Enhanced performance analysis of optical CDMA communication system

Optical CDMA technique is required to meet the increased demand for high speed, large capacity communications in optical networks. OCDMA assures data access security and supports asynchronous burst data transmission. In this paper, improvement in optical CDMA has been done by implementing forward error correction technique and a distance of 100 km was achieved BER 10⁻¹⁹ at bit rate 3.75 Gbps with forward error correction in optical CDMA.     

基于2D-OOC的OCDMA系统的地址码结构研究

提出了二维光正交码 (2D OOC)设计方案 ,围绕光码分多址 (OCDMA)技术实用化的两个主要指标—最大并发用户数和系统误码率的改进和提高 ,系统地研究了基于 2D OOC的OCDMA的性能 ,分析了 2D OOC码字的相关性 ,导出了系统的最大用户容量 ,详细研究了 2D OOC系统以多址干扰为主的系统误码率 .研究表明 :较之一维时域扩频码或一维谱域扩频码 ,2D OOC系统性能大大提高 ,在大信息量传输时 ,适当选择码长和码重 ,系统总容量可高达Tbits/s量级 ,性能优于基于 1D OOC的OWDM和OCDMA混合系统 .     

基于2D-OOC的OCDMA系统的地址码结构研究

提出了二维光正交码(2D-OOC)设计方案,围绕光码分多址(OCDMA)技术实用化的两个主要指标-最大并发用户数和系统误码率的改进和提高,系统地研究了基于2D-OOC的OCDMA的性能,分析了2D-OOC码字的相关性,导出了系统的最大用户容量,详细研究了2D-OOC系统以多址干扰为主的系统误码率.研究表明:较之一维时域扩频码或一维谱域扩频码,2D-OOC系统性能大大提高,在大信息量传输时,适当选择码长和码重,系统总容量可高达Tbits/s量级,性能优于基于1D-OOC的OWDM和OCDMA混合系统.     

卫星振动对星间光码分多址系统性能的影响

卫星振动是影响星间光CDMA通信系统性能的一个重要因素.考虑多用户干扰、背景光噪音、热噪音、接收机噪音和卫星振动,给出了基于PPM信号格式的星间二维光CDMA通信系统的系统模型.采用数值分析的方法,详细分析了卫星振动对该系统误码率性能的影响.结果表明,码速率、通信波长和卫星振动都会影响星间二维光CDMA通信系统的误码率性能.当卫星振动标准偏差σ≤4×10－7时,卫星振动对系统误码率性能的影响较小;当卫星振动标准偏差σ≥1.2×10－6,卫星振动对系统误码率性能的影响很大,得到的误码率难以满足系统的通信要求,需要采用卫星振动抑制或补偿等技术提高星间二维光CDMA通信系统的误码率性能.     

大气闪烁对无线光CDMA通信系统性能的影响

将光CDMA技术应用到无线光通信中。提出了基于脉冲位置调制的二维大气无线光CDMA通信系统，并分析了该通信系统的性能。考虑多用户干扰、APD噪声以及热噪声等干扰因素的影响，采用数值分析的方法，详细分析了大气湍流引起的大气闪烁对二维无线光CDMA通信系统误码率的影响。结果表明，大气闪烁是影响二维无线光CDMA系统误码率性能的一个重要因素；当大气闪烁的对数方差较小(如σ2s=0.1)时，该系统可以实现高速率通信；当大气闪烁的对数方差较大(σ2s≥0.2)时，在有限的光功率条件下，该系统难以实现通信，需要采用信道编码技术来提高系统的误码率性能。     

用于二维光址多分址系统的代数同余码研究

总结了线性、二次和双曲等有限域内的代数同余运算在光码分多址系统中的应用,将代数同余码分为三类：跳频码、扩时码和跳频扩时码,并给出它们的构造公式。提出了一种同时采用这三类编码分别用以提供不同类型用户服务的二维光码分多址方案,该系统方案尤其适合于大量用户需要常规服务,而少部分用户需要高速或高服务质量(QoS)服务的情况,具有对速率和服务质量的强适应能力。为了证明该系统方案的可行性,首先证明了三种码字的互相关常数,然后推导了达到各种互相关值的概率,最后得出混合系统中不同类型干扰对系统误码率的影响,并进行了数值分析与讨论。     

Fiber-wireless (FiWi) access network: Performance evaluation and scalability analysis of the physical layer

The fiber-wireless (FiWi) access network is a prestigious architecture for next generation (NG) access network. NG access networks are proposed to provide high data rate, broadband multiple services, scalable bandwidth, and flexible communication for manifold wireless end-users (WEUs). In this paper, the FiWi access network is designed based on a wavelengths division multiplexing/time division multiplexing passive optical network (WDM/TDM PON) at the optical backhaul with data rate of 2.5 Gb/s and wireless fidelity-worldwide interoperability for microwave access (WiFi–WiMAX) technologies at the wireless front-end along a 50 m–5 km wireless links with data rate of 54–30 Mb/s, respectively. The performance of the optical backhaul and the wireless front-end, in the proposed FiWi access network, has been evaluated in terms of bit error rate (BER), error vector magnitude (EVM), and signal-to-noise ratio (SNR) of the physical (PHY) layer. The scalability of the optical backhaul based on maximum split ratio of 1/32 for each wavelength channel and a fiber length of 24 km from the central office (CO) to the access point (AP) is analyzed with bit error rate (BER) of 10⁻⁹.     

2.50 Gbit/s optical CDMA over FSO communication system

Optical CDMA over FSO communication system is very effective to provide high data rate transmission with very low bit error rate and low amount of multiple access interference. In this paper, we have presented optical CDMA over FSO communication system to the range of 8000 m. The simulative results reveal that the transmission distance is limited mainly by the multi-access interference (MAI) which arises when there are number of users in the system because of the fact that one user data becomes noisy for all other users in the channel.     

Measurement of apex angle of the prism using total internal reflection

Total internal reflection technique is examined critically for the measurement of apex angle of the prism. Recognition of the critical boundary completely specifies the apex angle ‘A’ of the prism by keeping constant the ratio of refractive index of sample fluid (n₁=1.4) to that of glass (n₂=1.7). Recording of observations in the laboratory is dependent upon whether the apex angle ‘A’ of prism is greater than or less than or equal to critical angle ‘r_c’. The minimum value of error function ‘χ’ in the proximity of Abbe angle explain the choice of Abbe type instruments and shown invariant by relating positive ‘₊’ and the negative angle of emergence ‘₋’. Using ray-tracing technique, an expression for the length of the critical boundary ‘l’ is derived and is shown as a measure of the apex angle ‘A’ of the prism.

The choice of the realistic grazing condition leads us to two turning points for which error function ‘χ’ is minimal and yields appropriate option of the measurement of apex angle ‘A’ of the prism. It is further shown that the small error ‘ζ’ crept in the apex angle ‘A’ of the prism can lead to a significant error in the design of Pulfrich type instruments.

Experimental measurements are in-line with the predictions made.     

Cardinality enhancement of spectral/spatial modified double weight code optical code division multi-access system by PIIN suppression

A new two-dimensional (2-D) optical code division multiple access (OCDMA) scheme to increase the achievable system capacity is proposed. The code exhibits good cross-correlation property time and wavelength shift. Performances are analyzed on code size and correlation properties affecting two important system parameters, bit error rate (BER) as a function of cardinality generated and optical power transmission requirement. The proposed system can effectively suppress phase-induced intensity noise (PIIN) and has multi-access interference (MAI) cancellation property. Results in a good agreement indicate that 2-D modified double weight (MDW) offers 163.7% and 336.2% larger cardinality compare to 2-D perfect difference code (PDC) and 2-D modified quadratic congruence (MQC) code. By increasing spatial code (N) and keeps similar code length system performance can be further optimized. 2-D MDW (M = 45, N = 18) accommodates 252.2% and 18.3% cardinality increment and low effective transmitted power (P_sr) at −17.9 dBm, compare to 2-D MDW (M = 247, N = 3) and (M = 84, N = 9) at 10⁻⁹ BER error floor. The architecture of the spectral/spatial MDW OCDMA system with property of MAI cancellation is presented.     

一种基于扩展素数码和单重合序列的二维光正交码EPC/OCS

以扩展素数码(EPC)作为时间扩频伪随机序列，单重合序列(OCS)作为波长跳频伪随机序列，构造了一种新的二维光正交码EPC/OCS，并分析了码字的互相关性能.与修正素数跳频码MPHC相比，EPC/OCS的波长数并不局限于素数，可以是任意整数，不仅构造灵活，而且可充分利用MW OCDMA系统的有效波长数.当系统的有效波长数大于某个素数时，EPC/OCS不仅码字容量大于MPHC，而且互相关性能也有所改善.理论分析表明，EPC/OCS可降低MW OCDMA系统误码率.     

基于光纤布拉格光栅编解码器的直接序列扩频跳频光码分多址系统

提出了一种混合直扩跳频光纤码分多址（DS／FH－OCDMA）系统的实现方法，该系统采用跳频素数码作为扩频码，运用了光纤布拉格光栅序列作为编解码器，并在解码器两端安置了双硬限幅器以提高系统的性能，理论分析和仿真结果表明，该系统具有大用户容量，高功率效率和低误码率等方面的优点。     

Dual-Output Mach–Zehnder Modulator for Optical Access Networks

This paper employs dual-output Mach–Zehnder Modulator (MZM) for optical access networks without optical filters. Light waves generated from multiple laser sources are multiplexed and fed into dual-output MZM. Biasing the dual-output MZM at null point generates central carriers in one output port and first-order sidebands in another output port. Reflective semiconductor optical amplifier modulates both the central carriers and sidebands with wired and wireless data, respectively. The modulated optical signals are combined by polarization beam splitter and transmitted through 25-km single-mode fiber. The performance of the proposed scheme is proved by clear eye-diagrams and great bit error rate (BER) curves. Moreover, the power penalty at the BER of 10^-9 is less than 1 dB for both wired and wireless signals. Therefore, the proposed system simultaneously transmits wired and wireless signals.     

Analysis of physical layer performance of hybrid optical-wireless access network

The hybrid optical-wireless access network (HOWAN) is a favorable architecture for next generation access network. It is an optimal combination of an optical backhaul and a wireless front-end for an efficient access network. In this paper, the HOWAN architecture is designed based on a wavelengths division multiplexing/time division multiplexing passive optical network (WDM/TDM PON) at the optical backhaul and a wireless fidelity (WiFi) technology at the wireless front-end. The HOWAN is proposed that can provide blanket coverage of broadband and flexible connection for end-users. Most of the existing works, based on performance evaluation are concerned on network layer aspects. This paper reports physical layer performance in terms of the bit error rate (BER), eye diagram, and signal-to-noise ratio (SNR) of the communication system. It accommodates 8 wavelength channels with 32 optical network unit/wireless access points (ONU/APs). It is demonstrated that downstream and upstream of 2 Gb/s can be achieved by optical backhaul for each wavelength channel along optical fiber length of 20 km and a data rate of 54 Mb/s per ONU/AP along a 50 m outdoor wireless link.     

基于最大似然准则的多波长OCDMA接收机的最佳判决门限研究

采用最大似然准则,研究了多波长光码分多址接收机的最佳判决门限值.通过检测信道的光信号能量,接收端估计出信道的并发用户数,并通过最大似然准则得到了最佳判决门限值与并发用户数、码字参量的变化关系.当并发用户数超过一定值后,最佳判决门限值将随并发用户数量变化,相应地改变接收机的判决门限值.根据最佳判决门限值,得到了多波长光码分多址系统误码率与并发用户数的关系.理论分析和模拟结果表明,采用最大似然准则得到的最佳判决门限值,MW OCDMA系统的误码率将明显降低.     

无线光码分多址中一种改进的盲多用户检测算法

干扰消除是无线光码分多址(CDMA)通信系统中的一项关键技术。分析了基于递归最小二乘(RLS)算法的盲多用户检测算法的原理及性能,并将其应用于采用光正交码的无线光CDMA系统中。讨论了该系统中遗忘因子的选取原则及其对算法的收敛性和系统误码率性能的影响。为了克服固定遗忘因子所带来的矛盾,提出了一种改进的变遗忘因子RLS盲多用户检测算法,并对遗忘因子进行了修正。结果表明,采用改进的方法后系统能获得较快的收敛速度和跟踪速度,收敛时的估计误差也较小,而且信号干扰比由传统算法的6dB提高到9dB。说明采用变遗忘因子的RLS算法不仅适合于采用光正交码的无线光CDMA系统,而且跟踪期望用户信号的性能良好。     

Design and performance analysis in multiuser optical CDMA systems

The article presents comparative performance analysis of the proposed Optical CDMA system for 32 and 16 users with two dimensional codes. Numerical simulations have been done under interference significant environment, considering noise and dark current at data rates 2.5, 5, 7.5 and 10 Gbps over single mode fiber for transmission distance of more than 270 km. Results illustrate overall good performance, degraded with augment in bit rate and transmission distance, impervious with raise in number of simultaneous active users. Depicts significant performance improvement with inclusion of forward error correction RS (255,239), for low attenuation and chirp factor. It perceived, this is one of the efficient functional techniques for next generation broadband optical networks together with higher security owing to encoding and decoding, as it allow multiple users in the network to access the same fiber channel asynchronously.     

Frequency-swept coherently detected spectral amplitude code for flexible implicit optical label switching

A new optical label switching system with coherently detected implicit spectral amplitude code (SAC) labels is proposed in this letter. The implicit SAC labels are recognized using a frequency-swept local light source oscillator. Intensity modulation payloads of 625 Mb/s and 1.25 Gb/s are considered. Label and payload bit error rate (BER) performances are assessed and compared by simulations. The results reveal that, at a BER value of 10 9 , 32.4-dBm label received power can be obtained. In addition, 8.3-dB optical signal-to-noise ratio (OSNR) is obtained when carrying a payload of 625 Mb/s. The label BER value hardly reaches 10 -9 if the payload bit rate is at 1.25 Gb/s; however, a high payload bit rate only has little influence on received payload quality at a BER value of 10 -9 . Finally, a payload of 1.25 Gb/s could obtain 28.2 dBm received power and 9.5-dB OSNR.     

Performance evaluation of Fi-Wi network based on SCM–optical code division multiple access architecture

This paper investigates the performance of a spectral amplitude coding (SAC)–OCDMA system design utilizing subcarrier multiplexing techniques for use in the popular fiber-wireless (Fi-Wi) technology. Zero cross correlation code is employed because of its ability to eliminate phase induced intensity noise (PIIN) and the simplicity of using a direct detection technique at the receiving end. The performances are evaluated theoretically to derive the signal-to-noise ratio and results are presented in terms of bit error rates (BER). Results are analyzed to investigate the effect of code weight and variable data rates on the system performance. To validate the results, simulative analysis is also done using Optisys version 6.0. We demonstrated that the ZCC code performed better compared to other code for a BER floor of 10⁻⁹ with larger cardinality of subscribers. Results obviously indicate that system performance can be improved with larger code weight. In addition, the choice of lower bit rate is observed to perform better and more suitable for application in this hybrid Fi-Wi network.     

On the application of the optical method of caustics to the investigation of transient elastodynamic crack problems: Limitations of the classical interpretation

Several possible sources of inaccuracy that occur in the classical interpretation of caustics patterns generated during transient crack growth in elastic materials are examined using a ‘Bifocal Caustics’ set-up and a new full field optical technique called ‘Coherent Gradient Sensing’. During unsteady dynamic crack growth, strict K^d_I-dominance is generally absent, especially at times close to crack initiation and arrest, even in regions outside the crack-tip 3-D zone where plane stress conditions persist. In such cases a truly transient higher order expansion is found to be essential for correctly describing stress fields outside the 3-D zone.