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Optics Express

All-optical differential detection for suppressing multiple-access interference in coherent time-addressed optical CDMA systems

Sun‐Jong Kim, Tae Young Kim, Chul Soo Park, Chang‐Soo Park, Young Yun Chun

Published Jan 1, 2004• 7 citations
Publisher

Abstract

A novel scheme for suppressing the multiple-access interference (MAI) in coherent time-addressed optical CDMA systems is proposed. This is based on a differential detection using the dual-control NOLM. The basic principle for MAI suppression is described. For experimental demonstration, two encoded channels are constructed and decoded. These decoded signals are sent to the dual-control NOLM and a high autocorrelation peak with suppressed MAI at the output of NOLM is observed. Signal-to-interference ratio is improved by 7 dB.

DOI
10.1364/opex.12.001848

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A novel scheme for suppressing the multiple-access interference (MAI) in coherent time-addressed optical CDMA systems is proposed. This is based on a differential detection using the dual-control NOLM. The basic principle for MAI suppression is described. For experimental demonstration, two encoded channels are constructed and decoded. These decoded signals are sent to the dual-control NOLM and a high autocorrelation peak with suppressed MAI at the output of NOLM is observed. Signal-to-interference ratio is improved by 7 dB.
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.
(OCDMA) is an alternative method, which performs encoding and decoding through an optical signature code, in order to allow the selection of the desired signal so that different users can share the same bandwidth. In such a system, the data signal overlap both in time and wavelength. Tradeoffs between security and performance in optical CDMA are presented. Confidentiality against eavesdropper interception strategies for optical CDMA is considered. Coherent detection and combining signals shows better confidentiality than the incoherent one. Spectral amplitude optical CDMA based on Modified Double Weight (MDW) systems, with different code dimension, is investigated. Increasing the network system capacity or maximizing authorized specified SNRs will lead to eavesdropper to detect high SNRs.
Summary form only given.There been increasing interest in realizing optical signal processing functions in optical communication systems based on nonlinear optical parametric processes. In the proposed ultrashort pulse optical code-division multiple access (CDMA) systems, in order to realize multiplexing and demultiplexing for multichannel operation, a nonlinear element is needed to distinguish between correctly decoded signals and incorrectly decoded multiaccess interference, which have comparable average optical power but carry different spectral information. It had been proposed to realize all-optical recognition of optical phase-coded ultrashort waveforms by using second-harmonic generation in long nonlinear crystals, which could have advantages in contrast ratio, operating power and detection efficiency over other proposed schemes. While high contrast ratios have been demonstrated based on such a scheme using bulk periodically poled lithium niobate (PPLN) crystals, the high efficiency has not yet been demonstrated. We use a simple combination of PPLN SHG waveguides and conventional photodetectors to realize all-optical recognition of spectrally phase coded optical pulses, which shows the potential of much higher conversion efficiency compared to previous bulk experiments, which is crucial for real-time operation with a high signal to noise ratio.