Direct Sequence Optical Code Division Multiple Access Research Articles

Performance improvements of a VLC system, in a V2X context, using a different multiplexing technique

This paper scrutinizes visible light communication (VLC) employing hybrid optical time division multiplexing (OTDM) wavelength division multiplexing (WDM). It also demonstrates the simulation of VLC using the direct sequence optical code division multiple access (DS-OCDMA) system developed by OptiSystem-Matlab Co-simulation. The profits of WDM and OTDM capacities have been combined using two levels of freedom to build up of the system. The performances are demonstrated in the simulated findings of the bit error rate (BER) in relationship with the data rate and the number of users. First, the simulated results display the usefulness of the system. Then, the number of users has been significantly enhanced. Besides, the OCDMA functions by appointing a specific code to each user and could provide asynchronous and simultaneous access to numerous users. One drawback is multiple access interference (MAI). The use of very long code sequences is crucial in order to lessen this effect. Hence, very large bandwidths are essential. This study aims at demonstrating, through system simulation, the practicability of an all-optical conventional correlation receiver dedicated to a DS-OCDMA access network link. Again, OptiSystem has been used to realize our blocks.

Optimal Path Configuration with Coded Laser Pilots for Charging Electric Vehicles Using High Intensity Laser Power Beams

Wireless power transmission (WPT) for wireless charging has been gaining wide attention as a promising approach to miniaturizing the battery size and increasing the maximal total range of an electric vehicle (EV). With an appropriate charging infrastructure, WPT holds great potential to accelerate the acceptance of EVs through users’ higher satisfaction, reducing EV cost, and increasing the driving range and capability. A WPT system based on high-intensity laser power beaming (HILPB) provides an optimal solution for wirelessly charging electric vehicles from a distance of several meters. Despite a large number of WPT approaches, the problem of optimal path configuration for charging EV remains an unexplored area. This paper proposes a method to determine the optimal power transmission path in environments where multiple power transmitters (PTXs) and power receivers (PRXs) are operated simultaneously. To this end, we modeled the HILPB power that reaches a PRX equipped with a photovoltaic (PV) array and validated the model by simulating the WPT process in an environment with multiple PTXs and PRXs using a direct-sequence optical code division multiple access (DS-OCDMA) system. In the simulation environment, upon receiving a request from a PRX, a PTX sent its power channel information through optically encoded laser pulses using each available wireless power channel (WPC). The PRX calculated the maximum deliverable power of a PTX and WPC based on the received channel power indicator of the incident laser beam. Based on the calculation results, it selected the optimal PTX and WPC for its maximum power requirement (MPQ). The MPQ of each PRX was satisfied by applying the algorithm for selecting the PTX according to the alignment and characteristics of the PTXs and PRXs. We modeled a power reception model of the PRX based on a PV array using coded laser pilots and validated it through experimentation. We discussed some algorithms that select the most suitable PTX among several PTXs for which several EVs receive the power it needs.

LIDAR pulse coding for high resolution range imaging at improved refresh rate.

In this study, a light detection and ranging system (LIDAR) was designed that codes pixel location information in its laser pulses using the direct- sequence optical code division multiple access (DS-OCDMA) method in conjunction with a scanning-based microelectromechanical system (MEMS) mirror. This LIDAR can constantly measure the distance without idle listening time for the return of reflected waves because its laser pulses include pixel location information encoded by applying the DS-OCDMA. Therefore, this emits in each bearing direction without waiting for the reflected wave to return. The MEMS mirror is used to deflect and steer the coded laser pulses in the desired bearing direction. The receiver digitizes the received reflected pulses using a low-temperature-grown (LTG) indium gallium arsenide (InGaAs) based photoconductive antenna (PCA) and the time-to-digital converter (TDC) and demodulates them using the DS-OCDMA. When all of the reflected waves corresponding to the pixels forming a range image are received, the proposed LIDAR generates a point cloud based on the time-of-flight (ToF) of each reflected wave. The results of simulations performed on the proposed LIDAR are compared with simulations of existing LIDARs.

레이저 펄스 부호화를 이용한 원거리 고해상도 3D 스캐닝 라이다

본 논문에서는 DS-OCDMA(direct sequence optical code division multiple access)와 스캐닝 방식의 MEMS (microelectromechanical system) 거울을 이용하여 픽셀별로 스캐닝하는 라이다 시스템(light detection and ranging, LIDAR)의 설계와 시뮬레이션 결과를 기술한다. 제안하는 라이다는 <TEX>$848{\times}480$</TEX> 해상도의 거리 영상을 1초에 60번 측정한다. 영상을 구성하는 각각의 픽셀마다 픽셀 정보와 체크섬을 DS-OCDMA로 부호화한 레이저 펄스로 방출하므로, 반사파를 검출하기 위하여 대기할 필요없이 연속으로 거리 측정이 가능하다. MEMS 거울은 부호화된 레이저 펄스를 반사하여 측정을 원하는 방향으로 보내기 위한 용도로 사용한다. 하나의 거리 영상을 구성하는 픽셀 정보의 처리가 모두 완료되면, 픽셀 개개의 반사파 비행시간을 이용하여 포인트 클라우드를 생성한다. This paper presents the design and simulation of a three-dimensional pixel-by-pixel scanning light detection and ranging (LIDAR) system with a microelectromechanical system (MEMS) scanning mirror and direct sequence optical code division multiple access (DS-OCDMA) techniques. It measures a frame with <TEX>$848{\times}480$</TEX> pixels at a refresh rate of 60 fps. The emitted laser pulse waves of each pixel are coded with DS-OCDMA techniques. The coded laser pulse waves include the pixel's position in the frame, and a checksum. The LIDAR emits the coded laser pulse waves periodically, without idle listening time to receive returning light at the receiver. The MEMS scanning mirror is used to deflect and steer the coded laser pulse waves to a specific target point. When all the pixels in a frame have been processed, the travel time is used by the pixel-by-pixel scanning LIDAR to generate point cloud data as the measured result.

Successive Interference Cancellation for DS-Optical PPM-CDMA Systems

AbstractIn 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

Impact of four wave mixing on the performance of optical CDMA transmission system

A theoretical analysis is presented to evaluate the bit error rate (BER) performance of a direct sequence optical code division multiple access (DS-OCDMA) system with dispersion shifted fiber (DSF) considering the effect of four wave mixing (FWM). To verify the analytical expression of BER, the BER performance is evaluated using Matlab simulation with 63 chips per bit m-sequence as well as Gold sequence and compared with analytical results. It is investigated that simulation and analytical results are similar in all cases. Further, BER performance is evaluated for 1023 chips per bit encoding using analytical expression. The BER performance is found to be degraded due to FWM effect if the transmitted power per channel and number of simultaneous users are increased. It is investigated that maximum allowable transmitted power per channel is limited due to FWM effect. It is found that the maximum allowable transmitted power per channel is limited to 15dB for 10 users and 20dB for 20 users for a 100Mb/s, 1000km system with 1023 chips per bit encoding at a BER of 10–9.

Penalised and doubly‐penalised parallel/successive interference cancellation multi‐user detectors for asynchronous upstream optical code division multiple access passive optical network

Optical code division multiple access (OCDMA) is one of the emerging technologies for next generation passive optical networks that can offer customers a Gb/s-class experience in the upstream. However, OCDMA suffers from multiple access interference (MAI) that significantly reduces the capacity of the system. To mitigate MAI, three novel interference cancellation (IC) techniques have been proposed for incoherent direct sequence OCDMA systems that make use of some prior information to enhance their performance. Some of the IC structures exploit the non-negativity of the solution, while others exploit in addition to the non-negativity information the noise variance information. The proposed IC detectors outperform the conventional correlation receiver, the decorrelator detector, the linear parallel/successive interference cancellation detectors and even the linear minimum mean square error detector.

New code for spectral-amplitude coding optical code-division multiple-access system using avalanche and phase intensity–induced noise photodiodes

A new code for a spectral-amplitude coding optical code-division multiple-access (OCDMA) system called the random diagonal (RD) code is proposed. This code is constructed using code segment and data segment. One of the important properties of this code is that the cross correlation at data segment is always zero, which means that phase intensity–induced noise (PIIN) is reduced. The performances of a RD realistic optical direct-sequence OCDMA network have been evaluated with PIIN and avalanche photodiode (APD) receivers. The comparison carried out included realistic system parameters, such as chromatic dispersion, multiple-access interference (MAI), receiver noises (dark current shot noise, and receiver thermal noise), and photodiode bandwidth. We analyze the error probability of the network taking into account these parameters. Bit-error rate (BER) performance is compared to Hadamard and modified frequency hopping (MFH) codes. It is shown that the system using this new code matrix not only suppresses PIIN, but also allows a larger number of active users compared to other codes. Simulation results show that using point-to-point transmission with three encoded channels, RD code using APD has better BER performance than PIN photodiode. Also, it is found that at 20 km the BER is 1.28×10−20 and 6.26×10−13 for APD and PIN photodiodes, respectively.

Performance analysis of quadratic congruence codes using superstructured fiber Bragg gratings for a flexible data rate coherent DS-OCDMA system

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.

Design of novel DS-OCDMA encoders/decoders using phase sampled long period grating

A novel long period grating structure with phase sampled technique is proposed to act as encoders/decoders in the direct sequence optical code division multiple access (DS-OCDMA) system. The structure with address code of 31 chips is designed and the relation between the sample period, the input pulse width and the refractive index difference is given. The encoded and decoded results are demonstrated to verify the feasibility of utilizing the coupling ratio to the cladding mode. The whole structure is expected to move on the photonic crystal fiber, for it can support the large refractive index difference between the core mode and cladding mode, as well as minimize the wavelength shift caused by the temperature change.

Impact of encoder-decoder wavelength mismatch on the performance of direct-sequence optical code-division multiple-access system

The impact on the performance of the direct-sequence optical code-division multiple-access (DS-OCDMA) system due to wavelength mismatch between encoder and decoder is investigated. Simulation results show that the encoder-decoder wavelength mismatch will degrade the bit-error-rate performance of the DS-OCDMA system drastically by decreasing the contrast ratio of the correctly and incorrectly decoded signals and lower data rate systems are more vulnerable to wavelength mismatch than higher data rate systems. In practical DS-OCDMA systems, the wavelength mismatch between encoder and decoder should be minimized, and some protective measures, such as temperature stabilization and fiber Bragg grating (FBG) packaging, should be taken to enhance the immunity of the superstructured FBG encoder/decoder to ambient conditions.

Source coherence impairments in a direct detection direct sequence optical code-division multiple-access system

We demonstrate that direct sequence optical code- division multiple-access (DS-OCDMA) encoders and decoders using sampled fiber Bragg gratings (S-FBGs) behave as multipath interferometers. In that case, chip pulses of the prime sequence codes generated by spreading in time-coherent data pulses can result from multiple reflections in the interferometers that can superimpose within a chip time duration. We show that the autocorrelation function has to be considered as the sum of complex amplitudes of the combined chip as the laser source coherence time is much greater than the integration time of the photodetector. To reduce the sensitivity of the DS-OCDMA system to the coherence time of the laser source, we analyze the use of sparse and nonperiodic quadratic congruence and extended quadratic congruence codes.

DS-OCDMA receivers based on parallel interference cancellation and hard limiters

In an incoherent direct-sequence optical code-division multiple-access (DS-OCDMA) system, multiple-access interference (MAI) is one of the principal limitations. To mitigate MAI, the parallel interference cancellation (PIC) technique can be used to remove nondesired users' contribution. In this paper, we study four DS-OCDMA receivers based on the PIC technique with hard limiters placed before the nondesired users or before the desired user receiver, or both. We develop, for the ideal synchronous case, the theoretical upper bound of the error probability for the four receivers. Significant performance improvement is obtained by comparison with conventional receivers in the case of optical orthogonal codes. The paper highlights that the number of active users with null error probability is doubled, compared with conventional receivers. Finally, we show that, thanks to their good performances, the PIC structures permit considerably reducing the minimal code length required to have 30 users with bit-error rate <10 -9. So, the hardware constraints are relaxed for realistic application

DS-OCDMA encoder/decoder performance analysis using optical low-coherence reflectometry

Direct-sequence optical code-division multiple-access (DS-OCDMA) encoder/decoder based on sampled fiber Bragg gratings (S-FBGs) is characterized using phase-sensitive optical low-coherence reflectometry (OLCR). The OLCR technique allows localized measurements of FBG wavelength and physical length inside one S-FBG. This paper shows how the discrepancies between specifications and measurements of the different FBGs have some impact on spectral and temporal pulse responses of the OCDMA encoder/decoder. The FBG physical lengths lower than the specified ones are shown to affect the mean optical power reflected by the OCDMA encoder/decoder. The FBG wavelengths that are detuned from each other induce some modulations of S-FBG reflectivity resulting in encoder/decoder sensitivity to laser wavelength drift of the OCDMA system. Finally, highlighted by this OLCR study, some solutions to overcome limitations in performance with the S-FBG technology are suggested

Spectral efficiency of OCDMA systems with coherent pulsed sources

We present a model to evaluate the upper limit of the spectral efficiency of optical code-division multiple-access (OCDMA) systems with coherent sources. Phase-encoded and direct-sequence OCDMA systems are evaluated using this model. The results show that a spectral efficiency of 2.24/spl times/10/sup -2/ b/s/spl middot/ Hz can be achieved with a maximum bit error rate of 10/sup -10/ in these systems of the number of users. This result demonstrates that the maximum spectral efficiency of OCDMA systems with coherent sources is at least a factor of 5 higher than OCDMA systems with incoherent sources.

Reduction of interchannel interference noise in a two-channel grating-based OCDMA system using a nonlinear optical loop mirror

We show that a nonlinear optical switch can be used to suppress the interchannel noise generated under multiuser operation within a coherent, direct-sequence optical code-division multiple-access (OCDMA) system. By incorporating a simple nonlinear optical loop mirror (NOLM) within the receiver, we demonstrate a 3.6-dB power penalty reduction in a two-channel 1.25-Gb/s 64-chip 160-Gchip/s grating-based direct-sequence OCDMA system. Even greater improvements in system performance were obtained at a data rate of 2.5 Gb/s, where the noise due to the overlap of adjacent decoded data bits also needs to be suppressed. In both instances, the system performance under two-channel operation with nonlinear filtering was shown to be comparable to that achieved under single-channel operation using the conventional matched-filter approach.

An ATM cross-connecting node using optical CDMA

An asynchronous transfer mode (ATM) cross-connecting node is proposed based on direct sequence optical code division multiple access (DS-OCDMA). No buffering is necessary to facilitate switching because code conversion is instead emulating the switching function. The switch is also capable of limited dynamic bandwidth allocation. It is free from timing jitters and switching delay is significantly reduced. The switch provides a new approach to asynchronous cross-connection in the ATM core network, locally or over a wider area. The performance with the new proposed switch is evaluated using prime and modified prime OCDMA codes.