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Asilomar Conference on Signals, Systems and Computers (ACSSC)

Impact of beat noise on an incoherent OCDMA system with temporal spreading

Bin Ni, J.S. Lehnert, Jin Zhang

Published Mar 31, 2005• 10 citations
Publisher

Abstract

An incoherent optical CDMA (OCDMA) system with temporal spreading that uses a light source with nonzero coherence time is studied in this paper. A model is established to simulate the incoherent combining of multiple light pulses at the receiver. System performances for different ratios of chip duration to coherence time are evaluated. Results show that the performance with a nonideal source is noticeably degraded from that of the ideal case due to beat noise.

DOI
10.1109/acssc.2004.1399097

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An incoherent optical CDMA (OCDMA) system with temporal spreading that uses a light source with nonzero coherence time is studied in this paper. A model is established to simulate the incoherent combining of multiple light pulses at the receiver. System performances for different ratios of chip duration to coherence time are evaluated. Results show that the performance with a nonideal source is noticeably degraded from that of the ideal case due to beat noise.
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.
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 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.
Confidentiality performance is analyzed in detail for a 2-D incoherent optical code-division multiple access (OCDMA) system, especially a multi-user two-code-keying wavelength- hopping time-spreading (WHTS) code system. The WHTS system's robustness against brute-force attacks is discussed. Other than brute-force attacks, several practical approaches are proposed to compromise the WHTS system. The effectiveness of these practical approaches is theoretically analyzed and an experimental demonstration is provided. Possible approaches to improve WHTS system's confidentiality are proposed and discussed.