Optical CDMA System Research Articles

New Design of Flexible Cross Correlation (FCC) Code for SAC-OCDMA System

Abstract In this paper, we present, several aspects of unipolar optical code-division multiple access (OCDMA) codes, focusing on the flexible in-phase cross-correlation code algorithm and its potential for future optical networks. We briefly present a new class of code namely Flexible Cross Correlation (FCC) code for Spectral-Amplitude Coding (SAC) OCDMA approaches. The main coding properties are reviewed. The FCC code provides simple tridiagonal matrix constructions compared to the other SAC-OCDMA codes such as MDW, MQC and MFH codes. This code possesses such a various advantages, including the easier code construction, less complexity of encoder/decoder design and flexible in-phase cross-correlation for uncomplicated to implement using Fiber Bragg Gratings (FBGs) for the OCDMA systems.

Code acquisition of random optical codes in optical code-division multiple-access

In this study, the authors present a technique for code acquisition in optical code-division multiple-access systems using the recently introduced random optical codes. In these systems, the communication is divided in two parts: a synchronisation process and the data transmission. The main target of the study is to describe and analyse the new algorithm of synchronisation using Markov processes. Once described the code acquisition technique and established an estimation for the probability of false alarm (FA) of the proposed scheme, two different proposals for the optimisation of the code parameters are described. They are based on two different estimators for the probability of FA. The first one considers the expected value for the probability of FA. The second one is very much restrictive. It guarantees that the probability of FA is in a confidence interval. It takes the upper bound of the confidence interval as estimator for the probability of FA. Although the second one presents worse acquisition time, the authors consider that the gain provided by the first method is not sufficient enough to assume its loss in the reliability. Thus, the authors propose to guarantee the probability of FA in a confidence interval. A compromise between reliability and acquisition time is obtained with this second method.

Effect of Variations in Environmental Temperature on 2D-WH/TS OCDMA Code Performance

Extensive research has been carried out on the performance investigation of two-dimensional wavelength hopping/time spreading optical code division multiple access (OCDMA) codes, which are based on picosecond multiwavelength pulses under the influence of temperature variations resulting from changing environmental conditions. Equations have been derived to theoretically evaluate the extent to which such temperature changes will degrade the overall OCDMA system performance. To mitigate these negative effects on the OCDMA system, several steps have been introduced to improve the code robustness. System design improvements have then been investigated. We also found they would help to improve the spectral efficiency.

Constructions of optimal variable-weight OOCs via quadratic residues

Variable-weight optical orthogonal code (OOC) was introduced by G. C. Yang [IEEE Trans. Commun., 1996, 44: 47–55] for multimedia optical CDMA systems with multiple quality of service (QoS) requirements. In this paper, seven new infinite classes of optimal (v, {3, 4, 6}, 1,Q)-OOCs are constructed.

Efficient Use of PPM in Spectral-Amplitude-Coding Optical CDMA Systems

A prominent technique, for reducing the effect of phase-induced intensity noise (PIIN) in spectral-amplitude-coding optical code-division multiple-access (SAC-OCDMA) systems, is proposed. In this technique, each user is assigned two orthogonal code sequences, selected from a minimum cross-correlation code set. In addition, two-level M -ary pulse-position modulation (MPPM) scheme is adopted, where each level corresponds to a code sequence. This technique decreases the effect of PIIN and allows transmission at high data rates. The bit error rate (BER) of the proposed system is derived, taking into account the effects of PIIN, shot noise, and thermal noise. Two performance measures of this system, namely the BER and maximum achievable number of users, are compared to that of traditional systems adopting complementary and non-complementary keying. Our results reveal that the proposed technique allows each user to transmit at high data rates, compared to traditional techniques, while keeping the BER well below a prescribed threshold. Finally, our results are extended to higher levels of MPPM schemes with multicode modulation.

Performance analysis of two-dimensional optical code-division multiple-access systems using novel multi-code pulse-position modulation

Previous works show that pulse-position modulation (PPM) is an effective signalling method for mitigating multiple-access interference, and hence is able to increase the number of users in two-dimensional (2-D) optical code-division multiple-access (OCDMA) systems. However, in order to achieve high bit-rates, 2-D OCDMA systems using PPM signalling require very high transmitted power because of the negative impact of dispersion. In this study, the authors propose a novel modulation technique of multi-code PPM (MCPPM), which is the combination of the PPM and multi-code modulation (MCM). As the proposed technique inherits advantages from both MCM (in mitigating the dispersion) and PPM, 2-D OCDMA systems using MCPPM signalling are able to support higher user bit-rates for a larger number of users at low transmitted powers. Numerical results show that 2-D OCDMA systems using 4-4-MCPPM can support 60 users with 5 Gbps per user at the transmitted power of −7 dBm. The power gain in this case is 11 dB compared to 2-D OCDMA systems using 4-PPM signalling. The proposed systems using 4-2-MCPPM can support as many as 36 users with the bit-rate of 10 Gbps per user and transmitted power is −2 dBm.

Spectral efficiency analysis of bipolar spectral-amplitude coding optical code-division multiple-access systems using different one-dimensional codes

In this study, the spectral efficiency (SE) of bipolar spectral-amplitude coding optical code-division multiple-access (SAC-OCDMA) systems operating with incoherent sources is analysed. The analytical expressions of the model were derived to assess the performance of SAC-OCDMA systems, taking into account the intensity noise as the primary source of noise. Four different SAC codes were used to evaluate and compare the SE against the number of active users and code length for a bit-error rate (BER) of 10−11. The investigated codes are Hadamard, modified quadratic congruence (MQC), balanced incomplete block design (BIBD) and zero cross-correlation (ZCC). Our analytical results revealed that ZCC code has the ability to support more users at a high data rate up to 2.5 Gbps and offers significant SE enhancement in bipolar SAC-OCDMA system over other SAC codes under investigation. In addition, bipolar encoding improves the SE compared with unipolar technique, for example bipolar ZCC has 7.7-dB SE advantage than unipolar ZCC. Furthermore, when BER requirement of the system decreases, the SE can be increased.

Some Infinite Classes of Optimal (v, {3, 4}, 1, Q)-OOCs with $${Q \in \{(\frac {1}{3}, \frac {2}{3}), (\frac {2}{3}, \frac{1}{3})\}}$$

Variable-weight optical orthogonal code (OOC) was introduced by Yang for multimedia optical CDMA systems with multiple quality of service requirements. It is proved that optimal (v, {3, 4}, 1, (1/2, 1/2))-OOCs exist for some complete congruence classes of v. In this paper, for $${Q \in \{(1/3, 2/3), (2/3, 1/3)\}}$$ , by using skew starters, it is also proved that optimal (v, {3, 4}, 1, Q)-OOCs exist for some complete congruence classes of v.

Isolated user security enhancement in optical code division multiple access network against eavesdropping

A novel virtual user system is modeled for enhancing the security of an optical code division multiple access (OCDMA) network. Although the OCDMA system implementing code shift keying (CSK) is secure against a conventional power detector, it is susceptible to differential eavesdropping. An analytical framework is developed for the CSK-OCDMA system to show eavesdropper's code interception performance for a single transmitting user in the presence of a virtual user. It is shown that the eavesdropper's probability of correct bit interception decreases from 7.1×10 −1 to 1.85×10 −5 with the inclusion of the virtual user. Furthermore, the results confirm that the proposed virtual user scheme increases the confidentiality of the CSK-OCDMA system and outperforms the conventional OCDMA scheme in terms of security.

Energy-Efficient High-Capacity Optical CDMA Networks by Low-Weight Large Code-Set MPC

In this paper, we have proposed a novel spreading code-set based on the prime code families, hereby referred to as transposed sparse-padded modified prime code for synchronous optical code-division multiple-access (OCDMA) networks. The proposed code-set is compared with modified prime codes (MPCs) in terms of correlation properties, bit-error rate (BER), and cardinality. This new low-weight large code-set has better correlation characteristics as compared with MPC families and, as results indicated, the new code-set has considerably improved the BER and power consumption of the OCDMA networks. We have analyzed the performance of the proposed code-set in both incoherent ON-OFF keying and pulse position modulation OCDMA systems, while the multiple access interference was the major degrading quality. The results indicate that at the BER floor of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> , the proposed code-set can accommodate greater number of users and consume less power as compared with the MPC under the same transmission conditions.

Interference cancellation in optical CDMA systems via advanced binary optical logic gate elements

In this article we present an all passive optical iterative interference cancellation method for optical CDMA. We propose the receiver structure based on advanced binary optical logic gates (ABOLGs). Since the performance bottleneck in optical communication systems is the conversion from optical to electronic domains for optical detectors, the interference cancellation has been performed in the optical domain. In incoherent optical communication systems, we have only the positive values for the optical signals. In the proposed system, we bypass the negative values using logic arrays. In the simulations we consider multiple access interference and neglect other sources of noise. Simulation results show that in the future optical systems, the interference can be eliminated completely using iterative methods in the optical domains.

Comments on “An Enhanced Detection Technique for Spectral Amplitude Coding Optical CDMA Systems”

It has been shown in the above letter that the use of multiphotodiode balanced detection (MPBD), in spectral-amplitude coding optical code-division multiple-access systems, reduces the effect of phase-induced intensity noise (PIIN) at a cost of added complexity. In this letter, we show that this claim is not true, and the MPBD scheme retains the effects of PIIN as the conventional balanced detector does.

Improved BER based on intensity noise alleviation using developed detection technique for incoherent SAC-OCDMA systems

The major drawback of incoherent spectral-amplitude coding optical code-division multiple-access (SAC-OCDMA) systems is their inherent intensity noise originating due to the incoherency of the broadband light sources. In this paper, we propose a developed detection technique named the modified-AND subtraction detection for incoherent SAC-OCDMA systems. This detection technique is based upon decreasing the received signal strength during the decoding process by dividing the spectrum of the utilized code sequence. The proposed technique is capable of mitigating the intensity noise effect, as well as suppressing the multiple-access interference impact. Based on modified quadratic congruence (MQC) code, the analytical results reveal that the modified-AND detection offer best bit-error rate (BER) performance and enables MQC code to support higher transmission rate up to 1.25 Gb/s compared to conventional AND detection. Furthermore, we ascertained that the proposed technique enhances the system performance using a simulation experiment.

Mitigation of multiple access interference using two-dimensional modified double weight codes for optical code division multiple access systems

We proposed newly two-dimensional (2- D ) spectral amplitude coding optical code division multiple access (OCDMA) scheme using modified double weight (MDW) code capable of suppressing phase-induced intensity noise (PIIN). The architecture of the spectral/spatial MDW OCDMA system with the property of multi-access interference cancellation is presented. The proposed code exhibits good cross-correlation property. At the optimized data transmission rate of 0.745 Gbps, 2-D MDW, M = 63, N = 3, reaches maximum cardinality of 200% increases compared to 2- D perfect difference code, M = 57, N = 3. The performance is severely deteriorated if the data rate further increases above 0.745 Gbps. The proposed code meets the optical transmission requirements at 10−9 bit error rate error floor, with lowest effective transmitted power ( P sr), −17.5 dBm, in comparison to the others through minimizing interference noise that result in PIIN suppression. The proposed system reaches optimum requirements performance in terms of cardinality, data transmission rate, and low effective transmitted power.

Optical code-division multiple-access embedded with a polarisation diversity scheme for radio-over-fibre transmissions

This study presents a spectral polarisation-coding optical code-division multiple-access (SPC-OCDMA) system for radio-over-fibre (RoF) transmissions. The proposed system implements encoding and double-balanced detection processes using a polarisation diversity scheme to enlarge the transmitted radio frequency (RF) signal space. By combining the quasi-orthogonality of Walsh–Hadamard codes with arrayed waveguide grating (AWG) routers and a polarisation beam splitter (PBS), the proposed coder/decoder (codec) pair can encode–decode multiple Walsh–Hadamard code words and retain the ability of multiple access interference (MAI) cancellation. This study presents a series of investigations to evaluate the sensitivity of the proposed structure to the degree of matching in the state of polarisations (SOPs) of the transmitted light in the encoder and decoder, respectively. Results show that when the polarisation states are fully matched between the encoder and the decoder, the proposed system achieves a carrier-to-noise ratio (CNR) improvement of up to 3 dB compared with that obtained from a complementary spectral polarisation coding (SPC) system with a single-balanced detector. Numerical results demonstrate that maintaining the system with an orthogonal SOP for each radio base station (RBS) effectively reduces the phase-induced intensity noise (PIIN).

On Balanced Optimal Optical Orthogonal Codes

AbstractVariable‐weight optical orthogonal codes (OOCs) were introduced by G.‐C. Yang for multimedia optical CDMA systems with multiple quality of service (QoS) requirement. In this paper, by using incomplete difference matrices and perfect relative difference families, a balanced ‐OOC is obtained for every positive integer .

Performance Comparison of Coherent Time-spreading and Spectral Phase Encoding OCDMA Systems

Abstract We compared the bit-error-rate (BER) and throughput performance of coherent time-spreading (TS) and spectral phase encoding (SPE) optical code division multiple access (OCDMA) systems. The results indicate that for the short input pulse (250–300 fs pulsewidth), SPE-OCDMA can in principle yield a substantial throughput advantage over TS-OCDMA systems.

Performance enhancement of 2-D optical CDMA systems using MCM signalling and heterodyne detection receiver

We propose 2-D optical code-division multiple-access systems using multi-code modulation signalling and heterodyne detection receiver. The proposed system's performance is theoretically analysed and compared with conventional ones using on–off keying, pulse–amplitude modulation and pulse–position modulation. The numerical results show that the proposed system outperforms the conventional ones in terms of required transmitted power, the number of supportable users and spectral efficiency. Copyright © 2012 John Wiley & Sons, Ltd.

Ethernet-OCDMA system for multi-user visible light communications

An optical code-division multiple access system (OCDMA) for multi-user visible light communications is presented, which interfaces with the Ethernet network. The OCDMA scheme is based on the optical codes known as random optical codes, which do not present an optimal performance but are characterised by their ease of implementation. To combat the relative poor correlation properties of this kind of code, a synchronism mechanism has been proposed and evaluated. The results obtained with the final developed prototype demonstrate the suitability of the proposed scheme for multi-user communications in a visible light environment.

Performance analysis of AND gates based on four-wave-mixing for application in optical-code division multiple access systems

In this paper, we study the performance of all optical AND gates based on the four-wave-mixing (FWM) process, for application in the receivers of the spectral phase encoded optical code division multiple access (SPE-OCDMA) systems. For the performance analysis, the spectral domain auto-correlation function of the multiple access interference signal at the output of the optical AND gate is derived and then mean and variance of the photo-detector current are calculated. For solving the Fourier transform of FWM non-linear Schrodinger equations, spectral domain Volterra series are used and the solution of these coupled equations is approximated with the first three terms of the Volterra expansion. Finally, the performance of AND gating receivers in SPE-OCDMA systems is analysed and the bit error rate in terms of the pump power and the number of active users is depicted. In addition, a compression between the performance of the time-gating receivers, base on FWM in synchronous mode and other non-linear receivers such as second harmonic generation and self-phase modulation is made.