最新LTE关键技术之OFDM分析-外文翻译.docx

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1、Four short words sum up what has lifted most successful individuals above the crowd: a little bit more.-author-dateLTE关键技术之OFDM分析-外文翻译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： 二一四年三月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

2、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，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。图2-1 OFDM子载波间隔在OFD

3、M传输时，物理资源经常被描述成一个时域频域的网格坐标图。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。图2-2OFDM时频网格以一定的频率fs= N f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a0, a1,.aN-1。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，“OFDM调制”）。在实际中，OFDM调制是以快速

4、傅立叶反变换（IFFT）方式实现简单和快速的处理，通过选择IDFT size N 等于2m（m为整数）。在接收端，对接收信号以fs= N f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a0, a1,.aN-1。（参见图2-4，“OFDM解调”）。图2-3OFDM调制图2-4OFDM解调正如上面提到的，一个无干扰的OFDM信号可以解调出无任何子载波间干扰的信号。然而，在一个时间色散信道的情况下（如多径无线信道），子载波之间的正交性丢失，造成符号间干扰（ISI）。这是因为，解调器相关区间的一条路径将与不同路径的符号边界有重叠。（见图2-5，“时间的分散性和相应的接收信号”）

5、。图2-5 次分散和相应的接收信号要解决这个问题，使OFDM信号在无线信道传播时对时间色散完全不敏感，所谓的插入循环前缀通常被使用。如图2-6所示，“插入循环前缀”。循环前缀插入就意味着OFDM符号的最后部分（第N个cp）被复制并且被插入到OFDM块的开始部分。因此，OFDM符号的长度从TU 到TU +TCP ,其中TCP =NCPTU是循环前缀的长度。作为一个结果，OFDM符号率是减少的。因此，在时间色散信道里，只要时间色散的跨度小于循环前缀的长度，子载波的正交性就能被保持。图2-6插入循环前缀循环前缀插入的缺点是，在整个信号带宽没有减少，OFDM符号率减少的情况下，就意味着在吞吐量方面有相

6、应的损失。OFDM调制组合（IFFT处理），一个（分散的）无线信道，以及解调（FFT处理）可以被看作是一个频域信道。如图2-7，“频域模型的OFDM传输接收”，其中每个OFDM符号的时间期间，N个不同的调制码元被发送，每一个在相应的子载波上，在对比单一宽带载波系统时，如WCDMAwhere，每个调制符号被传输在整个带宽上。图2-7频率的OFDM传输接收域模型在频道k上，调制符号ak被缩放和相位转移，通过复杂的信道系数Hk（频域）。在接收端，解调后允许发送的信息准确解码。在接收端需要一个频域的信道抽头估计H0，H1， .， HN-1。这可以通过在OFDM时频网格内以一定规律的间隔插入已知参考符号

7、来实现，有时也称作导频符号或导频器。运用参考符号的相关知识，接收机可以估计信道抽头（频域）用于解码的必要。OFDM信号带宽一个OFDM信号的带宽等于Nf ，这就是说：子载波数乘以子载波间隔数。另一方面，通过设置这个传输符号从一侧组相邻子载波到零，这个基带被减少到NCf,其中NC 是非空子载波数目。然而，OFDM信号的频谱脱落到基本带宽以外的速度是很慢的，尤其比一个WCDMA信号慢的多。因此，在实际中，一个OFDM需要10%的保护间隔。这也就是说，举个例子，在一个 5 MHZ 的频谱分配中，OFDM基本带宽 NC f 大约是4.5 MHZ。做一个假设，例如，为LTE选择一个15 KHZ的子载波间

8、隔，那么，在 5MHZ内应对应于300个子载波。DFT OFDM传输离散的傅里叶变换扩展的正交频分复用（DFTS-OFDM）已被用作LTE上行链路的传输方案。DFTS-OFDM传输的基本原理在图2-8，“DFT的OFDM信号生成”中说明。类似于OFDM调制，DFTS-OFDM依赖于基于块的信号生成。在DFTS-OFDM中，一个M调制符号块来自于一些调制字母表，比如，QPSK 或者 16QAM,第一次被应用到size-m DTF。这个DFT输出被应用到一个size-N 的逆DFT的连续输入当中。其中，N 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2-8D

9、FT的OFDM信号的产生与图2-8，“DFT的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DFTS-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

10、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。图2-9 DFTS OFDM调制LTE multiple access techniquesLTE multiple accessOFDM transmis

11、sionOrthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmissiontechnique that has been adopted as the downlink transmission scheme for the 3GPPLong-Term Evolution (LTE) and is also used for several other radio technologies, e.g.WiMAX and the DVB broadcast technologies.It is char

12、acterized by a tight frequency-domain packing of the subcarriers with a subcarrier spacing f = 1/Tu, where Tu is the per-subcarrier modulation-symbol time. (See Figure 2-1, “OFDM subcarrier spacing”) .OFDM transmission is block-based. During each OFDM symbol interval, modulationsymbols are transmitt

13、ed in parallel. The modulation symbols can be from any modulation alphabet, such as QPSK, 16QAM, or 64QAM.For 3GPP LTE, the basic subcarrier spacing equals 15 kHz. On the other hand, thenumber of subcarriers depends on the transmission bandwidth, with in the order of 600 subcarriers in case of opera

14、tion in a 10 MHz spectrum allocation and correspondingly fewer/more subcarriers in case of smaller/larger overall transmission bandwidths.Figure 2-1 OFDM subcarrier spacingThe physical resource in case of OFDM transmission is often illustrated as atime-frequency grid where a column corresponds to on

15、e OFDM symbol (time) and a row corresponds to one OFDM subcarrier, as illustrated in (see Figure 2-2, “OFDM time-frequency grid” ).In the ideal case, despite the fact that the spectrum of neighbor subcarriers do overlap, the OFDM subcarriers do not cause any interference to each other after demodula

16、tion due to the specific choice of a subcarrier spacing f equal to the modulation symbol rate.Figure 2-2 OFDM time-frequency gridAn OFDM signal sampled at a rate fs = N f is the size-N Inverse Discrete FourierTransform (IDFT) of the block of modulation symbols a0, a1,.aN-1. Thus, OFDMmodulation can

17、be implemented by means of IDFT processing followed bydigital-to-analog conversion (see Figure 2-3, “OFDM modulation”) . In practice,the OFDM modulation can be implemented by means of Inverse Fast Fourier Transform (IFFT) easy and fast processing, by selecting the IDFT size N equal to 2m for some in

18、tegerm. At the receiver, by sampling the received signal at the rate fs = N f, efficient FFT processing is used to achieve OFDM demodulation and retrieve the block of modulation symbols a0, a1,.aN-1( see Figure 2-4, “OFDM demodulation”) .Figure 2-3 OFDM modulationFigure 2-4 OFDM demodulationAs menti

19、oned above, an uncorrupted OFDM signal can be demodulated without anyinterference between subcarriers. However, in case of a time-dispersive channel (such as multipath radio channels), the orthogonality between the subcarriers is lost, causing Inter Symbol Interference (ISI). The reason for this is

20、that the demodulator correlation interval for one path will overlap with the symbol boundary of a different path (see Figure 2-5,“Time dispersion and corresponding received signal”) Figure 2-5 Time dispersion and corresponding received signalTo deal with this problem and make an OFDM signal truly in

21、sensitive to time dispersion on the radio channel, so-called Cyclic Prefix insertion is typically used in case of OFDM transmission. As illustrated in(see Figure 2-6, “Cyclic Prefix insertion”) , cyclic-prefix insertion implies that the last part of the OFDM symbol (the last Ncp symbols) is copied a

22、nd inserted at the beginning of the OFDM block, increasing thus the length of the OFDM symbol from Tu to Tu + Tcp, where Tcp = Ncp,Tu is the length of the cyclic prefix.The OFDM symbol rate as is reduced as a consequence. Thus, subcarrier orthogonality is preserved in case of a time-dispersive chann

23、el, as long as the span of the time dispersion is shorter than the cyclic-prefix length.Figure 2-6 Cyclic Prefix insertionThe 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 reduction in the

24、 overall signal bandwidth.The combination of OFDM modulation (IFFT processing), a (time-dispersive) radiochannel, and OFDM demodulation (FFT processing) can then be seen as afrequency-domain channel as illustrated in(see Figure 2-7, “Frequency domain model of OFDM transmission reception”) , where du

25、ring each OFDM symbol time period, N different modulation symbols are transmitted, each on a given subcarrier over the corresponding sub-band, in contrast to single wideband carrier systems, such as a WCDMA where each modulation symbol is transmitted over the entire bandwidth.Figure 2-7 Frequency do

26、main model of OFDM transmission receptionOn frequency channel k, modulation symbol ak is scaled and phase rotated by thecomplex (frequency-domain) channel coefficient Hk. At the receiver side, to allow forproper decoding of the transmitted information after demodulation, the receiver needs an estima

27、te of the frequency-domain channel taps H0, H1,.,HN-1. This can be done by inserting known reference symbols, sometimes also referred to as pilot symbols or pilots,at regular intervals within the OFDM time/frequency grid. Using knowledge about the reference symbols, the receiver can estimate the (fr

28、equency-domain) channel taps necessary for the decoding.OFDM signal bandwidthThe basic bandwidth of an OFDM signal equals N f, i.e. the number of subcarriersmultiplied by the subcarrier spacing. On the other hand, by setting the symbols to betransmitted on a group of side contiguous subcarriers to z

29、ero, the basic bandwidth isreduced to Nc f where Nc is the number of non-null subcarriers. However, the spectrum of an OFDM signal falls off slowly outside the basic OFDM bandwidth and especially much slower than for a WCDMA signal. Thus, in practice, typically in the order of 10% guard-band is need

30、ed for an OFDM signal, implying 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. Assuming, for example, a subcarrier spacing of 15 kHz as selected for LTE, this corresponds to 300 subcarriers in 5 MHz.DFTS OFDM transmissionDiscre

31、te Fourier Transform Spread OFDM (DFTS-OFDM) is a transmission 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 signal generation”) . Similar to OFDM modulation, DFTS-OFDM relies on blo

32、ck-based signal generation. In case of DFTS-OFDM, a block of M modulation symbols from some modulation alphabet, e.g. QPSK or 16QAM, is first applied to a size-M DFT. The output of the DFT is then applied to consecutive inputs of a size-N inverse DFT where N M and where the (N-M) unused inputs of th

33、e IDFT are set to zero. Also similar to OFDM, a cyclic prefix is inserted for each transmitted block.Figure 2-8 DFTS OFDM signal generationComparing (see Figure 2-8, “DFTS OFDM signal generation” ),with the IFFT-based implementation of OFDM modulation, it is obvious that DFTS-OFDM can alternatively

34、be seen as OFDM modulation preceded by a DFT operation.If the DFT size M equals the IDFT size N, the cascaded DFT and IDFT blocks of (see Figure 2-8, “DFTS OFDM signal generation”), will completely cancel out each other.However, if M is smaller than N and the remaining inputs to the IDFT are set to

35、zero, the output of the IDFT will be a signal with low power variations, similar to a single-carrier signal. Besides, by varying the block size M the instantaneous bandwidth of the transmitted signal can be varied, allowing for flexible-bandwidth assignment. The main benefit of DFTS-OFDM, compared t

36、o a multi-carrier transmission scheme such as OFDM, is reduced variations in the instantaneous transmit power, implying the possibility for increased power-amplifier efficiency. The power variations are generally measured by the Peak-to-Average-Power Ratio (PAPR), defined as the peak power within on

37、e OFDM symbol normalized by the average signal power. The PAPR is significantly lower for DFTS-OFDM, compared to OFDM, making thus this transmission technique very useful in the uplink considering the transmit power capabilities of the mobile terminal.The basic principle of DFTS-OFDM signal demodula

38、tion is illustrated in (see Figure 2-9,“DFTS OFDM demodulation”) . The operations are basically the reverse of those for the DFTS-OFDM signal generation of (see Figure 2-9, “DFTS OFDM demodulation”) , i.e. size-N DFT (FFT) processing, removal of the frequency samples not corresponding to the signal to be received, and size-M Inverse DFT processing.Figure 2-9 DFTS OFDM demodulation-