Direct-detection Optical Code-division Multiple-access Research Articles

Performance Analysis of Polarization Modulated DirectDetection Optical CDMA Systems over Turbulent FSO LinksModeled by the Gamma-Gamma Distribution

This paper proposes a theoretical study to characterize the transmission of optical code division multiple access (CDMA) systems deploying polarization shift keying (PolSK) over a free space optical (FSO) link under the impact of atmospheric turbulence. In our analysis, a novel transceiver architecture for atmospheric OCDMA FSO systems based on polarization modulation with direct detection is proposed and discussed. A detailed analytical model for PolSK-OCDMA systems over a turbulent FSO link is provided. Further, we derive a closed-form bit error ratio (BER) and outage probability expressions, taking into account the multiple-access interference (MAI), optical noise and the atmospheric turbulence effect on the FSO link modeled by the Gamma-Gamma distribution. Finally, the results of this study show the most significant parameters that degrade the transmission performance of the PolSK-OCDMA signal over FSO links and indicate that the proposed approach offers improved bit error ratio (BER) performances compared to the on-off-keying (OOK) modulation scheme in the presence of turbulence.

Direct-detection synchronous O-CDMA system with interference estimation and cancellation

An M-ary coded synchronous optical code-division multiple-access (O-CDMA) system with pulse-position modulation (PPM) is investigated. One novel class of optical spreading codes based on combinatorial designs is adopted in the multiplexing process. Their ideal correlation properties facilitate the discrimination between desired signals and jamming. However, the multiple-access interference (MAI) with high intensity significantly deteriorates the system performance even if the number of interferers is small. In this paper, we present an interference reduction technique for direct-detection O-CDMA to suppress the noise effect and increase the system capacity. The MAI from reference signals can be estimated by utilizing the uniform cross-correlation (CC) among its sequences and considerably cancelled out after the photodetection process. The upper bound on the error probability of optical synchronous PPM-CDMA is then derived. The proposed system is shown to be effective to improve the bit error performance and to alleviate the error floor when the number of simultaneous users and the received optical power are not appreciably small.

Channel interference cancellation using electrooptic switch and optical hardlimiters for direct-detection optical CDMA systems

This paper proposes new channel interference cancellation technique using an electrooptic (EO) switch and optical hardlimiters for direct-detection optical code-division multiple-access (CDMA) systems. In the proposed system, the received signal is split into two paths, a signal path and a reference path. The channel interference in a reference path is reduced by double optical hardlimiters. The output of a reference path drives the EO switch to reduce the effect of the channel interference in a signal path. When the reference signal is larger than or equal to the threshold of the driver circuit, the EO switch is turned on, that is, set in the bar state and the signal in a signal path is passed to an avalanche photodiode (APD); Otherwise, the EO switch is turned off, that is, set in the cross state and the signal is not passed to the APD. It is shown that the proposed system is effective to reduce the effect of the channel interference when the received optical power is large and when the effect of the channel interference is large. Moreover, it is shown that the proposed system can improve the floor probabilities of the systems with and without double optical hardlimiters.