实时高速DDO-OFDM系统中的符号同步与采样频率同步技术

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"这篇研究论文探讨了在实时高速DDO-OFDM系统中符号同步和采样频率同步技术的应用和实验验证。实验在100公里标准单模光纤（SSMF）上进行，使用低成本的直接调制分布式反馈（DFB）激光器。" 在实时高速数字直接检测光正交频分复用（DDO-OFDM）系统中，符号同步（Symbol Synchronization）和采样频率同步（Sampling Frequency Synchronization）是两个至关重要的技术。它们对于确保信号的有效接收和正确解码具有决定性作用。 1. 符号同步：在通信系统中，符号同步是指确保接收端的信号解码时，其符号边界与发送端保持一致。在DDO-OFDM系统中，由于信号经过长距离传输和信道影响，可能会出现符号时序漂移，导致符号间的干扰，从而降低系统性能。因此，符号同步技术的目标是精确地确定每个OFDM符号的起始时刻，以减少因时间偏移引起的误码率。 2. 采样频率同步：采样频率同步涉及确保接收端的采样频率与发射端完全匹配。如果存在频率偏差，可能会导致频率混叠或信息丢失。在高速DDO-OFDM系统中，这种误差可能导致频谱重叠，影响解调过程，从而降低系统的数据传输速率和误码率性能。 3. OFDM技术：正交频分复用是一种高效的数据传输方法，通过将宽带信号分割成多个正交子载波来传输数据。OFDM技术在高速光通信中广泛应用，因为它能有效对抗多径传播和信道衰落，但同时也对同步精度提出了更高要求。 4. 实验设置：实验在100公里标准单模光纤上传输数据，采用直接调制DFB激光器，这是一种经济且常见的光源。这种方法虽然成本较低，但可能引入更多的噪声和失真，因此需要更精确的同步技术来补偿这些影响。 5. 结果与贡献：论文提出的技术经过实验验证，能够在长距离传输后保持良好的同步效果，提高了系统的稳定性和可靠性。这表明所提出的同步方法对于实时高速DDO-OFDM系统在实际应用中的性能提升具有重要意义。这篇研究论文的重点在于开发并实验验证了适用于实时高速DDO-OFDM系统的符号同步和采样频率同步技术，这对于提高长距离光纤通信的性能和稳定性具有重大价值。这些技术的实施有助于解决由同步问题引起的各种挑战，为未来高速光通信系统的设计和优化提供了新的思路。

Symbol synchronization and sampling frequency synchronization

techniques in real-time DDO-OFDM systems

Ming Chen

, Jing He

, Zizheng Cao

, Jin Tang

, Lin Chen

, Xian Wu

Key Laboratory for Micro/Nano Optoelectronic Devices of Ministry of Education, School of Information Science and Engineering,

Hunan University, Changsha 410082, China

School of Computer and Information Technology, Beijing Jiaotong University, Beijing 100 044, China

article info

Article history:

Received 14 February 2014

Received in revised form

2 April 2014

Accepted 7 April 2014

Available online 21 April 2014

Keywords:

Symbol synchronization

Sampling frequency synchronization

Orthogonal frequency-division multiplexing

(OFDM)

Real-time signal processing

abstract

In this paper, we propose and experimentally demonstrate a symbol synchronization and sampling

frequency synchronization techniques in real-time direct-detection optical orthogonal frequency

division multiplexing (DDO-OFDM) system, over 100-km standard single mode ﬁber (SSMF) using a

cost-effective directly modulated distributed feedback (DFB) laser. The experiment results show that the

proposed symbol synchronization based on training sequence (TS) has a low complexity and high

accuracy even at a sampling frequency offset (SFO) of 5000-ppm. Meanwhile, the proposed pilot-assisted

sampling frequency synchronization between digital-to-analog converter (DAC) and analog-to-digital

converter (ADC) is capable of estimating SFOs with an accuracy of o ±5-ppm for initial SFOs as large as

1000-ppm. Moreover, the sampling frequency synchronization technique can also compensate SFO

effects within a small residual SFO caused by deviation of SFO estimation and low-precision or unstable

clock source. The two synchronization techniques are suitable for high-speed DDO-OFDM transmission

systems.

1. Introduction

Due to the growing demand for communication bandwidth, the

current wire- and wireless-based access techniques will not meet

the ever-increasing bandwidth requirement. The real-time (RT)

optical OFDM systems have attracted a lot of attention due to its

powerful digital signal processing (DSP), ﬂexible dynamic band-

width allocation and high spectral efﬁciency. Compared with

coherent optical OFDM (CO-OFDM), intensity-modulation and

direct-detection optical OFDM (DDO-OFDM) are simpler and cost-

effectiave. Moreover, OFDM-based passive optical network (OFDM-

PON) has been proposed as a key optical access network technique

for the future [1].

The synchronization techniques in OFDM systems are very

important. For the DDO-OFDM system, the synchronization tech-

niques are mainly focused on symbol synchronization and sam-

pling frequency synchronization. A symbol synchronization error

brings about a fraction of a fast Fourier transform (FFT) window to

occur in an extended region of an adjacent symbol, leading to the

degradation of system performance. On the other hand, the

sampling frequency offset (SFO) between the digital-to-analog

converter (DAC) in the transmitter and analog-to-digital converter

(ADC) in the receiver can lead to three different effects: amplitude

reduction, phase rotation of the mapped QAM symbol and inter-

carrier interference (ICI) caused by the loss of orthogonality

between the sub-carriers [2].

For the high-sp ee d optica l OFDM system, the dat a rate i s

several orders of magnitud e highe r than wire l es s counterp ar t,

but the state-of- the-art FPGA can only operate at about a few

hundreds megahertz. Instead of the serial data processing, the

parallel algorithm should be c onside red. Therefore, in orde r to

achieve a hardware implementation of symbol sync hronization

for optical OFDM transmission systems efﬁciently, a novel 16-

parallel symbol synchronization method for real-time coherent

optical OFDM system was proposed [3].However, this approach

haslesstolerancetointer-symbolinterference(ISI).Asimple

symbol syn chronization technique utilizing subtraction and

Gaussian windowing was proposed and implemented in DD-

OOFDM re ceiver [4,5]. However, it requires a large number of

training symbols and may be not suitable for burst transmission

system. I n [6 ], a simple training sequence (TS)-based symbol

synchronization was proposed for the homodyne coherent

optical receiver. In order to avoid the heavy computation, the

accuracy of this synchronization technique was decreased. In

addition, to reduce or avoid SFO effects, a method was used to

estimate the SFO in the receiver, and then feedback the value to

a voltage co ntroller o scillator (VCO) f or eliminating the SFO

effects [5], but this method needs a stable and high-precision

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/optcom

Optics Communications

http://dx.doi.org/10.1016/j.optcom.2014.04.011

Corresponding author. Tel.: +86 073188821097.

E-mail address: hnu_jhe@hotmail.com (J. He).

Optics Communications 326 (2014) 80–87

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