Faraday. Hertz demonstrated that electromagnetic

waves could be transmitted and caused to travel through space at straight

lines and that they were able to be received by an experimental apparatus.

The experiments were not followed up by Hertz. Jagadish Chandra Bose of

India around this time developed an early wireless detection device and help

increase the knowledge of millimeter length electromagnetic waves. Practical

applications of wireless radio communication and radio remote control

technology were implemented by later inventors, such as Nikola Tesla.

APPLICATIONS OF WIRELESS TECHNOLOGY:

Security systems: Wireless technology may supplement or replace hard

wired implementations in security systems for homes or office buildings.

Television remote control: Modern televisions use wireless (generally

infrared) remote control units. Now radio waves are also used.

Cellular telephone (phones and modems): Perhaps the best known

example of wireless technology is the cellular telephone and modems. These

instruments use radio waves to enable the operator to make phone calls from

many locations worldwide. They can be used anywhere that there is a cellular

telephone site to house the equipment that is required to transmit and receive

the signal that is used to transfer both voice and data to and from these

instruments.

WiFi: Wi-Fi is a wireless LAN technology that enables laptop PCs, PDAs, and

other devices to connect easily to the internet. Technically known as IEEE

802.11 a,b,g,n, Wi-Fi is less expensive and nearing the speeds of standard

Ethernet and other common wire-based LAN technologies. Several Wi-Fi hot

spots have been popular over the past few years. Some businesses charge

customers a monthly fee for service, while others have begun offering it for

free in an effort to increase the sales of their goods.


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Wireless energy transfer: Wireless energy transfer is a process whereby

electrical energy is transmitted from a power source to an electrical load that

does not have a built-in power source, without the use of interconnecting

wires.

Computer Interface Devices: Answering the call of customers frustrated with

cord clutter, many manufactures of computer peripherals turned to wireless

technology to satisfy their consumer base. Originally these units used bulky,

highly limited transceivers to mediate between a computer and a keyboard

and mouse, however more recent generations have used small, high quality

devices, some even incorporating Bluetooth. These systems have become so

ubiquitous that some users have begun complaining about a lack of wired

peripherals. Wireless devices tend to have a slightly slower response time

than their wired counterparts, however the gap is decreasing. Initial concerns

about the security of wireless keyboards have also been addressed with the

maturation of the technology.

Many scientists have complained that wireless technology interferes with their

experiments, forcing them to use less optimal peripherals because the

optimum one is not available in a wired version. This has become especially

prevalent among scientists who use trackballs as the number of models in

production steadily decreases.

Modulation:

Modulation is the process in which some characteristics of the high frequency

signal is varied in accordance with the instantaneous value of the modulating

signal. An unmodulated signal is known as a carrier. While doing modulation a

carrier signal, which is a pure sine wave and modulating signal, carrying

information is required. Out of the various types of modulation techniques, the

most common are amplitude modulation and frequency modulation.

The need for modulation aroused due to following reasons:

Difficulty in radiating the audio signal due to antenna size. To give the identity i.e. to make it possible to separate out the signals

at the receiving end. Had all the audio signals been radiated in the

same frequency range, there would have been a mess at the receivingstation.


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In digital communication different modulation techniques are used for

spectral efficiency. In line communication, modulation is necessary for multiplexing i.e. to

send the different signals in the same frequency band over the samecable.

Amplitude Modulation

In amplitude modulation the amplitude of carrier wave is varied in accordancewith the instantaneous value of the modulating signal.Modulation Index

The modulation index of amplitude-modulated wave is given by the ratio ofamplitudes of Modulating voltage to carrier voltage. Distortion in the

modulated signal occurs, if the amplitude of modulating voltage is greater thanthat of the carrier signal. Thus in amplitude modulation, the amplitude ofcarrier wave shall be less than the amplitude of modulating signal.m = Vm / Vc

Forms of Amplitude Modulation

Double sideband full carrier: In this carrier & both sidebands are transmitted. Itis used for commercial broadcasting. The reason is if the carrier issuppressed then it will be required at the time of demodulation in the receiver.It is difficult for all the receivers to possess the carrier source of exactly thesame frequency.

Single sideband full Carrier: In this one of the sideband is suppressedthus there is saving of bandwidth & 25% power. As carrier is presentalong with the transmitted signal, there is no necessity of generating acarrier at the receiver.

Single sideband with suppressed carrier: In this only one side band istransmitted without carrier, which is introduced at the receiver.

Single sideband with reduced carrier: This is used in maritimecommunication. Carrier is transmitted at low level for tuning purposes.

Two independent side bands

Vestigial Sideband: It is used for video transmission. A trace of sideband istransmitted usually with full carrier.

Out of the above, the vestigial sideband and single sideband with full carrier is ofimportance to us.


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Frequency Modulation

Frequency modulation is a process in which the frequency of the carrier is

varied in accordance with the instantaneous value of the modulating signal.As the amplitude of the carrier remains constant it does not play any role atthe time of demodulation & any variations in the amplitude has no effect onthe original signal, after demodulation, thus it is immune to noise.Deviation

The amount by which carrier frequency is varied from its un-modulated valueis called deviation and the rate at which this deviation takes place is equal tothe frequency of modulating signal.

Frequency deviation = KVmfc

Modulation Index

Modulation index = Deviation / Modulating frequency= KVmfc / fm

From the above formula we find that modulation index is function of bothamplitude and frequency of modulating signal. If the amplitude of modulatingsignal is increased modulation index also increases.

Advantages & Disadvantages of FM

The main advantages of Frequency Modulation over Amplitude Modulationare:Improved signal to noise ratio (about 25dB) w.r.t. to man made interference

Smaller geographical interference between neighboring stations.Less radiated powerWell defined service areas for given transmitter powerDisadvantages of Frequency Modulation:More Bandwidth requirementMore complicated receiver and transmitter

Digital Modulation

The objective of digital modulation is to bring the base band signal(modulating signal in digital form) onto the RF carrier using the minimumbandwidth. Economical use of the frequency spectrum is a particular concern

of the digital modulation in view of the fact that a digital telephone channelrequires 16 times more bandwidth than its analog counterpart i.e. against4KHz for an analog telephone channel, digital telephone channel requires64KHz. Furthermore, the transmission capacity for digital signals has to beaccommodated in existing frequency plans that were originally defined foranalog transmission. This bandwidth economy, which is known as spectralefficiency, is defined in bits/s/Hz (transmission capacity/ RF carrierbandwidth).

PSK Modulation

Digital signals basically have two amplitude states binary 0 and 1

corresponding to phases of 00 & 1800. In the simplest digital modulation


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mode, this two state condition is keyed on the RF carrier by shifting the phaseof the carrier.

It is therefore known as phase shift keying modulation.

In two state PSK or 2 PSK, shifting the carrier phase by 180 degrees requiresone hertz of the carrier frequency for each bit of the base band, so thespectral efficiency is 1 bit/ s /Hz.A 2 Mbps base band modulated with 2 PSK thus requires an RF carrier with abandwidth of 2 MHz.A first improvement is obtained by using 4PSK or quaternary PSK also

known as QPSK modulation. In this case, the binary signal is converted into aquaternary signal and the four possible phases of the quaternary signals arekeyed onto the RF Carrier, shifting the carrier phases in 900 steps andresulting in a spectral efficiency of 0.5 bits/ s / Hz.Thus a 1 MHz RF carrier is needed to transport a 2 Mbps signal.

Similarly there is 8PSK having the spectral efficiency of 3 bits/ s/Hz. Eachhigher PSK modulation mode requires a better signal to noise performance,which is difficult to achieve. Consequently, 16 PSK is no longer practical.

Quadrature Amplitude Modulation

For higher spectral efficiency, quadrature amplitude modulation is used. QAM is

combination of phase shift keying and amplitude modulation of the carrier. Twocarriers 90 degrees out of phase hence quadrature are amplitude modulated by a

digital signal (base band) with a finite number m of amplitude levels, and are

subsequently added to one another. It is thus known as m-QAM. With 16 QAM, 16

different signals states are detected and amplitude & phase shift modulated on the RF

Carrier, resulting in spectral efficiency of 4. A 140Mbps base band thus requires a

bandwidth of 140/4 = 35 MHz. However 16 QAM cannot be used for the transmission

of 140Mbps in the 2, 4, 6.2 and 8GHz bands with an RF Channel spacing of 29/30

MHz. The next logical step to 64-QAM has to be made, resulting in a spectral

efficiency of 6 and enabling the transmission of a 140Mbps signal in 23 MHz

bandwidth, which fits with sufficient selectivity in the 29/30MHZ RF Channel

spacing. Implementation of 128QAM, with a spectral efficiency of 7bits/Hz and 256QAM with a spectral efficiency of 10 have already been realized


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A Quarature Amplitude Modulated signal for the values given in the table is shown in

the slide above.

GMSK

The enhancement behind increasing the data rate is the introduction of the 8-PSK

(octagonal Phase Shift Keying) modulation in addition to the existing GMSK

(Gaussian Minimum Shift Keying). An 8-PSK signal is able to carry 3 bit per

modulation symbol over the radio path, while a GMSK signal carries only 1 bit per

symbol. The carrier symbol rate (270.833 Kbps) of standard GSM is kept the same for

8-PSK and the same pulse shape as used in GMSK is applied to 8-PSK. The increase

in data throughput does not come for free, the price being paid in the decreased

sensitivity of the

This affects the radio network planning and the highest data rates can only be

provided with limited coverage. The GMSK spectrum mask was the starting point for

the spectrum mask of the 8-PSK signal but along the standardization process, the 8-

PSK spectrum mask was relaxed few dB in the 400 KHz offset from the center

frequency. This was found to be a good compromise between the linearity

requirement of the 8-PSK signal and the overall radio network performance.

By introducing the second modulation method, 8-PSK, there is a need to blindly

recognize the transmitted modulation in the mobile station receiver (DL). This is due

to the characteristics of the EDGE link quality control, where the used modulation and

coding scheme (MCS) is adjusted according to the channel condition to the mostsuitable one, and in DL, no prior information is sent to the receiver but the receiver

should be able to find out the used MCS based on the blind modulation identification.

The decoding of the RLC/MAC header field, which contains indication of the coding

scheme.

The modulation identification is based on different phase rotation characteristics in

the GMSK and 8-PSK training sequence. In the GMSK training sequence, the

symbol-by-symbol phase rotation is /2 whereas in the 8-PSK training sequence, the

rotation is 3 /8. Otherwise, the set of 8-PSK training sequence has identical

information content (the same 26 bit sequence) as the GMSK training sequence.


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Multiple AccessMultiple access is the connection of a switching center to two or more users by

separate access lines using a single message routing indicator or telephone number.

Access system refers to the manner in which a number of stations may use a repeater

or may interact with a central station (or any input output device) simultaneously.

There are three different methods that permit this simultaneous multi usage.

Different Multiple Access technologies are:

FDMA

TDMA

CDMA

Frequency Division Multiple Access System

In FDMA, the allotted bandwidth is shared between different locations. Each

location interacts in the sub band within the allotted bandwidth. Each station is

assigned a segment of that usable/allotted bandwidth. Sufficient guard band is

allocated between segments to ensure that one user will not interfere with

another, by drifting into a splatter in the other users segment.

Time Division Multiple Access

Time division multiple access (TDMA) is digital transmission technology that allows

a number of users to access a single radio-frequency (RF) channel without

interference by allocating unique time slots to each user within each channel.Pulse

code modulation is the example of TDMA.

Let us consider the E1 frame, which consists of 32 time slots from 0 to 31. The

duration of each slot is 3.906 ms. Each such slot carries the digital data of one

channel. In another slot digital data from other stations can be accommodated. All the

32 slots are combined & transmitted in 125 ms. 8000 such frames are transmitted in

one sec. From the above explanation we find that single frequency is used to transmit


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In telecommunication the signal strength is commonly judged by its power at any

point. Parameters like the Gain of an amplifier, Insertion Loss offered by an

attenuator, is described in decibel (dB) units.

The gain of this black box is given by the ratio of Output power P2 to Input power

P1and is denoted by G. Since gain is a ratio, it does not have any unit.

dB Calculation Thumb Rules

We find that when the power is doubled, dB power gain is 3 dB. For other values, we

can find out the corresponding value of dB by the following thumb rules:

Gain or Power ratio dB

1 0dB

2 3dB

THUMB RULE:

(A) If the power ratio value is doubled then dB value is incremented by 3. For

example:

4 6dB

4X2 = 8 6dB+3dB=9dB

(B) If the Power ratio is multiplied by 10 then the dB value is incremented by 10. For

example:

2X10 =20dB 3dB + 10 = 13dB

To identify whether a particular dB value is Gain (increase in Power) or a Loss

(reduction in Power), +ve andve sign is prefixed to the dB value to denote Gain or

Loss respectively.

dBm

We have seen the units Bel (B) and Decibels (dB). If you noticed, these units are only

ratios or relative units. These units do not define absolute power. For example: wecannot say that the output of an amplifier is 33dB - we can only say that amplifier has

gain of 33dB. These units do not give any idea of the absolute power levels i.e. the

actual power output of an amplifier etc.

To express absolute power levels, we use a unit called dBm. This unit is used to

describe power level relative to 1 mW (m in dBm stands for 1milli watt).

dBm = 10 Log P2 (Power in milli watts)

1milliwatt

To understand better, let us consider an amplifier whose output is 20 watts. We can

calculate its power output (Po), in dBm, as:

Po = 10 Log 20 X103 1mW

= 10 Log 20 X 103

= +43dBm


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Let us try another example, where the input to a network is 0.0004W. We can

calculate its Power Input (Pi), in dBm as:

Pi = 10 Log10 0.0004 X 103

1mW

= -4dBm (Approximately)

The minus sign indicates that the value is less than the reference value of 0dBm or

1mW.

dBW

The unit dBW is used where very high power is to be represented such as for radio

broadcasting and satellite transmitters. It is an absolute decibel unit and may bedefined as decibel referred to 1Watt (Instead of 1 mW in dBm).

Here , P1 = 1W

Power level in dBW = 10 Log P2

P1

dBi is a unit used to denote the gain of a directional antenna.

To understand the term better, let us briefly look at some antenna types:

Antennae can be broadly classified into two major types

Omni directional or isotropic antenna

Directional antenna.

In order to compare the performance of different types of directional antennae, a term

Antenna gain orDirectional Gain is used. In order to measure the antenna gain, it is

compared with respect to isotropic antenna gain

dBi = 10 Log P2

P1

Here P2 is the power at any point I in the direction of radiation, due to directional

antenna, and P1 is the power at the same point, (with same transmitter power), due to

isotropic antenna.

Thus an Isotropic antenna will give gain of 0dBi. The i in the term dBi denotes that

the antenna gain is as compared to isotropic antenna.

Telecom Basics

What is Antenna?

Radiate and receive radio wave ,convert high frequency current to

electromagnetic wave when transmitting, and convert electromagnetic wave

to high frequency current when receiving


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Functions of the antenna

Convert high frequency current to electromagnetic wave when transmitting

Convert electromagnetic wave to high frequency current when receiving

Antenna can not amplify the transmission power, just concentrate RF power

to one direction

Types of Antenna

Resonant antennas

Non-resonant antennas

Omni-directional antennas

Directional antennas

Resonant Antennas

Resonant antenna lengths are multiples of half wavelength

Length of resonant antenna and the number of side lobes in its

radiation pattern are directly proportional to each other

Non-Resonant Antennas

The radiation pattern of a non-resonant antenna is unidirectional

Omni-directional Antennas

The omni-directional antenna radiates and receives equally well in all

horizontal directions. The gain of an omni-directional antenna can be

increased by narrowing the beamwidth in the vertical or elevation plane. The

net effect is to focus the antennas energy toward the horizon.

Directional Antennas

Directional antennas focus energy in a particular direction. Directional

antennas are used in some base station applications where coverage over a

sector by separate antennas is desired.


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Point to point links also benefit from directional antennas. Yagi and panel

antennas are directional antennas.

Antenna Parameters

Gain and Directivity

Antenna Efficiency

Beamwidth

Bandwidth

Polarization

Radiation Pattern Envelope

Antenna gain: The maximum gain of an antenna is simply defined as theproduct of the directivity by efficiency. If the efficiency is not 100 percent, thegain is less than the directivity. When the reference is a loss less isotropicantenna, the gain is expressed in dBi. When the reference is a half wavedipole antenna, the gain is expressed in dBd (1 dBd = 2.15 dBi ).

Antenna directivity: The directivity of an antenna is given by the ratio of the maximumradiation intensity (power per unit solid angle) to the average radiation intensity (averagedover a sphere). The directivity of any source, other than isotropic, is always greater than unity.

Antenna efficiency: The total antenna efficiency accounts for the following losses: (1)

reflection because of mismatch between the feeding transmission line and the antenna and

(2) the conductor and dielectric losses

Bandwidth requirements

The carrier wave is a sine wave for almost any communication system. A sine wave

exists at only one frequency and therefore occupies zero bandwidth. As soon as the

signal is modulated to transmit information, however, the bandwidth increases.

Bandwidth in radio systems is always a scarce resource. Not all frequencies are useful

for a given communication system, and there is often competition among users for the

same part of the spectrum. In addition, as we have seen, the degrading effect of noise

on signals increase with bandwidth. Therefore, in most communication systems it is

important to conserve bandwidth to the extent possible.

Beam Width:


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Antenna gain is defined by the horizontal and vertical beamwidth along the efficiency

of the antenna and in general lesser the beam width higher the gain will be.

The beamwidth is defined the appending angle b/w the two pints on each side of the

main lob direction where the radiated power is 3 dB lower than in the main direction.Both the horizontal and vertical beam width are of prime importance in selecting an

antenna system.

By using the 650 or 900 antenna excessive overlap is avoided as excessive overlap

can cause higher bit error rate and can degrade quality because of lot of hand over b/w

adjacent sectors. Please note that a better gain will also be achieved for a reduced

beam width.

Besidetal beamwidth, vertical beamwidth is of great importance to RF Engineers as in

combination will knowledge of both, overall gain of an antenna can be defined if

antenna efficiency is known.

Radiation Pattern

The relative distribution of radiated power as a function of direction space is the

radiation pattern of an antenna.

Front to Back Ratio :

The Front to Back Ratio is an important aspect of horizontal beamwidth. The F/B

typically varies 20dB and 45dB, which is very useful for rejecting c0-channel andadjacent channel interference as signal coming from the back of antenna may cause

multipath interference which will increase bit error rate.

Front to Back Ratio = Back lobe level / Front lobe level.

PolarizationRadio waves are built by two fields, one electric and one magnetic. These two field

are perpendicular to each other. The sum of the fields is the electromagnetic field.

Energy flows back and forth from one field to the other - This is what is known as

"oscillation".

The position and direction of the electric field with reference to the earths surface

(the ground) determines wave polarization. In general, the electric field is the same

plane as the antenna's radiator.

Horizontal polarization the electric field is parallel to the ground.

Vertical polarization -- the electric field is perpendicular to the ground.

Voltage Standing Wave Ratio (VSWR)

VSWR is a measure of impedance mismatch between the transmission line and its

load. The higher the VSWR, the greater the mismatch. The minimum VSWR, i.e., that

which corresponds to a perfect impedance match, is unity.


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VSWR can also be taken as measure of return loss of the antenna.

VSWR = Reflected power / Transmitted power

Return loss : 20 log (VSWR+1) /(VSWR 1)

In base station antenna it is desirable to have low value of VSWR, normally upto 1.3,

as low VSWR means high quality.

Radio Propagation Model:

Structure of Earths Atmosphere

Since the radio waves are influenced by the earths atmosphere, an understanding of

the earths atmospheric structure is necessary. The Earth's atmosphere is divided into

three separate layers the Troposphere, the Stratosphere, and the Ionosphere.

The Troposphere is the portion of the Earth's atmosphere that extends from thesurface of the Earth to a height of about 3.7 miles (6 km) at the North Pole or the

South Pole and 11.2 miles (18 km) at the equator. Virtually all weather phenomena

take place in the troposphere. The temperature in this region decreases rapidly with

altitude. Space wave communication takes place in this layer.

The Stratosphere is located between the troposphere and the ionosphere. The

temperature throughout this region is considered to be almost constant and there is

little water vapour present.

The Ionosphere extends upward from about 31.1 miles (50 km) to a height of about

250 miles (402 km). It contains four cloud-like layers of electrically charged ions, the

existence and heights of these layers vary from time to time in a day. Some of these

layers disappear in the night. This is the most important region of the atmosphere for

long distance, HF communication.

Types of Propagation

Electromagnetic (radio) energy travels from a transmitting antenna to a

receiving antenna, in three principal ways:

1. Ground wave

2. Space wave

3. Sky wave

Ground waves are radio waves that travel near the surface of the Earth (surface and

space waves).Sky waves are radio waves that are reflected back to Earth from the ionosphere.


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Space waves are radio waves that travel in straight lines.

Cellular Mobile Communication

Cellular Concept

Frequency Re-use

This principle of reusing a set of frequencies in different cells of the coverage area is

the main concept of Cellular Technology.

While Planning for frequency reuse, the network planner has to define at what

distance the frequency can be reused again and how much should be the radius of the

cell. There are various other considerations, such as adjacent channel interference, to

decide the size of the cells and also where which frequency set is to be used. We will

go into this in detail in some of our advanced courses.

Generally speaking:

Divide the available service area (where all coverage is required) into small

areas.

Allot the different set of frequencies to all the adjacent channels of the

center cell.

Use the same set of frequencies for cells at the specified distance with

specified radius.

Advantages over wire line Telephony.

Mobility Convenience


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Flexibility

In cellular telephone system, Calls can be originated from a mobile station

subscriber for any other subscriber of the network (Also those of PSTN

network). Calls can be terminated on a Mobile station, irrespective of the

location of the Mobile station in the coverage area.

Unlike conventional systems, cellular mobile system, may use a large number

of small power transmitters, each covering a small area. Because of the short

distance covered by each transceiver, the particular channel frequency can be

used over and over in multiple non-adjacent cells.

Cellular Radio Principles

Some major Cellular principles, like Registration, Call originations and

terminations and Hand-Offs are below

Registration and Location

A unique identity is given for each MS that is registered in a network. The

MSC holds information on the location of the active mobiles. The network will

check the unique identity number that is transmitted by the Mobile station. As

the Mobile subscriber moves form one cell to another, the strength of the

signal between the base station and the mobile will weaken and the level of

the noise will increase. The system detects this and instructs the neighboring

cells to listen out for the signal and transfers the phone into the appropriate

cell. In this way the network can keep a record of the current location area of

each cell phone. And, when there is an incoming call, the network only needs

to page the MSin that particular area.

A MS that is not in use, but switched ON, will be tracked continuously by the

signaling messages in the control channel. Control channel information

contains information about Network identities, frequencies, notification for a

mobile about incoming call, channel assignment messages.


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Call Initiation

Mobile originated call

Once a number is dialed, upon detecting the idle control channel, MS

transmits its identity and the telephone number to the MSC. The MSC, after

receiving the call request, validates the status and commences to route the

call. The MS retunes to the frequency assigned on receipt of the message

from MSC. The MSC connects the voice channel to the required route

enabling the caller to monitor the ringing tone and commence conversation.

Mobile terminated call

The PSTN recognizes the dialed number as a mobile and forwards the same

to the MTSO. The MTSO, in turn, sends a paging message to certain cell

sites. Each cell site transmits the page on its own setup channel. The mobile

unit recognizes its own identification on a strong set up channel, locks onto it ,

and responds to page. The mobile unit also follows the instruction to tune to

an assigned channel and initiates user alert.

Call Termination

As soon as the mobile user transmitter is turned OFF, cell site receives a

signal and it frees the voice channel on both sides.

Handoff

This is a process of automatically changing the frequencies as the mobile unit

moves into a different frequency zone so that the conversation can be

continued in the new frequency zone without redialing.

The system continuously monitors the signals received from the MS engaged

in the calls, checks the signal strength, and quality of the signal. When the

signal falls below a preset threshold, the system will check whether any base

station can receive the MS with stronger signal. If there is, the system will

allocate a radio channel for the call on the new base station and cell phone

will be asked by a signaling message to switch to the new frequency. The


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whole process takes a few seconds to complete and a break in conversation

is for hardly 200- 300ms.

Cell Splitting

System blockage

As the traffic within a cell increases towards the point where service quality is

affected, the cell can be split into smaller cells. If this is not done, blockage

will increase. Blockage occurs when a user attempts to make a call and the

system is so loaded that the call cannot be completed. A measure of

telephone system performance is the amount of blockage that occurs within

the system. To prevent the blockage of the system cell splitting is used. As

traffic grows within a cell a condition is reached where it is desirable to revise

the cell boundaries in order to handle more traffic. So, a single cell is now

divided into a number of cells, but all within the original cell boundary.

Let us assume that the cell designated as F1 in the figure has reached

capacity. To increase traffic handling capacity within the original F1 boundary,

the cell is split into four cells, H3 , I3 , B6 , and C6 . As the demand continues to

grow the original coverage area may ultimately be split into small cells.

This technique of frequency reuse and cell splitting makes the cellular system

unique and makes it possible to meet the important objectives of serving a

large number of customers in a small coverage area using a small spectrum

allocation. Additionally, cell splitting makes it possible of matching the density

of available channels to the spatial density of demand for channels.

The motive behind implementing a cellular mobile system is to improve the

utilization of efficiency. The frequency reuse scheme is one concept, and cell

splitting is another concept. When traffic density starts to build up and the

frequency channels Fi in each cell Ci cannot provide enough mobile calls, the

original cell can be split into smaller cells. Usually the new radius is one-half

the original radius. There are two ways of splitting: In fig a, the original cell site

is not used, while in fig b, it is:

New cell radius = Old cell radius/2; then based on the above equation


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New cell area = Old cell area/4

Let each new cell carry the same maximum traffic load of the old cell; then:

New traffic load traffic load

--------------------- = 4 x -----------------

Unit area unit area

Spectrums:

Understand the frequency ranges in the Radio Frequency spectrum

Learn about the usage of the spectrum

Review the need for frequency management

Overview frequency management process

Radio Frequency Spectrum

Perhaps the most familiar part of the electromagnetic spectrum is the Visible

Light Spectrum. The light with which you are reading this page is, in reality,

radiation covering part of the electromagnetic spectrum. In fact, the term

"spectrum" was originally limited to light. Physicists of the 17th through 19th

centuries were the first to realize that what we think of as white light is really a

broad range of different colors of light from the brightest red at one end to the

deepest purple at the other. Thus, white light is a spectrum of different colors.

The electromagnetic spectrum extends in both directions from the visible

range. Shorter-wavelength, higher frequency "light" includes ultraviolet, x-rays, and cosmic rays. Longer-wavelength, lower-frequency "light" includes


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first infrared light then, as wavelengths become longer and longer, radio

waves.

The early physicists also found that electrons traveling through wires are

surrounded by both electric and magnetic fields, and that a wire carrying an

alternating current is surrounded by electric and magnetic fields varying in

intensity at the same frequency as the electric current. Furthermore, the wire

radiates energy that propagates just as do light waves with a frequency and

wavelength corresponding to the frequency of the alternating current in the

wire.

Usage of the Spectrum

Broadcasting Services

Mobile Communication Services

AM and FM Radios

VHF and UHF Television Stations

Paging Systems

Trunked Radio Systems

Aeronautical Communications

Satellite and Microwave Communication System

Frequency Management Process

Policy - making in Radio Frequency Management

Making policies and criteria for appropriate radio frequency assignment.

Preparation of Radio Frequency Plans


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Preparing radio frequency plans for each band to attain most uses and avoid

interference of frequencies.

Radio Frequency Assignment

Managing and assigning radio frequencies in accordance with the plans and

allocating

Radio Communications Licensing

Issuing, revoking and suspending the radio communications licenses which

are the licenses to import, export, manufacture and sell radio communications

equipment or accessories, as well as the licenses to establish radio

communications stations and radio operator licenses for government

agencies, private enterprises and private individuals.

Inspect the technical specifications of radio communications

equipment

Inspection the technical specifications of radio communications equipment

and accessories to be manufactured to ensure efficiency of communications,

and protection of national security against harmful interference.

Radio Communications Coordination

Regional coordination committees are formed for quick processing of

frequency assignments and Licensing.

Radio Frequency Monitoring and Direction-Finding

Inspecting the use of radio frequencies of government agencies, private

enterprises and private individuals according to laws and regulations in order

to ensure orderly and efficient uses of frequencies, prevent illegality of

utilization of frequencies and protect national security against harmful

interference.

.

Mobile Station Power Classes (GSM 900)


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Class Maximum

TX Power

Minimum

TX Power

Power Steps

(*)

1 (20W) Deleted from

specifications

3.2mW (5dBm)

2 8W (39dBm) 3.2mW (5dBm) 18

3 5W(37dBm) 3.2mW (5dBm) 17

4 2W (33dBm) 3.2mW (5dBm) 15

5 0.8W (29dBm) 3.2mW (5dBm) 13

Base Station Power Classes for GSM BTS

Typical maximum power out of a transceiver for most manufacture would be

40-60W. After combining losses are accounted for actual power radiated will

be much lower.

Adaptive power control is not applied to the BCCH carrier on a BTS. This

carrier is continuously transmitted at the BTSs maximum power in all

timeslots. All other carriers at the site may be subject to adaptive power control

on a timeslot basis.

In addition to these hardware limits to transmitted power, there are also

software parameter values set for both the MS and the BTS. Whenever the

lower of these two limits is reaches, on the uplink or the downlink, further

power control is inhibited in that direction, implying the probable need for

future handover.

BTS power control requirements are less rigid than for the MS. Manufacturers

can provide up to 15 steps, each giving a 2db reduction. Typically 6 steps

giving a total range of 12dB are used.


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INTRODUCTION TO GSM

GSM ARCHITECTURE

2.1 FUNCTIONAL BLOCK DIAGRAM:

GSM is divided into two separate entities the Switching System (SS)

and the Base Station System. Each of these contains a number of functional

units, where all systems functions are realised. These functional units are

implemented into various hardware components.

Figure 2.1: GSM Architecture

Functional units within the system are separated by interfaces. Such

interfaces are the Air interface (MS-BSS), the Abis interface (BTS-BSC) and


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the A interface (BSC-MSC). However before undertaking to study the main

functional components a brief overview of the complete system is deemed

necessary.

The SS Includes the Following Subsystems:

Mobile services Switching Centre (MSC)

Visitor Location Register (VLR)

Home Location Register (HLR)

Authentication Centre (AUC)

Equipment Identity Register (EIR)

The Base Station System (BSS) includes:

Base Station Controller (BSC)

Base Transceiver Station (BTS)

Transcoder Rate Adapter Unit (TRAU).

2.1.2 Base Transceiver Station:

Each cell has a Base Transceiver Station (BTS) operating on a set of

radio channels. These are different from the channels used in neighbouring

cells to avoid interference. The BTS handles the radio interface to the mobile

station. The BTS is the radio equipment (transceivers and antennas) needed to

service each cell in the network.

2.1.3 Base Station Controller:

A base station controller (BSC) controls a group of BTS. BSC controls

such information as handover and power control. BSC can be implemented as a

stand-alone node or as integrated with the MSC. The BSC provides all the

control functions and physical links between the MSC and BTS. It is a high-

capacity switch that provides functions such as handover, cell configuration

data, and control of radio frequency (RF) power levels in base transceiver


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stations. A number of BSC's are served by an MSC.

2.1.4 Mobile services Switching Centre:

A number of BSC are served by a MSC which controls calls to and

from other telephony and data communication systems, such as the public

switched telephone network (PSTN), integrated services digital network

(ISDN), public land mobile networks (PLMN), public data networks (PDN)

and possibly, various private networks. The MSC performs the telephony

switching functions of the system. It controls calls to and from other telephone

and data systems. It also performs such functions as toll ticketing, network

interfacing, common channel signalling, and others.

2.1.5 Databases:

The above-mentioned units are all involved in carrying out speech

connections between an MS and for example a subscriber in a PSTN. If it were

not for the possibility of making calls to an MS we would not need any further

equipment. The problem arises when we want to make an MS terminated call.

The originator hardly ever knows where the called MS is. Due to this we need a

number of databases in the network to keep track of the MS.

The most important of these databases is the Home Location Register

(HLR). When someone buys a subscription from one of the GSM operators, he

will be registered in the HLR of that operator; he will be registered in the HLR

of that operator. The HLR contains subscriber information, such as

supplementary services and authentication parameters. Furthermore, there will

be information about the location of the MS, i.e. in which MSC area the MS

resides presently. This information changes as the MS moves around. The MS

will send location information to its HLR, thus providing means to make a call.

Authentication Centre (AUC) is connected to the HLR. The function of

the AUC is provided the HLR with authentication parameters and ciphering

keys, both used for security reasons. AUC provides authentication andencryption parameters that verify the user's identity and ensure the


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confidentiality of each call. The AUC protects network operators from different

types of fraud found in today's cellular world.

The Visitor Location Register is a database containing information about

all the MSs currently located in the MSC area. As soon as an MS roams into a

new MSC area, the VLR connected to that MSC would request data about the

MS from the HLR. At the same time the HLR will be informed in which MSC

area the MS resides.

If, later on the MS wants to make a call, the VLR will have the

information needed for the call set-up without having to interrogate the HLR

each time. The VLR can be seen as a distributed HLR. The VLR will alsocontain more exact information about the location of the MS in the MSC area.

The VLR is a database that contains temporary information about subscribers

that is needed by the MSC in order to service visiting subscribers. The VLR is

always integrated with the MSC. When a mobile station roams into a new MSC

area, the VLR connected to that MSC would request data about the mobile

station from the HLR. Later, if the mobile station makes a call, the VLR will

have the information needed for call set-up without having to interrogate the

HLR each time.

2.1.6 Gateway:

A gateway is a node used to interconnect two networks. The gateway is

often implemented in an MSC. The MSC is then referred to as the GMSC. If

someone in a fixed network (PSTN) wants to make a call to a GSM subscribe,

the exchange in the PSTN will connect the call to a gateway. The gateway is

often realised in an MSC. It can be any one of the MSC in the GSM network.

The GMSC will have to find the location of the searched MS; interrogating the

HLR where the MS is registered can do this. The HLR will reply with the

address to the current MSC area. Now the GMSC can re-route the call of the

current MSC. When the call reaches that MSC, then the VLR will know in

more detail where the MS is. The call can then switched through.

2.1.7 Mobile Station:


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In GSM there is a difference between the physical equipment and the

subscription. The mobile station is piece of equipment, which can be vehicle

installed, portable or hand-held.

In GSM there is a small unit called the Subscriber Identity Module

(SIM), which is a separate physical entity e.g. an IC-card, also called a smart

card. SIM and the mobile equipment together make up the mobile station.

Without SIM, the MS cannot get access to the GSM network, except for

emergency traffic. While the SIM-card is connected to the subscription and not

to the MS, the subscriber can use another MS as well as his own. This then

raises the problem of stolen MS, since it is no use barring the subscription if

the equipment is stolen.

We need a database that contains the unique hardware identity of the

equipment, the Equipment Identity Register (EIR). The EIR is connected to the

MSC over a signalling link. This enables the MSC to check the validity of the

equipment. An non-type-approved MS can also be barred in this way. The

authentication of the subscription is done by parameters from AUC.

2.1.8 Operation and Maintenance:

The Operations and Maintenance Centre is connected to all equipment

in the Switching System and to all the BSCs. The objective of the OMC is to

offer the operator cost-effective support for the centralised regional and local

operational and maintenance activities required for a cellular network. The

main purpose of the OMC is to provide a network overview and support the

maintenance activities of different O&M organisations. The internal O&M

functions of both SS and BSS can be reached from the OSS by means of X.25

Links.


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The OSS is the functional entity from which the network operator

monitors and controls the system. The purpose of OSS is to offer the customer

cost-effective support for centralised, regional, and local operational and

maintenance activities that are required for a GSM network. An important

function of OSS is to provide a network overview and support the maintenance

activities of different operation and maintenance organisations. The Operation

Subsystem enables the operator to monitor and control the GSM network.

According to Telecommunications Management principles, on the one hand the

OSS is linked to major network elements such as the MSC, BSC, HLR and

others (BSTs are accessed through BSC's), on the other hand it provides a

man-machine interface for the operation personnel. The network element that is

in contact with BSS and NSS machines is called Operation and Maintenance

Centre (OMC). An OMC typically consists of a database for network data and

a couple of workstations

Which are in charge of managing the OMC database and are in

connection with other network elements. A GSM network can include several

OMCs; in such a case OMCs are linked together.

The OSS enables the operator to continuously check the quality of the

service provided for users through measuring parameters like traffic,

congestion, handovers, dropped calls, interference, etc. This feature helps to

find the bottlenecks and problematic areas in the system.

It also provides means to modify the network once a reaction to a given

problem is decided. OMC plays an important role in the daily maintenance, too.

It collects and displays alarms from all network elements and, thus, allows the

operator to detect, locate and correct faults and breakdowns in the system.


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2.1.9 Switching System:

The VLR is mostly built into the MSC. This makes signalling between

the two nodes over the GSM network unnecessary, and the internal signalling

can be used instead, decreasing the signalling load over the network.

HLR can be implemented together with the MSC/VLR or implemented

as a stand-alone node. The HLR is a database used for storage and management

of subscriptions. The HLR is considered the most important database, as it

stores permanent data about subscribers, including a subscriber's service

profile, location information, and activity status. When an individual buys a

subscription from one of the PCS operators, he or she is registered in the HLR

of that operator.

The AUC & EIR are either implemented as stand alone or as a

combined AUC/EIR node. The MXE is the node handling SMS service, Cell

broadcast, Voice mail and FAX mail. These services are optional in GSM, so

the whole node is optional and does not belong to the basic system structure.

The main role of the network and switching subsystem is to manage

communications between GSM users and other telecommunications network

users. It has two functional parts: the exchange system and the subscriber and

terminal equipment databases. The exchange system comprises the Mobile

Services Switching Centre (MSC) and potentially other service centres, such as

e.g. the Short Message Service Centre (SMSC). The subscriber and terminal

equipment databases contain the Visitor Location Register (VLR), Home

Location Register (HLR), Authentication Centre (AUC) and the Equipment

Identity Register. Another functional unit of the NSS is the Voice Mail System

(VMS) that does not actually fit in either of the above functional parts and is

not defined by GSM specifications.


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The MSC performs the basic switching and routing functions within the

NSS. Its main function is to co-ordinate the setting-up of calls to and from

GSM users within its service area. The difference between the MSC and an

ordinary telephone exchange is that the MSC has additional functions to take

into account the allocation of radio resources and to cope with the mobility of

subscribers.

These functions include location registration, paging, the handover

procedure and transferring encryption parameters and dual tone multi

frequency signalling.

The MSC is also a gateway for communicating with other networks,

what needs adaptation. This is carried out by the interworking functions

(IWFs). The IWF is basically transmission protocol adaptation equipment that

adapts the GSM transmission peculiarities to those of the partner networks such

as PSTN, ISDN, PSPDN or CSPDN.

The NSS usually contains more than one MSC. In this case, one or moreMSC's are designated as gateway MSCs, which are in charge of fetching the

location information and of routing the calls towards the MSC that, can serve

the subscriber or towards external networks such as e.g. the PSTN.

The role of the Short Message Service Centre for written messages is

identical to the role of the gateway MSC for incoming speech and data calls.

The GSM specifications do not exactly define all the protocols related to the

SMSC and, thus, leave some freedom for the manufacturer. Nevertheless, each

SMSC should include lower layer protocols which enable the delivery of short

messages between the mobile station and the SMSC as well as other protocols

which interrogate the HLR searching the address of the subscriber when

reachable, and alert the SMSC if a user becomes reachable again. It should be

emphasised that the short message service is the only service in GSM, which

does not require the end-to-end establishment of a traffic path. Short messagesmake use of signalling channels (namely the SDCCH and the SACCH


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channels), therefore, they can be transmitted even when the mobile is engaged

in full circuit communications.

The HLR is a database that contains subscriber-specific information

relevant to the provision of telecommunications services and the current

location. The HLR identifies whether a given teleservice or bearer service can

be provided for a subscriber. Information on supplementary services is not

necessarily stored in the HLR.

Two numbers belong to each subscriber in the home location register:

the Mobile Station International ISDN number (MSISDN) and the International

Mobile Station Identity (IMSI). The MSISDN is the directory number that is

dialled in order to contact a mobile. It defines the service of a subscriber and

not the subscribers telephone equipment. This means that subscribers have

different MSISDN's for different services. The IMSI is the unique

identification number of a SIM card, used within the GSM network. It is

allocated and cross referenced with MSISDN at initial subscription and stored

in the HLR, AUC and SIM.

The HLR enables to forward calls towards the MSC/VLR within the

service area of which the moving subscriber is situated by storing some

location information, including at least the address of the visited MSC/VLR

and the identification of the local MS, and by requesting the visited MSC/VLR

to provide a Mobile Station Roaming Number (MSRN).

Beside HLR, another database function is realised in GSM: the Visitor

Location Register (VLR). VLR's are connected to one or several MSC's, each

controlling a number of cells and being in charge of temporarily storing

subscription data for the subscribers currently situated in the service area of the

corresponding MSC(s), as well as of holding data on their location at a more

precise level than the HLR.

In GSM cells are grouped to compose location areas. Each time a mobile

crosses the boundary of two location areas or it is switched on in a different


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location area than the one where it was last successfully registered, it attempts

to register the subscriber by performing a location updating procedure. The

result of the last location update attempt is stored in the SIM. During location

updating, information on the subscriber is fetched from the HLR to the VLR.

By doing so, VLR takes part in the authentication and handover procedure,

supports encryption and handles supplementary services and short messages.

The management of security data for the authentication of subscribers is

carried out in the Authentication Centre (AUC). In order to protect the network

against unauthorised use, the authentication of the GSM subscriber identity can

be applied at each registration, each call set-up attempt and before performing

activation, deactivation, registration, or erasure of supplementary services. The

principle of authentication is to compare the subscriber authentication key (the

so-called KI number) on the network side with the KI in the SIM without ever

sending it. The AUC is the network element that stores the KI number on thenetwork side. It contains encryption parameters and a random generator as

well. The AUC is actually a functional subdivision of the HLR but it can be a

separate network element, too.

The GSM specifications identify a network element specific to MS

management, called Equipment Identity Register. It is a database that contains

information about mobile terminals. Here their unique International Mobile

Equipment Identity (IMEI) number refers to, MSs. Three different lists are

used for IMEI's in the EIR. The white list includes the range of IMEI's

allocated to type approved mobile equipment, the grey list is for terminals that

need to be observed for some reason and finally the black list includes the

IMEI's of mobile stations which need to be barred, either because they have

been stolen or because of severe malfunctions.

Voice Mail


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The Voice Mail System enables to store voice messages. Incoming calls

can be forwarded into the subscribers voice mailbox when he is busy, is out of

the coverage, is switched off, does not answer or activates unconditional call

forwarding into his voice mailbox. Some VMS's can also provide an intelligent

alert system. Repeated delivery calls can inform the subscriber of a new

message in his voice mailbox. The timing of such calls follows a timing matrix

of which the rows correspond to the possible reasons why the call was

forwarded into the voice mailbox. When the GSM system contains an SMSC,

delivery calls can be combined with short messages: a short message is

delivered to the customer subsequent to receiving a message in his voice mail

box and delivery calls are only activated if the short message was

unsuccessfully delivered. From architectural point of view the VMS is divided

into message storage units (Winchesters) and call message and alarm

management units.

2.1.10 TRAU:

The Trans coder rate adopter unit functionally belongs to the BTS. The

TRAU enables the use of lower rate (32, 16 or 8 kbps) over the Abis interface

instead of the 64kbps ISDN rate that the MSC is designed to handle. The

TRAU can be located at the BTS, the BSC or immediately before the MSC.

GSM Identities

In order to allow a mobile subscriber free movement in the GSM

Network and in the GSM visited networks,the GSM system requires a little

different numbering system compared to analogue mobile networks or a fixed

network.

The numbering system implemented in the GSM network also takes

security security aspects into account.These numbers are concerned with

Mobility management security management and subscriber administration


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IMEI International Mobile Equipment Identity.

IMSI International Mobile Subscriber Identity.

MSISDN Mobile Station International ISDN Number

TMSI Temporary Mobile Station Identity.

MSISDN

MSISDN Numbers are the directory numbers of the Mobile subscribers.

The structure of the GSM directory number, also called MSISDN because it is

part of the same numbering plan as ISDN numbers.

Subscriber may have more than one MSISDN due to the fact that the

MSISDN actually defines the service used, not the telephony equipment

For a mobile terminating call, the number dialed by the calling party is

MSISDN number, this does not refer to a telephone line or location, but points

to some HLR. For all mobile terminating calls, HLR is interrogated by the

GMSC, which tells the routing information to GMSC, thus the call is routed to

respective MSC. Within the GSM network, a mobile is identified by IMSI, the

corresponding IMSI number is produced by the HLR for MSISDN.

The maximum length of the MSISDN number can be 15 digits, prefix

not included. Example : +358 50 5009999

CC Country code 358

NDC National destination code 50

SN Subscriber Number 5009999

IMSI

Uniquely identifies subscriber in a GSM PLMN.

The International Mobile subscriber Identity is a 15 digit number(GSM

Recommendation), allotted by the network operator . The mobile subscriber is

identified by the IMSI, by its home network as well as by other networks. The

visited Network identifies the subscribers Network by MCC & MNC part of


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the IMSI.

The network uses this number for identification and also for security

reasons.This is defined by the operator(partly) and it is stored in the

HLR,VLR ,the AUC and the SIM. HLR is the place where both IMSI and

MSISDN are tied together This number is stored in the SIM card & protected

against changes

Compared to the MSISDN,the mobile subscriber has only one IMSI but

may have many MSISDN numbers.

IMSI is exclusively for the internal business of the Network.

It is composed of 3 parameters

MCC Mobile country code 3 Digits

MNC Mobile network code 2 Digits

MSIN Mobile subscriber Identification number 10 Digits maximum

MSRN

It is composed of 3 parameters

CC Country code

NDC National destination code

SN Subscriber Number

The MSRN & MSISDN have the same format but there is a

difference.In MSRN,the subscriber Number is the address to the serving MSC.


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MSRN (Mobile Station Roaming Number) number is important when

the Mobile station is roaming in the other network i.e. it is not located in the

home network.

Both MSISDN & MSRN are the routing numbers and part of the CCITT

E.164 numbering plan. While the MSISDN gives the routing information for

the GMSC to MS with in the same PLMN, but MSRN gives the routing

information about the second leg of the call i.e. from GMSC to the visited

MSC. MSRN is not visible to the GSM users or calling subscriber, but it is

used exclusively between the Home PLMN & visited PLMN. It is not allocated

permanently to a subscriber & its purpose is only to route the call to visited

MSC.

When a mobile terminated call lands in the home GMSC/MSC,

respective HLR is interrogated, which contains the subscribers record

and location information of the subscriber (address of the visited

MSC/VLR) i.e. where the subscriber is currently located.

The home HLR also contains the MSRN, provided by the visited

MSC/VLR at the time of location updating by the MS.

If the MSRN number is not available in the Home HLR, then it

interrogates the visited MSC/VLR to get the routing information.

There is pool of MSRN in the MSC/VLR; it is linked with the IMSI.

When the call reaches the visited MSC, using the MSRN as the address,

the MSC can retrieve the IMSI from its records can go ahead with the

establishment of the call towards the mobile station.

TMSI

The temporary Mobile Subscriber identity is an alias for the subscriber


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exchanged between the network and MS>

IMEI International Mobile Equipment identity

By performing the IMEI check procedure the network knows what

mobile equipment use the network

Basically an operator may have 3 lists of mobile stations hardware:

black,white and grey

When a mobile station hardware is on the black list, it is not allowed to

be used except for emergency calls.

The grey list contains mobile station hardware which is potentiallyfaulty or suspect, I.e. the mobile station hardware on the grey list is

under observation.

The white list is composed of all number series of equipment identities

allocated in any GSM network.

IMEI has a length of 15 digits:

TAC Type approval code 6 digits

FAC Final assembly code 2 digits

SNR Manufacturer serial number 6 digits

SP Spare for future 1 digit

The hardware of the Mobile station is called Mobile Equipment which is

allotted a unique 15 digits number, which is called IMEI (International Mobile

equipment Identity. This number does not tell the information of the subscriber.

Badly designed or damaged equipment may not only degrade the quality of

service for its user but also to other users of the network. The correct

functioning of the Mobile is the concern for the Network Operator. Such


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16 bits

Location Area

The location area is a group of cells. It is the area in which the subscriber is

paged. Each LA is served by one or more base station controllers, yet only by a

single MSC Each LA is assigned a location area identity (LAI) number.

LAI Location Area Identity

LAI has 3 parameters

MCC Mobile country code

MNC Mobile network code

LAC Location area code

MCC is a 3 digit code

MNC is a 2 digit code ,identifies the GSM PLMN of that country

LAC identifies a Location Area within a GSM PLMN.

The maximum length of an LAC is 16 bits, enabling 65536 different location

areas to be defined in one GSM PLMN.

GSM Services

Various types of services in GSM System.

Teleservices

Bearer Services

Supplementary Services

Short Message Services

Broadcast Message Services


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Teleservices

Major Teleservices supported by GSM network

Speech

- Telephony

- Emergency calls

Short Message Service

- Mobile Terminated

- Mobile Originated

- Cell broadcast

Voice mail

Facsimile Transmission

Telephony is the most important service provided by the GSM.

Emergency callingis a distinct service, derived from telephony.

All security functions apply to all Teleservices, i.e., if, for instance,

authentication procedure is unsuccessful, the call may be rejected.

Speech Services

Telephony

Speech, Telephony is a Teleservices offering a normal, traditional call. This

service enables a GSM user, establishment of bi-directional speech calls with


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Interface

The interface between MSC and MS is called A, Abis, Um interfaces. On these

interface only three layers are defined

A-interface: A interface between the BSC and the MSC. The A interface provides two

distinct types of information, signalling and traffic, between the MSC and the BSC.

Abis-Interface: The Abis interface responsible for transmitting traffic and signalling

information between the BSC and the BTS.

Um (Air) Interface: This is the interface between the mobile station and the Base

station. The Air interface uses the Time Division Multiple Access (TDMA) technique

to transmit and receive traffic and signalling information between the BTS and MS.

The TDMA techniques used to divide each carrier into eight time slots. These time

slots are then assigned to specific users, allowing up to eight conversations to be

handled simultaneously by the same carrier.

This interface is the radio interface between the mobile station and the network and

uses layer three messages. On layer three messages we have the division of message

types into CM (Connection Management), MM (Mobility Management), RR (Radio

Resource Management).

Connection Management(CM):

1. Call Control: Which handles the procedures concerning call control? Eg.

Setup, change of bearer service.

2. Supplementary Services: Which handles such as call bearing, Call waiting,

Call forwarding etc?

3. Short Message Service : Enables the MS to handle short message transfer to

and from the network.

4.

Mobility Management(MM) :

Mobility management handles functions for authentication, location updating,

identification and others concerning the mobility of the mobile station.

Radio Resources Management :

It contains the functions concerning the radio link. Here we find the capability

to establish, maintain and release the radio connection between the network and the

mobile station, which includes the handover procedure.

GSM Channels


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Preamble

With the current exponential growth in radio based communication throughout world,

microwave links are increasingly being employed over more difficult paths, requiring

graeter accuracy in LOS survey methods.

Line-of-sight(LOS) surveys form an important part of ther engineering considerations

for the planning of microwave radio links.

A microwave link installed over a non-viable path involves a lot of on-costs. These

on-costs include de-installation , re-engineering for alternative communications,

project delays, and damage to reputations. One mistake in a contract could cost more

than the profit margin.

A microwave radio links is usally engineered on the basis of there being a clear line-

of-sight(LOS) between the antennas at opposite ends of the link. For short paths,

particularly at the higher frequeincies, often a simple check as to whether a site can be

seen from the roof of a building, possible-using binocular is all that is needed.

Matters are not quite so simple for longers links; say those of 10kms or more between

antenna positions. It is not easy to identify a particular radio tower (or building) at

these distances, even with binoculars, unless that tower is in a landmark position and

the visibility is very clear.

Microwave engineering procedure has been to check the radio profiles, a path profile

being a cross section graph of counters between antenna positions, modified to show

the Earths curvature and the increase in that curvature due to refraction of the

miceowave radio wave.

To this path profile are added features such as trees and buildings, and occasionally,

man-made earth works not detaild in survey maps. It is then necessary to calculate the

diameter of the first fersenal zone (the major addative elements of the radio wave in


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waveront theory) and add this to the graph to ensure there is sufficient clearance over

any potential obstructions along the path during refraction fades ( the effect of an

increase in the Earths curvature due to atmospheric conditions), to ensure the link

will meet its performance objectives.

Typical Microwave radio systems and network considerations

Modern digital microwave radio systems provide a feasible technical solution for

telecommunications transmission links at distances up to 80km ( much greater

distances are achievable under specila path engineering conditions) and can carry

capacities up to N x 155Mbps.

Furthermore, digital radio relay systems in the microwave and milli metric bands

provide economic transmission options, coupledn with the advantages of rapid

deployment and network control and ownership. Such systems are increasingly being

deployed in both cellular and fixed telecommunications networks, and in the latter

case, particularly in wireless based networks.

A typical microwave radio terminal consists of an indoor mounted base bend shelf, an

indoor or outdoor mounted radio frequency(RF) transceiver and a parabolic antenna.

Each terminal transmits and receives information to and from the opposite terminal

simultaneously providing full duplex operation.

The based band shelf provides the interfaces to the traffic data and thereby to outside

world. In older, lower frequency products, the baseband unit is co-located with the RF

unit typically in a 19 inch or slim rack whereas modern products allow the baseband

and RF unit to be separated by up to 300 meters of commercially available coaxial

cable.

The RF transceiver, if based outdoors, can be mounted directly behind the parabolic

antenna separated by a shot run of waveguide. Optionally, waveguide runs can be

longer allowing the RF unit to be mounted remotely from the antenna. Mounting the

RF unit outdoors allows the system to be designed with minimum use of waveguide,


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which saves on costs, reduces losses, and reduce efforts in installation and

commissioning.

Microwave terminals are available in non-protected and protected configurations. A

protected terminal provides full duplication of all active elements, i.e. bothe the RF

transceiver an the base band components. A number of protection schemes are

available including frequency diversity, space diversity, and monitored hot standby

(MHSB).

Both space diversity and frequency diversity provide protection against path fading

due to multipath propogation in addition to providing protection against equipment

failure. Such techniques are typically only required in bands below 10 GHz, specially

for long paths over flat terrain or over areas subject to atmospheric inversion layers.

Space diversity requires use of additional antenna, which must be separated vertically

in line with engineering calculations. Frequency diversity can be achieved with one

antenna per terminal configured with a dual-pole feed. Frequency diversity has the

disadvantages of reqiring two frequency channels paer link, and the frequency

inefficiency of this technique is therefore a major consideration in many parts of the

world.

MHSB protection can be used at frequencies below 10 GHz if the path conditions are

suitable. It is alsi the normal protection scheme at the higher frequencies where

multipath fading is of negligible concern. MHSB systems are available using one

single-feed antenna per terminal, utilizing only one frequency channel per link.

MHSB thus seems an efficient protection scheme in relation to equipment and

frequency usage.

The transmission section of the netwok is a critical compnenet of any network and

care must be taken to plan it accordingly. There are many general principles applying

to planning a transmission network of leased lines or self-provide cable-based

systems, which also apply to microwave radio relay systems, in addition to a few

specifics for microwave systems. The following lkist can be considered as insight into

some of the planning processes required for microwave radio systems. There will


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obviously be variations due to specific operating conditions and objectives of different

operators, and therefore this should not be considered a definitive list. Also, planning

is an iterative process, and the following list does not necessarily follow sequentially

in every case.

Produce preliminary network design.

Determine local frequency availability and regulations relating to frequency

management.

Microwave path availabilities.

Select sites.

Establish line-of-sight.

The propagation characteristics of electromagnetic waves dictate that the higher the

frequency the greater the free space loss, or attenuation due to the atmosphere, i.e. the

shorter the achievable distances. However, this also means that frequency re-use

distances are shorter: essentially, the distance between links operating on the same

frequency can be shorter without fear of interference. As a result using lower

frequency bands for longer paths and higher frequencies for shorter paths can make

most efficient use of the frequency spectrum.

Path availability targets should also be established and the user should calculate its

taret availability, taking into account overall network availability required and

network integrity as a function of the topology chosen. Preliminary path budgets are

normally calculated either in the form of a spreadsheet, or using software tools

available from equipment manufacturers.

Path availability of a specific microwave link is a factor of a number of components in

relation to the path budget which will take into account net output power expressed as

an equivalent isotropic ally radiated power (EIRP) figure at the antenna, free-space

attenuation in the frequency management authorities will v

Documents

Basics to Survey EWC Notes