MIMO相关技术介绍

1、MIMO相关知识讨论主讲 ：秦闯定义At each subcarrier, the relationship between the received and transmitted resource elementson different antennas is expressed by a system of linear equations. In this system, the vector ofreceived resource elements on receive antennas results from the multiplication of the MIMOchan

2、nel matrix by the vector of transmitted resource elements on transmit antennas.动机Theoretically, the best way to increase data rates over a communications link is to increasethe overall received signal power for a given transmit power 1. An effective way of increasing the received power is to use add

3、itional antennas at the transmitter and/or the receiver.This represents a class known as multi-antenna or MIMO techniques. Impressive improvements in capacity and Bit Error Rates (BERs) brought about by the use of MIMO techniqueshave spurred a lot of interest in multi-antenna radio systems. Along wi

4、th the gains, however,comes added computational complexity. The complexity of a MIMO technique is usually inproportion to the number of antennas used.当信道为独立的瑞利衰落信道，发送天线和接收天线的个数分别为N和M且都很大时，信道容量C近似为：Cmin(N，M)Blog2(/2) （1）式中，B为带宽，为接收端的平均信噪比。在中MIMO使用算法 A successful implementation of a MIMO system hinges

5、 on solving systems of linear equations at the receiver inorder to correctly recover the transmitted data. In the presence of channel degradations, thefull spectrum exhibits a frequency-selective response. At each sub-band, however, the channelresponse is flatter and may be approximated by a scalar

6、gain value. In a MIMO system, ateach subcarrier the relationship between any pair of transmitted and received symbols can beexpressed with a single gain value. This means that the relationship between multiple transmitters and receivers can be expressed with a MIMO system of linear equations, which

7、aresolved at the receiver for each and every subcarrier of the full spectrum in order to recover the transmitted signal.The MIMO algorithms used in the LTE standard can be subdivided intofour broad categories: receiver-combining, transmit-diversity, beamforming, and spatial-multiplexing. Wewill prov

8、ide a short discussion of three of these techniques in this section.接收合并算法 信号在L条独立衰落信道上传输，设第k条信道的等效低通输出为： 其中 和 分别表示第k条衰落信道的幅度衰和相移； 是等效低通复高斯噪声，设它的实部和虚部的功率谱密度均为于是第k条信道接收机的输出信噪比为: 其中 为复包络信号 的能量。 选择合并器通过测量各条支路的信噪比，选择其中最大信道比支路的输出进行判决。选择性合并kak( )ktkN2ksksaErNsE( )ls t最大合并比（MRC） 在最大比合并中，合并器把各衰落支路所接收到的信号线性组

9、合，即 选择合并系数 使 的信噪比最大，所以最大比合并是一种最佳的合并器。1()()Llkl kkrtrtk( )lr t式中得信号分量为 (2-2) 噪声分量为 （2-3） 由于 相互独立，所以噪声总功率为： （2-4） 于是输出信噪比为：1()()kLjlkkkvtstae1()()Lkkktt( )kt21LkkkNN2121kLjkkskLkkkeErN利用Schwartz不等式，有 （2-6）式（2-6)成立的充分条件为 现令 222111LLLkkkkkkka bb ,1, 2 ,kkbakL,kjkkkkkka eabNN代入式（2-6）得到 所以 当选择 时，式等号成立，也就是

10、说这时合并器输出地信噪比 是最大的，等于各支路信噪比之和。222111kLLLjkkkkkkkkkeNN 1Lkkrr,1,2,kjkkkekLNr发射分集发射分集In transmit diversity, redundant information is transmitted on different antennas at each subcarrier. In this mode, LTE does not increase the data rate but only makes the communications link more robust. Transmit diver

11、sity belongs to a class of multi-antenna techniques known as spacetime coding. Spacetime codes are capable of delivering a diversity order equal to the product of the number of receive and transmit antennas. SFBC, a technique closely related to SpaceTime Block Coding (STBC), is the transmit-diversit

12、y technique used in the LTE standard.开环方式（空时分组码）发射端不需要知道信道信息无反馈回路，典型的空间发射分集闭环方式 （利用信道状态信息）发射端需要知道信道信息利用上行信道从移动台向基站及时报告信道信息，用于发射加权。闭环发射分集信号在接收端的平均SINR是开环发射分集信号在接收端的平均SINR的两倍空间复用空间复用信道In a flat-fading scenario, the relationship between any given pair of transmit and receiveantennas at any point in tim

13、e is given by a scalar gain value known as the channel path gain.The collection of these path gains specifies the channel matrixH. In a multipath fading scenario, the relationship between any given transmit and receiveantenna at any point in time is characterized by the channel-path gain vector. So

14、each receivedsignal at any point in time depends on the present and past values of transmitted signals. Thisnecessitates the introduction of one more parameter: the number of path delaysL. To computethe received signals in a multipath case, the MIMO operations mentioned in the flat-fadingscenario mu

15、st be repeated for each value of the path-delay vector.LTE-Specific Channel ModelsThe 3GPP (Third Generation Partnership Project) Technical Recommendation (TR) 36.104 3specifies three different multipath fading channel models: the Extended Pedestrian A (EPA),Extended Vehicular A (EVA), and Extended

16、Typical Urban (ETU). The channel-modelingfunctions used in this book explicitly take advantage of these models. We will not use thehigher-mobility profiles as the closed-loop spatial-multiplexing mode is applicable to highdata-rate and low-mobility scenarios only. Together with the generic channel m

17、odels describedearlier, these models enable us to evaluate the performance of the transceiver in various reference channel conditions.MIMO 共同特点资源网格结构MIMO信道估计在MIMO-OFDM系统中，发送端编码和接收端信号检测都需要真实准确的信道状态信息。信道状态信息的准确性将直接影响着MIMO-OFDM系统的整体性能。然而对于MIMO-OFDM系统，不同的信号同时从不同的天线发射出去，对于每一个天线、每一个子载波都会对应很多个信道参数，信道参数太多，对

18、信道估计带来了较大的困难。但对于不同的子载波，同一空分信道的参数是相关的，我们可以利用这一相关特性得到参数的估计方法。基于的信道估计Considering,for a 22 configuration for the MIMOchannel, the MIMO system of equation at agiven time index can be expressed as理想信道估计MIMO详细特点两天线空时块码 1.Alamouti STBC编码 在这种编码方案中，每组m比特信息首先调制为M=2m进制符号。然后编码器选取连续的两个符号，根据下述变换将其映射为发送信号矩阵。 天线1发送信

19、号矩阵的第一行，而天线2发送信号矩阵的第二行。编码器结构如下图所示。 *12*21xxxxX信源调制器编码器*1212*21 ,xxx xxx 时 域空域Tx1Tx21*12,xxx2*21,xxxAlamouti空时块编码器结构 由图可知，Alamouti空时编码是在空域和时域上进行编码。令天线1和2的发送信号向量分别为：这种空时编码的关键思想在于两个天线发送的信号向量相互正交 ，编码矩阵具有如下性质 ：1*2*1221,xxx xxx 其中I2是22的单位矩阵。 假设接收机采用单天线接收。发送天线1和2的块衰落信道响应系数为： 在接收端，相邻两个符号周期接收到的信号可以表示为： 其中，n1和n2表示第一个符号和第二个符号的加性白高斯噪声样值。这种