LTE的多址接入技术外文翻译

中文 2180字 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 OFDM 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 2-2 OFDM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 2-3 OFDM调制 图 2-4 OFDM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 ”）。 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 N f ，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 NC f,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 2-8 DFT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 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