Lte Course - Mimo Basics

Slide titleIn CAPITALS

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Slide subtitle 32 pt

MIMO Basics

Top right corner for field-mark, customer or partner logotypes. See Best practice for example.

Slide title 40 pt


Text 24 pt

Bullets level 2-520 pt

Ericsson Confidential 2007-03-232

Multiple antenna benefits

Diversity“Reduce fading”

Example

Transmit the signal in all Transmit the signal in all directionsdirections

Multiplexing“Capacity multiplication”

Example

Transmit several signals in Transmit several signals in different directionsdifferent directions

S-PDela

y

DirectivityAntenna/Beamforming gain

Example

Transmit the signal in the Transmit the signal in the best directionbest direction

Channel knowledge (average/instant)


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Directivity


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Beamforming

Beamforming provides antenna gain– Cell edge coverage

Constructive summation of signals (in the air)– Align phases between several transmitted copies of the

signal

Spatial isolation between users– Multi-user scheduling (SDMA)


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Beamforming solutions

”Traditional” beamforming– Same fading on all antennas– Stear beam by phase shift on antennas

Estimate ”direction” from UL OR Slow feedback

Antenna constallation do not match ”MIMO multiplexing”

Reference symbols provided in ”beam”– Cannot be used by others– Cannot estimate CQI when not scheduled

/2


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More beamforming solutions

Pre-coded based beamforming– Align signals by fixed BF vectors– A ”code-book” of vectors

UE pick ”best” vector Do not require same fading on antennas Blends with ”MIMO multiplexing” Different fading over frequencies

– Feedback/resource block Reference symbol per ”antenna”

– Possible to estimate CQI even when not scheduled

4-10


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Pre-coding based BF

Code-book Maximize SNR, rate, throughput, ...

From RS

The UE feeds back an index to the preffered pre-coding vector

4/74/54/34/

1111 jjjj eeee

kHH

kkP wHHw


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Diversity


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Transmit Diversity (open loop)

Common channels– Link adaptation not possible

Feedback not possible– High Doppler– Cell edge

Large Tx Antenna Distance is desirable Two antenna ports:

– Space-Frequency Block Coding

Four antenna ports: – SFBC + Frequency Shift Transmit Diversity (FSTD)


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LTE Downlink transmit diversity Two antenna ports:

– Space-Frequency Block Coding SFBC– Like WCDMA STTD (Alamuthi) but in frequency domain

SFBCcoder

s1

s2

IFFT

s1

s2

IFFT

-s2*

s1*

Antenna port #1

Antenna port #2


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LTE Downlink transmit diversity

s1

s2

s3

s4

IFFT

s1

s2

0

0

IFFT

0

0

s3

s4

IFFT

s2*

-s1*

0

0

IFFT

0

0

s4*

-s3*

Antenna port #1

Antenna port #2

Antenna port #3

Antenna port #4

SFBCcoder

Four antenna ports: – SFBC + Frequency Shift Transmit Diversity (FSTD)– Like Time Switched Transmit Diversity but in frequency domain


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Multiplexing


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Why Spatial Multiplexing?

Directivity and Diversity techiques improves SNR Capacity = Bandwidth × log2(1 + SNR)

– Low SNR: Capacity ~ Bandwidth × SNR – High SNR: Capacity ~ Bandwidth × log2(SNR) Saturation

If SNR is high then it is better to share it between several r streams with SNR/r each

– This works if r ≤ min{NT, NR} channel rank r

If SNR is low then the spatial multiplexing provides no gains over directivity and/or diversity techniques


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Spatial Multiplexing

Spatial Multiplexing may give rmax times higher throughput!– Transmit rmax = min(NT, NR) parallel symbol streams

– Gain depends on channel properties Well-conditioned channel – a.k.a. full rank channel

Maximum Likelihood receiver– Transmitted modulation alphabet known– Channel realization is known– Conceptually simple – computationally complex

Calculate probability of the received signal vector conditioned on every possible transmitter signal vector.

Select maximum.

Tx Rec

NRNT


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Multi-stream transmission

Same rate on each layer One CQI feedback Joint coding gain

Adapt rate on each layer One CQI feedback per layer Successive interference

cancellation (SIC) gain

Multi code word

S/P

Coding &modulation

R1

R2

R3

R4

RX

Coding &modulation

Coding &modulation

Coding &modulation

Transmitter

Feedback (channel state)

Single code word

S/P

R

R

R

R

4R Jointdetection

Feedback (channel state)

Coding &modulation


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Downlink Multi-Antenna Transmission in LTE


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Multi-antenna transmission OFDM works particularly well with MIMO

– MIMO becomes difficult when there is time dispersion– OFDM sub-carriers are flat fading (no time dispersion)

3GPP supports one, two, or four transmit Antenna Ports Multiple antenna ports Multiple time-frequency grids Each antenna port defined by an associated Reference Signal

Antenna port #1Antenna port #2Antenna port #3Antenna port #4

Single-antenna transmisson

Multi-antenna transmisson


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Multi-stream transmission in 3GPP

MIMO on/off on slow basis (several 100 ms)

– Unreliable feedback– Rank adaptation: Adapt # of streams to channel conditions

Multi code word– Exploit capacity gain by SIC– Maximum # of codewords: 2

UE feedback– Pre-coding matrix indicator (frequency selective)– Channel quality indicator (frequency selective)– Rank-indicator (for the full bandwidth)


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LTE Multi-antenna transmission

Spatial Multiplexing w. Pre-coding based Beam-forming– Multiple parallel data streams Higher data rates– One or two code words– Number of layers Number of antenna ports– Pre-coding weights based on UE feedback (beam-forming)

Transmit Diversity (”open-loop”)– Transmission of same information from multiple antenna ports Diversity– One code word– Number of layers = Number of antenna ports

Layermapping

Precoding

Up to four layers

RE mappingUp to four

”Antenna Ports”

RE mapping

One or two code words

Multi-antenna processing


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Downlink spatial multiplexingLayer mapping

Maximum of two code words Mapping to up to four layers

– Number of layers depends on channel ”rank”– Dynamically adjusted based on UE reports

One layer(No MIMO)

Two layers Three layers Four layers

One code word Two code words Two code words Two code words

Transport format (modulation scheme and code rate) may differ between the code words

Same number of symbols on each layer

Layermapping

Up to four layers


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Downlink spatial multiplexingPrecoding

One symbol from each of NL layers linearly mapped to NA antenna ports

UE reports recommended precoder matrix W (including channel rank)– Set of available precoder matrices = The precoder ”code book”

Used precoder matrix signaled by network (1 bit)– Network does not need to follow UE recommendation

One layer ”Closed-loop” TX diversity = ”Beam forming”

x

Precoding

W

yxWy

4 antenna portsLayer 1

Layer 2

Layer 3

Example: NL = 3, NA = 4


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Precoder code booksTwo antenna ports (NA = 2)

10

01

2

1

1

0

11

2

1

1

1

2

1

0

1

11

11

2

1

j

1

2

1

j

1

2

1

jj

11

2

1

6

5

4

3

2

1

NL = 2Codebook

index

Number of layersNL = 1


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Precoding with Spatial Multiplexing

Codeword 1

Codeword 2

Mod

Mod

CW2layer Wf

IDFT

IDFT

sf

DFT

DFT

yf

Hf

yf = HfWfsf + ef

RTTTTRRR

T

T

2

1

2

1

21

22221

11211

21

22221

11211

,,,

N

f

r

f

rNNN

r

r

f

NNNN

N

N

f

e

e

e

s

s

s

www

www

www

hhh

hhh

hhh

esWH

#receive antennas: NR, #transmit antennas: NT, #layers: rNote: r is also referred to as transmission rank

Precoder


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Codebook based Precoding Precoding weight Wf selected from a countable and finite set

where W(r) is the codebook for rank r containing Qr different NT x r precoding matrices

Rank and precoder element selection– Exhaustive search over all precoder and rank combinations– Select the combination giving the highest estimated throughput

Codebook based precoding is a form of channel quantization FDD: Selection is carried out on UE side and recommendations fed

back to eNodeB TDD: Channel reciprocity may also be used to conduct precoder

selection on eNodeB side– Still under discussion

)()(2

)(1 ,,, r

Qrr

rrW WWW


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3GPP LTE – Advanced antenna solutions

Two codewords + beam-forming (4x2)Two codewords + beam-forming (4x2)

Two codewords (2x2)Two codewords (2x2)

Single-codeword + Single-codeword + beam-forming (4x2)beam-forming (4x2)

Single-codeword 1x2Single-codeword 1x2

Coverage

Th

rou

gh

pu

t

Different antenna solutions needed Different antenna solutions needed depending on what to achievedepending on what to achieve

Documents

Lte Course - Mimo Basics