Performance analysis and simulation of OCDMA system based on FBGs encoder/decoder

International audience

A parallel combinatory (PC)-hybrid pulse interval modulation-optical code division multiple access (hPIM-OCDMA) system is proposed. In the PC-hPIM-OCDMA system, a user chooses e among K code sequences assigned to the user and sends e2b-ary PIM symbols for a given frame, where b is the number of bits per PIM symbol. The PC-hPIM-OCDMA system can offer higher bit rate than hPIM-OCDMA systems, and then it can reduce the effect of MAI, without reducing the bit rate. The bit error rate (BER) of PC-hPIM-OCDMA is derived, by considering that the dominant interference is MAI, which is caused by cross-correlation of strict optical orthogonal codes (SOOCs). Numerical results show that the PC-hPIM-OCDMA system can offer lower BER and accommodate more users than hPIM-OCDMA systems.

This paper presents recent proposals that have been made in the field of optical pulse shaping techniques based on the space-time duality, with an especial attention to their application for the design of encoder/decoder pairs in all-fibre Optical Code Division Multiple Access (OCDMA) systems. Two different proposals are presented, one based on an Optical Fourier Transformer using a dispersive media and the other based on the temporal self-imaging effect. Fiber Bragg Gratings are used as dispersive elements and electro-optic modulators for the amplitude or phase coding, allowing the electrical reconfiguration of the codes. Experimental results, both for coherent and incoherent schemes, are shown for the two systems transmitting at data rates of 1.25 Gbps and 10 Gbps respectively with a fixed codification rate of 10 Gcps and using a subset of the Hadamard codes with a length of 8 chips.

In this paper, three code families for spectral amplitude coding optical code division multiple access (SAC-OCDMA) systems in terms of performance are compared to each other under the impact of group velocity dispersion (GVD). The investigated code families are m-sequence, Walsh-Hadamard and modified quadratic congruence (MQC). The performance of these codes is evaluated for three types of transmission media such as single-mode fiber (SMF), dispersion shifted fiber (DSF) and nonzero dispersion shifted fiber (NZ-DSF). Results show that in each of the three transmission media m-sequence code family offers the best performance among the investigated code sets under the impact of GVD. Systems which employ m-sequence codes can achieve longer transmission length compared to those using Walsh-Hadamard and MQC codes. In addition, a chirped fiber Bragg grating (CFBG) is designed as a compensating element to compensate the impact of GVD in a SAC-OCDMA system. Employing the designed grating in the system results in a wide opening in the eye diagram as well as improved bit error rate (BER) of the system which indicate that the impact of GVD is effectively compensated.

We proposed the multipolar inserting zeros (MIZ) code for direct-sequence optical code division multiple access (OCDMA) systems. This code is designed to correspond with the judgment based on linear convolution. Simulation results showed that it can provide a lower multiuser interference than the conventional Gold sequence. With the same number of users, the bit error rate in the OCDMA system coded by MIZ can be reduced by several decuples than that coded by Gold, and the source power needed to reach a BER of 1×10?9 can also be reduced by several decibels. So the performance of OCDMA can be improved.

In this paper, we analyze the bit error rate (BER) performance of synchronous optical code division multiple access (OCDMA) system using an optical zero-correlation zone (ZCZ) sequence set over diffuse link by computer simulation, and evaluate the influence of inter-symbol interference (ISI) on the system. The optical ZCZ sequence set is a set of pairs of binary and bi-phase sequences with zero-correlation zone. As a result, the ISI can suppress by extension of zero-correlation zone.

A superstructured fibre Bragg grating (SSFBG) decorrelator for rejecting multi-user interference (MUI) in a synchronous optical code division multiple access (OCDMA) system is proposed and investigated. The decorrelator is formed by appropriately defining the superstructure of a weak fibre Bragg grating. Theoretical results arrived at using coupled mode theory show that it is possible to make the MUI practically zero at the sampling instant provided there is perfect bit synchronisation among the users accessing the OCDMA system. A theoretical investigation on the degradation of optical signal to interference ratio (OSIR) is also conducted by statistically modelling the jitter likely to result in the bit synchronisation mechanism of the system.

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.

In this paper we introduce a successive interference cancellation (SIC) scheme for direct sequence optical code division multiple access (DS-OCDMA) systems using pulse position modulation (PPM). Considering double-padded modified prime code (DPMPC) as a signature sequence code, results show that the system has better performance in terms of both capacity and bit error rate (BER) as compared to the one without cancellation scheme, where the system with SIC scheme can support up to 88 users while the system without SIC scheme can support only 38 users at similar BER=10

The performance of a coherent direct sequence optical code division multiple access (DS-OCDMA) system using quadratic congruence codes implemented with superstructured fiber Bragg gratings is experimentally analyzed. We point out that nonperiodic and sparse quadratic congruence codes reduce multipath beat noise and intersymbol interference, respectively. These two properties of the structure of the quadratic congruence codes are shown to improve the bit-error-rate (BER) performance of the coherent system. As a result, we demonstrate that the performance enhancement of the coherent DS-OCDMA system allows the data bit rate of the system to be varied from 1 Gbit/sto2.5 Gbits/s and the receiver bandwidth to be decreased from 15 GHzto5 GHz respecting the low-cost requirement of the optical access network. These data bit rate and receiver bandwidth performances are achieved with moderate BER penalty.

In this paper, we introduce the optimization of bit error rate (BER) in parallel cancellation of multiple access interference (PIC) using the bias compensation with a direct sequence unipolar optical code division multiple access (DS-OCDMA) system. This receiver is dedicated to mitigate Multi access Interference (MAI). The principle of our proposed receiver consists in reducing the output error by amplification of the interfering signals. We develop the PIC compensated theoretical error probability expression. The performance of such a technique is analyzed in a synchronous network using orthogonal optical codes (OOCs) and the results are compared with those for different receivers.

Analysis of electrically configurable spectral phase encoding techniques for optical CDMA

In this paper, a theoretical analysis is presented to evaluate the influence of polarization mode dispersion (PMD) on the bit error rate (BER) performance of direct sequence optical code division multiple access system. In this analysis, intensity modulation direct detection technique is employed in single mode fiber operating at 1550nm, and optical orthogonal codes are used as address sequence. Optoelectronic conversion is performed by an avalanche photodiode in an optical correlator receiver. The system BER performance is determined on account of receiver, optical amplifier, and multiuser access interference noises. The power penalty suffered by the system is evaluated at BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> as a function of number of simultaneous users, chip rates, and PMD co-efficient. The system performance is found to degrade more at higher chip rate, and longer fiber length due to the PMD.

This paper presents the Direct Sequence Optical Code Division Multiple Access (DS-OCDMA) system using two configurations of the optical source. To encode/decode transmitted data, we used Superstructured Fiber Bragg Grating (S-FBG) as encoders/decoders. We consider three unipolar family codes which are the Prime Sequence (PS), Quadratic Congruence (QC) and the Extended Quadratic Codes (EQC) codes. In order to evaluate the performances of our system in term of Bit Error Rate (BER), we implement the Importance Sampling (IS) technique, which is a variant of the wellknown Monte-Carlo (MC) method. Our simulation results depict that EQC codes outperform QC and PS codes for the DS-OCDMA system using either coherent or incoherent source. We show also that using EQC codes with increasing the optical bandwidth and maintaining the electrical filter leads to improve the performance of incoherent system.

Intensity noise (IN) or phase induced intensity noise (PIIN) is the key limiting factor of the performance in spectral amplitude coding optical code division multiple access (SAC OCDMA) systems. The PIIN can be mitigated in SAC OCDMA by splitting the signals and detecting each signal by a different photodiode if the independence between the split signals can be achieved. In this paper, we derive the upper and lower bounds for the bit error rate (BER) for a system that implements multiple photodiode structured balanced detectors. Additionally, a simulation using Optisystem is conducted to compare the performance of the conventional balanced detector and an ideal multi photodiode balanced detector.