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Cooperative Communications
Neelesh B. Mehta
ECE Department
IISc, Bangalore
Collaborators:
Andreas Molisch (MERL), Ritesh Madan (Flarion), Raymond Yim (Olin College),
Hongyuan Zhang (Marvell), Natasha Devroye (Harvard), Jin Zhang (MERL),
Jonathan Yedidia (MERL), Vinod Sharma (IISc), Gaurav Bansal (IISc)
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Motivation Behind Cooperative Communications
Multiple antenna spatial diversity
using only single antenna nodes
Exploit two fundamental aspects
of wireless channels:
Broadcast
Multiple access
s
r1
dr2
r3
r4
Cooperative relays
d
s2
Two cooperative sources
s1h1d
h2d
h12
h1d
h4d
h2d
h3dhsd
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Outline
Various cooperation schemes
Cooperation in ad hoc networks
Cooperation in infrastructure-based networks
Cross-layer issues
Other interesting topics
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Cooperative Communication Schemes
Amplify and forward
Decode and forward
Estimate and forward
Possibilities: Orthogonal / Non-orthogonal cooperation
Coded / Uncoded cooperation
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Analysis of Basic 3 Node Scenario
Performance metrics
Outage
Power consumption
Diversity
BER (Coded/Uncoded)
d
s2
Two sources
s1h1d
h2d
h12
S1 transmits S2 transmits
d receives d receivesConventional
model
Tx
Rx
S1 tx S2 repeats S2 tx S1 repeats
d, S2 rx d rx d,S1 rx d rxCooperativesource model
Tx
Rx
[Laneman & Wornell, IEEE Trans. on Inf. Theory, 2004][Stefanov, Erkip, IEEE Trans. on Communications, 2004]
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Outage Analysis: Amplify and Forward
[1][1]
[2] [2]
sd dd
d rd sr rd r d
h wyx
y h h h w w
2
0
r
sr s
P
h P N
2 2
2
2 2
SNR SNRlog 1 SNR
SNR SNR
sr rd sr rd
AF sd sd
sr sr rd sr
h hI h
h h
d
r
s hsd
hrd
hsrx
yd
yr = hsr x + wr
222 2
2 2 2 2
2 11( , ) Pr
2 SNR
sr
sd sr
R
rd
out AF
rd
P SNR R I R
Relay power
constraint:
Tx. rate
Outage prob.
Diversity order = 2
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Outage Analysis: Decode and Forward
Case 1: Destination can decode only if relay decodes
rx x d rd d y h x w
2 2 21 min log 1 , log 1
2DF sr sd rdI SNR h SNR h SNR h
2
2
1 2 1( , ) Pr
R
out DF
sr
P SNR R I R
SNR
(Assume codeword level decoding)
Diversity order = 1
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Outage Analysis: Adaptive Decode and Forward
Case 2: Source forwards to destination instead of relay if SR channel is
poor
r
x x d rd d y h x w
22 2
2 2
1 2 1log 1 2 ,2
1log 1 , else
2
R
sd sr
DF
sd rd
SNR h hSNRI
SNR h SNR h
222 2
2 2 2 2
2 11( , ) Pr
2
R
sr rd out DF
sd sr rd
P SNR R I RSNR
(Similar results apply for non-orthogonal scheme in which source transmits
to destination in both time slots, and relay repeats in second time slot)
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DF Coded Cooperation: An Explicit Example
Codeword of N bits divided into two parts: N1 and N2
In next frame:
S2 relays N2 bits of S1 if it can decode it correctly Else, S2 sends its own N2 bits
[Hunter & Nosratinia, IEEE Trans. on Wireless Commn., 2006]
S1 bits S2 bits relay Inactive
Inactive S2 bits S1bits relay
S1
S2 Rx S1 bits
Rx S2 bits
N1 bits N2 bits N1 bits N2 bits
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Analysis: Pairwise Codeword Error Probability
Slow fading
1 1 2 2
1 1 1
( ) 2 1 1d dP d d SNR d SNR
1 1 2 2( ) 2 2d dP d Q d d
Fast fading
1 2
1 2( ) 2 ( ) 2 ( )
d dn n
P d Q n n
1 2
1 1
1 1 1( )
2 1 1
d d
d d
P dSNR SNR
Diversity order = 2
Diversity order = Hamming distance
(Same for non-cooperation case)
SNR in first frame
SNR in second frame
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Other Cooperation Schemes
Estimate and forward
[Cover & El Gamal, IEEE Trans. Inf. Theory, 1979]
Non-orthogonal transmission schemes
Perform better at the expense of a more complicated destination
receiver [Nabar, Bolczkei, Kneubuhler, IEEE JSAC 2004]
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Cooperation in Ad Hoc Networks
Basic 3 node scenario
Multiple sources/relays case
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Indian Institute of Science, BangaloreExtension to Multiple Node Scenarios
Non-orthogonal schemesOpen-loop scenario
Each relay that decodes
chooses its column of a pre-
specified ST code matrix
(e.g., Orthogonal ST design)[Chakrabarti, Erkip, Sabharwal,
Aazhang, IEEE Sig. Proc. Mag.,
2007]
Relay subset selection
Closed-loop scenario Relays that decode beamform
together to destination
2 Repeats 11 Tx 3 Repeats 1 ... N Repeats 11 Repeats 22 Tx 3 Repeats 2 ... N Repeats 21 Repeats 33 Tx 2 Repeats 3 ... N Repeats 3
1 Repeats NN Tx 3 Repeats N ... N-1 repeats Ntime
frequency
Orthogonal scheme
[Laneman & Wornell, IEEE Trans. on Inf. Theory, 2003]
1 Tx D(1) subset repeats
2 Tx D(2) subset repeats
N Tx D(N) subset repeats
time
freque
ncy
Non-orthogonal scheme
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C
2
2
2
2
22
Cooperative Beamforming and its Feasibility
Relays phase align and power control transmit signal
Equivalent to a multi-antenna array at transmitter
Two important practical issues
CSI needs to be acquired
Beamforming nodes need to be synchronized
1
1
1
1
1
1
C
Inter-cluster
communications
[Ochiai, Mitran, Poor & Tarokh, IEEE Trans. Sig. Proc. 2005]
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Acquiring CSI in Cooperative Beamforming
s
r1
tr2
r3
r4
r5
xx
1. Broadcast data 2. Acquire CSI
3. Select relays
[Madan, Mehta, Molisch, Zhang, To appear in IEEE Trans. Wireless Commn., 2008]
Acquiring CSI requires extra energy and time
s
r1
tr2
r3
r4
r5
Relay subsetselection by
destination
g1
g3
g2
h1
h2
h3
h5
s
r1
tr2
r3
r4
r5
4. Beamform data
|g1|/(|g1|+|g3|)
|g3|/(|g1|+|g3|)
x
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Trade-offs and Design Goals
Broadcast power: Less power: Signal reaches fewer relays, lose out on diversity
More power: Signal reaches more relays, but increases relay
training overhead
Relay selection by destination:
Select few relays: Lose out on diversity when transmitting data
Select many/all relays: More feed back energy spent to reach less
and less useful relays
Questions:
Optimum relay subset selection rule (subject to outage constraint)?
Energy savings achieved by cooperative beamforming?
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Average Energy Consumption: Including Cost of CSI
As a function of number of relayswho decode message
Total energy consumed: Effect ofrelay selection rule
Rule of thumb: Broadcast to reach 3-4 (best) relays, some of then
beamform upon selection
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Synchronization for Cooperative Beamforming
Performance robust to imperfect synchronization
Example: Two equal amplitude signals from two
transmitters. Signals are offset by a phase w
Resulting amplitude: |1+ ej| = 2 cos(/2)
Even if = 300, amplitude = 1.93 (instead of 2) Off by only 4% !
[Mudumbai, Barriac & Madhow, IEEE Trans. Wireless Commn. 2007]
General case:
2
2
1
2
1.
12. 2 ( 1) cos
i
N
jR i
i
R i
P g e
E P N EN
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Receive Power Distribution
Phase uniformly distributedbetween [-/10, /10]
[Mudumbai, Barriac & Madhow, IEEE Trans. Wireless Commn. 2007]
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Relay Selection: Relays Help Even When Not Used
Full diversity achieved by just selecting single best relay
Well understood classical result
[Win & Winters, IEEE Trans. Commn. 1999]
E.g., Antenna selection, Partial Rake CDMA receivers
Simple to implement
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Relay Selection: Selection Criteria and Mechanisms
s
r1
dr2
r3
r4
h1
h2
h3
h4
g1
g2
g3
g4
Selection criteria: Depends on SR and RD channels
Criteria: 2 22 2
2 2
1. min ,
2.
i i i
i ii
i i
h g
h gh g
[Blestsas, Khisthi, Reed & Lippman, IEEE JSAC, 2006; Luo et al, VTC 2005;
Lin, Erkip & Stefanov, IEEE Trans. on Commn., 2006]
Multiple access relay selection mechanism:
Relays overhear a RTS (request to send) from source, and
CTS (clear to send) from destination to estimate channels
Each relay sets a timer with expiry 1/i it
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Indian Institute of Science, BangaloreOpportunistic Relay Selection and Cooperation UsingRateless Codes
Rateless codes (e.g., digital fountain codes)
Convert a finite-length source word into an infinitely long
bitstream
Receiver decodes successfully when received mutual information
exceeds the entropy of the source word
Receiver only needs to send a 1-bit ACK
Ideal binning properties of rateless codes
1. Order in which bits received doesnt matter2. If destination receives data streams from N nodes, it accumulates
mutual informationfrom all N nodes
[Shokrollahi, ISIT 2004; Mitzenmacher, ITW 2004; Luby, FOCS 2002;
Palanki & Yedidia, ISIT 2004; Erez, Trott & Wornell, CoRR 2007]
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Asynchronous Cooperation With Rateless Codes
s
r1
dr2
r3
r4
s
r1
dr2
r3
r4
s
r1
dr2
r3
r4
Broadcast Best relay receives packetand starts transmitting to
destination
Second best relay alsoreceives packet and starts
transmitting to destination
[Molisch, Mehta, Yedidia, Zhang, IEEE Trans. Wireless Commn, 2007]
Time taken for best relay to decode packet:
2log 1 maxi iB
th
h1
h4
h2
h3
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Performance: Transmission Energy & Time
Mean transmission time and energy usage Energy usage statistics
Performance primarily depends on inter-relay link strength
Meantx.energy M
eantx.time
Number of relays
CDF(tx.time)
Tx. time (normalized)
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Cooperation in Infrastructure-Based Networks
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Cooperation in Infrastructure-Based Networks
Downlink
Base station cooperation
Relay cooperation
Uplink Similar to schemes we have seen thus far
[Lee & Leung, IEEE Trans. Vehicular Technology, 2008]
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Base Station (BS) Cooperation
Much more capable base stations (source nodes) Each base station possesses multiple transmit antennas
CSI shared between base stations
Extreme case: Full CSI at all BSs
Benefit: Significantly better co-channel interference
management BS1 BS2
MS1 MS2
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Giant MIMO Array: Transmission Techniques
Linear precoding
Generalized Zero Forcing (GZF)
SLNR criterion based designs
Sum rate criterion based designs
Non-linear techniques
Dirty paper coding
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Indian Institute of Science, BangaloreBase Station Cooperation: Is It Giant MIMO?
No!
BS1 BS2
MS1 MS2
1H
2H
Super BS
MS1 MS2
1 2, H H
I di I i f S i B l
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Interference is fundamentally asynchronous
Even with perfect timing-advance!
(1)
2H
(1)1H
( 2)1H
( 2)
2H
(1)
1
(2)
2
(1)
2
(2)
1
BS1 BS2
MS2
MS10 0
(1) (1)
2 1 (2) ( 2)2 1
[Zhang, Mehta, Molisch & Zhang, IEEE Trans. Wireless Commn. 2008]
I di I i f S i B l
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Implications on Fundamental System Model
( ) ( ) ( ) ( ) ( )
1 1 1( ) ( ) ( )
B K Bb b b b b
k k k k k k jk k b j bm m m
y H T s H T i n
( ) ( ) ( ) ( )
1 1 1
( ) ( ) ( ) ( )B K Bb b b b
k k k k k j j k
b j b
m m m m
y H T s H T s n
Changes the basic model!
Should be:
Was:
Generalized zero forcing constraint is no longer sufficient
Channel from BS b to MS k
Precoding at BS b for MS k
I di I tit t f S i B l
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Asynchronous Interference-Aware Precoding
Linear precoding design methods
1. Sum rate maximization (CISVD)
Non-trivial, non-convex
Game theoretic approach in DSL: [Yu, Ginis, Cioffi 02]
2. Mean square error minimization (JWF)
[Zhang, Wu, Zhou, Wang 05]
3. Signal to leakage plus noise ratio criterion (JLS)
[Tarighat, Sadek, Sayed 05][Dai, Mailaender, Poor 04]
I di I tit t f S i B l
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Modeling Asynchronicity Helps
-5 0 5 10 15 200
2
4
6
8
10
12
Transmit SNR per User(dB)
AverageSpectrumE
ffic
iencyPerUser(bps/HZ)
JWF
JWF: Ignoring async. intf.
JLS
JLS: Ignoring async. intf.
CISVD
CISVD: Ignoring async. intf.
Rate penalty for ignoring asynchronicity is significant
JWF
JLS
CISVD
Transmit SNR per user [dB]
A
ve.spectralefficiency(bits/s/Hz)
2 cell, 2 UE set up
I di I tit t f S i B l
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Relay Cooperation System Model
1 11 21 11 21 1 1
2 12 22 12 22 2 2
Y h h b b U N
Y h h b b U N
Receivedsignals
BS-MSchannel
Linearprecoding
Informationsymbols
AWGN
Linear precoding at relays
I di I tit t f S i B l
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Asymmetric Relaying Arises Naturally
Optimal asymmetric linear precoder is unknown!
Can reduce the dimensionality of the optimization problem
considerably
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Cross Layer Aspects of Cooperation
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Cross-Layer Aspects of Cooperation
Cooperative MAC
[Liu, Lin, Erkip, Panwar, IEEE Wireless Commn., 2006]
Cooperative Hybrid ARQ
[Zhao & Valenti, IEEE JSAC 2005]
Cooperative routing
General routing problem
Progressive accumulative routing
Queued cooperation
[Mehta, Sharma, Bansal, Submitted, 2008]
Impact of physical layer non-idealities
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Cooperative Multi-Hop Routing
Which relay subset should cooperate in which step? Number of possibilities/step: 2N instead of N
Channel fading: Drives how local the cooperation can be
s
r1
tr2
r3
r4
r5
r6
r7
r9
[Khandani, Abounadi, Modiano & Zheng, Allerton 2003]
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Reducing Problem to Conventional Routing Problem
Only allow nodes k edges/hops apart to cooperate
Construct hyper graph of neighbour nodes
Determine optimal cooperation/non-cooperation scheme to transmit between
neighbours
Assign energy cost to each edge in hyper graph
Distributed conventional routing algorithms now applicable to determine best
multihop route from source to destination, e.g., Belman-Ford routing
[Madan, Mehta, Molisch, Zhang, Allerton 2007]
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Indian Institute of Science, BangaloreProgressive (Energy) Accumulative Routing
s
r1
tr2
r3
r4r6
Nodes do not discard previous transmissions in a route
Energy-efficient unicast, multicast and broadcast
Unicast: [Yim, Mehta, Molisch & Zhang, IEEE Trans. Wireless Commn., 2008]
Broadcast/Multicast routing: [Maric & Yates, IEEE JSAC 2002, 2005]
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1st Relay Addition: Necessary & Sufficient Conditions
A node rhelps if and only if
(Any eligible node can
overhear source to
destination transmission)
Source (s) and relay (r) transmit powers for maximal power savings
s thrt > hst(Relaydoesnt help)
hsr > hst(Relaydoesnt help)
hst < min{hsr,hrt} (Relay saves power)
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Indian Institute of Science, BangaloreProgressive Accumulative Routing: Protocol Design
s
r
t
s
r
t
q
s
r
t
q
s t
u v
l
w
Update routes without tearingthem down
Sufficient conditions to add a
relay turn out to be nice!
Packet header fields can be
designed so that only local
CSI is needed
How to select optimal relays?
Optimal relay transmission
power?
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s t
u v
l
w
s t u v whwt hwv
MSrc MDest RSrc RDest RelayIDGainD GainR
Ready to cooperate packet
Data Packet and Cooperation Packet Structures
PAR Protocol q
s t u v hst/hsq + hqt/hqu hut huv
MSrc MDest RSrc RDest FracDelivered GainD GainR
Data
Local CSI info
u to v
w to u
1 1 1
wt ut
uw uw uv
h h
h h h
Sufficient conditionsto be a useful relay
Energy accumulatedthus far
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Simulations: Gains from PAR
100 nodes distributed uniformly
in a grid of size 20 x 20 grid
Source at (5,10) and destination
at (15,10)
Total power consumption
decreases from 100% to 13.6%
to 2.84% to 1.47% and 1.35% in
5 iterations.
Box plot
Number of iterations
Totalpowerco
nsumed
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Other Aspects
Network lifetime maximization and cooperation
[Himsoon, Siriwongpairat, Han & Liu, IEEE JSAC 2007]
Distributed detection and estimation using cooperation in
sensor networks [Nayagam, Shea & Wong, IEEE JSAC 2007]
Cognitive radios and cooperation
[Ganesan & Li, IEEE Trans. Wireless Commn 2007]
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Summary and Conclusions
Cooperation effectively exploits three essential wireless
characteristics:
Physical layer spatial diversity
Broadcast advantage
Multiple access characteristics of wireless
Affects physical layer and higher layer design
Some key problems:
General multihop scenarios Cross-layer design with cooperation
Robust synchronization schemes
Infrastructure-based cooperation in next generation wireless