LTE关键技术之OFDM分析外文翻译.docx

1、LTE关键技术之OFDM分析外文翻译201x届本科毕业设计（外文翻译）学院:专业:姓名:学号:指导教师：完成时间：二0四年三月LTE的多址接入技术LTE的多址接入OFDM传输正交频分复用（OFDM ）是一种多载波传输技术，已被采纳为 3gpplong长期演化（LTE）的下行链路传输方案，也可用于其他几个无线技术，例如：wimax和DVB广播技术。它的特点是在一个频域内分布着许多带有间隔的子载波 f=1/Tu其中，Tu是每个子载波的调制符号时间。如图 2-1所示，OFDM子载 波间隔”。OFDM的传输是基于块的。每个 OFDM符号间隔之间，调制符号是并行发 送的。调制符号可以通过调制字母表得到，

2、如 QPSK, 16QAM或64QAM，对于 3GPP组织LTE，子载波间隔是相等的为15 kHz。另一方面，子载波的数目取决 于传输带宽，在一个10MHZ的频谱分配下，600个子载波可以有序传输。当然， 带宽减小了，子载波数目也相应减少，带宽增加了，子载波数目也相应增加。Af = 1/Tu*图2-1 OFDM子载波间隔在OFDM传输时，物理资源经常被描述成一个时域 一频域的网格坐标图。 在这个坐标图里一列对应一个 OFDM子载波，一行对应一个 OFDM子载波。如 图2-2所示，OFDM时频网格”。尽管子载波的频谱有重叠，但在理想情况下，是对 OFDM子载波解调后不 引起任何干扰的，这是因为对

3、每一个子载波间隔的特殊选择， 让它等于相应的解 调符号率。以一定的频率fs= N x 进行采样的OFDM信号，是该size-N的逆离散傅立叶变换（IDFT）的调制符号块ao, ai,.aN-i。因此，OFDM调制可以通过IDFT处理再到数字-模拟的转换来实现。（见图2-3， “OFD碉制”）。在实际中，OFDM调制 是以快速傅立叶反变换（IFFT）方式实现简单和快速的处理，通过选择IDFT sizeN等于2m （m为整数）。在接收端，对接收信号以fs= N X的频率采样，高效的FFT处理是用来实现OFDM的解调和检索调制符号块ao, ai,.aN-i。（参见图2-4,OFDM解调”Freque

4、ncy lo M且未使用的输入(N-M )设置为零。和OFDM 一样，每个传输块插入一个循环前缀。图2-8 DFT的OFDM信号的产生与图2-8, “ DFT勺OFDM信号生成”相比，基于IFFT OFDM调制的实现，很显然，DFTS-OFDM可以看作是OFDM调制之前的DFT运算。如果DFT的M的大小等于IDFT的N的大小，那么级联DFT和IDFT的块图2-8 DFT的OFDM信号生成”将完全抵消。如果M小于N且IDFT的剩余输入被设置为零，则IDFT的输出将是一个低功率变化的信号，类似于一个单载波信号。此外，不同块大小为 m的瞬时带宽发送的信号可以是多种多样的，允许灵活的带宽分配。与DFT

5、S-OFDM的主要好处想比，多载波传输方案，如 OFDM,减少变化的瞬 时发射功率，对提高功率放大器效率是可能的。功率的变化一般根据测得的峰值 平均功率比（PRPA）来判断。定义为在峰值功率一个 OFDM符号的平均信号功 率的归一化。对于DFTS-OFDM,PRPA明显降低，相比OFDM,再考虑到移动终端 的电源能力，这种传输技术在上行链路的传输中是非常有用的。DFTS-OFDM信号解调的基本原理如图2-9所示，DFT的OFDM解调”。这些 操作和图2-9 DFT的OFDM解调”基本上是相反的。即size-n离散傅里叶变换处理 中，和接受信号不对应的频率采样会被移除。Time Io frequ

6、iencydomain confers Ion图 2-9 DFTS OFDM 调希 9LTE multiple access tech niq uesLTE multiple accessOFDM tran smissio nOrthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission tech nique that has bee n adopted as the dow nli nk tran smissi on scheme for the 3GPP Lon g-Term Evolutio

7、 n (LTE) and is also used for several other radio tech no logies, e.g. WiMAX and the DVB broadcast tech no logies.It is characterized by a tight frequency-domain packing of the subcarriers with a subcarrier spac ingf = 1/Tu, where Tu is the per-subcarrier modulatio n-symbol time. (SeeFigure 2-1, “ O

8、FDM subcarrier spacing ”OFDM tran smissi on is block-based. During each OFDM symbol in terval, modulati on symbols are transmitted in parallel. The modulation symbols can be from any modulation alphabet, such as QPSK, 16QAM, or 64QAM.For 3GPP LTE, the basic subcarrier spaci ng equals 15 kHz. On the

9、other hand, thenumber of subcarriers depends on the transmission bandwidth, with in the order of600 subcarriers in case of operation in a 10 MHz spectrum allocation and corresp on di nglyfewr/more subcarriers in case of smaller/larger overall tran smissi on bandwidths.rI I I I I i I I IFigure 2-1 OF

10、DMsubcarrier spaci ngsub匚忌ier spacing = f ission is often illustrated as a s to one OFDM symbol (time) and ancy = 3 f 丁 , pite the .fact that lhe spectrum of n eighbor subcarriers do overlap, theOFDM subcarrie+sdo not cause any in terfere nee to each other after demodulati on due tothe specific choi

11、ce of a subcarrier spacing f equal to the modulation symbol- rate. The physica time-frequer row corres time-frequer In the ideal case,resoincy gri vh ponds tojncase of OFDMiere . a column cai ie OFDMsubcai血stated . in .(seFigure 2-2, “卩罔册I 11 I Hi WionFigure 2-2 OFDM time-freque ncy gridAf= 1/Tu* *F

12、requen匚y - AfTime to frequonKy dGcnafn conver-&ioiiOFDM modulati on can be impleme nted by mea ns of In verse Fast Fourier Tran sform (IFFT) easy and fast process ing, by selecti ng the IDFT sizN equal to 2m for some in tegerm. At the receiver, by sampli ng the received sig nal at the rates = N xf,

13、efficient FFT processing is used to achieve OFDM demodulation and retrieve the block of modulation symbols ao, ai,.aN-i( seeFigure 2-4, “OFDM demodulation. ” )Figure 2-3 OFDM modulation皿呻呻怖伽Figure 2-4 OFDM demodulationAs men ti oned above, an un corrupted OFDM sig nal can be demodulated without any

14、in terfere nee betwee n subcarriers. However, i n case of a time-dispersive cha nn el (such asmultipath radio channels), the orthogonality between the subcarriers is lost, causingIn ter Symbol In terfere nce (ISI). The reas on for this is that the demodulator correlati on interval for one path will

15、overlap with the symbol boundary of a different path (see Figure 2-5, “ Time dispersion and corresponding received signal ”Figure 2-5 Time dispersi on and corresp onding received sig nalCyclic prefix permits to facilitate demodulationki1 V M ITo deal witroblem and make an OFDM sig nal truly insen si

16、tive to种门 el, so-called Cyclic Prefix inserti on is typically used intransmiss in. As illustrated in( seeFigure 2-6； yclic Prefix：,”)clic-prefiX/ nsertio n implies that the last part of the OFDM symbol (the last Ncp symbols) is copieda nd in serted at the begi nning of the OFDM block, increasi ng th

17、us the len gth of theOFDM symbol from Tu to Tu + Tcp, where Tcp = NcplTJ is the len gth of the cyclic prefix. 心忖 /( ij The OFDM symbol rate as is reduced as a con sequence. Thus, subcarrier orthogonality is preserved in case of a ti-dispersivechan the time dispersionis shorter than the cyclic-prefix

18、 length.Figure 2-6 Cyclic Prefix in sertio ni 1 Jil f i li EXiMfcilong as the spa n o.一4 MrililiiiilmbeNode-BRadi)匚nannel respon別 h(t)fIThe drawback of cyclic-prefix insertion is that it implies a corresponding loss in terms of throughput as the OFDM symbol rate is reduced without a corresponding re

19、duction in the overall signal bandwidth. The comb in ati on of OFDM modulatio n (IFFT process in g), a (time-dispersive) radio cha nn el, and OFDM demodulatio n (FFT process ing) can the n be see n as a freque ncy-doma in cha nnel as illustrated in (seigure 2-7, “ Freque ncy doma in modeloOFDMttrafl

20、smsERnrecffl他七 3 W”rMuM 靶chOFDM symbl time period，N differe nt modulati on symbols are tran smitted, each on a give n subcarrier over the corresp onding sub-ba nd, in con trast to sin gle wideba nd carrier systems, such as a WCDMA where each modulati on symbol is tran smitted over the en tire ban dw

21、idth.Useful OFW symbolFigure 2-7 Freque ncy doma in model of OFDM tran smissi on recepti onOn frequency channel k, modulation symbol a is scaled and phase rotated by the complex (freque ncy-doma in) cha nnel coefficie nt Hk. At the receiver side, to allow for proper decod ing of the tran smitted in

22、formatio n after demodulatio n, the receiver n eeds an estimate of the frequency-domain channel taps H H1,.,Hn-1. This can be done by in sert ing known refere nee symbols, sometimes also referred to as pilot symbols or pilots,at regular in tervals with in the OFDM time/freque ncy grid. Using kno wle

23、dge about therefere nee symbols, the receiver can estimate the (freque ncy-domai n) cha nnel tapsnecessary for the decod ing.OFDM sig nal ban dwidthThe basic bandwidth of an OFDM signal equals N f, i.e. the Number of subcarriers multiplied by the subcarrier spaci ng. On the other hand, by sett ing t

24、he symbols to be transmitted on a group of side contiguous subcarriers to zero, the basic bandwidth is reduced to Nc f where Nc is the number of non-null subcarriers. However, the spectrumof an OFDM signal falls off slowly outside the basic OFDM bandwidth and especially much slower than for a WCDMA

25、signal. Thus, in practice, typically in the order of 10% guard-ba nd is n eeded for an OFDM sig nal, impl ying that, as an example, in a spectrum allocation of 5 MHz, the basic OFDM bandwidth Nc f could be in the order of 4.5 MHz. Assu ming, for example, a subcarrier spaci ng of 15 kHz as selected f

26、or LTE, this corresponds to 300 subcarriers in 5 MHz.DFTS OFDM tran smissio nDiscrete Fourier Tran sform Spread OFDM (DFTS-OFDM) is a tran smissi on scheme that has been selected as the uplink transmission scheme for LTE. The basic principle of DFTS-OFDM transmission is illustrated in(see Figure 2-8, “ DFTS OFDM signalgen eratio n)【Similar to OFDM modulatio n, DFTS-OFDM relies on block-based sig nal gen erati on. In case of DFTS-OFDM, a block of M modulatio n symbols from somemodulation alphabet, e.g. QPSK or 16QA