Modulation Techniques

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Chapter 5 Signal Encoding and Modulation
Techniques
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Encoding and Modulation Techniques
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Digital Signaling Versus Analog Signaling

• Digital signaling
• Digital or analog data is encoded into a digital
  signal
• Encoding may be chosen to conserve bandwidth or
  to minimize error
• Analog Signaling
• Digital or analog data modulates analog carrier
  signal
• The frequency of the carrier fc is chosen to be
  compatible with the transmission medium used
• Modulation the amplitude, frequency or phase of
  the carrier signal is varied in accordance with
  the modulating data signal
• by using different carrier frequencies, multiple
  data signals (users) can share the same
  transmission medium

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Digital Signaling

• Digital data, digital signal
• Simplest encoding scheme assign one voltage
  level to binary one and another voltage level to
  binary zero
• More complex encoding schemes are used to
  improve performance (reduce transmission
  bandwidth and minimize errors).
• Examples are NRZ-L, NRZI, Manchester, etc.
• Analog data, Digital signal
• Analog data, such as voice and video
• Often digitized to be able to use digital
  transmission facility
• Example Pulse Code Modulation (PCM), which
  involves sampling the analog data periodically
  and quantizing the samples

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Analog Signaling

• Digital data, Analog Signal
• A modem converts digital data to an analog signal
  so that it can be transmitted over an analog line
• The digital data modulates the amplitude,
  frequency, or phase of a carrier analog signal
• Examples Amplitude Shift Keying (ASK), Frequency
  Shift Keying (FSK), Phase Shift Keying (PSK)
• Analog data, Analog Signal
• Analog data, such as voice and video modulate the
  amplitude, frequency, or phase of a carrier
  signal to produce an analog signal in a different
  frequency band
• Examples Amplitude Modulation (AM), Frequency
  Modulation (FM), Phase Modulation (PM)

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Digital Data, Digital Signal

• Digital signal
• discrete, discontinuous voltage pulses
• each pulse is a signal element
• binary data encoded into signal elements

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Periodic signals

• Data element a single binary 1 or 0
• Signal element a voltage pulse of constant
  amplitude
• Unipolar All signal elements have the same sign
• Polar One logic state represented by positive
  voltage the other by negative voltage
• Data rate Rate of data (R) transmission in bits
  per second
• Duration or length of a bit Time taken for
  transmitter to emit the bit (Tb1/R)
• Modulation rate Rate at which the signal level
  changes, measured in baud signal elements per
  second. Depends on type of digital encoding used.

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Interpreting Signals

• Need to know
• timing of bits when they start and end
• signal levels high or low
• factors affecting signal interpretation
• Data rate increase data rate increases Bit Error
  Rate (BER)
• Signal to Noise Ratio (SNR) increase SNR
  decrease BER
• Bandwidth increase bandwidth increase data rate
  
• encoding scheme mapping from data bits to signal
  elements

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Comparison of Encoding Schemes

• signal spectrum
• Lack of high frequencies reduces required
  bandwidth,
• lack of dc component allows ac coupling via
  transformer, providing isolation,
• should concentrate power in the middle of the
  bandwidth
• Clocking
• synchronizing transmitter and receiver with a
  sync mechanism based on suitable encoding
• error detection
• useful if can be built in to signal encoding
• signal interference and noise immunity
• cost and complexity increases when increases
  data rate

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Encoding Schemes
Positive level (5V) Negative level (-5V)
Positive level (5V)No line signal (0V)Negative
level (-5V)
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Encoding Schemes
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NonReturn to Zero-Level (NRZ-L)

• Two different voltages for 0 and 1 bits
• Voltage constant during bit interval
• no transition, i.e. no return to zero voltage
• more often, negative voltage for binary one and
  positive voltage for binary zero

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NonReturn to Zero INVERTED (NRZI)

• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of signal
  transition at beginning of bit time
• transition (low to high or high to low) denotes
  binary 1
• no transition denotes binary 0
• Example of differential encoding since have
• data represented by changes rather than levels
• more reliable detection of transition rather than
  level

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Advantages and disadvantages of NRZ-L, NRZI

• Advantages
• easy to engineer
• good use of bandwidth
• Disadvantages
• dc component
• lack of synchronization capability
• Unattractive for signal transmission applications

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Multilevel BinaryBipolar Alternate Mark
Inversion (AMI)

• Use more than two levels (three levels, positive,
  negative and no line signal)
• Bipolar-AMI
• zero represented by no line signal
• one represented by positive or negative pulse
• one pulses alternate in polarity
• no loss of sync if a long string of ones
• long runs of zeros still a problem
• no net dc component
• lower bandwidth
• easy error detection

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Multilevel BinaryPseudoternary

• Binary one represented by absence of line signal
• Binary zero represented by alternating positive
  and negative pulses
• No advantage or disadvantage over bipolar-AMI
• Each used in some applications

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Multilevel Binary Issues

• Advantages
• No loss of synchronization if a long string of
  1s occurs, each introduce a transition, and the
  receiver can resynchronize on that transition
• No net dc component, as the 1 signal alternate in
  voltage from negative to positive
• Less bandwidth than NRZ
• Pulse alternating provides a simple mean for
  error detection
• Disadvantages
• receiver distinguishes between three levels A,
  -A, 0
• a 3 level system could represent log23 1.58
  bits
• requires approx. 3dB more signal power for same
  probability of bit error

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Theoretical Bit Error Rate (BER) For Various
Encoding Schemes
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Manchester Encoding

• has transition in middle of each bit period
• low to high represents binary one
• transition serves as clock and data
• high to low represents binary zero
• used by IEEE 802.3 (Ethernet) LAN standard

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Differential Manchester Encoding

• midbit transition is clocking only
• transition at start of bit period representing
  binary 0
• no transition at start of bit period representing
  binary 1
• used by IEEE 802.5 token ring LAN

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Advantages and disadvantages of Manchester
Encoding

• Disadvantages
• at least one transition per bit time and possibly
  two
• maximum modulation rate is twice NRZ
• requires more bandwidth
• Advantages
• synchronization on mid bit transition (self
  clocking codes)
• has no dc component
• has error detection capability (the absence of an
  expected transition can be used to detect errors)

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Modulation Rate versus Data Rate

• Data rate (expressed in bps)
• Data rate or bit rate R1/Tb1/1µs1Mbps
• Modulation Rate (expressed in baud) is the rate
  at which signal elements are generated
• Maximum modulation ratefor Manchester is
  D1/(0.5Tb)2/1µs2Mbaud

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Scrambling

• Use scrambling to replace sequences that would
  produce constant voltage
• These filling sequences must
• produce enough transitions to maintain
  synchronization
• be recognized by receiver replaced with
  original
• be same length as original
• Design goals
• have no dc component
• have no long sequences of zero level line signal
• have no reduction in data rate
• give error detection capability

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B8ZS and HDB3
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Bipolar with 8-Zero Substitution (B8ZS)

• To overcome the drawback of the AMI code that a
  long string of zeros may result in loss of
  synchronization, the encoding is amended with the
  following rules
• If 8 zeros occurs and the last voltage pulse was
  positive, then the 8 zeros are encoded as
  0000
• If zeros occurs and the last voltage pulse was
  negative, then the 8 zeros are encoded as
  0000

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High Density Bipolar-3 zeros (HDB3)

• The scheme replaces strings with 4 zeros by
  sequences containing one or two pulses
• In each case, the fourth zero is replaced with a
  code violation (V)
• successive violations are of alternate polarity

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Digital Data, Analog Signal

• Main use is public telephone system
• has freq range of 300Hz to 3400Hz
• use modem (modulator-demodulator)
• The digital data modulates the amplitude A,
  frequency fc , or phase ? of a carrier signal
• Modulation techniques
• Amplitude Shift Keying (ASK)
• Frequency Shift Keying (FSK)
• Phase Shift Keying (PSK)

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Modulation Techniques
Amplitude Shift Keying (ASK)
Binary Frequency Shift Keying (BFSK)
Binary Phase Shift Keying (BPSK)
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Amplitude Shift Keying (ASK)

• In ASK, the two binary values are represented by
  to different amplitudes of the carrier frequency
• The resulting modulated signal for one bit time
  is
• Susceptible to noise
• Inefficient modulation technique
• used for
• up to 1200bps on voice grade lines
• very high speeds over optical fiber

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Binary Frequency Shift Keying (BFSK)

• The most common form of FSK is Binary FSK (BFSK)
• Two binary values represented by two different
  frequencies ( f1 and f2 )
• less susceptible to noise than ASK
• used for
• up to 1200bps on voice grade lines
• high frequency radio (3 to 30MHz)
• even higher frequency on LANs using coaxial cable

0 0 1 1 0 1 0 0 0 1 0
f2 f2 f1 f1 f2 f1 f2 f2 f2 f1 f2
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Full-Duplex BFSK Transmission on a Voice-Grade
line

• Voice grade lines will pass voice frequencies in
  the range 300 to 3400Hz
• Full duplex means that signals are transmitted in
  both directions at the same time

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Multiple FSK (MFSK)

• More than two frequencies (M frequencies) are
  used
• More bandwidth efficient compared to BFSK
• More susceptible to noise compared to BFSK
• MFSK signal

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Multiple FSK (MFSK)

• MFSK signal
• Period of signal element
• Minimum frequency separation
• MFSK signal bandwidth

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Example

• With fc250KHz, fd25KHz, and M8 (L3 bits), we
  have the following frequency assignment for each
  of the 8 possible 3-bit data combinations
• This scheme can support a data rate of

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Example

• The following figure shows an example of MFSK
  with M4. An input bit stream of 20 bits is
  encoded 2bits at a time, with each of the
  possible 2-bit combinations transmitted as a
  different frequency.

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Phase Shift Keying (PSK)

• Phase of carrier signal is shifted to represent
  data
• Binary PSK (BPSK) two phases represent two
  binary digits

0 0 1 1 0 1 0 0 0 1 0
p p 0 0 p 0 p p p 0 p
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Differential PSK (DPSK)

• In DPSK, the phase shift is with reference to the
  previous bit transmitted rather than to some
  constant reference signal
• Binary 0signal burst with the same phase as the
  previous one
• Binary 1signal burst of opposite phase to the
  preceding one

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Four-level PSK Quadrature PSK (QPSK)

• More efficient use of bandwidth if each signal
  element represents more than one bit
• eg. shifts of ?/2 (90o)
• each signal element represents two bits
• split input data stream in two modulate onto
  the phase of the carrier
• can use 8 phase angles more than one amplitude
• 9600bps modem uses 12 phase angles, four of which
  have two amplitudes this gives a total of 16
  different signal elements

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QPSK and Offset QPSK (OQPSK) Modulators
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Example of QPSK and OQPSK Waveforms
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Performance of ASK, FSK, MFSK, PSK and MPSK

• Bandwidth Efficiency
• ASK/PSK
• MPSK
• MFSK

• Bit Error Rate (BER)
• bit error rate of PSK and QPSK are about 3dB
  superior to ASK and FSK (see Fig. 5.4)
• for MFSK MPSK have tradeoff between bandwidth
  efficiency and error performance

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Performance of MFSK and MPSK

• MFSK increasing M decreases BER and decreases
  bandwidth Efficiency
• MPSK Increasing M increases BER and increases
  bandwidth efficiency

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Quadrature Amplitude Modulation (QAM)

• QAM used on asymmetric digital subscriber line
  (ADSL) and some wireless standards
• combination of ASK and PSK
• logical extension of QPSK
• send two different signals simultaneously on same
  carrier frequency
• use two copies of carrier, one shifted by 90
• each carrier is ASK modulated

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QAM modulator
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QAM Variants

• Two level ASK (two different amplitude levels)
• each of two streams in one of two states
• four state system
• essentially QPSK
• Four level ASK (four different amplitude levels)
• combined stream in one of 16 states
• Have 64 and 256 state systems
• Improved data rate for given bandwidth
• but increased potential error rate

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