FM (Frequency Modulation) subcarrier multiplexing using multitone modulation for optical coherent communication system have been firstly introduced to maximize the bandwidth utilization, where the optimized number of bits allocated in each subcarrier is defined. Each subcarrier transmitting different number of data bits is modulated by the M-QAM. When we implement FDM to divide the given bandwidth within FM channel, the spectrum of the subcarrier signals in the given FM bandwidth is the quadratic noise.
We perform the numerical analysis the multicarrier modulation for only 4 subcarriers to prove applicability to FM subcarrier modulation, so the bit loading algorithm is not used. Firstly, we calculate the SNRs to obtain the BER of 10-9 for the cases of 4, 16, 64, 128-QAM at the given 4 subcarrier frequencies, 0.1, 0.15, 0,2, and 0.25 MHz. Secondly, we make the optimum choice the group composed of 20.7 dB for fsc=0.1 MHz, 18.4 dB for fsc=0.15 MHz, 20.3 dB for fsc=0.20 MHz, and 19.1 dB for fsc=0.25 MHz, where the SNR difference is lass than 2.3 dB. Hence the optimum modulators of FM subcarrier modulation are QPSK for fsc=0.1 MHz, 16-QAM for fsc=0.15 MHz, and 64- QAM for both fsc=0.20 MHz and fsc=0.25 MHz.