# FSK symbol error probability

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#### johnkaz77

##### Junior Member level 1
Hi!I'm trying to plot in matlab the symbol error probability (fsk modulation through AWGN channel) vs snr (0-30 db) using repetition codes (n,1) for n=1,3,5,7. I must be doing something completely wrong because instead of improving the error probability as I increase n,it only gets worse!Has anyone got a code to suggest because i'm new in matlab and I've come to a dead end here...Thanks.

#### kalyanasv

##### Full Member level 4
for the FSK you should plot on semilogy axis typically for measuring BER performance.
Code:
 Eb_No=1:10;
pb=0.5*exp(-0.5*Eb_N0);
semilogy(Eb_N0,pb);

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#### johnkaz77

##### Junior Member level 1
Thanks (again) for your reply!I'm trying to calculate the symbol error probability using repetition codes,not only BER.I've attached the code i've written if you could take a look and see what's wrong...It's not very well written so i have written a readme.txt to explain.View attachment AWGN.rar

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#### kalyanasv

##### Full Member level 4
Firstly, just try to keep code by pasting it on the board use "CODE and /CODE".

I presume your talking about the binary case only. Your confusion in the code seems to be that you have incorporated the Symbol error rate by by the factorial notation. The equation is for M-ary symbol error rate.

In the binary case it is simple and pb=1/2exp(-0.5*snr/n); nothing else.

If your using quadrature FSK or something then you need to add all the factorial stuff.e.g. If it quad-FSK then you sum the factorial from 2 to 4 and and calculate symbol error probability.

You should get straight lines which improve.

#### zorro

##### Advanced Member level 4
Hi Johnkaz77,

Don't expect improvement from repetition codes.
For a (n,1) code, you split the symbol (source bit) in n parts (channel bits) and more than half of them must be well detected in order to recover the right source bit.
Splitting the symbol in n parts reduces its energy by a factor n. Because of this the channel bit error rate increases dramatically. This impairment is more deletereous than the advantage gained by the error tolerance.
Error correcting codes more elaborate than repetition code are really effective.
Regards

Z

johnkaz77

### johnkaz77

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