LTE的多址接入技術(shù)外文翻譯

中文 2180字 201x 屆 本 科 畢 業(yè) 設(shè) 計（外文翻譯） 學 院： 專 業(yè)： 姓 名： 學 號： 指導教師： 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 QPSK， 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。當然，帶寬減小了，子載波數(shù)目也相應減少，帶寬增加了，子載波數(shù)目也相應增加。 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)格坐標圖。在這個坐標圖里一列對應一個 OFDM子載波，一行對應一個 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，這是因為對每一個子載波間隔的特 殊選擇，讓它等于相應的解調(diào)符號率。 圖 2-2 OFDM時頻網(wǎng)格 以一定的頻率 fs= N f進行采樣的 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)換來實現(xiàn)。（見圖 2-3， “OFDM調(diào)制 ”）。在實際中， OFDM調(diào)制是以快速傅立葉反變換（ IFFT）方式實現(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) ”）。 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）。 這是因為， 解調(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的接收信號 ”）。 圖 2-5 次分散和相應的接收 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）被復制并且被插入到 OFDM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，每一個在相應的子 載波上 ，在對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現(xiàn)，有時也稱作導頻符號或?qū)ьl器。運用參考符號的相關(guān)知識，接收機可以估計信道抽頭（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，在實際中，一個 OFDM需要 10%的保護間隔。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)應對應于 300個子載波。 DFT OFDM傳輸 離散的傅里葉變換擴展的正交頻分復用（ 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D 2-8，“ DFT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,第一次被應用到 size-m DTF。這個 DFT輸出被應用到一個 size-N 的逆DFT的連續(xù)輸入當中。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)制的實現(xiàn)，很顯然， DFTS-OFDM可以看作是 OFDM調(diào)制之前的 DFT運算。如果 DFT的 M的大小等于 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，和接受信號不對應的頻率采樣會被移除。 圖 2-9 DFTS OFDM調(diào)制 LTE multiple access techniques LTE multiple access OFDM transmission Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission technique that has been adopted as the downlink transmission scheme for the 3GPP Long-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 = 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, modulation 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 spacing equals 15 kHz. On the other hand, the number 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 smaller/larger overall transmission bandwidths. Figure 2-1 OFDM subcarrier spacing The physical resource in case of OFDM transmission is often illustrated as a time-frequency grid where a column corresponds to one 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 demodulation due to the specific choice of a subcarrier spacing f equal to the modulation symbol rate. Figure 2-2 OFDM time-frequency grid An OFDM signal sampled at a rate fs = N f is the size-N Inverse Discrete Fourier Transform (IDFT) of the block of modulation symbols a0, a1,.aN-1. Thus, OFDM modulation can be implemented by means of IDFT processing followed by digital-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 integerm. 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 modulation Figure 2-4 OFDM demodulation As mentioned above, an uncorrupted OFDM signal can be demodulated without any interference 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 symbol boundary of a different path (see Figure 2-5,“Time dispersion and corresponding received signal”) Figure 2-5 Time dispersion and corresponding received signal To deal with this problem and make an OFDM signal truly insensitive 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 and 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 channel, as long as the span of the time dispersion is shorter than the cyclic-prefix length. Figure 2-6 Cyclic Prefix insertion The 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 (time-dispersive) radio channel, and OFDM demodulation (FFT processing) can then be seen as a frequency-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, 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 reception On frequency channel k, modulation symbol ak is scaled and phase rotated by the complex (frequency-domain) channel coefficient Hk. At the receiver side, to allow for proper 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 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 (frequency-domain) channel taps necessary for the decoding. OFDM signal bandwidth The basic bandwidth of an OFDM signal equals N f, i.e. the number of subcarriers multiplied by the subcarrier spacing. On the other hand, by setting the 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 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 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 transmission Discrete 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 block-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 the IDFT are set to zero. Also similar to OFDM, a cyclic prefix is inserted for each transmitted block. Figure 2-8 DFTS OFDM signal generation Comparing (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. 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 vary