【作者】 王涛；

【导师】 侯永宏；

【作者基本信息】 天津大学 ， 信息与通信工程， 2020， 博士

【摘要】 在“互联网+”加速发展背景下,传统射频（Radio Frequency,RF）网络频谱资源已经非常拥挤,无法满足海量通信和减少功耗的要求。基于发光二极管（Light Emitting Diode,LED）的可见光通信（Visible Light Communication,VLC）凭借新频段、高速率、低功耗等诸多优势,被期望用于对RF通信进行有力互补。VLC中的主要矛盾表现为高速率需求受到LED的有限调制带宽的约束,光正交频分复用（Optical Orthogonal Frequency Division Multiplexing,O-OFDM）是一种高效调制解调技术,具有高频谱效率,可以缓解这一矛盾。但是受强度调制方式、多径效应及LED非线性等因素影响,现有的O-OFDM方案存在峰均功率比（Peak-to-average Power Ratio,PAPR）高、计算复杂度高、误码性能受幅度截断噪声和LED非线性影响、多径传播时信道估计的准确性受幅度截断噪声影响、频谱效率有损失等问题。本论文针对上述问题展开研究,完成的主要研究工作如下:1.混合非对称幅度截断光正交频分复用（Hybrid Asymmetrically Clipped Optical Orthogonal Frequency Division Multiplexing,HACO-OFDM）方案中传统接收机需要进行幅度截断噪声的再生和消除,这增加了计算复杂度,并导致脉冲幅度调制离散多音（Pulse Ampltude Modulated Discrete Multitone,PAM-DMT）支路的性能取决于幅度截断噪声估计的准确性。本文提出了一种充分利用时域信号结构的时域处理接收机,该接收机通过时域处理,消除了幅度截断噪声,避免了误差传播。与传统接收机相比,在采用512点快速傅里叶变换时,提出接收机的计算复杂度降低了58.3%。仿真结果表明,提出接收机中非对称幅度截断光正交频分复用（Asymmetrically Clipped OFDM,ACO-OFDM）支路误码率与传统接收机的完全相同。在中低信噪比区域,ACO-OFDM支路的误码率较高时,提出接收机的PAM-DMT支路误码性能比竞争方案的误码性能显著改善。2.本文提出了一种用于HACO-OFDM系统的信道估计方案。该方案利用最小二乘（Least Square,LS）算法估计出ACO-OFDM子载波上携带的信道特性,通过插值方法获得PAM-DMT子载波上的信道响应。仿真结果表明,该方案能够有效地对HACO-OFDM系统进行信道估计。根据信道估计的结果进行均衡后,ACO-OFDM支路在信噪比大于18.2d B时平均误码率低于10^-3;而PAM-DMT支路在信噪比大于19.4d B时平均误码率低于10^-3。3.由于没有利用偶数索引序号子载波的实部,HACO-OFDM方案具有明显的频谱效率损失。本文提出了一种提高频谱效率的方案,在该方案中,选定的偶数索引序号子载波组的实部被用于不同层上的数据调制,利用了更多的频谱资源。当采用相同的符号映射尺寸时,提出方案可以提高HACO-OFDM方案的频谱效率。当该方案采用5层,联合符号映射方案使用16QAM-4PAM和64QAM-4PAM时,频谱效率可以分别提高31.33%和23.48%。此外,与HACO-OFDM方案相比,该方案具有较低的PAPR和较好的LED非线性免疫力等优点。4.O-OFDM信号具有较高的PAPR,为了降低PAPR以缓解LED的非线性影响,并避免出现非对称幅度截断干扰,本文提出了一种降低PAPR的O-OFDM叠加调制方案,该方案通过联合μ律映射和叠加调制降低PAPR,而且在发射机处以无干扰的方式叠加O-OFDM分量,接收机不需要进行干扰消除的操作,可以独立地解调每个支路上的符号。仿真结果表明,该方案可以显著降低PAPR,同时,在LED非线性和有限动态范围的约束下,该方案的综合性能优于其他对比方案。更多还原

【Abstract】 Under rapid development of"Internet+",the conventional radio frequency（RF）network with increasingly crowded spectrum resources can not meet the requirements of mass communications and low power consumption.With the advantages of new spectrum resources,high data rates,and low power consumption,Visible Light Communication（VLC）based on light-emitting diode（LED）can complement RF communications.The main contradiction in VLC is that high-speed requirements are constrained by the limited modulation bandwidth of LEDs.Optical Orthogonal Frequency Division Multiplexing（O-OFDM）is an efficient modulation and demodulation technology,and its high spectrum efficiency can alleviate this contradiction.However,due to factors such as the intensity modulation method,multipath effect,and LED nonlinearity,there are some problems with existing superimposed O-OFDM,such as high peak-to-average power ratio（PAPR）,high complexity,bit error rate is affected by clipping noise and LED nonlinear distortion,the accuracy of channel estimation in the case of multipath propagation is affected by clipping noise,and the spectral efficiency is impaired.This thesis conducts research on the above issues,the main research work completed is as follows:1.The conventional receiver of hybrid asymmetrically clipped optical orthogonal frequency division multiplexing（HACO-OFDM）requires additional processing to regenerate and cancel the clipping noise,which increases the complexity and makes the performance of pulse-amplitude modulated discrete multitone（PAM-DMT）branch dependent on the accuracy of clipping noise estimation.In this thesis,a novel receiver of HACO-OFDM is proposed by fully exploiting the structure of time-domain signals.Through time-domain processing,the clipping noise can be removed and error propagation is avoided.Compared with the conventional receiver,the proposed receiver can achieve 58.3%complexity reduction in the case of a fast Fourier transform with size of 512.Simulation results show that the bit error rate（BER）of the proposed receiver is exactly the same as that of the conventional receiver for the asymmetrically clipped optical OFDM（ACO-OFDM）branch,but when the BER of the ACO-OFDM branch is high in the low-to-medium signal-to-noise ratio region,the BER performance of the PAM-DMT branch is significantly improved over other competing schemes.2.In this thesis,a channel estimation method for hybrid ACO-OFDM（HACO-OFDM）based visible light communications systems is proposed.The channel characteristics carried on the ACO-OFDM subcarriers are estimated by the least square（LS）method,and the channel response on the PAM-DMT subcarriers can be obtained by interpolation of which on the ACO-OFDM subcarriers.While considering the multipath channel in the thesis,the simulation results show that the proposed scheme is able to estimate the channel effectively for HACO-OFDM system.After equalization based on the results of the channel estimation,the average BER on the ACO-OFDM branch will be lower than10^-3 when SNR>18.2d B,the average BER on the PAM-DMT branch will be lower than10^-3 when SNR>19.4d B.3.The HACO-OFDM scheme has an obvious spectral efficiency loss due to the fact that the real part of the even subcarriers are not utilized.This thesis proposes a scheme to augment the spectral efficiency.In this scheme,more spectrum resources are exploited because the real part of selected even subcarrier group is utilized for data modulation at different depths.When the same modulation orders are used,the proposed scheme can enhance the spectral efficiency of HACO-OFDM scheme.When this scheme takes 5 depths,the joint modulation scheme uses 16QAM-4PAM and 64QAM-4PAM,the spectral efficiency of the proposed scheme can be increased by 31.33%and 23.48%,respectively.Moreover,compared with HACO-OFDM,the proposed scheme has advantages of lower PAPR and higher immunity to LED nonlinearity.4.Due to the high PAPR of O-OFDM signals,the inherent nonlinearity of LEDs is a challenge for O-OFDM schemes in visible light communication systems.In order to reduce PAPR to mitigate the effects of the nonlinearity of LEDs,and avoid the asymmetric clipping interference in the conventional hybrid O-OFDM schemes,a novel superimposed O-OFDM scheme is proposed in this thesis,which reduce the PAPR by combining the-law mapping and the superposition modulation.Moreover,at the transmitter,the hybrid O-OFDM components are superimposed in an interference-free way,the receiver does not require the operation of interference cancellation,and the symbols of each branch can be demodulated independently.Simulation results verify that the proposed scheme can significantly reduce the PAPR.At the same time,the comprehensive performance of the proposed scheme outperforms other competing schemes under the constraints of the nonlinearity and limited dynamic range of LEDs.更多还原