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It 802 d_intro&wlan

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  • 1. IT 802DMobile Communications
    Debasis Das
  • 2. Recommended Books
    J. Schiller, Mobile Communications, Addison –Wesley, 2003
    2. T. S. Rapport, Wireless Communications, Principle and Practices
    3. Forouzan, Data Communications and Networking, TMH
    2/20/2011
    2
    Debasis Das Mallabhum Institute of Technology
  • 3. Introduction
    2/20/2011
    3
    Debasis Das Mallabhum Institute of Technology
  • 4. Module Topics
    General Overview: History, Transmission Medium,
    Need, Advantages, Disadvantages
    Different Standards. AMPS, GSM, GPRS, 3G.
    2/20/2011
    4
    Debasis Das Mallabhum Institute of Technology
  • 5. Usage Scenarios
    Fixed & wired
    Desktops connected to a wired set up
    Mobile & wired
    Laptops get connected to wired networks in hotels
    Fixed & wireless
    Temporary arrangements, places where fixed wiring may not be possible
    Mobile & wireless
    Topic of this course, users are completely free to move around
    2/20/2011
    Debasis Das Mallabhum Institute of Technology
    5
  • 6. Mobile Devices That Need to Communicate
    Cellular handsets
    Walkie-talkies
    Pager
    PDA/Pocket computer
    Cordless phones
    Laptops on wireless LANs
    Tablet computers
    eReaders
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    6
  • 7. What Do We Need!
    A means of carrying information from one party to another
    A means of modifying the carrier to convey information
    Standardized formats for communication
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    7
  • 8. RF Can Do the Job
    Electromagnetic waves in RF frequencies can travel long distances
    The carrier frequency can be modulated to carry information
    The carrier needs to be changed in some way to be proportional to the input information
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    8
  • 9. RF Bands
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  • 10. Electromagnetic Bands
    Up to 300 GHz
    RF band
    Up to 300 THz
    IR band
    Above 300 THz
    Visible light, UV
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    10
  • 11. Wireless Communication Scenarios- 1
    Lambda=c/f wavelength
    C=3*10^8 meters/s
    LF – submarine communications, penetrates water and other obstacles
    MF & HF - radio broadcasts
    AM band; 520 kHz to 1605.5 kHz
    SW band; 5.8 MHz to 26.1 MHz
    FM band; 87.5 MHz to 108 MHz
    UHF & VHF – TV broadcasts 174 MHz to 230 MHz & 470 to 790 MHz
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    11
  • 12. Wireless Communication Scenarios- 2
    VHF & UHF; Digital audio 232 to 230 MHz, 1452 to 1472 MHz
    Digital TV 470 to 862 MHz
    Analog mobile phone 450-465 MHz
    Digital GSM 490-960 MHz, 1710-1880 MHz
    DECT cordless 1880-1900 MHz
    3G cellular, UMTS standard 1900-1980 MHz, 2020-2025 MHz, 2110-2190 MHz ….
    Most wireless mobile communication use the VHF and UHF bands. Small antennas.
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  • 13. Basic RF Communication
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    Receiver
    Transmitter
  • 14. Propagation Basics
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    Receivers
    Transmitter
    Transmission range
    Detection range
    Interference range
  • 15. Various Range in Transmission
    Transmission range : The zone around the transmitter where communication is possible. Receiver receives enough signal so that error rates are very low
    Detection range : transmitted power can be detected, error rate is high however
    Interference zone : not high enough to be detected but adds to the interference with other signals
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  • 16. Radiation Pattern
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    Transmitters
    Directional
    Omni-directional
  • 17. Antenna Issues
    Theoretical isotropic antenna is one that radiates equal energy in all directions.
    Practical antenna is a dipole, length lambda/2, centre fed, two halves of lambda/4
    Dipoles are Omni-directional
    Sectorized antennas are multiple directional antenna on a single pole
    Multiple element antennas help combat fading effects
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  • 18. Multiplexing
    Space division multiplexing (SDM)
    Frequency division multiplexing (FDM)
    Time division multiplexing (TDM)
    Code division multiplexing (CDM)
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    18
  • 19. Channel Allocation
    Borrowing channel allocation (BCA)
    Borrow unused channel frequencies from neighboring cells
    Fixed channel allocation (FCA)
    Channel allocations per cell is fixed
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    19
  • 20. Inverse Square Law
    Radio energy available at any point some distance away from the transmitter
    Is inversely proportional to the square of the distance
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    Surface area= 4.pi.d^2
    d
  • 21. Modes of Propagation
    Ground wave
    Sky wave
    Space wave
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  • 22. Ground Wave propagation
    Radio waves in the low frequency ranges (< 2 MHz)
    Radio energy travels along the ground mainly
    Enclosed between the ground and the Ionosphere
    Radio transmissions in medium wave are examples
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  • 23. Sky Wave Propagation
    There is a ionized belt around the earth( 2 to 30 MHz)
    Radio wave is reflected in this ionosphere and travels long distance
    Wavelengths are shorter than the frequency band that uses the ground wave mode
    Radio shortwave transmissions are examples
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    23
  • 24. Space Wave Propagation
    Wavelengths are short enough to penetrate the ionosphere (> 30 MHz)
    Energy needs to travel in straight line to the receiver
    TV transmissions are an example
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  • 25. Other Signal Propagation Effects
    Blocking/shadowing
    Reflection
    Refraction
    Scatter
    Diffraction
    Multi path propagation
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  • 26. Multipath Situation
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    Skyscraper
    Transmitter
    Car
    Obstacle
  • 27. Multipath + Channel Characteristics
    Multipath causes delay spread
    Effect= Inter-symbol interference
    short term fading
    (It can help if an estimate can be made of these multi path delays, at least the main path. Sender transmits a “training sequence : known to receiver, programs an equalizer to compensate
    Long term fading, due varying distance from sender, compensate by raising/ lowering power transmitted so that received power is within specified range
    Doppler shift
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  • 28. Mobile Communication Schemes
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    (A)
    CDMA
    (B)
    Cellular
  • 29. North American Systems-1
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  • 30. North American Systems-2
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  • 31. European Systems
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  • 32. Japanese Systems
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  • 33. Basic Modulation Schemes(Digital)
    In wireless network analog modulation need to be used, these include following basic schemes
    Amplitude Shift Keying ASK
    Frequency Shift Keying FSK
    Phase shift keying PSK
    The digital input is converted to analog baseband signal before modulation
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  • 34. Reasons for Going Analog
    Antenna
    Antenna need to comparable to wavelength of carrier
    Frequency Division multiplexing
    Analog modulation shifts baseband signal to different carrier signal. Higher carrier frequency gives you higher bandwidth
    Medium Characteristics
    Path loss, penetration of obstacles, reflection, scattering, diffraction are wavelength dependent
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  • 35. Modulation in a Transmitter
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    Analog baseband signal
    Digital
    modulation
    Analog Modulation
    Modulated
    signal
    Digital data
    Radio carrier
  • 36. Demodulation in Receiver
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    Analog baseband signal
    Analog
    demodulation
    Synchronization
    decision
    Digital
    signal
    Radio carrier
  • 37. ASK
    Simple scheme, send one level of carrier amplitude for 1 and another for 0
    Needs low bandwidth
    But amplitude is affected by multipath , noise, path loss etc.
    ASK is not used for RF communication
    However, is used in optical communication
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  • 38. FSK
    Used often in RF communication, binary FSK or BFSK
    Different frequencies are send for the two digital levels
    Sudden phase changes can generate high frequencies
    Frequency modulation in continuous phase modulation (CPM) is used
    Demodulation can use two fixed frequency filters
    Needs larger bandwidth, less susceptible to noise
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  • 39. Phase Shift Keying
    Change phase of carrier by 180 degrees, every time baseband changes from 1 to 0 or 0 to 1. binary PSK
    BPSK can be simply multiplying the carrier by +1 when input is 1 and multiplying by -1 when input is 0
    Reception uses phase locked loop for reference
    Compared to FSK, PSK is more resistant to interference
    Receiver and transmitter are more complex to design
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  • 40. MSK(Min Phase Shift Keying)
    Avoids abrupt phase changes
    Data bits are separated into odd and even bits, duration of bits are doubled
    Two frequencies f1 & f2, such that f2=2*f1
    Phase is shifted based different criteria
    Gaussian MSK is a variation, GMSK is used in most European wireless standards
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  • 41. Advanced PSK
    2 bits are considered together, smaller phase shifts for each group of 2 bits, 4 phase shifts are used
    This is the QPSK scheme
    Carrier reference is a must, frequent synchronization required
    Differential PSK, decides phase shifts with respect from the last phase, not absolute
    DQPSK is one of the most efficient schemes
    IS-136, PACS of US and Japanese PHS of Japan are examples
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  • 42. QAM
    Phase shift keying and amplitude keying is combined
    QPSK and several levels of amplitude
    64QAm for example can combine QPSK and 4 levels of amplitude
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  • 43. Multi Carrier Modulation
    MCM, orthogonal frequency division multiplexing(OFDM), coded OFDM
    Used in European digital audio broadcast (DAB), WLAN standards such as IEEE 802.11, HIPERLAN2
    Good ISI tolerance
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    43
  • 44. Spread Spectrum
    Direct sequence spread spectrum
    Frequency hopping spectrum system
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    Spread spectrum signal
    X
    Modulator
    Transmission
    signal
    User
    data
    Radio
    carrier
    Chipping
    sequence
    Correlator
    Demodulator
    X
    Integrator
    Decision
    Data
    Carrier
  • 45. Frequency Hopping Spread Spectrum
    Available bandwidth is split into many channels plus guard spaces
    Transmitter and receiver stay on a channel for some time and then jump to another, the sequence of this change is the hopping sequence
    Slow hopping: transmitter uses one frequency for several bit times.
    Fast hopping: transmitter changes even during one bit period
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  • 46. FHSS System
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    Narrowband signal
    Spread
    spectrum
    signal
    Data
    Modulator
    Modulator
    Frequency
    synthesizer
    Hopping
    sequence
    Narrowband signal
    Demodulator
    Demodulator
    Data
    Frequency
    synthesizer
    Hopping
    sequence
  • 47. Cellular Systems
    One base station serving a cell (limited area)
    Higher the user density, smaller the cell
    Frequency re-use is possible
    3 and 7 cell clusters are common
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  • 48. Advantages of Small Cells
    Higher capacity
    Space division multiplexing allows frequency reuse, more users can be supported
    Less transmission power
    Mobile station power need to be limited, smaller cells allow better communication
    Local interference only
    Need to worry about local interference only
    Robustness
    More base stations mean that the system would not fail as a whole if some base stations fail
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  • 49. Disadvantages of Small Cells
    Infrastructure needed
    Larger amount of infrastructure, in terms of base stations, towers and other s will be needed
    Handover needed
    With mobile users, users moving from one cell to another, calls will have to be maintained
    Frequency planning
    Careful frequency planning needed to avoid interference when you have only a handful of frequencies allocated
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