【作者】 杨建国；

【导师】 蒋宇飞；

【作者基本信息】 哈尔滨工业大学 ， 信息与通信工程， 2020， 硕士

【摘要】 随着无线设备和无线服务的爆炸式增长,现有的无线射频频谱资源稀缺,无法满足人们对高速率的需求。可见光通信(Visible Light Communication,VLC)具有丰富可见光频谱资源,利用发光二极管(Light Emitting Diode,LED)来发送无线数据,是未来有潜力技术之一。但是,LED限制带宽会引起码间干扰,从而降低误码率性能以及限制数据传输速率。直流偏置光正交频分复用(Direct Current Biased Optical Orthogonal Frequency Division Multiplexing,DCO-OFDM)应用于可见光通信系统能有效抵抗码间干扰。然而DCO-OFDM系统对时间同步十分敏感,微小的时间同步误差会降低系统性能。所以,本文提出了三个应用于可见光通信DCO-OFDM系统的时间同步算法。本文设计了一个已知的DCO-OFDM导频块,并基于零相关码对提出了抗LED限制带宽的增强型零相关码算法,实现了时间同步。本文证明了文献中传统零相关码算法受LED限制带宽严重干扰,无法实现有效的时间同步。因此,本文定义了一个具有鲁棒性的崭新定时度量函数,利用零相关码对的脉冲型相关特性,在接收端相关性运算后再进行峰值检测得到定时偏移估计值。与文献中传统零相关码算法相比,提出的增强型零相关码算法对LED限制带宽具有更强的鲁棒性。仿真结果表明,提出的增强型零相关码算法比文献中传统零相关码算法具有更好的时间同步性能和误码率性能。本文面向可见光通信DCO-OFDM系统,首次提出了一个基于直流偏置的半盲时间同步算法和一个基于虚子载波的半盲时间同步算法。提出的直流偏置半盲算法和虚子载波半盲算法都是不需要训练序列,比基于训练序列的时间同步算法具有更高的频谱效率。提出的直流偏置算法是利用了DCO-OFDM系统中直流偏置的性质来定义定时度量函数,接着用阈值的方式得到精确的定时偏移估计值。提出的虚子载波算法是利用DCO-OFDM系统固有的虚子载波的功率来定义定时度量函数,然后利用比值的最大值来得到精确的定时偏移估计值。本文对直流偏置算法使用的阈值进行了理论分析,推导出了阈值的上下界、最优阈值的表达式、最优阈值与同步所需的DCO-OFDM块数量的关系。仿真结果表明,提出的直流偏置半盲算法和虚子载波半盲算法在时间同步性能方面都优于现有的同步算法,并提供了接近于在完美时间同步和完美信道信息情况下的误码率性能,同时具有更高的频谱效率以及较低的算法复杂度。更多还原

【Abstract】 With the explosive growth of wireless devices and wireless services,the existing radio frequency spectrum resources are scarce and cannot meet people’s demand for high-speed communications.Visible light communication(VLC)has a wide range of visible light spectrum,which uses light emitting diode(LED)to transmit wireless data and becomes one of the potential technologies in the future.However,the limited bandwidth of LED can cause inter-symbol interference(ISI),resulting in bit error rate(BER)performance degradation and data transmission rate limitation.Direct current biased optical-orthogonal frequency division multiplexing(DCO-OFDM)has been considered for VLC systems,as it can effectively combat ISI problem.However,DCOOFDM VLC systems are very sensitive to timing synchronization,and small timing synchronization errors degrade the system performance.Therefore,three timing synchronization algorithms are proposed for DCO-OFDM VLC systems in this paper.Based on the zero correlation code(ZCC)pair,an enhanced zero correlation code(e ZCC)algorithm is proposed to achieve timing synchronization by designing a single DCO-OFDM block,which is robust against the limited bandwidth of LED.We prove that the traditional ZCC algorithm in the literature is seriously disturbed by the limited bandwidth of LED,and can not achieve timing synchronization.Therefore,a new robust timing metric is designed,which uses the impulse-shaped correlation property of the ZCC pair to perform timing synchronization via peak detection in the correlation operation.Compared with the traditional ZCC algorithm in the literature,the proposed e ZCC algorithm is much more robust against the limited bandwidth of LED.Simulation results show that the proposed e ZCC algorithm has better performance of timing synchronization and BER than the traditional ZCC algorithm.This is the first work to propose a direct current bias(DCB)based semi-blind timing synchronization algorithm and a null subcarrier(NS)based semi-blind timing synchronization algorithm for DCO-OFDM VLC systems.The proposed both DCB and NS semi-blind algorithms provide higher spectral efficiency,requiring no pilot than training sequence based timing synchronization methods.The proposed DCB algorithm defines the timing metric with respect to direct current bias in DCO-OFDM systems,and then uses the threshold to refine the timing index.The proposed NS algorithm defines the timing metric with respect to the power of the null subcarrier inherent in DCO-OFDM systems and then refines the timing index by maximizing the ratio of the timing metric.In this paper,the threshold used in the proposed DCB algorithm is analyzed theoretically.The upper and lower bounds of the threshold,the optimal threshold,and between the optimal threshold and the number of DCO-OFDM blocks used for timing synchronization are derived.Simulation results show that the proposed DCB semi-blind timing synchronization and NS semi-blind timing synchronization algorithms outperform the state-of-the-art methods in terms of timing synchronization performance,provide the BER performance close to the ideal case with perfect timing synchronization and perfect channel state information,and provide higher spectral efficiency and lower complexity.更多还原