I am designing a GMSK demodulator, and trying to measure the SNR I can achieve, So I am constructing a GSMK signal centered at F0=26 MHz, using Matlab sampling=260 MHz, and passing this through an AWGN channel, with mode (Eb/No), input power=0.5 Watts(as I am sending cosine) and Eb/No=15 dB.

To make sure the actual SNR is 15 dB, I measure the signal power at the Rx (either I or Q branch after filtering), noise power(by subtracting clean signal power from the received signal at I or Q branch ) and subtract them in dB, but I get the actual SNR to be lower by 5 dB (11 dB) . Can somebody explain this to me ? What is the correct SNR, and what could I be doing wrong ?