c14_jakes.m

無線通訊系統仿真原理與應用 詳細介紹通訊的仿真方法

% File: c14_Jakes.m
% Software given here is to accompany the textbook: W.H. Tranter, 
% K.S. Shanmugan, T.S. Rappaport, and K.S. Kosbar, Principles of 
% Communication Systems Simulation with Wireless Applications, 
% Prentice Hall PTR, 2004.
%
% This program builds up a two-tap TDL model and computes the output 
% for the two inpput signasl of interest.
%
% Generate tapweights. 
%
fd = 100; impw = jakes_filter(fd);
%
% Generate tap input processes and Run through doppler filter.
%
x1 = randn(1,256)+i*randn(1,256); y1 = filter(impw,1,x1);
x2 = randn(1,256)+i*randn(1,256); y2 = filter(impw,1,x2);
%
% Discard the first 128 points since the FIR filter transient.
% Scale them for power and Interpolate weight values.
% Interpolation factor=100 for the QPSK sampling rate of 160000/sec.
%
z1(1:128) = y1(129:256); z2(1:128) = y2(129:256);
z2 = sqrt(0.5)*z2; m = 100;
tw1 = linear_interp(z1,m); tw2 = linear_interp(z2,m);
%
% Generate QPSK signal and filter it.
%
nbits = 512; nsamples = 16; ntotal = 8192;
qpsk_sig = random_binary(nbits,nsamples)+i*random_binary(nbits,nsamples);
%
% Genrate output of tap1 (size the vectors first). 
%
input1 = qpsk_sig(1:8184); output1 = tw1(1:8184).*input1;
%
% Delay the input by eight samples (this is the delay specified 
% in term of number of samples at the sampling rate of 
% 16,000 samples/sec and genrate the output of tap 2.
%
input2 = qpsk_sig(9:8192); output2 = tw2(9:8192).*input2;
%
% Add the two outptus and genrate overall output.
%
qpsk_output = output1+output2;
%
% Generate the 1000 Hz  complex exponential and run it through the TDL 
% model. This could be done at the higher sampling rate of 16,0000
% samples per sec or at a lower rate. At the lower rate the tap spacing
% must be recomputed in number of samples at the lower rate. Also the 
% interpolation of the tap gain functions must now be at the lower rate. 
% In this example we will use the higher sampling rate.
%
ts = 1/160000; time = (ts:ts:8200*ts); 
cexp = exp(2*pi*i*1000*time);
input1 = cexp(1:8184); output3 = tw1(1:8184).*input1;
input2 = cexp(9:8192); output4 = tw2(9:8192).*input2;
%
% Add the two outputs and genrate overall output.
%
cexp_out = output3+output4;
[psdcexp,freq,ptotal,pmax] = linear_psd(cexp(1:8184),8184,ts);
[psdcexp_out,freq,ptotal,pmax] = linear_psd(cexp_out(1:8184),8184,ts);
%
% Plot results.
%
subplot(2,1,1)
plot(freq(4100:4180), psdcexp(4100:4180)); grid;
xlabel('Frequency (Hz)'); ylabel('PSD')
subplot(2,1,2)
plot(freq(4100:4180), psdcexp_out(4100:4180),'r'); grid;
xlabel('Frequency (Hz)'); ylabel('PSD')
figure; subplot(2,1,1)
plot(real(qpsk_sig(501:1000)),'r'); grid;
xlabel('Sample Index'); ylabel('Direct Input');
axis([0 500 -2 2])
subplot(2,1,2)
plot(real(qpsk_output(501:1000)));grid;
xlabel('Sample Index'); ylabel('Direct Output');
figure; 
plot(abs(output3(3000:6000))); grid
xlabel('Sample Index'); ylabel('Envelope Magnitude')
% End script file.