Training document e ran2.2_lte tdd system multiple antenna techniques(mimo and beamforming)-20111010-a-1.0

08/23/12 Internal

LTE System Multiple
Antenna Techniques

www.huawei.com
eRAN2.2 (MIMO and Beamforming)

HUAW TECHNOLOGIES CO., LTD.
EI Huawei Confidential

Training Objectives

 After completing this course, you will be able to:

Understand the concepts relevant to the MIMO and Beamforming.
 Understand basic principle of MIMO and Beamforming.
 References:
 3GPP TS 36.211: Physical Channels and Modulation
 3GPP TS 36.213: Physical layer procedures
 3GPP TS 36.306: User Equipment (UE) radio access capabilities
 FPD: MIMO and Beamforming Feature Documentation

EI Huawei Confidential Page2

Contents

 Background and Overview of the LTE MIMO Techniques

 Principles and Application of the MIMO Techniques

 Principles and Application of Beamforming


Background of Multi-Antenna Techniques
 Fifty years ago, Shannon gave the maximum efficiency that a time and
frequency communication system can achieve.
 S
C = B × log 2 1 + ( bit / s )
 N
 The rapid development of wireless communications poses increasingly higher
requirement for system capacity and spectral efficiency. Various algorithms
are invented, such as spreading the system bandwidth, optimizing the
modulation scheme, or using complex code division multiple access. These
methods are limited: Bandwidth cannot be expanded indefinitely; modulation
orders cannot increase indefinitely; channels between a CDMA system are
not ideally orthogonal. Another dimension, that is, MIMO, is invented to better
use the spatial resource. As expressed in the following equation, if multiple
antennas are used, the capacity is increased by a multiplication of the
number of antennas used.
 S
C = B × log 2 1 + ( bit / s ) × M
 N

Advantages of Multi-Antenna Techniques
 The LTE system improves system performance for cell edge users and brings
stable and reliable service experience for users. Therefore, multi-antenna
techniques can make use of the spatial resource and increase the wireless
transmission capacity many folds without increasing the transmit power and
bandwidth.

Array gain Improved Increased
system coverage spectral
efficiency
Diversity gain Improved
system capacity

Spatial multiplexing
gain
Increased peak
rate
Co-channel interference
reduction

EI Huawei Confidential Page 5

Contents





Principles of the MIMO Techniques
 MIMO is an important technique in the LTE system. MIMO means use of
multiple antennas at both the transmitter and receiver. MIMO can better
utilize the spatial resource and increase spectral efficiency, achieving array
gain, diversity gain, multiplexing gain, and interference rejection gain,
providing higher system capacity, wider coverage, and higher user rate.


Classification of MIMO Techniques
 Depending on whether the spatial channel information is used, MIMO techniques are
classified into open-loop MIMO and closed-loop MIMO.
• Open-loop MIMO: The UE does not feed back information, the eNodeB is not informed of the
UE situation. The protocols support single-stream (TM2) or multi-stream (TM3).
• Closed-loop MIMO: The UE feeds back information. The gain has a positive correlation with the
accuracy of the feedback information. The protocols support single-stream (TM4) or multi-
stream (TM6). At present, the feedback granularity supported by the reference signal in port 2
is large and closed-loop MIMO can hardly achieve gains. Closed-loop MIMO requires low UE
mobility. At present, the eNodeB cannot accurately estimate the UE movement speed with an
error of more than 30 km/h.
 Depending on the number of simultaneously transmitted spatial data streams, MIMO
techniques are classified into spatial diversity and spatial multiplexing.
 These modes are described in detail in the following pages.

MIMO Technique MIMO Mode Feature List in FDD Feature List in TDD
UL 2-Antenna Receive Diversity UL 2-Antenna Receive Diversity
UL 4-Antenna Receive Diversity UL 4-Antenna Receive Diversity
Receive diversity
Multi-antenna UL Interference Rejection UL Interference Rejection Combining
receive Combining UL 8-Antenna Receive Diversity
UL 2x2 MU-MIMO UL 2x2 MU-MIMO
MU-MIMO
UL 2x4 MU-MIMO UL 2x4 MU-MIMO
Open-loop transmit diversity
2x2 MIMO
Closed-loop transmit diversity 2x2 MIMO
Multi-antenna
Open-loop spatial multiplexing 4x2 MIMO
transmit 4x2 MIMO
Closed-loop spatial DL 4x4 MIMO
multiplexing
Page 8

Principle of Multi-Antenna Receive MIMO
 eRAN2.2 supports UL 2-Antenna Receive Diversity and optional UL 4-Antenna Receive
Diversity and UL 8-Antenna Receive Diversity.
 The following figure shows the block diagram of receive diversity. The UE uses one
antenna to transmit signals; different UEs use different time and frequency resources.
The eNodeB uses multiple antennas to receive signals and combine the received
signals to maximize SINR, therefore obtaining diversity gain and array gain, increasing
the cell coverage and improving single-user capacity.


Principle of Multi-Antenna Receive MIMO
 Mechanism of Signal Combination:
 An MMSE receiver uses receive beamforming targeted at a UE. The receiver adjusts the
combined weight and changes the direction of the major lobe and side lobe to maximize
the SINR of the received signals.
 There are two combination algorithms for UL receive diversity.
Maximum ratio combining (MRC) and interference rejection combining (IRC) can both
obtain diversity gain and array gain, improving system performance. MRC and IRC are
suitable for environments with different interference characteristics. MRC receivers and
IRC receivers are implementation of MMSE receivers in different scenarios.


Specification of Multi-Antenna Receive MIMO
 Adaptive Switchover Between MRC and IRC
 For eNodeBs V2.2, IRC is optional. If IRC is not selected, an eNodeB
uses MRC. If IRC is selected, an eNodeB adaptively selects IRC or MRC
depending on the current radio channel quality.
 If there is separable strong colored interference, the system automatically
uses IRC algorithm.
 If there is no separable strong colored interference, the system
automatically rolls back to MRC algorithm.
 In UL 2x2 MU-MIMO mode, the eNodeB does not support UL Interference
Rejection Combining or UL 2-Antenna Receive Diversity
 In UL 4-Antenna Receive Diversity mode, the eNodeB supports UL
Interference Rejection Combining.


Principle of Multi-User MIMO (MU-MIMO)
 Theoretically, the number of virtual MIMO users in the same RB cannot exceed the
number of receive antennas of the eNodeB. eNodeBsV2.2 support MU-MIMO 2x2.
The following figure shows MU-MIMO 2x2.
 eNodeBV2.2 ， The protocols support a maximum of MU-MIMO 4x4.


Multi-Antenna Transmit MIMO
 The eNodeB supports multi-antenna transmission and the UE does not. DL 2x2 MIMO, DL
4x2 MIMO, and DL 4X4 MIMO are described. R9 defines nine multi-antenna transmission
modes (TMs). The eNodeB adaptively selects one TM according to the channel condition
and service requirement.

Supported by
No. Name Applicable Scenario Current
eNodeB
1 Single antenna (port 0) Single-antenna transmission. Yes
Used by FDD/TDD

Open-loop transmit Suitable for cell edge where the channel condition is complex and
2 Yes
diversity interference is large, or high-mobility or low SNR situations.
Open-loop spatial
3 Suitable for high UE mobility and complex reflection environment. Yes
multiplexing
Closed-loop spatial Yes （ FDD
4 multiplexing Suitable for good channel condition. Provides high data transmission rate. ）
5 MU-MIMO Suitable for two orthogonal UEs. Used to increase cell capacity. Yes
Closed-loop transmit Yes （ FDD
6 diversity Suitable for cell edge, low mobility, and low SINR. ）
7 Single antenna (port5) Suitable for cell edge to reject interference. Yes
Adaptive single-stream
8 and dual-stream Suitable for cell edge, low mobility, and high SNR. Yes
beamforming
Used by TDD

Adaptive single-
stream, dual-stream, A new mode in LTE-A. Supports a maximum of eight layers. Increases data
9 No
and 4-stream transmission rate. Suitable for low mobility and high SNR.
beamforming


Concepts

 Port
 A port is a logical port and does not necessarily correspond to an antenna. There can be
multiple ports. The LTE protocols support a maximum of eight physical antennas. Ports
correspond to pilot formats, whereas the number of physical antennas has not direct
relationship with the pilot formats.
 Port 0 to port 3: Ports for transmitting common pilots. Usually the number of ports for physical
broadcast channels and downlink control channels is the same as that for common pilots.
 Port 5: A port defined in the LTE for supporting single-stream beamforming. The data of a
single port can be weighted and mapped to multiple physical antennas.
 Port 6: A port for locating the pilot.
 Port 7 to port 14: Similar to port 5. Supports a maximum of 8 layers. The data of 8 ports can
be weighted and mapped to 8 physical antennas. Used for dual-stream beamforming.
 Port 15 to port 22: CSI-RS port.
 Maximum number of streams = Number of logical antenna ports [2 ports, 4 ports, or 8 ports]


Concepts
 Pilots in the LTE system
 Cell-specific reference signal (CRS): CRS is known as common pilot. CRS is
used by the control channels for channel estimation and demodulation. CRS is
used for demodulation of TM1 to TM6 and RSRQ measurement.
 UE-specific reference signal at port 5: It is used for demodulating TM7.
 DM RS at ports 7 to 14: It is used for demodulating TM8 to TM9 and is the
reference signal in R9 and R10. It supports MU-MIMO and demodulation of a
maximum of eight layers.
 Reference signal at port 6: It is used for locating the UE.
 Channel status information measurement RS (CSI-RS): It is used for measuring
the channel quality indication, precoding matrix indication, and RI. CSI-RS
supports measurement of eight ports.
 Sounding reference signal (SRS): It is used for measuring the uplink channels
and supports uplink scheduling.


Ce ll-s pe cific Re fe re nce S igna l (CRS )
 Normal CP ， downlink reference signal map relationship.


Open-Loop Transmit Diversity

 In open-loop transmit diversity (TM2), space-frequency block coding (SFBC) is
used if the number of transmit antennas is 2; SFBC and frequency switched
transmit diversity (FSTD) are used if the number of transmit antennas is 4.
 SFBC: For two-way transmit (DL 2x2 MIMO), the transmit diversity uses SFBC,
where X1 and x2 are the information to be transmitted before SFBC, * indicates
conjugate operation, f1 and f2 are different subcarriers, and Tx1 and Tx2 are different
transmit antennas. SFBC codes x1 and x2 to different antennas and subcarriers for
transmission: x1 over Tx1 f1, x2 over Tx1 f2, -x2* over Tx2 f1, and x1* over Tx2 f2. Therefore,
by transmitting copies of x1 and x2 over different antennas and frequencies, SFBC
achieves diversity gain.


Open-Loop Transmit Diversity
 SFBC+FSTD
 For 4-way transmit (DL 4x2 MIMO or DL 4X4 MIMO), SFBC and FSTD are used together.
In FSTD, some of the transmit antennas are selected sequentially in frequency for
transmission.
 The transport format of SFBC+FSTD is as follows: x1, x2, x3, and x4 are information to be
transmitted before coding; f1 to f4 are different subcarriers; Tx1 and Tx4 are different
transmit antennas; * indicates conjugate operation; 0 indicates no information
transmitted. In SFBC+FSTD, x1 to x4 are coded to different antennas and subcarriers for
transmission; the transmit antennas are selected. Like SFBC, SFBC+FSTD achieves
diversity gain by transmitting copies over different antennas and frequencies.


Spatial Multiplexing
 Spatial multiplexing means transmission of multiple spatial data streams over different
antennas in the same RB. The dimension of spatial channels is increased compared
with the single-antenna technique. Therefore, spatial multiplexing increases system
capacity and achieves spatial multiplexing gain. Spatial multiplexing includes two
operations: layer mapping and precoding. Depending on whether the precoding matrix is
obtained based on the feedback information of the UE, spatial multiplexing is classified
into open-loop spatial multiplexing (TM3) and closed-loop spatial multiplexing (TM4).
The following figure shows the 2x2 spatial multiplexing


Adaptive Mode Configuration
 Mulit-Antenna transmit technologies can support different scenario transmit and
mode. According to different scenarios, eNodeB support choose the most best
MIMO mode.
 Mode choice and switch four type:
 Open and close loop mode adaptive choose and switch
 Open loop adaptive mode choose and switch
 Close loop adaptive mode choose and switch
 Fix mode choose
 DL 2x2 MIMO and DL 4x2 MIMO support four mode choose and switch.
 DL 4X4 MIMO only support open loop adaptive mode choose and switch.


Configura tions of MIMO


Configura tion of MU-MIMO


Application of MIMO
 At persent, LTE TDD can support by RRU3232 ， RRU3235
 Specification of eNodeB：

Configurati MIMO LBBPc RRU3232 RRU3231
on type
3 × 10MHz 2 × 2 MIMO 1 LBBPc 2 (2T2R) 3

3 × 10MHz 4 × 2 MIMO 1 LBBPc 3 -

3 × 20MHz 2 × 2 MIMO 1 LBBPc 2 (2T2R) 3

3 × 20MHz 4 × 2 MIMO 3 LBBPc 3 -


Contents





Principles of Beamforming
 Beamforming is a downlink multi-antenna technique. The transmitter of an
eNodeB weights the data before transmission, forming narrow beams and
aiming the energy at the target user, as shown in the following figure.
 Beamforming does not require the UE to feed back information or use multiple

antennas to transmit data. The direction of incoming wave and the path loss
information are obtained by measuring the uplink received signal.

The benefits of beamforming are as follows:
Increased SINR in the direction of incoming wave from the UE.
Increased system capacity and coverage.


Classification of Beamforming Techniques

 DOA Beamforming and MIMO Beamforming:
 Direction of Arrival (DOA) beamforming: The eNodeB estimates the direction of arrival of the
signal, uses the DOA information to calculate the transmit weight, and targets the major lobe of the
transmit beam at the best direction.

MIMO beamforming: The eNodeB uses the channel information to calculate the transmit weight,
forming a beam.
 In the industry, the TDD system uses open-loop Beamforming and the FDD
system uses closed-loop Beamforming. Huawei eNodeB supports open-
loop Beamforming.


Classification of Beamforming (Single-Stream)
Single-stream beamforming means transmission of a single data stream in the same
OFDM resource block. It is suitable for situations of poor channel quality.
Single-stream beamforming achieves diversity gain by 1 dB by increasing the SNR.
Take 4-antenna as an example. The following figure shows single-stream
beamforming. The data stream S is weighted by w1 to w4 and is sent to the four antenna
ports for transmission.


Classification of Beamforming (Dual-Stream)
Dual-stream beamforming means transmission of two data streams in the same OFDM
resource block, leading to spatial multiplexing. It is suitable for situations of good channel
quality.
Take 4-antenna as an example. The following figure shows dual-stream beamforming.
There are two data streams S1 and S2; each antenna has two weights wi1 and wi2. S1 is

weighted by four weights: w11 to w41; S2 is weighted by another four weights w12 to w42. The
weighted streams are summed and sent to the four antenna ports for transmission.


Engineering Guidelines of Beamforming
 Before configuring beamforming antennas, you need to understand the correspondence
between the port No. and the co-polarization of cross-polarized antennas. The following
figure shows the connection between RRU ports and antenna element of the four or
eight antennas.
 At present, the RRU models in LTE TDD that support beamforming are RRU3232,
RRU3233, and RRU3235.

4-antenna cross 4-antenna linear 4-antenna circular 8-antenna cross
polarization mapping polarization mapping polarization mapping polarization mapping


Beamforming Cell Configuration

 Add an LBBP by running the ADD BRD command with Mode set to TDD_ENHANCE.

 After adding the cell, run the following commands to turn on the beamforming
measurement switch and algorithm switch:
 MOD MEASURESWITCH: UlintfMeasSwitch=SW_BfNValidMeas-
1&SW_BfNRankMeas-1&SW_BfSrsMeas-1;
 MOD CELLALGOSWITCH: LocalCellId=0, BfAlgoSwitch=BfSwitch-1;


S pe cifica tion of Be a mforming

Configuration Type MIMO LBBPc RRU3232
3 × 10MHz 4T4R Beamforming 1 LBBPc 3

Configuration Type MIMO LBBP RRU3232

Configuration Type MIMO LBBP RRU3233
3 × 20MHz 8T8R 3 LBBPc 3(each RRU need two
Beamforming fibers ）


KPI of Beamforming
Leading 4x2 Beamforming Enhanced the Capacity

Always Leading in Beamforming Test Result in Japan SBM Network

3GPP R8 3GPP R9 3GPP R10
single- dual- Multi-User
stream stream Beamformin
beamformi beamformi g
ng ng
1st to support
1st to launch
Dual-stream Beamforming
Single-stream
Beamforming
+10%
+15%

+15% Hisilcon Balong710 Chipset
is the first to support
dual-stream beamforming
Hisilcon Balong700 Chipset
is the first to
support single-stream
beamforming 　 >2Mbps >4Mbps >6Mbps
TM7 91.50% 73.40% 60.10%
2011H1 2011H2 2012H1 TM2 82.80% 61.90% 56.10%


KPI of Beamforming
Relevant features
Single-stream beamforming must be enabled before dual-stream beamforming.

Influence on the KPI
Single-stream or dual-stream beamforming has the following influence on the KPI:
Cell average throughput
If the single-stream and dual-stream beamforming is enabled, the signal energy received by
the UE is increased, the MCS is increased at the same UE position, beamforming achieves
higher cell average throughput than transmit diversity. In comparison with no beamforming,
single-stream beamforming increases the cell average throughput by 15% to 25%. In
comparison with single-stream beamforming, adaptive single-stream and dual-stream
beamforming increases the cell average throughput by more than 10%.

 Beamforming compared with 2R diversity (UL)
• ~ 30% gain in cell average throughput
• ~ 50% gain in cell edge user throughput
 Beamforming compared with 2x2 MIMO (DL)
• ~ 15% gain in cell average throughput
23%~90% increasing in edge user throughput
• ~ 40% gain in cell edge user throughput


Adaptive MIMO and Beamforming
 With adaptive beamforming and MIMO, the UE always uses TM of high spectral efficiency under the
same channel condition. In comparison with non-adaptive MIMO or beamforming, adaptive MIMO and
beamforming significantly increases average cell throughput.
 If beamforming is used, due to the overhead of UE-specific reference signal, the number of resource
blocks is reduced. Therefore, in case of good channel quality, beamforming throughput is slightly lower
than MIMO throughput. At high UE mobility (higher than 120 km/h), the eNodeB cannot track the
channel change accurately according to the sounding reference signal. In this situation, beamforming
is not suitable.
Adaptive beamforming and MIMO (low Adaptive beamforming and MIMO (high
mobility) mobility)


Adaptive MIMO and Beamforming
 The BFMIMOADAPTIVESWITCH parameter is used to select adaptive beamforming or MIMO. The eNodeB selects
beamforming or MIMO according to the value of the parameter, the UE movement speed, and SINR.
 If the value of the parameter is NO_ADAPTIVE, the eNodeB does not support adaptive Beamforming and MIMO.
 If the value of the parameter is TxD_BF_ADAPTIVE, the eNodeB supports adaptive TM2 (transmit diversity) and
beamforming. There are two scenarios: low UE mobility and high UE mobility. Low UE mobility: For UEs that do not
support R9, single-stream beamforming (TM7) is used; for UEs that support R9, single-stream beamforming (TM7 or
TM8) is used at low SINR and dual-stream beamforming (TM8) is used at high SINR. High UE mobility: Transmit
diversity is used.
 If the value of the parameter is MIMO_BF_ADAPTIVE, the eNodeB supports adaptive transmit diversity, dual-stream
MIMO (TM3), and beamforming. There are two scenarios: low UE mobility and high UE mobility. Low UE mobility: For
UEs that do not support R9, single-stream beamforming (TM7) is used at low SINR and dual-stream MIMO (TM3) is
used at high SINR; for UEs that support R9, single-stream beamforming is used at low SINR and dual-stream
beamforming (TM8) is used at high SINR. High UE mobility: Transmit diversity is used at low SINR and dual-stream
MIMO (TM3) is used at high SINR.


Comparison Between Beamforming and Other
Techniques

 Though a space diversity system or intelligent antenna system
has multiple transmit or receive antennas, they can transmit only
single-stream data. A MIMO system can transmit single stream or
multiple streams depending on the channel quality.
 MIMO requires that the number of receive antennas is not less
than the number of transmit antennas. Space diversity and
intelligent antennas do not have this requirement.


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