Multiple-shift code acquisition of optical orthogonal codes in optical CDMA systems

In this paper, we introduce a new and advanced algorithm, namely, multiple-shift algorithm for code acquisition in optical code-division multiple access (CDMA) systems using unipolar optical orthogonal codes (OOCs) as signature sequences. We analyze the performance of the newly proposed algorithm and obtain a bound on its performance and show its advantage in reducing the mean time of synchronization when compared with other synchronization methods. The algorithm can be used with many different receiver structures, like active or passive correlator with or without hardlimiter(s). However, in this paper, we only consider the simple active correlator structure for further discussions and analysis.     

An OCDMA Scheme to Reduce Multiple Access Interference and Enhance Performance for Optical Subscriber Access Networks

We propose a new optical code division multiple access (OCDMA) scheme for reducing multiple access interference (MAI) and enhancing performance for optical subscriber access networks using modified pseudorandom noise (PN)‐coded fiber Bragg gratings with bipolar OCDMA decoders. Through the bipolar OCDMA decoder and the modified PN codes, MAI among users is effectively depressed. As the data are encoded either by a unipolar signature sequence of the modified PN code or its complement according to whether the data bit is 1 or 0, the bit error ratio (BER) can be more improved with the same signal to interference plus noise ratio over the conventional on‐off shift keying‐based OCDMA system. We prove by numerical analysis that the BER of the proposed bipolar OCDMA system is better than the conventional unipolar OCDMA system. We also analyze the spectral power distortion effects of the broadband light source.     

Atmospheric Optical CDMA Communication Systems via Optical Orthogonal Codes

We propose and consider using a class of multiple-access sequences, namely, optical orthogonal codes (OOCs) in atmospheric optical code-division multiple-access systems. We obtain analytical solutions to the error probability for various channel models using positive-intrinsic-negative diode and avalanche photodiode photodetectors. In our analysis, the effects of atmospheric turbulence, ambient light, thermal noise, and multiuser interference are considered, in the context of a semiclassical photon-counting approach. The performance of the systems taking advantage of space diversity and error-correcting codes are also evaluated. Two common and widely used optical modulations, on–off keying and pulse-position modulation, are considered. Receiver structures based on correlator and chip level are used for OOC detection. Unlike the traditional chip-level receiver, here a generalized form of chip-level structure with two threshold levels is considered. Upper and lower bounds on the error probability for the above-chip-level receiver structure is obtained. From our analytical results, we can deduce that the chip-level receiver outperforms a simple correlator in the absence or weak atmospheric fading; however, in a strong fading environment, the simple correlator outperforms the chip-level receiver.     

完全互补OCDMA系统编解码方案与系统设计

对双极性光码分多址(OCDMA)系统采用码的单/双极性转换思路,将相关性能更优秀的双极性码应用于传统单极性OCDMA信道中,提出了一种采用双极性完全互补序列的OCDMA编解码方案及系统模型.仿真结果表明,完全互补的OC-DMA系统与基于Gold序列的OCDMA系统性能相比有较大的优势.采用完全互补序列作为地址码,可以有效降低OCD-MA系统中的多址干扰,提高通信质量.     

Optical CDMA Passive Optical Network Using Prime Code With Interference Elimination

One novel decoding scheme of prime codes for interference elimination is proposed for optical code-division multiple-access network. This scheme uses the correlation property of prime codes for interference elimination without additional processing, and thus simplifies the coder implementation. By taking advantage of the cyclic property of the prime codes in the same code groups, the proposed compact encoder is low cost and suitable for the application of passive optical network (PON). In addition, the information encoding of each optical network unit (ONU) can adopt two-code keying using a simplified ONU decoder for performance enhancement. Thus, this is an economical solution for end users in the PON     

Optical CDMA network codecs with merged-M-coded wavelength-hopping and prime-coded time-spreading

This paper proposes a new hybrid code for optical CDMA systems based on modified maximal-length sequence (M-sequence) codes for wavelength-hopping and prime codes for time spreading. Using the proposed hybrid code, two optical encoder/decoders are constructed using either fiber-Bragg gratings (FBGs) and optical delay lines, or arrayed-waveguide gratings (AWGs) and optical delay lines, respectively. The system performance is compared with that provided by the well-known prime-hop sequence code. The results show that the merged-M-sequence codes not only provide a higher capacity, but can also be realized via a simpler configuration. Furthermore, the proposed code is less expensive than prime-hop code under virtually identical BER conditions.     

An SNR improvement of passive SAW tags with 5-bit Barker code sequence

Passive surface acoustic wave (SAW) tags require a large signal-to-noise ratio (SNR) in order to increase the interrogation range. For the purpose of achieving high SNR for radio frequency identification (RFID) communication systems, Barker codes, a binary phase shift keying (BPSK) modulation technique, have been adopted in this study. Passive SAW RFID tags were designed with 5-bit Barker code sequences to generate BPSK modulated signals. Through the SNR analysis, the improvements in SNR were about 11?dB using Barker codes along with a correlator, which can be further improved by optimisation in the correlator.     

Direct-detection optical synchronous CDMA systems with doubleoptical hard-limiters using modified prime sequence codes

An optical code-division multiple-access (CDMA) system with double optical hard-limiters is proposed where the optical hard-limiters are placed before and after an optical correlator. Moreover, the-effect of the optical hard-limiter on the performance of the optical synchronous CDMA systems using modified prime sequence codes as signature codes is analyzed under the assumption of a Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists. We evaluate the performance under average power and bit rate constraints. Our results show that using the single optical hard-limiter slightly degrades the performance of the optical CDMA systems under the assumption of Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists. Moreover, we show that the optical CDMA systems with double optical hard-limiters have better performance than other conventional CDMA systems with and without the optical hard-limiter when the number of simultaneous users is not so large     

高效光电混合的联合变换相关器作多目标检测

提出一种使用高效光电混合的联合变换相关器作多目标检测的方法。这一方法将联台功率谱复制N份构成N个全同的联合功率谱列阵，可以更有效地利用空间光调制器的面积以及读出无效率，因而在输出面可获得增强的光学相关信号。     

Optical spectral amplitude CDMA communication

We report the first demonstration of bipolar coding techniques in the optical spectral domain for incoherent optical code division multiple access (CDMA) communication. Based on the modulation and detection principles that we have developed, the power spectrum of an erbium-doped superfluorescent fiber source was encoded using bipolar codes and decoded using an optical bipolar correlator. A CDMA testbed consisting of two encoders and one decoder was implemented with bulk optics and free-space transmission. Our measurements verify the correlations between the bipolar codewords and demonstrate the rejection of multiple access interference     

Realization of OSW/AWG-based bipolar wavelength–time optical CDMA for wired–wireless transmissions

This study proposes a novel radio-over-fiber (RoF) system using two-dimensional (2-D) optical code-division multiple-access (OCDMA) scheme using pseudorandom (PN) codes for the time-spreading and wavelength-hopping (t-spreading/λ-hopping) codes. The 2-D system is implemented using optical switches (OSWs) and arrayed-waveguide grating (AWG) routers. By constructing 2-D codes using bipolar PN codes rather than unipolar codes provides a significant increase in the maximum permissible number of active radio base stations (RBSs). In general, the phase-induced intensity noise (PIIN) generated at high optical intensities significantly degrades the performance of a conventional multi-wavelength scheme. However, the OSW-based time-spreading method employed in the current 2-D OCDMA scheme effectively suppresses the PIIN effect. Additionally, multiple-access interference (MAI) is suppressed by the use of a wavelength/time balanced detector structure in the network receivers. The numerical evaluation results demonstrate that under PIIN- and MAI-limited conditions, the proposed system outperforms a conventional multi-wavelength OCDMA scheme by using the spectral spreading scheme to suppress beating noise. Especially, the t-spreading encoder/decoder (codec) groups share the same wavelength codec and the overall complexity is reduced and system network becomes more compact.     

All-optical CDMA with bipolar codes

A method for the transmission and detection of bipolar sequences in a unipolar system is presented. It allows all-optical implementation, in noncoherent optical CDMA systems, of the bipolar codes that have been developed for the radio domain. A practical design is described that encodes the spectrum of a broadband optical source to support a large number of subscribers     

New unipolar codes allowing electrooptical correlation utilizing a semiconductor laser amplifier

This letter reports on the development of a new code allowing the use of a semiconductor laser amplifier (SLA) to execute correlation in the optical domain. Semiconductor laser amplifiers act as AND gates and can only correlate codes in the unipolar domain and therefore have been limited by the lack of unipolar codes with good autocorrelation properties. The authors propose a new unipolar code derived from bipolar Golay codes which has a high probe duty cycle (50%) and is ideal for use with an SLA based correlator. Computer modeling of an SLA correlator used to interrogate a sensor network is described, indicating crosstalk levels of less than 0.7%.     

光码分多址系统中双极性码的应用研究

基带双极性光码分多址(OCDMA)系统，采用码的极性转换思想，实现在考虑信道非理想下的单极性信道中传输双极性码。研究了Gold序列，m序列在系统中的应用和传输信道对光码分多址系统性能的影响。仿真得到了双极性码在光码分多址系统的自相关和互相关特性。基于不同码字的光码分多址系统用户数与误码率的关系和基于非理想传输信道的系统性能影响曲线。结果表明，Gold序列和m序列适合双极性光码分多址系统，但Gold序列较m序列更适合应用到双极性光码分多址系统，当系统激活用户数为20，系统误码率分别达到了10^-9，10^-7，非理想传输信道对系统性能有较大的影响(恶化近20dB)，在系统研究中不可忽略。     

二维光码分多址的地址码及干扰估计接收机

阐述了自相关旁瓣为0，互相关值小于等于1的二维光正交码(2D-OOC)的新构造方法及非同步频域跳频时域扩频二维光码分多址(2D-OCDMA)系统中的干扰估计接收机模型。二维光正交码的新构造方法以两个单极性码分别作为频域跳频模式和时域扩频模式。用作频域跳频模式的地址码是自相关旁瓣及互相关值小于等于1的一维光正交码(1D-OOC)，用作时域扩频模式的地址码其码长为素数或码长与码中任意两个非零码元的位置差值互质。将一个没有分配给任何用户并且和接收用户地址码正交的地址码用作干扰估计码来得到多用户干扰(MUI)估计值，从而得到最佳的判决门限。系统误码率(BER)的分析显示干扰估计接收机系统的性能要优于传统的接收机系统。     

实时自适应阈值二值化联合变换相关器

提出了采用参考图像功率谱为阈值的自适应阈值二值化联合变换相关器 ( BJTC)的方法。和传统的联合变换相关器结果比较 ,自适应阈值法对于输出的相关峰值、信噪比等主要参数都有明显提高。对这种方法进行了理论分析和计算机模拟 ,并给出了光学实验结果 ,为自适应阈值二值化联合变换相关器的实用化奠定了基础。     

数字图像处理在光学相关识别中的应用

光学模式识别信息处理容量大,运算速度快,但精度不高,存储不灵活,不易判断控制和分析.联合变换相关器在光学图像相关识别中得到了广泛的应用.为提高联合变换相关器的性能,将数字图像处理技术引入光学相关识别中,主要体现在对联合图像进行预处理以及对联合功率谱的滤波处理.详细阐述了国内外常用的联合图像预处理算法和功率谱滤波算法,并对这些算法在光学相关识别中的应用效果进行了比较研究.     

New architecture for incoherent optical CDMA to achieve bipolarcapacity

A simple and efficient method for using bipolar codes in noncoherent optical code-division multiple access systems is proposed. The proposed system requires only incoherent optical delay lines and direct detection receivers. Practical architectures for the encoder/decoder are given. The Letter shows that combining Gold codes of period M=127 and Barker code of period K=4 into composite sequences of period N=508 allows 20 simultaneous active users for P_e=10 ^-9, while requiring only four nonprogrammable delay lines     

Two-dimensional optical orthogonal codes for fiber-optic CDMA networks

Several constructions of two-dimensional (2-D) codes have been proposed to overcome the drawbacks of nonlinear effects in large spread sequences of one-dimensional (1-D) unipolar codes in fiber-optic code-division multiple-access (FO-CDMA) networks. Wavelength-time (W/T) encoding of the 2-D codes is practical in FO-CDMA networks. W/T codes reported so far can be classified mainly into two types: 1) hybrid codes, where one type of sequence is crossed with another to improve the cardinality and correlation properties and 2) conversion of 1-D sequences to 2-D codes to reduce the "timelike" property. This paper describes the basic principles of a new family of wavelength/time multiple-pulses-per-row (W/T MPR) codes, for incoherent FO-CDMA networks, which have good cardinality, spectral efficiency, and minimal cross correlation values. In addition, an expression for the upper bound on the cardinality of W/T MPR codes is derived. Another feature of the W/T MPR codes is that the aspect ratio can be varied by a tradeoff between wavelength and temporal lengths. The correlation properties of W/T MPR codes are verified by simulation using Matlab. For given wavelength /spl times/ time dimensions, various W/T codes, whose cardinalities are known, are compared, and it is shown that the W/T MPR family of codes have better cardinality and spectral efficiency than the other W/T codes. Performance analysis of the W/T MPR codes and their limiting cases is carried out for various parameter variations such as the dimensions of wavelength, time, and weight of the code.