Simulating multipath channel I'm trying to simulate a multipath channel to see how it would affect my signal. So considering my channel is LTI, I can model it as an FIR filter. I've been reading Telecommunication Breakdown and looking at some examples and there's one thing I don't quite understand in page 183 (Chapter 9: Stuff happens, Section 9.4: Other Impairments: "More What Ifs")

if cdi &lt; 0.5, % channel ISI
    mc=[1 0 0 ] ; % distortion−free channel
elseif cdi&lt;1.5 , % mild multipath channel
    mc=[1 zeros (1 ,M) 0.28 zeros (1, 2.3*M) 0.11 ] ;
el se % harsh multipath channel
    mc=[1 zeros (1, M) 0.28 zeros (1, 1.8*M) 0.44 ] ;
end


So cdi is a parameter to choose between distortion-free, mild or harsh multipath channel and mc is the resulting FIR filter. What I don't understand is:


What's the point on inserting zeros in between?
How does that make it mild or harsh?


Would you mind explaining that to me? Thanks
You put zeros in between to show delay between the channel taps. I don't know what $M$ is in your code but suppose $M=10$ and say your sample rate is 10 MHz. Then you can interpret mc = [1 zeros(1, M) 0.28 zeros(1, 2.3*M) 0.11];  as you get the first multipath component with zero delay (gain = 1), then 10 zeros later (which is equal to 1 microsecond) you get another multipath component (gain = 0.28), and finally after another 2.8 microseconds you receive the last multipath component (gain = 0.11). 
It looks like the "mild" vs "harsh" is just language that the author uses to let you know that mild happens to rarely cause errors where the harsh causes many errors.