A modified topology achieved in OFDM/SAC-OCDMA-based multi-diagonal code for enhancing spectral efficiency

In this paper, a modified topology-based orthogonal frequency division multiplexing–spectral amplitude coding in optical code division multiple access (OFDM–SAC-OCDMA) has been introduced. Such analysis aims to explain the effect of spectral efficiency with respect to multi-diagonal (MD) code. The new proposed technique introduces a fusion system between the two-code keying scheme and the forward error correction (FEC). The introduced analysis has targeted the effect of signal-to-noise ratio, bit error rate, and spectral efficiency with the presence of beat noise, phase incoherent intensity noise, and thermal noise. Numerical simulations have been utilized and tested in order to illustrate the validation of the (MD) code method to eliminate dominant noise. The calculation results show the following; the SNR and BER for MD under the presence of the previous discussed signal degradation is equal 161 and 1.2?×?10^?10, respectively, according to a predefined number of users 150 comparing with some existing codes such as: modified frequency hopping code, Hadamard (HD) code, modified quadratic congruence code. The proposed unipolar encoding with direct detection technique-based OFDM/SAC-OCDMA scheme achieves an enhancement in the performance of SE over the unipolar encoding with direct detection technique-based SAC-OCDMA technique by 3.455 dB. The proposed two keying scheme-based OFDM/SAC-OCDMA introduces an enhancement in the performance of the SE over the unipolar encoding with direct detection technique-based OFDM/SAC-OCDMA technique by 2.8859 dB. FEC has been applied in order to increase the spectral efficiency for different BERs, which the numerical results show that FEC provides 1.7457 dB. Finally, the total enhancement for SE was about 8.0866 dB.