function [lsignal, lr, lw, lalpha] = verify_channel(pmultipath, pnoise, pfading)

    global model;
    global options;
    
    setup_model;

    lsignal = sign(rand(1,model.n_t) - 0.5);

    if (pnoise == 1)
	subplot(1, 1, 1); clg; hold off; axis; leg='';
	model.noise = 1; model.fading = 0; model.multipath = 'none';

	[lr, lalpha, lw] = channel(lsignal, 0, -3);
	subplot(2, 2, 1); clg; hold on; 
	plot(t(1:100), lr(1,1:100), 'b'); leg = [leg; 'r'];
	plot(t(1:100), lw(1,1:100), 'g'); leg = [leg; 'w'];
	plot(t(1:100), lsignal(1,1:100), 'rL'); leg = [leg; 's'];
	title('Senyal (r) i soroll (w) SNR=-3dB');
	axis([t(1), t(100), -3*median(lw(1,:)), 3*median(lw(1,:))]);

	[lr, lalpha, lw] = channel(lsignal, 0, 0);
	subplot(2, 2, 2); clg; hold on; 
	plot(t(1:100), lr(1,1:100), 'b'); leg = [leg; 'r'];
	plot(t(1:100), lw(1,1:100), 'g'); leg = [leg; 'w'];
	plot(t(1:100), lsignal(1,1:100), 'rL'); leg = [leg; 's'];
	title('Senyal (r) i soroll (w) SNR=0dB');
	axis([t(1), t(100), -3*median(lw(1,:)), 3*median(lw(1,:))]);

	[lr, lalpha, lw] = channel(lsignal, 0, 3);
	subplot(2, 2, 3); clg; hold on; 
	plot(t(1:100), lr(1,1:100), 'b'); leg = [leg; 'r'];
	plot(t(1:100), lw(1,1:100), 'g'); leg = [leg; 'w'];
	plot(t(1:100), lsignal(1,1:100), 'rL'); leg = [leg; 's'];
	title('Senyal (r) i soroll (w) SNR=3dB');
	axis([t(1), t(100), -3*median(lr(1,:)), 3*median(lr(1,:))]);

	[lr, lalpha, lw] = channel(lsignal, 0, 300);
	subplot(2, 2, 4); clg; hold on; 
	plot(t(1:100), lr(1,1:100), 'b'); leg = [leg; 'r'];
	plot(t(1:100), lw(1,1:100), 'g'); leg = [leg; 'w'];
	plot(t(1:100), lsignal(1,1:100), 'rL'); leg = [leg; 's'];
	title('Senyal (r) i soroll (w) SNR=6dB');
	axis([t(1), t(100), -3*median(lr(1,:)), 3*median(lr(1,:))]);
    end

    if (pfading == 1)
	subplot(1, 1, 1); clg; hold off; axis; leg='';
	model.noise = 0; model.fading = 1; model.multipath = 'none';

	[lr, lalpha, lw] = channel(lsignal, 200, 300);
	subplot(2, 2, 1); clg; hold on; 
	plot(t(1:100), lr(1,1:100), 'b'); leg = [leg; 'r'];
	plot(t(1:100), lalpha(1,1:100), 'g'); leg = [leg; 'a'];
	plot(t(1:100), lsignal(1,1:100), 'rL'); leg = [leg; 's'];
	title('Senyal (r) i fading (a) f_d=200Hz');
	axis([t(1), t(100), 1.15*min(lr(1,:)), 1.15*max(lr(1,:))]);

	[lr, lalpha, lw] = channel(lsignal, 700, 300);
	subplot(2, 2, 2); clg; hold on; 
	plot(t(1:100), lr(1,1:100), 'b'); leg = [leg; 'r'];
	plot(t(1:100), lalpha(1,1:100), 'g'); leg = [leg; 'a'];
	plot(t(1:100), lsignal(1,1:100), 'rL'); leg = [leg; 's'];
	title('Senyal (r) i fading (a) f_d=700Hz');
	axis([t(1), t(100), 1.15*min(lr(1,:)), 1.15*max(lr(1,:))]);

	[lr, lalpha, lw] = channel(lsignal, 1200, 300);
	subplot(2, 2, 3); clg; hold on; 
	plot(t(1:100), lr(1,1:100), 'b'); leg = [leg; 'r'];
	plot(t(1:100), lalpha(1,1:100), 'g'); leg = [leg; 'a'];
	plot(t(1:100), lsignal(1,1:100), 'rL'); leg = [leg; 's'];
	title('Senyal (r) i fading (a) f_d=1200Hz');
	axis([t(1), t(100), 1.15*min(lr(1,:)), 1.15*max(lr(1,:))]);

	[lr, lalpha, lw] = channel(lsignal, 1700, 300);
	subplot(2, 2, 4); clg; hold on; 
	plot(t(1:100), lr(1,1:100), 'b'); leg = [leg; 'r'];
	plot(t(1:100), lalpha(1,1:100), 'g'); leg = [leg; 'a'];
	plot(t(1:100), lsignal(1,1:100), 'rL'); leg = [leg; 's'];
	title('Senyal (r) i fading (a) f_d=1700Hz');
	axis([t(1), t(100), 1.15*min(lr(1,:)), 1.15*max(lr(1,:))]);
    end

end