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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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.
DL a x b MIMO means that the eNodeB uses a antennas to transmit data and the UE uses b antennas to receive data. UL a x b 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.
Principles of receive diversity: Signal x transmitted by the UE arrives at antennas r 1 to r m of the eNodeB over different channels. The eNodeB multiplies the received signals with weights w i * and combines the signals to obtain signal y. The combined signal y can be expressed as follows: y=W H (Hx+N) where W=(w 1 * …… w m * ) H is the weight vector of the antennas. H=(h 1 …… h m ) H is the spatial channel matrix and h i 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=(n 1 …… n m ) 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: m aximum ratio combining and i nterference rejection combining . Relevant concepts Minimum m ean s quare e rror (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.
where x 0 is the signal, n i (i=1,2,…m) is the interference, h i (p) (p=0, 1) is the channel coefficient, n’ and n’’ are white noise (additive white Gaussian noise), w (p) is the receive weight, r P+1 is the received signal, y is the combined signal, p is the antenna sequential No. x 0 and n pass their respective radio channels h i (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 r P+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.
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-00202001 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.
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 (TM 2) and closed-loop transmit diversity (TM6 ).
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.
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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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.
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.
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.
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.
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 DocumentationHUAW TECHNOLOGIES CO., LTD. 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 HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page3
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 NHUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential
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 HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 5
Contents Background and Overview of the LTE MIMO Techniques Principles and Application of the MIMO Techniques Principles and Application of Beamforming HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page6
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. HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 7
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 diversityMulti-antenna UL Interference Rejection UL Interference Rejection Combiningreceive 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 MIMOMulti-antenna Open-loop spatial multiplexing 4x2 MIMOtransmit 4x2 MIMO Closed-loop spatial DL 4x4 MIMO multiplexing HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential 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. HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 9
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. HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 10
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. HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 11
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. HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 12
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. YesUsed 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 beamformingUsed 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 HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 13
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] HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 14
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.HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 15
Ce ll-s pe cific Re fe re nce S igna l (CRS ) Normal CP ， downlink reference signal map relationship. HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 16
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.HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 17
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. HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 18
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 HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 19
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. HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 20
Configura tions of MIMOHUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 21
Configura tion of MU-MIMO HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 22
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 -HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 23
Contents Background and Overview of the LTE MIMO Techniques Principles and Application of the MIMO Techniques Principles and Application of Beamforming HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page24
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. HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential
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. HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 26
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. HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 27
Classification of Beamforming (Dual-Stream)Dual-stream beamforming means transmission of two data streams in the same OFDMresource block, leading to spatial multiplexing. It is suitable for situations of good channelquality.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 isweighted by four weights: w11 to w41; S2 is weighted by another four weights w12 to w42. Theweighted streams are summed and sent to the four antenna ports for transmission. HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 28
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 crosspolarization mapping polarization mapping polarization mapping polarization mapping HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 29
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; HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 30
S pe cifica tion of Be a mforming Configuration Type MIMO LBBPc RRU3232 3 × 10MHz 4T4R Beamforming 1 LBBPc 3 3 × 20MHz 4T4R Beamforming 3 LBBPc 3 Configuration Type MIMO LBBP RRU3232 6 × 20MHz 4T4R Beamforming 6 LBBPc 6 Configuration Type MIMO LBBP RRU3233 3 × 20MHz 8T8R 3 LBBPc 3(each RRU need two Beamforming fibers ）HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 31
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 beamformingHisilcon 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% HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 32
KPI of BeamformingRelevant featuresSingle-stream beamforming must be enabled before dual-stream beamforming.Influence on the KPISingle-stream or dual-stream beamforming has the following influence on the KPI:Cell average throughputIf the single-stream and dual-stream beamforming is enabled, the signal energy received bythe UE is increased, the MCS is increased at the same UE position, beamforming achieveshigher cell average throughput than transmit diversity. In comparison with no beamforming,single-stream beamforming increases the cell average throughput by 15% to 25%. Incomparison with single-stream beamforming, adaptive single-stream and dual-streambeamforming 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 HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 33
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) HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 34
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. HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 35
Comparison Between Beamforming and OtherTechniques 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. HUAW TECHNOLOGIES CO., LTD. EI Huawei Confidential Page 36