The definition of $\rm SNR$ seems to be somewhat of a tower of babel in industry. What definitions of $\rm SNR$ are there (feel free to site application), and how exactly can it be measured for that applications?
My specific questions on $\rm SNR$ are:
How may we measure $\rm SNR$ for a communications system, if we have not yet been able to attain optimal bit sampling timing, and all we have to work with are all the signals in the receiver up to and including the envelope of the I and Q channels? See this post for context.
Once we attain optimal bit sampling, and softbits are attained, how best to measure $\rm SNR$ (or $E_bN_0$)? One way I use is: $$ 10\log_{10}\left[ \frac{\textrm{mean}\left\{\lvert s_n \rvert^2\right\}}{\textrm{var}\textrm\{\lvert s_n \rvert\textrm\}}\right], $$
however I understand this is not suitable for low $\rm SNR$ cases. What other ways exist?