You can use this block to make a pulse random generator:

Set the sample time of the model to 6 nanosecond:

You can consider the below mentioned MATLAB function to be used in the pulse random generator:
function y = fcn(u)
persistent counter
if isempty(counter)
counter = 0;
end
id = rand(1);
if id > 0.5 && u == 0
counter = 6;
end
if counter > 0
y = 1;
counter = counter - 1;
else
y = 0;
end
end
Hope this helps!
