Time-spreading Optical CDMA System Research Articles

Quasi-synchronous Coherent Time-spreading Optical CDMA System using LA Sequence

We propose a novel quasi-synchronous coherent time-spreading optical CDMA (TS-OCDMA) system, where the synchronization among users can be controlled within permissible time delay. Relationship between the system performance and code parameters is analyzed. In quasi-synchronous coherent TS-OCDMA system, beat noise (BN) and multiple-access interference (MAI) are depend- ent on the mean intensity of aperiodic cross-correlation within permissible time delay. LA sequence is employed in this network, and code performance is analyzed. Within permissible time delay, the mean intensity of aperiodic cross-correlation equals zero, and hence the BN and MAI can be eliminated in quasi-synchronous coherent TS- OCDMA system. When the time delay is out of zero correlation zone, crosstalk value of LA sequence is less than that of Gold sequence. Therefore, quasi-synchronous coherent TS-OCDMA using LA sequence is beneficial in a low-user-load OCDMA network.

Performance analysis of chip-asynchronous coherent time-spreading optical CDMA system

Multiple-access interference(MAI) and beat noise(BN) in chip-asynchronous coherent time-spreading OCDMA system are evaluated by the aperiodic cross-correlation function of up-sampled sequence. Relationship between the mean intensity of aperiodic cross-correlation and MAI and BN is deduced, and then the relationship between BER and mean intensity of aperiodic cross-correlation is discussed. For 127-long gold sequence, mean BER performance of chip-asynchronous coherent time-spreading OCDMA is derived and compared with that of the chip-synchronous system. It can be shown that the chip-synchronous assumption provides a performance upper limit. For realistic asynchronous coherent time-spreading OCDMA system, BER will reduce by about one order of magnitude.