hello everyone 

Please i need your help in a project on which I work.

This project consists to apply the pulse compression to a sinusoidal signal sin (2 * pi * f * t) with f = 25KHz, using a bi-phase modulation (barker code).

But what I got is that the SNR after the autocorrelation between signal transmitted and signal received for the modulated signal is lower than that of an unmodulated signal. So please is that normal, even That one of the advantages of pulse compression is to increase the SNR. And to have good compression a modulated signal must be used.

and thank you for your help .

(all the results I got using simulation in matlab)

No, that's not normal.   Be careful about how you are measuring the signal and noise power in the receiver, as that is a common area for mistakes.  Since you are relying on signal processing gain, where you measure signal power and noise power in the process will matter.

You don't say much about what processing you are doing in the receiver or what information you're really trying to extract, so there could be a problem elsewhere as well.