seminar sc 2011sc.enseeiht.fr/doc/seminar_escrig.pdf · performance criterion–capacity spatial...
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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
Cooperative Communications - BE
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:
Cooperative Communications - BE
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
Cooperative Communications - BE
System model
8
Fixed Wireless Mesh Networks (WMNs)
MIMO issues for fixedrelay architectures
BASE STATION
Cooperative Communications - BE
Focus on one hop of the route (relays are subscriber stations)
STATION
RELAY STATION
SUBSCRIBER STATION
Outline
9
� Introduction
� Physical layer
� MAC layer
� Conclusion
Cooperative Communications - BE
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
Cooperative Communications - BE
( ) ( )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
Cooperative Communications - BE
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
Cooperative Communications - BE
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
Cooperative Communications - BE
SD
S
SRSRSR wxhy +=SDSDSD wxhy +=
at R
max[d(r)] max(r)
Direct Transmission 1 1
Selective DF w. 1 relay 2 1/2
Cooperation with (m-1) relays15
SD
Issues:
Allocating resources to relaysOptimizing the capacity-robusteness trade-off
Cooperative Communications - BE
max[d(r)] max(r)
Direct Transmission 1 1
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
Cooperative Communications - BE
� 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)
Cooperative Communications - BE
S S
max[d(r)] max(r)
Direct Transmission 1 1
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
Cooperative Communications - BE
SD
S S
max[d(r)] max(r)
Direct Transmission 1 1
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
Cooperative Communications - BE
S S S
max[d(r)] max(r)
Direct Transmission 1 1
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
Cooperative Communications - BE
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
Cooperative Communications - BE
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 ?
Cooperative Communications - BE
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 ?
Cooperative Communications - BE
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
Cooperative Communications - BE
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
Cooperative Communications - BE
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 ?
Cooperative Communications - BE
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
Cooperative Communications - BE
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
Cooperative Communications - BE
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 Communications - BE
Cooperative transmissions
30
STEP 1 STEP 2
SD
R
SD
R
Cooperative Communications - BE
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
Cooperative Communications - BE
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
Cooperative Communications - BE
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