摘要
摘要
在宽带业务快速发展的推动下，光短距离互联系统的带宽需求比骨干网流量增长的更快。尽管结合先进的调制方式和数字信号处理技术，相干检测在骨干传输系统中得到了广泛的应用。然而，短距互联系统对成本和功耗问题十分敏感，强度调制和直接检测（IM/DD）系统更适合短距互联。但是，低成本的IM/DD系统在传输中容易受到如直调激光器啁啾，光纤散（CD）、光电器件带宽受限和光纤非线性的影响。基于上述问题，非线性补偿问题必将成为光学短距离系统研究的热点。在深入了解自适应均衡的关键技术的基础上，我们在实验中利用沃尔泰拉（V olterra）滤波器补偿在正交频分复用（OFDM）和脉冲幅度调制（PAM）系统中的非线性失真。本文的主要研究内容如下：
(1) 本文阐述了基于最小均方（LMS）和递归最小二乘（RLS）的V olterra自适应算法原理，并通过实验数据对两种自适应算法进行对比分析。然后介绍了正OFDM和PAM-4两种高阶调制格式和各自的一些优缺点。
(2) 通过实验中平台搭建了IM/DD OFDM实验系统，首先使用V olterra均衡器补偿非线性并分析对信噪比和误码率的影响，然后在此基础上结合Bitloding算法和V olterra算法提升系统的传输容量。实验系统中，最后实现了36 Gbps的OFDM 信号传输50 km光纤的实验，系统传输容量提升了22%。
(3) 通过实验中平台搭建了IM/DD PAM-4实验系统，首先对PAM-4系统中传统的前馈均衡器（FFE）和V olterra算法进行对比，并根据PAM-4系统的特点对算法作出改进。讨论了V olterra滤波器不同阶记忆长度对系统性能的影响，确定了主要干扰来自线性项和二次项。针对PAM-4系统中V olterra均衡器复杂度较高的问题引入了一种截断算法，在保证系统传输性能的基础上极大的降低了算法的复杂度。
关键词：非线性，沃尔泰拉，正交频分复用，脉冲幅度调制，算法复杂度
I
ABSTRACT
Driven by the rapid development of broadband services, the increasing bandwidth demand of short-reach optical transmissions grows even faster than that in backbone networks. Combined with advanced modulation formats and digital signal processing, coherent detection has been widely used in backbone transmission systems. However, the cost and energy consumption are much more sensitive in short reach optical transmissions. Intensity modulation and direct detection (IM/DD) system is more suitable for short-reach applications.However, there are some transmission impairments which seriously degrade the transmission performance of the DML-based IM/DD transmission system, such as laser chirp, chromatic dispersion (CD), limited bandwidth of the optoel
ectronic devices and fiber nonlinearity. Based on the above problems, the compensation of nonlinear has become a hot topic in optical short-reach systems. On the basis of the key technologies in adaptive equalizer, we use the V olterra filter to compensate for nonlinear distortions in Orthogonal Frequency Division Multiplexing (OFDM) and Pulse Amplitude Modulation (PAM) systems in the experiments. The main contents of this paper are as follows:
(1) This paper describes the principle of adaptive V olterra algorithm based on  Least mean square (LMS) and Recursive Least Square (RLS) algorithms. The two adaptive algorithms are compared by experimental setup. Then, two kinds of high order modulation formats, i.e. OFDM and PAM-4, and their advantages and disadvantages are introduced.
(2) The IM/DD OFDM system has been built through experimental platform. Firstly, the V olterra equalizer has been used to compensate for the nonlinear distortions. Then, on the basis of the bitloding algorithm , V olterra algorithm is used to enhance the transmission capacity of the system. In the OFDM IM/DD system, we finally realize the 36 Gbps OFDM signal transmission over 50 km standard single mode fiber(SSMF) experiment. The system transmission capacity has been increased by 22%.
(3)The IM/DD PAM-4 system has been built through experimental platform. Firstly, the traditional Feed-Forward Equalizer (FFE) and V olterra algorithm in PAM-4 system are compared, and the V olterra algorithm is modified according to the characteristics of the PAM-4 system. Then, the influence of different memory lengths
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on the performance of V olterra filter has been discussed, and the main interference is derived from linear and the second order terms. At last, in order to solve the problem of high complexity of V olterra equalizer in PAM-4 system, a truncated algorithm is introduced, which greatly reduces the complexity of V olterra algorithm and of guarantee the transmission performance of the PAM-4 system.
Key words: nonlinearity, V olterra,OFDM,PAM-4, algorithm complexity
III
目录
第一章绪论 (1)
1.1 研究背景 (1)
1.2 研究意义 (2)
1.3国内外研究现状 (3)
1.4本论文的主要研究内容 (4)
第二章基于Volterra级数的自适应信道估计 (6)
2.1 V olterra级数 (6)
2.1.1 V olterra级数研究现状 (6)
2.1.2 V olterra级数模型 (7)
2.2 自适应算法 (8)
2.2.1基于LMS的V olterra自适应算法 (8)
2.2.2 V olterra RLS自适应算法 (11)
2.2.3 RLS和LMS算法对比 (13)
2.2.4本章小结 (15)
第三章光OFDM系统中的非线性补偿 (16)
3.1IM/DD OFDM实验原理框图及环境配置 (16)
3.1.1 实验原理框图 (16)
3.1.2 实验环境配置 (17)
3.1.3 实验参数优化 (19)
3.2实验结果分析 (20)
3.2.1 B2B实验结果 (20)
3.2.2传输25km光纤实验结果 (22)
3.2.3传输50km光纤实验结果 (26)
3.2.4传输容量和距离关系 (30)
3.3本章小结 (30)
第四章PAM-4系统中的非线性补偿 (32)
4.1实验框图和环境配置参数 (32)
4.1.1实验框图 (32)
4.1.2实验参数设置 (33)
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4.2实验结果分析 (34)
4.2.1背靠背实验结果分析 (35)
4.2.2光纤传输结果讨论 (39)
4.2.2.1光纤传输实验结果 (39)
4.2.2.2 V olterra滤波器改进前后对比 (41)
4.2.2.3记忆长度对传输性能的影响 (41)
4.3算法复杂度问题 (43)
4.4本章小结 (45)
第五章总结与展望 (46)
5.1论文主要工作 (46)
5.2下一步工作展望 (47)
致谢 (48)
degrade参考文献 (49)
攻读硕士期间取得的成果 (53)
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