【作者】 刘云；

【导师】 季飞；

【作者基本信息】 华南理工大学 ， 信息与通信工程， 2018， 博士

【摘要】 OFDM(Orthogonal Frequency Division Multiplexing,正交频分复用)技术是多载波传输方案的实现方式之一,是频率选择性信道下实现高速通信的有效技术,其实现简单,频谱利用率高,已被广泛用于各种数字数字传输系统中。然而,OFDM具有三个显著的缺点:1)系统对时间同步要求高,时间同步误差可以导致明显的ISI(Inter-symbol Interference,符号间干扰)和ICI(Inter-carrier Inteference,子载波间干扰);2)系统对信道的时变及频率同步误差非常敏感,它们可以轻易地摧毁子载波之间的正交性,导致ICI;3)信号的峰均比很高,即信号动态范围大,增加了发射端功率放大器的成本,且耗电量大。OFDM的上述缺点直接导致了其在某些场景中的应用受限,例如在高铁移动通信及水下声通信中,信道的时变性对实现高速可靠的OFDM通信形成了很大的挑战。针对这些问题,本文研究OFDM系统的性能增强技术,内容涵盖OFDM时间同步、功率分配和子载波索引调制等。为了实现时间同步,发射端通常在OFDM符号帧的起始位置插入一段被称为导言的辅助数据,接收端再运用相关操作对其进行检测。根据相关方式,早期时间同步算法可以划分为两大类:1)基于接收信号自相关的算法;2)基于接收信号与本地导言互相关的算法。2011年,Hamed与Mahrokh提出了一种自相关与互相关相结合的算法(HM算法),该算法与导言结构无关,可以抗任何程度的载波频偏或多普勒频移,在低信噪比下也能稳健工作,鲁棒性很高。然而,该算法的计算复杂度较高。为了降低计算复杂度,本文对HM算法进行了改进,所得新算法的计算复杂度是HM算法的一半。蒙特卡罗仿真结果表明,本文提出的简化算法的性能与HM算法完全相同。基于IM(Index Modulation,索引调制)的OFDM是一种性能增强的OFDM技术,简称OFDM-IM。在OFDM-IM中,系统的所有子载波被划分为若干个子块,每个子块中仅有固定数量的部分子载波被激活,其余子载波保持沉默。激活子载波的位置是可变的,部分信息比特可以调制到激活子载波的索引上,被称为索引比特;与传统OFDM一样,在激活子载波上传输经典的数字调制符号,所传输的信息称被为符号比特。在本文中,我们提出了一种名为ECI-OFDM-IM(Enhanced Coordinate Interleaved OFDM,增强的正交交织OFDM)的调制方案。在ECI-OFDM-IM中,通过增加子块子载波激活图案之间的最小距离,索引比特的误码性能得到显著提高;通过对激活子载波上的数据符号进行性能增强的正交交织,符号比特的误码性能也得到显著提升。理论分析及蒙特卡洛仿真结果都表明,在信道利用率相同的前提下,ECI-OFDM-IM的误码性能显著优于传统OFDM、OFDM-IM和CI-OFDM-IM。在移动场景下,很多无线信道都可以建模为MSML(Multi Scale Multi Lag,多时间尺度扩展多延迟)模型,例如移动水声信道。在MSML信道下,信号经多条路径到达接收端,接收信号的各多径分量不但经历不同的传输延迟及信号衰减,还经历不同程度的时间尺度扩展。信号的时间尺度扩展会导致子载波频移,对于具有宽带本质(信道带宽与载波频率可比拟)的信道,如水声信道,各子载波上的多普勒频移并不相同,即子载波经历非一致的频偏。对接收信号进行重采样,有助于缓解信号因时间尺度扩展引起的非一致多普勒频移问题,但由于个路径的扩展因子不同,重采样操作无法完全补偿各子载波的多普勒频移/扩展,因而在MSML信道下,OFDM信号的ICI固然存在。本文在信道路径的时间尺度扩展、衰减因子和传输时延的统计特性都给定的条件下,推导了OFDM信号各子载波的ICI系数,并得到了闭式解。进一步证明了子载波的ICI系数随子载波频率单调递增,也即低频子载波的信道质量在统计意义上优于高频子载波。基于该结论,我们提出一种新型的子载波功率分配算法。在该算法中,我们将所有子载波依频率高低划分为两部分:低频段与高频段。考虑到低频子载波符号间干扰系数小,我们让该段的子载波全部激活,使得信道自由度得到充分利用;高频子载波的符号间干扰系数较大,我们将奇数序号的子载波关闭,避免了符号间干扰引起的低能效问题;最后在所有激活子载波上平均分配发射功率。仿真结果表明,本功率分配算法在MSML信道下显著强于传统的平均分配功率算法及统计的注水算法,可以获得更大的平均可达信息速率。更多还原

【Abstract】 OFDM(Orthogonal Frequency Division Multiplexing)is one of the existing modulation schemes based on multi-carrier technology.With high spectral efficiency and low complexity,it has been applied in many practical high-speed digital communication systems.However,OFDM has three main disadvantages: First,the receiver makes great demands on the accuracy of timing synchronization,because severe ISI(Inter-symbol Interference)and ICI(Inter-carrier Interference)can be resulted by even small synchronization errors of timing.Secondly,the performance of OFDM is very sensitive to the carrier frequency offset and the variance of the channel,which can destroy the orthognality among subcarriers and result in ICI.Thirdly,the OFDM signal has a high PAPR(Peak-to-average),which increases the cost of the power amplifier,and also the power assumption at the transmitter.The above disadvantages of OFDM make it very challenging to achieve reliable communicate in some scenarios,such as in underwater acoustic channels,or RF(Radio Frequency)channels between the base station and terminals in a high-speed train.In this paper,we investigate the performance-enhancing techniques for OFDM,including timing synchronization,power allocation,and index modulation.To achieve timing synchronization,at the sender,an auxiliary sequence termed preamble is always padded at the header of the OFDM frame.At the receiver,the position of the data frame is estimated by performing correlation operations on the preamble.The corresponding algorithms can be divided into two categories: 1)those performing self-correlation among different parts of the received sequence;2)those performing inter-correlation between the local preamble with the received sequence.In 2011,by combining the self-correlation and inter-correlation operations,Hamed and Mahrokh proposed a robust timing method(HM algorithm).This timing estimator is independent of the structure of the preamble,immune against the CFO(Carrier Frequency Offset),and working even at low SNR(Signal-to-noise Ratio).However,it suffers from high computational complexity.In this paper,based on HM algorithm,we propose a modified timing estimator,which can halving the computational complexity of HM algorithm without performance loss.OFDM-IM(OFDM with Index Modulation)is a kind of enhanced modulation schemes based on OFDM.In OFDM-IM,In OFDM-IM,the available sub-carriers are first divided into several sub-blocks.In each sub-block,only a fixed number of sub-carriers are activated,and the remaining sub-carriers keep silent.The position of the active subcarrier is variable sub-block by sub-block,and thus the indices of the activated subcarrier can convey some data bits,which is called index bits.Like the traditional OFDM,the constellation symbols transmitted on the active subcarriers deliver data bits,which is called symbol bits.In this paper,we propose a modulation scheme called ECI-OFDM-IM(Enhanced Coordinate Interleaved OFDM).In ECI-OFDM-IM,by increasing the minimum distance between different sub-carrier activation patterns,the indexbit error performance of the index bits is significantly improved;through performance-enhanced coordinate interleaving of data symbols on active sub-carriers,the symbol-bit error performance has also been significantly improved.Both theoretical analysis and Monte Carlo simulation results show that the error performance of ECI-OFDM-IM is significantly better than that of conventional OFDM and OFDM-IM at the same spectral efficiency.In mobile communication scenarios,many wireless channels can be described by MSML(Multi-scale Multi-lag)model,such as mobile UWA(Underwater acoustic)channels.Under a MSML channel,the received signal can be viewed as a superposition of many path components with different delays,attenuation factors,and time scales.For wide-band OFDM systems,the time-scaled signals always suffer from non-uniform frequency shifts across the sub-carriers.The resampling has been widely used to compensate the Doppler scaling induced non-uniform frequency shifts.However,in MSML channels,the ICI resulted by the Doppler distortion cannot be cleared out completely even with the optimal resampling factor,because the different pathscales cannot be compensated simultaneously with a single resampling factor.In this paper,given some statistics of the delays,attenuation factors,and path-scales of the channel paths,we derive the ICI coefficients of the sub-carriers in closed-form.It is also proved that the ICI coefficient increases with the sub-carrier frequency monotonously,i.e.,the sub-carriers of lower frequency are better than those of higher frequency.Based on this observation,we propose a novel power allocation scheme,in which the total subcarriers are divided into two consecutive parts with respect to the carrier-frequency.For the part of lower frequency,by considering that the ICI coefficients is negligible,all the sub-carriers are activated to make full use of the degrees of freedom of the channel.For the part of higher frequency,the subcarriers of odd indices keep silent to avoid the low energy efficiency resulted by ICI.The total power then is allocated uniformly across all the active sub-carriers.As shown in the simulation result,the proposed power allocation performs better than the existing statistical water-filling method and uniform power allocation in average achievable rate.更多还原