OFDM之信道建模仿真

通过MATLAB实现OFDM的信息传送，对不同的调制方式进行了测试。

clear all
close all
clc


%% simulation parameters
%modulation method: BPSK, QPSK, SPSK, 16QAM, 32QAM, 64QAM
mod_method = 'QPSK';


% IFFT/FFT size
n_fft = 64;


%size of cyclic prefix extension
n_cpe = 16;


%target SNR (dB)
snr = 20;


%number of channel taps (1 == no channel)
n_taps = 8;


%channel estimation method: none, LS
ch_est_method = 'LS';


%option to save plot to file
save_file = 0;


%calculate modulation order from modulation method
mod_methods = {'BPSK', 'QPSK', '8PSK', '16QAM', '32QAM', '64QAM'};
mod_order = find (ismember (mod_methods, mod_method));


%% input data do binary stream
im = imread('slika.bmp');
im_bin = dec2bin(im(:))';
im_bin = im_bin(:);


%% binary stream to symbols
%parse binary stream into mod_order bit symbols
%pads input signal to appropriate length
sym_rem = mod( mod_order-mod( length( im_bin), mod_order), mod_order);
padding = repmat ( '0', sym_rem, 1);
im_bin_padded = [im_bin; padding];
cons_data = reshape( im_bin_padded, mod_order, length(im_bin_padded)/mod_order)';
cons_sym_id = bin2dec(cons_data);


%% symbol modulation
%BPSK
if mod_order == 1
mod_ind = 2^(mod_order-1);
n = 0:pi/mod_ind:2*pi-pi/mod_ind;
in_phase = cos(n);
quadrature = sin(n);
symbol_book = (in_phase + quadrature*1i)';
end


%phase shift keying about unit circle
if mod_order == 2 || mod_order == 3
mod_ind = 2^(mod_order-1);
n = 0: pi/mod_ind: 2*pi-pi/mod_ind;
in_phase = cos(n+pi/4);
quadrature = sin(n+pi/4);
symbol_book = (in_phase + quadrature*1i)';
end


%16QAM, 64QAM modulation
if mod_order == 4 || mod_order == 64
mod_ind = sqrt(2^mod_order);
in_phase = repmat(linspace(-1, 1, mod_ind), mod_ind, 1);
quadrature = repmat(linspace(-1, 1, mod_ind)', 1, mod_ind);
symbol_book = in_phase(:) + quadrature(:)*1i;
end


%32QAM modulation
%generates 6x6 constellation and removes corners
if mod_order == 5
mod_ind = 6;
in_phase = repmat(linspace(-1, 1, mod_ind), mod_ind, 1);
quadrature = repmat(linspace(-1, 1, mod_ind)', 1, mod_ind);
symbol_book = in_phase(:) + quadrature(:)*1i;
symbol_book = symbol_book([2:5 7:30 32:35]);
end


%modulate data according to symbol_book
X = symbol_book(cons_sym_id+1);


%% use IFFT to move to time domain
%pad input signal to appropriate length
fft_rem = mod(n_fft-mod(length(X), n_fft), n_fft);
X_padded = [X; zeros(fft_rem,1)];
X_blocks = reshape(X_padded, n_fft, length(X_padded)/n_fft);
x = ifft(X_blocks);


%add cyclic prefix extension and shift from parallel to serial
x_cpe = [x(end-n_cpe+1:end,:);x];
x_s = x_cpe(:);


%add AWGN
%calculate data power
data_pwr = mean(abs(x_s.^2));


%add noise to channel
noise_pwr = data_pwr/10^(snr/10);
noise = normrnd(0,sqrt(noise_pwr/2),size(x_s)) + normrnd(0, sqrt(noise_pwr/2), size(x_s))*1i;
x_s_noise = x_s + noise;


%measure SNR
snr_meas = 10*log10(mean(abs(x_s.^2))/mean(abs(noise.^2)));


%% apply fading channel
g = exp(-(0:n_taps-1));
g = g/norm(g);
x_s_noise_fading = conv(x_s_noise, g, 'same');


%% use fft to move to frequency domain
%remove cyclic prefix extension and shift from serial to parallel
x_p = reshape(x_s_noise_fading, n_fft+n_cpe, length(x_s_noise_fading)/(n_fft+n_cpe));
x_p_cpr = x_p(n_cpe + 1:end,:);


%move to frequency domain
X_hat_blocks = fft(x_p_cpr);


%% estimate channel
if n_taps > 1
switch(ch_est_method)
case 'none'
case 'LS'
G = X_hat_blocks(:,1)./X_blocks(:,1);
X_hat_blocks = X_hat_blocks./repmat(G,1,size(X_hat_blocks,2));
end
end


%% symbol demodulation
%remove fft padding
X_hat = X_hat_blocks(:);
X_hat = X_hat(1:end-fft_rem);


%recover data from modulated symbols
rec_syms = knnsearch([real(symbol_book) imag(symbol_book)], [real(X_hat) imag(X_hat)]) - 1;


%parse to binary stream & remove symbol padding
rec_syms_cons = dec2bin(rec_syms);
rec_im_bin = reshape(rec_syms_cons', numel(rec_syms_cons),1);
rec_im_bin = rec_im_bin(1:end-sym_rem);
ber = sum(abs(rec_im_bin - im_bin))/length(im_bin);


%% recover image
rec_im = reshape(rec_im_bin,8, numel(rec_im_bin)/8);
rec_im = uint8(bin2dec(rec_im'));
rec_im = reshape(rec_im,size(im));


%% generate plots
%transmit constellation
subplot(2,2,1);
plot(X, 'x', 'linewidth', 2, 'markersize', 10);
xlim([-2 2]);
ylim([-2 2]);
xlabel('In Phase');
ylabel('Quadrature');
if n_taps > 1
title(sprintf('Poslani konstelacijski dijagram\n\\rm%s Modulacija\nMultipath Channel Taps: %d', mod_method, n_taps));
else
title(sprintf('Poslani konstelacijski dijagram\n\\rm%s Modulacija', mod_method));
end
grid on;


%recovered constellation
subplot(2,2,2);
plot(X_hat(1:500:end),'x','markersize',3);
xlim([-2 2]);
ylim([-2 2]);
xlabel('In Phase');
ylabel('Quadrature');
if n_taps > 1
title(sprintf('Primljeni konstelacijski dijagram\n\\rmMeasured SNR: %.2f db\nChannel Estimation: %s', snr_meas, ch_est_method));
else
title(sprintf('bfPrimljeni konstelacijski dijagram\n\\rmMeasured SNR: %.2f dB', snr_meas));
end
grid on;


%original image
subplot(2,2,3);
imshow(im);
title('\bfPoslana slika');


%recovered image
subplot(2,2,4);
imshow(rec_im);
title(sprintf('\\bfOporavljena slika\n\\rmBER: %.2g', ber));


%position figure
%set(gcf,'Position',[680 287 698 691]);


%save figure
if save_file
print(sprintf('Plots/%s_%.0ffft_%.0fcpe_%0fdB_%.0ftaps_%s',mod_method, n_fft, n_cpe, snr, n_taps, ch_est_method), '-dmeta');
end