07 ct82357en01gla1 mimo for lte
TRANSCRIPT
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MIMO for LTE
LTE Air Interface Course
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Nokia Solutions and Networks Academy
Legal notice
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At the end of this module, you will be able to:
• List the main types of MIMO and their characterist ics• Describe the princip les of antenna mapping and precoding
• Explain the transmit d iversity and the spatial multip lexing
• Describe application options of MIMO in relation to radio planning and
performance issues
• Explain Advanced MIMO techniques
Module Objectives
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MIMO for LTE
Transmission Modes in 3GPP
Antenna Mapping and Precoding
Transmission Diversity
Spatial Multiplexing
Examples
Advanced MIMO techniques
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Multiple-Input Mult iple-Output MIMO Principle
Tm
T2
T1
Rn
R2
R1
•••
•••
Input
M x NMIMO
system
Output
• MIMO: Multiple-Input Multiple Output
• M transmit antennas, N receive antennas form MxN MIMO system
• Huge data stream (input) distributed toward m spatial distributed
antennas; m parallel bit streams (Input 1..m)
• Spatial Multiplexing generate parallel “virtual data pipes”
• Using Multipath effects instead of mitigating them
Signal from jth Tx antenna
S j
MIMO
Processor
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Multiple Antenna Systems
SIMO MISO MIMO
Improved
Transmission
Reliability
Greater
Coverage or
Range
Reduced UE
Power
Consumption
Increased
Transmission
Throughput
Multiple Antenna Systems
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Multiple Antenna Systems, cont.
LTE Physical Layer services assume multiple port antenna systems are used.Multiple port antenna
systems are implemented for the following reasons:
• Improved transmission reliability
• Greater coverage or range
• Reduced UE power consumption
• Increased transmission throughput
Multiple port antenna systems include the following:• Single Input Multiple Output (SIMO)
• Multiple Input Single Output (MISO)
• Multiple Input Multiple Output (MIMO)
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Single Input Multiple Output (SIMO)
• Switched Diversity
• Equal Gain Combining
• Maximum Ratio Combining
Rx Tx
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Single Input Multiple Output (SIMO), cont.
In a SIMO configuration the transmitter (usually the UE) has one transmitter and the receiver (the
eNodeB) has two physically separated antenna ports. The receiver picks up multiple versions of
the same signal but separated spatially. SIMO receivers use the following techniques to compute
the best received signal.
Switched Diversity
In Switched Diversity, the input with the best signal is chosen as the best source. The “best”
signal may be based on Signal-to-Noise Ratio (SNR) or Bit Error Rate (BER). Switched
diversity is the most simple and inexpensive SIMO technique.
Equal Gain Combining
Equal Gain Combining is a summation of all available received signals.
Maximum Ratio Combining
In Maximum Ratio Combining (MRC), each received signal has compensation applied to it
before being combined to produce a composite single signal. This technique is particularly
effective where the signal undergoes deep fading. Because fading probably occurs at different
frequencies on each antenna port, the reliability of the radio link is increased.
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Multiple Input Single Output (MISO)
• Space-Time Transmit Diversity
Tx Rx
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Multiple Input Single Output (MISO), cont.
A MISO (eNodeB) transmitter has two or more physically separated antenna ports, while the
MISO (UE) receiver has one antenna. Each Tx port transmits the same information bits. In
addition to data signals, reference signals are also transmitted via both antenna ports. The
normal reference signal pattern is sent via the first antenna port and the diversity reference
signal pattern via the second antenna port.
In Space-Time Transmit Diversity (STTD) the same data is transmitted simultaneously over
both Tx ports. On each port, the channel-coded data is processed in blocks of four bits, thenthe bits are time reversed and complex conjugated. The physical separation of the antenna
ports provides the space diversity, and the time difference derived from the bit-reversing
process provides the time diversity. These features together make the decoding process in
the receiver more reliable.
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Multiple Input Multiple Output (MIMO)
• Improved Transmission Reliability
• Increased Coverage or Range
• Reduced UE Power Consumpt ion
RxTx
MIMO systems contain multiple antenna ports at both the transmitter and receiver. The MIMO transmitter
transmits signals using time, frequency, and space diversity. The MIMO receiver recovers the data
across multiple receiving antenna ports.
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MIMO Techniques
• Space-Time Coding (STC) – 1 Data Stream
• Spatial Multiplexing – 2 Data Streams
RxTx
Data Stream 2
Data Stream 1
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MIMO Techniques, cont.
Space-Time Coding (STC) provides diversity gain to combat the effects of unwanted
Multipath propagation. Similar to STTD, time delayed and coded versions of the same signal
are sent from the same transmitter antenna. The codes that are used are mainly: trellis and
block (less complex) codes.
This improves the SNR for cell edge performance.
Spatial Multiplexing (SM)
With Spatial Multiplexing, unique (different) data streams are transmitted over different
antenna ports.
Spatial multiplexing can double (2x2 MIMO) or quadruple (4x4 MIMO) capacity and
throughput. This technique gives higher capacity when RF conditions are favorable and
users are closer to the eNodeB. The graphic shows spatial multiplexing with a 2x2 MIMO
configuration. The receiver can identify the transmitting antenna port for each received
signal
Si l U MIMO
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UE
Single User MIMO
• Improved Performance (STC), or
• Improved Throughput (SM) for Single UE
eNodeB
Data Stream 2
Data Stream 1
MIMO supports single user MIMO and multi-user MIMO. Single User MIMO improves the
performance for a UE (via space time coding), or increases the throughput for a UE (using
spatial multiplexing).
M lti U MIMO
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UE
UE
Multi-User MIMO
• Improved Number of UEs
• No Increase in System Bandwidth
eNodeB
In multi-user MIMO, the data for different users is multiplexed onto a single time-frequency resource, so
the capacity of the cell can increase in terms of users without increasing the system bandwidth.
Switching between SU-MIMO and MU-MIMO is supported on a per UE basis. The use of codes and
reference signals not only allows the receiver to differentiate between antenna streams and users, but also
allows accurate channel estimation
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3GPP MODE 1
• Single antenna port; port 0
• 1 TX antenna transmitt ing alwayson port 0
3GPP Mode 4
• Closed Loop spatial mult iplexing
• Multiple antennas transmitting
different signals• Feedback from the UE used
• Improves user data rate
3GPP MODE 2
• Transmit diversity
• Multiple antennas transmit samesignal
• Improves SINR
3GPP Mode 3• Open loop spatial mult iplexing
• Multiple antennas transmittingdifferent signals
• No feedback f rom the UE used
• Improves user data rate
Transmission Modes in 3GPP (1/2)
G ( / )
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3GPP MODE 5
• Multi user MIMO
• Multiple antennas transmittingto different UEs in the cell
• Increase sector capacity
3GPP Mode 6• Closed-loop Rank=1 precoding
• Beamforming
• UE signals back the suitableprecoding for the beamforming
operation
3GPP Mode 7
• Single Antenna port; port 5
• Beamforming• UE specific reference signals are
generated for feedback
Transmission Modes in 3GPP (2/2)
MIMO f LTE
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MIMO for LTE
Transmission Modes in 3GPP
Antenna Mapping and Precoding
Transmission Diversity
Spatial Multiplexing
Examples
Advanced MIMO techniques
LTE DL B b d Si l G ti
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scrambling
scrambling
Modulation
mapper
Modulation
mapper
Layer
mapperPrecoding
Resource
element
mapper
Resource
element
mapper
OFDM signal
generation
OFDM signal
generation
Code words layers Antenna
ports
Split into several
streams if needed
Weighting data streams
for transmission
Source: 3GPP TS 36.211 V8.6.0
LTE DL: Baseband Signal Generation
A t M i
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Layer
mapperPrecoding
Maximum oftwo
CODEWORDS
Up to NL LAYERS
N A ANTENNA
PORTS
RANK
1 CODEWORD = 1 Transport Blockwhich is coded and modulated (input)
LAYERS = different s treams used by spatialmultiplexing
Mapping of symbols onto antenna ports
ANTENNA PORT = defined by spec if icreference signals
RANK = number of layers transmitted
Antenna Mapping
Antenna mapping
MIMO f LTE
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MIMO for LTE
Transmission Modes in 3GPP
Antenna Mapping and Precoding
Transmission Diversity
Spatial Multiplexing
Examples
Advanced MIMO techniques
Transmission Modes in 3GPP
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3GPP MODE 1
• Single antenna port; port 0
• 1 TX antenna transmitting alwayson port 0
3GPP Mode 4
• Closed Loop spatial multiplexing
• Multiple antennas transmittingdifferent signals
• Feedback from the UE used• Improves user data rate
3GPP MODE 2
• Transmit diversity
• Multiple antennas transmit samesignal
• Improves SINR
3GPP Mode 3
• Open loop spatial mult iplexing• Multiple antennas transmitting
different signals
• No feedback from the UE used
• Improves user data rate
Transmission Modes in 3GPP
eNodeB Same data
stream
Diversity 2x2 MIMO
MIMO for LTE
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MIMO for LTE
Transmission Modes in 3GPP
Antenna Mapping and Precoding
Transmission Diversity
Spatial Multiplexing
Examples
Advanced MIMO techniques
Transmission Modes in 3GPP
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3GPP MODE 1
• Single antenna port; port 0
• 1 TX antenna transmitting alwayson port 0
3GPP Mode 4
• Closed Loop spatial mult iplexing
• Multiple antennas transmittingdifferent signals
•Feedback from the UE used
• Improves user data rate
3GPP MODE 2
• Transmit diversity• Multiple antennas transmit same
signal
• Improves SINR
3GPP Mode 3
• Open loop spatial mult iplexing
• Multiple antennas transmittingdifferent signals
• No feedback from the UE used
• Improves user data rate
Transmission Modes in 3GPP
eNodeB
Data stream 1
Data stream 2
Spatial multiplexing 2x2 MIMO
Spatial Multiplexing
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Spatial Multiplexing
Layermapper
Pre-coding
RE mapping
OFDM signal
Layer
mapper
Pre-
coding
Symbols after scrambling and
modulation, 2 code words
Two data stream
are supported
Spatial multip lex rank 2
Transmission on a single antenna port
Closed Loop MIMO
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Closed Loop MIMO
eNodeB
Pilot Channel on All Antenna Ports1
Evaluate
Codebook
Options2
Preferred Codebook Index3
Adjusted MIMO Transmission4
UE
Closed Loop MIMO cont
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Closed Loop MIMO, cont.
MIMO supports both open loop and closed loop control. Open loop MIMO transceivers adjust their
transmission based on received (reference signal) measurements. This assumes no rapid feedback
technique is available from the UE receiver back to the eNodeB transmitter. Unfortunately, in open loop
operation, the transmitter receives no feedback regarding antenna port operation or signal strength in
the forward direction.
Closed loop MIMO suppor ts a feedback loop describing eNodeB transmitter operation and UE
recommendations. Both the eNodeB and UE contain a codebook which describes possible RF
parameters, for example, the phase shift between antenna ports. In closed loop MIMO, the UE
describes eNodeB transmitter operation by returning an index into the shared codebook.
Closed loop operation uses the following steps.
1. The eNodeB transmits a DL pilot channel as a reference signal on all antenna ports.
2. The UE evaluates various codebook options that specify the RF parameters.
3. The UE transmits its recommendations in the form of a codebook index to the eNodeB.
4. The eNodeB adjusts its DL transmission to the UE based on the recommended parameters.
Differences between Mode 3 and 4
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3GPP Mode 4
• Closed Loop spatial mult iplexing
• Multiple antennas transmittingdifferent signals
• Feedback from the UE used
• Improves user data rate
3GPP Mode 3
• Open loop spatial mult iplexing
• Multip le antennas transmittingdifferent signals
• No feedback from the UE used
• Improves user data rate
In case of open loop spatial multiplex two cases have to be distinguished. If the transmitted rank indication (TRI) = 1 the
transmission mode corresponds to transmit diversity.
If TRI >1 large delay CDD is used. The number of layers is 2, 3 or 4.
In case of closed loop spatial multiplexing feedback from the UE it is used.
The UE feedbacks values of the RI = Rank Indicator and PMI = Precoding Matrix Indicator.
In case of 2 antenna ports the codebook consists of 2 matrices, in case of 4 antenna ports there are 16 entries. A restriction
may be signaled so that only a subset thereof can be used.
MIMO for LTE
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MIMO for LTE
Transmission Modes in 3GPP
Antenna Mapping and Precoding
Transmission DiversitySpatial Multiplexing
Examples
Advanced MIMO techniques
Examples of MIMO Usage
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Examples of MIMO Usage
Spatial
multiplexing
Transmission
diversity
Typically, close to the eNodeB Spatial multiplexing could be used to improve the throughput
At the cell edge Transmission diversity could be used to improve the coverage
MIMO type Gain downlink
Transmission
diversity 2x2 MIMO
+3…5dB downlink
link budget due to
SINR improvement
Spatial multiplexing
2X2 MIMO
+100% peak data
rate
Spatial multiplexing
4X4 MIMO
+300% peak data
rate
Single Antenna Port and DL Reference Signals
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Reference
Signal
DL RS, Normal TCP
Resource
Element
DL RS, Extended TCP
S=0 S=6
f =1
f =2
f =3
f =12
f =11
S=0 S=5
2 Port DL Reference Signals, Normal TCP
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Port 0
Reference
Signal
DL RS, Normal TCP
R0
R0
R0
R0
Port 0
Port 1
R1
R1
R1
R1
Not used
on this port
Port 1
Reference
Signal
Not used
on this port
eNodeB
2 Port DL Reference Signals, Extended TCP
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Port 0
Reference
Signal
DL RS, Extended TCP
R0
R0
R0
R0
Port 0
Port 1
Not used
on this port
Port 1
Reference
Signal
Not used
on this port
eNodeB
R1
R1
R1
R1
Antenna Port Layering
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eNodeB
0
1
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4 Port RS, Normal TCP – Continued
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R3
R3
R3
R3
R2
R2
R2
R2
eNodeB
Port 2
Port 3
Even Slot Odd Slot
Port 3
Reference
Signal
Not used
on this
port
Port 2
Reference
Signal
Antenna Port Layering
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eNodeB
0
1
2
3
MIMO for LTE
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Transmission Modes in 3GPP
Antenna Mapping and Precoding
Transmission DiversitySpatial Multiplexing
Examples
Advanced MIMO techniques
Enhanced Mult i-antenna Techniques in Downlink
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Extension up to 8-stream transmission
• Rel. 8 LTE supports up to 4-stream transmiss ion, LTE-Advanced suppor ts up to 8-stream transmiss ion
Satis fy the requirement for peak spectrum efficiency, i.e., 30 bps/Hz
Specify addit ional reference signals (RS)
• Two RSs are specified in addition to Rel. 8 common RS (CRS)
- Channel state information RS (CSI-RS)
- UE-specif ic demodulation RS (DM-RS)
UE-specific DM-RS, which is precoded, makes it possible to apply non-codebook-based
precoding
UE-specific DM-RS will enable application of enhanced multi-user beamforming such as zeroforcing (ZF) for, e.g., 4-by-2 MIMO
Max. 8 streams
Enhanced
MU-MIMO
Higher-order MIMO up
to 8 streams
CSI feedback
Enhanced Mult i-antenna Techniques in Downlink, cont.
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Release 10 has enhanced the reference signal design with user specific reference symbols
for signal demodulation and common reference symbols for feedback purposes in downlink
and more orthogonal reference signal structure in uplink. The enhanced design enables
better performance when the number of antenna branches is high.
Downlink MIMO has already been included in LTE Release 8. The LTE Release 8 codebook
and reference symbol design was found to be quite optimum for two and four transmit
antennas (2x2, 2x4 and 4x4 antenna configurations), but the channel state information
feedback from UE to eNB could have been more accurate. This limitation is overcome by
the new reference symbol design of Release 10, which is also more effective when the
number of transmit antennas is higher. Based on the studies and numerous contributions in
3GPP, it can be safely concluded that the higher the number of antennas, the higher is the
gain that Release 10 MIMO provides in downlink. With two eNB and two UE antennas,
Release 10 downlink MIMO provides no improvements over release 8 in SU-MIMO mode
but small performance improvements have been gained in MU-MIMO mode. In most cases
it is best to operate two TX antenna eNBs in Release 8 SU-MIMO mode. When eNB has
four transmit antennas, Release 10 downlink MIMO gain is more than 20% over Release 8
and with eight transmit antennas a bit higher. Reference symbol overhead effects on system
performance are significant with four and eight transmit antennas. Therefore the selection of
MIMO operating modes and system parameters for both Release 8 and 10 UE is a critical
network optimization task.
LTE-A DL MIMO
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Up to 8-stream transmission (8x2 MIMO)
single user (SU)-MIMO up to 4-stream transmission
Additional reference signals (RS):
• Channel state information RS (CSI-RS)
• UE-specific demodulation RS (DM-RS)
4 antenna por ts
LTE-A DL MIMO, cont.
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An important point worth remembering is that the network should also support Release 8and 9 UE which does not benefit from the Release 10 enhancements. The capacity gainfrom Release 10 downlink MIMO enhancements could even be negative since newreference symbols create overhead for all UE. However, these overheads can be decreased
by decreasing the Release 8 and 9 specific reference symbols, but this would prevent non-LTE-A UE to operate in MIMO mode and thus lower their data rates. Additionally, therewould be negative effects on common control channel performance. Consequently, thetiming of the introduction of the new features and the configuration of the system parametersare essential for an optimum performance of the LTE network.
CSI - For downlink channel sounding / Sparse, low overhead (configurable)
CSI = PMI(precoding matrix indicator) + RI(rank indicator) + CQI (channel quality indicator)
DM - UE-specific DM-RS, which is precoded, makes it possible to apply non-codebook-based precoding (precoding based on CSI feedback and/or UL sounding)- UE-specific DM-RS will enable application of enhanced multi-user beamformingsuch as zero forcing (ZF) for,e.g., 4-by-2 MIMO - DM RS pattern for higher numbers of layers is extended for 2-layerformat for transmission mode 8 in Rel-9 //CDM between RS of two layers// E.g. for 4
antenna ports:
Enhanced Multi-antenna Techniques in Uplink
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Introduction of s ingle user (SU)-MIMO up to 4-stream transmission
• Whereas Rel. 8 LTE does not suppor t SU-MIMO, LTE-Advanced supports up to 4-stream transmiss ion
Satis fy the requirement for peak spectrum efficiency, i.e., 15 bps/Hz
Signal detection scheme with affinity to DFT-Spread OFDM for SU-MIMO
• Turbo serial interference canceller (SIC) is assumed to be used for eNB receivers to achieve higher
throughput performance for DFT-Spread OFDM
Improve user throughput, while maintaining single-carrier based signal transmission
Max. 4 streams
SU-MIMO up to 4 streams
Uplink MIMO provides significantly higher peak rates and improved spectrum efficiency in uplink direction.SU-MIMO provides mainly increased data rates in lightly loaded networks for high-end multi-transmitter UE,
whereas MU-MIMO can offer significant improvement of spectrum efficiency even with single transmitter
UE. This can boost network capacity at low costs The LTE-A system can operate in both SU and MU-MIMO
modes at the same time using dynamic user specific MIMO transmission configuration.
LTE-A UL MIMO
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UL transmit diversity for PUCCH
Single user (SU)-MIMO up to 4-stream transmission (2x4)MU – MIMO supported
LTE-A DM-RS
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LTE-A MBSFN subframe
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LTE-A can configure MBSFN subframes to schedule non MBSFN data (PDSCH).
Physical Multicast Channel (PMCH) is used instead of PDSCH.
Special RS pattern with higher density in frequency domain supports longer “delay spread”
from multi-cell transmission.
Multimedia Broadcast Single Frequency Network(MBSFN) mode of operation is supported by E-UTRAN to
enable efficient multi-cell transmission of E-MBMS services