Wavelength-time codes with maximum cross-correlation function of two for multicode-keying optical CDMA

In this paper, a new family of wavelength-time codes with expanded code cardinality and the maximum cross-correlation function of 2 (i.e., /spl lambda//sub c/=2) is constructed and analyzed. One application of the large code cardinality of our /spl lambda//sub c/=2 codes is multicode-keying wavelength-time optical code division multiple access (O-CDMA), in which each user is allocated with multiple code matrices, instead of just one code matrix in the conventional ON-OFF keying (OOK) O-CDMA. System throughput is increased because a lower baud rate O-CDMA system can be used to support higher bit-rate transmission since each code matrix is used to represent a "symbol" of several data bits. User code confidentiality is improved because of symbol transmission. The performances of two multicode-keying O-CDMA schemes with the new /spl lambda//sub c/=2 wavelength-time codes are also analyzed. The results in this paper show that there is a tradeoff between the performance and the number of code matrices per user.     

Multiple-length multiple-wavelength optical orthogonal codes for optical CDMA systems supporting multirate multimedia services

Multiple-wavelength optical orthogonal codes (MWOOCs) with autocorrelation sidelobes and cross-correlation values of both at most one were recently proposed for wavelength-time optical code-division multiple-access (O-CDMA) systems. The codes have cardinality as a quadratic function of the number of wavelengths and find applications in high bit-rate O-CDMA systems with broadband supercontinuum lasers, in which the number of available wavelengths is larger than the number of time slots. To support multimedia services with different bit-rate and quality-of-service requirements, a new class of multiple-length constant-weight MWOOCs with autocorrelation sidelobes of zero and cross correlations of at most one is constructed algebraically in this paper. The performance of these new codes in an O-CDMA system with double-media services is analyzed. In contrary to conventional single-length codes, our study shows that the performance of these multiple-length codes improves as the code length decreases. This unique property supports "prioritization" in O-CDMA.     

Confidentiality performance of spectral-phase-encoded optical CDMA

Enhanced security has often been cited as an important benefit of optical code-division multiple-access (O-CDMA) signaling but has seldom been analyzed in detail. This paper presents a theoretical analysis of the degree of confidentiality that can be provided by spectral-phase-encoded O-CDMA. Two eavesdropping detector structures are presented that can theoretically break the confidentiality of spectral-phase-encoded signals by detecting the code words in use by a specific user. One of them, an optical beat detector, is quantitatively analyzed to determine the probability of correctly detecting user code words. The confidentiality of user signals is shown to be vulnerable to such a detector if an eavesdropper can isolate a single user signal with a sufficiently high signal-to-noise ratio (SNR). At lower SNRs, combining multiple bits is shown to dramatically increase the probability of an eavesdropper correctly detecting user code words; even for codes long enough to strain implementation capabilities (e.g., 2048 code elements), the probability of correct detection is shown to rise from negligibly low values to virtually 100% by the combining of less than 100 transmitted bits at the eavesdropper's receiver.     

Optical S-ALOHA/CDMA systems for multirate applications: architecture, performance evaluation, and system stability

In this paper, a new multirate optical network based on a hybrid slotted-ALOHA (S-ALOHA)/overlapped code division multiple access (O-CDMA) system is proposed as an effective way of integrating multiclass traffic. The key issue in this proposal is to exploit the potential of the optical O-CDMA using fiber Bragg gratings when jointly used with the S-ALOHA protocol in a link layer. The newly proposed system is modeled using a general Markov chain from which both the system throughput and the average packet delay are derived. The system stability passing through a general finite user model is also envisaged. The system is then compared to the classical S-ALOHA/variable processing gain (VPG) CDMA system. Numerical results show that the proposed system outperforms the latter one, especially at high transmission rates. In addition, stability measures demonstrate that the overlapped system tends to be more stable than the VPG system.     

Performance Analysis of Variable-Weight, Multilength Optical Codes for Wavelength-Time O-CDMA Multimedia Systems

To support multimedia services with different discrete bit-rate requirements, families of multilength optical codes, such as the carrier-hopping prime code (CHPC), extended CHPC, and multiwavelength optical orthogonal code, were recently constructed for wavelength-time optical code-division multiple-access (O-CDMA). In this paper, the performance of these multilength optical codes in a multimedia O-CDMA system with a variable-weight operation are analyzed. Our study shows that short-length codes generate stronger interference than long-length codes. This supports services prioritization in O-CDMA. Our study also shows that code weight is a more important factor than code length in determining code performance (i.e., quality of service).      

Performance Analysis of Variable-Weight Multilength Optical Codes for Wavelength–Time O-CDMA Multimedia Systems

To support multimedia services with different discrete bit-rate requirements, families of multilength optical codes, such as the carrier-hopping prime code (CHPC), extended CHPC, and multiwavelength optical orthogonal code, were recently constructed for wavelength-time optical code-division multiple-access (O-CDMA). In this paper, the performances of these multilength optical codes in a multimedia O-CDMA system with a variable-weight operation are analyzed. Our study shows that short-length codes generate stronger interference than long-length codes. This supports services prioritization in O-CDMA. Our study also shows that code weight is a more important factor than code length in determining code performance.     

Multiple-length extended carrier-hopping prime codes for optical CDMA systems supporting multirate multimedia services

Extended carrier-hopping prime codes (ECHPCs) with ideal correlation properties (i.e., zero autocorrelation sidelobes and cross-correlation values of at most 1) and significantly expanded cardinality were recently constructed for wavelength-hopping time-spreading optical code division multiple access (O-CDMA). To support multimedia services with different bit-rate requirements, a new family of multiple-length constant-weight ECHPCs with ideal correlation properties is constructed algebraically in this paper. The performance of these new codes in an O-CDMA multimedia system is analyzed. Contrary to conventional single-length codes, our study shows that the performance of these multiple-length codes improves as the code length decreases, thus supporting prioritization in O-CDMA.     

Performance enhancement of visible light communication (VLC) system incorporating WMZCC-OCDMA codes and PDM-QPSK-DSP data encoding

Multi-user optical code division multiplexing (OCDMA) in visible light communication (VLC) is an imminent domain for providing an even distribution of available bandwidth and improved security. Q4R1Zero cross-correlation (ZCC) codes are widely reported, and improved performance is offered by them due to insignificant multiple access interference (MAI). However, practically, MAI exists in ZCC codes due to adjacent weights (W) in the code matrix rows for each user. Therefore, a novel ZCC code with a nonadjacent W is designed in this work to eradicate MAI limitations and is named weight managed ZCC codes (WMZCC). An indoor 100 Gbps/User OCDMA-VLC system is realised using polarisation division multiplexed quadrature phase shift keying (PDM-QPSK) and digital signal processing (DSP). Incorporation of DSP in the coherent receiver compensates the effects of dispersion and nonlinear effects and also performs phase estimation. Further, a state-of-the-art comparison of WMZCC codes with diagonal identity matrix (DIM) and zero cross-correlation resultant weight (ZCCRW) codes has been performed in terms of log symbol error rate (SER), Q factor, error vector magnitude percentage (EVM%) and log bit error rate (BER) log at different VLC lengths, Q4R1Transmitter half angles (THA), incidence half angles (IHA) and optical concentrators (cm²). Constellations are analysed with and without DSP, and according to the author's best knowledge, a 100 Gbps VLC system for five users covering 6 m at −3.42 log BER is demonstrated for the first time. Results revealed that weight management in WMZCC codes makes its performance improve as compared with ZCCRW and DIM codes.     

Reconfigurable all-optical code translation in spectrally phase-coded O-CDMA

Reconfigurable all-optical code translation are demonstrated in a spectrally phase-coded optical-code-division multiple-access (O-CDMA) testbed with an interference user. For both one-stage and two-stage code translations, less than 0.9-dB power penalties are induced at each code translation. Multistage code translations are investigated via simulation and experimental emulation in a loop pulse shaper to show the potential application of the proposed method for up to several tens of code translations.     

Security performance of optical CDMA Against eavesdropping

Enhanced security has often been cited as an important benefit of optical CDMA (O-CDMA) signaling. However, the quality and degree of security enhancement has not been closely examined in the literature. This paper examines the degree and types of security that may be provided by O-CDMA encoding. A quantitative analysis of data confidentiality is presented for O-CDMA encoding techniques that use both time spreading and wavelength hopping. The probability of successful data interception is calculated as a function of several parameters, including signal-to-noise ratio and fraction of total available system capacity. For reasonable choices of system and encoding parameters, it is shown that increasing code complexity can increase the signal-to-noise ratio (SNR) required for an eavesdropper to "break" the encoding by only a few dB, while the processing of fewer than 100 bits by an eavesdropper can reduce the SNR required to break the encoding by up to 12 dB. The overall degree of confidentiality obtainable through O-CDMA encoding is also compared with that obtainable through standard cryptography. time-spreading/wavelength-hopping in particular, and O-CDMA in general, are found to provide considerably less data confidentiality than cryptography, and the confidentiality provided is found to be highly dependent on system design and implementation parameters.     

基于LDPC码的无线光通信副载波误码性能分析

为了研究信道编码技术对无线光通信副载波系统差错性能的影响,基于无线光通信链路噪声特性,建立了大气信道等效数学模型。采用低密度奇偶校验(LDPC)码作为信道编码方式引入无线光通信,在不同光强闪烁指数下对基于副载波相移键控调制无线光通信系统进行了差错性能仿真,比较分析了LDPC编码前后副载波调制系统的误比特率,并对二相相移键控及四相相移键控两种系统进行了对比。结果表明,副载波二相相移键控调制系统的差错性能优于四相相移键控系统,同时LDPC码在弱湍流信道具有优越的纠错能力,可以获得比未编码系统较高的编码增益,在无线光通信领域具有一定的应用价值。     

Demonstration of 160- and 320-Gb/s SPECTS O-CDMA network testbeds

This letter presents a high-capacity optical code-division multiple-access (O-CDMA) network testbed based on the spectral phase-encoded time-spreading technique. Two 10-Gb/s/user O-CDMA network architectures (time-slotted and time-slotted polarization multiplexed) are investigated. The first O-CDMA network testbed architecture utilizes eight encoders and a decoder to produce 16 users equally distributed in two time slots while the second architecture evenly distributes 32 users in two time slots and two polarizations. The 16-user testbed achieved error-free performance. The 32-user testbed obtained bit-error rates below 10/sup -8/ without using forward-error-correction techniques.     

Code Design for MIMO Broadcast Channels

Recent information-theoretic results show the optimality of dirty-paper coding (DPC) in achieving the full capacity region of the Gaussian multiple-input multiple-output (MIMO) broadcast channel (BC). This paper presents a DPC based code design for BCs. We consider the case in which there is an individual rate/signal-to-interference-plus-noise ratio (SINR) constraint for each user. For a fixed transmitter power, we choose the linear transmit precoding matrix such that the SINRs at users are uniformly maximized, thus ensuring the best bit-error rate performance. We start with Cover's simplest two-user Gaussian BC and present a coding scheme that operates 1.44 dB from the boundary of the capacity region at the rate of one bit per real sample (b/s) for each user. We then extend the coding strategy to a two-user MIMO Gaussian BC with two transmit antennas at the base-station and develop the first limit-approaching code design using nested turbo codes for DPC. At the rate of 1 b/s for each user, our design operates 1.48 dB from the capacity region boundary. We also consider the performance of our scheme over a slow fading BC. For two transmit antennas, simulation results indicate a performance loss of only 1.4 dB, 1.64 dB and 1.99 dB from the theoretical limit in terms of the total transmission power for the two, three and four user case, respectively.     

Performance analysis of spectrally encoded hybrid WDM-OCDMA network employing optical orthogonal modulation format against eavesdropper

In this paper, the spectrally encoded hybrid WDM-OCDMA network employing 60 Gbps non-return to zero/differential quadrature phase shift keying (NRZ/DQPSK) orthogonally modulated data signal operating over 100 km SMF+DCF has been proposed. In proposed hybrid WDM-OCDMA network, the orthogonal modulation formats are used to enhance per channel capacity and the spectral amplitude optical coding for enhancing the confidentiality of data from unauthorized user or eavesdropper. The impact of input power, transmission distance and energy per bit to noise power spectral density ratio (E_b/N₀) on the performance of WDM-OCDMA network in terms of output optical power, timing diagram, BER, Q-factor and probability of error free code detection has been investigated. It can also be determined that orthogonal modulation formats are promising option to increase per channel capacity as compared to conventional modulation format and these are less vulnerable to chromatic dispersion (CD) and polarization mode dispersion (PMD). Meanwhile, the performance of proposed hybrid network is compared with existing OCDMA network which show the feasibility of proposed WDM-OCDMA network in future generation optical networking.     

Error-free, 12-user, 10 Gbit/s/user O-CDMA network testbed without FEC

Error-free operation of a 12/spl times/10 Gbit/s synchronous, time-slotted spectral phase-encoded time-spreading optical code division multiple access (O-CDMA) network testbed without using forward-error-correction (FEC) is demonstrated. A nonlinear optical loop mirror time gate and nonlinear thresholding minimise multiuser interference.     

A 10 Gbit/s OCDMA system based on electric encoding and optical transmission

An electric encoded/optical transmission system of code division multiple access （CDMA） is proposed. It encodes the user signal in electric domain, and transfers the different code slice signals via the different wavelengths of light. This electric domain encoder/decoder is compared with current traditional encoder/decoder. Four-user modula- tion/demodulation optical CDMA （OCDMA） system with rate of 2.5 Gbit/s is simulated, which is based on the optical orthogonal code （OCC） designed in our laboratory. The results show that the structure of electric encoding/optical transmission can encode/decode signal correctly, and can achieve the chip rate equal to the user data rate. It can over- come the rate limitation of electronic bottleneck, and bring some potential applications in the electro-optical OCDMA system.     

The role of optical CDMA in access networks

We investigate the possible role of optical CDMA (O-CDMA) in future access networks. We begin with a short review of the O-CDMA technique for those unfamiliar with the technology. Next, we investigate in detail those characteristics of O-CDMA that make it an attractive technology for application in metro access networks: fairness, flexibility, simplified network control and management, service differentiation, and increased security. Although O-CDMA has many favorable attributes, it also has several actual or perceived drawbacks. We discuss the technical, economic, and perception barriers that may have limited the widescale deployment of O-CDMA access networks. We try to determine which of these drawbacks may be surmountable in the near future and which may be true "showstoppers"     

基于极化码的无线光通信副载波误码性能分析

提出了一种基于极化码的无线光信道副载波调制方法,给出了极化编译码的具体算法过程,并将其应用于无线光通信系统。在不同大气湍流强度下,对系统的差错性能进行了仿真分析,其重对基于极化码的二相相移键控和四相相移键控两种调制方法的误码率进行了对比,结果表明,采用极化码的副载波二相相移键控调制系统的性能优于四相相移键控系统。最后,实验比较分析了极化编码前后副载波调制系统的误比特率,结果表明,在无线光通信大气湍流信道模型下,采用副载波极化码编码调制技术可使误码率性能改善一个量级。     

Multiple access interference cancelation technique in optical CDMA systems

This paper presents a simple and efficient multiple access interference (MAI) cancelation technique in optical code division multiple access (OCDMA) system. The proposed technique is based on hybrid frequency shift keying (FSK) with an enhanced modified prime code as a signature sequence for coding techniques. Coherent FSK modulation along with incoherent demodulation using Arrayed-Waveguide Grating has been examined in the transceiver structure. In the proposed technique, a reference signal is constructed by using one of the addressed spreading sequences, and MAI cancelation is performed by subtracting the reference signal from the received signal of the desired user. The performance of the proposed FSK-OCDMA system is compared with the performance of the existing pulse position modulation (PPM)–OCDMA system. The simulation results reveal that the bit-error rate performance of the proposed technique is superior to the performance of the pulse position modulation (PPM) technique. Also, the results indicate that the proposed technique is very power efficient, and when the bit rate is constant, the network capacity can be expanded to accommodate a large number of simultaneous active users with low error rate. Moreover, the proposed technique simplifies the hardware of the receiver design.     

Effects of atmospheric weather and turbulence in MSK based FSO communication system for last mile users

Telecommunication Systems - In this paper, a minimum shift keying (MSK) based free space optical (FSO) communication system for the terrestrial user is modeled. In our setup 10 Gbps...