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  • pg 1

     Communication systems                                                          The components of a communications system:
                                                                                         Input transducer: The device that converts a physical signal
                                                                                          from source to an electrical, mechanical or electromagnetic
                                                                                          signal more suitable for communicating
     Digital                                                                             Transmitter: The device that sends the transduced signal
                                                                                         Transmission channel: The physical medium on which the
                                                                                          signal is carried

     Analog                                                                              Receiver: The device that recovers the transmitted signal
                                                                                          from the channel

                                                                                         Output transducer: The device that converts the received
                                                                                          signal back into a useful quantity
      •   The block diagram on the top shows the blocks common to all
          communication systems
               P.R.K.Reddy                                                                     P.R.K.Reddy

Major issues in communication system                                           •Modulation
    The purpose of a communication system is to transfer information from a
    source to a destination.                                                   •   Modulation is the process of encoding information
In practice, problems arise in baseband transmissions,                             from a message source in a manner suitable for
the major cases being:                                                             transmission.

•   Noise in the system –                                                      •   i.e. it involves translating a base band signal to a band
    external noise and circuit noise reduces the signal-to-noise (S/N) ratio
                                                                                   pass signal at frequencies that are very high when
    at the receiver (Rx) input and hence reduces the quality of the output.
                                                                                   compared to the base band frequency.
•    System is not able to fully utilise the available bandwidth,
     for example telephone quality speech has a bandwidth ≃ 3kHz, a                            Baseband             S(t)                                   M(t)
     co-axial cable has a bandwidth of 100's of Mhz.                                           signal Modulator                Comm     S(t)     De
                                                                                               m(t)                           Channel          modulator
•    Radio systems operating at baseband frequencies are very difficult.
•    Not easy to network.                                                                                 Intermediate Freq

               P.R.K.Reddy                                                                     P.R.K.Reddy


•   In analog modulation an information bearing analog waveform is
    impressed upon a carrier                                                            •   Demodulation is the process of extracting the base band message
•   In digital modulation a discrete time bit/symbol sequence representing                  from the carrier so that it maybe processed and interpreted by
    information is converted to continuous time waveform and impressed
    upon a carrier                                                                          the intended receiver

    The Messages
    The message or modulating signal may be either:
    analogue – denoted by m(t)
    digital – denoted by d(t) – i.e. sequences of 1's and 0's

    The message signal could also be a multilevel signal, rather than binary; this is
    not considered further at this stage.

    The Carrier
    The carrier could be a 'sine wave' or a 'pulse train'.

            P.R.K.Reddy                                                                              P.R.K.Reddy

      Types of Modulation:
                                                                                        Amplitude Modulation
    CONTINUOUS WAVE                               PULSE                                      • A carrier signal is modulated only in amplitude
    (SINE)                                        (RECTANGLE)
                                                                                             • The modulating signal is the envelope of the
     ANALOG                 DIGITAL              ANALOG                DIGITAL
                                                                                             • The required bandwidth is 2B, where B is the
     AMPLITUDE                  ASK                  PAM                   PCM
        (AM)                                                                                   bandwidth of the modulating signal
                                                                                             • Since on both sides of the carrier freq. fc, the
                                FSK                  PWM                  DPCM
        (FM)                                                                                   spectrum is identical, we can discard one half, thus
                                                                                               requiring a smaller bandwidth for transmission.
        PHASE                    PSK                                         DM

            P.R.K.Reddy                                                                              P.R.K.Reddy


  Amplitude Modulation                                      Amplitude modulation

                     Carrier wave

                     Baseband signal

                     Modulated wave
                                       Amplitude varying-
                                       frequency constant
       P.R.K.Reddy                                                    P.R.K.Reddy

                                                            Frequency Modulation

                                                             •   The modulating signal changes the freq. fc of the carrier signal
                                                             •   The bandwidth for FM is high

                                                             •   It is approx. 10x the signal frequency
       The total bandwidth required for AM
                can be determined
         from the bandwidth of the audio
                  signal: BAM = 2B.

       P.R.K.Reddy                                                    P.R.K.Reddy


 Frequency Modulation                                                   Frequency modulation

  Carrier wave

                   Small amplitude:   Large amplitude:
 Baseband signal                      high frequency
                   low frequency

   Modulated wave
                                            Frequency varying-
                                            amplitude constant

       P.R.K.Reddy                                                              P.R.K.Reddy

                                                                 AM vs. FM
                                                                 •   AM requires a simple circuit, and is very easy to generate.

                                                                 •   It is simple to tune, and is used in almost all short wave broadcasting.

Note                                                             •   The area of coverage of AM is greater than FM (longer wavelengths (lower
                                                                     frequencies) are utilized)

The total bandwidth required for FM can                          •   However, it is quite inefficient, and is susceptible to static and other forms of
                                                                     electrical noise.
  be determined from the bandwidth
                                                                     The main advantage of FM is its audio quality and immunity to noise. Most forms
  of the audio signal: BFM = 2(1 + β)B.                          •

                                                                     of static and electrical noise are naturally AM, and an FM receiver will not
          Where  is usually 4.                                      respond to AM signals.

                                                                 •   The audio quality of a FM signal increases as the frequency deviation increases
                                                                     (deviation from the center frequency), which is why FM broadcast stations use
                                                                     such large deviation.

                                                                 •   The main disadvantage of FM is the larger bandwidth it requires
       P.R.K.Reddy                                                              P.R.K.Reddy


  •Frequency Vs Amplitude Modulation                        Phase Modulation (PM)
  • In FM ,the signals have all their information in the    •   The modulating signal only changes the phase of the carrier signal.
    phase or frequency of the carrier. This provides a
    non-linear and very rapid improvement in reception      •   The phase change manifests itself as a frequency change but the
    quality once a certain minimum received signal called       instantaneous frequency change is proportional to the derivative of
    FM threshold is achieved.
                                                                the amplitude.

  LINEAR                                                    •   The bandwidth is higher than for AM.
  • In AM ,there is a linear relationship between the
    quality of the received signal and the power of the
    received signal.

    P.R.K.Reddy                                                        P.R.K.Reddy

Phase modulation

                                                                  The total bandwidth required for PM can be
                                                                       determined from the bandwidth
                                                                and maximum amplitude of the modulating signal:
                                                                                BPM = 2(1 + β)B.
                                                                            Where  = 2 most often.

    P.R.K.Reddy                                                        P.R.K.Reddy


                                         Relationship between data rate and signal rate

                                           •   The data rate defines the number of bits sent per
            Digital signals                    sec - bps. It is often referred to the bit rate.

                                           •   The signal rate is the number of signal elements
                                               sent in a second and is measured in bauds.

                                           •   It is also referred to as the modulation rate.

                                           •   Goal is to increase the data rate whilst reducing the
                                               baud rate.

     P.R.K.Reddy                                  P.R.K.Reddy

Signal element versus data element
                                     Data rate and Baud rate

                                     •The baud or signal rate can be expressed as:
                                                      S = c x N x 1/r bauds
                                                      where N is data rate
                                            c is the case factor (worst, best & avg.)
                                     r is the ratio between data element & signal element

     P.R.K.Reddy                                  P.R.K.Reddy


                                                                 Amplitude Shift Keying (ASK)
A signal is carrying data in which one data element is
                                                                   •   ASK is implemented by changing the amplitude of a
encoded as one signal element ( r = 1). If the bit rate is 100
kbps, what is the average value of the baud rate if c is               carrier signal to reflect amplitude levels in the
between 0 and 1?                                                       digital signal.
Solution:                                                          •   For example: a digital “1” could not affect the signal,
                                                                       whereas a digital “0” would, by making it zero.
We assume that the average value of c is 1/2 . The baud
rate is then                                                       •   The line encoding will determine the values of the
                                                                       analog waveform to reflect the digital data being

       P.R.K.Reddy                                                        P.R.K.Reddy

Implementation of binary ASK                                      Binary amplitude shift keying

       P.R.K.Reddy                                                        P.R.K.Reddy


                                                        Example .2
         Bandwidth of ASK                           We have an available bandwidth of 100 kHz which spans
                                                    from 200 to 300 kHz. What are the carrier frequency
                                                    and the bit rate if we modulated our data by using ASK
                                                    with d = 1?
• The bandwidth B of ASK is proportional to
  the signal rate S.
                   B = (1+d)S
• “d” is due to modulation and filtering, lies      The middle of the bandwidth is located at 250 kHz.
  between 0 and 1.                                  i.e. our carrier frequency can be at fc = 250 kHz.
                                                    We can use the formula for bandwidth to find the bit rate
                                                    (with d = 1 and r = 1).

       P.R.K.Reddy                                         P.R.K.Reddy

Frequency Shift Keying                                   Binary frequency shift keying

 •   The digital data stream changes the
     frequency of the carrier signal, fc.

 •   For example,

     a “1” could be represented by f1=fc +f, and
     a “0” could be represented by f2=fc-f.

       P.R.K.Reddy                                         P.R.K.Reddy


                                                         Example :
      Bandwidth of FSK
                                                      We have an available bandwidth of 100 kHz which spans
                                                      from 200 to 300 kHz. What should be the carrier
    • If   the difference between the two             frequency and the bit rate if we modulated our data by
                                                      using FSK with d = 1?
     frequencies (f1 and f2) is 2f,
    • then   the required BW B will be:
                                                      The midpoint of the band is at 250 kHz.
                   B = (1+d)xS +2f                   We choose 2Δf to be 50 kHz; this means

           P.R.K.Reddy                                       P.R.K.Reddy

           Phase Shift Keying                                Implementation of BASK

• We vary the phase shift of the carrier signal
  to represent digital data.
• The bandwidth requirement, B is:

                 B = (1+d)xS
• PSK is much more robust than ASK as it is not
  that vulnerable to noise, which changes amplitude
  of the signal.

           P.R.K.Reddy                                       P.R.K.Reddy


     Binary phase shift keying                              Pulse Amplitude Modulation (PAM)

                                                     •   PAM is a general signaling technique whereby
                                                         pulse amplitude is used to convey the message

                                                     •   In digital PAM, where the pulse amplitudes are
                                                         constrained to chosen from a specific alphabet
                                                         at the transmitter.

    P.R.K.Reddy                                             P.R.K.Reddy

                                                         Pulse Width Modulation (PWM)
Pulse Amplitude Modulation (PAM)
                                                     In PWM the sample values of the analog waveform are used to
                                                     determine the width of the pulse signal.

                         •   According to Nyquist    Either instantaneous or natural sampling can be used.
                             theorem, the sampling
                             rate must be at least
                             two times the
                             frequency to ensure
                             the accurate
                             reproduction of the
                             original signal.

    P.R.K.Reddy                                             P.R.K.Reddy


 Pulse Phase Modulation (PPM)                                         Pulse Code Modulation (PCM)
In PPM the sample values of the analog waveform are used to   •     PCM consists of three steps to digitize an
determine the phase of the pulse signal.
                                                                    analog signal:
Either instantaneous or natural sampling can be used.              1. Sampling
                                                                   2. Quantization
                                                                   3. Binary encoding

                                                                   Before we sample, we have to filter the signal
                                                                    to limit the maximum frequency of the signal
                                                                    as it affects the sampling rate.
                                                                   Filtering should ensure that we do not distort
                                                                    the signal, ie remove high frequency
                                                                    components that affect the signal shape.

      P.R.K.Reddy                                                     P.R.K.Reddy

 Components of PCM encoder                                         Sampling
                                                              • Analog signal is sampled every TS secs.
                                                              • Ts is referred to as the sampling interval.
                                                              • fs = 1/Ts is called the sampling rate or sampling

                                                              •   The process is referred to as pulse amplitude
                                                                  modulation PAM and the outcome is a signal with
                                                                  analog (non integer) values

      P.R.K.Reddy                                                     P.R.K.Reddy


    Three different sampling methods for PCM

                                                         According to the Nyquist theorem, the
                                                                  sampling rate must be
                                                         at least 2 times the highest frequency
                                                                 contained in the signal.

                                                        Telephone companies digitize voice by assuming a
                                                        maximum frequency of 4000 Hz. The sampling rate
                                                        therefore is 8000 samples per second.

         P.R.K.Reddy                                            P.R.K.Reddy

     Quantization                                        Quantization Zones
• Sampling results in a series of pulses of varying       •   Assume we have a voltage signal with amplitutes
  amplitude values ranging between two limits: a min          Vmin=-20V and Vmax=+20V.
  and a max.
                                                          •   We want to use L=8 quantization levels.
• The amplitude values are infinite between the two
  limits.                                                 •   Zone width = (20 - -20)/8 = 5
• We need to map the infinite amplitude values onto a
                                                              The 8 zones are:
  finite set of known values.                             •
                                                          •   -20 to -15, -15 to -10, -10 to -5, -5 to 0, 0 to +5, +5 to +10, +10
• This is achieved by dividing the distance between           to +15, +15 to +20
  min and max into L zones, each of height 
                                                          •   The midpoints are: -17.5, -12.5, -7.5, -2.5, 2.5, 7.5, 12.5, 17.5
                     = (max - min)/L

         P.R.K.Reddy                                            P.R.K.Reddy


                                                                          Differential Pulse-Code Modulation (DPCM)
    Quantization and encoding of a sampled signal            Usually PCM has the sampling rate higher than the Nyquist rate .The
                                                             encode signal contains redundant information. DPCM can efficiently
                                                             remove this redundancy.

                                                                   Figure DPCM system. (a) Transmitter. (b) Receiver.
         P.R.K.Reddy                                                  P.R.K.Reddy

       Delta Modulation
                                                             The process of delta modulation
• This scheme sends only the difference between
  pulses, if the pulse at time tn+1 is higher in amplitude
  value than the pulse at time tn, then a single bit, say
  a “1”, is used to indicate the positive value.
• If the pulse is lower in value, resulting in a negative
  value, a “0” is used.
• This scheme works well for small changes in signal
  values between samples.
• If changes in amplitude are large, this will result in
  large errors.

         P.R.K.Reddy                                                  P.R.K.Reddy


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