Fundamentals of Mobile communication

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Unit 1

• Fundamentals of Wireless Communication

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Wireless Communication

• Transmitting voice and data using electromagnetic
  waves in open space
• Electromagnetic waves
• Travel at speed of light (c 3x108 m/s)
• Has a frequency (f) and wavelength (l)
• c f x l
• Higher frequency means higher energy photons
• The higher the energy photon the more penetrating
  is the radiation

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Electromagnetic Spectrum
104
102
100
10-2
10-4
10-6
10-8
10-10
10-12
10-14
10-16
IR
UV
X-Rays
Cosmic Rays
Radio Spectrum
Microwave
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106
108
1010
1012
1014
1016
1018
1020
1022
1024
1MHz 100m100MHz 1m 10GHz 1cm
Visible light
lt 30 KHz VLF30-300KHz
LF 300KHz 3MHz MF3 MHz 30MHz
HF 30MHz 300MHz VHF 300 MHz 3GHz
UHF 3-30GHz SHF gt 30 GHz
EHF
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Guglielmo Marconi, 1897

• Radios ability to provide continuous contact
  with ships sailing in English channel

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Evolution of mobile radio communication

• Historically the growth of communication field
  was slow
• coupled by technological developments
• Bell laboratories developed concept of wireless
  communication to entire population in 1960
• Highly reliable, miniature, solid state frequency
  hardware wireless era was born in 1970s

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Evolution of Mobile Radio Communications
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• Mobile Radiotelephone in the U.S.
• In 1934, AM mobile communication systems for
  municipal police radio systems.
• vehicle ignition noise was a major problem.
• In 1946, FM mobile communications for the first
  public mobile telephone service
• Each system used a single, high-powered
  transmitter and large tower to cover distances of
  over 50 km.
• Used 120 kHz of RF bandwidth in a half-duplex
  mode. (push-to-talk release-to-listen systems.)
• Large RF bandwidth was largely due to the
  technology difficulty (in mass-producing tight RF
  filter and low-noise, front-end receiver
  amplifiers.)
• In 1950, the channel bandwidth was cut in half to
  60kHZ due to improved technology.

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• By the mid 1960s, the channel bandwidth again was
  cut to 30 kHZ.
• Thus, from WWII to the mid 1960s, the spectrum
  efficiency was improved only a factor of 4 due to
  the technology advancements.
• Also in 1950s and 1960s, automatic channel
  truncking was introduced in IMTS(Improved Mobile
  Telephone Service.)
• offering full duplex, auto-dial, auto-trunking
• became saturated quickly
• By 1976, has only twelve channels and could only
  serve 543 customers in New York City of 10
  millions populations.

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• Cellular radiotelephone
• Developed in 1960s by Bell Lab and others
• The basic idea is to reuse the channel frequency
  at a sufficient distance to increase the spectrum
  efficiency.
• But the technology was not available to implement
  until the late 1970s. (mainly the microprocessor
  and DSP technologies.)
• In 1983, AMPS (Advanced Mobile Phone System,
  IS-41) deployed by Ameritech in Chicago.
• 40 MHz spectrum in 800 MHz band
• 666 channels ( 166 channels), per Fig 1.2.
• Each duplex channel occupies gt 60 kHz (3030)
  FDMA to maximize capacity.
• Two cellular providers in each market.

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• In late 1991, U.S. Digital Cellular (USDC, IS-54)
  was introduced.
• to replace AMPS analog channels
• 3 times of capacity due to the use of digital
  modulation ( DQPSK), speech coding, and TDMA
  technologies.
• could further increase up to 6 times of capacity
  given the advancements of DSP and speech coding
  technologies.
• In mid 1990s, Code Division Multiple Access
  (CDMA, IS-95) was introduced by Qualcomm.
• based on spread spectrum technology.
• supports 6-20 times of users in 1.25 MHz shared
  by all the channels.
• each associated with a unique code sequence.
• operate at much smaller SNR.(FdB)

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• Mobile Radio Systems Around the World

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• Examples of Mobile Radio Systems

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Examples of mobile radio system

• Classification of mobile radio transmission
  system
• Simplex communication in only one direction
• Half-duplex same radio channel for both
  transmission and reception (push-to-talk)
• Full-duplex simultaneous radio transmission and
  reception (FDD, TDD)
• Frequency division duplexing uses two radio
  channel
• Forward channel base station to mobile user
• Reverse channel mobile user to base station
• Time division duplexing shares a single radio
  channel in time.

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• In FDD,
• a device, called a duplexer, is used inside the
  subscriber unit to enable the same antenna to be
  used for simultaneous transmission and reception.
• to facilitate FDD, it is necessary to separate
  the XMIT and RCVD frequencies by about 5 of the
  nominal RF frequency, so that the duplexer can
  provide sufficient isolation while being
  inexpensively manufactured.
• In TDD,
• only possible with digital transmission format
  and digital modulation.
• very sensitive to timing. Consequently, only used
  for indoor or small area wireless applications.

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Paging Systems

• Conventional paging system send brief messages to
  a subscriber
• Modern paging system news headline, stock
  quotations, faxes, etc.
• Simultaneously broadcast paging message from each
  base station (simulcasting)
• Large transmission power to cover wide area.

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Cordless Telephone System

• Cordless telephone systems are full duplex
  communication systems.
• First generation cordless phone
• in-home use
• communication to dedicated base unit
• few tens of meters
• Second generation cordless phone
• outdoor
• combine with paging system
• few hundred meters per station

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Cellular Telephone Systems

• Provide connection to the PSTN for any user
  location within the radio range of the system.
• Characteristic
• Large number of users
• Large Geographic area
• Limited frequency spectrum
• Reuse of the radio frequency by the concept of
  cell.
• Basic cellular system mobile stations, base
  stations, and mobile switching center.

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• Communication between the base station and
  mobiles is defined by the standard common air
  interface (CAI)
• forward voice channel (FVC) voice transmission
  from base station to mobile
• reverse voice channel (RVC) voice transmission
  from mobile to base station
• forward control channels (FCC) initiating mobile
  call from base station to mobile
• reverse control channel (RCC) initiating mobile
  call from mobile to base station

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• How a Cellular Telephone Call is Made

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Cellular systems generations

• 1G (first generation) voice-oriented systems
  based on analog technology ex. Advanced Mobile
  Phone Systems (AMPS) and cordless systems
• 2G (second generation) - voice-oriented systems
  based on digital technology more efficient and
  used less spectrum than 1G ex. Global System
  for Mobile (GSM) and US Time Division Multiple
  Access (US-TDMA)
• 3G (third generation) high-speed voice-oriented
  systems integrated with data services ex.
  General Packet Radio Service (GPRS), Code
  Division Multiple Access (CDMA)
• 4G (fourth generation) still experimental, not
  deployed yet based on Internet protocol networks
  and will provide voice, data and multimedia
  service to subscribers

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High speed wireless technologies
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Cellular System Design Fundamentals
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• Traditional mobile service
• similar to television broadcasting
• powerful transmitter located at the highest spot
  
• in an area
• broadcast in a radius of up to 50 kilometers.

• Drawbacks
• High power consumption
• Large size of the mobile
• Low capacity

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Cellular Concept

• replacing, a single, high power transmitter with
  many low power transmitters
• each providing coverage to only small portion of
  the service area

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Cellular Networks

• Multiple low-power transmitters (100 W or less)
  are used
• The service area is divided into cells
• Each cell is served by its own antenna
• Each base station consists of a transmitter, a
  receiver, and control unit
• Base station placed in the middle or at the
  border of the cell
• Each base station is allocated a certain
  frequency band (frequency allocation)

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Cellular geometrics

• Adjacent circles cannot be overlaid upon a map
  without leaving gaps or creating overlapping
  regions
• Actual radio coverage of a cell is known as
  footprint and is determined from field
  measurements or propagation prediction models
• Square, equilateral triangle, hexagon

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Cellular geometrics

• For a given distance between the center of a
  polygon and its farthest perimeter points, the
  hexagon has the largest area
• the fewest number of cells can cover a geographic
  region, and hexagon closely approximates a
  circular radiation pattern
• A group of radio channels to be used within a
  small geographic area called cell

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Cellular Geometries

• Cells are classified based on their sizes
• Macrocells with radius of 1km or more (wide area)
• Hexagonal shape cells
• Microcells with radius of 100m or more (cities)
• Hexagonal shape cells
• Manhattan (city) type cell structure
• Picocells with radius in the meters (indoor)
• Shape depends on the room

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Cellular Geometries

• The most common model used for wireless networks
  is uniform hexagonal shape areas
• A base station with omni-directional antenna is
  placed in the middle of the cell

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Design of Wireless Networks

• The design is done in two steps
• Area coverage planning
• Channel (Frequency) allocation

• Outage area
• Coverage area

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Frequency Reuse

• An efficient way of managing the radio spectrum
  is by reusing the same frequency, within the
  service area, as often as possible
• This frequency reuse is possible thanks to the
  propagation properties of radio waves
• By limiting the coverage area to within the
  boundaries of a cell, the same groups of channels
  may be used to cover different cells that are
  separated from one another by distances large
  enough to keep the interference levels within
  tolerable limits

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Frequency reuse

• is a method used by service providers to improve
  the efficiency of a cellular network and to serve
  millions of subscribers using a limited radio
  spectrum
• is based on the fact that after a distance a
  radio wave gets attenuated and the signal falls
  bellow a point where it can no longer be used or
  cause any interference
• a transmitter transmitting in a specific
  frequency range will have only a limited coverage
  area
• beyond this coverage area, that frequency can be
  reused by another transmitter

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Frequency Reuse

• A cluster of cells is formed
• Divide the total number of channels (frequencies)
  between the cells of the cluster.
• All the channels within the cluster are
  orthogonal
• No interference between cells of the same cluster
• Cluster is repeated over the service area
• The distance between the clusters is called the
  reuse distance D
• The design reduces to finding D!

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Frequency reuse concept

• Consider cellular system with S duplex channels
  available,
• let each cell be allocated a group of k
  channels(klts) and if the S channels
• are divided among N cells.
• Available radio channels can be expressed as
• S
  KN
• The N cells which collectively use the complete
  set of available
• frequencies is called a cluster.
• If it is replicated M times within the system,
  total no.of duplex
• channels
• C, can be used as a measure of capacity and is
  given by
• C
  MKN
• 
  MS
• N Cluster size and typically equal to 4,7,12.

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From geometry of hexagons is such that the
number of cells per Cluster, N, can only have
the values which satisfy equation
N i2 ij j2 i
and j are non-negative integers. N can have
the values of 3, 4, 7, 9, 12, 13,19,
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19-cell reuse example (N19, i3, j2)

• To find the nearest co-channel of a neighboring
  cell
• Move i cells along any chain of hexagons.
• Turn 60 degrees counter clockwise.
• Move j cell.

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• Frequency reuse implies that in a given coverage
  area there are several cells that use the same
  set of frequencies
• These cells are called co-channel cells and
  interference between signals from these cells is
  called co-channel interference
• A larger cluster size causes the ratio between
  the cell radius and the distance between the
  co-channel cells to decrease, leading to weaker
  co-channel interference
• To reduce co-channel interference, co-channel
  cells must be physically separated by a minimum
  distance to provide sufficient isolation due to
  propagation
• Co-channel interference ratio is independent of
  the transmitted power and becomes a function of
  the radius of the cell (R) and the distance
  between the centers of the nearest co-channel
  cells (D)

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Frequency Reuse

• For hexagonal cells, the number of cells in the
  cluster is given by

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• The parameter Q is called the co-channel reuse
  ratio is related to the cluster size
• For a hexagonal geometry QD/Rv(3N) a small
  value of Q provides larger capacity since the
  cluster size is small
• whereas a large value of Q improves the
  transmission quality, due to smaller level of
  co-channel interference

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Allocation of channels

• Each Base station is allocated a portion of total
  number of channels available to the entire
  system, and nearby base stations are assigned
  different groups of channels to minimize the
  interference between base stations.

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Channel assignment strategies
Fixed channel assignment Each cell is allocated a
predetermined set of voice channels. Any call
attempt within the cells can only be served by
unused channels in that particular cell. If all
the channels in the cell are occupied, the call
is blocked and the subscriber does not receive
service. Fixed assignment with borrowing Before
a call is blocked, a BS might try to borrow" a
channel from a neighbouring BS. Dynamic channel
assignment The voice channels are not allocated
to different cells permanently, instead each time
a call request is made, the serving base station
request a channel from the mobile switching
center.
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Handoff strategies

• When a mobile moves into a different cell while a
  conversation is in progress, the MSC
  automatically transfers the call to a new channel
  belonging to the new base station.
• This handoff operation not only involves
  identifying a new base station, but also requires
  that the voice and control signals be allocated
  to channels associated with the new base station.

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Handoffs the basics
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• In the first generation analog cellular systems,
  Signal strength Measurements are made by the base
  station to determine the relative location of
  each mobile user with respect to the base
  station.
• In second generation systems that use digital
  TDMA technology,handoff decisions are made mobile
  assisted handoff (MAHO). Every mobile station
  measures the received power from surrounding base
  stations and continually reports the results of
  these measurements to the serving base station. A
  handoff is initiated when the power received from
  the base station of a neighboring cell begins to
  exceed the power received from the current base
  station by a certain level or for a certain
  period of time.

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Prioritizing handoffs

• Guard channel concept
• In this a fraction of total available
  channels in a cell is reserved exclusively for
  handoff requests from ongoing calls which may be
  handed off into the cell.
• Queuing of handoff requests is another method
• to decrease the probability of forced
  termination of a call due to lack of available
  channels.

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Practical handoff considerations

• Using different antenna heights and different
  power levels it is possible to provide large and
  small cells which are co-located at a single
  location. This technique is called umbrella cell
  approach and is used to provide large area
  coverage to high speed users while providing
  small area coverage to users traveling at low
  speeds.
• The umbrella cell approach ensures that the
  number of handoffs in minimized for high speed
  users and provides additional microcell channels
  for pedestrian users.

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Hard handoff and soft handoff

• Hard handoff When the signal strength of a
  neighboring cell exceeds that of the current
  cell, plus a threshold, the mobile station is
  instructed to switch to a new frequency band that
  is within the allocation of the new cell.
• Soft handoff a mobile station is temporarily
  connected to more than one base station
  simultaneously. A mobile unit may start out
  assigned to a single cell. If the unit enters a
  region in which the transmissions from two base
  stations are comparable (within some threshold of
  each other), the mobile unit enters the soft
  handoff state in which it is connected to the two
  base stations. The mobile unit remains in this
  state until one base station clearly
  predominates, at which time it is assigned
  exclusively to that cell.

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Co-channel reuse ratio

• Assuming same cell size and that the base
  stations transmit the same power
• The Co-channel interference ratio becomes
  independent of the Transmitted power and becomes
  function of Radius of the cell (R) and Distance
  between centers of the nearest co-channel cells
  (D)

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You are trying to design a cellular network that
will cover an area of at least 2800 km2. There
are K300 available voice channels. Your design
is required to support at least 100 concurrent
calls in each cell. If the co-channel cell centre
distance is required to be 9 km, how many base
stations will you need in this network?

• If 100 concurrent voice calls must be supported
  in each cell, each cell must be allocated 100
  voice channels.
• This necessitates the frequency re-use factor, N,
  to be 300/1003.
• The distance between co-channel cell centres D is
  related to R and N via the formula
• D 9 km, then, R 3 km
• cell area is
• 2800/23.38 120 base stations are required

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Interference

• Interference is a major limiting factor in the
  performance of cellular radio system
• Sources of Interference
• Another mobile in the same cell
• A call in progress in a neighboring cell
• Other base stations operating in the same
  frequency band or
• Any non cellular system which inadvertently leaks
  energy into the cellular frequency band

• The two major types of system generated
  interferences are
• Co-channel interference
• Adjacent channel interference

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Co-channel Adjacent channel Interference
Co-channel interference
Adjacent-channel interference
Co-channel cells
Adjacent-channel cells
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Co-channel cells for 7-cell reuse
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Signal to Interference Ratio (SIR)
where S desired signal power Ii Interference
power caused by the ith interfering
co-channel cell base station
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Assuming that the transmitting power of each
base station is equal and the path loss exponent
same through out the coverage area
io The number of Co-channel interfering cells
and S/I Signal to interference ratio at the
desired mobile receiver
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• Considering first layer of interfering cells. If
  all the
• interfering base stations are equidistant from
  the
• desired base station (by D between cell centers)

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• For the case where the mobile unit is at the cell
• boundary in a 7-cell cluster (the worst case).
  The distances from the co-channel interfering
  cells are approximated to
• D-R, D and DR .
• Assuming n 4

the worst case SIR
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• For N7 , Q 4.58 from
• From
• (worst case)
• S/I 49.56 (17 dB)
• But by using
• S/I 17.8 dB
• (average)
• Hence for a 7-cell cluster, the S/I ratio is
  slightly less that 18 dB in the worst case.

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Adjacent Channel Interference

• What is adjacent channel interference?
• Interference resulting from signals which are
  adjacent in frequency to desired signal.
• Why does it occur?
• This results from imperfect receiver filters
  which allow nearby frequencies to leak into the
  pass band.
• How can adjacent channel interference be reduced?
• careful filtering and channel assignment
• By keeping the frequency separation between each
  channel in a given cell as large as possible
• By sequentially assigning successive channels in
  the frequency band to different cells

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Power Control For Reducing Interference

• In practical cellular radio and personal
  communication systems the power levels
  transmitted by every subscriber unit are
  controlled by the serving base stations
• Need for Power Control
• Received power must be sufficiently above the
  background noise for effective communication
• Desirable to minimize power in the transmitted
  signal from the mobile. Reduce co-channel
  interference, alleviate health concerns, save
  battery power
• In Spread Spectrum systems using CDMA, its
  desirable to equalize the received power level
  from all mobile units at the Base station.

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