An Internally Coded TH/OCDMA Scheme for Fiber Optic Communication Systems and its Performance Analysis-Part II: Using Frame Time Hopping Code

In Part I, a new internally coded time hoping optical code division multiple access (TH/OCDMA) scheme for fiber optic communication systems has been proposed and its multiple access performance has been evaluated using optical orthogonal code (OOC). Due to low cardinality of OOCs with a correlation value of 1, the capability of the proposed scheme could not be utilized effectively for an increasing number of simultaneous users. In this part, we consider applying the internally coded technique introduced in part one to the frame time hopping/OCDMA scheme. We evaluate the multiple access performance of the system for the three detectors introduced in Part I. Our results demonstrate the capability of the internally coded TH/CDMA scheme to substantially increase the maximum number of simultaneous users when using FTH codes instead of OOCs.     

Frame time-hopping fiber-optic code-division multiple access using generalized optical orthogonal codes

A new spreading technique for intensity modulation direct detection fiber-optic code-division multiple-access (FO-CDMA) communication systems is proposed. This new spreading technique is based on generalized optical orthogonal codes (OOC) with large cardinality and minimal degradation in performance when compared with a more optimum system, namely, an optical CDMA system using OOC with autocorrelation and cross-correlation value bounded by one, i.e., OOC (/spl lambda/=1). To obtain the performance of such systems, we use a communication scheme, namely, frame time-hopping (FTH)-CDMA with random coding. Systems with generalized OOC patterns and random time-hopping coding are close in structure and performance. Furthermore, the performance of such systems is near the performance of optical CDMA with optimum but low cardinality OOC (/spl lambda/=1), which further renders the practicality of the proposed technique with very large cardinality. Two new receiver structures for FO-CDMA, namely, chip-level detector with optimum comparator threshold and correlation receiver with an electrical hard limiter, are also proposed. The performance analysis for a binary pulse position FTH-FO-CDMA network is considered for the correlation receiver, chip-level detector, correlation receiver with an optical hard limiter, optimum receiver, and the two newly proposed receiver structures. The results also show that a chip-level detector with optimum comparator threshold is superior to a chip-level detector for received low signal powers, and predict that the performance of the correlation receiver with an electrical hard limiter is superior to all other considered receiver structures, e.g., requiring one third of transmission power to achieve a desired bit error rate when compared with other receiver structures.     

A new family of 2-D optical orthogonal codes and analysis of its performance in optical CDMA access networks

A new family of two-dimensional optical orthogonal code (2-D OOC), one-coincidence frequency hop code (OCFHC)/OOC, which employs OCFHC and OOC as wavelength hopping and time-spreading patterns, respectively, is proposed in this paper. In contrary to previously constructed 2-D OOCs, OCFHC/OOC provides more choices on the number of available wavelengths and its cardinality achieves the upper bound in theory without sacrificing good auto-and-cross correlation properties, i.e., the correlation properties of the code is still ideal. Meanwhile, we utilize a new method, called effective normalized throughput, to compare the performance of diverse codes applicable to optical code division multiple access (OCDMA) systems besides conventional measure bit error rate, and the results indicate that our code performs better than obtained OCDMA codes and is truly applicable to OCDMA networks as multiaccess codes and will greatly facilitate the implementation of OCDMA access networks.     

Performance Improving of OCDMA System Using 2-D Optical Codes With Optical SIC Receiver

In this paper, we have studied the probability of error of optical code division multiple access (OCDMA) system using 2-D optical codes, namely prime hop and hybrid codes with a probability distribution approach. In order to increase the OCDMA system performance and increase the number of simultaneous users in the system, we have investigated the system when two different receivers are employed, namely, OCDMA serial interference cancellation (SIC) receivers and OCDMA correlation conventional receiver (CCR). Comparison in terms of OCDMA system's performance between these two receivers has been reported. In order to obtain the optimum OCDMA system's performance, we have studied several optical SIC receiver structures. Optimum choice of the number of stages in the SIC receiver and the threshold value on each stage has been reported.     

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.     

High-performance optical CDMA system based on 2-D optical orthogonal codes

Optical code-division multiple access (OCDMA) is an interesting subject of research because of its potential to support asynchronous, bursty communications. OCDMA has been investigated for local area networks, access networks, and, more recently, as a packet label for emerging networks. Two-dimensional (2-D) OCDMA codes are preferred in current research because of the flexibility of designing the codes and their higher cardinality and spectral efficiency (SE) compared with direct sequence codes based on on-off keying and intensity modulation/direct detection, and because they lend themselves to being implemented with devices developed for wavelength-division-multiplexed (WDM) transmission (the 2-D codes typically combine wavelength and time as the two dimensions of the codes). This paper shows rigorously that 2-D wavelength/time codes have better SE than one-dimensional (1-D) CDMA/WDM combinations (of the same cardinality). Then, the paper describes a specific set of wavelength/time (W/T) codes and their implementation. These 2-D codes are high performance because they simultaneously have high cardinality (/spl Gt/10), per-user high bandwidth (>1 Gb/s), and high SE (>0.10 b/s/Hz). The physical implementation of these W/T codes is described and their performance evaluated by system simulations and measurements on an OCDMA technology demonstrator. This research shows that OCDMA implementation complexity (e.g., incorporating double hard-limiting and interference estimation) can be avoided by using a guard time in the codes and an optical hard limiter in the receiver.     

An Internally Coded TH/OCDMA Scheme for Fiber Optic Communication Systems and Its Performance Analysis—Part I: Using Optical Orthogonal Code

In this paper, a new internally coded time-hopping optical code-division multiple-access (TH/OCDMA) scheme for fiber-optic communication systems is proposed, and its multiple-access performance is evaluated using optical orthogonal codes (OOCs). In the proposed method, the duration of each bit is divided into N_s frames, each one containing L chips. Two signature codes, namely, one pseudonoise (PN) sequence and one OOC, are assigned to each user. During each bit interval, based on the output of an channel encoder and the user's dedicated PN sequence, one of the N_s frames is selected, in which W pulses are transmitted in W chips, marked by the user's OOC. We consider three detectors at the receiver front end, namely, correlation, soft chip-level, and hard chip-level detectors. We evaluate the multiple-access performance of the system for each detector considering the effects of shot noise, dark current, and thermal noise. We compare the results with those obtained for the conventional OCDMA systems. Our numerical results indicate that, for the same bit rate and bandwidth, our proposed method substantially outperforms the conventional OCDMA system. Our results also show that the soft chip-level detector outperforms the other detection techniques in all cases considered     

OCDMA系统和相关器件研究进展

全面综述了光码分多址(OCDMA)系统的发展概况.对不同系统方案的性能进行了对比并分析了系统中的噪声,重点对编/解码器、光源、光阈值器件等OCDMA系统中的关键部分进行了讨论.评述了当前多用户OCDMA系统的最新研究进展,并对OCDMA的发展趋势和应用前景进行了预测.     

Code-Empowered Lightwave Networks

In this paper, an architecture for code-empowered optical CDMA (OCDMA) lightwave networks is presented. The architecture is based on reconfigurable optically transparent paths among users of the network to provide high-bandwidth optical connections on demand over small areas such as local area networks or access networks. The network operates on the transmission of incoherent OCDMA codes, each network station being equipped with an OCDMA encoder and decoder. The routing at a network node is based on the OCDMA code itself. The destination address, as well as the next node on the path, is given by the code as in a code-empowered network. A node consists of an OCDMA router built from parallel code converter routers that perform switching, routing, and code conversion. The latter enables a virtual code path for increased scalability. Commonly available delay lines enable the tunability of the encoder, decoder, and router for a reconfigurable and flexible network. Flexibility and granularity are also accentuated by OCDMA encoding. An OCDMA lightwave network can therefore respond to changes in traffic load, traffic conditions, failure, and other network impairments. We describe the possible architectures and the routing constraints of such OCDMA lightwave networks. We present a power analysis and focus on the performance issues of dynamic routing. The effect of coding, topology, load condition, and traffic demand is analyzed using simulations. The obtained results show that the flexibility of OCDMA and the large offered cardinality can be a solution to the needs of local area and access networks.     

基于WDM/OCDMA的混合PON技术研究

基于WDM/OCDMA的混合PON作为一种大容量、易升级、对称性和高度保密性的新一代光接入网技术,被认为是未来宽带光接入网的重要发展方向.文章分析了波分复用和码分多址技术结合的可能性,介绍了混合WDM/OCDMA-PON的基本结构和工作原理,并给出了三种混合PON系统的实现方案,提出了应用前景.     

Design and performance analysis of wavelength/time (W/T) matrix codes for optical CDMA

Two-dimensional (2-D) codes for optical CDMA (OCDMA) are increasingly important because the code set size (cardinality) of such codes is large and the codes have good spectral efficiency, especially when compared to linear or direct sequence codes. As an example, the 2-D codes described in this paper (that use intensity modulation and direct detection, IM/DD) have a cardinality of 32, and their spectral efficiency is /spl sim/0.5 bit/s/Hz when a guard-time is used to avoid intersymbol interference. The cardinality is readily increased to 64-80, using the techniques described in the paper. The next best 2-D codes of comparable cardinality that use IM/DD tend to have a lower spectral efficiency (going like 1/K, where K is the cardinality) because they do not support multiple entries per row or per column of the code matrix. To improve on the spectral efficiency of the codes described in this paper, bipolar codes must be considered. Two-dimensional codes or matrices can be generated from pseudoorthogonal (PSO) sequences by means of simple quasigraphical operations. Important results of this construction are that both the cardinality and the spectral efficiency or information spectral density of the set of matrices is higher than that of the generating set of sequences. The matrices can be interpreted (implemented) as space/time (S/T) or wavelength/time (W/T) matrix codes for OCDMA applications. The resultant matrix codes are robust, have high information spectral density, and are effective wavelength multipliers. This paper describes the design and construction of the matrices; analyzes their performance from a communications viewpoint; describes their use as codes for the asynchronous, concurrent communication of multiple users; and analyzes the bit error rate performance based on capturing and modeling a typical network topology and performing a numerical modeling of the system.     

Code-division multiple-access techniques in optical fiber networks - part III: optical AND logic gate receiver structure with generalized optical orthogonal codes

In this paper, we present a deep insight into the behavior of optical code-division multiple-access (OCDMA) systems based on an incoherent, intensity encoding/decoding technique using a well-known class of codes, namely, optical orthogonal codes (OOCs). As opposed to parts I and II of this paper, where OOCs with cross-correlation /spl lambda/=1 were considered, we consider generalized OOCs with 1/spl les//spl lambda//spl les/w, where w is the weight of the corresponding codes. To enhance the performance of such systems, we propose the use of an optical and logic gate receiver, which, in an ideal case, e.g., in the absence of any noise source, except the optical multiple-access interference, is optimum. Using some basic laws on probability, we present direct and exact solutions for OOCs with /spl lambda/=1,2,3,...,w, with the optical and logic gate as receiver. Using the exact solution, we obtain empirical solutions that can be easily used in optimizing the design criteria of such systems. From our optimization scheme, we obtain some fresh insight into the performance of OOCs with /spl lambda//spl ges/1. In particular, we can obtain some simple relations between P/sub emin/ (minimum error rate), L/sub min/ (minimum required OOC length), and N/sub max/ (maximum number of interfering users to be supported), which are the most desired parameters for any OCDMA system design. Furthermore, we show that in most practical cases, OOCs with /spl lambda/=2,3 perform better than OOCs with /spl lambda/=1, while having a much bigger cardinality. Finally, we show that an upper bound on the maximum weight of OOCs are on the order of /spl radic/2/spl lambda/L where L is the length of the OOCs.     

A new family of 2D variable-weight optical orthogonal codes for OCDMA systems supporting multiple QoS and analysis of its performance

A new family of two-dimensional variable-weight and constant-length optical orthogonal codes (2D VWOOCs) is proposed, and the code cardinality and BER performance for the corresponding OCDMA system are analyzed in this article. It is shown that the cardinality of 2D VWOOC is larger than that of constant-weight 2D OOC and close to the upper bound in theory. In an OCDMA network, the users employing 2D VWOOC codewords with larger Hamming weight outperform the users using 2D VWOOC codewords with smaller Hamming weight in bit-error-rate performance. Therefore, the OCDMA network employing 2D VWOOC can support diverse quality-of-services (QoS) classes and multimedia services, and make the better use of bandwidth resources in optical networks.

Novel anti-jamming technique for OCDMA network through FWM in SOA based wavelength converter

In this paper, we propose a novel anti-jamming technique for optical code division multiple access (OCDMA) network through four wave mixing (FWM) in semiconductor optical amplifier (SOA) based wavelength converter. OCDMA signal can be easily jammed with high power jamming signal. It is shown that wavelength conversion through four wave mixing in SOA has improved capability of jamming resistance. It is observed that jammer has no effect on OCDMA network even at high jamming powers by using the proposed technique.     

Performance analysis of wavelet packet multirate multicarrier multicode CDMA for wireless multimedia communications

Different kinds of Multirate (MR) communication systems, such as multicode (MCD) scheme and variable spreading length (VSL) schemes, have been considered for accommodating information sources with different data rates in Multicarrier code-division multiple access (MC-CDMA). In this paper, we propose the use of MCD scheme for MR services in MC/MCD-CDMA system that employs wavelet packets (WPs) as subcarriers. The bit error rate (BER) performance for the system was investigated by means of analytical methods and numerical results in a slow fading frequency selective Nakagami channel. The performance analysis includes the effects of diversity techniques, channel intensity profile, diversity order and fading parameter. Also, the effects of different service rates and number of users in each service rate were investigated. The performance of the system was compared to that of MC/MCD-CDMA based on sinusoidal carrier. Results reveal that BER performance is proportional to the service rate and our proposed system outperform the other system.     

OCDMA系统SAC编解码实现方案及性能研究

讨论了基于FBG的谱幅编解码spectral-amplitude-coding(SAC)系统的原理；提出了(N，w,λ)=(q^2 q l,q l，1)光正交码的SAC编解码器实现方案：给出了该系统在码分功率不均匀时，以多址干扰为主的系统信噪比和误码率。分析了谱幅编解码OCDMA系统的应用前景。     

Wireless optical CDMA LAN: digital design concepts

In this paper we study and present an in-depth analysis on the operability and the viability of a typical wireless optical CDMA (OCDMA) local area network. Three receiver structures for OCDMA systems, using optical orthogonal codes (OOC) with minimum auto and cross-correlations as signature sequence, namely, correlation, correlation with hard-limiter, and chip-level detection are studied and proposed for such a network. For the synchronization circuit design the performance of two algorithms for OOC based OCDMA networks, namely, simple serial-search and multiple-shift in the context of wireless OCDMA LAN are studied. Furthermore, we study a synchronization method based on matched filtering and show that it presents a much better performance in our wireless OCDMA system. The effect of sampling rate and its performance on tracking circuit is analyzed. Bit-error-rate (BER) analysis is performed by photon counting methodology. Multi-user interference (MUI), ambient light, and photodetector dark current are considered in our analysis. Our analysis strongly indicates the viability and practicality of such systems in certain important wireless optical communications systems.     

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.     

New Two-dimensional Modified Quadratic Congruence Code/Optical Orthogonal Code for OCDMA

A new family of two-dimensional optical orthogonal code（2-DOOC）, namely, modified quadratic congruence code（MQCC ）/optical orthogonal code（OOC） is proposed who employs MQCC and OOC as wavelength hopping and time-spreading patterns, respectively. Through analyzing the performance of MQCC/ OOC, we can see that the correlation properties of the MQCC/OOC are still ideal. Simultaneously, our analysis shows that the proposed new code families can get more cardinalities than other codes and can improve the bit error rate（BER） of optical code division multiple access（OCDMA） effectively.     

Analysis and Simulation of a Plastic Optical Fiber Access Network Based on OCDMA Technology

In this paper a plastic optical fiber (POF) access network based on optical code-division multiple access (OCDMA) technology is proposed. Recently, optical transmission using POF has received much attention due to its low weight, large core diameter, flexibility, and easy installation. Specially, its high bandwidth makes POF a very attractive candidate for a transmission media in an access network based on OCDMA technology. A conventional OCDMA system only allows a finite number of units to transmit and access simultaneously according to the number of channels which is restricted by BER. To resolve this problem a novel protocol is also proposed in this paper. The protocol can efficiently support variable-sized messages, and any new unit can join the network at any time without requiring network initialization. To implement the demonstration, each optical network unit is equipped with a fixed and a tunable optical encoder/decoder. The optical encoder/decoder employing planar holographic optical processors (HOPs) in this system may be of low fabrication cost. The network throughput and average delay using various system parameters has been investigated by numerical analysis and simulation experiments. It is shown that the dynamic control protocol in this POF access network based on OCDMA technology is valid and efficient.