# Very high SNR BPSK modulation

I am using BPSK modulation to modulate a signal in continuous time (making rectangular waveform, multiplying by a carrier). The channel is band-limited and frequency-selective, and has the following magnitude response:

After that I passed the signal through AWGN channel to add noise and then multiplied by carrier again, and then performed detection. Here is the sequence of steps:

• convert binary signal to -1 and 1 and make continuous rectangular pulse
• multiply by cosine or sine carrier
• take the FFT of the modulated signal
• multiply by frequency response of channel
• take IFFT of the result
• multiply signal by carrier
• pass signal through bank of correlators
• detection

Frequency response of my bandpass signal is

Then I plotted the BER vs SNR. However what I noticed is that for a given BER my SNR value is very high. Here is what I got :

Can you please tell me why I am getting such high SNR? does the BER vs SNR make sense?

Thanks.

• There are too many things that could be happening here, it's impossible to give a a simple answer. A few things you might want to check: 1) It seems your sampling frequency is of the order of 20 GHz. Are you using that sampling frequency consistently in your simulation? 2) The channel is frequency selective and can introduce siginificant signal distortion. What is the signal's bandwidth? 3) Most importantly, why do you think the SNR required to achieve a given BER is too high? What did you expect, and why? – MBaz Jul 5 '15 at 17:35
• sampling Frequency 800000. Yes my sampling frequency is consistent. the channel bandwidth is 4000 Hz. I have a question can you please tell me if the steps I mentioned in the question for the modulation and demodulation is correct or not ? – user59419 Jul 5 '15 at 19:21
• The steps look correct. However, if the sampling frequency is 800 kHz, the frequency response you posted above is incorrect, since it goes up to 10 MHz. That would imply a sampling frequency of 20 MHz. (Not 20 GHz as I incorrectly pointed out in my first comment). – MBaz Jul 5 '15 at 19:44
• I posted the spectrum of my modulated signal as well. – user59419 Jul 5 '15 at 20:42
• I found the frequency response by bode plot then I sampled it with sampling frequency 800 kHz. its frequency range was from 0 to 10^7. – user59419 Jul 5 '15 at 20:58