Calculating SNR & Es/No and placing noise onto a signal
I assume the OP wants the SNR to be 20 dB just prior to decision in the receiver, in which case the noise would be 20 dB below the signal power AFTER being passed through the second RRC filter. The total noise in the bandwidth of this filter would be 20 dB below the total noise of the signal. Since neither if flat over bandwidth, I would not recommend units of dBW/Hz, but work with total power (or as the square root; the standard deviation of signal and noise).