2024年4月4日发(作者：)
MIMO 学习心得 --------Ellen wang
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。
6. close loop rank1——SM or BF，UE反馈信道信息使得基站选择合适的
Precoding。
7. UE Special RS——BF，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，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codeword，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. atenna 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0~3，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precoding matrix
vector。
From 36.213 Table 7.1-5
Table 7.1-5: PDCCH and PDSCH configured by C-RNTI
Mode 2 DCI format 1A
DCI format 1
Mode 3 DCI format 1A
DCI format 2A
Mode 4 DCI format 1A
DCI format 2
Mode 7 DCI format 1A
Common and
UE specific by C-RNTI
UE specific by C-RNTI
Common and
UE specific by C-RNTI
UE specific by C-RNTI
Common and
UE specific by C-RNTI
UE specific by C-RNTI
Common and
UE specific by C-RNTI
Transmit diversity (see subclause
7.1.2)
Transmit diversity (see subclause
7.1.2)
Transmit diversity (see subclause
7.1.2)
Large delay CDD (see subclause 7.1.3)
or Transmit diversity (see subclause
7.1.2)
Transmit diversity (see subclause
7.1.2)
Closed-loop spatial multiplexing (see
subclause 7.1.4)or Transmit diversity
(see subclause 7.1.2)
If the number 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)
Single-antenna port; port 5 (see
subclause 7.1.1)
DCI format 1 UE specific by C-RNTI
PDSCH process flow:
Layer mapping
Now codeword-to-layer mapping support 3 types:
1. Layer mapping for transmission on a single antenna port
1 codeword-to-1 layer
2. Layer mapping for spatial multiplexing
2 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 codewords-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 mapping
layer
i0,1,...,M
symb
1
1 1
x
(0)
(i)d
(0)
(i)
x
(0)
(i)d
(0)
(i)
layer(0)
M
symb
M
symb
2 2
x
(1)
(i)d
(1)
(i)
2 1
layer(0)(1)
M
symb
M
symb
M
symb
x
(0)
(i)d
(0)
(2i)
x
(1)
(i)d
(0)
(2i1)
layer(0)
M
symb
M
symb
2
x
(0)
(i)d
(0)
(i)
3 2
x(i)d
(1)(1)
(2i)
layer(0)(1)
M
symb
M
symb
M
symb
2
x
(2)
(i)d
(1)
(2i1)
x
(0)
(i)d
(0)
(2i)
x
(1)
(i)d
(0)
(2i1)
4 2
layer(0)(1)
M
symb
M
symb
2M
symb
2
x
(2)
(i)d
(1)
(2i)
x
(3)
(i)d
(1)
(2i1)
3. 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
layers
Number of
codewords
Codeword-to-layer mapping
layer
i0,1,...,M
symb
1
x
(0)
(i)d
(0)
(2i)
2 1
x
(1)
(i)d
(0)
(2i1)
layer(0)
M
symb
M
symb
2
x
(0)
(i)d
(0)
(4i)
x
(1)
(i)d
(0)
(4i1)
4 1
layer
M
symb
x
(2)
(i)d
(0)
(4i2)
x
(3)
(i)d
(0)
(4i3)
(0)
(0)(0)
M
symb
4if M
symb
mod40
(0)(0)
M
symb
24 if M
symb
mod40
If
M
symb
mod40
two null symbols shall be
appended to
d
(0)
(0)
(M
symb
1)
Preconding
Now precoding support 3 types:
1. Precoding for transmission on a single antenna port
2. Precoding for transmit diversity
y
(0)
(2i)
j0
Rex
(0)
(i)
10
(1)
010
(1)
j
1
y(2i)Rex(i)
(0)(0)
y(2i1)
0j
Imx(i)
2
01
(1)
(1)
10j0
y(2i1)Imx(i)
3. Precoding without CDD---- Closed-loop spatial multiplexing scheme(TM4)
y
(0)
(i)
x
(0)
(i)
W(i)
y
(P1)
(i)
x
(
1)
(i)
4. Precoding for large delay CDD ---- Large delay CDD scheme(TM3 )
y
(0)
(i)
x
(0)
(i)
W(i)D(i)U
y
(P1)
(i)
x
(
1)
(i)
U
and
D(i)
can get from below table according to different layer
Table 6.3.4.2.2-1: Large-delay cyclic delay diversity.
Number of
layers
2
U
1
1
1
j2
2
2
1e
1
1
1
j2
1e
3
j4
1e
1
3
3
D(i)
0
1
0e
j2
i2
3
3
0
1
0e
j2
i3
0
0
0
e
j4
i3
0
3
e
j4
e
j8
4
1
1
1e
j2
1
2
1e
j4
j6
1e
1
4
4
4
1
e
4
4
e
j4
4j6
4
e
j8
e
j12
e
j12
e
j18
4
4
0
1
0e
j2
i4
00
0
0
0
0
e
j4
i4
0
0
0
e
j6
i4
0
For
W(i)
can selected from codebook defined in 36.211 Table 6.3.4.2.3-1.
Table 6.3.4.2.3-1: Codebook for transmission on antenna ports
0,1
.
Codebook
index
1
Number of layers
2
0
1
1
2
1
1
10
2
01
1
1
1
1
2
1
11
2
11
2
1
1
j
2
1
1
j
2
1
11
2
jj
3
-
PMI
UE 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.
Transmission mode 3
Number of bits
A
c
2 antenna
ports
4 antenna
ports
2
4
Transmission mode 4
Transmission mode 5
Transmission mode 6
Transmission mode 8
6
64
4
4
16
16
6
32
➢ Transmission mode 3:
PMI will use 2 bits---
a
0
a
1
For 2 antenna ports:
a
1
means 2 layers ----codebook index 0 --------
1
10
01
2
a
0
means 1 layer ----- codebook index 0 -------means transmit diversity
➢ Transmission mode 4:
PMI will use 6 bits---
a
0
a
1
a
2
a
3
a
4
a
5
For 2 antenna ports, defined in 36.211 Table 7.2-1c:
a
0
means 1 layer ----codebook index 0-----
1
1
2
1
1
1
2
1
1
1
2
j
1
1
2
j
a
1
means 1 layer ----codebook index 1-----
a
2
means 1 layer ----codebook index 2------
a
3
means 1 layer ----codebook index 3-------
a
4
means 2 layers ----codebook index 1------
2
11
a
5
means 2 layers ----codebook index 2------
2
jj
Table 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
i
c
0
1
2
3
1
a
0
a
1
a
2
a
3
2
-
a
4
a
5
-
Number of layers
1
11
1
11
Precoding info in DCI
eNB will send the precoding 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-1
For 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
2
4
Number of bits for precoding information
0
2
The 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.
➢ 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
2
4
Number of bits for precoding information
3
6
For antenna ports 2, there are 3 bits for precoding information, detail info see
below table:
Precoding info bit field mapped to index:
For 2 layers:
Precoding info codebook index
1
11
0 0 ------------means 2 layers –precoding matrix
2
11
1
11
1 1 ------------means 2 layers –precoding matrix
2
jj
2 X ------------means 2 layers –precoding matrix used
the latest PMI report on PUSCH, using the precoder(s)
indicated by the reported PMI(s)
For 1 layer:
Precoding info codebook index
0 0 ------------means 2 layers: Transmit diversity
T
1 1 ------------means 1 layer –precoding matrix
11
/2
2 2 ------------means 1 layer –precoding matrix
11
/2
T
3 3 ------------means 1 layer –precoding matrix
1j
/2
T
T
4 4 ------------means 1 layer –precoding matrix
1j
/2
5 X ------------means 1 layer –precoding matrix will use the latest
PMI report on PUSCH, using the precoder(s) indicated by the
reported PMI(s),
if RI=2 was reported, using 1
st
column multiplied by
precoders implied by the reported PMI(s)
2
of all
6 X ------------means 1 layer –precoding matrix will use the latest
PMI report on PUSCH, using the precoder(s) indicated by the
reported PMI(s),
if RI=2 was reported, using 2
nd
column multiplied by
precoders implied by the reported PMI(s)
2
of all
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 disabled
Bit field
mapped to
index
0
Message
2 layers: Transmit
diversity
Two codewords:
Codeword 0 enabled,
Codeword 1 enabled
Bit field
mapped
to index
0
Message
2 layers: Precoding
corresponding to
precoder matrix
1
11
2
11
1 1 layer: Precoding
corresponding to
precoding vector
1 2 layers: Precoding
corresponding to
precoder matrix
11
T
/
2
2
1
11
2
jj
2 2 layers: Precoding
according to the latest
PMI report on
PUSCH, using the
precoder(s) indicated
by the reported
PMI(s)
1 layer: Precoding
corresponding to
precoder vector
11
/2
T
3 1 layer: Precoding
corresponding to
precoder vector
3 reserved
1
4
j
/2
T
1 layer: Precoding
corresponding to
precoder vector
4 reserved
1
5
j
/2
T
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 1
st
column
multiplied by
2
of all
precoders implied by the
reported PMI(s)
5 reserved
TM7 mode:
For transmission mode 7 , can support one port and two ports .
NumAntennaPort = 1
tansmissionMode = TM7
DCI 1 -port 5--------------beamforming
Cell-RS ----port 0 D-RS ----port 5
/////////////////////////////////////////////////////////////////////////////////////////////////////////
NumAntennaPort = 2
tansmissionMode = TM7
DCI 1 - port 5-------------- Beamforming
Cell-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 , Baseband 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, Cell-RS is sending on port 0
When 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 = 1
tansmissionMode = TM7
DCI 1A -port 0-----------single-antenna port, port 0
Cell-RS ----port 0
This mode is just like TM1
/////////////////////////////////////////////////////////////////////////////////////////////////////////
NumAntennaPort = 2
tansmissionMode = TM7
DCI 1A -port 0, port1 --transmit diversity
Cell-RS ----port 0, port 1
This mode is just like TM2
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