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Am_dem.m
Package: 现代通信系统matlab版源代码.zip　[view]
Upload User: loeagle
Upload Date: 2013-03-02
Package Size: 1236k
Code Size: 3k
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Communication Document

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Matlab

Am_dem.m：Code Content
  1. % am-dem.m
  2. % Matlab demonstration script for envelope detection. The message signal
  3. % is +1 for 0 < t < t0/3, -2 for t0/3 < t < 2t0/3 and zero otherwise.
  4. echo on
  5. t0=.15;                               % signal duration
  6. ts=0.001;                             % sampling interval
  7. fc=250;                               % carrier frequency
  8. a=0.85;                               % Modulation index
  9. fs=1/ts;                              % sampling frequency
  10. t=[0:ts:t0];                          % time vector
  11. df=0.25;                              % required frequency resolution
  12. % message signal
  13. m=[ones(1,t0/(3*ts)),-2*ones(1,t0/(3*ts)),zeros(1,t0/(3*ts)+1)];
  14. c=cos(2*pi*fc.*t);                    % carrier signal
  15. m_n=m/max(abs(m));                    % normalized message signal
  16. [M,m,df1]=fftseq(m,ts,df);            % Fourier transform 
  17. f=[0:df1:df1*(length(m)-1)]-fs/2;     % frequency vector
  18. u=(1+a*m_n).*c;                       % modulated signal
  19. [U,u,df1]=fftseq(u,ts,df);            % Fourier transform 
  20. env=env_phas(u);                      % find the envelope 
  21. dem1=2*(env-1)/a;                     % remove dc and rescale
  22. signal_power=spower(u(1:length(t)));   % power in modulated signal
  23. noise_power=signal_power/100;         % noise power
  24. noise_std=sqrt(noise_power);          % noise standard deviation
  25. noise=noise_std*randn(1,length(u));   % generate noise
  26. r=u+noise;                            % add noise to the modulated signal
  27. [R,r,df1]=fftseq(r,ts,df);            % Fourier transform 
  28. env_r=env_phas(r);                    % envelope, when noise is present
  29. dem2=2*(env_r-1)/a;                   % demodulate in the presence of noise
  30. pause  % Press any key to see a plot of the message
  31. subplot(2,1,1)
  32. plot(t,m(1:length(t)))
  33. axis([0 0.15 -2.1 2.1])
  34. xlabel('Time')
  35. title('The message signal')
  36. pause  % Press any key to see a plot of the modulated signal
  37. subplot(2,1,2)
  38. plot(t,u(1:length(t)))
  39. axis([0 0.15 -2.1 2.1])
  40. xlabel('Time')
  41. title('The modulated signal')
  42. pause  % Press a key to see the envelope of the modulated signal
  43. clf
  44. subplot(2,1,1)
  45. plot(t,u(1:length(t)))
  46. axis([0 0.15 -2.1 2.1])
  47. xlabel('Time')
  48. title('The modulated signal')
  49. subplot(2,1,2)
  50. plot(t,env(1:length(t)))
  51. xlabel('Time')
  52. title('Envelope of the modulated signal')
  53. pause  % Press a key to compare the message and the demodulated signal
  54. clf
  55. subplot(2,1,1)
  56. plot(t,m(1:length(t)))
  57. axis([0 0.15 -2.1 2.1])
  58. xlabel('Time')
  59. title('The message signal')
  60. subplot(2,1,2)
  61. plot(t,dem1(1:length(t)))
  62. xlabel('Time')
  63. title('The demodulated signal')
  64. pause  % Press a key to compare in the presence of noise 
  65. clf
  66. subplot(2,1,1)
  67. plot(t,m(1:length(t)))
  68. axis([0 0.15 -2.1 2.1])
  69. xlabel('Time')
  70. title('The message signal')
  71. subplot(2,1,2)
  72. plot(t,dem2(1:length(t)))
  73. xlabel('Time')
  74. title('The demodulated signal in the presence of noise')