Add pathloss to signal
To simulate 100dB pathloss from the transmitter to the receiver, you can do

tx = .....; % generate the TX signal
PL_db = 100; PL_lin = 10^(PL_db/10);
rx = tx / sqrt(PL_lin);  % the amplitude is scaled with the square root of the power scaling.

% add some noise at the receiver, according to the SNR