Codes for spectral-amplitude-coding optical CDMA systems

Codes with ideal in-phase cross correlation (CC) are required in spectral-amplitude-coding optical code-division multiple-access (CDMA) systems since these codes eliminate multiuser interference and also suppress the effect of phase-induced intensity noise. However, little research work has been done on such codes although codes with ideal CC have been studied for many years. In a paper by Zhou et al. (2000), such a code has been introduced, but no clear construction method has been presented. In this paper, we first review the construction method of this old code and then construct two new codes with ideal in-phase CC in algebraic ways. Both of the proposed new codes are obtained by modifying former codes with ideal CC. It has been shown that the system performance can be improved significantly by using codes with ideal in-phase CC instead of the Hadamard code. These codes can also be used in the synchronous optical CDMA systems for multiuser interference cancellation.     

Noncoherent Spectral/Time/Spatial Optical CDMA System Using 3-D Perfect Difference Codes

In this paper, we propose a family of newly constructed codes, named three-dimensional perfect difference codes (3-D PD codes), and a corresponding system structure for spectral/time/spatial optical code division multiple access (OCDMA). The 3-D PD codes, which are generated from the perfect difference set, can further suppress the phase-induced intensity noise (PIIN) and possess the multiuser interference (MUI) cancellation property. Compared with the OCDMA systems using modified quadratic congruence codes (MQC codes), maximal-area matrices codes (M-matrices codes), or 2-D PD codes, the proposed system can effectively suppress the PIIN and eliminate MUI. Hence, the proposed system not only has better performance, but also can accommodate more users.     

Unipolar codes with ideal in-phase cross-correlation for spectral amplitude-coding optical CDMA systems

Unipolar codes with ideal in phase cross-correlation are important in spectral amplitude-coding optical code-division multiple-access (CDMA) systems since these codes eliminate multiuser interference and also suppress the effect of phase-induced intensity noise. However, very little research work has been done on such codes although codes with ideal cross-correlation have been studied for many years. In the paper by Zhou et al.(see Electron. Lett., vol.36, p.728-29, 2000), such a code has been introduced without a clear construction method. In this paper, we firstly review this old code, and then construct two new codes with ideal in-phase cross-correlation in algebraic ways. Both of the proposed new codes are obtained by modifying former codes with ideal cross-correlation. It has been shown that the system performance can be improved significantly by using codes with ideal in-phase cross-correlation instead of the Hadamard code. These codes can also be used in synchronous optical CDMA systems for multiuser interference cancellation.     

Hard-Limiting Performance Analysis of 2-D Optical Codes Under the Chip-Asynchronous Assumption

The traditional chip-synchronous assumption used in the analyses of optical codes in optical code-division multiple access gives a pessimistic performance upper bound, while a more realistic chip-asynchronous assumption gives a more accurate performance. It is also known that a hard-limiter can be placed at the front end of an optical decoder to reduce the effects of multiple-access interference and the near-far problem. In this paper, the "hard-limiting" performance of two-dimensional (2-D) optical codes is analyzed under the chip-asynchronous assumption. We apply a Markov-chain method for a more accurate analysis, which can be generalized to 2-D optical codes with arbitrary maximum cross-correlation values. The performance of 2-D optical codes with the hard-limiting and chip-asynchronous assumptions is also compared with the soft-limiting and chip- synchronous assumptions.     

A non-coherent SAC-OCDMA system using extended quadratic congruence codes for two-code keying scheme in passive optical networks

In this paper, we propose a family of extended quadratic congruence codes for two-code keying (TCK) with the corresponding encoding/decoding architecture for passive optical networks (PONs) in spectral amplitude coding optical code division multiple access (OCDMA) systems. The proposed system can simultaneously eliminate multi-user interference (MUI) and further suppress phase-induced intensity noise (PIIN). We reduce the complexity of the encoding/decoding architecture of the optical line terminal reduced by exploiting arrayed waveguide gratings (AWGs) and the properties of the extended quadratic congruence codes (EQC codes). Moreover, we also design a deployment method to increase the number of simultaneous users. Our numerical results demonstrate that the proposed system outperforms the improved quadratic congruence codes (improved QC codes).     

Noncoherent spatial/spectral optical CDMA system with two-dimensional perfect difference codes

In this paper, the family of newly constructed codes, named 2-D perfect difference codes, is proposed to suppress the phase-induced intensity noise (PIIN) in noncoherent spatial/spectral optical code division multiple access (OCDMA) systems. A novel spatial/spectral transceiver structure employing the new codes to eliminate the multiuser interference (MUI) by using the MUI cancellation property of the new codes is also presented. Compared with the systems employing modified quadratic congruence codes (MQC codes) and maximal-area matrices codes (M-matrices codes), numerical results verify that our proposed system can more effectively suppress the PIIN and eliminate MUI. Hence, the number of simultaneous users and total transmission rate increase significantly.     

A new family of space/wavelength/time spread three-dimensionaloptical code for OCDMA networks

A new family of space/wavelength/time spread three-dimensional (3-D) optical codes for optical code-division multiple-access (OCDMA) networks has been proposed. Two types of 3-D codes have been constructed: 3-D codes with single pulse per plane and 3-D codes with multiple pulses per plane. Both codes are based on the prime sequence algorithm and have shown improved performance compared to the previously proposed two-dimensional (2-D) prime code. Effective implementation of the 3-D code has also been proposed. In order to eliminate the requirement of fiber ribbons and multiple star couplers in space/wavelength/time spread 3-D code based optical networks, a wavelength²/time scheme has been suggested, in which the periodic property of an arrayed waveguide grating (AWG) is used. It has been shown that the system performance can be maximized for given resources with a proper choice of the wavelength²/time scheme. Due to the improved performance of the 3-D code and the effective architecture of the wavelength²/time scheme, the feasibility of the OCDMA network is much enhanced     

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     

Comparison of novel coding techniques for a fixed wavelength hopping SAC-OCDMA

The optical code division multiple access (OCDMA), the most advanced multiple access technology in optical communication has become significant and gaining popularity because of its asynchronous access capability, faster speed, efficiency, security and unlimited bandwidth. Many codes are developed in spectral amplitude coding optical code division multiple access (SAC-OCDMA) with zero or minimum cross-correlation properties to reduce the multiple access interference (MAI) and Phase Induced Intensity Noise (PIIN). This paper compares two novel SAC-OCDMA codes in terms of their performances such as bit error rate (BER), number of active users that is accommodated with minimum cross-correlation property, high data rate that is achievable and the minimum power that the OCDMA system supports to achieve a minimum BER value. One of the proposed novel codes referred in this work as modified random diagonal code (MRDC) possesses cross-correlation between zero to one and the second novel code referred in this work as modified new zero cross-correlation code (MNZCC) possesses cross-correlation zero to further minimize the multiple access interference, which are found to be more scalable compared to the other existing SAC-OCDMA codes. In this work, the proposed MRDC and MNZCC codes are implemented in an optical system using the optisystem version-12 software for the SAC-OCDMA scheme. Simulation results depict that the OCDMA system based on the proposed novel MNZCC code exhibits better performance compared to the MRDC code and former existing SAC-OCDMA codes. The proposed MNZCC code accommodates maximum number of simultaneous users with higher data rate transmission, lower BER and longer traveling distance without any signal quality degradation as compared to the former existing SAC-OCDMA codes.     

Spectral amplitude-coding optical CDMA system using Mach-Zehnder Interferometers

In this paper, we propose a family of newly constructed codes to suppress the phase-induced intensity noise (PIIN) in spectral amplitude-coding (SAC) optical code division multiple access (OCDMA) systems. These new codes are derived from modified prime codes and their cross-correlation is not larger than one. We also present a novel SAC-OCDMA system employing the new codes together with Mach-Zehnder interferometers to eliminate the multi-user interference (MUI). Compared with the systems employing modified quadratic congruence codes (MQC codes), numerical results verify that our proposed system can more effectively suppress the PIIN and eliminate MUI. Hence, the number of simultaneously users and total transmission rate increases significantly.     

Performance comparison of multiwavelength CDMA and WDMA+CDMA forfiber-optic networks

A “multiwavelength” scheme has been proposed to support large numbers of subscribers and simultaneous users in optical code-division multiple-access (CDMA) networks without using very large bandwidth expansion or the need of complicated and not-yet-feasible optical processing. In this paper, multiwavelength optical orthogonal codes (MWOOCs), which consist of two-dimensional codewords (or matrices) with every pulse of a codeword encoded in a distinct wavelength, are constructed for this scheme. MWOOCs have larger cardinality than the one-dimensional codes used in the hybrid wavelength-division multiple-access (WDMA) and CDMA scheme. With the same hardware configuration, our analysis shows that the multiwavelength scheme, in general, performs better than the hybrid scheme, particularly when the traffic load is heavy. However, if a central controller (i.e., under the best scenario) is used to uniformly distribute all available wavelengths to simultaneous users in the hybrid scheme, both schemes have comparable performance for a medium traffic load and the hybrid scheme can theoretically achieve error-free transmission when the load is light. In addition, using multiple wavelengths, the requirements of fiber ribbons and multiple stars in temporal/spatial optical CDMA networks are eliminated     

Multipulse-Per-Row Codes for High-Speed Optical Wavelength/Time CDMA Networks

New wavelength/time code families constructed with short code lengths are proposed for optical code-division multiple- access networks. Since the code matrices inherit correlation properties from the underlined one-dimensional codes, much effort for codeword searching and code property validation is saved. In addition, since the proposed code matrices with multipulse-per-row (MPR) property have optimal cardinalities, the total number of supported users in the network is increased. Performance analysis shows that the proposed MPR matrix codes have bit-error rates comparable to other wavelength/time codes proposed previously.     

Construction of two-dimensional wavelength/time optical orthogonal codes using difference family

A new family of two-dimensional (2-D) wavelength/time optical orthogonal codes (OOCs) for asynchronous optical code division multiple access (OCDMA) systems is proposed. The construction scheme uses the difference family (DF), which is an assemblage of difference sets in the combinatorial theory. It is proven that the proposed codewords satisfy the correlation properties required for the asynchronous OCDMA systems. The code dimension of the proposed codes is more flexible than that of the conventional 2-D codewords. The performance of the system with the proposed codes is analyzed by using the Markov-chain method. Numerical results show that the bit error rate (BER) has a minimal value given the number of simultaneous users. It is also observed that the maximum number of simultaneous users of the system can be achieved by properly choosing both the code weight and cross correlation of the 2-D OOCs.     

Design of a special family of optical CDMA address codes for fullyasynchronous data communications

A special family of optical address codes, called strict optical orthogonal codes (OOCs), is proposed for fiber-optic code-division multiple access (FO-CDMA) networks. Such codes can strictly guarantee both crosscorrelation and autocorrelation constraints (i.e., λ _c and λ_a) to be one in fully asynchronous data communications and ultrafast switching. Theory of strict OOCs is presented and the code design is described by using the concepts of slot distances. Moreover, the use of strict OOCs can support variable-rate and multirate data communications in an FO-CDMA network with no violation of λ_c=λ_a=1 and no increase of system complexity compared to using conventional OOCs     

数字电视码流的分析转换

对数字电视码流的主要形式即传输流TS和节目流PS进行了分析,对TS的解析、显示时间标签PTS、解码时间标签DTS及视频帧结构信息等关键技术进行了研究,提出了TS转换成PS过程中PTS,DTS,SCR等重要参数的组建算法,最终实现了TS复合流到多个PS节目的转换.     

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.     

Construction and performance analysis of variable-weight optical orthogonal codes for asynchronous OCDMA systems

A construction scheme of variable-weight optical orthogonal codes （VW-OOCs） for asynchronous optical code division multiple access （OCDMA） system is proposed. According to the actual situation, the code family can be obtained by programming in Matlab with the given code weight and corresponding capacity. The formula of bit error rate （BER） is derived by taking account of the effects of shot noise, avalanche photodiode （APD） bulk, thermal noise and surface leakage currents. The OCDMA system with the VW-OOCs is designed and improved. The study shows that the VW-OOCs have excellent performance of BER. Despite of coming from the same code family or not, the codes with larger weight have lower BER compared with the other codes in the same conditions. By taking simulation, the conclusion is consistent with the analysis of BER in theory. And the ideal eye diagrams are obtained by the optical hard limiter.     

Design and analysis of three-dimensional OCDMA code families

A new code design algorithm for application in three-dimensional (3D) optical code division multiple access (OCDMA) for asynchronous optical fiber communication is proposed. 3D refers to space-wavelength-time codes. The performance analysis of the proposed algorithm in 3D multiple pulses per plane (MPP) codes is shown. This design ensures a maximum cross-correlation of ‘1’ between any two codes. The performance metrics that have been investigated are the bit error rate due to multiple access interference (MAI) for different values of the number of simultaneous users and, cardinality for different values of temporal length.     

用于光码分多址系统中的跳频地址码研究

研究了光正交码,EQC码和素数码与频率相结合构成的单跳频及多跳频二维地址码,并对它们的容量与采用波分复用系统时的容量进行了比较,得到了比采用跳频地址码容量更大的结论。     

基于FSO系统的多进制LT码性能分析

为克服空间光通信(FSO)链路受大气湍流效应的影响,将LT码应用到FSO系统中.多进制LT码具有较高的传输效率.提出一种新型度分布函数来改善多进制LT码的译码性能.计算不同进制LT码的译码失败率,并模拟强湍流信道,对采用不同编码方案的FSO性能进行仿真.结果表明:采用新型度分布的LT码能够降低译码失败率,且能有效改善FSO系统在强湍流情况下的性能.