Chapter 5 : Digital Communication Systems Chapter contents - PowerPoint PPT Presentation

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Chapter 5 : Digital Communication Systems Chapter contents


Chapter 5 : Digital Communication Systems Chapter contents 5.1 Overview of Digital Communication Systems Transmission schemes, communication link, Adv vs. Disadv – PowerPoint PPT presentation

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Title: Chapter 5 : Digital Communication Systems Chapter contents

Chapter 5 Digital Communication SystemsChapter
  • 5.1 Overview of Digital Communication Systems
  • Transmission schemes, communication link, Adv vs.
  • 5.2 Digital Transmission Pulse Modulation
  • Pulse modulation method PWM, PAM, PPM, PCM
  • 5.3 Pulse Code Modulation
  • PCM operation, sampling, quantization
  • 5.4 Information Capacity, Bits, Bit Rate, Baud,
    M-ary encoding
  • 5.5 Digital Modulation
  • 5.6 Applications of Digital Communication Systems

5.1 Overview
  • Digital communications is the transfer of
    information (voice, data etc) in digital form.
  • Basic diagram of digital/data communications

5.1 Overview
  • If the information is in the analog form, it is
    converted to a digital form for transmission. At
    the receiver, it is re-converted to its analog
  • In some case, data needs to be changed to analog
    form to suit the transmission line (ex
    internet/point-to-point data communication
    through the public switching telephone network)
    the use of modem
  • Modem (from modulator-demodulator) is a device
    that modulates an analog carrier signal to encode
    digital information, and also demodulates such a
    carrier signal to decode the transmitted
  • Function of modem at transmitter converts
    digital data to analog signal that are compatible
    to the transmission line characteristics.

5.1 Overview
  • Transmission schemes for analog and digital

5.1.1 Communication links in digital transmission
  • Basic protocol of transmission simplex,
    half-duplex, full duplex
  • Classification of communication link
  • Synchronous Channel the transmitted and
    received data clocks are locked together. This
    requires that the data contains clocking
    information (self-clocking data).
  • Asynchronous Channel the clocks on the
    transmitter and the receiver are not locked
    together. The data do not contain clocking
    information and typically contains start and stop
    bits to lock the systems together temporarily.

5.1.2 Digital vs Analog Communication Systems
  • Advantages
  • Noise immunity
  • Digital signals are less susceptible than analog
    signals to interference caused by noise
  • Simple determination is made whether the pulse is
    above or below the prescribed reference level
  • Signal processing capability
  • Digital signals are better suited than analog
    signals for processing and combining for
    multiplexing purpose.
  • Much simpler to store digital signals compare to
    analog signals
  • Transmission rate of digital signals can be
    easily changed to suit different environments and
    to interface with different types of equipment.
  • Can also be sample instead of continuously
  • A regenerative repeater along the transmission
    path prevent accumulation of noise along the
    path. It can detect a distorted digital signal
    and transmit a new clean signal

5.1.2 Digital vs Analog Communication Systems
  • Advantages
  • Simpler to measure and evaluate than analog
  • Easier to compare the error performance of one
    digital system to another digital system.
  • Transmission error can be detected and corrected
    more easily and more accurately (error bit
    check). This gives very low error rate and high
  • Digital hardware implementation is flexible and
    permits the use of microprocessors and digital
  • Ability to carry a combination of traffics, e.g.
    telephone signals, data, coded video and
    teletext, if the medium has enough capacity.

5.1.2 Digital vs Analog Communication Systems
  • Disadvantages
  • Bandwidth
  • Transmission of digitally encoded analog signals
    requires significantly more bandwidth than simply
    transmitting the original analog signal.
  • Circuit complexity
  • Analog signals must be converted to digital
    pulses prior to transmission and converted back
    to their original analog form at the receiver
    additional encoding/decoding circuitry.
  • Requires precise time synchronization between the
    clocks in the transmitter and receiver.

5.2 Digital Transmission Pulse Modulation
  • Mostly used modulation technique in digital
  • Consists of several processes
  • Sampling analog information signals
  • Converting those samples into discrete pulse
  • Transporting the pulses from a source to a
    destination over a physical transmission medium
  • Predominant method of pulse modulation pulse
    width modulation (PWM), pulse position modulation
    (PPM), pulse amplitude modulation (PAM), pulse
    code modulation (PCM)
  • Pulse Width Modulation (PWM)
  • The width (active portion of the duty cycle) of a
    constant amplitude pulse is varied proportional
    to the amplitude to the amplitude of the analog
    signal at the time the signal is sampled.
  • Maximum analog signal amplitude produces the
    widest pulse, and the minimum analog signal
    amplitude produces the narrowest pulse.
  • All pulses have the same amplitude.

5.2 Digital Transmission Pulse Modulation
  • Pulse Position Modulation (PPM)
  • The position of a constant-width pulse within a
    prescribed time slot is varied according to the
    amplitude of the sample of the analog signal.
  • The higher the amplitude of the sample, the
    farther to the right the pulse is positioned
    within the prescribed time slot.
  • The highest amplitude sample produces a pulse to
    the far right, and the lowest amplitude sample
    produces a pulse to the far left.
  • Pulse Amplitude Modulation (PAM)
  • the amplitude of a constant-width
    constant-position pulse is varied according to
    the amplitude of the sample of the analog signal.
  • The amplitude of a pulse coincides with the
    amplitude of the analog signal
  • PAM wave resemble the original analog signal more
    than the waveforms for PWM or PPM.

5.2 Digital Transmission Pulse Modulation
  • Pulse Code Modulation (PCM)
  • Analog signal is sampled and then converted to a
    serial n-bit binary code for transmission.
  • Each code has the same number of bits and
    requires the same length of time for transmission.

5.2 Digital Transmission Pulse Modulation
Figure Comparing between Pulse modulations
(a) analog signal (b) sample pulse (c) PWM (d)
PPM (e) PAM (f) PCM
5.3 Pulse Code Modulation (PCM)
  • Preferred method of communication within the
    public switched telephone network (PSTN).
  • with PCM it is easy to combine digitized voice
    and digital data into a single, high-speed
    digital signal and propagate it over either
    metallic or optical fiber cables.
  • Refer to figure of simplified block diagram of
    PCM system.
  • At the transmitter
  • The bandpass filter limits the frequency of the
    analog input signal to the standard voice-band
    frequency range of 300 Hz 3000 Hz.
  • The sample-and-hold circuit periodically samples
    the analog input signal and converts those
    samples to a multilevel PAM signal.
  • The analog-to-digital converter (ADC) converts
    the PAM samples to parallel PCM codes, which are
    converted to serial binary data in the
    parallel-to-serial converter. The output to the
    transmission line is a serial digital pulses.
  • The transmission line repeaters are placed at
    prescribed distances to regenerate the digital

5.3 Pulse Code Modulation (PCM)
  • At the receiver
  • The serial-to parallel converter converts serial
    pulses received from the transmission line to
    parallel PCM codes.
  • The digital-to-analog converter (DAC) converts
    the parallel PCM codes to multilevel PAM signals.
  • The hold circuit is basically a low pass filter
    that converts the PAM signals back to its
    original analog form
  • An integrated circuit that performs the PCM
    encoding and decoding is called a codec

5.3 Pulse Code Modulation (PCM)
  • Block diagram of a single channel, simplex PCM
    transmission channel

5.3.1 PCM Sampling
  • The function of the sampling circuit
  • to periodically sampled the continually changing
    analog input and convert those samples to a
    series of constant-amplitude pulse that easily be
    converted to binary PCM code
  • 2 basic techniques for the sampling function
  • 1) Natural sampling
  • Tops of the sample pulses retain their natural
    shape during the sample interval.
  • Difficult for an ADC to convert the sample to a
    PCM code due to un-constant voltage.
  • 2) Flat-top sampling
  • Most common method, used in the sample-and-hold
    circuit periodically sample the continually
    changing analog input voltage and converts those
    samples to a series of constant-amplitude PAM
    voltage levels.

5.3.1 PCM Sampling

Natural sampling
Flat-top sampling
5.3.2 Sampling Rate
  • Sampling is a process of taking samples of
    information signal at a rate based on the Nyquist
    Sampling Theorem.
  • Nyquist Sampling Theorem the original
    information signal can be reconstructed at the
    receiver with minimal distortion if the sampling
    rate in the pulse modulation signal is equal or
    greater than twice the maximum information signal
  • where fs minimum Nyquist sampling
  • fm(max) maximum information signal

5.3.2 Sampling Rate
  • If fs is less than 2 times fm(max) an impairment
    called as alias or fold-over distortion occurs.

5.3.3 Quantization
  • Quantization process of assigning the analog
    signal samples to a pre-determined discrete
  • The number of quantization levels, L depends on
    the number of bits per sample, n where
  • where L number of quantization level
  • n number of bits in binary to represent the
    value of the samples
  • The quantization levels are separated by a value
    of ?V that can be defined as
  • ?V is the resolution or step size of the
    quantization level.

5.3.3 Quantization
  • Ex

5.3.3 Quantization
  • Ex (continue)

5.3.3 Quantization
  • Quantization error/Quantization noise error
    that is produced during the quantization process
    due to the difference between the original signal
    and quantized signal magnitudes.
  • Since a sample value is approximated by the
    midpoint of the sub-internal of height ?V, in
    which the sample value falls, the maximum
    quantization error is ?V/2.
  • Thus, the quantization error lies in the range (-
    ?V/2, ?V/2).

5.3.4 Dynamic Range
  • the number of PCM bits transmitted per sample
    determined by determined by several factors
    maximum allowable input amplitude, resolution and
    dynamic range.
  • Dynamic range (DR) the ratio of the largest
    possible magnitude to the smallest possible
    magnitude (other than 0 V) that can be decoded by
    the DAC converter in the receiver.
  • mathematically expressed
  • where DR dynamic range (unitless ratio)
  • Vmin the quantum value (resolution)
  • Vmax the maximum voltage magnitude that can
    be discerned by the
  • DACs in the receiver

5.3.4 Dynamic Range
  • Dynamic range is generally expressed as a dB
  • where DR dynamic range (unitless ratio)
  • Vmin the quantum value (resolution)
  • Vmax the maximum voltage magnitude that can
    be discerned by the
  • DACs in the receiver
  • the number of bits used for a PCM code depends on
    the dynamic range. The relationship between
    dynamic range and the number of bits in a PCM
    code is
  • and for a minimum number of bits 2n 1 DR

5.3.4 Dynamic Range
  • Ex For a PCM system with the following
    parameters, determine (a) minimum sample rate (b)
    minimum number of bits used in the PCM code (c)
    resolution (d) quantization error
  • Maximum analog input frequency 4 kHz
  • Maximum decode voltage at the receiver
  • Minimum dynamic range 46 dB

5.3.4 Coding Efficiency
  • Coding efficiency ratio of the minimum number
    of bits required to achieve a certain dynamic
    range to the actual number of PCM bits used.
  • number of bits should include the sign bit !

5.3.5 Signal-to-Quantization Noise Ratio
  • Generally, the quantization error or distortion
    caused by digitizing an analog sample expressed
    as an average signal power-to-average noise power
  • For a linear PCM codes (all quantization
    intervals have equal magnitudes), the signal
    power-to-quantizing noise power ratio is
    determined by
  • where R resistance (ohms)
  • v rms signal voltage (volts)
  • q quantization intervals (volts)
  • v2/R average signal power (watts)
  • (q2/12)/R average quantization noise power
  • if R is assume to be equal

5.3.6 Companding
  • Companding is the process of compressing and
    expanding to improve the dynamic range of a
    communication system.
  • a companding process is done by firstly
    compressing signal samples and then using a
    uniform quantization. The input-output
    characteristics of the compressor are shown
  • the compressor maps input signal
  • increments ?x into larger increments
  • ?y for a large input signals.
  • 2 compression laws recognized by
  • µLaw North America Japan
  • A-Law Europe others

5.3.7 Line speed / Transmission bit rate
  • Line speed is the transmission bit rate at which
    serial PCM bits are clocked out of the PCM
    encoder onto the transmission line.
  • Line speed/transmission bit rate can be expressed
  • Line speed samples/seconds x bits/sample
  • line speed transmission rate (bps)
  • samples/second sampling rate fs
  • bits/sample no of bits in the compressed PCM

5.4 Parameters in Digital Modulation5.4.1
Information Capacity
  • Information capacity a measure of how much
    information can be propagated through a
    communication systems and is a function of
    bandwidth and transmission time.
  • represents the number of independent symbols that
    can be carried through a system in a given unit
    of time
  • the most basic digital symbol used to represent
    information is the binary digit, or bit.
  • Bit rate the number of bits transmission during
    one second and is expressed in bits per second
  • Bit rate is used to express the information
    capacity of a system.
  • mathematically expressed, information capacity I
  • refer to slides of chapter 1 !

5.4.2 M-ary encoding
  • in an M-ary encoding, M represents a digit that
    corresponds to the number of conditions, levels,
    or combination possible for a given number of
    binary variables.
  • the number of bits necessary to produce a given
    number of conditions is expressed mathematically
  • where N number of bits necessary
  • M number of conditions, levels, or
    combination possible with N bits
  • from above, the number of conditions possible
    with N bits can be expressed as
  • Ex with 1 bit ? 21 2 conditions
  • 2 bits ? 22 4 conditions
  • 3 bits ? 23 8 conditions

5.4.3 Baud and Minimum Bandwidth
  • Bit rate refers to the rate of change of
    digital information, which is usually binary.
  • Baud refers to the rate of change of a signal
    on a transmission medium after encoding and
    modulation have occurred.
  • Baud can be expressed as
  • where Baud symbol rate (baud per second)
  • ts time of one signaling element (seconds)
  • signaling element symbol
  • for a given bandwidth B, the highest theoretical
    bit rate is 2B. Using the multilevel signaling,
    the Nyquist formulation for channel capacity is

5.4.3 Baud and Minimum Bandwidth
  • where fb channel capacity (bps)
  • B minimum Nyquist bandwidth (Hertz)
  • M number of discrete signal or voltage
  • above formula can be rearranged to solve for the
    minimum bandwidth necessary to pass M-ary
    digitally modulated carrier as follow
  • since N log2M above formula can be expressed as
  • where N is the number of bits encoded into each
    signaling element (symbol).

5.5 Digital Modulation
  • Given an information signal which is digital and
    a carrier signal represented as follow
  • A digitally modulated signal is produced as
  • If the amplitude (V) of the carrier is varied
    proportional to the information signal, ASK
    (Amplitude Shift Keying) is produced.
  • If the frequency (f) of the carrier is varied
    proportional to the information signal, FSK
    (Frequency Shift Keying) is produced.
  • If the phase (?) of the carrier is varied
    proportional to the information signal, PSK
    (Phase Shift Keying) is produced.
  • If both amplitude and phase are varied
    proportional to the information signal, QAM
    (Quadrature Amplitude Modulation) is produced.

5.5.1 Amplitude Shift Keying
  • digital information signal directly modulates the
    amplitude of the analog carrier.
  • mathematically, the modulated carrier signal is
    expressed as follow
  • (5.5-1)
  • where vask(t) amplitude-shift keying wave
  • vm(t) digital information (modulating)
    signal (volts)
  • A/2 unmodulated carrier amplitude (volts)
  • ?c analog carrier radian frequency
  • in the above (5.5-1), modulating signal vm(t) is
    a normalized binary waveform, where 1V logic 1
    and -1V logic 0.

5.5.1 Amplitude Shift Keying
  • for a logic 1 input, vm(t) 1V, and (5.5-1)
    reduces to
  • and for logic 0 input, vm(t) -1V, and (5.5-1)
    reduces to
  • so the modulated wave vask(t), is either
    Acos(?ct) or 0, means the carrier is either on
    or off. ASK is sometimes referred as on-off
    keying (OOK).

5.5.1 Amplitude Shift Keying
5.5.2 Frequency Shift Keying
  • general expression for FSK
  • (5.5-2)
  • where vfsk(t) binary FSK waveform
  • Vc peak analog carrier amplitude
  • fc analog carrier center frequency (Hz)
  • vm(t) binary input (modulating signal)
  • ?f peak change (shift) in the analog
    carrier frequency
  • from (5.5-2), the peak shift in the carrier
    frequency (?f) is proportional to the amplitude
    of the binary input signal vm(t).
  • the direction of the shift is determined by the
    polarity of signal ( 1 or 0 ).
  • the modulating signal vm(t) is a normalized
    binary waveform where a logic 1 1V and a logic
    0 -1V.

5.5.2 Frequency Shift Keying
  • for logic 1 input, vm(t) 1, equation (5.5-2)
  • for logic 0 input, vm(t) -1, equation (5.5-2)
  • the carrier center frequency fc is shifted
    (deviated) up and down in the frequency domain by
    the binary input signal as shown below.

5.5.2 Frequency Shift Keying

5.5.2 Frequency Shift Keying
  • mark (fm) logic 1 frequency
  • space (fs) logic 0 frequency

5.5.3 Phase Shift Keying
  • modulation technique that alters the phase of the
  • in a binary phase-shift keying (BPSK), where N
    (number of bits) 1, M (number of output phases)
    2, one phase represents a logic 1 and another
    phase represents a logic 0.
  • as the input digital signal changes state (i.e.
    from 1 to 0 or 0 to 1), the phase of the output
    carrier shifts between two angles that are
    separated by 180º.

5.5.3 Phase Shift Keying
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