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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