DSP Techniques for Software Radio by 78zq4k7

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									DSP Techniques for Software
Radio

    DSP Front End Processing

        Dr. Jamil Ahmad
The DSP Front End

   DSP Front End for Software Radio
   Analog-to-Digital Conversion
    Techniques
   Direct Digital Synthesis



                                       2
               The DSP Front End
                  DSP Front End Processing
                        Digital Channel Selection
                        Digital Multiplexing




                   ADC     DDC
RF Interface




                                   Channelizer
                                                              Frequency Band
                                                               Channelization

                   DAC                       DDS   DUC
                           Channelizer


                                                         Up Conversion and Multiplex
                           (Base Station)
                                                                                  3
          Data Conversion
                                               ADC

                             Discrete Time
                           Continuous Valued
              Sampling          Signals                  Quantization

Analog                                                                   Digital
Domain                                                                   Domain
                                                      Discrete-Time
                                                     Discrete Valued
Continuous-Time                                          Signals
Continuous Valued
     Signals                                                   0 1 0 0 1 0, 0 1 1 0 0 1, …

                                 Continuous Time
          Reconstruction         Discrete Valued
                                     Signals                    Voltage Mapping


                                               DAC
                                                                                       4
Sampling and Aliasing
Why Filter Before Sampling


                          10KHz




    -40KHz   -20KHz       20KHz   40KHz   60KHz   80KHz
                      0




                                                          5
Sampling and Aliasing
       Applying Anti-Aliasing Filter



               -10KHz 0   10KHz




            -10KHz   0    10KHz




  -40KHz   -20KHz    0       20KHz   40KHz   60KHz   80KHz

                                                             6
Sampling Bandpass Signals
   Direct Sampling of BP Signals


                                    f
           -Fc      0        Fc




   What should be the Sampling Rate?
   Images in Bandpass Sampling
                                        7
 Bandpass Sampling
      Nyquist Second theorem                            fc
       Minimum Sampling Rate is
       Twice the Bandwidth

      2        f        f 
Fs         1 L  B M   L                         fL
    M 1       B        B                                B fH
      2  fH 
             B             x   largest integer smaller than x
                              
    M 1  B 
                                   e.g.,  47.78  47
                                               
     2 fH
                          Nyquist rate achievable only at integer
     fH                  multiples of highest frequency and the
    B
                         Bandwidth of the BP Signal
                                                                      8
Bandpass Sampling




    B / fH



                    9
     Bandpass Sampling
     Consider the case where   f H  LB (L an Odd Integer)



                                                         L = 5 for this case




                                                             L is Odd here


Whenever fH = LB, We can choose Fs = 2B to perfectly “interweave” the
shifted spectral images
                                                                               10
Bandpass Sampling
   Advantages of BP Sampling
                       Bandpass
 x(t )                 Sampling
                         ADC
                                       A BP-Sampling ADC
                                       Works like a Mixer and
                                       A Baseband-Sampling ADC
x(t )                       Baseband
                     LPF    Sampling
                              ADC

    cos(2 f c t )
                                                             11
Hilbert Transformer (HT)
   What is Hilbert Transform?
       90o Phase Shifter
       All Pass Magnitude Characteristics


                     2 sin 2 ( n / 2)
                                         n0
           hd (n)           n
                     0                   n0
                    




                                                12
Hilbert Transformer

   1.5
                                        1
    1                  2 hd ( n )
                                             1/3
   0.5                                             1/5
                                                         1/7
    0

  -0.5     -1/7 -1/5
                         -1/3
    -1
                                -1
  -1.5
     -10        -5                   0             5           10




                                                                    13
   Hilbert Transform
      I/Q Conversion
                                                     N 1
                                                D
                                                      2


                       z -D

x(n)
                                          z (m)  xi (m)  jxq (m)
                        hd(n)

             Discrete Hilbert Transform



                                                                     14
 Digital Conversion
        Practical System

~ (t )
x                  x(t )                 x(n)                ˆ
                                                   Quantiza- x(n)
            AAF               Sampling                                  Coder
                                                     tion

Analog            Bandlimited            Sampled            Quantized           Bit Stream
Signal            Analog Signal          Signal             Signal



                           Sampling Clock




                                                                                     15
Digital Conversion
   Quantization
       Signal level divided into discrete steps
       Samples rounded to the nearest step value
       Introduces errors in the signal which is
        treated as „Quantization Noise‟ or
        „Quantization Error‟
       Quantization Error depends upon the
        quantization step size

                                                16
        Digital Conversion -
        Quantization
                                              ˆ
                                              x
                                              3D
                                              2D
                                               D

D  Step Size                                       D   x
                                                   2D
Quantization error = e(n)  x(n)  x(n)
                                   ˆ               3D
                                                   4D
                                   D2
Quantization Noise =    Nq   2 
                                   12
Signal-to-Quantization Noise = SQNR  6.02B

 B = Number of Bits

                                                         17
Digital Conversion
   Binary Coding
                           Two’s   Offset   Fraction
               ˆ
               x          Comple
                           ment
                                   Binary
                                    Code
                                             Value

                           Code

               3D          011      111       3/4
               2D          010      110       1/2
               D           001      101       1/4
                           000      100        0
                    D     111      011       -1/4
                    2D    110      010       -1/2
                    3D    101      001       -3/4
                    4D    100      000        -1



                                                       18
Digital Conversion -
Quantization




                       19
Data Conversion
   Dynamic Range
        SDR utilizes wide band ADCs
        Critical for SDR Design to accommodate all
         type of analog input signals
        Full-Scale Range Utilization

                                           % FSR 
        Dynamic Range  6.02 B  20 log10        
                                            100 

                                                      20
Data Conversion
   Quantifying Dynamic Performance
       Harmonic Distortion
       Total Harmonic Distortion (THD)
       Thermal Noise
       Signal-to-Noise and Distortion Ratio (SINAD)
       Effective Number of Bits (ENOB)
       Signal-to-Noise Ratio
       Spurious-Free Dynamic Range (SFDR)
       Intermodulation Distortion (Two Tone and Multi-
        Tone)

                                                          21
Data Conversion
   Total Harmonic Distortion (THD)
                          
                          Pi 
         THD  10 log10  i 1    dB
                         P0 
                              
                              
   Signal-to-Noise and Distortion Ratio
    (SINAD)
                                  
                            P0    
         SINAD  10 log10     
                                    dB
                          N P
                          
                          
                               i
                              i 1 

                                           22
Data Conversion
   Thermal Noise
                           k = 1.38 ×10-23 J/K
       P  kTe B
        T                 Te = Effective Noise Temperature
                          B = Signal Bandwidth



   Effective Number of Bits (ENOB)

       ENOB  (SINAD  1.763) / 6.02

                                                         23
Data Converters
   AD9220 12Bit 10MBPS ADC SINAD and ENOB
         at Different Input Signal Level




                                            24
    Data Conversion
       Spurious-Free Dynamic Range (SFDR)


                P0 
SFDR  10log10             dB
                max( Pi ) 




                                             25
Data Converters
   Improvement Techniques
       Dithering
            Out of Band
            Subtractive
       Automatic Gain Control (AGC)
       Response Time



                                       26
The DSP Front-End



 Direct Digital Synthesis (DDS)




                                  27
Direct Digital Synthesis (DDS)
   What is DDS?
       Digital Waveform Generator
       Flexibility in Control and Precision
       Fundamental Block in SDR
       No Manual Tuning




                                               28
 Direct Digital Synthesis
    DDS Approaches
        ROM Lookup Table


                   Accumulator
Frequency                        Lookup
                                  Table   DAC
Word


                   Delay




                                                29
      DDS Approaches
         ROM Lookup




To generate 1100Hz tone with
Sampling Rate of 8KHz and 0.5Hz
Resolution, Calculate
         i) Number of Points in LUT
         ii) Input Frequency Word Value


                                          30
   DDS Approaches
      ROM Lookup Approach
Design Solution:
              Required Frequency Resolution      Df  0.5
                                                            Df   
                                                 D  2       
                                                            Fs 8000
              In the Phase Wheel                       2
                                                 D 
                                                       N
                                                 2      2
                                              N             16000
                                                 D      
  Frequency Word Value                                  8000
                                                 f      1100
                                           2     2       0.86393
                                                Fs      8000             31
DDS Approaches
   Issues with ROM LUT Method
       ROM Size directly proportional to Fs
       ROM Size Inversely Proportional to
        Frequency Resolution
       Memory Problem
       Phase Noise



                                               32
DDS Approaches
   Taylor Series Approximation
     For small 

                           1 3 1 5 1 7
           sin( )              
                           3!    5!  7!
     Its true when
                      Fs      f0



                                            33
DDS Approaches
   Digital Oscillator
                                                                 y(n)
       Widely used in
        DTMF Tone                                a1        Z-1

        Generation                                         Z-1
       Issues with Higher                       -1


        Frequencies
                             y (2)   A sin  0
                                                      f0
                             y (1)  0,  0  2
                                                      Fs
                             y (n)  2 cos 0  y (n  1)  y (n  2)


                                                                        34

								
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