Performance Improvement of Optical Pulse Position Modulation Code Division Multiple Access using Successive Interference Cancellation Technique

Multiplexing techniques are suffering from major problem known as multiple access interference (MAI). A rejection interference called successive interference cancellation (SIC) is employed in optical code division multiple access (Optical CDMA), which has the potential to reduce the effect of MAI and therefore can increase the capacity of optical CDMA systems and improve its performance. The basic principle of SIC scheme is that, detection is carried out in multiple stages and during each stage, only the most reliable symbols are selected. In subsequent stages, the interference contributed by the already detected symbols will be removed. In this project we theoretically analyzed direct sequence (DS) optical CDMA system based on SIC scheme using pulse position modulation (PPM) signaling, considering the interference between the users, imperfection cancellation occurred during the cancellation process and receiver noises. In addition, balanced detection scheme known as spectral amplitude coding (SAC) is used to suppress the overlapping between the users and reduce the receiver noises effect

Suppression Of Four Wave Mixing Impairment In Hybrid Optical Code Division Multiple Access And Dense Wavelength Division Multiplexing System

Improving the performance and capacity of optical networks is an on-going research goal. Two technologies which have been the subject of much research are dense wavelength-division multiplexing (DWDM) and optical code-division multiple access (OCDMA). Although the OCDMA system features distinct advantages over the DWDM system, each system suffers from significant limitations. Of the most serious of these limitations for DWDM is inter and intra-channel four-wave mixing (FWM), while OCDMA suffers from multiple access interference (MAI) and intersymbol interference (ISI). We propose and analyse the effectiveness of an optical hybrid system, which combines the OCDMA and the DWDM technologies, to increase system capacity, and in conjunction with an electro optic phase modulator (EOPM), simultaneously overcome the most serious limitation of the DWDM system. With this hybrid system, each wavelength/channel has the ability to carry N number of OCDMA users; hence, the capacity of the system is multiplied by a factor of N. The system is examined using both theoretical and simulation analysis with and without EOPM at different transmitted powers and different transmission distances based on the RZ-OOK modulation format

Performance enhancement of optical PPM-CDMA using successive interference cancellation scheme

A comprehensive mathematical model of Successive Interference Cancellation (SIC) using Direct Sequence (DS) and Spectral Amplitude Coding (SAC), which is employed in incoherent optical CDMA using On-Off Keying (OOK) Pulse Position Modulation (PPM) and OnOff Keying (OOK) considering the presence of Phase–Induced Intensity Noise (PIIN), shot noise and thermal noise is presented in this work. SIC scheme has the potential to suppress Multiple Access Interference (MAI). The concept of SIC is the following: detection is taken place in successive stages where the most reliable symbols are chosen at each stage. Where the interference contributed by the already detected symbols will be removed. It has been found out that, theoretically, the size of M-ary on optical CDMA system limits the MAI effect. Thus, the system performance improves with the increasing of M-ary PPM. Hence, the system using PPM has better performances if compared to the one using OOK, due to a lower average number of interfering optical pulses. In addition, a lot of users can be accommodated with SIC/SAC-optical CDMA based PPM signaling system instead of the SIC/SAC-optical CDMA based OOK one

Performance Investigation of Hybrid Optical CDMA/ DWDM System Under the Influence of FWM

The significance of nonlinear effects arising from inter- and intra-channel four-wave mixing (FWM) in a hybrid optical code-division multiple-access (OCDMA) and wavelength division multiplexing (WDM) network is thoroughly investigated in this research. The Electro-Optic Phase Modulator (EOPM) module, a crucial component introduced to the system to introduce a phase shift of an ideal value of 2π/3 between adjacent bits, takes center stage. Furthermore, it was discovered that compressing the code sequence into a quarter of the bit duration reduces inter-symbol interference (ISI) and enhances system performance for optical CDMA users. In this investigation, a Multi-diagonal (MD) code is employed as the optical signature sequence code. Even at a high-power level of 18 dBm, the proposed system effectively mitigated the nonlinear effects of inter- and intrachannel FWM. The entire system's performance aligns consistently with both theoretical predictions and simulation results, demonstrating excellent performance for all optical CDMA users

Hybrid optical CDMA and DWDM system implemented under the influence of Non-linear effects

A hybrid optical CDMA-DWDM system accommodating 12 optical CDMA users carried by 5 DWDM wavelengths at a data rate of 60Gb/s/wavelength with channel spacing of 0.4nm is implemented under the effect of four-wave mixing (FWM). It was found that the FWM effect could be minimized by the use of CDMA technology, where the energy of each bit is spread over the optical sequence code. Over a distance of 105.075km, significant performance of all optical CDMA users in terms of the BER is achieved. The results reveal that the inter-symbol interference (ISI) can be mitigated when the interval of optical signature sequence code is squeezed into 25% of the bit duration

Signal Quality Improvement in Optical PPM-CDMA Systems Based SIC Scheme

Optical pulse position modulation-code division multiple access (PPM-CDMA) system with an interference rejection scheme is theoretically analyzed. Successive interference cancellation (SIC) scheme has the ability to suppress the overlapping between the users by subtracting the strongest one from the original received signals one by one until all users detected. Different optical codes have been used for the comparison purpose; results show that the system with SIC scheme using modified quadratic congruence codes has better performance than the system using modified prime code as signature codes. In addition, the system with SIC scheme significantly superior to the one without SIC scheme

Performance Comparison of Different Modulation Formats for a 40 Gbps Hybrid Optical CDMA/DWDM System against ISI and FWM

This paper carries out a numerical simulation investigating three different modulation formats: carrier suppressed return to zero (CSRZ), modified duobinary return to zero (MDRZ) and return to zero (RZ). The purpose of this investigation is to find the optimum modulation format for the hybrid optical code division multiple accesses–dense wavelength division multiplexing (optical CDMA/DWDM) system with the implementation of an electro-optic phase modulator (EOPM) at a data rate of 40 Gbps per channel, with a transmitted power of 22 dBm and transmission distance of 105.075 km. The results revealed that CSRZ was superior to MDRZ and RZ and was more tolerant to optical fiber nonlinearity. Furthermore, unlike the DWDM systems, the performance of the proposed hybrid system based RZ format was better than the performance of MDRZ. Hence, the CSRZ modulation format is the best candidate for the optical CDMA/DWDM with EOPM module due to its high performance

Reduction of Four-Wave Mixing in DWDM System using Electro-Optic Phase Modulator

In this paper, electro-optic phase modulator (EOPM) is used to reduce the effect of four-wave mixing (FWM), which is placed after 64 DWDM-channels multiplexer. It was found that the FWM is very sensitive to the phase deviation of the EOPM, and it can be reduced by introducing a phase shift between pulses. The simulation results confirmed the ability of the EOPM in improving the system performanceas indicated by the bit error rates. In term of comparison, the system of 64 channels based intensity modulated/ direct detection (IM/DD) transmission achieved bit error rate of 10-26 over 30 km and 70km without and with EOPM, respectively

Suppression of inter and intra channel four wave mixing effects in optical CDMA over DWDM hybrid system

In this study, a hybrid system of optical Code Division Multiple Access(optical CDMA) and Dense Wavelength Division Multiplexing(DWDM) is proposed with a comprehensive investigation into the effect of four-wave mixing(FWM). In such system, two major FWM problems exist, inter and intra-channel FWM, including multiple access interference(MAI) and inter-symbol interference(ISI). Results show that the optimum transmitted power is 18dBm in order to control the trade-off between inter and intra-channel FWM, where an increase in the BER of the hybrid system at transmitted power above 18 dBm is indicated. Hence, an electro-optic phase modulator(EOPM) module is proposed and placed after the WDM multiplexer to simultaneously modulate the phase of all wavelengths signals to increase the nonlinear tolerance in the hybrid system by suppressing the impact of intra-channel FWM, which is shown to greatly improve the performance of the optical CDMA-DWDM hybrid system based OOK transmission. In addition, the effect of MAI can be reduced by the use of multi-diagonal(MD) identification sequence code, due to the zero cross-correlation property of MD. The results also reveal that the CDMA technology in conjunction with chromatic dispersion helps to reduce the effect of inter-channel FWM. Moreover, the identification sequence code interval plays crucial role in the mitigating of ISI as the results expose that the best performance of the proposed hybrid system can be achieved when the identification sequence code interval squeezed into 25% of bit duration where the avoidance of ISI is guaranteed

Successive Interference Cancellation for DS-Optical PPM-CDMA Systems

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=10−9. Although the receiver sensitivity is affected by the overlapping between users, which limits the system performance, the theoretical analysis of this issue indicates the influence of the size of M-ary PPM on OCDMA system. Hence, the BER value is still sufficient for good system performance