U N I T I I Baseband Demod V S H

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  • 1. V. S. Hendre Department of E&TC, TCOER, Pune
    1
    UNIT-II
    Baseband Demodulation/ Detection Techniques
    9/27/2011
  • 2. UNIT-II: Baseband Demodulation/Detection Techniques
    Signals & noise,
    Data formats,
    Synchronization
    multiplexing,
    Intersymbol interference,
    Equalization,
    Detection of binary signals in presence of Gaussian noise,
    Matched and optimum filters.
    9/27/2011
    V. S. Hendre Department of E&TC, TCOER, Pune
    2
  • 3. INTRODUCTION
    In case of baseband signaling, the waveforms at Rx are in pulsed form, but these pulses are not in ideal form.
    Due to such degradation & filtering at the transmitter, the problem of Intersymbol Interference occurred.
    The goal of the demodulator are:
    1) Recovered the baseband signal with less degradation
    2)The SNR should be as high as possible
    3) The received signal should be free from ISI.
    9/27/2011
    V. S. Hendre Department of E&TC, TCOER, Pune
    3
    Baseband Signaling Transmitter
    Baseband Signaling Receiver
  • 4. INTRODUCTION
    9/27/2011
    4
    Line Coding/ Data Formats
    Signal Source
    Channel encoder
    Source encoder
    Multiplexer
    modulator
    Communication channel
    UNIT-II Baseband Demod/Detection Tech
    Unit-III
    Line codes
    (Unipolar, polar)
    -NRZ, RZ, AMI
    -Manchester
    Digital Mux
    • Synchronous
    -Asynchronous
    -quasi-sync
    Synchronization-bit, frame
    Scrambling & unscrambling
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 5. V. S. Hendre Department of E&TC, TCOER, Pune
    5
    Binary line codes
    Line coding: waveform pattern of voltage or current used to represent the 1s & 0s on a transmission link Line coding
    Because of the ac coupling in the transformers & repeaters it is desirable to have a ‘0’ dc in the waveform generated by PCM
    Line
    9/27/2011
  • 6. V. S. Hendre Department of E&TC, TCOER, Pune
    6
    How DC component is generated???Reason 1
    9/27/2011
  • 7. V. S. Hendre Department of E&TC, TCOER, Pune
    7
    Reason 2
    Digital data representation
    Well suited inside the machines (computers)
    Not suitable for long distance-due to presence of stray capacitance in the transmission medium
    For sufficient capacitance on line-adds DC component to data stream
    5 V
    0 V
    9/27/2011
  • 8. V. S. Hendre Department of E&TC, TCOER, Pune
    8
    Conti…
    Problem of synchronization
    Receivers clock oscillator locks on the signal level
    for long string of 1s & 0s-no level shift
    receiver oscillator frequency drifts
    Unsynchronise
    9/27/2011
  • 9. V. S. Hendre Department of E&TC, TCOER, Pune
    9
    Figure 4.3 Effect of lack of synchronization
    9/27/2011
  • 10. Line Coding Formats/ Data Formats/ Transmission Coding Formats
    Binary signaling in the original form suffers degradation & ISI occurs.
    To avoid these problems we are converting this digital pulses into another form of digital pulses which make this data suitable for line or channel.
    Hence this digital to digital conversion is called as Line Coding or Data Formats.
    If baseband data is itself in digital form, then it is necessary to convert that data into a PAM suitable format i.e. Data format
    There are variety of Line coding or Data formats available but they are selected based upon different properties.
    9/27/2011
    V. S. Hendre Department of E&TC, TCOER, Pune
    10
  • 11. V. S. Hendre Department of E&TC, TCOER, Pune
    11
    Requirements
    1) Small BW: to send more signals in a communication channel
    2) Enough Timing content: for receiver to extract the clock information & decode the signal
    3) small probability of error: increases reliability of line codes
    4) Good power efficiency: for a specific BW; transmitted power should be small
    5)Transparency: the coded signal should be received correctly
    9/27/2011
  • 12. Properties of Line Coding Formats
    9/27/2011
    Transmission Bandwidth: It should be as small as possible.
    Power Efficiency: For a given bandwidth and specified detection error probability, it should be as small as possible
    Error Detection & Correction Capability: eg in bipolar case single bit error will be indicated by polarity variation
    Favorable PSD: PSD should be zero at w=0, i.e. D.C. component should be zero.
    Adequate Timing Content: possible to extract timing or clock information from signal
    Transparency: Possible to transmit digital signal correctly regardless of continuous ‘1’ & ‘0’
    V. S. Hendre Department of E&TC, TCOER, Pune
    12
  • 13. V. S. Hendre Department of E&TC, TCOER, Pune
    13
    Line codes: category
    I) Level codes-
    -They are independent on past data.
    -They carry information on their voltage levels
    -two common formats-RZ & NRZ
    -NRZ:Pulse level remains constant during the bit duration
    -RZ:Pulse level zero for a portion of bit duration
    II) Transition codes-
    -Current bit level depends on the previous levels
    -codes have memory
    Ex: Miller code, Split phase (mark), Bi-phase (mark), Code mark inversion (CMI), Dou-Binary, Dicode
    9/27/2011
  • 14. V. S. Hendre Department of E&TC, TCOER, Pune
    14
    Line Coding Formats
    Split phase manchester
    9/27/2011
  • 15. Mapping of Data Symbols into Signal Levels
    9/27/2011
    15
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 16. Unipolar
    All signal levels are on one side of the time axis - either above or below
    NRZ - Non Return to Zero scheme is an example of this code. The signal level does not return to zero during a symbol transmission.
    Scheme is prone to baseline wandering and DC components. It has no synchronization or any error detection. It is simple but costly in power consumption.
    9/27/2011
    16
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 17. Unipolar NRZ scheme
    Advantage:
    1) only one power supply
    2) Easy to generate
    Disadvantage:
    1) more power consumption
    2) prone to DC component
    3)spectrum is not approaching zero near DC
    9/27/2011
    17
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 18. V. S. Hendre Department of E&TC, TCOER, Pune
    18
    Spectrum: Unipolar NRZ
    BW= R (Hz), data rate
    Application: magnetic tape recording
    9/27/2011
  • 19. V. S. Hendre Department of E&TC, TCOER, Pune
    19
    Unipolar RZ
    Advantage:
    1)relatively simple to implement
    2)DC level is lower than NRZ
    Disadvantage:
    1)BW=2R (Hz)
    2) needs 3 dB more power than polar signaling for the same probability of error
    +A
    0
    9/27/2011
  • 20. V. S. Hendre Department of E&TC, TCOER, Pune
    20
    Spectrum: Unipolar RZ
    Bandwidth: 2R
    Application: In baseband data transmission & magnetic tape recording
    9/27/2011
  • 21. Polar - NRZ
    The voltages are on both sides of the time axis.
    Polar NRZ scheme can be implemented with two voltages. E.g. +V for 1 and -V for 0.
    There are two versions:
    NRZ - Level (NRZ-L) - positive voltage for one symbol and negative for the other
    NRZ - Inversion (NRZ-I) - the change or lack of change in polarity determines the value of a symbol. E.g. a “1” symbol inverts the polarity a “0” does not.
    9/27/2011
    21
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 22. Polar NRZ-L and NRZ-I schemes
    Advt.:1)relatively easy to generate
    2) better error probability
    Disadvt.:1)requires two different voltages
    2) Large PSD near zero
    9/27/2011
    22
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 23. V. S. Hendre Department of E&TC, TCOER, Pune
    23
    Spectrum: Polar NRZ
    Bandwidth=R (Hz)
    9/27/2011
  • 24. 4.24
    Note
    In NRZ-L the level of the voltage determines the value of the bit. In NRZ-I the inversion or the lack of inversion determines the value of the bit.
    9/27/2011
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 25. 4.25
    Note
    NRZ-L and NRZ-I both have an average signal rate of N/2 Bd.
    9/27/2011
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 26. 4.26
    Note
    NRZ-L and NRZ-I both have a DC component problem and baseline wandering, it is worse for NRZ-L. Both have no self synchronization &no error detection. Both are relatively simple to implement.
    9/27/2011
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 27. Polar - RZ
    The Return to Zero (RZ) scheme uses three voltage values. +, 0, -.
    Each symbol has a transition in the middle. Either from high to zero or from low to zero.
    This scheme has more signal transitions (two per symbol) and therefore requires a wider bandwidth.
    No DC components or baseline wandering.
    Self synchronization - transition indicates symbol value.
    More complex as it uses three voltage level. It has no error detection capability.
    9/27/2011
    27
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 28. 4.28
    Polar - RZ
    9/27/2011
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 29. 4.29
    Polar - Biphase: Manchester and Differential Manchester
    Manchester coding consists of combining the NRZ-L and RZ schemes.
    Every symbol has a level transition in the middle: from high to low or low to high. Uses only two voltage levels.
    Differential Manchester coding consists of combining the NRZ-I and RZ schemes.
    Every symbol has a level transition in the middle. But the level at the beginning of the symbol is determined by the symbol value. One symbol causes a level change the other does not.
    9/27/2011
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 30. 4.30
    Figure 4.8 Polar biphase: Manchester and differential Manchester schemes
    9/27/2011
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 31. V. S. Hendre Department of E&TC, TCOER, Pune
    31
    Manchester NRZ/Bi--L/split phase/self clocking line code
    Binary 1positive half bit period pulse followed by negative half bit period pulse
    Binary 0 negative half bit period pulse followed by positive half bit period pulse
    Advt:1)always 0 DC value regardless of data sequence
    2)a string of 0’s will not cause a loss of clocking signal
    3) In built ‘single error detection’ capacity
    Disadvt: needs twice the BW of Unipolar NRZ/Polar NRZ codes
    9/27/2011
  • 32. V. S. Hendre Department of E&TC, TCOER, Pune
    32
    spectrum
    Applications:1) LAN like Ethernet & cheaper net 2) IEEE 802.3 baseband coaxial & twisted pair CSMA/CD bus LANS
    9/27/2011
  • 33. 4.33
    Note
    In Manchester and differential Manchester encoding, the transition
    at the middle of the bit is used for synchronization.
    9/27/2011
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 34. 4.34
    Note
    The minimum bandwidth of Manchester and differential Manchester is 2 times that of NRZ. The is no DC component and no baseline wandering. None of these codes has error detection.
    9/27/2011
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 35. 4.35
    Bipolar - AMI and Pseudo ternary
    Code uses 3 voltage levels: - +, 0, -, to represent the symbols (note not transitions to zero as in RZ).
    Voltage level for one symbol is at “0” and the other alternates between + & -.
    Bipolar Alternate Mark Inversion (AMI) - the “0” symbol is represented by zero voltage and the “1” symbol alternates between +V and -V.
    Pseudoternary is the reverse of AMI.
    9/27/2011
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 36. V. S. Hendre Department of E&TC, TCOER, Pune
    36
    Alternate Mark Inversion (AMI)/Bipolar RZ/Polar RZ
    BW=R (Hz)
    9/27/2011
  • 37. V. S. Hendre Department of E&TC, TCOER, Pune
    37
    Pseudo Ternary: 3 encoded signal levels for two level data
    Advt: 1) Low BW, 2) zero DC value
    3) In built ‘single error detection’ capacity
    4) capable of recording clock information
    Disadvt: 1) a long string of successive 0s will adversely affect the precision of synchronization
    2)The receiver has to distinguish 3 different levels instead of just 2
    3) Needs @ 3dB more signal power than polar signal for same probability of error
    Application: Telephone systems
    9/27/2011
  • 38. V. S. Hendre Department of E&TC, TCOER, Pune
    38
    All Line Codes
    Unipolar NRZ
    Polar NRZ
    Unipolar RZ
    Bipolar RZ
    Split-phase or Manchester code
    9/27/2011
  • 39. 9/27/2011
    V. S. Hendre Department of E&TC, TCOER, Pune
    39
  • 40. V. S. Hendre Department of E&TC, TCOER, Pune
    40
    PSD of All Line codes
    9/27/2011
  • 41. V. S. Hendre Department of E&TC, TCOER, Pune
    41
    Comparison of all PSD’s
    9/27/2011
  • 42. V. S. Hendre Department of E&TC, TCOER, Pune
    42
    MULTIPLEXING
    • Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared.
    • 43. Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link.
    • 44. As data and telecommunications use increases, so does traffic.
    9/27/2011
  • 45. V. S. Hendre Department of E&TC, TCOER, Pune
    43
    9/27/2011
  • 46. V. S. Hendre Department of E&TC, TCOER, Pune
    44
    Digital Multiplexing
    Many base-band signals common channel
    Analog system: 1) TDM, FDM. Digital system: interleaving ~TDM
    9/27/2011
  • 47. V. S. Hendre Department of E&TC, TCOER, Pune
    45
    Digital Multiplexing
    The MUX has to perform four functional operations:
    1) Establish a frame as the smallest time interval containing at least one bit from every input.
    2) Assign to each input a number of unique bit slots within the frame
    3) Insert control bit for frame identification & synchronization
    4) Make allowance for any variations of the bit rate.
    Analog system: 1) TDM, FDM. Digital system: interleaving ~TDM
    9/27/2011
  • 48. V. S. Hendre Department of E&TC, TCOER, Pune
    46
    Interleaving Process
    Ex. TRINITY COE PUNE………..
    9/27/2011
    Data Entry Row wise
    Shift Register Bank
    Read Column wise
    Interleaved Sequence : TTEERY . I P.NCU.ION.
  • 49. V. S. Hendre Department of E&TC, TCOER, Pune
    47
    DATA INTERLEAVING/INTERLEAVED CODES/INTERLACED CODE
    Impulse noise-lightening & switching transients-
    burst of error.
    Burst of error of length b
    -sequence of b bit error (1st & last bits-1)
    -in between (b-2) digits-either errornous or correct
    Bursty channels-1.channel causing multipath &
    feding
    2.magnetic recording channel
    (tape/disks)
    9/27/2011
  • 50. V. S. Hendre Department of E&TC, TCOER, Pune
    48
    Read out bits to modulator
    data
    m
    rows
    entry
    (n-k)
    Parity bits
    K data bits
    9/27/2011
  • 51. V. S. Hendre Department of E&TC, TCOER, Pune
    49
    Advantages over TDM
    1) Free from compulsion of periodic sampling
    2)waveform preservation-not necessary while multiplexing
    Multiplexer used-”Binary Multiplexer”
    Multiplexed signal-source digits interleaved
    -bit by bit/characters/words
    For demultiplexing –constant bit rate at Tx
    9/27/2011
  • 52. V. S. Hendre Department of E&TC, TCOER, Pune
    50
    Problem of bit rate variation-solved
    A) Synchronous multiplexer
    -master clockgoverns all sources & eliminates bit rate variations
    -highest efficiency
    Disadvantage: needs provision of distributing the master clock, design becomes complex.
    B) Asynchronous Multiplexer
    -used for data source that operates in start/stop modewith burst of characters with variable spacing betn bursts
    -principle-buffering & interleaving
    -Application-computer networks
    C) Quasi-synchronous multiplexer
    -used when input bit rates have the same nominal value butvaries within specified bound
    9/27/2011
  • 53. V. S. Hendre Department of E&TC, TCOER, Pune
    51
    Multiplexing hierarchy for digital telecommunications:Increasing BIT RATES
    • Multiplexing patterns- 1) American Telephone & Telegraph Company (AT &T)hierarchy, 2) International Telegraph and Telephone Consultative Committee (CCIT) hierarchy
    -both 64 kbps voice PCM unit
    Layout
    Digital
    Data
    Channel Bank
    Only
    Multiplexing
    Other MUX  point to point transmission
    9/27/2011
  • 54. V. S. Hendre Department of E&TC, TCOER, Pune
    52
    Parameters of AT&T and CCITT hierarchies
    Out put bit rate>sum of input bit rates
    Surplus-control bit+ stuff bits (for steady output rate)
    9/27/2011
  • 55. V. S. Hendre Department of E&TC, TCOER, Pune
    53
    Illustrative configuration of the AT&T hierarchy
    9/27/2011
  • 56. V. S. Hendre Department of E&TC, TCOER, Pune
    54
    Figure 6.23 Digital hierarchy
    9/27/2011
  • 57. V. S. Hendre Department of E&TC, TCOER, Pune
    55
    Example-synchronous multiplexers
    Basic TDM scheme
    Wide Band co-axial cable
    Framing &
    Sequencial sampling
    Local clock
    repeaters
    9/27/2011
  • 58. V. S. Hendre Department of E&TC, TCOER, Pune
    56
    Figure 6.24 T-1 line for multiplexing telephone lines
    9/27/2011
  • 59. V. S. Hendre Department of E&TC, TCOER, Pune
    57
    Figure 6.25 T-1 frame structure
    9/27/2011
  • 60. V. S. Hendre Department of E&TC, TCOER, Pune
    58
    Bits/frame
    :commutator speed-8000 revolution/sec
    :sampling rate-8000 samples/sec
    : each sample 8 bits
    • no. of output bits=24x8=192 bits
    *frame synchronisation
    :synchronisation information-extra bits/frame
    Channel 1
    Channel 23
    Channel 24
    Frame
    bit
    8 bits
    8 bits
    8 bits
    193 bits, 125 s
    9/27/2011
  • 61. V. S. Hendre Department of E&TC, TCOER, Pune
    59
    Figure 6.22 Framing bits
    9/27/2011
  • 62. V. S. Hendre Department of E&TC, TCOER, Pune
    60
    Bit rate
    -frame time Tp=1/8000=125 s
    -Tp occupies 193 bits
    -bit rate on T1 channel
    fb(T1)=no. of bits/Time=193/125 Mb/s=1.544 Mbps
    Signaling information/supervisory information
    -Dial pulses, busy signal with speech signal
    -added to voice signal-method “BIT ROBBING”
    -8th (LSB) bit of 6th sample- Voice transmission + signaling
    -1st five samples-Eight bit, 6th sample- 7 bits + 8th bit for signaling
    No. of bits in six frames=[5 frames x 8 bits]+[1 frame x 7 bits]
    =47 bits
    Avg. bits/sample=47/6= 7 (5/6) bits
    9/27/2011
  • 63. V. S. Hendre Department of E&TC, TCOER, Pune
    61
    Signalling bit frequecny=(1/6)xframe bit rate
    =(1/6) x 8000
    fb(T1) signalling = 1333 Hz
    Signalling Technique-
    -“Channel Associated Signalling”
    9/27/2011
  • 64. V. S. Hendre Department of E&TC, TCOER, Pune
    62
    Synchronization Techniques
    Types: 1) Symbol / Bit synchronization
    2) Frame Synchronization
    3) Carrier synchronization
    Synchronization in a Binary Receiver
    y(t)
    Bit Synchronization:-Open loop bit synchronization
    -closed loop bit synchronization
    -Early Late Synchronization
    9/27/2011
    Output Message
    LPF
    Regenerator
    Bit Sync.
    Frame Sync.
    CLK.
    Frame Identification
  • 65. V. S. Hendre Department of E&TC, TCOER, Pune
    63
    Carrier Sync.:1)Mth power Law carrier synchronization
    2)Costas Loop
    Bit Synchronization:-Open loop bit synchronization
    Used when y(t)=unipolar RZ format
    y(t) from polar to unipolar conversion
    9/27/2011
  • 66. V. S. Hendre Department of E&TC, TCOER, Pune
    64
    9/27/2011
  • 67. V. S. Hendre Department of E&TC, TCOER, Pune
    65
    Closed loop bit synchronization
    9/27/2011
  • 68. V. S. Hendre Department of E&TC, TCOER, Pune
    66
    Disadvantages
    Sync will suffer from timing jitter when 1) zero crossing of y(t) are not spaced by integer multiples of Tb
    2)message includes a long string of 1’s & 0’s
    Problem is solved by message scrambling
    9/27/2011
  • 69. V. S. Hendre Department of E&TC, TCOER, Pune
    67
    Early-late bit synchronization (a) waveform (b) block diagram
    Filtered digital signal
    • Independent of zero crossings
    • 70. Properly filtered digital signal has peaks at the optimum sampling times & symmetric on either side
    • 71. tksinchronised & <Tb/2
    |y(tk-)|~ |y(tk+)|<|y(tk|
    • Early synchronisation
    |y(tk-)|<|y(tk+)|
    v(t)=|y(tk-)|-|y(tk+)|>0
    speeds up clock
    Late sync
    |y(tk-)|>|y(tk+)|
    9/27/2011
  • 72. V. S. Hendre Department of E&TC, TCOER, Pune
    68
    Scrambler & message sequence generator
    Coding technique at transmitter—long string of likely bits occure —randomized
    Eliminates periodic bit patterns-undesired discrete frequency components in the power spectrum
    Tapped shift registers are used
    9/27/2011
  • 73. V. S. Hendre Department of E&TC, TCOER, Pune
    69
    Binary scrambler (b) un-scrambler at receiver
    tap gains 1=2=0,
    & 3=4=1
    a)
    M”k=M’k-3 M’k-4
    M’k=Mk M”k
    9/27/2011
  • 74. V. S. Hendre Department of E&TC, TCOER, Pune
    70
    Input sequence:10 11 00 00 00 00 01
    9/27/2011
  • 75. V. S. Hendre Department of E&TC, TCOER, Pune
    71
    Frame synchronization
    Receiver should know-when signal is present
    Aspects of frame sync.
    1) Identify start of frame
    2) Identify subdivisions/subframes within the message
    For frame sync-special N bit sync word
    prefix-time for bit-sync acquisation
    Start of message
    Message bits
    t
    N bit sync. word
    9/27/2011
  • 76. V. S. Hendre Department of E&TC, TCOER, Pune
    72
    Start of message-different codeword followed by Prefix
    Frames are labeled by sync words-inserted periodically in the bit stream
    Frame synchronizer
    o/p bits with polar format
    ~sync word bits in polar form
    V
    9/27/2011
  • 77. V. S. Hendre Department of E&TC, TCOER, Pune
    73
    Phase locked loop
    Phase
    Compa-
    rator
    LPF
    VCO
    Narrow band F
    ()M
    Divide by M
    BPF
    Received signal
    Carrier Synchronizer-1) Mth power carrier recovery circuit
    Recovered carrier
    Removes noise outside the Band
    Spectral components + Mfc
    Mfc
    fc
    • Due to jitter-carrier at transmitter is unstable
    • 78. Oscillator frequency drifts: f & larger
    • 79. Narrowband filter cannot be made as narrow as our requirements
    • 80. Replace narrowband filter by PLL
    • 81. PLL-filter whose bandpass is determined by LPF
    • 82. PLL will follow oscillator jitter
    9/27/2011
  • 83. V. S. Hendre Department of E&TC, TCOER, Pune
    74
    Phase comparator
    LPF, fc
    900 phase
    shift
    VCO
    Loop filter
    Vm
    LPF, fc
    Phase comparator
    b) The Costas Loop
    Involves two PLL’s with common VCO & loop filter
    9/27/2011
  • 84. V. S. Hendre Department of E&TC, TCOER, Pune
    75
    If VCO frequency differs form carrier frequency progressive change in phase difference -
    This change change in Vm—increases/decreases VCO frequency
    9/27/2011
  • 85. V. S. Hendre Department of E&TC, TCOER, Pune
    76
    Signal & Noise
    Error Performance Degradation in Digital Communication System: The task of the detector is to retrieve the bit stream from the received waveform as error free as possible, notwithstanding the impairments to which the signal have been subjected.
    Two Causes of error:
    1) The effect of filtering at the transmitter, channel, and receiver which causes ISI.
    2) Electrical noise and Interference produced by a variety of sources, such as atmospheric noise switching transients, as well as interfering signals from other sources.
    9/27/2011
  • 86. Intersymbol Interference
    • No channel has infinite bandwidth
    • 87. Most transmission schemes require higher bandwidth than available in the channel.
    • 88. Square wave requires infinite bandwidth.
    • 89. Synch function is not possible due to causality violation.
    • 90. Modified synch function to satisfy the causality requires higher bandwidth.
    • 91. Each symbol may be smeared into adjacent time slots.
    • 92. Intersymbol Interference (ISI) is the spreading of symbol pulses from
    one slot into adjacent slots.
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    77
    V. S. Hendre Department of E&TC, TCOER, Pune
  • 93. V. S. Hendre Department of E&TC, TCOER, Pune
    78
    ISI & EYE Pattern
    Fundamental limitation (digital transmission)
    relationship betn ISI, BW & signaling rate
    “Given an ideal Low pass channel of BW ‘B’, it is possible to transmit independent symbols at a rate r≤2B boud without Inter- Symbol Interference”
    ‘No transmission at r>2B’
    Practically, no channel-ideal freqn response
    Linear distortion=amplitude & delay
    Solution-channel+equalisationideal freqn response
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  • 94. V. S. Hendre Department of E&TC, TCOER, Pune
    79
    Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
    (a) Baseband transmission system (b) signal-plus-noise (ISI) waveform
    Actual signal
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  • 95. V. S. Hendre Department of E&TC, TCOER, Pune
    80
    Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
    Baseband binary receiver
    Figure 11.2-1
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  • 96. V. S. Hendre Department of E&TC, TCOER, Pune
    81
    Hc(f)
    Heq(f)
    x(t)
    y(t)
    channel
    Equalizer
    Overall T.F.=Hc(f).Heq(f)filtering characteristic
    If Heq(f)-chosen to minimize ISIfilter at Rx ‘Equalizing filter’
    Final o/p - distortion less (minimum ISI) if
    Time delay
    Gain factor
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  • 97. V. S. Hendre Department of E&TC, TCOER, Pune
    82
    Methods to eliminate ISI
    a) Nyquist First Method:Zero ISI
    b) Nyquist Second Method:control of ISI
    c) Nyquist Third Method
    Eye-Pattern
    :effect of channel filtering & channel noise-seen by observing received line codes on an Analog Oscilloscope-displayeye-pattern
    Eye pattern provides excellent way of assessing the quality of the received line code & the ability of the receiver to combat bit errors
    General eye pattern
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  • 98. V. S. Hendre Department of E&TC, TCOER, Pune
    83
    • Under normal operating conditions (No detected bit error)
    eye –opened
    • For noise/ISI eye closedbit error at receiver
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  • 99. V. S. Hendre Department of E&TC, TCOER, Pune
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    Eye Pattern Seen in oscilloscopeThe Cleaner, the betterGood indication of transmission quality
  • 100. V. S. Hendre Department of E&TC, TCOER, Pune
    85
    (a) Distorted polar binary signal (b) eye pattern
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  • 101. V. S. Hendre Department of E&TC, TCOER, Pune
    86
    Binary Baseband Demodulation / Detection
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  • 102. V. S. Hendre Department of E&TC, TCOER, Pune
    87
    Demodulation / Detection
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    • For any binary channel, the transmitted signal over a symbol interval (0, T) is represented by
    • 103. The received signal r(t) degraded by noise n(t) and possibly degraded by the pulse response of the channel hc(t) was described
  • V. S. Hendre Department of E&TC, TCOER, Pune
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    Demodulation & Detection
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    • Demodulation: Demodulation is a recovery of a waveform (to an undistorted baseband pulse),
    • 104. Detection: To mean the decision-making process of selecting the digital meaning of that waveform.
    • 105. Frequency down-conversion block:performs frequency translation for band pass signals operating at some radio frequency (RF). It may take place within the front end of the receiver, within the demodulator, shared between the two locations, or not at all.
  • V. S. Hendre Department of E&TC, TCOER, Pune
    89
    Demodulation & Detection
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    • The Receiving filter: which performs waveform recovery in preparation of the next important step-detection.
    • 106. The goal of the receiving filter is to recover a baseband pulse with the best possible signal-to-noise ratio (SNR), free of any ISI.
    • 107. The optimum receiving filter for accomplishing this function is called a matched filter or correlator.
    • 108. An optional equalizing filter follows the receiving filter; it is only needed for those systems where channel induced ISI can distort the signals.
  • V. S. Hendre Department of E&TC, TCOER, Pune
    90
    Baseband Signal Detection: Integrate & Dump Switch
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  • 109. V. S. Hendre Department of E&TC, TCOER, Pune
    91
    Baseband Signal Detection: Integrate & Dump Switch
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  • 110. V. S. Hendre Department of E&TC, TCOER, Pune
    92
    Baseband Signal Detection: Integrate & Dump Switch
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  • 111. V. S. Hendre Department of E&TC, TCOER, Pune
    93
    Baseband Signal Detection: Integrate & Dump Switch
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    • We will be interested in a quantity called Peak pulse signal-to-noise ratio at the output which is given by,
    To find , we need transfer function of integrator which is given by,
  • 112. V. S. Hendre Department of E&TC, TCOER, Pune
    94
    Baseband Signal Detection: Integrate & Dump Switch
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  • 113. V. S. Hendre Department of E&TC, TCOER, Pune
    95
    Baseband Signal Detection: Integrate & Dump Switch
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    Thus. we can see from above equation that-
    The signal-to-noise ratio at the output of integrate-and-dump circuit increases with bit duration T.
    It also depends on A2Twhich is normalized energy of the bit (symbol).
    Since and the signal voltage increases linearly with T and the noise voltage increases slowly with. Hence we can say that integrator enhances the signal more than the noise.
  • 114. V. S. Hendre Department of E&TC, TCOER, Pune
    96
    Probability of Error
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    Probability density function (pdf) of the Gaussian random noise no can be represented as
    The conditional PDF:
  • 115. V. S. Hendre Department of E&TC, TCOER, Pune
    97
    Probability of Error
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  • 116. V. S. Hendre Department of E&TC, TCOER, Pune
    98
    Example
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    Find the error probability of a binary baseband receiver with the binary pulse S(t) = +0.5 V and -0.5 V with bit rate 1 kbps. The noise power spectral density Is 10-5 W/Hz. What is the probability of error if the transmitted amplitudes are reduced by
    50%? Given erf (3.535)=0.999999445
  • 117. V. S. Hendre Department of E&TC, TCOER, Pune
    99
    Example
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  • 118. V. S. Hendre Department of E&TC, TCOER, Pune
    100
    Optimum Filter
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    The integrate-and-dump circuit emphasizes signal output in comparison with the noise voltage. The error probability of this circuit is dependent on Eb/No ratio. But then, is it an optimum value of probability that we get?
  • 119. V. S. Hendre Department of E&TC, TCOER, Pune
    101
    Error Probability of Optimum Filter
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    In order to find error probability in this receiver, consider that S2 (t) was transmitted.
    • Let no(T) be positive and SOl (T) > SO2 (T). If no(T) is larger than error will be made in decision-making i.e. we will be deciding in favor of Sl(t).Thus, error in detection occurs when,
  • 120. V. S. Hendre Department of E&TC, TCOER, Pune
    102
    Error Probability of Optimum Filter
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  • 121. V. S. Hendre Department of E&TC, TCOER, Pune
    103
    Error Probability of Optimum Filter
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    Let
  • 122. V. S. Hendre Department of E&TC, TCOER, Pune
    104
    Transfer function of Optimum Filter
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  • 123. V. S. Hendre Department of E&TC, TCOER, Pune
    105
    Matched Filter
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    The optimum filter is considered with generalized Gaussian noise. An optimum filter which gives a maximum ratio
    when input noise is white Gaussian noise, is called a matched filter
  • 124. V. S. Hendre Department of E&TC, TCOER, Pune
    106
    Properties of Matched Filter
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    1. The spectrum of output signal of a matched filter with matched signal as input is proportional to energy density of input signal.
    Hence, spectrum of output signal [Y(t)] is proportional to its Energy Spectral Density
  • 125. V. S. Hendre Department of E&TC, TCOER, Pune
    107
    Properties of Matched Filter
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    2. The output signal of a matched filter is proportional to a shifted version of autocorrelation function of input signal to which the filter is matched.
    3. The output signal-to-noise ratio of matched filter depends only on the ratio of the signal energy to P.S.D. of white noise at filter input.
  • 126. Conclusion : Baseband Demodulation/Detection Techniques
    Signals & noise,
    Data formats,
    Synchronization
    multiplexing,
    Intersymbol interference,
    Equalization,
    Detection of binary signals in presence of Gaussian noise,
    Matched and optimum filters.
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  • 127. V. S. Hendre Department of E&TC, TCOER, Pune
    109
    Maximum length /Pseudo noise sequence Generator
    Shift resister-non-zero state & output is fed back to input
    Unit acts-periodic sequence generator
    Ex:5 stage shift resister with [5,2] configurations with initial non zero states
    periodic
    1111100110100100001010 111 011 000……
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  • 128. V. S. Hendre Department of E&TC, TCOER, Pune
    110
    Longest possible sequence for n stage shift resister
    L=2n -1, output MLS/PN sequence
    Pseudo noise-correlation properties of PN sequence
    PN signal –acts like-white noise with small DC component
    Application of PN sequence:
    1) in test instruments
    2)radar ranging
    3)spread spectrum communication
    4) Digital framing
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  • 129. V. S. Hendre Department of E&TC, TCOER, Pune
    111
    Pseudorandom Numbers
    Generated by algorithm using initial seed
    Deterministic algorithm
    Not actually random
    If algorithm good, results pass reasonable tests of randomness
    Need to know algorithm and seed to predict sequence
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  • 130. V. S. Hendre Department of E&TC, TCOER, Pune
    112
    Properties of PN sequence
    PN sequence is periodic
    In each period-number of 1’s >0’s by 1
    Among the runs of consecutive 1’s & 0’s
    -(1/2)-of the runs of each kind are of length 1
    -(1/4)-are of length 2
    -(1/8)-are of length 3 etc.
    Ex: calculate PN sequence for 4 bit shift resister with [3,2] feedback connections
    Autocorrelation of a PN sequence
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  • 131. V. S. Hendre Department of E&TC, TCOER, Pune
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  • 132. V. S. Hendre Department of E&TC, TCOER, Pune
    114
    [1]
    [2,1]
    [3,2]
    [4,3]
    [5,3]
    [6,5]
    [7,6]
    [8,4,3,2]
    [9,5]
    [10,7]
    Unit-III-ENDS
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  • 133. V. S. Hendre Department of E&TC, TCOER, Pune
    115
    Maximum length shift register codes /PN sequence
    Class of cyclic codes with (n,k)=(2 m-1,m) , where m= +ve integer
    M stage digital shift register –feedback based on parity polynomial
    Ex. 3 stage(m=3) shift register with feedback
    Source
    m bits
    2
    1
    Out
    put
    Flip-flop
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  • 134. V. S. Hendre Department of E&TC, TCOER, Pune
    116
    For each code word- m info. Bits ->SR
    Switch –from 1 to 2
    Shift register –shifted 1 bit left for 2m-1 shifts
    Systematic code –length n= 2m-1
    Data undergo cyclical shift for 2m-1 shifts
    SR-original state –in 2m-1 shifts
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  • 135. V. S. Hendre Department of E&TC, TCOER, Pune
    117
    Consider example with I/p: 0 0 1
    0 0 1
    2
    1
    Flip-flop
    0
    0
    0
    1
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  • 136. V. S. Hendre Department of E&TC, TCOER, Pune
    118
    0
    0
    1
    1
    1
    1
    1
    1
    1
    1
    0
    1
    1
    1
    0
    1
    0
    0
    1
    0
    1
    1
    0
    0
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  • 137. V. S. Hendre Department of E&TC, TCOER, Pune
    119
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  • 138. V. S. Hendre Department of E&TC, TCOER, Pune
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    Output seqn-periodic & length n= 2m-1
    Length-largest possible period
    Hence , 2m-1 codewords –different cyclic-shift of a single codeword
    Not all f/b arrangements- MLS
    To check-polynomial f(x)= 0+ 1x+ 2x2 +-------+(n-1)x(n-1)+xn
    Check if irreducible.
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  • 139. V. S. Hendre Department of E&TC, TCOER, Pune
    121
    Shift register connection – MLSR code (for 2<=m<=34)
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  • 140. V. S. Hendre Department of E&TC, TCOER, Pune
    122
    Properties of MLSR code
    Sequence –periodic
    Each codeword (except all zero)- 2m-1 ones & 2m-1 -1 zeros
    All codewords-identical weights w = 2m-1 =dmin
    Codeword compared-cyclical shift of itself no of agreements differ from no of
    disagreements - by one
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  • 141. V. S. Hendre Department of E&TC, TCOER, Pune
    123
    Same shift register arrangements- a periodic binary sequence –period n= 2m-1
    Seqn-periodic autocorrelation(m) =n for m=0,+-n,+-2n,--- (m) =-1 for all other shifts
    ->impulse like autocorrelation =>power spectrum ~white
    Seqn resembles white noise
    Maximum length sequence-Pseudo noise sequence -used-for data scrambling -SS generation
    All zero state-prohibited
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  • 142. V. S. Hendre Department of E&TC, TCOER, Pune
    124
    PN sequence
    mls –long string of likely bits- at receiver -disturbs synchronisation
    Randomized at transmitter
    ->PN sequence
    Device-scrambler -eliminates periodic bit pattern
    At receiver- Unscrambler
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  • 143. V. S. Hendre Department of E&TC, TCOER, Pune
    125
    Scrambler & unscrambler –4 stage SRtap gains 1= 2=0 & 3= 4=1
    Scrambler unscrambler
    m”k
    m”k
    mk
    m’k
    mk
    m’k
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  • 144. V. S. Hendre Department of E&TC, TCOER, Pune
    126
    m’’k=m’k-3 m’k-4 & m’k=mk m’’k (a)
    m’k m’’k = [mk m’’k] m’’k = mk[ m’’k  m’’k] = mk 0 = mk
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  • 145. V. S. Hendre Department of E&TC, TCOER, Pune
    127
    I/p seqn: 1011 0000 0000 01*SR-initially- zeros
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  • 146. V. S. Hendre Department of E&TC, TCOER, Pune
    128
    Application of PN sequence
    In Test equipments
    Radar ranging
    Spread spectrum communication
    Digital framing
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  • 147. V. S. Hendre Department of E&TC, TCOER, Pune
    129
    Mean square value of r.v. x
    Putting II) into I), mean square value of noise voltage
    =
    At R=1, noise power is normalized
     Normalized noise power/ quantization noise power =
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