1. LTE System Multiple Antenna Techniques
2017/4/17
LTE System Multiple Antenna Techniques
eRAN2.2 (MIMO and Beamforming)

2. Training Objectives After completing this course, you will be able to:
课程名称
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 : Physical Channels and Modulation
3GPP TS : Physical layer procedures
3GPP TS : User Equipment (UE) radio access capabilities
FPD: MIMO and Beamforming Feature Documentation
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3. Contents Background and Overview of the LTE MIMO Techniques
课程名称
Contents
Background and Overview of the LTE MIMO Techniques
Principles and Application of the MIMO Techniques
Principles and Application of Beamforming
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4. Background of Multi-Antenna Techniques
Fifty years ago, Shannon gave the maximum efficiency that a time and frequency communication system can achieve.
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.

5. 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
Diversity gain
Spatial multiplexing gain
Co-channel interference reduction
Improved system coverage
Improved system capacity
Increased peak rate
Increased spectral efficiency
Array gain: A power gain achieved by combining signals from different antennas based on the correlation between signals and the non-correlation between noises. Signal combining increases the signal to interference plus noise ratio (SINR) of the combined signal.
Diversity gain: The performance gain obtained by reducing the fading amplitude (the covariance of SNR). The fading amplitude is reduced by combining signals from different antennas on which the deep fading of the signals are unrelated.
Spatial multiplexing gain: A throughput gain achieved by adding spatial channels without increasing the total bandwidth and total TX power.
Interference rejection gain: A gain achieved by interference rejection combining or other multi-antenna interference rejection algorithms.
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6. Contents Background and Overview of the LTE MIMO Techniques
课程名称
Contents
Background and Overview of the LTE MIMO Techniques
Principles and Application of the MIMO Techniques
Principles and Application of Beamforming
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7. 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.
MIMO uses the idea of spatial coding.
Transmitter: N streams are sent to a channel simultaneously. Each TX signal input can use the same frequency, the same codeword, and be transmitted simultaneously. As long as the channel response of each transmit and receive antennas is independent of each other, MIMO can create multiple parallel spatial channels and use the characteristics of each spatial channel to identify the streams.
Receiver: Each antenna receives the code streams of each transmit antenna, uses the characteristics of the parallel spatial channels to combine and decode the received signals, and combine the streams.
Multiple streams are transmitted by independent parallel spatial channels to improve the overall data transmission rate.
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8. 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
Multi-antenna receive
Receive diversity
UL 2-Antenna Receive Diversity
UL 4-Antenna Receive Diversity
UL Interference Rejection Combining
UL Interference Rejection Combining
UL 8-Antenna Receive Diversity
MU-MIMO
UL 2x2 MU-MIMO
UL 2x4 MU-MIMO
Multi-antenna transmit
Open-loop transmit diversity
2x2 MIMO
4x2 MIMO
DL 4x4 MIMO
Closed-loop transmit diversity
Open-loop spatial multiplexing
Closed-loop spatial multiplexing
DL axb MIMO means that the eNodeB uses a antennas to transmit data and the UE uses b antennas to receive data.
UL axb MU-MIMO means that a UEs use the same resource block to transmit data and the eNodeB uses b antennas to receive data.
The MimoAdaptiveSwitch and FixedMimoMode parameters are used to set the MIMO mode to one of the following four modes: Open-loop and closed-loop adaptive, open-loop adaptive, closed-loop adaptive, and fixed mode.
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9. 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.
Principles of receive diversity:
Signal x transmitted by the UE arrives at antennas r1 to rm of the eNodeB over different channels. The eNodeB multiplies the received signals with weights wi* and combines the signals to obtain signal y. The combined signal y can be expressed as follows:
y=WH(Hx+N)
where
W=(w1* …… wm*)H is the weight vector of the antennas.
H=(h1 …… hm)H is the spatial channel matrix and hi is the channel coefficient. The amplitude and phase of a signal are changed after passing a channel. The signal is multiplied by the channel coefficient to obtain the signals that passes the channel.
N=(n1 …… nm)H is the noise vector of the antennas.
x is the transmit signal.
Due to the fading characteristics of a radio channel, the radio channel between a transmitter and a receiver experiences deep fading (10 dB to 20 dB) periodically, causing SINR fluctuation. However, deep fading in different antennas does not occur simultaneously, or the probability of simultaneous occurrence is low. When signals received by different antennas are combined, the probability of deep fading is greatly reduced, achieving diversity gain.
The white noise in different antennas is not correlated. The combined noise power is unchanged, but the signal energy is increased by many folds, achieving array gain. Array gain is usually proportional to the number of receive antennas. That is, the array gain of a 2-antenna receive system is 3dB and that of a 4-antenna receive system is 6dB. The key of receive diversity is in signal combination algorithms. There are two types of signal combination algorithms: maximum ratio combining and interference rejection combining.
Relevant concepts
Minimum mean square error (MMSE): MMSE between the estimated transmit signal and actual transmit signal.
White noise: Noise whose power spectrum density is evenly distributed in the frequency or space domain.
Colored interference: Interference whose power spectrum density is unevenly distributed in the frequency or space domain.
MMSE receiver: 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 combined signals.
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10. 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.
where
x0 is the signal,
ni(i=1,2,…m) is the interference,
hi(p) (p=0, 1) is the channel coefficient,
n’ and n’’ are white noise (additive white Gaussian noise),
w(p) is the receive weight,
rP+1 is the received signal,
y is the combined signal,
p is the antenna sequential No.
x0 and n pass their respective radio channels hi(p) and are summed in the antennas. Also summed is the additive white Gaussian noise n’ and n’’ brought by the intermediate frequency system. The received signal is rP+1. The MMSE receiver adjusts the w(p) of each antenna to combine the received signals in maximum ratio and in minimum mean square error.
Due to the constraint in the number of antennas, the MMSE receiver cannot simultaneously minimize the gain of the interference achieved by the side lobe while target the major lobe at the signal source. Rather, the receiver finds a tradeoff between minimizing interference gain and maximizing signal gain to maximize the SINR of the combined signal.
Assuming that the interference and noise are both white in the space, MRC receivers use MRC algorithm to achieve MMSE.
Assuming that there is colored interference, IRC receivers use IRC algorithm to achieve MMSE.
The interference rejection performance of IRC algorithm depends on the interference characteristics. Only separable spatial colored interference can be rejected by IRC algorithm. The performance of IRC algorithm depends on the accuracy of estimating the interference characteristics by the algorithm. In the following scenarios, IRC algorithm provides no advantage.
If the interference to the antenna channels is strongly correlated to the signals to the antenna channels, the interference and signals are inseparable. In this case, IRC performance is worse than MRC performance.
If the interference is white or weak, theoretically IRC algorithm is equivalent to MRC algorithm; their performance is the same. In practice, there is an error in estimating the interference characteristics. Without interference, IRC performance is slightly worse than MRC performance.
The eNodeB measures the spatial color of the interference to determine whether a user is under white interference or colored interference.
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11. 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.
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12. 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.
Principles of MU-MIMO
UL 2x2 MU-MIMO: In MU-MIMO mode, multiple users use the same resource block. In addition to the diversity gain and array gain achieved by uplink transmit diversity, MU-MIMO also achieves multiplexing gain, providing higher performance for the LTE system.
MU-MIMO gain depends on the SINR of the multiple users and the correlation between the user channels.
If the SINR of the two users is high and the user channel correlation is orthogonal, the interference between the two users can be eliminated satisfactorily. Virtual MIMO makes use of the good channel quality to provide additional system capacity.
If the user channel correlation is strong or the SINR is low, the interference between the two users cannot be eliminated. In this case, virtual MIMO causes deteriorated system throughput.
The key of MU-MIMO is in signal combination algorithm and user pairing algorithm.
Combination algorithm of MU-MIMO
UE1 and UE2 use the same resource block to send data x1 and x2. x1 and x2 arrive at receive antennas 1 to m after passing their respective channels. The MIMO decoder weights and combines the signals in the antennas to obtain the y1 and y2, which are the estimated x1 and x2. MU-MIMO combination algorithm is one that calculates the weight and performs multi-user detection for users that use the same resource block. The estimation of x1 and x2 is regarded as two independent receive diversity. x1 is an interference to x2. So is x2 to x1. Therefore, virtual MIMO achieves array gain and diversity the same as receive diversity does.
Like receive diversity, there are two MU-MIMO combination algorithms: MRC and IRC. The MRC algorithm assumes that the noise and interference in the environment are white and rejects them by adjusting the weight. The IRC algorithm assumes that there is a strong interference source in the environment and rejects it. In 2x2 MU-MIMO, due to constraint in the number of antennas, the IRC performance is not satisfactory. In this antenna configuration, eNodeBV1.5 in virtual MIMO mode supports MRC only.
User pairing for MU-MIMO
If MU-MIMO is enabled, the eNodeB scheduler flexibly schedules each user by the maximum pairing policy and selects the most suitable UEs to pair. For example the scheduler selects UEs whose channels are orthogonal to achieve maximum gain, improving system throughput while maintaining channel robustness. The eNodeB measures, filters, pairs, and schedules virtual MIMO users in each TTI. The procedure is as follows:
Measuring SINR: The eNodeB measures the average SINR of each user in the full bandwidth. Filtering SINR: The eNodeB selects those users whose SINR exceeds the threshold value as the candidate virtual MIMO users.
Usually, users with good channel quality (large SINR) provide satisfactory pairing.
Pairing: The eNodeB selects two candidate users to attempt pairing. If the pairing index (such as increased spectral efficiency and increased system capacity after pairing) exceeds the efficiency threshold, the pair succeeds and the eNodeB pairs these two users. If the pairing index is lower than the efficiency threshold, the pair fails. The purpose of virtual MIMO pairing is to increase the system capacity, or spectral efficiency. The spectral efficiency threshold stipulates the threshold that must be achieved by virtual MIMO user pairing. Scheduling: The eNodeB schedules two pairing users to transmit data in the same resource block.
Adaptive mode selection and switchover
If the UlSchSwitch (UlVmimoSwitch) is enabled, the eNodeB adaptively selects and switches between receive diversity and MU-MIMO depending on the user channel condition.
If the eNodeB is configured with 2 antennas, the system adaptively switches between UL 2-Antenna Receive Diversity and UL 2x2 MU-MIMO. The eNodeB measures the SINR and channel correlation of each user in each TTI. The eNodeB selects users with high SINR and channel orthogonality for pairing and switches to MU-MIMO. If the SINR or channel orthogonality of a user deteriorates, the system rolls back to the receive diversity mode.
If the value of the UlSchSwitch parameter is UlVmimoSwitch, the system adaptively switches between UL 2-Antenna Receive Diversity and UL 2x2 MU-MIMO depending on the channel quality.
If the value of the UlSchSwitch parameter is not UlVmimoSwitch, the system supports LBFD UL 2-Antenna Receive Diversity only.
In UL 2x2 MU-MIMO mode, the system throughput is increased. This mode is not suitable for high-speed mobility at 120 km/h or 350 km/h and frequency hoping.
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13. 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.
No.
Name
Applicable Scenario
Supported by Current eNodeB 1
Single antenna (port 0)
Single-antenna transmission.
Yes 2
Open-loop transmit diversity
Suitable for cell edge where the channel condition is complex and interference is large, or high-mobility or low SNR situations. 3
Open-loop spatial multiplexing
Suitable for high UE mobility and complex reflection environment. 4
Closed-loop spatial multiplexing
Suitable for good channel condition. Provides high data transmission rate.
Yes（FDD） 5
MU-MIMO
Suitable for two orthogonal UEs. Used to increase cell capacity. 6
Closed-loop transmit diversity
Suitable for cell edge, low mobility, and low SINR. 7
Single antenna (port5)
Suitable for cell edge to reject interference. 8
Adaptive single-stream and dual-stream beamforming
Suitable for cell edge, low mobility, and high SNR. 9
Adaptive single-stream, dual-stream, and 4- stream beamforming
A new mode in LTE-A. Supports a maximum of eight layers. Increases data transmission rate. Suitable for low mobility and high SNR. No
Used by FDD/TDD
LTE downlink transmission modes include the following: 1. TM1: Single-antenna transmission. 2. TM2: Transmit diversity. Suitable for cell edge where the channel condition is complex and interference is large. Sometimes, TM2 is used in high mobility situation. TM2 provides diversity gain. 3. TM3: Large delay diversity. Suitable for high UE mobility situation. 4. TM4: Closed-loop spatial multiplexing. Suitable for good channel condition. Provides high data transmission rate. 5. TM5: MU-MIMO. Increases cell capacity. 6. TM6: Rank 1 transmission. Suitable for cell edge. 7. TM7: Single-stream beamforming. Suitable for cell edge. Effectively rejects interference. 8. TM8: Dual-stream beamforming: Suitable for cell edge and other situations. 9. TM9: A new mode in LTE-A. Supports a maximum of eight layers. Increases data transmission rate.
Transmit diversity uses weak correlation of spatial channels and selectivity in time and frequency to combine copies of signals that experience different fading and to lower the probability of deep fading, achieving diversity gain and increasing transmission reliability.
Depending on whether the transmitter uses the channel information provided by the UE, transmit diversity is classified into open-loop transmit diversity (TM2) and closed-loop transmit diversity (TM6).
Used by TDD
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14. 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]
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15. 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.
LTE reference signal pattern: The figure in the first line indicates the cell-specific reference signal (CSRS) of a single antenna port; the figure in the second line indicates the CSRS of 2-antenna ports; the figure in the third line indicates the CSRS of 4-antenna ports. In R8, UE demodulation uses the CSRS, except TM7 port 5 that uses UE-Specific reference signal of independent pattern.
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16. Cell-specific Reference Signal (CRS)
Normal CP，downlink reference signal map relationship.
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17. 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.
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18. 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.
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19. 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
Relevant concepts:
Codeword
Data streams that are channel coded and rate controlled differently and separately are codewords.
CDD
CDD refers to cyclic delay diversity. The traditional delay diversity means transmission of the same signal of different delay versions in different antennas, therefore manually increasing the delay of the channel that the signal passes. CDD is designed for the OFDM system. Before a cyclic prefix (CP) is inserted, the same OFDM symbol is cyclic shifted by Dm samples (m=1, ……, M indicates the sequential antenna number), and then each antenna inserts its own CP to the corresponding cyclic shifted version.
where
x is the transmitted signal,
y is the received signal,
H is the spatial channel matrix,
Hij is the channel coefficient from the jth transmit antenna and the ith receive antenna.
y=Hx
y1=h11x1+h12x2+n1
y2=h21x1+h22x2+n2
The accuracy of the receiver in estimating the data transmitted by the transmitter has a negative correlation with the statistic correlation between vector (h11, h12) and vector (h21, h22).
To lower the receiver complexity and reduce the signal interference between antennas, the eNodeB performs layer mapping and precoding for the modulated data before sending the data to the antenna ports, and converts the cross spatial channels into equivalent independent parallel channels.
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20. 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.
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21. Configurations of MIMO
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22. Configuration of MU-MIMO
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23. Application of MIMO At persent, LTE TDD can support by RRU3232，RRU3235
Specification of eNodeB：
Configuration type
MIMO
LBBPc
RRU3232
RRU3231
3 × 10MHz
2 × 2 MIMO
1 LBBPc
2 (2T2R) 3
4 × 2 MIMO -
3 × 20MHz
3 LBBPc
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24. Contents Background and Overview of the LTE MIMO Techniques
课程名称
Contents
Background and Overview of the LTE MIMO Techniques
Principles and Application of the MIMO Techniques
Principles and Application of Beamforming
Page24
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P-24

25. 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.
Beamforming is an antenna array multi-antenna technique for small distance. Its principle is to use the new strong spatial correlation and the interference to generate strong directional radiation pattern, so that the major lobe adaptively points at the direction of arrival, therefore increasing the SINR, system capacity, or coverage radius.
The benefits of beamforming are as follows:
Increased SINR in the direction of incoming wave from the UE.
Increased system capacity and coverage. 25

26. 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.
Beamforming:
Beamforming is similar to closed-loop MIMO but does not require the UE to feed back information. The TDD system performs measurement accurately by using the uplink channels. Single-stream beamforming (TM7) or multi-stream beamforming (TM8/TM9) is supported.
If the UE supports single input, the system cannot use one antenna to estimate the channel of another antenna, leading to some loss.
Beamforming requires low UE mobility.
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27. 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.
Beamforming requires use of dedicated pilot channels. The reason is that multiple antenna units are required to achieve beamforming gain. At present, the LTE system supports a maximum of 4 common pilot channels and does not support antenna array in exceeds of four antennas.
The design of dedicated pilot channels for LTE users is compliant with forward compatibility with LTE-A demodulation reference signal.
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28. 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.
Dual-stream beamforming achieves large capacity gain but requires high UE SNR (near the eNodeB). The following is a case of China Mobile.
The LTE TDD system uses 8-antenan dual-stream beamforming. In comparison with the 2-antenna MIMO, the sector throughput is increased by 80% and the edge throughput is increased by 130%.
The cell coverage radius of the 8-antenna dual-stream beamforming is significantly increased compared with that of the 2-antenna MIMO to 300 m, by 1.5 to 2 times. 8-antenna dual-stream beamforming and TD-SCDMA can be co-site and co-coverage, lowering the LTE TDD network construction cost.
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29. 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 polarization mapping
4-antenna linear polarization mapping
4-antenna circular polarization mapping
8-antenna cross polarization mapping
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30. 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;
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31. Specification of Beamforming
Configuration Type
MIMO
LBBPc
RRU3232
3 × 10MHz
4T4R Beamforming
1 LBBPc 3
3 × 20MHz
3 LBBPc
Configuration Type
MIMO
LBBP
RRU3232
6 × 20MHz
4T4R Beamforming
6 LBBPc 6
Configuration Type
MIMO
LBBP
RRU3233
3 × 20MHz
8T8R Beamforming
3 LBBPc
3(each RRU need two fibers）
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32. KPI of Beamforming +10% +15% 3GPP R8 3GPP R9 3GPP R10
Leading 4x2 Beamforming Enhanced the Capacity
3GPP R8
2011H1
2011H2
1st to launch
Single-stream Beamforming
1st to support
Dual-stream Beamforming
+15%
+10%
3GPP R9
3GPP R10
single-stream beamforming
dual-stream beamforming
Multi-User Beamforming
Always Leading in Beamforming
2012H1
Hisilcon Balong700 Chipset
is the first to
support single-stream
beamforming
Hisilcon Balong710 Chipset
is the first to support
Test Result in Japan SBM Network
>2Mbps
>4Mbps
>6Mbps
TM7
91.50%
73.40%
60.10%
TM2
82.80%
61.90%
56.10%
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33. 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
~ 40% gain in cell edge user throughput
23%~90% increasing in edge user throughput
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34. 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 mobility)
Adaptive beamforming and MIMO (high mobility)
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35. 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.
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36. 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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