# Rayleigh BER in Simulink

I'm using the Rayleigh Channel in Simulink but I can't calculate the BER. I always get values near to 0.5 which doesn't make sense (even using high EbNo values). [In this simulink model][1] I found a trick to calculate the BER but I also would like to use delays and attenuation vectors.

Any idea of how to calculate the BER? I have tried also with a Matlab script comparing bit to bit but I also get values near 0.5.

Edit1: I can calculate the BER but it is always around 0.5, regardless EbNo values. I'm using this simulink model.

Edit2: I'm using delay and attenuation vector so I have problems with the array dimensions:

• Welcome to DSP.SE! Why can't you calculate the BER? What is stopping you? If you "can't" calculate it, where did you get 0.5 from? Can you edit your question and show or explain how you are currently calculating the BER?
– Peter K.
Aug 31 '15 at 19:30
• The theoretical BER on a Rayleigh fading channel is indeed close to $\frac 12$ even at high (average) SNR. This is because the large number of errors made when the channel is in a deep fade (and so the effective SNR is very small) swamp the good performance when the channel is not faded and the effective SNR is high. In an AWGN channel, the SNR decays exponentially with increasing SNR; on a Rayleigh fading channel, only inversely with increasing average SNR. Sep 1 '15 at 3:07

After the AWGN block you need to divide with the channel coefficient value before you apply the BPSK demodulator.

If you click on the Rayleigh block it gives the option to enable a "Complex Gain Path Port". You can use this output to accomplish the division mentioned above.

Instead of division you can also multiply the output of the AWGN block with the complex-conjugate of the channel gain.

• I'm using delay vector and attenuation vector so I have problems with the matrix dimensiones (see Edit2 on the post). I don't really understand what to do. Sep 1 '15 at 14:26

When you pass your signal into a Rayleigh Fading Channel, it distorts your signal, causing Intersymbol Interference (ISI). You need to Equalize your signal at the receiver in order to perform the detection correctly. You can do it in various ways. I suggest you to search about zero-forcing and MSE equalization.

• Also see Dilip's comments on the question.
– Peter K.
Sep 1 '15 at 13:24