Optical Code-division Multiple-access Scheme Research Articles

Packet Switching Strategy and Node Architecture of Extended Spectral-Amplitude-Coding Labels in GMPLS Networks

We present packet switching applications based on extended spectral-amplitude-coding (SAC) labels in generalized multi-protocol label switching (GMPLS) networks. The proposed approach combines the advantages of wavelength-division multiplexing (WDM) and optical code-division multiple access (OCDMA). The extended SAC labels preserve the orthogonal property to avoid the effect of multiple access interference (MAI) shown at the decoder. We investigate the node architecture of label generation/recognition based on arrayed waveguide grating (AWG). Combining cyclic-shifted maximal length sequence (MLS) codes with the wavelength routed property of AWG simplifies the node structure. The simulation results show that the proposed labels achieve good performances against receiver noise due to the low average cross-correlation values. Under a given bit-error-rate (BER), the switching efficiency of the extended SAC labels outperforms the previous OCDMA schemes, as the network nodes are capable of processing a large number of labels simultaneously.

Novel secure and bandwidth efficient optical code division multiplexed system for future access networks

Abstract In this paper, a spectrally coded optical code division multiple access (OCDMA) system using a hybrid modulation scheme has been investigated. The idea is to propose an effective approach for simultaneous improvement of the system capacity and security. Data formats, NRZ (non-return to zero), DQPSK (differential quadrature phase shift keying), and PoISk (polarisation shift keying) are used to get the orthogonal modulated signal. It is observed that the proposed hybrid modulation provides efficient utilisation of bandwidth, increases the data capacity and enhances the data confidentiality over existing OCDMA systems. Further, the proposed system performance is compared with the current state-of-the-art OCDMA schemes.

Two New Families of Synchronous Optical Codes for CDMA-Based Passive Optical Networks

In this paper, two new families of "synchronous" optical codes, called 1-D and 2-D synchronous multilevel prime codes (SMPCs), are constructed and analyzed for incoherent optical code-division multiple access (O-CDMA). With the benefit of synchronous transmission, the new algebraic codes have expanded cardinality and substantially increase the number of possible subscribers, thus potentially supporting rapid code switching and obscure-nature (privacy and military) communications. In addition, the SMPCs can be partitioned into a tree structure of multiple levels of subsets of codewords. Different cross-correlation functions are obtained from codewords coming from different subsets; code orthogonality is achieved among the codewords within each subset of the lowest level. As code performance is a strong function of the cross-correlation function, the performance of the codewords in use can be adjusted by choosing the subsets from which the codewords originate. The performances of the SMPCs are formulated and also verified by computer simulation. Our analysis shows that code performance can be optimized by applying an "in-order" subset-by-subset code-assignment method to simultaneous users. Besides supporting time-spreading and wavelength-time O-CDMA schemes, the new codes also find applications in passive optical networks and in spectral-amplitude-coding schemes.

Confidential Enhancement with Multi-Code Keying Reconfiguration over TS/WH OCDMA Networks

In order to overcome the vulnerability to eavesdropping on-off-keying based OCDMA scheme, a new multi-code keying reconfiguration is presented to enhance confidentiality over two dimensional time-spreading and wavelength-hopping (2D TS/WH) optical code division multiple access (OCDMA) schemes. The time-shifted carrier-hopping prime codes (CHPCs) are proposed and assigned as signature address code. This proposed scheme provides the integration of electronic and optical layer domain, which inherently comes from the data encryption and spectrum-spreading of the transmitted signals, and then achieves data-confidential enhancement in synchronous networks. Moreover, multi-code keying reconfiguration in a set of distinct codeword is assigned to individual user and random selection by using thin film filter, tunable delay line and optical switch. Here, the probability of interception performance in terms of various multi-code key numbers per user is analyzed. The result shows that the confidentiality is enhanced to combat against eavesdropping while signature address code number per user is increased in proposed multi-code key reconfiguration scheme.

Isolated user security enhancement in optical code division multiple access network against eavesdropping

A novel virtual user system is modeled for enhancing the security of an optical code division multiple access (OCDMA) network. Although the OCDMA system implementing code shift keying (CSK) is secure against a conventional power detector, it is susceptible to differential eavesdropping. An analytical framework is developed for the CSK-OCDMA system to show eavesdropper's code interception performance for a single transmitting user in the presence of a virtual user. It is shown that the eavesdropper's probability of correct bit interception decreases from 7.1×10 −1 to 1.85×10 −5 with the inclusion of the virtual user. Furthermore, the results confirm that the proposed virtual user scheme increases the confidentiality of the CSK-OCDMA system and outperforms the conventional OCDMA scheme in terms of security.

Study of a Diversity O-CDMA Scheme for Optical Wireless

Borrowing from wireless communications, a “diversity” optical code-division multiple-access (O-CDMA) scheme for optical wireless is studied in this paper. The scheme transmits multiple copies of an optical code per data bit of one for improving code performance and spectral efficiency. An approximate performance-analytical model is also formulated for quick computation and comparison with a conventional O-CDMA scheme. Under the assumptions of same code power and hardware complexity, our study finds an important application of the diversity scheme, which circumvents a heavy code-weight requirement (for achieving good code performance) in conventional O-CDMA by transmitting multiple copies of lower weight codes with little penalty in spectral efficiency and without depletion in code cardinality (i.e., the number of possible subscribers.)

Design of multi-channel optical code-division multiple-access encoders and decoders based on sampled fiber Bragg gratings

An encoding/decoding scheme of multi-channel optical code-division multiple-access (OCDMA) system using sampled fiber Bragg gratings (SFBGs) is proposed and numerically investigated. A multiple-phase-shift (MPS) technique based SFBG is first developed as a dense wavelength-division multiplexing (DWDM) device to accommodated multiple channels. The OCDMA scheme is achieved through phase coding/encoding on each individual channel of the SFBG and the auto correlative ratio for each user is larger than 14.3 dB. The whole system exhibits enhanced performance in terms of the capacity and security since the OCDMA technique is simultaneously implemented for all channels using the single SFBG.

Optical Code Division Multiple Access Network Transmission With M-ary Chip Symbols

This paper uses M-ary chip symbols to increase the spectral efficiency of optical code division multiple access (OCDMA) even though, when compared to its binary counterpart, M-ary OCDMA can have significantly increased multi-user interference (MUI). The presented numerical channel capacity results show that appropriate non-uniform M-ary signaling allows the reduction of MUI and the achievement of improved OCDMA spectral efficiency. Using insights from the OCDMA capacity results, we design a coded M-ary OCDMA transmission scheme that can achieve very high spectral efficiencies. The proposed time/wavelength OCDMA scheme relies on intensity modulation, appropriate non-uniform signaling, M-ary trellis-coded modulation and error-correcting codes. The corresponding receiver is based on direct detection of optical signals, soft-decision single-user demodulation and error-control decoding. The presented simulation results illustrate that the designed OCDMA scheme can support hundreds of active users at the target bit error rate of 10−9 and is robust to impairments encountered in optical transmission (shot noise, thermal noise). The achieved spectral efficiency of up to 1.34 bits per OCDMA chip is significantly better than the best binary OCDMA result reported in the literature.

Performance analysis of asynchronous time-spreading phase coding and wavelength-time OCDMA systems

We compare the theoretical bit-error-rate (BER) and throughput performance of asynchronous time-spreading (TS) phase coding and two-dimensional (2D) wavelength-time (WT) optical code division multiple access (OCDMA) communication systems. Particular consideration is given, however, to both beat noise (BN) and multiple access interference (MAI) effects. BER performance of the TS OCDMA scheme is much superior to that of the WT scheme in the MAI limited case, while the WT scheme has better tolerance to beat noise. Improvement of BER performance and throughput can be obtained by utilization of optimum threshold detection. The two OCDMA schemes have comparable bit-error probability, and the WT scheme achieves a higher throughput for a heavy load when the optimum threshold is employed.

Spectral Efficiency Study of Two Multirate Schemes for Asynchronous Optical CDMA

Multirate transmission techniques, such as multiple-code (MC) and multicode-keying (MK) schemes, have been proposed for asynchronous optical code-division multiple-access (O-CDMA) systems supporting multimedia services. A user in the MC scheme transmits several codes at the same time to represent the transmission of several data bit ones. The number of parallel codes transmitted at a time is proportional to the user's bit rate and the number of data bit ones. While the MK scheme only transmits one code at a time, each code represents a symbol and each transmitted symbol represents multiple data bits. Data rate is varied by changing the symbol size, which, in turn, changes the number of data bits per baud, similar to MODEM communications. In this paper, the spectral efficiencies (SEs) of these two asynchronous schemes are derived for the comparison of their performances and efficiencies as a whole. The SE of the conventional single-rate on-off-keying (OOK) O-CDMA scheme is also derived for a baseline comparison. Our numerical results show that the MC scheme always has a better SE than the single-rate OOK scheme, while the MK scheme can have a better SE than the single-rate OOK scheme under certain conditions.

Theoretical Analysis of Optimum Threshold Detection in Coherent Time-spreading OCDMA Systems

The performance of coherent time-spreading (TS) and asynchronous TS phase coding optical code division multiple access (OCDMA) systems is investigated and analyzed. Particular consideration is given however to both beat noise (BN) and multiple-access interference (MAI) effects, and the impact of decision threshold on the bit-error-rate (BER) performance is evaluated. Improvement of the BER performance can be obtained by use of optimum threshold detection, and the improvement is more apparent in the asynchronous TS OCDMA scheme for a low number of simultaneous users with long code length.

Effects of the Random Diagonal code link parameters on the performance of an OCDMA scheme for high-speed access networks

In this paper, we study the performance of optical code-division multiple access (OCDMA) systems using Random Diagonal (RD) code under various link parameters. This code which can be constructed using address segment and data segment. One of the important properties of this code is that the cross correlation at data level is always zero, which means that Phase Intensity Induced Noise (PIIN) is reduced. The impact of the fiber dispersion effects on the MAI is reported using a commercial optical systems simulator, OptSim TM . The RD code is compared mathematically with other codes which use similar techniques. We analyzed and optimized the data rate, fiber length, and channel spacing in order to reduce the BER without the need to deploy dispersion compensating devices. The performance and optimization of RD code in OCDMA system is reported. We have demonstrated that, for a high data rate (higher than 2.5 Gb/s), even if dispersion compensated devices are not deployed, the BER can be significantly improved when the RD code desired parameters are selected. We have shown that when compensation dispersion devices are not deployed in the system, there is a trade off between the limited dispersion effects and the MAI.

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.

A New Approach to Achieve High Spectral Efficiency in Wavelength-Time OCDMA Network Transmission

We design a new wavelength-time optical code-division multiple-access (OCDMA) modulation scheme that does not use spreading sequences for information transmission, i.e., the spreading length is N=1. The proposed transmitter sends (error-control) coded data directly through the optical channel and exploits a probabilistic method to reduce the amount of multiuser interference. Simulation results show that by using turbo codes, Reed-Solomon codes, and soft-decision demodulation, the proposed OCDMA scheme can support hundreds of active users at target bit-error rate =10-9. Furthermore, the achieved spectral efficiency of 0.740 bits per channel use is almost twice as large as the best previous OCDMA results

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., lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> =2) is constructed and analyzed. One application of the large code cardinality of our lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> =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 lambda <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> =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

Using Bipolar Codes in Non-coherent Optical CDMA Systems to Stabilize Decision Threshold for BER Performance Improvement

We use bipolar codes in non-coherent optical code-division multiple-access (O-CDMA) systems to stabilize decision threshold and improve system performance. The bipolar optical system also obviates the need for complicated threshold injection process. Theoretical error rate results under threshold instability are presented. Simulation results for an O-CDMA bipolar system using light-emitting diodes (LED) as the light sources and 21dB Erbium-doped fibre amplifier (EDFA) gain at 622 Mbit/s show that, for 10 -8 bit error rate (BER), the O-CDMA bipolar system can perform up to 15 km, a 7 km enhancement in comparison with the unipolar O-CDMA scheme.

Shifted Carrier-Hopping Prime Codes for Multicode Keying in Wavelength-Time O-CDMA

In this paper, we study the use of shifted carrier-hopping prime codes (CHPC) for multicode keying in wavelength-time optical code-division multiple-access (O-CDMA) systems. By using the code's good properties of zero autocorrelation sidelobes and cross-correlation functions of at most one, each user is assigned M=2/sup m/ shifted copies of its own code matrix to represent m data bits per symbol with minimal interference. The advantages of our scheme are that: 1) a lower baud rate O-CDMA system can now support a higher bit rate; 2) no network synchronization is required; 3) no M-fold increase in code cardinality is needed, as compared with standard M-code keying; 4) the numbers of optical encoders and decoders can be reduced from M to one per user, a major hardware cost savings; and 5) user code confidentiality is enhanced, as compared with the transmission of one code matrix for each data-bit one in conventional "on-off keying" O-CDMA schemes. Finally, our performance analysis shows that there is a tradeoff between the error probability and the choice of M.

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.

Using advanced optical multiple-access techniques in high-speed avionic local area networks for future aircraft applications. Part I: optical code-division multiple-access networks

In the first part of this paper, we propose high-speed fiber optic code-division multiple-access (CDMA) local area networks (LANs) for future real-time on-board command and control communications in aircrafts. We give new design concepts to implement avionic LANs which use optical CDMA schemes with a full tunability and can support sensor data distribution and computer interconnects. Reliable short-pulse generation and optical sampling are presented to optical CDMA transmitters to facilitate the application of avionic LANs. Optical architectures for fastly tunable CDMA encoders and correlators using prime codes are given. Their simple delay control logic is described, and it can be easily performed by using optical fiber delay lines. Moreover, the power losses of all-optical CDMA encoders and correlators are analyzed. Topologies for avionic fiber optic CDMA LANs are also discussed. We show that optical CDMA can be used in an avionic LAN to support high-speed parallel data communications with a low latency and multiple connections with a very fast reconfiguration time. This in turn can ensure real-time on-board command and control communications for future high-performance aircraft applications.