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Journal of Optical Communications

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

Jianhua Ji, Fangping Gong, Qing Wu

Published Jan 1, 2009• 2 citations
Publisher

Abstract

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.

DOI
10.1515/joc.2009.30.3.169

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Journal of Optical Communications•Jan 1, 2004•0 citations
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.
The performance of coherent time-spreading (TS) and asynchronous TS phase coding optical code division multiple access (OCDMA) systems is investigated and analyzed. Particular consideration is given however to both beat noise (BN) and multiple-access interference (MAI) effects, and the impact of decision threshold on the bit-error-rate (BER) performance is evaluated. Improvement of the BER performance can be obtained by use of optimum threshold detection, and the improvement is more apparent in the asynchronous TS OCDMA scheme for a low number of simultaneous users with long code length.
The performance of WDM-based coherent time-spreading OCDMA system is studied. Upper bound and lower bound of normalized throughput are obtained with 8 wavelength channels and 127 length and 511 length Gold codes. It is shown that when all simultaneous users are equally allocated to different wavelength channels, WDM+ TS OCDMA has much better performance. However, if there is no central control to allocate wavelength channels equally, WDM+ TS OCDMA system has the in-between performance of normalized throughput.
In phase encoding optical CDMA (OCDMA) the spreading is achieved by encoding the phase of signal spectrum. In this paper we first derive a mathematical model for the output of phase encoding OCDMA systems. Based on this model we introduce a metric to design spreading sequences for asynchronous transmission. Then we connect the phase encoding sequence design problem to OFDM PMEPR (peak to mean envelope power ratio) problem. Using this connection we conclude that designing sequences with good properties for samples of timing delay guarantees that the same sequence to be good for all timing delays. Finally using generalized bent function we manage to construct a family of sequences which are good for asynchronous phase encoding OCDMA systems and using these sequences we introduce an M-ary modulation scheme for phase encoding OCDMA
Optical code division multiple access (OCDMA) systems are generally interference limited. To alleviate the adverse effect of optical multiple access interference (OMAI), use of forward error correcting (FEC) codes and optical double hard-limiters (DHLs) are explored. The system analysis is carried out under chip synchronous case among different users with PIN+OA as the receiver. The analysis incorporates the effect of amplifier noise, thermal noise and OMAI. It is observed that optical CDMA systems with FEC and double hard-limiters give rise to satisfactory performance even when the number of simultaneous users is large. It has been observed that with the usage of DHLs there is a significant improvement in the performance. When FEC is also used along with DHLs, there is slight enhancement in the performance.