Optical CDMA System Research Articles

Performance enhancement of the 3D OCDMA system by using dynamic cyclic shift and multi-diagonal codes

A new three-dimensional (3D) code is developed for spectral/time/spatial optical code division multiple access (OCDMA) system namely 3D dynamic cyclic shift/multi-diagonal (3D-DCS/MD). Its construction is based on the dynamic cyclic shift code for spectral and temporal encoding and multi-diagonal code for spatial one. The data of users are retrieved correctly using the MAI cancelation property. Compared with the OCDMA systems using 3D perfect/multi-diagonal, and 3D perfect difference, the 3D-DCS/MD system has a low phase-induced intensity noise. Hence, this system overcomes these codes in terms of data rate, required source power and spectral width. Also, it can accommodate more than 200 users at 622 Mbps.

Development of a novel two-dimensional-SWZCC – Code for spectral/spatial optical CDMA system

In order to overcome the effect of the Multiple Access Interference (MAI) and noises, and increase the number of the simultaneous users, the multiplexing capacity and data rate, this paper proposes a novel design of a two dimensional spectral/spatial code named Two Dimensional-Single Weight Zero Cross Correlation (2D-SWZCC) based on a single weight code which is characterized by a zero cross correlation and a high number of users. The proposed code is developed to be implemented in non-coherent Optical Code Division Multiple Access (OCDMA) systems. Numerical results of the design construction show that the proposed code enhances the system performance by improving its capacity, reducing the signal-to-noise ratio, and increasing the data rate. The system output performance for active user number is equal to 1.09, 1.54, 2.14, 2.215, 3.89 and 5.14 times than those of, Two Dimensional Multi-Diagonal (2D-MD), Two Dimensional Dynamic Cyclic Shift (2D-DCS), (1D-SWZCC), Two Dimension Perfect Difference (2D-PD), Two Dimensional Hybrid Flexible Cross Correlation- Modified Double Weight (FCC/MDW) and Two Dimensional Extended Enhanced Double Weight (2D-extended-EDW) codes, respectively. It can save an effective power around −3.2 dBm and −1.4 dBm at the level of the receiver compared to 2D-DCS and 2D-MD codes. Moreover, the system performance has been studied using Optisystem software. The simulation results show that the system respects the optical transmission property accommodating four users for a power of the source equal to −10 dBm, and the data rate is 1 Gbps.

Application of Semiconductor Optical Amplifier (SOA) in Managing Chirp of Optical Code Division Multiple Access (OCDMA) Code Carriers in Temperature Affected Fibre Link

Chromatic and temperature induced dispersion can both severely affect incoherent high data rate communications in optical fibre. This is certainly also true for incoherent optical code division multiple access (OCDMA) systems with multi-wavelength picosecond code carriers. Here, even a relatively small deviation from a fully dispersion compensated transmission link can strongly impact the overall system performance, the number of simultaneous users, and the system cardinality due to the recovered OCDMA auto-correlation being strongly distorted, time-skewed, and having its full width at half maximum (FWHM) value changed. It is therefore imperative to have a simple tunable means for controlling fibre chromatic or temperature induced dispersion with high sub-picosecond accuracy. To help address this issue, we have investigated experimentally and by simulations the use of a semiconductor optical amplifier (SOA) for its ability to control the chirp of the passing optical signal (OCDMA codes) and to exploit the SOA ability for dispersion management of a fibre link in an incoherent OCDMA system. Our investigation is done using a 19.5 km long fibre transmission link that is exposed to different temperatures (20 °C and 50 °C) using an environmental chamber. By placing the SOA on a transmission site and using it to manipulate the code carrier’s chirp via SOA bias adjustments, we have shown that this approach can successfully control the overall fibre link dispersion, and it can also mitigate the impact on the received OCDMA auto-correlation and its FWHM. The experimental data obtained are in a very good agreement with our simulation results.

Improvement of physical-layer security and reliability in coherent time-spreading OCDMA wiretap channel

Optical code division multiple access (OCDMA) can enhance the physical-layer security performance of optical fiber communication systems. In asynchronous coherent time-spreading OCDMA employing m sequence and Gold code, the reliability, capacity and physical-layer security of the system will degrade due to multiple access interference and beat noise. In order to enhance the physical-layer security and reliability of coherent OCDMA systems, a quasi-synchronous coherent time-spreading OCDMA wiretap channel is proposed in this paper. The influences of the extraction location, the extraction ratio, the number of active users on the physical-layer security are analyzed quantitatively. System performances are characterized by bit error rate, secrecy capacity as well as security leakage factor. The numerical results prove that the proposed scheme can improve the physical-layer security and reliability simultaneously.

Management of OCDMA Auto-Correlation Width by Chirp Manipulation Using SOA

In order to preserve the performance of incoherent optical code-division multiple access (OCDMA) systems, it is imperative to maintain the width of the OCDMA auto-correlation function. This can be challenging in OCDMA systems when using multi-wavelength pico-second pulses as code carriers. In this respect, a semiconductor optical amplifier was used to control the chirp of OCDMA code carriers before fiber transmission, and the resulting effect on the recovered OCDMA auto-correlation was investigated at the receiving end after a 16-km long transmission in a fiber optic testbed with imperfectly balanced chromatic dispersion.

Rate-Maximized Scheduling in Adaptive OCDMA Systems Using Stochastic Optimization

In this letter, we use stochastic optimization to schedule data transmissions in an adaptive optical code division multiple-access (OCDMA) system to maximize transmission rate. We employ Lyapunov drift theory to formulate a general optimization problem for rate-maximized scheduling in which user codes are adaptively assigned such that time-averaged transmission rate is maximized and important system parameters such as stability and BER performance are maintained. Simulation results show that the overall transmission rate in a rate-maximized scheduled OCDMA system can be up to 3 (1.2) times better than the transmission rate in its counterpart OCDMA system with fixed code assignment scheme (with rate-efficient code allocation algorithm).

Performance enhancement of optical code-division multiple-access systems using transposed modified Walsh code

This paper presents the construction of unipolar transposed modified Walsh code (TMWC) and analysis of its performance in optical code-division multiple-access (OCDMA) systems. Specifically, the signal-to-noise ratio, bit error rate (BER), cardinality, and spectral efficiency were investigated. The theoretical analysis demonstrated that the wavelength-hopping time-spreading system using TMWC was robust against multiple-access interference and more spectrally efficient than systems using other existing OCDMA codes. In particular, the spectral efficiency was calculated to be 1.0370 when TMWC of weight 3 was employed. The BER and eye pattern for the designed TMWC were also successfully obtained using OptiSystem simulation software. The results indicate that the proposed code design is promising for enhancing network capacity.

Bounds and Constructions for Optimal $(n, \{3, 4, 5\}, \Lambda _a, 1, Q)$ -OOCs

Let $W=\{w_{1}, \ldots , w_{r}\}$ be a set of positive integers, $\lambda _{c}$ a positive integer, $\Lambda _{a}=(\lambda _{a}^{(1)}, \ldots \lambda _{a}^{(r)})$ an $r$ -tuple of positive integers, and $Q=(q_{1}, \ldots q_{r})$ an $r$ -tuple of positive rational numbers whose sum is 1. In 1996, Yang introduced variable-weight optical orthogonal code, $(n, W, \Lambda _{a}, \lambda _{c}, Q)$ -OOC, for multimedia optical CDMA systems with multiple quality of service (QoS) requirements. Some work had been done on the constructions of optimal $(n, W, \Lambda _{a}, 1, Q)$ -OOCs with unequal auto-correlation constraints for $W=\{3, 4\}$ and {3, 5}, while little is known on optimal $(n, W, \Lambda _{a}, 1, Q)$ -OOCs for $|W|\geq 3$ . In this paper, we focus our main attentions on $(n, \{3, 4, 5\}, \Lambda _{a}, 1, Q)$ -OOCs with $\Lambda _{a}\in \{(2, 1, 1), (2, 1, 2), (2, 2, 1), (2, 2, 2)\}$ . Tight upper bounds on the maximum code size of $(n, \{3, 4, 5\}, \Lambda _{a}, 1, Q)$ -OOCs are obtained, and infinite classes of optimal $(n, \{3, 4, 5\}, \Lambda _{a}, 1, Q)$ -OOCs are constructed.

Multi-rate optical CDMA system using OVSF codes for visible light communication in diffuse channel

Multi-rate optical CDMA system using OVSF codes for visible light communication in diffuse channel

Bi-orthogonal Symbol Mapping and Detection in Optical CDMA Communication System

AbstractIn this paper, the bi-orthogonal symbol mapping and detection scheme is investigated in time-spreading wavelength-hopping optical CDMA communication system. The carrier-hopping prime code is exploited as signature sequence, whose put-of-phase autocorrelation is zero. Based on the orthogonality of carrier-hopping prime code, the equal weight orthogonal signaling scheme can be constructed, and the proposed scheme using bi-orthogonal symbol mapping and detection can be developed. The transmitted binary data bits are mapped into corresponding bi-orthogonal symbols, where the orthogonal matrix code and its complement are utilized. In the receiver, the received bi-orthogonal data symbol is fed into the maximum likelihood decoder for detection. Under such symbol mapping and detection, the proposed scheme can greatly enlarge the Euclidean distance; hence, the system performance can be drastically improved.

Two-dimensional multi-service code for spectral/spatial optical CDMA system

A new two-dimensional(spectral/spatial) optical code division multiple access is developed by using one-dimensional Multi-Service code. The spectral/spatial proposed system can suppress the multiple access interference (MAI) by using the MAI cancellation property. The numerical analysis shows that two-dimensional multi-service code has a better performance compared to one-dimensional multi-service, two-dimensional perfect difference (2D-PD), two-dimensional Hybrid «flexible cross correlation/modified double weight» (2D-FCC/MDW)and two-dimensional extended enhanced double weight (2D-extended-EDW) codes; It accommodates 1.5, 2.5 and 3.3 times the maximum number of simultaneous users allocated by 2D-PD, 2D-FCC/MDW and 2D-Extended-EDW codes, respectively. By keeping the similar code length and increasing the spatial code length, the capacity and the available data rate are enhanced; also, the effective transmitted power and spectral width are decreased. The transit from 1D to 2D enhances around three times the system capacity.

Mitigation of Temperature-Induced Dispersion in Optical Fiber on OCDMA Auto-Correlation

We have experimentally investigated the influence of temperature on the optical code-division multiple-access (OCDMA) auto-correlation recovered after transmission in an optical fiber link when exposed to different temperature settings. The OCDMA system uses two-dimensional wavelength-hopping time-spreading (2D-WH/TS) codes based on picosecond multi-wavelength pulses. To the best of our knowledge, we have confirmed for the first time experimentally that a recovered OCDMA auto-correlation will be affected by transmission link temperature changes. Our simulation results are in good agreement with experimental observations. We have also demonstrated that the observed auto-correlation width change can be mitigated by introducing a semiconductor optical amplifier as part of the OCDMA receiver.

OCDMA and its Applications in Fiber Optics Communication Networks

As multiple accessing technique that can be used to provide access to multiple users to transmit data to same channel simultaneously without any scheduling or delay in transmission, Optical Code Division Multiple Access (OCDMA) has been an alluring for the past few decades. All users share all bandwidth simultaneously and access the network asynchronously. To provide access to multiple users simultaneously, many coding techniques to increase possible users and improve performance has been analyzed for OCDMA systems. In this paper, many different proposed methods of coding are reviewed. Performance, cardinality and capacity of these codes are discussed. Significance of introducing dimensions in optical code is also be discussed.

Performance Analysis of Physical-Layer Security in an OCDMA-Based Wiretap Channel

Based on information-theoretic security, physical-layer security in an optical code division multiple access (OCDMA) system is analyzed. For the first time, the security leakage factor is employed to evaluate the physicallayer security level, and the safe receiving distance is used to measure the security transmission range. By establishing the wiretap channel model of a coherent OCDMA system, the influences of the extraction location, the extraction ratio, the number of active users, and the length of the code on the physical-layer security are analyzed quantitatively. The physical-layer security and reliability of the coherent OCDMA system are evaluated under the premise of satisfying the legitimate user’s bit error rate and the security leakage factor. The simulation results show that the number of users must be in a certain interval in order to meet the legitimate user’s reliability and security. Furthermore, the security performance of the coherent OCDMA-based wiretap channel can be improved by increasing the code length.

Utilization of spatial ternary coding scheme in optical CDMA system

In this paper, we propose a spatial ternary coding scheme for optical code-division multiple access systems. Since the multiple access interference (MAI) and beat noise leads to the error floor effect, using error correction mechanism is a preferable manner to further enhance the system performance. The proposed spatial ternary coding scheme possesses larger free distance, which is an effective way to relieve such deteriorating effect. It is expected that more users accommodated in the network can obtain higher throughput, while the side effects of MAI and beat noise will turn out to be more serious. To further reduce such adverse impact, the optical hard-limiter is also proposed. Once the channel condition is improved, it will give rise to the more powerful utilization of error correction code. The countermeasures utilized in the proposed system can effectively fight against the MAI and beat noise, and can allow more simultaneous users accommodated in the network. Therefore, it is a promising way for future high-speed optical access networks.

Three-dimensional multi-diagonal codes for OCDMA system

In this paper, we propose a new three dimensional multi diagonal (3D-MD) code for optical code division multiple access (OCDMA) system. Its corresponding system structure reduces 75% and 50% the complexity of 3D-perfect difference (3D-PD) and 3D-perfect/multi-diagonal (3D-PD/MD) receivers, respectively. Moreover, the numerical analysis shows that the proposed code outperforms the performance of one and two dimensional multi-diagonal (1D-MD and 2D-MD), 3D-PD and 3D-PD/MD codes.

Development of wireless optical CDMA system for biosignal monitoring

This paper investigates the use of infrared technology in healthcare monitoring to develop a mobile medical system. This is because existing technology is generally based on radio frequency (RF) systems, which might suffer from electromagnetic interference (EMI). Additionally, the effect of the radiation field on medical equipment may lead to misdiagnosis. In answer to these issues, optical wireless links between the medical sensors and the receiver in the middle of the ceiling in a hospital room was examined. From the results, the power efficiency of the mobile optical wireless system to ensure the communication between medical sensors and the receiving point was established. Furthermore, the minimum required transmitted power for the required performance was determined to achieve a higher power autonomy. The results also show that the requirements of a healthcare monitoring application for medical nodes of up to 10 nodes, considering a data rate lower than 1Mbps, can be achieved using infrared technology. The results prove the superiority of the optical wireless technology over the conventional radio-based communication network in healthcare monitoring systems.

Pulse-Power-Detection Analysis of Incoherent O-CDMA Systems Under the Influence of Fiber Temperature Fluctuations

In this paper, a pulse-shift technique, which divides every time slot (or chip) into equal-width subchips, is used to model the effect of fiber temperature fluctuations in incoherent (asynchronous) optical code-division multiple-access (O-CDMA) systems. With the advances in all-optical thresholding technology, power detection of ultrashort optical pulses is possible. This paper also formulates a new pulse-power-detection model for incoherent O-CDMA and applies it to the analysis of the pulse-shift technique. Numerical studies and computer simulations are presented to validate the new analytical model. Our study shows that the pulse-power model results in better performance than the conventional pulse-energy model in incoherent O-CDMA.

Performance of 1‐D and 2‐D OCDMA systems in presence of atmospheric turbulence and various weather conditions

In this study, the authors compare the performance of one-dimensional (1D) and two-dimensional (2D) optical code division multiple access (OCDMA) systems in presence of turbulence and various weather conditions. Lognormal fading model is used for analysis of weak turbulence and gamma–gamma model for moderate and strong turbulence. It is observed that presence of moderate and strong turbulence may lead to a complete link failure in 2D OCDMA system depending upon the number of users and turbulence level. The various weather conditions considered are very clear air, light fog and thick fog. The results show that the performance degradation due to thick fog is more than the light fog and very clear air in all turbulence regimes.

A Novel Coherent OCDMA Scheme Over Atmospheric Turbulence Channels

For reducing the impairment from the turbulence-induced scintillation and improving the receiver sensitivity, a novel coherent optical code division multiple access (OCDMA) system for free space optical (FSO) communication is proposed. Based on the scheme designed, the laser is modulated by phase shift keying and demodulated without the help of local laser. The error probability expression over atmospheric channels is derived. Simulation results show the effectiveness of the proposed scheme, and the performance of the novel proposed scheme is much better than that of the traditional coherent OCDMA system in overcoming the phase fluctuation. Therefore, the scheme is more suitable for multi-user access FSO communication in atmospheric turbulence channel.