KRnet2011

Uplink/Downlink MIMO

June 28, 2011

Juho Lee

Samsung Electronics juho95.lee@samsung.com

KRnet2011

Table of Contents

• LTE-Advanced Downlink MIMO

• LTE-Advanced Uplink MIMO

• Performance Evaluation

• Real Life Deployment Issues

1

KRnet2011

DL MIMO Overall Procedure

2

•PDCCH = Downlink Control

•PDSCH = Downlink Data

•CQI = Channel Quality Indicator

•PMI = Precoding Matrix Indicator

•RI = Rank Iindicator

•ACK/NACK = feedback for Hybrid ARQ operation

KRnet2011

DL Transmission Chain

3

ScramblingModulation

mapperLayer

mapper

ScramblingModulation

mapper

layerscodewords

Precoding

Resource element mapper

OFDM signal

generation

Resource element mapper

OFDM signal

generation

antenna ports

layers

Identity in LTE-Adv MIMO

KRnet2011

DL MIMO Overview

4

LTE

(up to 4 TX antennas)

LTE-Advanced

(up to 8 TX antennas)

Spatial

Multiplexing

•Closed-loop codebook based precoding

•Open-loop

Closed-loop non-codebook based

precoding

Transmit

diversity

•SFBC for 2 TX antennas

•SFBC+FSTD for 4Tx antennas

No additional transmit diversity

defined for 8TX antennas

(8 physical antennas virtualized to 2 or

4 logical antennas)

PMI feedback Yes No

Closed-loop Open-loop

Set of precodings at

transmitter

Specified Not specified

Codebook based Non-codebook based

KRnet2011

DL MIMO with 8 TX Antennas

• MIMO with 8 Tx antennas

– Peak spectral efficiency up to 30bps/Hz, i.e., 3Gbps @ 100MHz

– Reference signal (=pilot) structure to reduce overhead

• Common CSI-RS for downlink channel quality measurement

– CSI-RS: Channel Status Indicator Reference Signal

• Dedicated demodulation RS

• Note: in LTE, only common RS is used

– Feedback mechanism design

• CQI (Channel Quality Indicator)

• PMI (Precoding Matrix Indicator)

• RI (Rank Indicator)

5

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MU-MIMO with 8 TX Antennas

6

0 2 4 6 8 10 120.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

5%

User

Thro

ughput

(bps/H

z)

# of UEs

Baseline-1x2

MUBF2x2

MUBF4x2

MUBF8x2

0 2 4 6 8 10 121

2

3

4

5

6

7

8

9

Syste

m T

hro

ughput

(bps/H

z)

# of UEs

Baseline-1x2

MUBF2x2

MUBF4x2

MUBF8x2

Cell edge user throughput Sector throughput

• Up to 8 Tx antennas and mostly 2 Rx antennas at UE

• Favorable to MU-MIMO

• 8Tx MU-MIMO provides

• Both throughput and coverage gain

• Cell edge performance gain of up to 150% against 4x2 case with 12 UEs/cell

• SU-/MU-MIMO dynamic switching supported

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Layer-to-Codeword Mapping

7

Number of

layers

Number of

codewords

Number of layers per

codeword Note

1 1 1

2 1 2 Only for retransmitted codeword

2 (1, 1)

3 1 3 Only for retransmitted codeword

2 (1, 2)

4 1 4 Only for retransmitted codeword

2 (2,2)

5 2 (2,3) Added in LTE-Advanced

6 2 (3,3) Added in LTE-Advanced

7 2 (3,4) Added in LTE-Advanced

8 2 (4,4) Added in LTE-Advanced

•Maximum two codewords are transmitted to reduce the UL ACK/NACK feedback overhead

•Multiple layers may be mapped to a single codeword depending on the rank value

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Dedicated Demodulation RS

8

0l 6l 0l 6l 0l 6l 0l 6l

even-numbered slots odd-numbered slots

Antenna port 7, 8, 11, 13

even-numbered slots odd-numbered slots

Antenna port 9, 10, 12, 14

Time-frequency mapping

Demodulation RS of antenna ports 7, 8, …, 6+L for L layers transmission

Orthogonal covers for S1

Antenna port Orthogonal cover

7 +1 +1 +1 +1

8 +1 -1 +1 -1

11 +1 +1 -1 -1

13 +1 -1 -1 +1

Antenna port Orthogonal cover

9 +1 +1 +1 +1

10 +1 -1 +1 -1

12 -1 -1 +1 +1

14 -1 +1 +1 -1

Orthogonal covers for S2

Set S1 Set S2

SF2

SF4

SF2

SF4

KRnet2011

Common CSI-RS

9

even-numbered slots odd-numbered slots even-numbered slots odd-numbered slots even-numbered slots odd-numbered slots even-numbered slots odd-numbered slots

0l 6l 0l 6l 0l 6l 0l 6l 0l 6l 0l 6l 0l 6l 0l 6l

0l 6l 0l 6l 0l 6l 0l 6l 0l 6l 0l 6l 0l 6l 0l 6l

15R 15R 16R 16R

17R 17R 18R 18R

19R 19R20R 20R

21R 21R22R 22R

Orthogonal patterns Orthogonal patterns

Orthogonal patterns Orthogonal patterns

•CSI-RS occupies much less overhead than CRS

- CRS transmitted each subframe (i.e., 1ms)

- CSI-RS transmission period: 5ms, 10ms, 20ms, 40ms, 80ms

CSI-RS patterns

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Codebook for UE feedback

10

W1W2

precoder W = W1W2

...

...

8 Tx antenna configuration

W1 =X(k) 0

0 X(k) X(k)= diag{ej0,ejfk,ej2fk,ej3fk}´DFT4(k)

Linear Phase Shift DFT

•For 2TX and 4TX, Rel-8 codebook is reused

•For 8TX, double codebook structure is introduced

KRnet2011

UE feedback (1/3)

• Feedback framework

– Double codebook structure

• exploits the long-term and wideband properties of the MIMO channel

• decreases the feedback overhead

• improves the feedback accuracy (decreases the quantization error)

11

codebook 1:

long-term and/or widebandcodebook 2:

short-term and/or subband

W1

W2

W = W1W2 W = W2W1

TI, Nokia Samsung

W =f(W1,W2)

channel H

HW1

W2

channel H

W2

W1

codebook 1:

long-term and/or widebandcodebook 2:

short-term and/or subband

W1

W2

W = W1W2 W = W2W1

TI, Nokia Samsung

W =f(W1,W2)

channel H

HW1

W2

channel H

W2

W1

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UE feedback (2/3)

• PUCCH RI/W1/W2/CQI reporting modes – Wideband/subband report (UE selection)

– Wideband reports only

…

…

…

time

time

time

…

Wideband report (PTI=0) subband report (PTI=1)

12

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UE feedback (3/3)

• PUSCH RI/W1/W2/CQI reporting modes – self contained: RI/W1/W2/CQI reported in the same subframe

– Subband W2 & wideband CQI

– Wideband W2 & subband CQI

– Subband W2 and subband CQI

13

subband W2, wideband W1, wideband CQI

Wideband W1, wideband W2, subband CQI

t=k

t=k

Wideband W1 & wideband W2

Subband W2, subband CQI on selected subbands

t=k

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UL MIMO Overall Procedure

14

•SRS = Sounding Reference Signal

•PUSCH = Uplink data

•PDCCH = Downlink Control

•ACK/NACK = feedback for Hybrid ARQ operation

KRnet2011

UL Transmission Chain

15

ScramblingModulation

mapperLayer

mapper

ScramblingModulation

mapper

layerscodewords

Precoding

Resource element mapper

SC-FDMA signal gen.

Resource element mapper

SC-FDMA signal gen.

Transform precoder

Transform precoder

antenna portslayers

KRnet2011

UL MIMO Overview

16

LTE

(up to 2 TX antennas)

LTE-Advanced

(up to 4 TX antennas)

Spatial

Multiplexing None

•Closed-loop codebook based

precoding

•15 bps/Hz with 4x4 configuration, i.e.,

1.5Gbps @ 100MHz

Transmit

diversity

•Closed loop antenna selection diversity

•Open loop antenna selection diversity

SORTD for control signaling with up to

2 TX antennas

- SORTD: Spatial Orthogonal Resource Transmit

Diversity

KRnet2011

Layer-to-Codeword Mapping

17

Number of

layers

Number of

codewords

Number of layers per

codeword Note

1 1 1

2 1 2 Only for retransmitted codeword

2 (1, 1)

3 2 (1, 2)

4 2 (2,2)

•Maximum two codewords are transmitted to reduce the DL ACK/NACK signaling overhead

•Multiple layers may be mapped to a single codeword depending on the rank value

KRnet2011

UL MIMO HARQ Operation

18

CW0initial

CW1initial

CW0reTX

NDI0toggled

NDI1 toggled

Downlink

Uplink

NACK0

ACK1 ACK0

Stop reTX

PDCCH PHICH

PHICH

(NACK,NACK) (ACK,NACK) or (NACK,ACK)

Precoding indicated by the most recent PDCCH

Predefined precoding

Use of PHICH allows HARQ operation with smaller overhead

KRnet2011

PUCCH SORTD

19

CCE0

PUCCH resource

(CCEn)

CCE1 CCE2 CCEn+1 CCEn+2 CCEn+3 CCEn+4... ...CCEn

PDCCH for DL assignment

UE

PUCCH single port transmission (Rel-8)

A/N

CCE0

PUCCH resource

(CCEn)

CCE1 CCE2 CCEn+2 CCEn+3 CCEn+4... ...CCEn

UE

PUCCH TxD (Rel-10)

A/N

PDCCH for DL assignment

PUCCH resource (CCEn+1)

SORTD

CCEn+1

PDCCH: Physical Downlink Control Channel

CCE: Control Channel Element

PUCCH: Physical Uplink Control Channel

SORTD: Spatial Orthogonal Resource Transmit Diversity

SORTD

•Keep single carrier property while achieving diversity gain

•Defined only for control signaling

KRnet2011

Performance Evaluation

20

(1) Closely-spaced cross-

polarized set-up

(2) Widely-spaced cross-

polarized set-up(3) Closely-spaced co-

polarized set-up

(1) Closely-spaced cross-polarized set-up (2) Closely-spaced co-polarized set-up

(3) Widely-spaced cross-polarized set-up

4 antenna set-ups

8 antenna set-ups

KRnet2011

Performance Evaluation

21

FDD DL MIMO in 3GPP Case 1 (urban macro) (angular spread of 8 degrees)

Scheme

Closely-spaced cross-polarizeet-up at the base station

Closely-spaced co-polarized set-up at the base station

Cell spectral efficiency

[b/s/Hz/cell]

Cell-edge user spectral efficiency

[b/s/Hz/user]

Cell spectral efficiency

[b/s/Hz/cell]

Cell-edge user spectral efficiency

[b/s/Hz/user]

Rel-8 4x2 SU-MIMO 2.41 (100%) 0.064 (100%) 2.23 (100%) 0.081 (100%)

4x2 SU/MU-MIMO dynamic switching

2.56 (106%) 0.075 (117%) 3.19 (143%) 0.106 (131%)

8x2 SU/MU-MIMO dynamic switching

3.72 (154%) 0.113 (177%) 4.38 (196%) 0.142 (175%)

KRnet2011

Performance Evaluation

22

FDD UL MIMO in 3GPP Case 1 (urban macro) (angular spread of 8 degrees)

Scheme

Closely-spaced cross-polarizeet-up

Widely-spaced co-polarized set-up

Cell Spectral Efficiency

[bps/Hz/cell]

Cell Edge Spectral

Efficiency [bps/Hz/cell]

Cell Spectral Efficiency

[bps/Hz/cell]

Cell Edge Spectral

Efficiency [bps/Hz/cell]

Rel-8 1x4 SIMO 2.00 (100%) 0.075 (100%) 1.95 (100%) 0.075 (100%)

Rel-10 2x4 SU-MIMO

2.27 (114%) 0.091 (121%) 2.36 (121%) 0.097 (129%)

KRnet2011

Real Life Deployment Issues

• Distributed antennas

23

Antenna port 0

Antenna port 1

Antenna port 0

Antenna port 1

0 10 20 30 40 50 60 70 80 90 100-40

-35

-30

-25

-20

-15

-10

-5

0

Time [seconds]

Rela

tive R

SR

P [

dB

]

RSRP

0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

60

70

80

90

100

Time [Seconds]

Rela

tive t

hro

ughput

[%]

Throughput

A BA-port 0 A-port 1 A-port 0A BA-port 0 A-port 1 A-port 0

Experiment environment (3GPP R1-111330)

Received Signal Power

Throughput

KRnet2011

Real Life Deployment Issues

• Different antenna configurations (3GPP R1-111434)

24

UE covered by RRH 4 only

Central

Antenna

RRH 4

RRH 1

RRH 5

UE covered by RRH1 and

central antenna

RRH 3

UE covered by central antenna

and RRH2 & 3

RRH 2 and RRH 3

RRH 2

UE covered by central antenna

only

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