Optical CDMA System Research Articles

Cellular Underwater Wireless Optical CDMA Network: Performance Analysis and Implementation Concepts

In this paper, we introduce and investigate a cellular underwater wireless optical code division multiple-access (OCDMA) network based on optical orthogonal codes (OOC). The structures, principles, and performance of the underwater wireless OCDMA network in various water types are presented. Since underwater wireless optical links are considered for high-bandwidth underwater communications at short ranges, we will place a set of optical base transceiver stations (OBTS) each in the center of a hexagonal cell to cover a larger underwater area. The OBTSs are connected via fiber optic to an optical network controller (ONC) which operates as the core of the network. An integral expression for system error probability is obtained for both uplink and downlink schemes under a specific channel model expressed in this paper, among many channel models in underwater optical communication systems. Furthermore, an experimental prototype of underwater an OCDMA system including the OBTS, ONC, transmitters, and receivers are implemented using field programmable gate arrays (FPGA).

Eradication of Multiple Access Interference Using a Modified Multi-service Code for SAC–OCDMA

Phase induced intensity noise (PIIN) and multiple access interference (MAI) are the main parameters that affect the performance of optical code division multiple access (OCDMA) systems, leading to degradations in system performance and reduction in the number of active users. So, this paper presents an efficient modified multi-service (MMS) code that can be used for encoding the spectral amplitude of the optical source in OCDMA systems to avoid these limitations. The proposed code disposes the effect of MAI, PIIN, and gives better performance than the other traditional codes. It can support a large number of active users (80 at bit error rate of 10?9) with a small code weight (w = 4). Also, it achieves a practical code length with a simple receiver structure.

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.

Enhancing Optical-CDMA Confidentiality With Multicode-Keying Encryption

Optical codes with large cardinality and tree structures of multiple subsets of codewords for adjustable code performance and cardinality have recently been proposed. As studied in this paper, these characteristics support multicode-keying encryption for enhancing physical-layer confidentiality in optical code-division multiple-access systems and networks. The concept of the multicode-keying encryption technique is introduced. The associated all-optical hardware is designed and validated with OptiSystemTM simulation. The theoretical analyses of confidentiality improvement by means of rapid codeword switching and multicode-keying encryption are formulated.

A new two-code keying scheme for SAC-OCDMA systems enabling bipolar encoding

In this paper, we propose a new two-code keying scheme for enabling bipolar encoding in a high-rate spectral-amplitude coding optical code-division multiple-access (SAC-OCDMA) system. The mathematical formulations are derived for the signal-to-noise ratio and bit-error rate (BER) of SAC-OCDMA system based on the suggested scheme using multi-diagonal (MD) code. Performance analyses are assessed considering the effects of phase-induced intensity noise, as well as shot and thermal noises in photodetectors. The numerical results demonstrated that the proposed scheme exhibits an enhanced BER performance compared to the existing unipolar encoding with direct detection technique. Furthermore, the performance improvement afforded by this scheme is verified using simulation experiments.

Performance comparison of proposed 3-d coherent spatial-phase-time coding/decoding with super structured fiber Bragg grating-based optical CDMA systems

In this paper, performance comparison in terms of bit error rate (BER) of proposed WDM compatible optical CDMA system incorporating 3-D spectral-phase-time encoding/decoding to a 7 chip-super-structured fiber Bragg grating (SSFBG)-based optical code division multiple access (OCDMA) system is investigated. Coding and decoding using binary [0, π] phase chips is demonstrated for six users at 5 Gb/s, and a single coded signal is separated with acceptable bit-error rate ≤10−9. In our proposed optical CDMA system encoding and decoding is done by converting hadamard codes (used for conventional CDMA system) to phase codes. It is then compared with two optical pulse retiming and reshaping systems incorporating super structured fiber Bragg gratings (SSFBGs) as pulse shaping elements. Simulation results show that with all input bands having same sample rate, size data rates our proposed codes with even larger number of channels perform better in terms of eye opening & BER.

Further results on balanced [formula omitted]-OOCs

Let W={w1,…,wr} be an ordering of a set of r integers greater than 1, Λa=(λa(1),…,λa(r)) be an r-tuple of positive integers, λc be a positive integer, and Q=(q1,…,qr) be an r-tuple of positive rational numbers whose sum is 1. In 1996, Yang introduced variable-weight optical orthogonal code ((n,W,Λa,λc,Q)-OOC) for multimedia optical CDMA systems with multiple quality of service (QoS) requirements. Most existing works on variable-weight optical orthogonal codes assume that λa(1),…,λa(r)=λc=1. In this paper, new balanced (n,{3,4},Λa,1)-OOCs are constructed, where Λa∈{(1,2),(2,1),(2,2)}.

Performance Analysis of Multirate Multiservice Optical CDMA Networks Adopting Overlapping PPM Signaling

In this paper, an optical code-division multiple-access (OCDMA) system employing overlapping pulse-position modulation (OPPM) is proposed to support multimedia services with different bit-rate and quality-of-service (QoS) requirements in OCDMA networks. Both wrapped OPPM (WOPPM) and unwrapped OPPM (UOPPM) are considered. The proposed system achieves multirate and multi-QoS transmission by using multilength variable-weight optical orthogonal codes (MLVW-OOCs) as signature sequences and using different values of OPPM modulation parameters. Furthermore, numerical analysis is performed for two different receiver structures, namely, correlation receivers with and without hard-limiters. In our analysis, the multiple-access interference (MAI) is considered as the main performance limiting factor. In addition, the performance of the proposed system is evaluated and compared to that of traditional OOK-OCDMA system. Our results reveal that under both pulsewidth and throughput constraints, the performances of the proposed WOPPM- and UOPPM-OCDMA systems are significantly superior to that of OOK-OCDMA system.

Construction and performance analysis of variable-weight optical orthogonal codes for asynchronous OCDMA systems

A construction scheme of variable-weight optical orthogonal codes (VW-OOCs) for asynchronous optical code division multiple access (OCDMA) system is proposed. According to the actual situation, the code family can be obtained by programming in Matlab with the given code weight and corresponding capacity. The formula of bit error rate (BER) is derived by taking account of the effects of shot noise, avalanche photodiode (APD) bulk, thermal noise and surface leakage currents. The OCDMA system with the VW-OOCs is designed and improved. The study shows that the VW-OOCs have excellent performance of BER. Despite of coming from the same code family or not, the codes with larger weight have lower BER compared with the other codes in the same conditions. By taking simulation, the conclusion is consistent with the analysis of BER in theory. And the ideal eye diagrams are obtained by the optical hard limiter.

Spectrally Efficient 3D Optical CDMA Using Coherent Spatial Phase–Time Coding/Decoding

We demonstrate a WDM compatible optical CDMA system incorporating 3D spectral phase–time encoding/decoding. We provide coding and decoding using binary [0, π] phase chips for six users at 6 Gb/s, with a single coded signal separated with an acceptable bit-error rate ≤10−9. The coding and decoding method is based on 3D coding of tightly spaced phase-locked laser lines that is compatible with conventional WDM networking. In optical CDMA systems, we propose to provide encoding and decoding done by converting Hadamard codes (used for conventional CDMA system) to the phase codes. We report that duo-binary modulation format is the best with adequate bandwidth compression. We confirm that better simulation results are reached in terms of the Q factor and bit error rate.

Error probability upper bound for perfect sequences implemented with super‐structured fibre Bragg gratings

The success of coherent optical code-division multiple-access (OCDMA) systems is strongly dependent on the optical encoder/decoder technology and on the selection of the correct OCDMA codes/sequences. For this reason, in this study, the authors present a method to implement perfect sequences with Super-Structured Fibre Bragg Gratings (SSFBGs). A new SSFBG power reflection model has been found. They have also derived a property that explains why the SSFBGs should use codes derived from m -sequences. Usually, OCDMA researchers try many different codes into SSFBGs in order to select the SSFBG encoders that result in lower error probability. In the authors work, they show that a SSFBG can be considered to be a perfect sequence encoder. For this reason, the codes written into the SSFBGs should be selected based on their new property. This property permits to design and select quickly the correct codes with low power contrast ratios. In addition, a new error probability upper bound, which is a function of the code family and of its power contrast ratio is also presented. With this new bound, it is not necessary to use an optical simulator to estimate the maximum bit error rate of an OCDMA system, if some power contrast ratios of the selected SSFBG code set are known.

Design and performance analysis in multiuser optical CDMA systems

The article presents comparative performance analysis of the proposed Optical CDMA system for 32 and 16 users with two dimensional codes. Numerical simulations have been done under interference significant environment, considering noise and dark current at data rates 2.5, 5, 7.5 and 10Gbps over single mode fiber for transmission distance of more than 270km. Results illustrate overall good performance, degraded with augment in bit rate and transmission distance, impervious with raise in number of simultaneous active users. Depicts significant performance improvement with inclusion of forward error correction RS (255,239), for low attenuation and chirp factor. It perceived, this is one of the efficient functional techniques for next generation broadband optical networks together with higher security owing to encoding and decoding, as it allow multiple users in the network to access the same fiber channel asynchronously.

A new two dimensional spectral/spatial multi-diagonal code for noncoherent optical code division multiple access (OCDMA) systems

A new two dimensional codes family, namely two dimensional multi-diagonal (2D-MD) codes, is proposed for spectral/spatial non-coherent OCDMA systems based on the one dimensional MD code. Since the MD code has the property of zero cross correlation, the proposed 2D-MD code also has this property. So that, the multi-access interference (MAI) is fully eliminated and the phase induced intensity noise (PIIN) is suppressed with the proposed code. Code performance is analyzed in terms of bit error rate (BER) while considering the effect of shot noise, PIIN, and thermal noise. The performance of the proposed code is compared with the related MD, modified quadratic congruence (MQC), two dimensional perfect difference (2D-PD) and two dimensional diluted perfect difference (2D-DPD) codes. The analytical and the simulation results reveal that the proposed 2D-MD code outperforms the other codes. Moreover, a large number of simultaneous users can be accommodated at low BER and high data rate.

Utilization of balanced detection with optical hard-limiters in T/W OCDMA system

In this paper, we utilize optical hard-limiters to further enhance the system performance in balanced detection T/W optical CDMA system. In T/W optical CDMA systems, it had been investigated that the balanced detection scheme is not superior to the OOK scheme because of the adverse impact of beat noise. The employment of optical hard-limiter is an effective way to alleviate the deteriorating effect of beat noise. The carrier-hopping prime code is utilized as signature sequences whose out of phase autocorrelation is zero and the cross-correlation is at most one. Due to the orthogonality of signature sequence, the correlator of shifted version of carrier-hopping prime code is utilized in the receiver. The numerical results reveal that our proposed scheme can greatly reduce the error floor; also it is superior to that using OOK scheme with optical hard-limiter. Most important of all, our proposed scheme has fixed and optimum thresholds which do not vary with system parameters; and this will lead to dynamic estimation of optimal thresholds unnecessary in the receiver.

Bounds and constructions for [formula omitted]-OOCs

In 1996, Yang introduced variable-weight optical orthogonal code for multimedia optical CDMA systems with multiple quality of service (QoS) requirements. Let W={w1,…,wr} be an ordering of a set of r integers greater than 1, λ be a positive integer (auto- and cross-correlation parameter), and Q=(q1,…,qr) be an r-tuple (weight distribution sequence) of positive rational numbers whose sum is 1. A (v,W,λ,Q) variable-weight optical orthogonal code ((v,W,λ,Q)-OOC) is a collection of (0,1) sequences with weights in W, auto- and cross-correlation parameter λ. Some work has been done on the construction of optimal (v,W,1,Q)-OOCs, while little is known on the construction of (v,W,λ,Q)-OOCs with λ≥2. It is well known that (v,W,λ,Q)-OOCs with λ≥2 have much bigger cardinality than those of (v,W,1,Q)-OOCs for the same v,W,Q. In this paper, a new upper bound on the number of codewords of (v,W,λ,Q)-OOCs is given, and infinite classes of optimal (v,{3,4},2,Q)-OOCs are constructed.

Enhanced performance analysis of optical CDMA communication system

Optical CDMA technique is required to meet the increased demand for high speed, large capacity communications in optical networks. OCDMA assures data access security and supports asynchronous burst data transmission. In this paper, improvement in optical CDMA has been done by implementing forward error correction technique and a distance of 100km was achieved BER 10−19 at bit rate 3.75Gbps with forward error correction in optical CDMA.

Assessment of an Optical Burst Switched Core Node With a Restricted Number of Simultaneous Active Users Using the Burst Error Loss Rate

The technical superiority of spectral-amplitude coding optical code-division multiple-access (SAC-OCDMA) systems over traditional wavelength division multiplexing (WDM) systems in optical burst switched (OBS) networks is mainly attributed to the former’s better medium access control (MAC) layer performance. Nevertheless, in order to conduct an accurate comparison, a thorough study of the physical layer performance should be involved, especially because in many cases the physical layer noise would affect the maximum achievable number of simultaneously active users. Hence, in this work, we develop a novel assessment approach that combines both MAC and physical layer capabilities by introducing a new burst error loss rate parameter. In particular, the approach targets cases with limitations on the number of simultaneous active users. Next, as an example of a noisy physical layer, the effect of phase-induced intensity noise on the number of active users in OBS/SAC-OCDMA systems is analyzed. Our analysis shows that this effect introduces a burst error rate (BurstER) in the multiple-access interference cancellation operation (not investigated before). This BurstER is an increasing function of the number of active users and hence would suppress the system MAC layer performance. Finally, assuming an ideal WDM physical layer, we employ the developed approach to present an illustrative performance comparison between OBS/SAC-OCDMA and OBS/WDM systems. The results show that OBS/SAC-OCDMA performance outperforms that of OBS/WDM when the number of tolerated bits in error per burst exceeds a certain value.

Implementation and performance analysis of a new family of coherent 3-dimensional space/phase/frequency spread optical code for OCDMA networks

In his paper, we have demonstrated the feasibility of a spectrally efficient wavelength-division-multiplexing-compatible optical code-division multiple-access system using 16 phase-locked laser lines within a 40-GHz tunable window as frequency and an ultrahigh frequency resolution spectral phase encoder–decoder. Coding and decoding slots using binary [0,π ] phase chips were demonstrated for four users at 5 Gb/s, and a single coded signal was separated from four copropagating signals, with bit-error rate <10 -9.

An efficient MAI cancellation technique in Optical CDMA

Optical code division multiple access (OCDMA) is a promising operational system for future backhaul optical networks especially for switching and optical signal multiplexing. However, OCDMA operation is restricted by multiple access interference (MAI) due to the asynchronous nature of transmission. In this study, we discuss an efficient MAI technique for OCDMA systems that depends not only on using hybrid frequency shift keying (FSK) modulation but also on employing the double padded modified prime code (DPMPC) as a signature sequence. MAI cancellation is performed by subtracting a reference signal from the received signal of the desired user. The simulation results show that the performance of the FSK-OCDMA technique is superior to the performance of the PPM-OCDMA technique. Furthermore, the obtained results illustrate that the FSK-OCDMA technique is able to accommodate a large number of simultaneous active users with low error-rate.

40G-OCDMA-PON System With an Asymmetric Structure Using a Single Multi-Port and Sampled SSFBG Encoder/Decoders

In the hybrid optical code division multiple access (OCDMA) architectures using different types of optical encoder/decoder (E/Ds) between an optical line terminal and optical network units, a hybrid 40G-OCDMA-PON with a single multi-port and super-structured fiber Bragg grating (SSFBG) E/Ds is studied as a candidate system beyond next-generation PON stage 2 (NG-PON2). In this paper, we have developed uniformed and sampled SSFBG E/Ds with different refractive index profiles and have compared their code performances in a hybrid 40G-OCDMA-PON system. The experimental results show that the sampled profile improves the code dependent performances presented in many OCDMA systems. We have also demonstrated a full-duplex 4-user × 40 Gb/s hybrid OCDMA-PON system by applying the sampled SSFBG E/Ds. An asynchronous full-duplex 50 km transmission with a total capacity of 160 Gb/s (4-user × 40 Gb/s) has been achieved for the first time. Furthermore, we focus on major requirements in NG-PON2 and discuss several issues to introduce the hybrid 40G-OCDMA-PON for optical access networks.