基于超结构光纤光栅的混沌序列OCDMA系统

Chaotic OCDMA system based on superstructure fiber Bragg grating

提出了一种双极性编解码的光码多分址(OCDMA)系统,采用混沌序列作地址码,并使用超结构光纤布拉格光栅(SSFBG)作编/解码器。三阶Chebyshev映射能够产生具有良好相关性且适用于异步CDMA系统的双极性混沌序列,通过对三阶Chebyshev映射赋予一定初值,并进行反复迭代,得到一定长度的双极性混沌序列。根据地址码的特征,通过控制FBG的折射率分布形成SSFBG,利用SSFBG产生相应的双极性混沌序列作OCDMA系统的编/解码器。对系统性能进行了定量分析,在完全异步的情况下,推导了信噪比(SNR)与误码率(BER)的表达式。仿真分析了系统在不同码长下的BER随接入用户数的变化,并和使用Gold序列的情况进行了比较。结果表明,随着码长的增加,在接入用户数一定的情况下,系统具有较低的BER;由于混沌序列良好的自互相关性,相应的混沌序列OCDMA系统与使用Gold序列的系统相比具有较低的BER,并可以容纳更多的用户数。

This paper presents a bipolar en/decoding optical code division multiple access （OCDMA） sys- tem based on chaotic sequences, whose en/decoder is superstructure fiber Bragg grating （SSFBG）. The three-order Chebyshev map can generate bipolar chaotic sequences which are appropriate for asyn- chronous CDMA system. With an initial condition and repeating application of the three-order Chebyshev map,chaotic sequences with a certain length can be obtained. According to the characteristics of address code and through control of refractive index distribution of fiber grating, the SSFBG can be fabricated which can generate bipolar chaotic sequences,so SSFBG can be used as the en/decoder of the OCDMA system. The performance of this system is tested by the quantitative analysis. In the case of asynchroni- zation, the formulas of SNR and BER are derived and the curves of BER versus the number of simulta- neous transmissions are simulated. Our numerical simulations show that with the increase of code length,the system has a lower BER with identical number of users, and the system based on chaotic spreading sequences performs better than that using Gold sequences.