OFDM原理.ppt

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,1,多载波调制,1、多载波调制(MCM)的基本思想,MCM：Multi-Carrier Modulation，将待传输的数据流分解成M个子数据流，每个子数据流的传输速率降为原数据流的1/M，然后用这些子数据流去并行调制M个载波,。,串,/,并变换,+,cos2,f,1,t,cos2,f,2,t,cos2,f,M,t,a,k,S,MCM,(t),1,2,2、,MCM,的意义,时间弥散（时延扩展，,time delay spread,）：,移动信道中，由于存在多条不同距离的传播路径，使发送端发送的一个脉冲，到达接收端时却是多个不同时延脉冲在时间轴上的叠加。,将最大时延与最小时延的差值称为时延扩展，记为,。若发送的脉冲宽度为,T,s,，则接收脉冲的宽度为：,T,s,+,。,时延扩展将导致,ISI,，并且码间串扰的严重程度与,/,T,s,成正比。,多载波调制可以降低信道传输的符号速率，即增加,T,s,，因此可以有效地减小甚至消除码间串扰。,2,3,3、MCM的实现方式,多音实现的,MCM,（,Multitone,Realization MCM,）,主要用于有线传输系统,正交频分复用的,MCM,（,OFDM,）,主要用于无线传输系统,多载波码分复用,MCM,（,MC-CDMA,）,主要用于扩频通信系统,3,4,OFDM,1、OFDM的基本思想,OFDM：Orthogonal Frequency Division Multiplexing，正交频分复用。,将信道分成N个子信道，每个子信道上一个载波，称为子载波，各个子载波之间相互正交。实现时，将一路高速串行输入的数据信号流转换成N路并行的低速子数据流，调制到每个子载波上进行传输。,4,Real Part of Complex Exponential Time Series:Integer Number of Cycles per Interval,5,5,Imaginary Part of Complex Exponential Time Series:Integer Number of Cycles per Interval,6,6,Spectra Of Complex Exponential Time Series:Integer Number of Cycles per Interval,7,7,8,OFDM,系统子载波正交特性,在中心频率时振幅达到峰值，而在此频率的整数倍时振幅为零。,8,9,Processing steps,9,Power and Bandwidth of OFDM,Power,frequency,bandwidth,The throughput is the sum of the data rates of,each individual(or used)tones while the power,is distributed to all used tones,10,10,11,2、,OFDM,的特点,正交信号可以通过在接收端采用相关技术分离，这样可以减少子信道之间的相互干扰；,通过把信道分成多个带宽很窄的子信道，可以使,OFDM系统比单载波系统具有更强的抗频率选择性衰落的能力，减少突发错；原因是，对于每一个子信道而言，遭受的是平坦衰落；,与采用自适应均衡的单载波系统相比，OFDM系统的信道均衡更简单；,=,11,12,Each subcarrier is flat faded.Channel influence can be described by a complex valued factor for each subcarrier,Example:Splitting of a broadband channel into,N,=32 subchannels,narrowband“,subchannel,12,13,由于每个子载波之间相互正交，所以每个子载波的频带之间互相重叠，可以获得很高的频带效率，这也是OFDM技术区别于FDM技术的本质。,由于,OFDM,信号是由多个不同频率，不同振幅，不同相位的信号相互叠加的结果，因此，具有很高的峰平功率比（,PAPR,），这就要求RF功率放大器要有较大的动态范围。,与单载波系统相比，,OFDM,系统对频偏和频漂更敏感。,FDM,OFDM,13,14,OFDM,Modulator,14,15,OFDM Demodulator,15,16,OFDM is a Block Process,16,17,发端：串/并变换后,N,路并行数据同时输出，分别调制在,N,个子载波上。,N,个子载波可以表示为,则第,m,时刻的一个,OFDM,信号可以表示为,收端：采用混频后积分清洗的方式对各子载波解调。,17,18,Adjacent Symbol Interference(ASI)Symbol Smearing Due to Channel,18,19,Guard Interval Inserted Between Adjacent Symbols to Suppress ASI,19,20,Cyclic Prefix Inserted in Guard Interval to Suppress Adjacent Channel Interference(ACI),A copy of the signal tail(length TG)is inserted at the beginning of each OFDM symbol.,Absorbs multipath components.,20,21,OFDM time frequency representation,21,22,4、OFDM,的,FFT,实现,如果不考虑保护时间，那么,OFDM,信号可以表示为,X,(,t,),为复等效基带信号。对,X,(,t,),进行抽样，抽样速率为,1/,T,s,，则当,t=k,T,s,时，抽样值,X,(,k,),满足,X,(,k,),恰好是,d,(,n,),的N点反离散傅立叶变换（,IDFT,）的结果。因此可以用,IDFT,运算完成,OFDM,复等效基带信号的子载波调制过程，用离散傅立叶变换（,DFT,）完成其解调过程。,22,用离散傅立叶变换（DFT）完成解调过程,23,23,24,信息,序列,串并,变换,IFFT,并串,变换,插入保,护间隔,e,j2,f,c,t,QAM,调制,(,a,),发射机工作原理,并串,变换,FFT,串并,变换,移去保,护间隔,QAM,解调,信道估计,捕获与同步,(,b,),接收机工作原理,信息,序列,e,-j2,f,c,t,24,25,Basic OFDM Transceiver System,25,Input Vector FFT Mapped to Output Time Series,Up-Sampled,Converted Via DAC to Waveform,and I-Q Up-Converted,26,26,27,5,、,OFDM,对同步的要求,载波同步：,OFDM,符号由多个子载波信号叠加构成，各个子载波之间利用正交性来区分。系统对由多普勒频移或收发端载波频偏产生的频率偏差非常敏感，频偏会破坏子载波之间的正交性，引入,ICI,，而且还会引起传输信号的相位旋转，造成系统性能大幅度降低。,采样时钟同步：,指发射端的,D/A,变换器和接收端的,A/D,变换器的工作频率保持一致。一般地，两个变换之间的偏差较小，相对于载波频偏的影响来说也较小。,符号定时：,IFFT,和,FFT,起止时刻一致。理想的符号同步就是选择最佳的,FFT,窗，使子载波保持正交，且,ISI(,符号间干扰,),被完全消除或者降至最小。由于使用了循环前缀技术，,OFDM,系统能够容忍一定的符号定时误差而不受到性能上损失。所以,OFDM,系统对定时偏差不像对频率偏差那么敏感。,帧同步：,就是要在,OFDM,符号流中找出帧的开始位置在帧结构已知的情况下，帧同步和符号同步可以认为是一样的。,27,28,符号定时误差的影响,FFT,处理窗位置与,OFDM,符号的相对关系,符号定时同步误差将导致,FFT,处理窗与发送窗对不准，一个,OFDM,符号由保护间隔和有效数据采样构成，保护间隔在前，有效数据在后。如果,FFT,处理窗延迟放置，则,FFT,积分处理包含了当前符号的样值与下一个符号的样值，从而引入了,OFDM,符号间干扰,(ISI),，可能严重影响系统性能。而如果,FFT,处理窗超前放置，则,FFT,积分处理包含了当前符号的数据部分和保护时间部分，因此后者不会引入码间干扰。,28,29,符号定时同步算法,基于循环前缀的同步：,以,Van de,Beek,的算法为代表，利用,CP,是,OFDM,符号末尾的复制这一特性，给出了时间偏移的最大似然估计,基于训练符号的同步：,用两个特殊结构的前导符号来做定时和频偏估计,联合循环前缀及导频的同步：,一个依赖,CP,和导频信号的最大似然估计算法,29,频率同步误差的影响,小数倍频偏：,破坏子载波之间的正交性，引入了,ICI,，也会造成系统性能的下降,整数倍频偏：,虽然不会破坏子载波之间的正交性，但是引起,OFDM,信号的频谱结构错位，导致接收机恢复的数据码元序列的循环移位和相位旋转,频率误差造成,OFDM,系统产生载波间干扰,30,30,频偏估计算法,数据辅助,(data-aided),估计，,即基于训练符号,这类算法的优点是捕获快、精度高，适合分组数据通信，缺点是由于插入训练符号而带来的资源浪费。具体的实现是在分组数据包的包头加一个前导符号，专门用来做定时、频偏估计。,非数据辅助,(non-data-aided),估计，,即盲估计，它利用,OFDM,信号的结构，例如，循环前缀使,OFDM,的前端和后端有一定的相关性、利用虚子载波来做估计以及利用数据经过成形滤波之后的循环平稳特性等方法来做估计。盲估计的最大优点是避免由于插入导频符号而带来的资源浪费，缺点是为了获得高精度需要几十个甚至上百个,OFDM,符号，捕获时间长。,31,31,峰均功率比Peak-to-Average Power Ratio(PAPR),OFDM suffers from high PAPR defined as,Maximum PAPR of an,N,sub-carrier OFDM signal,32,32,33,Example with 8 carriers and BPSK modulation,33,34,For an OFDM system with,N,sub-carriers,an oversampling rate of,L,can be achieved by inserting(,L,1),N,zeros in the middle of the modulated symbol vector to form a 1,LN,data vector,X,i.e.,The PAPR computed from the,L-,times oversampled time-domain signal samples is given by,PAPR in discrete-time case,34,35,The cumulative distribution function(CDF)of the PAPR is one of the most frequently used performance measures for PAPR reduction techniques.In the literature,the complementary CDF(CCDF)is commonly used instead of the CDF itself.,The CCDF of the PAPR denotes the probability that the PAPR of a data block exceeds a given threshold.,From the central limit theorem,the real and imaginary parts of the time domain signal samples follow Gaussian distributions,assuming each distribution with a mean of zero and a variance of 0.5 for a multicarrier signal with a large number of subcarriers.,The CCDF of the PAPR,35,36,Hence,the amplitude of a multicarrier signal has a Rayleigh distribution,while the power distribution becomes,a central chi-square distribution with two degrees of freedom,.,The CDF of the,instantaneous power,of a signal sample is given by,The CCDF of the PAPR of a data block with,Nyquist,rate sampling is derived as,This expression assumes that the,L,N,time domain signal samples are mutually independent and uncorrelated.,The CCDF of the PAPR,36,PAPR distribution,37,37,Multiuser OFDM,OFDM-TDMA,OFDM-FDMA,OFDMA,User 1,User 2,User 3,Subcarrier,Time,OFDM-TDMA,Subcarrier,Time,OFDM-FDMA,Subcarrier,Time,OFDMA,38,39,6,、,OFDMA,Orthogonal Frequency division multiple access,39,40,40,41,Uplink Transmission,Single carrier FDMA based system,Orthogonal transmission within cell,Modulation:,QPSK,16QAM,Optional:8PSK,64QAM,Channel coding,LDPC and convolutional Turbo code,Code rate:4/154/5,MIMO,Up to 2 transmit antennas,Up to 4 receive antennas,Inter-cell interference avoidance/concentration with resource coordination,41,42,Comparison OFDMA vs.SC/FDMA,42,43,SC-FDMA(1),Low PAPR,Cyclic prefix guard interval:enable cost-effective frequency domain block processing at receiver side,Two types of SC transmission,Localized transmission:multi-user scheduling gain in frequency domain,Distributed transmission:robust transmission for control channels and high mobility UE,43,44,SC-FDMA(2),Localized transmission,Need to feedback channel state information,Mainly for low-to-medium mobility users,Distributed transmission,Mainly for high mobility users,Orthogonal resource subspace division,Transmission bandwidth is divided into localized band and distributed band,Each band is further divided into several subbands for inter-cell interference avoidance/concentration,A subband out of each band in a cell is operated in,whispering,mode;UEs using a channel belonging to the same subband in neighboring cells can be operated in,speaking,mode,44,45,Processing Steps,45,46,PAPR,(Peak-to-average power ratio),comparison,46,47,47,48,The IEEE 802.11a Standard,48,49,Scope of 802.16a(3/3),System specifications of 802.16a OFDM mode.,ETSI(fs/BW=8/7),49,