1、MIMO 学习心得 LTE 的 7 个传输模式中 6 个分别应用了四种 MIMO 技术方案：传输分集（TD），波束赋型（Beamforming），空间复用（SM），多用户 MIMO（MU-MIMO）：1. 为普通单天线传输模式。 2. TransmitDiversity 模式：分 2 发送天线的 SFBC，和 4 发送天线的SFBC+FSTD 两种方案。2 发送天线的 SFBC : SFBC 是由 STBC（Space Time Block Code）演变而来，由于 OFDM 一个 slot 的符号数为奇数，因此不适于使用 STBC，但频域资源是以 RB=12 个子载波来分配的，因此可以用连续
2、两个子载波来代替连续两个时域符号，从而组成 SFBC。而当使用 4 发送天线时，SFBC+FSTD（Frequency Switched Transmit Diversity）被采用。 3. SM-open loop，UE 仅仅反馈信道的 RI（Rank Indicator）。此时基站会使用 CDD（Cycle Delay Diversity）技术。 4. SM-close loop，UE 根据信道估计的结果反馈合适的 PMI（Precoding Matrix Indicator）。(如利用系统容量最大计算合适的 PMI) 5. MU-MIMO，该方案将相同的时频资源通过空分，分配给不同的用户
3、。 6. close loop rank1SM or BF，UE 反馈信道信息使得基站选择合适的Precoding。 7. UE Special RSBF，和 BeamForming 的前一种方式不同，这种方式无需UE 反馈信道信息，而是基站通过上行信号进行方向估计，并在下行信号中插入 UE Special RS。基站可以让 UE 汇报 UE Special RS 估计出的 CQI。 空间复用是为了提高传输数据数量，基于多码字的同时传输，即多个相互独立的数据流通过映射到不同的层，再由不同的天线发送出去。码字数量与天线数量未必一致。（当然天线数量码字数量）。传输分集主要用于提高信号传输的可靠性，
4、例如采用空时编码（STC）、循环延时分集（CDD)及天线切换分集等，LTE 中用的比较多的是 SFBC 编码。传输分集（TM2）用来提高信号传输的可靠性，主要是针对小区边缘用户，TM3，TM4 主要是针对小区中央的用户，提高峰值速率。MU-MIMO 是为了提高吞吐量，用于小区中的业务密集区。TM6，TM7 是用于增强小区覆盖，也是用于边缘用户。不过 6 是针对 FDD，7 是针对 TDD 而已。在学习理解 MIMO 之前，需要理解几个基本概念：1. codeword: 相当于 TranportBlock, 即物理层需要传输的原始数据块. LTE 可支持在同一块资源同时传输 2 个相对独立的 c
5、odeword，这是通过空间复用（SM）技术实现的。 2. layer：数据被分为不同 layer 进行传输，layer 总数 =天线个数。和信道矩阵的 rank 是对应的。相当于空分的维度。 3. rank：相当于总的 layer 数。 4. Antenna port：其实并不等同于天线个数，而是相当于不同的信道估计参考信号 pattern。对端口 03，确实对应多天线时，RS 的发送 pattern；对于端口 4，对应于 PMCH，MBSFN 情况的 RS；对于端口 5，对应于 UE Special RS。 TDD-LTE 中支持的传输模式使用的 DCI format 。在每一种模式中都可
6、以通过改变DCI format 切换到发射分集模式（ TM2），并且 TM3、TM4 模式下，也支持在不改变 DCI format 的情况下从空分复用切换到发射分集模式（TM2），这时需要通过在 DCI format 中的 precoding information field 来指示所使用的 preceding matrix vector。From 36.213 Table 7.1-5Table 7.1-5: PDCCH and PDSCH configured by C-RNTIDCI format 1A Common andUE specific by C-RNTITransmit di
7、versity (see subclause 7.1.2)Mode 2DCI format 1 UE specific by C-RNTI Transmit diversity (see subclause 7.1.2)DCI format 1A Common andUE specific by C-RNTITransmit diversity (see subclause 7.1.2)Mode 3DCI format 2A UE specific by C-RNTI Large delay CDD (see subclause 7.1.3) or Transmit diversity (se
8、e subclause 7.1.2)DCI format 1A Common andUE specific by C-RNTITransmit diversity (see subclause 7.1.2)Mode 4DCI format 2 UE specific by C-RNTI Closed-loop spatial multiplexing (see subclause 7.1.4)or Transmit diversity (see subclause 7.1.2)DCI format 1A Common andUE specific by C-RNTIIf the number
9、of PBCH antenna ports is one, Single-antenna port, port 0 is used (see subclause 7.1.1), otherwise Transmit diversity (see subclause 7.1.2)Mode 7DCI format 1 UE specific by C-RNTI Single-antenna port; port 5 (see subclause 7.1.1)PDSCH process flow:Layer mappingNow codeword-to-layer mapping support 3
10、 types:1. Layer mapping for transmission on a single antenna port1 codeword-to-1 layer2. Layer mapping for spatial multiplexing2 antenna ports:1) 1 codeword-to-1 layer 2) 2 codewords-to-2 layers 4 antenna ports:1) 1 codeword-to-1 layer 2) 1 codeword-to-2 layers 3) 2 codewords-to-2 layers 4) 2 codewo
11、rds-to-3 layers 5) 2 codewords-to-4 layers Table 6.3.3.2-1: Codeword-to-layer mapping for spatial multiplexing.Number of layers Number of codewords Codeword-to-layer mapping1,.0layersmbMi1 1 )()(0dix)0(lri2 2 )()(1dix)1(symb)0(slayersmb2 1 )2()01(iilayer(0)sbsb2M(dx3 2 )12()2(1ii 2)1(symb)0(slayersm
12、b)()01(idix4 2 )12()3(2ii 2)1(symb)0(syblaersmM3. Layer mapping for transmit diversity 1 codeword-to-2 layers (2 antenna ports)1 codeword-to-4 layers (4 antenna ports)Table 6.3.3.3-1: Codeword-to-layer mapping for transmit diversity.Number of layersNumber of codewordsCodeword-to-layer mapping1,.0lay
13、ersmbMi2 1 )12()()01idix 2)0(syblaers4 1 )34()(21)()()03210idixii 04mod if42)0(syb)0(symblaersybMMIf two null symbols shall be od)(sappended to )1(0syb)PrecondingNow precoding support 3 types:1. Precoding for transmission on a single antenna port2. Precoding for transmit diversity )(ImRe)(0102)()( 1
14、0)10 ixijjiyi3. Precoding without CDD- Closed-loop spatial multiplexing scheme(TM4)()(10)1()0( ixiiWiyiP4. Precoding for large delay CDD - Large delay CDD scheme(TM3 )()()(1)0(1)0( ixiUiDiyiPand can get from below table according to different layer Ui Table 6.3.4.2.2-1: Large-delay cyclic delay dive
15、rsity.Number of layers U)(iD2 21je 201ije3 3834421jje 34320ijije4 4184124628621jjj jjj ee 4644200ijijij eeFor can selected from codebook defined in 36.211 Table 6.3.4.2.3-1.)(iWTable 6.3.4.2.3-1: Codebook for transmission on antenna ports .1,0Codebook indexNumber of layers 1 202 101 12122 j2j23 j1-P
16、MIUE will report the suggested codebook index by PMI and eNB will send the codebook info which eNB will use for PDSCH by DCI.For different transmission mode, the PMI will use different number of bits.Table 7.2-1b: Number of bits in codebook subset restriction bitmap for applicable transmission modes
17、.Number of bits cA2 antenna ports4 antenna portsTransmission mode 3 2 4Transmission mode 4 6 64Transmission mode 5 4 16Transmission mode 6 4 16Transmission mode 8 6 32 Transmission mode 3:PMI will use 2 bits- 0a1For 2 antenna ports: means 2 layers -codebook index 0 -1a 102means 1 layer - codebook in
18、dex 0 -means transmit diversity0 Transmission mode 4:PMI will use 6 bits- 0a12345aFor 2 antenna ports, defined in 36.211 Table 7.2-1c:means 1 layer -codebook index 0-0a 1means 1 layer -codebook index 1-1 2means 1 layer -codebook index 2-2a j1means 1 layer -codebook index 3-3 j2means 2 layers -codebo
19、ok index 1-4a 12means 2 layers -codebook index 2-5 jTable 7.2-1c: Association of bits in codebookSubSetRestriction bitmap to precoders in the 2 antenna port codebook of Table 6.3.4.2.3-1 in 3.Codebook index ciNumber of layers 1 20 a0 -1 a1 a42 a2 a53 a3 -Precoding info in DCIeNB will send the precod
20、ing info in DCI format 2/2A. Transmission mode 3:DCI format 2A:Precoding information number of bits as specified in Table 5.3.3.1.5A-1For antenna ports 2, the precoding information field is not present.Table 5.3.3.1.5A-1: Number of bits for precoding information.Number of antenna ports at eNodeB Num
21、ber of bits for precoding information2 04 2The number of transmission layers is equal to 2 if both codewords are enabled; transmit diversity is used if codeword 0 is enabled while codeword 1 is disabled. But the PDCCH format will use DCI format 2A no matter spatial multiplexing or transmit diversity
22、. Transmission mode 4:DCI format 2:Precoding information number of bits as specified in Table 5.3.3.1.5-3(36.212)Table 5.3.3.1.5-3: Number of bits for precoding information.Number of antenna ports at eNodeB Number of bits for precoding information2 34 6For antenna ports 2, there are 3 bits for preco
23、ding information, detail info see below table:Precoding info bit field mapped to index:For 2 layers:Precoding info codebook index 0 0 -means 2 layers precoding matrix 121 1 -means 2 layers precoding matrix j2 X -means 2 layers precoding matrix used the latest PMI report on PUSCH, using the precoder(
24、s) indicated by the reported PMI(s)For 1 layer:Precoding info codebook index 0 0 -means 2 layers: Transmit diversity1 1 -means 1 layer precoding matrix 2/1T2 2 -means 1 layer precoding matrix 3 3 -means 1 layer precoding matrix /j4 4 -means 1 layer precoding matrix T5 X -means 1 layer precoding matr
25、ix will use the latestPMI report on PUSCH, using the precoder(s) indicated by the reported PMI(s),if RI=2 was reported, using 1st column multiplied by of all 2precoders implied by the reported PMI(s)6 X -means 1 layer precoding matrix will use the latestPMI report on PUSCH, using the precoder(s) ind
26、icated by the reported PMI(s),if RI=2 was reported, using 2nd column multiplied by of all 2precoders implied by the reported PMI(s)36.212 Table 5.3.3.1.5-4: Content of precoding information field for 2 antenna ports.One codeword: Codeword 0 enabled, Codeword 1 disabledTwo codewords: Codeword 0 enabl
27、ed, Codeword 1 enabledBit field mapped to indexMessageBit field mapped to indexMessage0 2 layers: Transmit diversity0 2 layers: Precoding corresponding to precoder matrix 11 1 layer: Precoding corresponding to precoding vector 2/T1 2 layers: Precoding corresponding to precoder matrix j2 1 layer: Pre
28、coding corresponding to precoder vector 2/T2 2 layers: Precoding according to the latest PMI report on PUSCH, using the precoder(s) indicated by the reported PMI(s)3 1 layer: Precoding corresponding to precoder vector 2/Tj3 reserved4 1 layer: Precoding corresponding to precoder vector /Tj4 reserved5
29、 1 layer: Precoding according to the latest PMI report on PUSCH, using the precoder(s) indicated by the reported PMI(s),if RI=2 was reported, using 1st column multiplied by of all 2precoders implied by the reported PMI(s)5 reservedTM7 mode:For transmission mode 7 , can support one port and two ports
30、 .NumAntennaPort = 1tansmissionMode = TM7DCI 1 -port 5-beamformingCell-RS -port 0 D-RS -port 5/NumAntennaPort = 2tansmissionMode = TM7DCI 1 - port 5- BeamformingCell-RS -port 0, port 1 D-RS -port 5/For beamforming ,the PDSCH will use port5 ,and no mater RRU has 2 or 4 or 8 physical antenna , Baseban
31、d will send the same signal in all these physical antennas, just every antenna has different phase and amplitude. So in this mode, RRU has no primary and secondary antenna .And for NumAntennaPort =1 and NumAntennaPort =2, the beamforming the main difference is for Cell-RS ,When NumAntennaPort =1, Ce
32、ll-RS is sending on port 0When NumAntennaPort =2, Cell-RS is sending on port 0 and port 1.This difference will just impact the RE which used for PDSCH data./Under TM7 mode, can support beamforming and transmit diversity using different DCI format .DCI 1 for beamforming and DCI 1A for transmit diversity.NumAntennaPort = 1tansmissionMode = TM7DCI 1A -port 0-single-antenna port, port 0Cell-RS -port 0This mode is just like TM1 /NumAntennaPort = 2tansmissionMode = TM7DCI 1A -port 0, port1 -transmit diversityCell-RS -port 0, port 1This mode is just like TM2
