Comparison of modulation formats based on generalized mutual information

16QAM is found to achieve the lowest SNR penalty against the Shannon limit at the GMI of 2~3 bits/channel use compared to the analyzed 2- and 4-dim modulation formats. The FEC OSNR limit of 16QAM is measured to require ~1 dB less than 8QAM at the same GMI of 2.4 bits/channel use.