MIMO相關(guān)技術(shù)介紹

1、MIMO相關(guān)知識討論主講 ：秦闖定義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.動機(jī)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.當(dāng)信道為獨(dú)立的瑞利衰落信道，發(fā)送天線和接收天線的個數(shù)分別為N和M且都很大時，信道容量C近似為：Cmin(N，M)Blog2(/2) （1）式中，B為帶寬，為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 信號在L條獨(dú)立衰落信道上傳輸，設(shè)第k條信道的等效低通輸出為： 其中 和 分別表示第k條衰落信道的幅度衰和相移； 是等效低通復(fù)高斯噪聲，設(shè)它的實(shí)部和虛部的功率譜密度均為于是第k條信道接收機(jī)的輸出信噪比為: 其中 為復(fù)包絡(luò)信號 的能量。 選擇合并器通過測量各條支路的信噪比，選擇其中最大信道比支路的輸出進(jìn)行判決。選擇性合并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號線性組

9、合，即 選擇合并系數(shù) 使 的信噪比最大，所以最大比合并是一種最佳的合并器。1()()Llkl kkrtrtk( )lr t式中得信號分量為 (2-2) 噪聲分量為 （2-3） 由于 相互獨(dú)立，所以噪聲總功率為： （2-4） 于是輸出信噪比為：1()()kLjlkkkvtstae1()()Lkkktt( )kt21LkkkNN2121kLjkkskLkkkeErN利用Schwartz不等式，有 （2-6）式（2-6)成立的充分條件為 現(xiàn)令 222111LLLkkkkkkka bb ,1, 2 ,kkbakL,kjkkkkkka eabNN代入式（2-6）得到 所以 當(dāng)選擇 時，式等號成立，也就是

10、說這時合并器輸出地信噪比 是最大的，等于各支路信噪比之和。222111kLLLjkkkkkkkkkeNN 1Lkkrr,1,2,kjkkkekLNr發(fā)射分集發(fā)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.開環(huán)方式（空時分組碼）發(fā)射端不需要知道信道信息無反饋回路，典型的空間發(fā)射分集閉環(huán)方式 （利用信道狀態(tài)信息）發(fā)射端需要知道信道信息利用上行信道從移動臺向基站及時報告信道信息，用于發(fā)射加權(quán)。閉環(huán)發(fā)射分集信號在接收端的平均SINR是開環(huán)發(fā)射分集信號在接收端的平均SINR的兩倍空間復(fù)用空間復(fù)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 共同特點(diǎn)資源網(wǎng)格結(jié)構(gòu)MIMO信道估計(jì)在MIMO-OFDM系統(tǒng)中，發(fā)送端編碼和接收端信號檢測都需要真實(shí)準(zhǔn)確的信道狀態(tài)信息。信道狀態(tài)信息的準(zhǔn)確性將直接影響著MIMO-OFDM系統(tǒng)的整體性能。然而對于MIMO-OFDM系統(tǒng)，不同的信號同時從不同的天線發(fā)射出去，對于每一個天線、每一個子載波都會對應(yīng)很多個信道參數(shù)，信道參數(shù)太多，對

18、信道估計(jì)帶來了較大的困難。但對于不同的子載波，同一空分信道的參數(shù)是相關(guān)的，我們可以利用這一相關(guān)特性得到參數(shù)的估計(jì)方法。基于的信道估計(jì)Considering,for a 22 configuration for the MIMOchannel, the MIMO system of equation at agiven time index can be expressed as理想信道估計(jì)MIMO詳細(xì)特點(diǎn)兩天線空時塊碼 1.Alamouti STBC編碼 在這種編碼方案中，每組m比特信息首先調(diào)制為M=2m進(jìn)制符號。然后編碼器選取連續(xù)的兩個符號，根據(jù)下述變換將其映射為發(fā)送信號矩陣。 天線1發(fā)送信

19、號矩陣的第一行，而天線2發(fā)送信號矩陣的第二行。編碼器結(jié)構(gòu)如下圖所示。 *12*21xxxxX信源調(diào)制器編碼器*1212*21 ,xxx xxx 時 域空域Tx1Tx21*12,xxx2*21,xxxAlamouti空時塊編碼器結(jié)構(gòu) 由圖可知，Alamouti空時編碼是在空域和時域上進(jìn)行編碼。令天線1和2的發(fā)送信號向量分別為：這種空時編碼的關(guān)鍵思想在于兩個天線發(fā)送的信號向量相互正交 ，編碼矩陣具有如下性質(zhì) ：1*2*1221,xxx xxx 其中I2是22的單位矩陣。 假設(shè)接收機(jī)采用單天線接收。發(fā)送天線1和2的塊衰落信道響應(yīng)系數(shù)為： 在接收端，相鄰兩個符號周期接收到的信號可以表示為： 其中，n1和n2表示第一個符號和第二個符號的加性白高斯噪聲樣值。這種