When we use following Matlab function
signal=awgn(signal,5,'measured')
How much SNR do we have added to original signal? I mean the SNR is 5 dB? How can I add 20 dB noise to signal by using this command?
Like Dan has mentioned in the comments, the second argument is used to set the value of SNR.
However, this is under the assumption that your signal has a power of 0 dBW. In case the signal power is different, you need to increase (or decrease) the value of the snr argument appropriately.