week 5 principles of mimo-ofdm technology

8/13/2019 Week 5 Principles of MIMO-OfDM Technology

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International

TelecommunicationUnion

Week 5. Principles of MIMO-OFDMTechnology

ITU Asia-Pacific Centres of Excellence Online Trainingon

4G LTE Mobile Systems and Applications

2 December 2013Republic of Korea

Hyung-Jin Choi, Professor,

Sungkyunkwan University, Suwon, Republic of Korea


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Content

Background

SIMO: Receive spatial diversity

MISO: Transmit spatial diversity

MIMO: Spatial multiplexing

Application in 3GPP LTE system

Content


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Background


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Aspirations (Mathematical) of a system designer

High data rate Achieve channel capacity (C)

Quality Minimize probability of error (Pe)

Real-life Issues Minimize complexity/cost of

implementation of proposed system

Minimize transmission power required

(translates into SNR)

Minimize bandwidth (frequency

spectrum) used

Background


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Spectral efficiency

Shannons capacity (Csh)

Given a unit of BW (Hz), the max error-free transmission rate is

Let us define

R : data rate (bits/symbol), RS : symbol rate (symbols/second), W : allotted BW (Hz)

Spectral efficiency is defined as the number of bits transmitted per second per Hz

As a result of filtering/signal reconstruction requirements, RS W. Hence spectral efficiency

= R (if RS = W)

If we transmit data at a rate of R Csh, an arbitrarily low Peis acheived

Background

sh 2log (1 SNR) bits/s/HzC = +

bits/s/HzSR R

W


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Spectral efficiency (contd)

Spectral efficiencies of some widely used modulation schemes

The whole point: Given an acceptable Pe, realistic power and BW limits, MIMO

systems using smart modulation schemes provide much higher spectral efficiencies

than traditional SISO system

Background

Scheme b/s/Hz

BPSK 1

QPSK 2

16-QAM 4

64-QAM 6


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Multiple antenna types

Single-input multi-output (SIMO): A single transmit antenna and Nrreceive antennas

Multi-input single-output (MISO): Nt transmit antennas and a single receive antenna

Multi-input multi-output (MIMO): Nttransmit antennas and Nrreceive antennas

Background


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Array gain

It means a power gain of transmitted signals that is achieved by using multiple-

antennas at transmitter and/or receiver, with respect to SISO case

It can be simply called power gain

It increases coverage and quality of service (QoS)

Multiplexing gain

It can be interpreted as multiple streams are multiplexed in spatial domain compared

to single stream of SISO case

It increases spectral efficiency

It can be archived through spatial multiplexing and space division multiple access

techniques

Background


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Diversity gain

It is the increase in signal-to-interference-plus-noise ratio (SINR) due to some of

transmit and receive diversity schemes

How much the transmission power can be reduced when a diversity scheme is

introduced, without a performance loss compared to SISO case

Time diversity: Channel coding (FEC) with interleaver, ARQ

Frequency diversity: Wideband system, coded-OFDM

Space diversity: Multiple antennas with low correlation

Polarization diversity: Vertical & horizontal

Background


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Trends of MIMO technologies

A promising technology for 3G mobile communication system

3GPP long-term evolution (LTE), 3GPP2 revision C: MIMO technology

Mobile Wimax (WiBro): Smart antenna, transmit diversity, and MIMO technologies

A key technology for 4G (or 5G) communication system

Maximum required spectral efficiency of cdma2000 1xEVDO system is 1.92 bps/Hz

For 100Mbps transmission, at least bandwidth of 50MHz is required, which is higher by 5

times than current system

Thus, MIMO technologies are strongly required!

Background


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Trends of MIMO technologies

Background


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Typical multiple antenna techniques in mobile communication system

SIMO: obtain receive spatial diversity gain

Receiver maximal ratio combining scheme (Rx-MRC)

MISO: obtain transmit spatial diversity gain

Space time/frequency block coding (STBC/SFBC), cyclic delay diversity (CDD)

MIMO: obtain spatial multiplexing gain

V-BLAST, channel matrix decomposition, pre-coding, SIC/MMSE receiver

Background


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SIMO: Receive Spatial Diversity


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Maximum ratio combing (MRC) technology at base station

It has been adopted for use in down-link of IS-95 system

Average receive SNR increases as the number of antenna increases

It obtains a spatial diversity gain by reducing the effect of deep or attenuated fading

channel components


[ ]

[ ]

T

1 2

T* * * *

1 2 1 2 1 2

2 2

MRC 1 2

MRC 2 MRC

( ) ( )

log (1 ) [bps / Hz]

h h

h h h h h h

SNR h h

C

= +

= +

= +

= +

r s n

y s n

Capacity increases logarithmically

with number of receive antennas...


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MRC at mobile and fixed terminals

Diversity gain improves received SNR: 2 branch diversity: 3~7dB gain

Increasing spectral efficiency without upgrade of existing systems

It can be readily used for 3GPP, 3GPP2, Wimax, and LTE systems



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Benefit of MRC technology

Channel gain according to the use of two receive antennas



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The other receive spatial diversity techniques

Selection diversity technology

Select antenna element with the highest SNR (or other metric)

Greatest SNR improvement when

Desired signal subject to independent (uncorrelated) fading and signal receiver with the same

average power at each element

Background noise is AWGN, equal power and uncorrelated across elements

Switched Diversity

Choose antenna element only when SNR undergoes a deep fade and the received SNR

crossed a threshold.

Perform worse than selection diversity

Requires only a single RF chain to serve all its antennas



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MISO: Transmit Spatial Diversity


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Space-time block coding (STBC)

Transmit diversity puts the extra antenna at the BS instead of the MS

STBC has emerged as an efficient means of achieving near optimal transmitter

diversity gain [98]

The basic orthogonal principle has been applied to the space-frequency block coding

(SFBC) formulation


Siavash M. Alamouti, A Simple Transmit Diversity Technique for Wireless Communications, IEEE

Journal on Selected Areas in Communications, vol. 16, no. 8, October 1998

2 2* * *0 10 1 0 1

* 2 2

1 0 1 0 0 1

0

0

S SS S S S

S S S S S S

+ =

+


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Comparisons: (Tx/Rx) MRC and STBC w.r.t SNR

Rx-MRC (Receiver MRC)

STC (Space Time Code, Transmit Diversity)

Tx-MRC (Transmit MRC, require channel information in transmitter)


1 0 1

2 1 2

x h ns

x h n

= +

x hs n= +

2 2

0 1

2

h hSNR

+=

0 11 1 1

* **

1 02 2 2

( ) 1

( ) 2

h hx T s n

h hx T s n

= +

x Hs n= +

[ ]0 0 1 1x w h w h s n= + +* 2 2

0 0 0 1

* 2 2

1 1 0 1

/

/

w h h h

w h h h

= +

= +

2 2

0 1

22

h hSNR

+=

2 2

0 1

2

h hSNR

+=


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Space-time block coding (STBC)

Transmit diversity using STBC reduces the required fade margin at receiver and

improves link margin by 5-10dB.

3-dB power penalty compared to receiver MRC on the basis of equal transmit power



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STBC-OFDM system

STBC-OFDM achieves near optimal diversity gain in slow fading.

It still outperforms non-diversity OFDM system at fd=100Hz.



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SFBC-OFDM system

SFBC-OFDM achieves similar diversity gain as STBC-OFDM in slow fading

SFBC-OFDM performs better in fast fading



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Cyclic Delay Diversity (CDD)

The OFDM symbols of the CDD signal can be generated from the reference signal OFDM symbols

just by applying a cyclic time shift cyto the reference OFDM symbols and subsequent insertion of

the cyclic prefix.

CDD is independent of the existence of a cyclic prefix (guard interval) and are capable to increase

the channel frequency selectivity without increasing the overall channel delay spread because

these operations are done before guard interval insertion and are restricted to the OFDM symbol

itself.



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Cyclic Delay Diversity (CDD) (contd)

An additional transmitter antenna with CDD increases the frequency selectivity, i.e. decreases the

coherence bandwidth.

A lower coherence bandwidth lead to a better error performance.

In order to achieve any diversity effects, i.e. to get constructive and destructive interference within

the OFDM signal bandwidth B, the inserted (cyclic) delays

cyhave to fulfill

cy1/B.


Indoor channel snapshot for single antenna system Indoor channel snapshot for CDD system


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Cyclic Delay Diversity (CDD) (contd)

Problem in CDD

CDD may raise synchronization ambiguities where the OFDMA signals transmitted with

different delays on the two Tx antennas, making synchronization more difficult.

CDD decreases the coherence bandwidth of the actual channel and therefore may incur

channel estimation performance degradation.



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MIMO: Spatial Multiplexing


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Spatial multiplexing (SM) technique

Representative SM system: V-BLAST [Wolniansky et al. 1998]

Maximize transmission rate (optimistic approach)

MIMO uses multi-antenna-path to advantage to multiply data rate

Transmits different data along different paths

Transmit data rate becomes Nttimes compared with SISO case (Nt: # of Tx antenna)



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Basic assumptions of SM system

The number of Tx antenna is not more than that of Rx antenna (N t Nr)

All of paths have random Rayleigh distribution

Channel component hjiis a complex Gaussian random variable

Assuming rich-scattering channel (non line-of-sight)

If line-of-sight components exist, the full rank of channel matrix is not guaranteed, which

means that parallel independent data transmission is difficult or impossible



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MIMO channel capacity

Capacity interpretation (without feedback information from receiver) of SM system in terms of

eigenvalues of HHH

The eigenvalue (the number of orthogonal basis (or rank)) means a maximum channel capacity

(thickness) capable of transmitting different information symbols to corresponding receiver with

parallel over the channel at any given time


11 12 13

21 22 23

31 32 33

h h h

H h h h

h h h

=

3 3MIMO 2

2 2 2 3

log [ ( / 3) ]

log [1 ( / 3) ] log [1 ( / 3) ] log [1 ( / 3) ]

HC I SNR HH

SNR SNR SNR

= +

= + + + + +


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MIMO channel capacity (contd)

Case of specular channel (non rich-scattering, LOS channel)

Extreme example for AWGN channel, i.e., all of channel components have 1 (without normalization) which

gives understanding of the effect of specular channel.

Eigenvalue is [0 0 3]

The number of orthogonal basis (rank) of H is one.

Actual available channel link is only one.


11 12 13

21 22 23

31 32 33

1 1 1

1 1 1

1 1 1

h h h

H h h h

h h h

= =

3 3MIMO 2 2log [ ( / 3) ] log [1 ]H

C I SNR HH SNR = + = +


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Spectral Efficiency comparison



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Optimal SM system

The design of optimal SM system through MIMO channel decomposition and precoding

Singular value decomposition (SVD)

Eigen value decomposition (EVD)

The power is optimally allocated to the different streams by using the waterfilling scheme.


H


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Optimal SM system - MIMO channel decomposition


11 1

21 2

1

r

r

n n r

N

N

N N N

h h

h hH

h h

=

1

2

0 0

0 0

0n

D

=

11 1

21 2 *

1

1

,

r

n

r

n n r

N

NNH

ij ik jk

k

N N N

g g

g gR HH g h h

g g

=

= = =

1

2

0 00 0

0

H

n

D D

=

Channel Transfer Matrix Channel Correlation Matrix

H = UDVH R = VAAH =VDHDVH

Singular

Value

Decomposition

Eigen

Value

Decomposition


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Optimal SM system - Precoding

Through estimated CSI, receiver selects a precoder (W), used for a transmitter, capable

of providing a maximum channel capacity. Then it informs precoder index through

feedback link

Transmitter make a beam by using the selected precoder, then it performs

beamforming



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Unitary precoded MIMO system (3GPP 3GPP2)



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Scheduling-based MIMO system

Single-user MIMO vs. multi-user MIMO

User selection (single-user MIMO) Stream selection (multi-user MIMO)



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MIMO receiver technologies

for mitigating the ICI and ISI effects (Practical and efficient approaches)

Ordering-based signal detection

SIC (Successive interference cancellation)

Channel equalization

MMSE (minimum mean

squared error)

ZF (Zero-forcing)



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Basic principle of ordering and SIC (with simple ordering)

In classical SIC, the receiver arbitrarily takes one of the estimated symbols (for

example the symbol transmitted in the first spatial dimension), and subtract its effect

from the other received symbols.



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Basic principle of ordering and SIC (With optimal ordering)

More intelligence in choosing which symbol should be subtracted its ICI effect. To

make that decision, let us find out the transmit symbol (after multiplication with the

channel) which came at higher power at the receiver.

The received power at the antennas corresponding to each transmitted symbol is



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Overall structure of SIC and channel equalization

According to ordering, symbols are detected and subtracted as shown in below figure


Hs x(M) x(M-1) x(M-2)

- -

fMhM

fM-1hM-1

f1

Some ISI has

been canceled

Linear estimator

of the M-th elementCancel the response

of the M-th element

s(M) s(M-1) s(1)

Decision

v


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Simple example of ordering and SIC procedures



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Advantages and disadvantages of SIC

Advantages

It realizes efficient MIMO detection in the implementation point of view.

It provides a Rx-MRC gain through a cancellation procedure.

Disadvantages (inevitable problem)

It suffers from imperfect ICI cancellation and error propagation problems caused by a wrong

decision symbol.



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Channel equalization - Usual algorithms

ML Algorithm

MMSE algorithm

ZF algorithm


2 , where argmin = =

x W y W Wy x

( ) 1

= +

x H HH nn y

( )# #

with (pseudo-inverse)=

-1

x H y H = H H H


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Channel equalization Performance comparison

ML, MMSE, and ZF algorithms



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BER plot for 2 transmit 2 receive MIMO channel for BPSK modulation



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Practical Applicationin 3GPP LTE System


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Transmission modes (TM) in LTE release 9 downlink

Practical Application in 3GPP LTE System


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TM 2 Transmit diversity

It sends the same information via various antennas, whereby each antenna stream

uses different coding and different frequency resources

Improves the signal-to-noise ratio and makes transmission more robust

PBCH and PDCCH are also transmitted using transmit diversity

For two antennas, a frequency-based version of the Alamouti codes (SFBC) is used

For four antennas, a combination of SFBC and frequency switched transmit diversity

(FSTD) is used



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TM 3 Open loop spatial multiplexing with CDD

It is used when channel information is missing or when the channel rapidly changes

e.g. for UEs moving with high velocity

It requires less UE feedback regarding the channel situation (no precoding matrix indicator is

included)

the signal is supplied to every antenna with a specific delay (CDD), thus artificially

creating frequency diversity


l l


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TM 4 Closed loop spatial multiplexing

Supports spatial multiplexing with up to four layers that are multiplexed to up to four

antennas, respectively, in order to achieve higher data rates

Requires CSI feedback and precoding matrix selection

CRS based channel estimation at UE

Precoding matrix indicators (PMI) defined

in the codebook

a table with possible precoding matrices

that is known to both sides


i l li i i 3 S


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TM 5 Multi-user MIMO

It uses codebook-based closed loop spatial multiplexing, however one layer is

dedicated for one UE

Multi-User diversity gain & Spatial multiplexing gain


P i l A li i i 3GPP LTE S


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TM 6 Closed loop spatial multiplexing using a single transmission layer

Special type of closed loop spatial multiplexing (TM 4)

Only one layer is used (corresponding to a rank of 1)

The precoding in the baseband of

the signals to the different antennas

results in a beamforming effect

Aiming at achieving a direct impact

on the antenna diagram

e.g. for illuminating particular areas

of a cell


P i l A li i i 3GPP LTE S


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TM 7 Beamforming (antenna port 5)

Uses UE-specific DM-RS

Both the data and the RS are transmitted using the same antenna weightings. Because the UE

requires only the UE-specific RS for demodulation of the PDSCH, the data transmission for the

UE appears to have been received from only one transmit antenna, and the UE does not see

the actual number of transmit antennas. Therefore, the transmission appears to be

transmitted from a single "virtual" antenna port 5.


P ti l A li ti i 3GPP LTE S t


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TM 8 Dual layer beamforming (antenna ports 7 and 8)

Uses UE-specific DM-RS for dual-layer

the same elements are used, the reference signals must be coded differently

UE can distinguish among them based on orthogonality of code

Both layers can be assigned to one UE (single-user MIMO), or the two layers can be

assigned to two separate UEs (multi-user MIMO)


time

frequency

1111

1111

1111

-11-11

1-11-1

-11-11

Antenna port 7 Antenna port 8

Data symbolDM-RS for

port 7

DM-RS for

port 8


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T l i i

Thank You

week 5 principles of mimo-ofdm technology

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