Group velocity walk-off effect on frequency hopping optical code division multiple access (FH-OCDMA) system is analyzed. We develop a systematic method which employs transfer function considering encoder, fiber channel and decoder to analysis the FH-OCDMA system. The difference of group velocities affects system performance more seriously than dispersion effect. The results show that single mode fiber is not proper for the FH-OCDMA system even the transmission length is no more than 1 kilometer while the dispersion-shifted fiber is more suitable for FH-OCDMA system.
Performance of asynchronous phase-encoded OCDMA system is evaluated on the condition that receiver noise and the impact of fiber channel are neglected. Phase-encoded optical signal (pseudo random optical signal with low intensity) is analyzed in the view of stationary random process. The pseudo random optical signal with low intensity is seen as a sample function of a certain stationary random process which is ergodic in strict sense. The analysis results reveal that the variance of the corresponding random process is only inversely proportional to the code length while the root-mean-square width of the phase-encoded optical signal is proportional to the width of initial optical pulse and the code length F. The numerical results demonstrate that the better system performance can be achieved in case of larger code length and shorter initial optical pulse.
By designating different frequencies to different mark positions of the modified prime code, a frequency-time spreading code is proposed. The side lobe of auto-correlation and cross-correlation of the proposed code have relatively lower values. The scheme greatly improves the total number of the available codes in asynchronous optical code division multiple access (OCDMA) system. Numerical results show that the asynchronous OCDMA system using the proposed code has excellent performance in the presence of double hard limiters. We also introduce an encoding/decoding device which is based on the frequency-time spreading modified prime code.
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