Advanced telecommunication systems

8/8/2019 Advanced telecommunication systems

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Advanced telecommunicationsystems

ar : mo e ne wor mens on ng

Salah Eddine El Ayoubi

October 2010


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objective: ensuring QoS in mobile networks

dimensioning for ensuring coverage

dimensioning for ensuring capacity

outline

2 Salah Eddine EL AYOUBI October 2010

GSM

UMTS

LTE

just before LTE: HSDPA

after LTE: LTE-A


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outline


GSM

UMTS

LTE


after LTE: LTE-A


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coverage targets

mobile operators have to ensure complete coverage:

minimize white zones

cover villages as well as cities

cover routes


limited power

loss due to propagation


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cellular networks

each base station covers a cell / sector

large cells required to reduce costs, however:

degraded QoS at cell edge: coverage problems

many users served: capacity problems



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QoS targets

coverage is not the only criterion:

QoS in coverage areas is important

QoS includes:

access rate

good communication probability

throughput


operator target: ensure coverage target and QoS

with lowest costs

operator dilemma:

low cost -> large cells -> more users in each cell -> more

spectrum needed

spectrum is limited and too costly


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What is spectrum ?

Radio waves are characterized by their frequency,

measured in Hertz (Hz)

f f


Spectrum is the continuous aggregation of these frequencies

VHF UHF SHF

30 MHz 300 MHz 3 GHz 30 GHz


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Main guidelines when managing spectrum

Spectrum shall beSpectrum shall beSpectrum shall beSpectrum shall be

usableusableusableusable (not all frequencies(not all frequencies(not all frequencies(not all frequenciesare valuable for every typeare valuable for every typeare valuable for every typeare valuable for every type

of radio access)of radio access)of radio access)of radio access)

coveragecoveragecoveragecoverage


FrequencyFrequencyFrequencyFrequency

(MHz)(MHz)(MHz)(MHz)400 1000 5000

Terminal

too big

overage

too smallCoveragefrequencies

Capacityfrequencies

Spectrum shallSpectrum shallSpectrum shallSpectrum shall

be managedbe managedbe managedbe managed

as efficientlyas efficientlyas efficientlyas efficiently

as possibleas possibleas possibleas possible


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How it works ?

f3 f3


f1 f1

f2

f2 f3 f1

f2


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High demand


Limited resource


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outline


GSM

UMTS

LTE


after LTE: LTE-A


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link budget

link budget objective

maximum distance between a user and its serving base station while guaranteeing

a given quality of service

equipment parameters propagation model cell rangereceived signals SINR



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equipment parameters

determine gains and losses due to equipments.

antenna gain GA:

directivity of antenna amplifies the signal in some directions. feeder loss LC:

due to the cable between amplifier and antenna.


B

due to the body of the user.

for an emitted power Pmax:

BF

AmaxLL

GPpoweruseful

=


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propagation model







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radio channel

channel variations are due to

pathloss attenuation

shadowing (slow fading)

fast fading

Path LossShadowingFast fading

Distance

Attenuation(dB) Path Loss

ShadowingFast fading

Distance

Attenuation(dB)


path loss is due to the distance between the transmitter and the receiver shadowing is due to the obstacles between the transmitter and the receiver

fast fading is due to multipath propagation (reflections on obstacles that createmultiple paths of the received signal)

for coverage dimensioning, focus is on the path loss, adding a margin forshadowing


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use of propagation models

Ptxpathloss

C

pathloss

Ptx

I


propagation models allow to compute:

The received signal power ( coverage maps)

The interfering power ( QoS maps) a propagation model is the first building block of (almost) any radio

planning tool

Serving BS Interfering BS


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path loss models

free space propagation

only valid for line of sight, without multiwithout multiwithout multiwithout multi----pathpathpathpath

22

44

=

=

cDfDPathloss

D


these conditions are not met in cellular networks

statistical models (e.g. Okumura-Hata)

simple models with A & B statistically tunedfor typical environments (urban, etc.)

no geographical data required

useful for dimensioning

( ) 4020withlog][ += BDBAdBPathlosse.g. urban environment

D


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received signals







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received signals

for a user situated at distance d from a base station:

)(10

10

dPLLL

GPpowerreceived

BF

Amax

=


PL(d)=path loss at distance d shadowing variable


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SINR







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interference in the dowlink

interference is received by the mobile

from the base stations:

it depends on the position of the

mobile in the cell cell-edge users are subject to

higher interference because they

are closer to interferers.


observations: the origin of interference is well

defined.

the intensity of this interference

is to be calculated.


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interference in the uplink

interference is received by the base

station from the mobiles in adjacent

cells:

it is independent from theposition of the mobile in the cell.

it depends on the distribution of

mobiles in interfering cells.


observations: the average interference is

uniform for all mobiles.

the position of interferers is

unknown.


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SINR calculations

collisions decrease the Signal to Interference Ratio (SINR):

noiseceinterferenreceived

powerreceivedSINR

+=


a lower SINR means a larger Bit Error Rate (BER):

degraded QoS


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cell range







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example coverage of a cell

exercise



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outline


GSM

UMTS

LTE


after LTE: LTE-A


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Erlang-like capacity

need to install resources:

until a target Quality of Service (QoS) is achieved for users

example: number of frequency carriers per cell user perceived QoS includes:

blocking rates for real-time calls

-


-

this is called Erlang-like capacity:

reference to mathematician Agner Krarup Erlang

example Erlang-B law.


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Erlang-B law

probability of call loss:

B=blockin rate 5055606570758085

9095

1000.0001 0.001 0.01

N

Erlang table


E=traffic intensity C= number of circuits

Each call uses one

circuit 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 805

101520253035

40

85

A simple Erlang calculator can be found at:

http://perso.rd.francetelecom.fr/bonald/Applets/erlang.html


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the race for bit rates in mobile networks

WIDE AREAMOBILITY

1995 2000 2005 2010

GSMGPRS 4G?

EDGE UMTS LTEHSPA

+

HSDPA

Mobile TV

HSUPA

WIDE AREAMOBILITY

Mobility

1995 2000 2005

4G?EDGEUMTS LTE

HSPA

+

HSDPA

DVB-x


SHORT RANGE

MOBILITY

Data Rate

10kbps 100kbps

FIXED

WLAN

Fix

1Mbps 10Mbps 100Mbps

. m

Data Rate

10kbps 100kbps

FIXED

WLAN

Fixed

Wimax

1Mbps 10Mbps

. m


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outline


GSM

UMTS

LTE


after LTE: LTE-A


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GSM operation

the spectrum assigned to GSM is divided into sub-bands of

200 KHZ each.

the subbands cannot be used in adjacent cells

due to inter-cell interference

a frequency reuse map is necessary


1/3 of sub-bands used in each cell 1/7 of sub-bands used in each cell

a transmitter (a dedicated amplifier) is necessary for each sub-

band in the cell.


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Time Division Multiple Access operation

several frequency sub-bands of 200 KHZ each

each sub-band is allocated for different users at different times

the time frame of 4.62 ms is divided into 8 time slots

but the transmitter serves up to 7 users (one TS for signalling)


Transmitters

Time slots


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example capacity of a GSM cell

exercise



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outline


GSM

UMTS

LTE

just before LTE: HSDPA after LTE: LTE-A


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outline: UMTS

physical layer

admission control

capacity calculations



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Code Division Multiple Access

everybody transmits at the same

time-frequency resources.

each transmitter has its own code

the receiver decodes the signal and

views the others' signals as residual

interference.



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spreading process



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downlink spreading codes

Walsh code:

W(0,1) = 1

W(0,2) = 1, 1

W(1,2) = 1,-1

W(0,4) = 1, 1, 1, 1

W(1,4) = 1,-1, 1,-1

W(2,4) = 1, 1,-1,-1

W(3,4) = 1,-1,-1, 1

W(0,8) = 1, 1, 1, 1, 1, 1, 1, 1

W(1,8) = 1,-1, 1,-1, 1,-1, 1,-1

W(2,8) = 1, 1,-1,-1, 1, 1,-1,-1

W(3,8) = 1,-1,-1, 1, 1,-1,-1, 1


W(4,8) = 1, 1, 1, 1,-1,-1,-1,-1

W(5,8) = 1,-1, 1,-1,-1, 1,-1, 1W(6,8) = 1, 1,-1,-1,-1,-1, 1, 1

W(7,8) = 1,-1,-1, 1,-1, 1, 1,-1

orthogonal codes, as synchronous transmissions

problem: multipath propagation that introduces delays


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uplink spreading codes (1/2)

Maximum Length (ML) sequence

sequence determined by the XOR feedbacks.

if register of length R, sequence of period L=2R-1

XOR of a sequence with a shifted version of it

gives another version of the same ML sequence.

characterized by irreductible polynom:

ak


=+=

+=+

+=+=

Rk

k

Rk

k

Rk

k

Rk

k

kndajncncnd

jknckncajncnc

jkncajnckncanc

1

2mod

1

2mod

1

2mod

1

2mod

)()()()(

)()()()(

)()(),()(

=

Rk

kkxaxf

0

2mod

)(


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uplink spreading codes (2/2)

inter-correlation between ML sequences may be large.

for obtaining good correlation properties, Gold codes are

generated by EXOR-ing some preferred pairs of ML-sequences

Gold demonstrates that, if we choose carefully two MLsequences of length L=2R-1, characterized by polynoms f(x)

and g(x), such that inter-correlation is low, the ML sequences of

=


not orthogonal but with low correlation for cases wheretransmitters are not synchronized

R=6

f(x)=x6+x+1, g(x)=x6+x5+x2+x+1

z(x)=x12+x11+x8+2x7+3x6+x5+x3+2x2+2x+1

=x12+x11+x8+x6+x5+x3+1

sequence of 22R-1, divided into 2R+1 sequences of length L.


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dealing with inter-cell interference


scrambling codes (Gold code) separate also cells in thedownlink.

inter-cell interference is reduced as if it were a transmission

from the same cell.


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outline: UMTS

physical layer

admission control




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cell decomposition

1. the SINR for a mobile depends on

the distance r0 from the BS, as

inter-cell interference increases at

cell edge.

2. to simplify the problem, divide the

cell into concentric rings

3. a mobile is thus charcterized b its


service and its position in the cell.

4. calculate powers and SINRs.

5. apply admission control: emitted

power< maximal power.

i d


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emitted power

zoneiis characterized by:

path loss qi,lwith celll

interference factor

service c characterized by target quality:

S: s readin factorc

cc

SINRS

SINR

.

+=

=

0 ,

0,

l li

ii

q

qF


multi-path propagation introduces a orthogonality factor a power PCom is used for signalling

adjacent cells have average load

number of users of class c in zoneiis Mi,c

the total transmitted power is

= =

= =

++

=n

i

C

c

cic

n

i

C

c

ciciiCom

tot

M

MqNFPP

P

1 1

,

1 1

,0max

)(1

))((

d i i t l


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admission control

power of base station limited by Pmax

admission control constraint:

Com

n

i

C

c

cicii PPMqNFPP ++ = =

max

1 1

,0maxmax ))((


intra-cell interference

intra-cell interference

noise

tli UMTS


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outline: UMTS

physical layer

admission control capacity calculations


capacit calc lations


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admission control constraint indicates that there is a resource

(power) shared by users of different demands

(position+service).

traffic c,i (Erlang) in zone i for class c.


-

= =

=C

c

n

i ic

Mic

nCMG

MMic

1 1 ,

,,1,1

!

1],...,Pr[

,



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Exercise


outline


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outline


GSM UMTS

LTE

just before LTE: HSDPA after LTE: LTE-A

outline: LTE


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outline: LTE

physical layer

throughput calculations capacity calculations

use case: mobile TV


Beyond 3G context and E-UTRAN requirements


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Beyond 3G context and E-UTRAN requirements

1 Tx antenna, 2 Rx antennas16 QAM modulation, code rate 5/6

56 Mbit/s

(71 Mbit/s for 64QAM)

Peak rate (Uplink)(in 20 MHz, FDD)

2 Tx and 2 Rx antennas- -

0.7 b/s/Hz/cell

Average cell spectrum

2 Tx and 2 Rx antennasMIMO transmission with linear

receiver

1.72 b/s/Hz/cell

(8.6 Mbit/s in 5 MHz)

Average cell spectrumefficiency (downlink)

2 Tx and 2 Rx antennas,64 QAM modulation, code rate 5/6

144 Mbit/sPeak rate (Downlink)(in 20 MHz, FDD)

Expected performance (based on analysis and simulations)

1 Tx antenna, 2 Rx antennas16 QAM modulation, code rate 5/6

56 Mbit/s

(71 Mbit/s for 64QAM)

Peak rate (Uplink)(in 20 MHz, FDD)

2 Tx and 2 Rx antennas- -

0.7 b/s/Hz/cell

Average cell spectrum

2 Tx and 2 Rx antennasMIMO transmission with linear

receiver

1.72 b/s/Hz/cell

(8.6 Mbit/s in 5 MHz)

Average cell spectrumefficiency (downlink)

2 Tx and 2 Rx antennas,64 QAM modulation, code rate 5/6

144 Mbit/sPeak rate (Downlink)(in 20 MHz, FDD)

Expected performance (based on analysis and simulations)


Assumptions:FDD, 30% retransmissions

~ 10 msUser plane latency(two way radio delay)

.

< 50 msecs (dormant->active)

< 100 msecs (idle ->active)

Connection setuplatency

Assumptions:FDD, 30% retransmissions

~ 10 msUser plane latency(two way radio delay)

.

< 50 msecs (dormant->active)

< 100 msecs (idle ->active)

Connection setuplatency

the 3M of Beyond 3G


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the 3M of Beyond 3G

similar principles are used by most beyond 3G air interfaces- the physics are the same for everybody !

Multi-carrier Frequency dimension

Allow for spectrum flexibility and higher bandwidths.

Data rate = Bandwidth [Hz] x Spectrum efficiency [bps/Hz]

Multi-antenna (MIMO)


Higher spectrum efficiencies

Information Theory:Max. spectrum efficiency increases linearly with the number ofantennas.

Multi-Layer

Cross-layer optimization (PHY, MAC, RLC) Packet oriented radio interface

Low latencies and higher spectrum efficiencies.

fast fading parameters (1/3)


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fundamental parameters of the fast fading channel

- delay spread (frequency selectivity)

- maximum delay: tmax

- coherence band: Bc = 1/tmax

- Bc=maximum bandwidth over which

Remote Scatterer

Terminal

Local-to-mobileScatterers


to experience correlated fast fading.- if the symbol duration is much largerthan tmax, impact of delay spread isnegligible.

Remote Scatterer

Basestation

tmax



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-Doppler spread (time selectivity)

- Mobile speed v

- serving frequency fC

- Maximum doppler: fD = fC x v/c0

- Coherence time: Tc = 1 / (2 fD)

Remote Scatterer

Terminal



- signal arrives at the receiver withinthe interval [fC-fD,fC+fD]

- if the baseband signal bandwidth ismuch greater than fD the effects of

Doppler spread are negligible.

Remote Scatterer

Basestation



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- angle spread (spatial selectivity)

- difference in angles of arrival/departure

- coherence distance is the maximumspatial separation over which the channelresponse can be assumed constant.

Remote Scatterer

Terminal



- or sma ang e sprea , co erence

distance is large

-for large angle spread, coherencedistance is small (e.g. in mobilecommunications).

Remote Scatterer

Base station

multi-carrier the frequency dimension


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q y

Orthogonal Frequency Division Multiplexing (OFDM)

Facilitates equalization at the receiver

Divides bandwidth in narrowband sub-carriers

Simple frequency domain equalization

OFDM Access (OFDMA) provides flexibility for resource allocation

Time-frequency resources can be allocatedency L1/L2

Control User A User Bency L1/L2

Control User A User B


to data and control channels

Various spectrum allocationscan be addressed with the same technology

Modified scheme may be needed in uplink

E-UTRAN uses Single Carrier FDMA (SC-FDMA)

Similar properties than OFDM, but allows for

cheap power amplifiers at the terminal.

TimeF

requ

Spectrumallocation

1.25 - 20 MHz

1ms sub-frame (LTE DL)

TimeF

requ

Spectrumallocation

1.25 - 20 MHz


multi-carrier the frequency dimension


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q y

OFDM parameters and signal design h*

0

IFFT

Symbolmapping

0

NC-1

.

.

.

+ TGP/S

Modulation

Coding

Modulation

Coding

User 1

User K

- TGFFT

S/P Symbol

de-mappingh*Nc-1


Nc narrowbandsub-carriers

design rules Avoid inter symbol interference: Guard interval (TG) > Maximum Channel delay (tmax) Avoid inter carrier interference: Carrier spacing (f=1/TS) >> max. Doppler spread (2fD)

Limit overhead and ensure time invariance: TG ~0.25TS, TS+TG


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q y

Basic parameters of E UTRAN Downlink

Frequenc

y L1/L2Control User A User B

Frequenc

y L1/L2Control User A User B




Multi-carrier the frequency dimension


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SC - FDMA signal design

IFFT

DFT

0

NC-1

.

.

.

+ TGP/S

ModulationCoding

User 1

- TG FF

T

S

/P

IDFT

0

g*0

g*N

User 1

IDFT

h*

0

h*N

User 2


SC - FDMA properties Lower Peak to Average Power Ratio

Flexible resource size in frequency

Contiguous resource allocation required

Some residual interference between users



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E-UTRAN uplink sub-frame format Same basic parameters as downlink

Contiguous resource allocation

Frequency hopping between slots

(half sub-frame) and betweensub-frames allowed for diversity.

Control only channels are

Fre

quency

Spectrum

allocation1.25 - 20 MHz L1/L2

Control

Modulated

User AFre

quency

Spectrum

allocation1.25 - 20 MHz L1/L2

Control

Modulated

User A


of the band. If data allocation exists

control is multiplexed with data

in the same resource.

1msNormal

Sub-frame

part of band

~ 60%

User B1msNormal

Sub-frame

part of band

~ 60%

User B



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frequency adaptive scheduling

Choose best time frequency resources

based on channel quality feedback

Additional scheduling dimensioncompared to HSDPA (time only)

Reliable feedback can only be obtained

for low speed users


interference coordination

Power restrictions allow for

soft/adaptive frequency re-use

Gains seen in particular for

varying load distributions1

2

3

4

5

6

7f

P(f)

f

P(f)

f

P(f)

Cell 1

Cells 2, 4, 6

Cells 3, 5, 7

f

P(f)

f

P(f)

f

P(f)

Cell 1

Cells 2, 4, 6

Cells 3, 5, 7

Multi-antenna the spatial dimension


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MIMO increases spectrum efficiency

NTX NRX


Theoretical Maximum: Spectrum Eff. = min(NTX, NRX) x Single antenna Eff.

Yes but

Additional antenna branches are costly especially on the terminal side

Achievable rates highly depend on propagation conditions

Mobile feedback required for high rates -> limitation of supported speeds

Different and adaptive solutions required depending on thedeployment scenario (coverage vs. rate trade-off).



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multi-antenna mechanisms in E-UTRAN downlink

Space diversitySpace diversitySpace diversitySpace diversity for improved robustness

of common control channels and

for users with high speed and/or low rate

BeamformingBeamformingBeamformingBeamforming for coverage

limited deployments

Spatial multiplexingSpatial multiplexingSpatial multiplexingSpatial multiplexing for high rates near

A) Transmit diversity-> Increased robustness

B) Beamforming-> Increased coverage


Adaptive selection of number of layers.

Spatial multiplexing of usersSpatial multiplexing of usersSpatial multiplexing of usersSpatial multiplexing of users in scenarios

with high user density and low rate traffic

Only single antenna transmission considered in E-UTRAN uplink

Spatial multiplexing of userswith multiple antennas at the

base station receiver.

C) Spatial multiplexing-> Increased throughput

D) Multi-user beamforming (SDMA)-> Increased capacity



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Transmit diversity

Space diversity takes advantage of spatial

de-correlation to mitigate fast fading

Large antenna spacing or cross-polarized setups are preferred.

Receive diversity does not require a specific scheme and

A) Transmit diversity


, .

Transmit diversity schemes rely on redundancy transmitted from thedifferent antennas and can work with single receive antenna.

Low correlation between antennas is essential since no power gain

is achievable at the transmitter (power is distributed over antennas).

Space-Time Block Codes (or Space-Frequency Block Codes withOFDM) are low complex transmit diversity schemes.



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Transmit diversity in E-UTRAN

Transmit diversity can be applied to all downlink

channels in E-UTRAN (broadcast, control, data)

Basic scheme is Space Frequency Block Coding (SFBC)

Orthogonal encoding avoids interference between symbols and

simplifies the receiver (linear receiver is sufficient)

A) Transmit diversity


Transmit diversity can be combined with multi-layer transmission

using so-called cyclic delay diversity (CDD).



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Beamforming

Beamforming concentrates energy toincrease transmission rates at cell edge.

Small antenna spacing and spatially correlated fading (small anglespreads) are preferred.

Channel state information (CSI) needed at transmitter

B) Beamforming


,

CSI can be obtained from uplink estimations (in particular in TDDsystems) or from terminal feedback (costly).

Beamformed dedicated (user specific pilots) are needed to enablechannel estimation at the terminal.

Broadcast and control channels cannot be beamformed.

DL Coverage is determined by these channels

Common reference signals are needed for broadcast & control.

Calibration of antenna arrays is a practical technical challenge.



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Single-user approach

maximisation of the

SNR.

implicit interference

reduction

knowled e of user DoA

Beamforming illustrated:


Antenna: ULA,M= 8Users: 2 (Car: 1 DOA/ Phone: 2 DOAs)

Multi-user approach Maximisation of the

SINR.

Explicit interference

reduction Knowledge of all DoAs



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Beamforming in E-UTRAN

Dedicated reference signals for a

single stream are supported.

Terminal estimates CQI from common reference signals,

BS estimates beamforming gain for link adaptation.

B) Beamforming


Codebook based pre-coding (~fixed beams) is supported and can

also be combined with multi-layer transmission.

Mobile feeds back index of preferred pre-coding vector and can

obtain channel estimates from common pilots multiplied by knownpre-coding vector.



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Spatial multiplexing

Exploits good channel conditions to

transmit via parallel layers.

Prefers rich scattering and un-correlated fading

(large antenna spacing's or cross-polarized setups)

C) Spatial multiplexing

-> Increased throughput


Receiver needs as many antennas as layers to be received.

FECPre-

codingwN

Mod.

FEC

Pre-coding

w1Mod.

N spatiallayers

M Tx-antennas

CQI feedbackfor link adaptation Feedback of

pre-coding vector index

FECPre-

codingwN

Mod.FECPre-

codingwN

Mod.

FEC

Pre-coding

w1Mod.FEC

Pre-coding

w1Mod.

N spatiallayers

M Tx-antennas

CQI feedbackfor link adaptation Feedback of

pre-coding vector index



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Spatial multiplexing receiver

Serial Interference Cancellation (SIC) receiver:

Detect first codeword, if CRC correct re-generate interference contribution

and subtract before decoding second codeword,




SerialInterferenceCancellation

Symboldetection

SpaceTime

LMMSE

Source: A. Saadani



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Spatial mutliplexing in E-UTRAN

Up to 2 codewords per user.

Coverage vs. Rate trade-off:




Source: Ericsson



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Multi-user MIMO

Different layers can be transmittedto different users in downlink.

E-UTRAN uses same codebook as for single user multiplexing. Challenge to estimate CQI at terminal, since potential interference of

other users is not known in advance.

D) Multi-user beamforming (SDMA)

-> Increased capacity


Multi-user MIMO can enhance capacity in the uplink. Transparent to the UE, only separable reference signals need to be

used.

Multi-user MIMO is only useful for medium/low rate serviceswith very high user densities.

Control signaling will become the limiting factor for user capacity.

Multi-layer packet oriented radio


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Fast packet scheduling in E-UTRAN

Reduced transmission interval of 1ms

Fast packet scheduling

Fast link adaptation and cross-layer design Benefits

Reduced latenc


Performance gains from adaptive configuration and multi-user diversity

Yes but

Amount of signaling is increased -> higher overheads

Robustness to feedback errors and high velocities

Multi-layer packet oriented radio


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Cross-layer design (Layer 1 Layer 2)

Time

Fast fading

~~~~AchievableThroughput

User 1User 1User 1User 1


Fixed ressourceallocation

userthroughputTransmission time

Time

Fast fading




Fixed ressourceallocationFixed ressourceallocation

userthroughputTransmission time

Circuit oriented andlayered design


Usage of terminal feedback for resource allocation and phy-layer configuration

Cross-layer mechanisms already implemented in HSDPA.

Extension to frequency adaptive scheduling and adaptive MIMO transmission

Time

Fast fading




Intelligentschedulingwith feedback

globalthroughput

Multi-userdiversity gain

bad

good

Time

Fast fading




Intelligentschedulingwith feedback

globalthroughput

Multi-userdiversity gain

bad

good

Packet oriented andcross layer design

Uplink power control in E-UTRANInterference


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Data interference

Intra-cell power control

Inter-cell power control

Data interference

Intra-cell power control

Inter-cell power control

coordination


Combination of open loop power control with closed loop adjustments

Closed loop updates are send les frequently than for UMTS (


78/111

Basic formula implemented in the terminal:

P = min ( Pmax , 10 log M + Po + x PL + delta_mcs + f(delta_i))

Po : UE specific offset

: Fractional Path-Loss compensation (cell specific)

M : the number of assi ned RBs in the u link rand onl for data


delta_mcs : MCS specific correction delta_i : cumulative or absolute correction value per UE signalled in the

UL grant (data channels) or periodically (control channels)

Discussion still ongoing in particular on the interactions with interecell

coordination


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what is interference in OFDMA?


80/111

no intra-cell


n er erence

inter-cell interferenceis due to collisions

between chunks

interference calculations


81/111

Exercise


link budget for throughput calculations


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equipment parameters propagation model throughputreceived signals SINR


link level curves


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provide throughput vs SNR

curves according to:

multiple antenna use

(SISO, MIMO) channel model (AWGN,

Vehicular A, ..)


main output


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stand-alone user throughput as a function of the distance to the base station

DL Cell Throughput versus Distance

18000

Max throughput


0

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6000

8000

10000

12000

14000

16000

0,000 0,050 0,100 0,150 0,200 0,250

Distance (Km)

D

LCellThroughput(K

bps)

Throughput @ cell edge

application: impact of some design parameters


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inter-site distance impact onDL average cell throughput:

when the cell is larger, a

larger proportion of users is

at cell edge neighboring cell load impact

on DL average cell throughput:

DL average cell throughput vs ISD

3.5

4.0

4.5

5.0

5.5

6.0

DLaveragecell

throughput(Mbps)


when the load of

neighboring cells increases,inter-cell interference

increases

DL average cell throughput vs DL load

4.0

6.0

8.0

10.0

12.0

14.0

16.0

0% 20% 40% 60% 80%DL load (%)

DLa

veragecell

throug

hput(Mbps)

Inter-site distance (km)

outline: LTE


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physical layer

throughput calculations


use case: mobile TV


how can link budget help capacity analysis?


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link budget gives the throughput vs distance:

throughput depends on position

cell can be decomposed into rings:

To simplify analysis

Homogeneous throughput in each ring

87Salah Eddine EL AYOUBI October 2010


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8000

10000

12000

14000

16000

18000

0,000 0,050 0,100 0,150 0,200 0,250

Distance (Km)

DLCellT

hroughput(Kbps)

voice traffic: multi-Erlang analysis


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Consider voice traffic

Calls arrive with Poisson rate

Stay in communication for an average time T=3min

Require each 20 Kbps, or are blocked otherwise.

Example: 2 rings

88

Salah Eddine EL AYOUBI October 2010

,

One cell center (edge) user occupies 2% (4%) of the resources

Admission control constraint: 2*Kcenter+4*Kedge


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Consider best effort traffic

Calls arrive with Poisson rate

Stay connected until transmitting a file of average size 1 Mbits

Example: 2 rings

1 Mbps for cell center, 500 Kbps for cell edge

89

Salah Eddine EL AYOUBI October 2010

in the cell until transmitting its file

the time necessary for the two users to transmit their files is 1+2=3

seconds

Within these three seconds, the volume of data transferred is equal

to 2 files= 2 Mbit.

The average throughput of the cell is then:T=2 Mbit/3 second=667 Kbps

Best effort traffic: Arithmetic versus harmonic mean


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The arithmetic mean of the throughput is:

Tarith=(1 Mbps+0.5 Mbps)/2=750 Kbps

This is different from the average throughput calculated

previously.

However, this corresponds to the harmonic mean:


harm= ps- + . ps - - = ps

This harmonic mean gives larger weights for cell edge users asthey stay longer in the cell

The harmonic mean is convenient to measure the cell

throughput

Best effort traffic: Harmonic mean calculations



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2000

4000

6000

8000

10000

12000

14000

16000

18000

DLCellThro

ughput(Kbps)


Represents the maximal traffic that can be carried by the cell.

Used since the paper of Bonald el al., 2003.

0

0,000 0,050 0,100 0,150 0,200 0,250

Distance (Km)

Best effort traffic: Processor sharing


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Objective:

Estimate QoS for a given traffic

Data users share the remaing resources

not used by streaming and voice ones (priority to

streaming/voice)

Fair in time but not fair in throu h ut


Processor sharing analysis can be used to assess capacity:

Several classes corresponding to the number of rings

Gives average individual throughput at each position of the cell.


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outline: LTE


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physical layer

throughput calculations


use case: mobile TV


Use case: TV traffic

mobile TV traffic expected to explode unicast too greedy in resources:


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mobile TV traffic expected to explode

TV traffic evolution

5

6

7

8

unicast too greedy in resources:

spectrum resources

4

5

6

MHz


0

1

23

4

2009 2010 2011 2012 2013

Erlang

0

1

2

3

2009 2010 2011 2012 2013

carrie

rsof5

15

broadcast solution

Point to Multipoint is the solution adapt to radio conditions


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Point to Multipoint is the solution adapt to radio conditions

QPSK 1/2

16QAM 1/2

16QAM 3/4

64QAM 3/4


transmit with QPSK

advantage: simple

drawback: suboptimal

transmit with 16QAM

advantage: optimal

drawback: needs

feedback

total broadcast: Single Frequency Network

if every body is watching TV


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if every body is watching TV

why not cooperating all base stations?


Interference is seen as a multipath propagation

drawback: tight synchronization between cells is needed

Weight function for the constructive portion of a received SFN signal:

Delay and multipath impact


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CP

CP uCP CP u

u

CP u

w delay if delay T

T T delayw delay if T delay T T

T

w delay if T T delay

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Advanced telecommunication systems