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CT82357EN01GLA1 ©2014 Nokia Solut ions and Networks. All righ ts reserved.

MIMO for LTE

LTE Air Interface Course



2 CT82357EN01GLA1 ©2014 Nokia Soluti ons and Networks . All righ ts reserved.

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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




MIMO for LTE

Transmission Modes in 3GPP

Antenna Mapping and Precoding

Transmission Diversity

Spatial Multiplexing

Examples

Advanced MIMO techniques




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




Multiple Antenna Systems

SIMO MISO MIMO

Improved

Transmission

Reliability

Greater

Coverage or

Range

Reduced UE

Power

Consumption

Increased

Transmission

Throughput

Multiple Antenna Systems




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)




Single Input Multiple Output (SIMO)

• Switched Diversity

• Equal Gain Combining

• Maximum Ratio Combining

Rx Tx




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.




Multiple Input Single Output (MISO)

• Space-Time Transmit Diversity

Tx Rx




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.




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.




MIMO Techniques

• Space-Time Coding (STC) – 1 Data Stream

• Spatial Multiplexing – 2 Data Streams

RxTx

Data Stream 2

Data Stream 1




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




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




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




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


Transmission Modes in 3GPP (1/2)

G ( / )




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




MIMO for LTE





Examples


LTE DL B b d Si l G ti




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




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




MIMO for LTE





Examples






3GPP MODE 1


• 1 TX antenna transmitting alwayson port 0

3GPP Mode 4

• Closed Loop spatial multiplexing


• 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



eNodeB Same data

stream

Diversity 2x2 MIMO

MIMO for LTE




MIMO for LTE





Examples






3GPP MODE 1


• 1 TX antenna transmitting alwayson port 0

3GPP Mode 4



•Feedback from the UE used


3GPP MODE 2

• Transmit diversity• Multiple antennas transmit same

signal

• Improves SINR

3GPP Mode 3

• Open loop spatial mult iplexing





eNodeB

Data stream 1

Data stream 2

Spatial multiplexing 2x2 MIMO






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




Closed Loop MIMO

eNodeB

Pilot Channel on All Antenna Ports1

Evaluate

Codebook

Options2

Preferred Codebook Index3

Adjusted MIMO Transmission4

UE

Closed Loop MIMO cont




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




3GPP Mode 4



• Feedback from the UE used


3GPP Mode 3

• Open loop spatial mult iplexing

• Multip le antennas transmittingdifferent signals



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




MIMO for LTE



Transmission DiversitySpatial Multiplexing

Examples


Examples of MIMO Usage




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




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




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




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




eNodeB

0

1



4 Port RS, Normal TCP – Continued




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




eNodeB

0

1

2

3

MIMO for LTE






Transmission DiversitySpatial Multiplexing

Examples


Enhanced Mult i-antenna Techniques in Downlink




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.




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




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.




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




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




UL transmit diversity for PUCCH

Single user (SU)-MIMO up to 4-stream transmission (2x4)MU – MIMO supported

LTE-A DM-RS




LTE-A MBSFN subframe




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