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This is probably a very simple case but I am not sure where I am going wrong. In order to estimate the effect of path loss in Matlab, the function fspl(R,lambda) gives a pathloss value for a distance Rin $dB$.

My question is that, for using this function should one directly multiply the path loss value in $dB$ or convert it to linear scale and then multiply. If I use the $dB$ value, the signal power increases and if I use the linear scale value, there is a lot of difference between the transmitted and received power. Here's a snippet of my code:

SNR =2;    
Fs=12e6;
Fc = 2.4e9;
d = 5; lambda = physconst('LightSpeed')/Fc;    
data=randi([0 1],3000,1);
hMod = comm.OQPSKModulator('BitInput',true);
modulated_sgl = step(hMod, data);
pathloss_dB = fspl(d,lambda);
tx_sgl = modulated_sgl * pathloss_dB ;

If I try to plot the power spectral density of tx_sgl and modulated_sgl using the following:

hss = dsp.SpectrumAnalyzer('SampleRate', Fs); 
step(hss,[tx_sgl modulated_sgl]);
release(hss);

I observe however that the power of tx_sgl is higher than the modulated_sgl. Can someone guide me where I may be going wrong.

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The function gives you the path loss in dB. This number is positive, i.e. a path loss of $3$ dB corresponds to a gain of $-3$ dB. So for a given path loss $x$ (in dB), the signal is attenuated by a factor $10^{-x/20}$. Note that multiplication with a dB value hardly ever makes any sense.

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