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CELLULAR AND MOBILE COMMUNICATIONS
by
D.MUTHILINGAM
Asst Professor
Dept of ECE
LITS-KHAMMAM.
Small Scale Fading
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Small-scale fading, or simply fading, is used to describe the rapid fluctuation of the amplitude of a radio signal over a short period of time or travel distance.
Multipath in the radio channel creates small-scale fading effects.
Rapid changes in signal strength -small travel distance or time interval
Random frequency modulation -varying Doppler shifts
Time dispersion (echoes) caused by multipath propagation delays.
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The path loss for the free space model when antenna gains are included is given by
When antenna gains are excluded, the antennas are assumed to have unity gain, and path loss is given by
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The gain of an antenna is related to its effective aperture( Ae).
The effective aperture is related to the physical size of the antenna.
The frequency is related as
66
The free space power received by a receiver is
Pt is the transmitted power
Pr(d) is the received power
Gt is the transmitter antenna gain,
Gr is the receiver antenna gain
d is the T-R separation distance in meters,
L is the system loss factor/spatial attenuation.
Factors influencing short term fading
70
Many physical factors in the radio propagation channel influence small scale fading.
Multipath propagation.
Speed of the mobile.
Speed of surrounding objects.
The transmission bandwidth of the signal.
Multipath Propagation.
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The presence of reflecting objects and scatterers in the channel creates a constantly changing environment that dissipates the signal energy in amplitude, phase, and time.
These effects result in multipath propagation.
The multipath propagation results fluctuations in signal strength, thereby inducing small-scale fading, signal distortion, or both.
Multipath propagation often lengthens the time required for the baseband portion of the signal to reach the receiver which can cause signal smearing due to intersymbol interference.
Speed Of The Mobile
72
The relative motion between the base station and the mobile results in random frequency modulation.
Different Doppler shifts on each of the multipath components.
Doppler shift will be positive- moving toward BS.
Doppler shift will be negative-away from the BS.
The phase change in the received signal due to the difference in path and results in change in frequency.
Doppler shift positive-increase in frequency.
Doppler shift negative-decrease in frequency.
Speed Of Surrounding Objects
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If objects in the radio channel are in motion, they induce a time varying Doppler shift on multipath components.
If the surrounding objects move at a greater rate than the mobile, then this effect dominates the small-scale fading.
Otherwise, motion of surrounding objects may be ignored, and only the speed of the mobile need be considered.
The Transmission Bandwidth Of The Signal
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If the transmitted radio signal bandwidth is greater than the "bandwidth" of the multipath channel, the received signal will be distorted, but the received signal strength will not fade much over a local area.
The bandwidth of the channel can be quantified by the coherence bandwidth which is related to the specific multipath structure of the channel.
The coherence bandwidth is a measure of the maximum frequency difference for which signals are still strongly correlated in amplitude.
If the transmitted signal has a narrow bandwidth as compared to the channel, the amplitude of the signal will change rapidly, but the signal will not be distorted in time.
Parameters Of Mobile Multipath Fading
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Many multipath channel parameters are derived from the power delay profile.
Power delay profiles are generally represented as plots of relative received power as a function of excess delay with respect to a fixed time delay reference.
Power delay profiles are found by averaging instantaneous power delay profile measurements over a local area in order to determine an average small-scale power delay profile.
Large Scale Fading
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The free space propagation model is used to predict received signal strength.
Transmitter and receiver have a clear, unobstructed line-of-sight path between them.
Satellite communication systems and microwave line-of-sight radio link.
Free Space Propagation Model
The free space power received by a receiver antenna which is separated from a radiating transmitter antenna by a distance d, is given by the Friis free space equation,
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propagation models that characterize the rapid fluctuations of the received signal strength over very short travel distances (a few wavelengths) or short time durations (on the order of seconds) are called small-scale or fading models.
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Scattering:
Scattering occurs when the radio wave travels through a medium consisting of objects with dimensions that are small compared to the wave's wavelength.
- e.g, foliage, street signs, lamp posts..
reflection
scattering
diffraction
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Reflection:
Reflection occurs when a radio wave collides with an object which has very large dimensions compared to the wavelength of the propagating wave.
- e.g., the surface of the Earth, buildings, walls, etc.
Diffraction:
Radio path between transmitter and receiver obstructed by surface with sharp irregular edges,
Waves bend around the obstacle, even when LOS (line of sight) does not exist.
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Radio wave propagation is affected by the following mechanisms:
Reflection - large obstacles
Scattering - small obstacles
Diffraction - edges
Electromagnetic Wave Propagation
Uniqueness Of Mobile Radio Environment
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The mobile radio channel places fundamental limitations on the performance of wireless communication systems.
The transmission Paths can vary from simple line-of-sight to ones that are severely obstructed by buildings, mountains, and foliage.
Radio channels are extremely random and difficult to analyze.
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Due to multiple reflections from various objects ,the electromagnetic waves travel along different paths of varying lengths. Which result in multipath propagation.
In multipath scenario, several copies of the same signal arrive at the receiver with different path lengths at different times and with varying amplitudes and phases.
The strengths of the waves decrease as the distance between the transmitter and receiver increases.
Fading is deviation of the attenuation affecting a signal over certain propagation media.
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Fading is of two types
Large scale fading
Small scale fading
Large scale fading: average signal power attenuation/path loss due to motion over large areas.
Small scale fading: Large variation in signal power due to small changes in the distance between the transmitter and receiver.
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Propagation models have traditionally focused on predicting the average received signal strength at a given distance from the transmitter.
Propagation models of two types depends on the power measured in certain distance /time.
Large-scale propagation model
Small-scale or fading model
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Propagation models that predict the mean signal strength for an arbitrary transmitter-receiver (T-R) separation distance are useful in estimating the radio coverage area of a transmitter and are called large-scale propagation models
since they characterize signal strength over large T-R separation distances (several hundreds or thousands of meters).
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Time Dispersion Parameters
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Multipath channel parameters can be given as
Mean excess delay
RMS delay spread
Excess delay spread
These parameters can be determined from power delay profile.
The time dispersive properties of multipath channels are most commonly quantified by their mean excess delay and rms delay spread .
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Mean excess delay
RMS delay spread
where
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Frequency selective fading is due to time dispersion of the transmitted symbols within the channel.
Induces inter symbol interference.
Frequency selective fading channels are much more difficult to model than flat fading channels.
Statistic impulse response model
2-ray Rayleigh fading model
computer generated
measured impulse response
For frequency selective fading
Frequency selective fading
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If the channel possesses a constant-gain and linear phase response over a bandwidth that is smaller than the bandwidth of transmitted signal, then the channel creates frequency selective fading.
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Time varying statistics: Rayleigh flat fading.
A signal undergoes flat fading if
and
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It can be seen from that if the channel gain changes over time, a change of amplitude occurs in the received signal.
Over time, the received signal r(t) varies in gain, but the spectrum of the transmission is preserved.
Flat fading channel is also called amplitude varying channel.
Also called narrow band channel: bandwidth of the applied signal is narrow as compared to the channel bandwidth
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Frequency selective fading channel characteristics
Fading Effects Due to Doppler Spread
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Small scale Fading Effects Due to Doppler Spread are of two types.
Fast Fading.
Slow Fading.
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Fast Fading: The channel impulse response changes rapidly within the symbol duration.
The coherent time of the channel is smaller then the symbol period of the transmitted signal.
Cause frequency dispersion due to Doppler spreading.
A signal undergoes fast fading if
and
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Slow Fading: The channel impulse response changes at a rate much slower than the transmitted baseband signal s(t).
The Doppler spread of the channel is much less then the bandwidth of the baseband signal.
A signal undergoes slow fading if
and
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The characteristics of a flat fading channel are illustrated in Figure
Flat Fading
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Time dispersion due to multipath causes the transmitted signal to undergo either flat or frequency selective fading.
If the mobile radio channel has a constant gain and linear phase response over a bandwidth which is greater than the bandwidth of the transmitted signal, then the received signal will undergo flat fading.
The received signal strength changes with time due to fluctuations in the gain fo the channel caused by multipath.
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Depends only on the relative amplitude of the multipath components.
Typical RMS delay spreads
Outdoor: on the order of microseconds
Indoor: on the order of nanoseconds
Maximum excess delay (X dB) is defined to be the time delay during which multipath energy falls to X dB below the maximum.
excess delay =
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Coherent Bandwidth(Bc)
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Coherent band width ,Bc , is a statistic measure of the range of frequencies over which the channel can be considered to be "flat".
A channel which passes all spectral components with approximately equal gain and linear phase.
Two sinusoids with frequency separation greater than Bc are affected quite differently by the channel.
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If the coherent bandwidth is defined as the bandwidth over which the frequency correlation function is above 0.9, then the coherent bandwidth is approximately.
If the frequency correlation function is above 0.5
Doppler Spread and Coherence Time
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Doppler spread and coherent time are parameters which describe the time varying nature of the channel in a small-scale region.
When a pure sinusoidal tone of fc is transmitted, the received signal spectrum, called the Doppler spectrum, will have components in the range fc-fd and fc+fd, where fd is the Doppler shift.
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Coherent time Tc is the time domain dual of Doppler spread.
Coherent time is used to characterize the time varying nature of the frequency dispersiveness of the channel in the time domain.
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Two signals arriving with a time separation greater than Tc are affected differently by the channel.
A statistic measure of the time duration over which the channel impulse response is essentially invariant.
If the coherent time is defined as the time over which the time correlation function is above 0.5, then
Types of Small-Scale Fading
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The type of fading experienced by a signal propagating through a mobile radio channel depends on the nature of the transmitted signal with respect to the characteristics of the channel.
signal parameters as bandwidth, symbol period.
channel parameters as rms delay and Doppler spread.
Multipath delay spread leads to time dispersion and frequency selective fading.
Doppler spread leads to frequency dispersion and time selective fading.
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Features of 3G Mobile Technology
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Providing faster communication.
Send/receive large email messages
Video conferencing and 3D gaming
High speed web
TV streaming-Mobile TV
Less time to download MP3 songs and data
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In the past, a total of 33 channels were all allocated to three mobile telephone systems.
Mobile Telephone Service (MTS)-40MHz
Improved MTS (IMTS) MJ-150MHz
Improved MTS (IMTS) MK -450MHz
6 channels of MJ serving 320 customers, with another 2400 customers on a waiting list.
6 channels of MK serving 225 customers, with another 1300 customers on a waiting list.
Poor Service Performance
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The handoff is a process of automatically changing frequencies as the mobile unit moves into a different frequency zone so that the conversation can be continued in a new frequency zone without redialing.
Another disadvantage of the conventional system is that the number of active users is limited to the number of channels assigned to a particular frequency zone.
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Conventional Mobile System
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Limited service capability:
Each area is allocated with one or more channels.
Which is large autonomous geographic zone.
The transmitted power should be as high as the federal specification allows.
The user who starts a call in one zone has to reinitiate the call when moving into a new zone because the call will be dropped.
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The large number of subscribers created a high blocking probability during busy hours.
Although service performance was undesirable, the demand was still great.
A high-capacity system for mobile telephones was needed.
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The frequency utilization measurement (Mo), is defined as the maximum number of customers that could be served by one channel at the busy hour.
Mo = Number of customers/channel
Mo = 53 for MJ system
37 for MK system
Inefficient frequency spectrum utilization
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The offered load can then be obtained by
A = Average calling time (minutes) x total customers / 60 min (Erlangs)
Assume average calling time = 1.76 min.
A1 = 1.76 x53 x 6 / 60 = 9.33 Erlangs (MJ system)
A2 = 1.76 x 37 x 6 / 60 = 6.51 Erlangs (MK system)
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If the number of channels is 6 and the offered loads are A1 = 9.33 and A2 = 6.51, then from the Erlang B model the blocking probabilities,
B1 = 50 percent (MJ system)
B2 =30 percent (MK system),
It is likely that half the initiating calls will be blocked in the MJ system, a very high blocking probability.
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If the actual average calling time is greater than 1.76 min, the blocking probability can be even higher.
To reduce blocking probability we must decrease Mo.
As far as frequency spectrum utilization is concerned, the conventional system does not utilize the spectrum efficiently since each channel can only serve one customer at a time in a whole area.
This is overcome by the new cellular system.
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Limitations of Conventional Mobile Telephone Systems
One of many reasons for developing a cellular mobile telephone system and deploying it in many cities is the operational limitations of conventional mobile telephone systems:
Limited service capability,
Poor service performance,
Inefficient frequency spectrum utilization.
Time division duplexing(TDD)
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Uses time for forward and reverse link
Multiple users share a single radio channel
Forward time slot
Reverse time slot
TDD-provides two simplex time slots on the same frequency.
Frequency division duplexing(FDD)
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Two bands of frequencies for every user
Forward band
Reverse band
Frequency separation between forward band and reverse band is constant.
FDD-provides two simplex channels at the same time.
Channel 1 825.030 MHz (Reverse channel)
870.030 MHz (forward channel)
Concept of Mobile Communication
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A conventional mobile telephone system is usually designed by selecting autonomous geographic zones.(50miles)
one or more channels from a specific frequency allocation for use in the geographic zones.
High powered transmitters are used for coverage.
Mobile Communication
It is the process of communication while moving around a wide geographic area.
Portable - hand-held devices used at walking speed.
Stay connected in everywhere we go.
Stay connected in many ways (e.g.Calls,video,etc)
Communication facility between stationary and mobile or mobile and mobile users ( units )
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UNIT-I
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INTRODUCTION TO CELLULAR MOBILE RADIO SYSTEMS:
Introduction,
Limitations of conventional mobile telephone systems,
Basic Cellular Mobile System,
Generations of cellular wireless systems,
Uniqueness of mobile radio environment-Long term fading,
Factors influencing short term fading,
Parameters of mobile multipath fading,
Time dispersion parameters, Coherence bandwidth
Doppler spread and coherence time
Types of small scale fading.
TEXT BOOKS
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1.Mobile and Cellular Telecommunications-W.C.Y.Lee 2nd Edn, 1989.
2. Wireless Communications – Theodore, S. Rappaport, PHI, 2nd Edn., 2002.
Mobile Communication Operation
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Examples of Mobile Communication Systems
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Pagers-Simplex
Hand held Walkie-Talkies-Half duplex
Cordless phones-Full duplex
Cellular telephones-Full duplex
pager
Walkie-Talkie
Cordless phone
Cellular telephone
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Classification of Wireless Systems
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Simplex- communication is possible in only one direction….e.g. Paging systems
Half duplex-Two way communication but not simultaneous…..e.g. Walkie-Talkies
Full duplex-Two way simultaneous communication…..e.g. cellular systems
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DUPLEXING
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In wireless communication systems ,it is often desirable to allow the user to send simultaneously information to the base station while receiving information from the base station.
Duplexing is done either using frequency or time domain techniques:
Frequency division duplexing (FDD)
Time division duplexing (TDD)
FDD - is more suitable for radio communication systems,
TDD- is more suitable for fixed wireless systems
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Limited service capability,
Poor service performance,
Inefficient frequency spectrum utilization.
Finally we need to increase the capacity(efficiency) of the system by giving service to maximum number of customers with available resources(frequency spectrum).
Mobile communications Cellular communications
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Cellular Concept:
Replacing a single, high power transmitter (large cell) with many low power transmitters (small cells).
Each providing coverage to only a small portion of the service area.
Each base station is allocated a portion of the total number of channels available to the entire system,
Nearby base stations are assigned different groups of channels.
All the available channels are assigned to a relatively small number of neighboring base stations.
2G Mobile Systems
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Global System for Mobile Communications (GSM)
Time Division Multiple Access (TDMA).
900 MHz, 1800 MHz and 1900 MHz
(IS-136 TDMA)
Time Division Multiple Access (TDMA).
IS-136 referred to as Digital AMPS (DAMPS).
CDMA(IS-95)
IS-95 is based on CDMA/DSSS and FDMA.
800 MHz & 1900 MHz bands.
2G Mobile Communication
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Digital cellular technology
Digital multiplexing-TDMA,CDMA
Developed in Europe and the US
Offer support for simple non-voice services like SMS (short messaging service)
Speed up to 64kbps
Systems using 2G
GSM,
TDMA(IS-136)
CDMA(IS-95)
1G Mobile Systems
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Advanced Mobile Phone Service (AMPS)
FDMA for control and FDD for two way transmission.
825 MHz to 890 MHz frequency range.
824 MHz -849 MHz (uplink), 869 MHz-894 MHz (downlink).
Total Access Communication System (TACS)
Sweden, Norway, Demark & Finland
890 MHz - 915 MHz –uplink &935 MHz -960 MHz downlink.
Nordic Mobile Telephone (NMT)
Introduced to Europe in 1981.
450 MHz and 900 MHz
Features of 2G Mobile Technology
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Digital system,
Better voice quality,
Higher capacity,
Lower power consumption.
Short Messaging Service
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Packet Based Cellular that have been enhanced to provide for advanced communication applications
Speed up to 64-144kbps.
Systems use 2.5G:
GPRS (General Packet Radio service)
EDGE (Enhanced Data rate for GSM Evolution)
2.5G Mobile Communication
Features of 2.5G Mobile Technology
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Wireless application protocol (WAP) access,
Multimedia Messaging Service (MMS),
Internet communication services such
E-mail and
World Wide Web access.
Camera
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3G Mobile Technology
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3rd generation was introduced in 2000.
Data speed up to 144kbps-2Mbps.
Smart phones
Systems using 3G:
IMT 2000
WCDMA
CDMA2000
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1G or 1st generation mobile phones were the earliest cellular systems that were developed in early 80's.
Analog cellular networks
Voice communication.
Were very limited in capacity.
Speed up to 2.4kbps.
Systems using 1G :
AMPS,
TACS,
NMT
Evolution of Mobile Communication
Features of 1G Mobile Technology
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Analogue system.
Mobility –can take the cellular where needed
Circuit switched technology.
Basic voice calls only.
Limited local & regional coverage.
Phones were large in size.
Low capacity.
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MTSO:
The switching office, the central coordinating element for all cell sites,
contains the cellular processor and cellular switch.
It interfaces with telephone company zone offices, controls call processing, provides operation and maintenance, and handles billing activities.
The cellular switch, switches calls to connect mobile subscribers to other mobile subscribers and to the nationwide telephone network.
Its processor provides central coordination and cellular administration.
Cellular Concept(contd)
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Neighboring base stations are assigned different groups of channels.
The interference between base stations (and the mobile users under their control) is minimized.
Frequency spectrum may be reused as many times by systematically spacing base stations and their channel groups in a given geographic area.
Co-channel interference.
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Cell:
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Cell is the small geographic area covered by the base station.
The area around an antenna where a specific frequency range is used.
Cell is represented graphically as a hexagonal shape, but in reality it is irregular in shape.
cell
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Basic Cellular Mobile System
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A basic cellular system consists of three subsystems.
A mobile unit,
A cell site, and
A mobile telephone switching office (MTSO)
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Mobile units:
A mobile telephone unit contains a control unit, a transceiver, and an antenna system.
Cell site:
The cell site provides interface between the MTSO and the mobile units.
It has a control unit, radio cabinets, antennas, a power plant, and data terminals.
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Cluster:
A cluster is a group of adjacent cells.
No frequency reuse is done within a cluster.
Number of cells in cluster N=i2+ij+j2
1
3
2
1
4
3
2
2
7
5
4
3
1
6
3-cell cluster
4-cell cluster
7-cell cluster
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Basic Cellular Mobile System
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As frequency spectrum experiences more traffic, spectrum efficiency becomes more important.
In digital systems, continuous transmission is not required because users do not use the allotted bandwidth all the time. In such systems,
TDMA is a complimentary access technique to FDMA. Global Systems for Mobile communications (GSM) uses the TDMA technique.
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