Orthogonal frequency division multiplexing (OFDM) is a suitable solution thanks to its many advantages known in wireless communications. On the other hand, optical communications is also used as a backbone to transmit and receive large data rates with economical and good performance. Recently, fiber optical communication and OFDM method have been combined to obtain both advantages in a communication link called Coherent Optical OFDM (CO-OFDM). In this study, Bit error rate (BER) versus distance variations are investigated for a constant signal to noise ratio in CO-OFDM systems. Results also show the performance of the CO-OFDM system at different data rates and distances for one RF carrier and one optical carrier. So far, the Telecommunication Standardization Sector standards have suggested 81 channels between 192.1 and 196.1 THz in C band. Extending the number of channels using 111 more channels between 185.9 and 191.4 THz in L band where optical amplifiers and laser sources are available, the total number of channels reaches up to 192. In this research, CO-OFDM technique is modeled and simulated designing a Monte Carlo simulation. Dense wavelength division multiplexing (DWDM) is the key factor to obtain 3 Tb/s (192*16 Gb/s) utilizing only one optical cable by covering whole C and L bands. To the best of our knowledge, this work shows the first BER versus Distance variations in a CO-OFDM communication link for 3 Tb/s.