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					EE445S Real-Time Digital Signal Processing Lab     Fall 2011



           Channel Impairments

              Prof. Brian L. Evans
 Dept. of Electrical and Computer Engineering
       The University of Texas at Austin


  Lecture 12                         http://courses.utexas.edu/
                        Outline
• Analog communication systems
  Transmitter
  Channel impairments
  Receiver

• Hybrid communication systems

• Analog and digital modulation methods

• Analog pulse amplitude modulation


                                          12 - 2
                                         Review



                 Communication Systems
       • Information sources
         Message signal m(t) is the information source to be sent
         Possible information sources include voice, music, images,
           video, and data, which are baseband signals
         Baseband signals have power concentrated near DC
       • Basic structure of an analog communication
         system is shown below

          Signal     Carrier           Transmission          Carrier      Signal
m(t)    Processing   Circuits            Medium              Circuits              ˆ
                                                                        Processing m(t )
                                s(t)                  r(t)
           TRANSMITTER                  CHANNEL                  RECEIVER
                                                                             12 - 3
                                         Review



                            Transmitter
       • Signal processing
         Conditions the message signal
         Lowpass filtering to make sure that the message signal
           occupies a specific bandwidth, e.g. in AM and FM radio,
           each station is assigned a slot in the frequency domain.
         In a digital communications system, we might add redundancy
           to the message bit stream m[n] to assist in error detection
           (and possibly correction) in the receiver
          Signal     Carrier           Transmission          Carrier      Signal
m(t)    Processing   Circuits            Medium              Circuits              ˆ
                                                                        Processing m(t )
                                s(t)                  r(t)
           TRANSMITTER                  CHANNEL                  RECEIVER
                                                                             12 - 4
                                         Review



                            Transmitter
       • Carrier circuits
          Convert baseband signal into frequency band appropriate for
            channel via upconversion
          Amplify signal for transmission
       • Analog continuous-time transmission over
         communication channel


          Signal     Carrier           Transmission          Carrier      Signal
m(t)    Processing   Circuits            Medium              Circuits              ˆ
                                                                        Processing m(t )
                                s(t)                  r(t)
           TRANSMITTER                  CHANNEL                  RECEIVER
                                                                             12 - 5
                                          Review



                                  Channel
       • Transmission medium
          Wireline (twisted pair, coaxial, fiber optics)
          Wireless (indoor/air, outdoor/air, underwater, space)
       • Propagating signals experience a gradual
         degradation over distance
       • Boosting improves signal and reduces noise, e.g.
         repeaters
          Signal      Carrier           Transmission          Carrier      Signal
m(t)    Processing    Circuits            Medium              Circuits              ˆ
                                                                         Processing m(t )
                                 s(t)                  r(t)
           TRANSMITTER                   CHANNEL                  RECEIVER
                                                                              12 - 6
    Wireline Channel Impairments
• Linear time-invariant effects
   Attenuation: dependent on channel frequency response
   Spreading: finite extent of each transmitted pulse increases, i.e.
     pulse widens (see next slide)
• Linear time-varying effects
   Phase jitter: sinusoid at same fixed frequency experiences
     different phase shifts when passing through channel
     (i.e. time-varying phase response)
• Nonlinear effects
   Additive noise: arises from many sources in transmitter,
     channel, and receiver (e.g. thermal noise)
   Harmonics: due to squaring & other nonlinear transformations
     (e.g. in power amplifiers)                             12 - 7
       Wireline Channel Impairments
• Linear time-invariant effect in time domain
     Spreading due to convolution of transmitted waveform and
       channel impulse response
      x0 (t )                                                                   y0 (t )


                Tb           input                         output                 Th      Th+Tb
                                      Communication                                                 t
                     t                  Channel
                              x(t)                          y(t)        -A Th
-A

      x1 (t )                                          h(t )                    y1 (t )
                             Model channel as
A                             LTI system with                   1       A Th
                             impulse response
                                                                                                    t
                Tb       t          h(t)                   Th                     Th      Th+Tb
                                                                    t

                                     Assume that Th < Tb                                   12 - 8
                                          Home Power Line Noise
                                                 Power Spectral Density Estimate
                                   -75

                                   -80
        Power/frequency (dB/Hz)




                                   -85

                                   -90

                                   -95

                                  -100

                                  -105

                                  -110

                                  -115

                                  -120

                                  -125
                                      0    10   20    30      40    50       60   70   80   90
                                                           Frequency (kHz)
Measurement taken on a wall power plug in an                                                     12 - 9
apartment in Austin, Texas, on March 20, 2011
                                          Home Power Line Noise
                                                    Power Spectral Density Estimate
                                   -75

                                   -80
        Power/frequency (dB/Hz)




                                   -85

                                   -90

                                   -95

                                  -100

                                  -105

                                  -110

                                  -115

                                  -120          Spectrally-Shaped
                                  -125          Background Noise
                                      0    10      20    30      40    50       60   70   80   90
                                                              Frequency (kHz)
Measurement taken on a wall power plug in an                                                        12 - 10
apartment in Austin, Texas, on March 20, 2011
                                          Home Power Line Noise
                                                    Power Spectral Density Estimate
                                   -75

                                   -80           Narrowband Noise
        Power/frequency (dB/Hz)




                                   -85

                                   -90

                                   -95

                                  -100

                                  -105

                                  -110

                                  -115

                                  -120          Spectrally-Shaped
                                  -125          Background Noise
                                      0    10      20    30      40    50       60   70   80   90
                                                              Frequency (kHz)
Measurement taken on a wall power plug in an                                                        12 - 11
apartment in Austin, Texas, on March 20, 2011
                                          Home Power Line Noise
                                                    Power Spectral Density Estimate
                                                                                             Periodic and
                                   -75                                                    Asynchronous Noise
                                   -80           Narrowband Noise
        Power/frequency (dB/Hz)




                                   -85

                                   -90

                                   -95

                                  -100

                                  -105

                                  -110

                                  -115

                                  -120          Spectrally-Shaped
                                  -125          Background Noise
                                      0    10      20    30      40    50       60   70   80   90
                                                              Frequency (kHz)
Measurement taken on a wall power plug in an                                                        12 - 12
apartment in Austin, Texas, on March 20, 2011
    Wireless Channel Impairments
• Same as wireline channel impairments plus others
• Fading: multiplicative noise
  Talking on a mobile phone and reception fades in and out
  Represented as time-varying gain that follows a particular
    probability distribution
• Simplified channel model for fading, LTI effects
  and additive noise

       a0         FIR          +


                             noise
                                                          12 - 13
                                          Review



           Receiver and Information Sinks
       • Receiver
         Carrier circuits undo effects of carrier circuits in transmitter,
           e.g. downconvert bandpass signal to a baseband signal
         Signal processing subsystem extracts and enhances the
           baseband signal
       • Information sinks
         Output devices, e.g. computer screens, speakers, TV screens

          Signal      Carrier           Transmission          Carrier      Signal
m(t)    Processing    Circuits            Medium              Circuits              ˆ
                                                                         Processing m(t )
                                 s(t)                  r(t)
           TRANSMITTER                   CHANNEL                  RECEIVER
                                                                             12 - 14
      Hybrid Communication Systems
• Mixed analog and digital signal processing in the
  transmitter and receiver
   Example: message signal is digital but broadcast over an
     analog channel (compressed speech in digital cell phones)
• Signal processing in the transmitter
                           Error
          A/D                                Digital           D/A
                         Correcting
 m(t)   Converter                           Signaling        Converter
                           Codes
• Signal processing in the receiver                          baseband signal
A/D      Equalizer        Detection    Decoder           Waveform        D/A
                                                         Generator
                     digital      digital         code
                                                                      12 - 15
                    sequence     sequence
Single-Carrier Modulation Methods
• Analog communication    • Digital communication
  Transmit and receive      Same but treat transmission
    analog waveforms          and reception as digitized
  Amplitude Modulation      Amplitude Shift Keying
    (AM)                      (ASK)
  Phase Modulation (PM)     Phase Shift Keying (PSK)
  Freq. Modulation (FM)     Freq. Shift Keying (FSK)
  Quadrature Amplitude      QAM
    Modulation (QAM)
  Pulse Amplitude           PAM
    Modulation (PAM)
                                                 12 - 16
Pulse Amplitude Modulation (PAM)
• Amplitude of periodic pulse train is varied with a
  sampled message signal m(t)
   Digital PAM: coded pulses of the sampled and quantized
     message signal are transmitted (lectures 13 and 14)
   Analog PAM: periodic pulse train with period Ts is the carrier
     (below)

              m(t)                           s(t) = p(t) m(t)


                      p(t)


                                               t
                         T     Ts T+Ts 2Ts                      12 - 17
                                Analog PAM
• Pulse amplitude varied                        • Transmitted signal
                                                       
  with amplitude of                                   s (t )     m(T
                                                                 n  
                                                                            s   n) h(t  Ts n)
  sampled message                                                         sample      hold

  Sample message every Ts                             h(t) is a rectangular pulse
  Hold sample for T seconds                             of duration T units
    (T < Ts)                                                    1     for 0  t  T
  Bandwidth  1/T                                              
                                                      h (t )  1 / 2 for t  0, t  T
                                                                0
                         s(t)                                          otherwise
                  m(0)                                    m(t)
   h(t)                             m(Ts)                                       As T  0,
   1                                                                            1
                                                                                  h(t )   (t )
              t                                            t                    T
          T                     T     Ts T+Ts   2Ts                                          12 - 18
                                            Analog PAM
• Transmitted signal                                               • Equalization of sample
                      
  s(t )   m(T n) h(t  T n)
                    n  
                                     s               s
                                                                     and hold distortion
                                                                    added in transmitter
            m(T              s   n)  (t  Ts n) * h(t ) 
           n                                                      H(f) causes amplitude
              
                                 
          m(Ts n)  (t  Ts n) * h(t )                             distortion and delay of T/2
         n                   
                                                                     Equalize amplitude
                               msampled(t)
                                                                       distortion by post-filtering
• Fourier transform                                                    with magnitude response
  S( f )                  M sam pled( f ) H ( f )                     1           1            f
                                                                                         
                   fs    M( f  f
                         k  
                                              s   k) H ( f )         H ( f ) T sinc( f T ) sin( f T )

                                                                     Negligible distortion       T
  H ( f )  T sinc( f T ) e j 2                        f T /2
                                                                                                     0.1
                                                                      (less than 0.5%) if        Ts
            T sinc( f T ) e j                        fT
                                                                                             12 - 19
                    Analog PAM
• Requires transmitted pulses to
   Not be significantly corrupted in amplitude
   Experience roughly uniform delay
• Useful in time-division multiplexing
   public switched telephone network T1 (E1) line
     time-division multiplexes 24 (32) voice channels
   Bit rate of 1.544 (2.048) Mbps for duty cycle < 10%
• Other analog pulse modulation methods
   Pulse-duration modulation (PDM),
     a.k.a. pulse width modulation (PWM)
   Pulse-position modulation (PPM): used
     in some optical pulse modulation systems.
                                                         12 - 20

				
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