seminar SC 2011sc.enseeiht.fr/doc/Seminar_Escrig.pdf · Performance criterion–CAPACITY Spatial...

1Cooperative Communications - BE

COOPERATIVE COMMUNICATIONS

Benoît ESCRIG – ENSEIRB-MATMECA/IRIT

Outline

2

� Introduction

� Physical layer

� MAC layer

� Conclusion

Cooperative Communications - BE

Outline

3

� Introduction

� Physical layer

� MAC layer

� Conclusion


Wireless communications

4

Spatial diversity :

MIMO techniques

Increase the number of uncorrelated transmitted

signals (antennas)

Short term fading (Rayleighfading)Time spreading of the signal

(inter symbol interference)Time variance of the channel

Long term fading

(free-space loss, shadowing)

Power control at the MAC layer Reduce the probability of

MAC: Medium Access ControlMIMO: Multiple Input Multiple Output

Cooperative Communications - BE10-4

10-3

10-2

10-1

100

101

time

Channel Gains

|h1|²

|h2|²

|h1|²+|h

2|²

Example: receiver diversity (Rayleigh fading) h1

h2

signals (antennas)the MAC layer Reduce the probability of

having all of them experiencing a deep fade at the same time

Limitation of spatial diversity techniques: coherence distance

5

Miminal seperation between antennas: half-wavelenth

Example: h1

Constraint for the implementation of MIMO techniques: uncorrelated transmitted signals

Example:


Example: ZigBee // IEEE 802.15.4Half-wavelength: from 17 cm down to 6 cm

Solution: cooperative communications

(distributed spatial diversity techniques)

SD

R

WiFi: Wireless Fidelity

h2

Example: WiFi // IEEE 802.11nHalf-wavelength: 6 cm and 3 cm

Issues in cooperative communications

6

STEP 1 STEP 2

Cooperative communication approachMIMO approach

Cooperative Communications - BEMANet: Mobile Ad hoc NETwork

State of the art

7

� Pioneering works in 2003 by Laneman

� 1300 journal papers between 2003 and 2011 (march)

� 2010: relaying is implemented in WiMAX (IEEE 802.16j Multihop Relay) and LTE (Long Term Evolution) advanced standards


System model

8

Fixed Wireless Mesh Networks (WMNs)

MIMO issues for fixedrelay architectures

BASE STATION


Focus on one hop of the route (relays are subscriber stations)

STATION

RELAY STATION

SUBSCRIBER STATION

Outline

9

� Introduction

� Physical layer

� MAC layer

� Conclusion


Channel model : short term fading (Rayleigh fading), frequency flat fading channel

10

x yhwhxy +=

Cooperative Communications - BEAWGN: Additive White Gaussian Noise

Several communications on uncorrelated channels :

increased capacity (low robustness)

Same communication on uncorrelated channels :

increased robustness (low capacity)

MIMO Issues

Performance criterion – CAPACITYSpatial multiplexing gain r

11

M emittingantennas

N receivingantennas

Spectral efficiency R(SNR) (in b/s/Hz)SISO

MIMO

Capacity = R(SNR) x Bandwidth


( ) ( )SNRrSNRR log×≈

Spectral efficiency of the SISO link

noise

signal

P

PSNR =

SNR: Signal to Noise RatioSISO: Single Input Single Output

max(x)

SISO 1

MIMO min{M,N}

antennas

( )( )SNR

SNRRr

SNR loglim

+∞→=

Approximation for the high SNR regime

Performance criterion – ROBUSTNESSSpatial diversity gain d(r)

12

( ) ( )[ ]( )

rSNRprd out ,log

lim −=M emittingantennas

N receivingantennas

SISO

MIMO

Outage probability:

pout(SNR,r) = Pr[I<R(SNR)]I is the mutual information

Diversity orderfor a SISO link: 1


noise

signal

P

PSNR =

Approximation for the high SNR regime

( ) ( )[ ]( )SNR

rSNRprd out

SNR log

,loglim −=

+∞→

( ) ( )rdout SNRrSNRp

1, ≈

max[d(r)]

SISO 1

MIMO M NSNR (in dB)

( )rSNRpout ,

Target

Margin on the emitted power

SISOMIMO

antennasfor a SISO link: 1

Fixed Amplify-and-Forward (AF)

13

� Same power emittedby S and R

+= 02

2

1/ NEE ssβ

STEP 1 STEP 2

SD

R

D

RThe received

signal isamplified at the relay terminal

SDSDSD wxhy +=

SRSRSR wxhy += ( ) RDSRRDRD wyhy += β


SD

SD

max[d(r)] max(r)

Direct Transmission 1 1

Fixed AF w. 1 relay 2 1/2 Issue: trade-off

Selective Decode-and-Forward (DF)

14

STEP 1 STEP 2

SD

R RDecision based on a cyclic redundancycheck (CRC) or on the observed SNRat R

The receivedsignal is decoded

at the relayterminal

=+

=otherwise0

ˆ when xxwxhy RDRD

RD


SD

S

SRSRSR wxhy +=SDSDSD wxhy +=

at R

max[d(r)] max(r)


Selective DF w. 1 relay 2 1/2

Cooperation with (m-1) relays15

SD

Issues:

Allocating resources to relaysOptimizing the capacity-robusteness trade-off


max[d(r)] max(r)


Selective DF w. (m-1) relays m 1/m

SD

Issue: improving the multiplexing gain whilemaintaining the diversity gain

Optimizing the capacity-robusteness trade-off (channel coding, space time coding, opportunistic relaying)

Cooperative communications and channel coding

16

STEP 2

SD

R R

DATA | P1

Or

DATA | P1

DATA | P2

STEP 1

Or

P2


� Example: use of punctured codes

SD

S P2

HARQ: Hybrid Automatic Repeat reQuest

Similar approach in HARQ mechanisms

Cooperative communications and space time coding

17

STEP 2

SD

S

STEP 1

STC

D

(m-1) relays = (m-1) antennas

P time-slots

One relay

P > (m-1)


S S

max[d(r)] max(r)


Selective DF w. (m-1) relays + STC m 1/2

Issue: allocatingspace time codes to relays

One relay

Example: 3 transmitting antennas, 4 time-slots for 3 symbols

Opportunistic cooperation

18

STEP 2

SD

R R

STEP 1 STEP 3

R


SD

S S

max[d(r)] max(r)


Selective DF w. 1 relay +

Opportunistic relaying2 1

Issue: opportunistic relayingand multiple relays

Contribution

19

STEP 2

SD

S

STEP 1 STEP 3

SD D

Best Relay


S S S

max[d(r)] max(r)


Selective DF w. (m-1) relays + Opportunistic relaying

m 1

BWA’09WCNC’10

Issue: efficient selection of the best relay

To summarize

20

STEP 1 STEP 2

SD

R

SD

R

Main transmission schemes:

Fixed Amplify-and-ForwardSelective Decode-and-Forward


S S

Gains in capacity or robustness can beconverted in gains in power margin, bandwidth

Options:

One or several relaysChannel and/or space-time coding

Opportunistic relaying

Issues:

Allocating relays to direct transmissionsImpact of cooperative transmissions on the network

Outline

21

� Introduction

� Physical layer

� MAC layer

� Conclusion


Main task: allocating one or several relays to a source-destination terminal pair

22

Collection of cooperative

information:

Channel State Information (CSI): channel gains (SNR) between pairs of terminalsAdditional Parameters: available resources at the relay (spreading

Relay selection and resource

optimization:

Processing CSI

Notification of the result:

Broadcasting the address(es) of the relay(s) and additional parameters

S

DR ?

R ?

R ?


resources at the relay (spreading code, STC, power)

relay(s) and additional parameters

Issues:Distributed / CentralizedReactive /ProactiveReducing the signaling overhead

MeshTech’08

Example 1: Persistent Relay Carrier Sensing Multiple Access (PRCSMA)

23

� J. Alonso-Zarate et al. (PIMRC ’06)

Features:

Optimal multiplexing gain through on-demand cooperation (reactive)Distributed selection

S

DR ?

R ?


S

D

Ri

Rj

CFC CTS

RTS DATA CTS

RTS DATA

R ?R ?

RTS

CTS

DATA

ACK

Issues:

Relay collisionsUnknown number of relaysSuboptimal spatial diversity

Example 2: A Simple Cooperative Diversity Method Based on Network Path Selection

24

� Bletsas et al. (IEEE JSAC 2006)

Features:

Optimal spatial diversity gain through the forwarding of the best relayDistributed selection S

DR ?

R ?R ?Issues:

=22

,min DRSR

i

iihh

Tλ


S

D

Ri

Rj

F

F DATA

R ?

RTS

CTS

DATA

ACK

Issues:

Relay collisionsSuboptimal spatial multiplexing gain

CONTENTION PERIOD

Improvement: Chou et al. (PerCom 2007) Relay candidates are pre-selected

Contribution

25

STEP 2

SD

R

S

R

STEP 1 STEP 3

S D

Best Relay


WCNC’10

Addtitional features:

Pre-selection of relay candidatesSelection of relay candidates by splitting algorithmsNakagami-m channels

Features: on-demand cooperation and forwarding by the best relay (distributed selection)

MeshTech’10 // CCNC’11Extended version in Elsevier PHYCOM

Optimal spatial multiplexing gain

Optimal spatial diversity gain

S S S

Example 3: CoopMAC: A Cooperative MAC for Wireless LANs

26

� P.Liu et al. (IEEE JSAC, 2007)

Features:

Optimal spatial diversity gainCentralized approachSuited for wireless communications with infrastructure (WiMAX)

S

DR ?

R ?R ?


with infrastructure (WiMAX)

S

D

Ri

Rj

DATA

DATA

R ?

RTS

HTS

CTS ACK

Issues:

Suboptimal spatial multiplexing gainNot suited to networks with a routing layer

Each station maintains a CoopTable of potential relaysThe creation and updating of the CoopTable is done by passively listening to all ongoing transmissions

Contribution

27

STEP 2

SD

R

S

STEP 1 STEP 3

S D

Best Relay


Optimal spatial multiplexing gain

Optimal spatial diversity gain

Features: on-demand cooperation and forwarding by the best relay (centralized selection)

MASS’10Extended version in IEEE TWC (submitted)

S S S

To summarize

28

3 main functions must be implemented

Collection of CSISelection ProcessNotification of the result


S

DR ?

R ?

R ?

Notification of the result

Most of the MAC protocols rely on the MAC layer of the IEEE 802.11 standard

Main limitation: interoperability issue

Outline

29

� Introduction

� Physical layer

� MAC layer

� Conclusion


Cooperative transmissions

30

STEP 1 STEP 2

SD

R

SD

R


Main transmission schemes:

Fixed Amplify-and-ForwardSelective Decode-and-Forward

S S

Gains in capacity or robustness can beconverted in gains in power margin, bandwidth

Options:

One or several relaysChannel and/or space-time coding

Opportunistic relaying

Issues:

Allocating relays to direct transmissionsImpact of cooperative transmissions on the network

Cooperative set up

31

3 main functions must be implemented

Collection of CSISelection ProcessNotification of the result


S

DR ?

R ?

R ?

Notification of the result

Most of the MAC protocols rely on the MAC layer of the IEEE 802.11 standard

Main limitation: interoperability issue

Main issues

32

Activation of the cooperative mode

Example: central terminalsProactive mode and reactive mode

Impact of cooperative transmissions on network

Interaction with power control and rate

adaptation

Studies are limited to Rayleigh fading channels

Coding issues

Turbo-codesNetwork coding


Impact of cooperative transmissions on network

performance (overall throughput)

Increased number of contention areas due to relaysIncreased values of NAV (Network Allocation Vector) timers

Interoperability issue with legacy

terminals

Optimizing the relay selection

Estimating the number of collidingterminals

seminar SC 2011sc.enseeiht.fr/doc/Seminar_Escrig.pdf · Performance criterion–CAPACITY Spatial...

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