Optical CDMA System Research Articles

Throughput of Coded Optical CDMA Systems with AND Detectors

Conventional detection techniques used in optical code-division multiple access (OCDMA) systems are not optimal and result in poor bit error rate performance. This paper analyzes the coded performance of optical CDMA systems with AND detectors for enhanced throughput efficiencies and improved error rate performance. The results show that the use of AND detectors significantly improve the performance of an optical channel.

Design of FBG En/decoders in Coherent 2-D Time-polarization OCDMA Systems

A novel fiber Bragg grating (FBG)-based en/decoder for the two-dimensional (2-D) time-spreading and polarization multiplexer optical coding is proposed. Compared with other 2-D en/decoders, the proposed en/decoding for an optical code-division multiple-access (OCDMA) system uses a single phase-encoded FBG and coherent en/decoding. Furthermore, combined with reconstruction-equivalent-chirp technology, such en/decoders can be realized with a conventional simple fabrication setup. Experimental results of such en/decoders and the corresponding system test at a data rate of 5 Gbit/s demonstrate that this kind of 2-D FBG-based en/decoders could improve the performances of OCDMA systems.

Effects of Fiber Dispersion on the Performance of Optical CDMA Systems

In recent years, Optical Code Division Multiple Access (OCDMA) system had received great attentions due to its potential to support applications with various data rates and Quality of Service requirements in physical layer. Since the main part of any fiber-based transmission system is the optical fiber itself, OCDMA also suffers from fiber impairments especially dispersion. In this paper, a review of fiber dispersion effects on system performance of different coding techniques is presented, focusing on single mode fiber. The different coding techniques include direct-sequence, spectral amplitude encoding and wavelength hopping/time spreading. In addition, the basic dispersion compensation approaches, which have been proposed in the literature, for these particular coding schemes are also discussed.

Two-Dimensional Optical CDMA Systems Based on MWOOC with Generalized Prime Sequences

Two-dimensional (2-D) codes for optical code-division multiple access (O-CDMA) systems can increase the number of subscribers and simultaneous users as compared to one-dimensional time-spreading codes. Multiple-wavelength optical orthogonal code (MWOOC), which is one of the 2-D codes, uses prime sequences as a wavelength-hopping code and an optical orthogonal code (OOC) as a time-spreading code. MWOOCs have some advantages over other 2-D codes especially in high bit-rate O-CDMA systems. The only drawback of MWOOC is that the performance degrades significantly when the number of wavelengths is not prime. Recently a generalized class of modified prime sequence codes (MPSCs), which includes the class of original MPSCs as its subclass, was presented. An important property of generalized MPSCs is that the codes can be constructed over not only prime fields but also extension fields. It has been shown that the correlation property of generalized MPSCs is the same as that of the original MPSCs. This paper investigates MWOOC with generalized prime sequences, which can be obtained in the process of generating the generalized MPSCs, as a wavelength-hopping code. Use of the generalized prime sequences can solve the nonprime problem of MWOOCs. The average error probability of the proposed MWOOCs is formulated theoretically and numerical results are compared with that of the original schemes. It is shown that nonprime numbers, such as 2m, 3m and 5m, can be also taken as the number of wavelengths without degrading the system performance in the proposed systems.

Improving spectral efficiency of SAC-OCDMA systems by SPD scheme

This letter presents a single photodiode detection (SPD) as an effective technique for eradicating both multiple-access interference (MAI) and phase-induced intensity noise (PIIN) in spectral amplitude coding optical code-division multiple-access (SAC-OCDMA) systems. Mathematical analysis and simulation experiments are used to investigate the spectral efficiency (SE) of SAC-OCDMA systems utilizing different detection techniques. Results show that the SPD technique significantly enhances the SE compared to AND as well as modified-AND subtraction detections.

Modified-AND Subtraction Detection Technique Based on Weight-Division for SAC-OCDMA Systems

In this paper, we investigate the use of the modified-AND subtraction detection to assuage the intensity noise in spectral-amplitude coding optical code-division multiple-access (SAC-OCDMA) systems. This detection approach is based upon reducing the received power to the decoders by dividing the weight of the utilised code sequence. The proposed technique has the ability to suppress the influence of multiple-access interference (MAI), in addition to alleviate the intensity noise effect. We simulate the system performance of the proposed detection technique using OptiSystem software Version 9.0. The software considered the entire practical impacts in the system, such as nonlinear effect, dispersion, and attenuation in the fiber. The simulation results based on the modified quadratic congruence (MQC) code show that the modified-AND subtraction detection offer best bit-error rate (BER) performance compared to traditional AND subtraction detection.

Photon-counting chip-interleaved iterative PIC detector over atmospheric turbulence channels

A photon-counting-based iterative parallel interference cancellation (PIC) scheme for free-space opticalcommunications in the presence of multiple-access interference, shot noise, background radiation, and turbulence fading is designed. An efficient chip-level iterative equivalent noise estimation algorithm is also derived. Simulation results show that the proposed scheme can achieve a single-user performance,bound with the fast convergence property. More importantly, it can eliminate the bit-error rate floor of the conventional optical code-division multiple-access system with the aid of a relatively short spreading code length.

New Infinite Classes of Optimal (v,{k,6},1,Q) Optical Orthogonal Codes via Quadratic Residues

Variable-weight optical orthogonal codes (OOCs) were introduced by Yang for multi-media optical CDMA systems with multiple quality of service (QoS) requirements. Some works had been done on the existence of optimal (v,W, 1, Q)-OOCs for W ∈ {{3, 4}, {3, 5}, {4, 5}}. In this paper, by using quadratic residues, seven new infinite classes of optimal (v, {k, 6}, 1, Q)-OOCs for k ∈ {3, 4} are constructed. key words: cyclic packing, optical orthogonal code, quadratic residue, relative difference family

Performance analysis of spectral-amplitude-coding optical CDMA systems with new subtract exclusive OR detection (SED) using vectors combinatorial (VC) code

This paper investigates the performance of vector combinatorial (VC) code for spectral amplitude coding optical code-division multiple-access (SAC-OCDMA) system using a newly proposed subtract exclusive OR detection (SED) technique. VC code is constructed using Euclidean vector and algebraic ways. One of the important properties of this code is that the maximum cross correlation is one, which means that multi-user interference (MUI) and phase induced intensity noise (PIIN) are reduced. The weight for the VC code can be any positive integer number. The system performance was evaluated by comparing the theoretical studies using related equations and simulation results taken from the commercial optical systems, “VPI™ TransmissionMaker 7.1”. Significant improvement in system performances has been observed when the proposed SED technique employing VC code was applied compared to other codes that utilize conventional balance detection techniques for spectral amplitude coding.

Study of Power Control on Double-Weight Codes With an Arbitrary Maximum Cross-Correlation Value in Variable-QoS Optical CDMA

In this paper, the use of power and weight control in multimedia, variable quality-of-services (QoS) incoherent optical code-division multiple-access (O-CDMA) systems in on-off keying is studied. An adjustable-power and variable-weight incoherent O-CDMA architecture is presented and the general performance model of 1-D and 2-D double-weight optical codes with an arbitrary maximum cross-correlation value λ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> is formulated as functions of code weight and chip power. Our results show that the performance of double-weight codes can be fine-tuned by varying both code weight and power, and they play different roles in the QoS differentiation. From the numerical examples, we show that λ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> =2 double-weight codes can perform as good as their λ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> =1 counterparts and, at the same time, have the advantage of supporting more possible subscribers. This general analytical model is important for power-sensitive applications, such as in-service monitoring and fiber-fault surveillance in optical networks and sensor-identification in fiber-sensor systems with the use of optical codes.

Time-spreading wavelength-hopping optical CDMA system using EWO and TCM

In this paper, time-spreading wavelength-hopping optical CDMA system using trellis coded modulation scheme is investigated. The carrier hopping prime code and its shifted version are employed as signature sequences. Since the carrier hopping prime code is a symmetric code, the out of phase autocorrelation is zero. Applying the orthogonality of the signature sequences, the ternary transmission mechanism can be achieved. The proposed coding scheme is simple in both transmitter and receiver. As the multiple access interference and beat noise are the major deterioration of system performance, using coding scheme is an effective way to reduce the error floor. The numerical results reveal that the proposed coding scheme can significantly improve the system performance.

Performance Characterization of Ultrahigh SpeedOptically Amplified Spectral-Phase EncodedOCDMA Systems with Second-Harmonic-Generation Effect in Thin and Thick Crystal Receiver Structures

In this paper we study and evaluate the performance of a spectral-phase encoded optical CDMA (SPE-OCDMA) system using advanced receiver structures based on second harmonic generation (SHG) effect imposed in a Thick or Thin crystal. This receiver structure is employed as the nonlinear pre-processor prior to the conventional low-speed photodetector. In our performance evaluation amplified spontaneous emission (ASE) of the optical amplifiers, receiver front end thermal noise and photodetector shot noise which are effective in low power conditions, and multiple access interference (MAI) noise which is the dominant source of noise in high power conditions in any coherent OCDMA communications system have been considered. We begin by studying the statistical behavior of Thick crystals in an optically amplified digital lightwave communication system in the context of a SPE-OCDMA communication system. The error probability for Thick crystal receiver structure is evaluated using Saddle-Point approximation. Furthermore, we obtain a closed form approximation for the probability density function (pdf) of the decision variable in SPE-OCDMA systems with Thin crystal SHG receiver. In this analysis, to obtain approximate probability density function, we employ Gram-Charlier expansion based on the corresponding first order moments of the decision variable. The first three moments are obtained in a closed form approximation and consequently a closed form expression for the error probability is obtained by integrating the approximated pdf of the decision variables for transmitting bit "1" and "0" in Thin crystal receiver structure. The precision of the approximation is verified by the Monte-Carlo simulation. Finally the performance of SPE-OCDMA network for both Thin and Thick SHG crystals are analytically evaluated and discussed for different number of interfering users and different code-lengths and different speed of the conventional photodetectors. We deduce that the performance of Thin SHG crystal receiver is more sensitive to the transmitted power level especially in low power conditions. It is concluded that in low power conditions where ASE, thermal and shot noises are the dominant noise sources, Thick SHG crystal receiver outperforms the Thin crystal receiver. However, in high power conditions where MAI noise is the most effective noise term, Thin SHG crystal receiver structure outperforms the Thick crystal receiver.

Performance Analysis of Optical CDMA Systems Utilizing Optical Encoding in Presence of Interference and Receiver Noises

Abstract In this paper, performance of incoherent and asynchronous optical CDMA (OCDMA) systems employing “optical encoding” is investigated and the results are compared with the results obtained when the encoding is performed in electrical domain. These investigations are performed for receiver structures that employ a single hard limiter (SHL), a double optical hard limiter (DHL) and also for the structures without a hard limiter (WHL). In addition, “electrical encoding” OCDMA systems using DHL in their receiver structures are investigated. In this work, conventional optical orthogonal codes are used and Gaussian approximation is assumed for the output of APD. Besides interference, other destructive effects such as APD and thermal noises are also taken into account. It is shown that when the number of users N is equal to or greater than pulsed laser's modulation extinction ratio Me , for all receiver structures (SHL, DHL and WHL) “optical encoding” systems are superior to their “electrical encoding” counterparts and this superiority becomes much more apparent in SHL and DHL receivers. We will also show that when N &lt; Me , “optical encoding” WHL systems outperform their “electrical encoding” counterparts. Our results also indicate that the performance of “optical encoding” SHL systems approaches the performance of DHL systems as the transmitted power increases.

Performance analysis of multiple interference suppression over asynchronous/synchronous optical code-division multiple-access system based on complementary/prime/shifted coding scheme

A complete complementary/prime/shifted prime (CPS) code family for the optical code-division multiple-access (OCDMA) system is proposed. Based on the ability of complete complementary (CC) code, the multiple-access interference (MAI) can be suppressed and eliminated via spectral amplitude coding (SAC) OCDMA system under asynchronous/synchronous transmission. By utilizing the shifted prime (SP) code in the SAC scheme, the hardware implementation of encoder/decoder can be simplified with a reduced number of optical components, such as arrayed waveguide grating (AWG) and fiber Bragg grating (FBG). This system has a superior performance as compared to previous bipolar-bipolar coding OCDMA systems.

Simple Code Switching Scheme for Security Enhancement of OCDMA Communication System

A simple code switching scheme for security enhancement of an optical code-division multiple-access (OCDMA) communication system is proposed. The scheme avoids the replacement of all the encoders and decoders in the system and all users' codes are changed by only tuning the center wavelength of the laser source. Each en/decoder contains multiple codes which are nonoverlapping in the spectrum. The scheme is demonstrated in a proof-of-principle transmission experiment. There are three codes on the encoder and decoder pairs, one 2-D code and two 1-D bipolar gold codes with chip number of 63 and 127, respectively. All three codes are decoded successfully after 12.7-km single-mode fiber link.

Impact of optical hard limiter on the performance of an optical overlapped-code division multiple access system

Throughout this paper, a closed form expression of the multiple access interference (MAI) limited bit error rate (BER) is provided for the multiwavelength optical code-division multiple-access system when the system is working above the nominal transmission rate limit imposed by the passive encoding-decoding operation. This system is known in literature as the optical overlapped code division multiple access (OV-CDMA) system. A unified analytical framework is presented emphasizing the impact of optical hard limiter (OHL) on the BER performance of such a system. Results show that the performance of the OV-CDMA system may be highly improved when using OHL preprocessing at the receiver side.

Bounds for the BER of Codes With Fixed Cross Correlation in SAC-OCDMA Systems

In this paper, two lower and upper bounds for the bit error rate of codes with fixed cross-correlation in spectral-amplitude-coding optical code-division multiple access systems are offered. In this study, we consider only the phase-induced intensity noise and neglect other noises such as shot and thermal noises, in the performance analysis. Also, it is assumed that users are synchronous. This bound is obtained using combinatorics approach and utilizing simple inequalities. In numerical results, the comparison between the upper bound, lower bound, and simulation results for MQC, MFH, BIBD, and Hadamard codes is presented, and the tightness of the bounds is evaluated.

Correlator receiver architecture with PnpN optical thyristor operating as optical hard-limiter

We propose novel correlator receiver architecture with a PnpN optical thyristor operating as optical hard-limiter, and demonstrate a multiple-access interference rejection of the proposed correlator receiver. The proposed correlator receiver is composed of the 1×2 splitter, optical delay line, 2×1 combiner, and fabricated PnpN optical thyristor. The proposed correlator receiver enhances the system performance because it excludes some combinations of multiple-access interference patterns from causing errors as in optical code-division multiple access systems with conventional optical receiver shown in all previous works. It is found that the proposed correlator receiver can fully reject the interference signals generated by decoding processing and multiple access for two simultaneous users.

General Constructions of Optimal Variable-Weight Optical Orthogonal Codes

Variable-weight optical orthogonal code (OOC) was introduced by Yang for multimedia optical CDMA systems with multiple quality of service (QoS) requirements. In this paper, four general constructions for optimal cyclic packings and optimal variable-weight OOCs are presented. Many new infinite classes of optimal (v, W,1,Q)-OOCs are obtained. New infinite classes of optimal (v, W,1,Q)-OOCs for |W| ≥ 4 are easily obtained by the constructions.

Design of multi-channel optical code-division multiple-access encoders and decoders based on sampled fiber Bragg gratings

An encoding/decoding scheme of multi-channel optical code-division multiple-access (OCDMA) system using sampled fiber Bragg gratings (SFBGs) is proposed and numerically investigated. A multiple-phase-shift (MPS) technique based SFBG is first developed as a dense wavelength-division multiplexing (DWDM) device to accommodated multiple channels. The OCDMA scheme is achieved through phase coding/encoding on each individual channel of the SFBG and the auto correlative ratio for each user is larger than 14.3 dB. The whole system exhibits enhanced performance in terms of the capacity and security since the OCDMA technique is simultaneously implemented for all channels using the single SFBG.