LTE關(guān)鍵技術(shù)之OFDM分析外文翻譯

1、 201x 屆 本 科 畢 業(yè) 設(shè) 計（外文翻譯）學(xué) 院： 專 業(yè)： 姓 名： 學(xué) 號： 指導(dǎo)教師： 完成時間： 二一四年三月LTE的多址接入技術(shù)LTE的多址接入OFDM傳輸正交頻分復(fù)用（OFDM）是一種多載波傳輸技術(shù)，已被采納為3gpplong長期演化（LTE）的下行鏈路傳輸方案，也可用于其他幾個無線技術(shù)，例如：wimax和DVB廣播技術(shù)。它的特點(diǎn)是在一個頻域內(nèi)分布著許多帶有間隔的子載波 f=1/Tu其中， Tu是每個子載波的調(diào)制符號時間。如圖2-1所示，“OFDM子載波間隔 ”。OFDM的傳輸是基于塊的。每個OFDM符號間隔之間，調(diào)制符號是并行發(fā)送的。調(diào)制符號可以通過調(diào)制字母表得到，如QP

2、SK，16QAM或64QAM，對于3GPP組織LTE，子載波間隔是相等的為15 kHz。另一方面，子載波的數(shù)目取決于傳輸帶寬，在一個10MHZ的頻譜分配下，600個子載波可以有序傳輸。當(dāng)然，帶寬減小了，子載波數(shù)目也相應(yīng)減少，帶寬增加了，子載波數(shù)目也相應(yīng)增加。圖2-1 OFDM子載波間隔在OFDM傳輸時，物理資源經(jīng)常被描述成一個時域頻域的網(wǎng)格坐標(biāo)圖。在這個坐標(biāo)圖里一列對應(yīng)一個OFDM子載波，一行對應(yīng)一個OFDM子載波。如圖2-2所示，“OFDM時頻網(wǎng)格” 。盡管子載波的頻譜有重疊，但在理想情況下，是對OFDM子載波解調(diào)后不引起任何干擾的，這是因?yàn)閷γ恳粋€子載波間隔的特殊選擇，讓它等于相應(yīng)的解調(diào)符

3、號率。圖2-2 OFDM時頻網(wǎng)格以一定的頻率fs= N ×f進(jìn)行采樣的OFDM信號，是該size-N的逆離散傅立葉變換（IDFT）的調(diào)制符號塊a0, a1,.aN-1。因此，OFDM調(diào)制可以通過IDFT處理再到數(shù)字-模擬的轉(zhuǎn)換來實(shí)現(xiàn)。（見圖2-3，“OFDM調(diào)制”）。在實(shí)際中，OFDM調(diào)制是以快速傅立葉反變換（IFFT）方式實(shí)現(xiàn)簡單和快速的處理，通過選擇IDFT size N 等于2m（m為整數(shù)）。在接收端，對接收信號以fs= N ×f的頻率采樣，高效的FFT處理是用來實(shí)現(xiàn)OFDM的解調(diào)和檢索調(diào)制符號塊a0, a1,.aN-1。（參見圖2-4，“OFDM解調(diào)”）。

4、圖2-3 OFDM調(diào)制圖2-4 OFDM解調(diào)正如上面提到的，一個無干擾的OFDM信號可以解調(diào)出無任何子載波間干擾的信號。然而，在一個時間色散信道的情況下（如多徑無線信道），子載波之間的正交性丟失，造成符號間干擾（ISI）。這是因?yàn)椋庹{(diào)器相關(guān)區(qū)間的一條路徑將與不同路徑的符號邊界有重疊。（見圖2-5，“時間的分散性和相應(yīng)的接收信號”）。圖2-5 次分散和相應(yīng)的接收信號要解決這個問題，使OFDM信號在無線信道傳播時對時間色散完全不敏感，所謂的插入循環(huán)前綴通常被使用。如圖2-6所示，“插入循環(huán)前綴”。循環(huán)前綴插入就意味著OFDM符號的最后部分（第N個cp）被復(fù)制并且被插入到OFD

5、M塊的開始部分。因此，OFDM符號的長度從TU 到TU +TCP ,其中TCP =NCPTU是循環(huán)前綴的長度。作為一個結(jié)果，OFDM符號率是減少的。因此，在時間色散信道里，只要時間色散的跨度小于循環(huán)前綴的長度，子載波的正交性就能被保持。圖2-6插入循環(huán)前綴循環(huán)前綴插入的缺點(diǎn)是，在整個信號帶寬沒有減少，OFDM符號率減少的情況下，就意味著在吞吐量方面有相應(yīng)的損失。OFDM調(diào)制組合（IFFT處理），一個（分散的）無線信道，以及解調(diào)（FFT處理）可以被看作是一個頻域信道。如圖2-7，“頻域模型的OFDM傳輸接收”，其中每個OFDM符號的時間期間，N個不同的調(diào)制碼元被發(fā)送，每一個在相應(yīng)的子載波上，在對

6、比單一寬帶載波系統(tǒng)時，如WCDMAwhere，每個調(diào)制符號被傳輸在整個帶寬上。圖2-7頻率的OFDM傳輸接收域模型在頻道k上，調(diào)制符號ak被縮放和相位轉(zhuǎn)移，通過復(fù)雜的信道系數(shù)Hk（頻域）。在接收端，解調(diào)后允許發(fā)送的信息準(zhǔn)確解碼。在接收端需要一個頻域的信道抽頭估計H0，H1， .， HN-1。這可以通過在OFDM時頻網(wǎng)格內(nèi)以一定規(guī)律的間隔插入已知參考符號來實(shí)現(xiàn)，有時也稱作導(dǎo)頻符號或?qū)ьl器。運(yùn)用參考符號的相關(guān)知識，接收機(jī)可以估計信道抽頭（頻域）用于解碼的必要。OFDM信號帶寬一個OFDM信號的帶寬等于N×f ，這就是說：子載波數(shù)乘以子載波間隔數(shù)。另一方面，通過設(shè)置這個傳輸符號從一側(cè)組相鄰

7、子載波到零，這個基帶被減少到NC×f,其中NC 是非空子載波數(shù)目。然而，OFDM信號的頻譜脫落到基本帶寬以外的速度是很慢的，尤其比一個WCDMA信號慢的多。因此，在實(shí)際中，一個OFDM需要10%的保護(hù)間隔。這也就是說，舉個例子，在一個 5 MHZ 的頻譜分配中，OFDM基本帶寬 NC ×f 大約是4.5 MHZ。做一個假設(shè)，例如，為LTE選擇一個15 KHZ的子載波間隔，那么，在 5MHZ內(nèi)應(yīng)對應(yīng)于300個子載波。DFT OFDM傳輸離散的傅里葉變換擴(kuò)展的正交頻分復(fù)用（DFTS-OFDM）已被用作LTE上行鏈路的傳輸方案。DFTS-OFDM傳輸?shù)幕驹碓趫D2-8，“DFT

8、的OFDM信號生成”中說明。類似于OFDM調(diào)制，DFTS-OFDM依賴于基于塊的信號生成。在DFTS-OFDM中，一個M調(diào)制符號塊來自于一些調(diào)制字母表，比如，QPSK 或者 16QAM,第一次被應(yīng)用到size-m DTF。這個DFT輸出被應(yīng)用到一個size-N 的逆DFT的連續(xù)輸入當(dāng)中。其中，N > M 且未使用的輸入（N-M）設(shè)置為零。和OFDM一樣，每個傳輸塊插入一個循環(huán)前綴。圖2-8 DFT的OFDM信號的產(chǎn)生與圖2-8，“DFT的OFDM信號生成”相比，基于IFFT OFDM調(diào)制的實(shí)現(xiàn)，很顯然，DFTS-OFDM可以看作是OFDM調(diào)制之前的DFT運(yùn)算。如果DFT的M的大

9、小等于IDFT的N的大小，那么級聯(lián)DFT和IDFT的塊圖2-8“DFT的OFDM信號生成”將完全抵消。如果M小于N且IDFT的剩余輸入被設(shè)置為零，則IDFT的輸出將是一個低功率變化的信號，類似于一個單載波信號。此外，不同塊大小為m的瞬時帶寬發(fā)送的信號可以是多種多樣的，允許靈活的帶寬分配。與DFTS-OFDM的主要好處想比，多載波傳輸方案，如OFDM,減少變化的瞬時發(fā)射功率，對提高功率放大器效率是可能的。功率的變化一般根據(jù)測得的峰值平均功率比（PRPA）來判斷。定義為在峰值功率一個OFDM符號的平均信號功率的歸一化。對于DFTS-OFDM,PRPA明顯降低，相比OFDM,再考慮到移動終端的電源能

10、力，這種傳輸技術(shù)在上行鏈路的傳輸中是非常有用的。DFTS-OFDM信號解調(diào)的基本原理如圖2-9所示，“DFT的OFDM解調(diào)”。這些操作和圖2-9“DFT的OFDM解調(diào)”基本上是相反的。即size-n離散傅里葉變換處理中，和接受信號不對應(yīng)的頻率采樣會被移除。圖2-9 DFTS OFDM調(diào)制LTE multiple access techniquesLTE multiple accessOFDM transmissionOrthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmissiontechnique

11、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 characterized by a tight frequency-domain packing of the subcarriers with a subcarrier spacing f =

12、 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 transmitted in parallel. The modulation symbols can be from any modulation alphabet, such as QPSK, 16QA

13、M, 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 operation in a 10 MHz spectrum allocation and correspondingly fewer/more subcarriers in case of sma

14、ller/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 one OFDM symbol (time) and a row corresponds to one OFDM subcarrier, as illustrated in (see Figu

15、re 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 demodulation due to the specific choice of a subcarrier spacing f equal to the modulation symbol rate.

16、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 be implemented by means of IDFT processing followed bydigital-to-analog conversion (see

17、 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 integerm. At the receiver, by sampling the received signal at the rate fs = N× f, ef

18、ficient 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 mentioned above, an uncorrupted OFDM signal can be demodulated without anyinterference

19、 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 that the demodulator correlation interval for one path will overlap with the symb

20、ol 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 insensitive to time dispersion on the radio channel, so-called Cyclic Prefix insert

21、ion 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 and inserted at the beginning of the OFDM block, increasing thus the length of the

22、 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 channel, as long as the span of the time dispersion is shorter than the cyclic-prefix

23、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 overall signal bandwidth.The combination of OFDM modulation (IFFT processing), a

24、 (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 during each OFDM symbol time period, N different modulation symbols are transmitted

25、, 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 domain model of OFDM transmission receptionOn frequency channel k, modulation symbo

26、l 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 estimate of the frequency-domain channel taps H0, H1,.,HN-1. This can be done by insert

27、ing 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 (frequency-domain) channel taps necessary for the decoding.OFDM signal bandwidthThe

28、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 zero, the basic bandwidth isreduced to Nc × f where Nc is the number o

29、f 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 needed for an OFDM signal, implying that, as an example, in a spectrum

30、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 transmissionDiscrete Fourier Transform Spread OFDM (DFTS-OFDM) is a transmissi

31、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 signal generation”) . Similar to OFDM modulation, DFTS-OFDM relies on block-based signal generation. In case of DFTS-OFDM, a block of

32、 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 the IDFT are set to zero. Also similar to OFDM, a cyclic

33、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 be seen as OFDM modulation preceded by a DFT operation.

34、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 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 si