Hello esakkimuthu
Generally to obtain the maximum link distance,system for maximum length for which your BER would between the range of 10^-9 to 10^-12 and Q-factor 10 is your maximum achievable distance.You can calculate the Q-factor from BER and BER fro Q factor using There are many ways to enhance the maximum achievable distance by using efficient modulation techniques,accurate amplification etc.
http://www.onsemi.com/pub_link/Collateral/AND9075-D.PDF
https://www.utdallas.edu/~dlm/3350%20comm%20sys/T-REC-G.Sup39-200602-I!!PDF-E.pdf