LTE Downlink OFDMA & MIMO Planning Guide

Jakarta
15 December 2012

Arief Hamdani Gunawan

OFDMA & MIMO Planning

OFDMA & MIMO

•OFDM and OFDMA •Introduction to MIMO?
•LTE Downlink •Different gains of multiple antenna systems
•OFDMA time-frequency multiplexing •Shannon capacity of Wireless Channels
•LTE Spectrum Flexibility •Multiple antennas at one end
•LTE Frame Structure type 1 (FDD) •Capacity of MIMO Links
•LTE Frame Structure type 2(TDD) •Principles of Data transmission over MIMO Systems
•Introduction to SC-FDMA and UL frame structure •Diversity using Space Time Block Codes
•How to generate SC-FDMA •Spatial Multiplexing
•How does SC-FDMA signal look like •Wireless Channel Modeling
•SC-FDMA Signal Generation •System Level Issues
•SC-FDMA PAPR •MIMO Transmission Scheme for HSPA and LTE
•SC-FDMA Parameterization

OFDM
• Single Carrier Transmission (e.g. WCDMA)

• Orthogonal Frequency Division Multiplexing

OFDM Concept: Mengapa OFDM

• Sinyal OFDM (Orthogonal Frequency Division
Multiplexing) dapat mendukung kondisi NLOS (Non
Line of Sight) dengan mempertahankan efisiensi
spektral yang tinggi dan memaksimalkan spektrum
yang tersedia.
• Mendukung lingkungan propagasi multi-path.
• Scalable bandwidth: menyediakan fleksibilitas dan
potensial mengurangi CAPEX (capital expense).

5

OFDM Concept: NLOS Performance

6

OFDM Concept: Mutipath Propagation

• Sinyal-sinyal multipath datang pada waktu yang berbeda dengan amplitudo dan pergeseran fasa yang
berbeda, yang menyebabkan pelemahan dan penguatan daya sinyal yang diterima.
• Propagasi multipath berpengaruh terhadap performansi link dan coverage.
• Selubung (envelop) sinyal Rx berfluktuasi secara acak.
7

OFDM Concept: FFT

• Multi-carrier modulation/multiplexing technique
• Available bandwidth is divided into several subchannels
• Data is serial-to-parallel converted
• Symbols are transmitted on different subcarriers
8

OFDM Concept: IFFT

Basic ideas valid for various multicarrier techniques:
• OFDM: Orthogonal Frequency Division Multiplexing
• OFDMA: Orthogonal Frequency Division Multiple Access

9

OFDM Concept: Single-Carrier Vs. OFDM

Single-Carrier Mode: OFDM Mode:
• Serial Symbol Stream Used to Modulate a • Each Symbol Used to Modulate a Separate
Single Wideband Carrier Sub-Carrier
• Serial Datastream Converted to Symbols
(Each Symbol Can Represented 1 or More
Data Bits) 10

OFDM Concept: Single-Carrier Vs. OFDM

Single-Carrier Mode OFDM Mode
• Dotted Area Represents Transmitted Spectrum
• Solid Area Represents Receiver Input

• OFDM mengatasi delay spread, multipath dan ISI (Inter Symbol Interference) secara efisien sehingga
dapat meningkatkan throughput data rate yang lebih tinggi.
• Memudahkan ekualisasi kanal terhadap sub-carrier OFDM individual, dibandingkan terhadap sinyal
single-carrier yang memerlukan teknik ekualisasi adaptif lebih kompleks. 11

OFDM Concept: Motivation for Multi-carrier Approaches

• Multi-carrier transmission offers various advantages over
traditional single carrier approaches:
– Highly scalable
– Simplified equalizer design in the frequency domain, also in cases of
large delay spread
– High spectrum density
– Simplified the usage of MIMO
– Good granularity to control user data rates
– Robustness against timing errors
• Weakness of multi-carrier systems:
– Increased peak to average power ratio (PAPR)
– Impairments due to impulsive noise
– Impairments due to frequency errors

12

OFDM Concept: Peak to Average Power Ratio (PAPR)

• PAPR merupakan ukuran dari fluktuasi tepat sebelum amplifier.
• PAPR sinyal hasil dari mapping PSK base band sebesar 0 dB karena semua symbol mempunyai daya yang
sama.
• Tetapi setelah dilakukan proses IDFT/IFFT, hasil superposisi dari dua atau lebih subcarrier dapat
menghasilkan variasi daya dengan nilai peak yang besar.
• Hal ini disebabkan oleh modulasi masing-masing subcarrier dengan frekuensi yang berbeda sehingga
apabila beberapa subcarrier mempunyai fasa yang koheren, akan muncul amplituda dengan level yang
jauh lebih besar dari daya sinyalnya. 13

OFDM Concept: Peak to Average Power Ratio (PAPR)

• Nilai PAPR yang besar pada OFDM membutuhkan amplifier dengan dynamic range yang lebar untuk
mengakomodasi amplitudo sinyal.
• Jika hal ini tidak terpenuhi maka akan terjadi distorsi linear yang menyebabkan subcarrier menjadi tidak
lagi ortogonal dan pada akhirnya menurunkan performansi OFDM.
14

Tipe Sub-Carrier OFDM

Data Sub-carriers
• Membawa simbol BPSK, QPSK, 16QAM, 64QAM
Pilot Sub-carriers
• Untuk memudahkan estimasi kanal dan demodulasi koheren pada receiver.
Null Subcarrier
• Guard Sub-carriers
• DC Sub-carrier 15

Guard Interval (Cyclic Prefix)

• Untuk mengatasi multipath delay spread
16
• Guard Interval (cyclic prefix) : 1/4, 1/8, 1/16 or 1/32

OFDM Transceiver

17

OFDM & OFDMA
OFDM OFDMA
• Semua subcarrier dialokasikan untuk satu • Subcarrier dialokasikan secara fleksibel
user untuk banyak user tergantung pada kondisi
• Misal : 802.16-2004 radio.
• Misal : 802.16e-2005 dan 802.16m

18

OFDM Parameters used in WiMAX

19

Difference between OFDM and OFDMA

• OFDM allocates users in time • OFDMA allocates users in time
domain only and frequency domain

OFDMA time-frequency multiplexing

LTE Downlink Physical Layer Design: Physical Resource

The physical resource can be seen as
a time-frequency grid

• LTE uses OFDM (Orthogonal Frequency Division Multiplexing) as its radio technology in downlink
• In the uplink LTE uses a pre=coded version of OFDM, SC-FDMA (Single Carrier Frequency Division
Multiple Access) to reduced power consumption
22

LTE Downlink Resource Grid

• Suatu RB (resource block) terdiri dari 12 subcarrier pada suatu
durasi slot 0.5 ms.

• Satu subcarrier mempunyai BW 15 kHz, sehingga menjadi 180
kHz per RB.

23

Parameters for DL generic frame structure

Bandwidth (MHz) 1.25 2.5 5.0 10.0 15.0 20.0

Subcarrier bandwidth (kHz) 15

Physical resource block (PRB)
180
bandwidth (kHz)

Number of available PRBs 6 12 25 50 75 100

24

Parameters for DL generic frame structure

Transmission BW 1.25 MHz 2.5 MHz 5 MHz 10 MHz 15 MHz 20 MHz

Sub-frame duration 0.5 ms

Sub-carrier spacing 15 kHz

192 MHz
7.68 MHz 15.36 MHz 23.04 MHz 30.72 MHz
Sampling frequency (1/2x3.84 3.84 MHz
(2x3.84 MHz) (4x3.84 MHz) (6x3.84 MHz) (8x3.84 MHz)
MHz)

FFT size 128 256 512 1024 1536 2048

OFDM sym per slot
7/6
(short/long CP)

(4.69/9) x 6, (4.69/18) x 6, (4.69/36) x 6, (4.69/72) x 6, (4.69/108) x 6, (4.69/144) x 6,
Short (5.21/10) x 1 (5.21/20) x 1 (5.21/40) x 1 (5.21/80) x 1 (5.21/120) x 1 (5.21/160) x 1
CP length
(usec/
samples) (16.67/32) (16.67/64) (16.67/128) (16.67/256) (16.67/384) (16.67/512)
Long

25

LTE – Spectrum Flexibility

• LTE physical layer supports any bandwidth from 1.4 MHz to 20
MHz in steps of 180 kHz (resource block).
• Current LTE specification supports a subset of 6 different
system bandwidths.
• All UEs must support the maximum bandwidth of 20 MHz.

Case Study
LTE Signal Spectrum (20 MHz case)

• The LTE standard uses an over-sized LTE. The actual used bandwidth is controlled by the number of used
subcarriers. 15 kHz subcarrier spacing is the constant factor!
• 18 MHz out of 20 MHz is used for data, 1 MHz on each side is used as guard band.
• LTE used spectrum radio = 90%
• WiMAX used spectrum radio = 82% 28

TDD & FDD

• Time Division Duplex (TDD)
• Frequency Division Duplex (FDD)
• Durasi Frame : 2.5 - 20ms 29

Generic LTE Frame Structure type 1 (FDD)
Tf = 307200 x Ts = 10 ms

Tslot = 15360 x Ts = 0.5 ms

• Untuk struktur generik, frame radio 10 ms dibagi dalam 20 slot yang sama berukuran 0.5 ms.
• Suatu sub-frame terdiri dari 2 slot berturut-turut, sehingga satu frame radio berisi 10 sub-frame.
• Ts menunjukkan unit waktu dasar yang sesuai dengan 30.72 MHz.
• Struktur frame tipe-1 dapat digunakan untuk transmisi FDD dan TDD.
30

LTE Frame Structure type 1 (FDD)

• 2 slots form one subframe = 1 ms
• For FDD, in each 10 ms interval, all 10 subframes are available for downlink transmission and uplink transmissions.
• For TDD, a subframe is either located to downlink or uplink transmission. The 0th and 5th subframe in a radio frame is
always allocated for downlink transmission.
31

Downlink LTE Frame Structure type 1 (FDD)

Generic LTE Frame Structure type 2 (TDD)

• Struktur frame tipe-2 hanya digunakan untuk transmisi TDD.
• Slot 0 dan DwPTSdisediakan untuk transmisi DL, sedangkan slot 1 dan UpPTS disediakan untuk transmisi
UL.
33

LTE Frame Structure type 2 (TDD)

34

Mobile WiMAX Frame Structure

35

LTE Frame Structure type 2 (TDD)

DL Peak rates for E-UTRA FDD/TDD
frame structure type 1
Downlink
64 QAM
Assumptions Signal overhead for reference signals and
control channel occupying one OFDM symbol
Unit Mbps in 20 MHz b/s/Hz
Requirement 100 5.0
2x2 MIMO 172.8 8.6
4x4 MIMO 326.4 16.3

UL Peak rates for E-UTRA FDD/TDD
Uplink
Single TX UE
Assumptions Signal overhead for reference signals and control
channel occupying 2RB
Unit Mbps in 20 MHz b/s/Hz
Requirement 50 2.5
16QAM 57.6 2.9
64QAM 86.4 4.3

Peak rates for E-UTRA TDD

Downlink Uplink
Single TX UE,
Assumptions 64 QAM, R=1
64 QAM, R=1
Mbps Mbps
Unit b/s/Hz b/s/Hz
in 20 MHz in 20 MHz
Requirement 100 5.0 50 2.5
2x2 MIMO in DL 142 7.1
62.7 3.1
4x4 MIMO in DL 270 13.5

3GPP TR 25.912
Technical Specification Group Radio Access Network;
Feasibility study for
evolved Universal Terrestrial Radio Access (UTRA)
and Universal Terrestrial Radio Access Network (UTRAN)

Release Freeze meeting Freeze date ::
Rel-7 RP-33 2006-09-22 ::
event version available
RP-27 0.0.0 2005-03-03
RP-31 0.0.4 2006-03-20
draft 0.1.0 2006-03-20
draft 0.1.1 2006-03-20
post RP-31 0.1.2 2006-03-30
R3-51b 0.1.3 2006-05-02
draft post Shanghai 0.1.4 2006-05-22
draft 0.1.5 2006-07-10
draft 0.1.6 -
draft 0.1.7 2006-05-29
RP-32 0.2.0 2006-06-12
RP-32 7.0.0 2006-06-23
RP-33 7.1.0 2006-10-18
RP-36 7.2.0 2007-08-13

3GPP TR 25.912
Technical Specification Group Radio Access Network;
Feasibility study for
evolved Universal Terrestrial Radio Access (UTRA)
and Universal Terrestrial Radio Access Network (UTRAN)
Rel-8 SP-42 2008-12-11 :: . ETSI
event version available remarks
RTR/TSGR-
SP-42 8.0.0 2009-01-02 Upgraded unchanged from Rel-7
0025912v800
Upgraded to Rel-9 with no technical change to enable
Rel-9 SP-46 2009-12-10 :: reference related to ITU-R IMT-Advanced submission ETSI
(reference in 36.912). .
RTR/TSGR-
RP-45 9.0.0 2009-10-01 Technically identical to v8.0.0
0025912v900
Upgraded from previous Release without technical
Rel-10 SP-51 2011-03-23 :: ETSI
change .
RTR/TSGR-
SP-51 10.0.0 2011-04-06 Automatic upgrade from previous Release version 9.0.0
0025912va00
Upgraded from previous Release without technical
Rel-11 SP-57 2012-09-12 :: ETSI
change .
SP-57 11.0.0 2012-09-26 Automatic upgrade from previous Release version 10.0.0 -

LTE Uplink Transmission Scheme: SC-FDMA
• Pemilihan OFDMA dianggap optimum untuk memenuhi persyaratan LTE
pada arah downlink, tetapi OFDMA memiliki properti yang kurang
menguntungkan pada arah Uplink.
• Hal tsb terutama disebabkan oleh lemahnya peak-to-average power ratio
(PAPR) dari sinyal OFDMA, yang mengakibatkan buruknya coverage uplink.
• Oleh karena itu, skema transmisi Uplink LTE untuk mode FDD maupun TDD
didasarkan pada SC-FDMA, yang mempunyai properti PAPR lebih baik.
• Pemrosesan sinyal SC-FDMA memiliki beberapa kesamaan dengan
pemrosesan sinyal OFDMA, sehingga parameter-parameter DL dan UL
dapat diharmonisasi.
• Untuk membangkitkan sinyal SC-FDMA, E-UTRA telah memilih DFT-
spread-OFDM (DFT-s-OFDM).

43

OFDMA and SC-FDMA
• The symbol mapping
in OFDM happens in
the frequency
domain.
• In SC-FDMA, the
symbol mapping is
done in the time
domain.
• Appropriate
subscriber mapping
in the frequency
domain allows to
control the PAPR.
• SC-FDMA enable
frequency domain
equalizer approaches
like OFDMA

44

Comparison of how OFDMA and SC-FDMA
transmit a sequence of QPSK data symbols

45

Comparison of how OFDMA and SC-FDMA
transmit a sequence of QPSK data symbols
Creating the time-
domain waveform of an
SC-FDMA symbol

Baseband and shifted
frequency domain
representations of an
SC-FDMA symbol

46

How to generate SC-FDMA?
• DFT “pre-coding” is performed on modulated data symbols to
transform them into frequency domain,
• Sub-carrier mapping allows flexible allocation of signal to available
sub-carriers,
• IFFT and cyclic prefix (CP) insertion as in OFDM,

• Each subcarrier carries a portion of superposed DFT spread data
symbols, therefore SC-FDMA is also referred to as DFT-spread-
OFDM (DFT-s-OFDM).

How does a SC-FDMA signal look like?

• Similar to OFDM signal, but…
– …in OFDMA, each sub-carrier only carries information
related to one specific symbol,
– …in SC-FDMA, each sub-carrier contains information of ALL
transmitted symbols.

SC-FDMA signal generation
Localized vs. distributed FDMA

SC-FDMA – Peak-to-average Power Ratio (PAPR)

Comparison of CCDF of PAPR for IFDMA, LFDMA, and OFDMA with M = 256 system subcarriers,
N=64 subcarriers per users, and a = 0.5 roll factor; (a) QPSK; (b) 16-QAM

Source:
H.G. Myung, J.Lim, D.J. Goodman “SC-FDMA for Uplink Wireless Transmission”,
IEEE VEHICULAR TECHNOLOGY MAGAZINE, SEPTEMBER 2006

SC-FDMA parameterization (FDD and TDD)
LTE FDD
•Same as in downlink

TD-LTE
•Usage of UL depends on the selected UL-DL configuration (1 to 8), each
configuration offers a different number of subframes (1ms) for uplink
transmission,
•Parameterization for those subframes, means number of SC-FDMA symbols
same as for FDD and depending on CP,
51

Improved UL Performance
SC-FDMA compared to ordinary OFDM

Single-carrier transmission in uplink enables low PAPR that gives more 4 dB better link
budget and reduced power consumption compared to OFDM
52

LTE Uplink SC-FDMA Physical Layer Parameters

53

Physical Channel Processing

• Scrambling: Scramble binary information
• Modulation Mapper: Maps groups of 2, 4, or 6 bits onto QPSK, 16QAM, 64QAM symbol constellation points
• Transform Precoder: Slices the input data vector into a set of symbol vectors and perform DFT transformation.
• Resource Element Mapper: Maps the complex constellation points into the allocated virtual resource blocks
and performs translation into physical resource blocks.
• SC-FDMA Signal Generation: Performs the IFFT processing to generate final time domain for transmission.

54

SC-FDMA and OFDMA Signal Chain
Have a High Degree of Functional Commonality

Cyclic
Single Carrier S/P Symbol M-Point Subcarrier
Bit N-Point Prefix &
RFE
Constellation Convert Mapping
Stream Mapping Block DFT IDFT Pulse
Shaping

Channel

Freq Cyclic
Bit Const. SC S/P Symbol M-Point N-Point
De-map Detector Convert Block IDFT Domain Prefix RFE
Stream DFT
Equalizer Removal

Functions Common to OFDMA and SC-FDMA

SC-FDMA Only

55

What is MIMO?
• MIMO: Multiple input – multiple output
• Given an arbitrary wireless communication system:
– ”A link for which the transmitting end as well as the receiving end is
equipped with multiple antenna elements”

• The signals on the transmit antennas and receive antennas are
”combined” to improve the quality of the communication (ber
and/or bps)
• MIMO systems use space-time processing techniques
– Time dimension is completed with the spatial dimension

Different gains of multiple antenna systems
• ”Smart antenna” gain
– Beamforming to increase the average signal-to-noise (SNR)
ratio through focussing energy into desired directions
• Spatial diversity gain
– Receiving on multiple antenna elements reduces fading
problems. The diversity order is defined by the number of
decorrelated spatial branches
• Spatial multiplexing gain
– A matrix channel is created, opening up the possibility of
transmitting over several spatial modes of the matrix
channel increasing the link throughput at no additional
frequency, timer or power expenditure

Multiple antenna fundamentals
Recovered data stream

Data
Tx antenna ports

Channel

Data
Rx antenna ports

Data stream


Data
Tx antenna ports

Data N transmit antennas Rx antenna ports

h11 h12 h13 h14
Data stream M receive
H h21 h22 h23 h24
antennas
h31 h32 h33 h34

Channel matrix


Data
Tx antenna ports

A1
A2
A3
A4
Data
Rx antenna ports

Data stream

Spatial multiplexing Recovered data stream

Data
Tx antenna ports

Data
Rx antenna ports

h11 h12 h13 h14
Data stream
The different data H h21 h22 h23 h24
streams are divided in
space
h31 h32 h33 h34

rank(H) determines how many streams are possible to transmit

Transmit diversity Recovered data stream

Data
Tx antenna ports

A1
A2
A3
A4
Data
Rx antenna ports

Data stream

Redundancy:
The data streams contain
the same data

Beamforming Recovered data stream

Data
Tx antenna ports

A1
A2
A3
A4
Data
Rx antenna ports

Data stream
Only the best spatial
channel is used to maximize
C/N

Fundamental limits of wireless transmission

• Shannon capacity of Wireless C log 2 (1 )
Channels: C log 2 (1 h )
2

– h is the unit power complex Gaussian
amplitude of the channel
• h is a random variable
C log 2 (1 hh * )
– Multiple antennas at one end:
C log 2 det I M HH *
– Capacity of MIMO Links: N

• Average capacity Ca
PC Co 99 .9..9%
• Outage capacity Co

65 01.03.2012

Shannon capacity of Wireless Channels
Ideal Rayleigh Channel
2
C log 2 (1 h )

C log 2 det I M HH *
N

C log 2 (1 hh * )

66 01.03.2012

Data transmission over MIMO systems
• Two main categories:
– Data rate maximization
• Sending as many independent signals as antennas
• Spatial multiplexing
– Diversity maximization
• The individual streams can be encoded jointly
• Protect against transmission errors caused by channel
fading
• Minimize the outage probability

Maximizing diversity with space-time
block codes
• Alamouti’s scheme:
– The block of symbols s0 and s1 is coded across time and space
*
– Normalization factor ensures total energy to be the same the
case of one transmitter
1 s0 s1
• Reception:
C *
– The receiver collects the observation, y, over two symbol 2 s1 s0
periods

T
ˆ 1 h0 h1
s s0 s1 h h0 h1 H * *
2 h1 h0
n
*
s0 , s Tx0 h0 y0 y1 n h C n
1

Rx
y0 *
y1
T
ˆ
n H s n
s1, s
*
0 Tx1 h1

Spatial multiplexing
Y HC N
• Extending the Space-
Time Block Coding C H Y
– Transmitting
independent data
over different
antennas
– The receiver must
un-mix the channel
– Limited diversity
benefit

Spatial multiplexing - decoding
• Zero-forcing (ZF)
Y HC N
– Inverting matrix H
– Simple approach ˆ
C H 1Y
– Dependent on low-correlation in H

• Maximum likelihood (ML)
– Optimum ˆ min ˆ
– Comparing all possible combination with C arg ˆ Y HC
C
the observation
– High complexity

• Nulling and cancelling
– Matrix inversion in layers
– Estimates one symbol, subtracts and
continues decoding successively

Transmission scheme performance
• Same transmission
rate

– Alamouti

– Spatial multiplexing
– zero forcing

– Spatial multiplexing
– maximum
likelihood

– Combined STBC
spatial multiplexing

Wireless channel modelling
• The promise of high MIMO capacities largely relies on the
decorrelation properties:
– Between antennas
– Full-rankness of the MIMO channel matrix H
• E.g. spatial multiplexing becomes completely inefficient if the channel
has rank 1

• Aim of channel modelling:
– Get an understanding of what performance can be reasonably
expected form MIMO systems
– To provide the necessary tools to analyze the impact of selected
antenna or propagation parameters
• Spacing, frequency, antenna height..
– To influence the system design in the best way

Wireless channel modelling
• Four approaches
– Theoretical Models
• E.g. the ”idealistic” channel matrix of perfectly uncorrelated
(i.i.d.) random Gaussian elements
– Heurestic Models
• In practice, MIMO channels will not fall completely into any
of the theoretical cases
– Broadband Channels
• Frequency selective fading is experienced a new MIMO
matrix is obtained at each frequency/sub-band
– Measured Channels
• Validate the models, provide acceptance of MIMO systems
into wireless standards

Theoretical channel models
• Ideal channel (i.i.d.):
– Corresponds to a rich multipath
environment

• Emphasizing the separate roles
– Antenna correlation (transmit or receive)
– Rank of the channel
• Uncorrelated High Rank (UHR aka i.i.d.)
• Correlated Low Rank (CLR)
– Antennas are placed too close to each other, or
– Too little angular spread at both transmitter
H g rx g tx u rx u*
*
tx
and receiver
• Uncorrelated Low Rank (ULR)
– ”pin-hole” model
H g rx g *
tx

Heuristic channel models
• Display a wide range of MIMO
channel behaviours through
the use of as few relevant
channel parameters as
possible, with as much realism
as possible
– What is the typical capacity of a
MIMO channel?
– What are the key parameters
governing capacity?
– Under what simple conditions
do we get full rank channel?
• The model parameters should
be controllable or measurable

Antenna correlation at transmitter or receiver

H R1/r2dr H0
• A MIMO channel with correlated ,

receive antennas:
– For ”large” values of the angle spread
and/or antenna spacing, R will
converge to the identity matrix
– For ”small” values of θr, dr, R becomes
H R1/r 2dr H0R1/t 2dt
rank deficient (eventually rank one) , ,

causing fully correlated fading
• Generalized model includes
correlation on both sides:

The double scattering model:
”pinhole” channels

• Uncorrelated low rank:
– Significant local scattering around both the BTS and the subscriber’s
antennas
– Local scatterer’s are considered as virtual receive antennas
• When the virtual aperture is small, either on transmit or receive, the rank of
the overall MIMO channel will fall

Broadband channels
• Frequency selective channels are
experienced
• MIMO capacity benefits OFDM systems
with MIMO
– Additional paths contribute to the
selectivity as well as a greater overall
angular spread
– Improving the average rank of the
MIMO channel across frequencies

H(f)

Measured channels

• Channel matrix is measured using multiple antennas at transmitter and
receiver
– Results confirm the high level of MIMO capacity potential, at least in urban and
suburban areas
– Eigenvalue analysis SISO
• A large number of the modes of MIMO channels can be exploited to transmit data

SNR mean value and difference 4x4
P Kvadraturen 01 15 21
30 800 MIMO

Capacity Mbits/s
NLOS 600
20
LOS
dB

400
10 2x2
200
MIMO
0 0
0 200 400 600 0 200 400 600
Route sample no. Route sample no.
RX= 10,14,12,16 TX= 2,6,1,5 Diversity gain, full CSI
ity < C-sum)

0
1 10
bility

System level issues:
optimum use of multiple antennas
• Multiple antenna usage is not new in mobile systems:
– Spatial diversity systems
• Different goals:
– Beamforming is optimum using a large number of closely
spaced antennas:
• Directional beamforming imposes stringent limits on
spacing, typically a half wavelength
• Best performance in line-of-sight (LOS)
– MIMO algorithms focusses on diversity or data rate
maximization:
• Antennas will use as much space as possible to realize
decorrelation between antennas
• Turning rich multipath into an advantage and lose the gain in LOS
cases

MIMO in mobile broadband
• A unfavourable aspect:
– Increased cost and size of the subscriber’s equipment
– Limits applicability on simple mobile devices

• A better opportunity:
– Wireless LAN modems – tablets - laptops

MIMO transmission schemes for LTE
• LTE supports downlink LTE Transmission modes
transmissions on one, two or four
cell-specific antenna ports 1 Single eNB antenna
– Up to two transport blocks can be 2 Tx diversity (SFBC)
transmitted simultaneously on up
to four layers 3 Open-loop SM
4 Closed-loop SM
• The use of multiple antennas in
the DL of LTE comprises several 5 Multi-user MIMO
modes 6 Beamforming
• The system adaptively switches 7 UE specific RS
between each mode to obtain
the best possible performance as
the propagation conditions vary

Downlink multi-antenna support in
LTE
• Up to 4x4 antennas on downlink 1 Single eNB antenna
– 8x8 on LTE-advanced 2 Tx diversity (SFBC)
• Single-user schemes 3 Open-loop SM
– Transmit diversity (2) 4 Closed-loop SM
– Spatial multiplexing (3, 4)
5 Multi-user MIMO
– Beamforming (6)
• Multi-user MIMO (5) 6 Beamforming
• A common physical layer architecture: 7 UE specific RS

code words layers antenna ports

Modulation Resource element OFDM signal
Scrambling mapper
mapper generation
Layer
Precoding
mapper
Scrambling mapper
mapper generation

Transmit Diversity with 2 Tx antennas
• Alamouti scheme
– Transmitted diversity streams are orthogonal:
Subcarrier (frequency)
Port (antenna)

y 0 (1) y 0 (2) x1 x2
* *
y1 (1) y1 (2) x2 x1
x1 x2

Antenna port 0

-x2* x1*

Antenna port 1

OFDM subcarriers

Downlink multi-antenna support in LTE

• Up to 4x4 antennas on downlink 1 Single eNB antenna
– 8x8 on LTE-advanced 2 Tx diversity (SFBC)
• Single-user schemes 3 Open-loop SM
– Transmit diversity (2) 4 Closed-loop SM
– Spatial multiplexing (3, 4)
5 Multi-user MIMO
– Beamforming (6)
• Multi-user MIMO (5) 6 Beamforming
• A common physical layer architecture: 7 UE specific RS

code words layers antenna ports

Scrambling mapper
mapper generation
Layer
Precoding
mapper
Scrambling mapper
mapper generation

Downlink spatial multiplexing for 2x2 antennas

• The number of codewords equals the transmission rank and codeword n is
mapped to layer n
• Rank one precoders are column subsets of the rank two precoders

1 0 1 1 1 1
, ,
0 1 1 1 j j
• Recommendations on transmission rank and which precoder matrix to use
is obtained via feedback from the subscriber equipment (UE)
– The base station (eNB) can override the rank recommended by the UE

• Codeword to layer mapping:

Codeword 1 Codeword 2
Rank 1 Layer 1
Rank 2 Layer 1 Layer 2
Rank 3 Layer 1 Layer 2 and 3
Rank 4 Layer 1 and 2 Layer 3 and 4

DL peak throughputs in LTE
64QAM Modulation

MIMO config

4 layer
Data rate (gross)

2 layer 326Mbps

245Mbps
Peak Throughput

1 layer 163Mbps
172.8Mbps
82Mbps
49Mbps 129.6Mbps
23Mbps
86.4Mbps
43.2Mbps 86.4Mbps
10.4Mbps 25.9Mbps
64.8Mbps
43.2Mbps
13Mbps 21.6Mbps
5.2Mbps

1.4 3 5 10 15 20
Carrier Bandwidth (MHz)

Downlink MIMO for HSPA (3G)
• HSPA supports downlink closed-loop MIMO rank 2

Other multiple antenna schemes
• Multi-user (MU-) MIMO
– Spatial multiplexing to different UEs in the same
cell
– Also called Spatial Division Multiple Access
(SDMA)

Summary
• MIMO is using multiple antennas at both transmitter and receiver ends to set up a
wireless link

• MIMO gains can be beamforming, diversity or spatial multiplexing

• Wireless link capacity can be multiplied by min(M,N)

• Data transmission exploits the spatial dimension by maximizing either data rate or
diversity

• Wireless channel modelling is a tool to get the necessary understanding of
perfoemence and be atool to analyze the impact of the design

• Optimum use of multiple antennas contain conflicting goals in the system
design, especially when it comes to antenna sizes and design

• Both HSPA and LTE enables practical use of MIMO

End of
15 December2012

Thank You

See you again at
16 December 2012

LTE Downlink OFDMA & MIMO Planning Guide

Empfohlen

Empfohlen

Weitere ähnliche Inhalte

Was ist angesagt?

Was ist angesagt? (20)

Andere mochten auch

Andere mochten auch (20)

Ähnlich wie LTE Downlink OFDMA & MIMO Planning Guide

Ähnlich wie LTE Downlink OFDMA & MIMO Planning Guide (20)

Mehr von Arief Gunawan

Mehr von Arief Gunawan (20)

Kürzlich hochgeladen

Kürzlich hochgeladen (20)

LTE Downlink OFDMA & MIMO Planning Guide

Hinweis der Redaktion