cooperative comm v3-1
TRANSCRIPT
Indian Institute of Science (IISc), Bangalore, India
Cooperative Communications
Neelesh B. MehtaECE DepartmentIISc, 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)
Indian Institute of Science, Bangalore
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 relaysd
s2
Tw
o co operativ e sourc es
s1
h1d
h2d
h12
h1d
h4d
h2d
h3dhsd
Indian Institute of Science, Bangalore
What’s Different Between MIMO and Cooperation?
• Distributed nature of relays/nodes
– Different channel gain amplitudes and phases
– Each relay runs on its own timer and VCO
• Relay capabilities
– Single antenna
– Full duplex or half duplex
• Channel state information (CSI)
– Relay might not know states of other relay links
Indian Institute of Science, Bangalore
Outline
• Various cooperation schemes
• Cooperation in ad hoc networks
• Cooperation in infrastructure-based networks
• Cross-layer issues
• Other interesting topics
Indian Institute of Science, Bangalore
Cooperative Communication Schemes
• Amplify and forward
• Decode and forward
• Estimate and forward
Possibilities:
• Orthogonal / Non-orthogonal cooperation
• Coded / Uncoded cooperation
Indian Institute of Science, Bangalore
Analysis of Basic 3 Node Scenario
Performance metrics
• Outage
• Power consumption
• Diversity
• BER (Coded/Uncoded)
d
s2
Tw
o so urces
s1
h1d
h2d
h12
S1 transmits S2 transmits
d receives d receivesConventionalmodel
Tx
Rx
S1 tx S2 repeats S2 tx S1 repeats
d, S2 rx d rx d,S1 rx d rx Cooperative source model
Tx
Rx
[Laneman & Wornell, IEEE Trans. on Inf. Theory, 2004]
[Stefanov, Erkip, IEEE Trans. on Communications, 2004]
Indian Institute of Science, Bangalore
Outage Analysis: Amplify and Forward
[1][1]
[2] [2]
sd dd
d rd sr rd r d
h wyx
y h h h w wβ β
= + +
2
0
r
sr s
P
h P Nβ ≤
+
2 22
2 2
SNR SNRlog 1 SNR
SNR SNR
sr rd sr rdAF sd sd
sr sr rd sr
h hI h
h h
÷= + + ÷+
d
r
s hsd
hrd
hsr
xyd
yr = hsr x + wr
( ) ( ) 222 2
2 2 2 2
2 11( , ) Pr
2 SNRsr
sd sr
Rrd
out AFrd
P SNR R I Rσ σ
σ σ σ−+
= < ≈
Relay power
constraint:
Tx. rate
Outage prob.
Diversity order = 2
Indian Institute of Science, Bangalore
Outage Analysis: Decode and Forward
Case 1: Destination can decode only if relay decodes
ˆrx x= ˆd rd dy h x w= +
( ) ( )2 2 21min log 1 , log 12DF sr sd rdI SNR h SNR h SNR h = + + +
( )2
2
1 2 1( , ) Pr
R
out DFsr
P SNR R I RSNRσ
−= < ≈
(Assume codeword level decoding)
Diversity order = 1
Indian Institute of Science, Bangalore
Outage Analysis: Adaptive Decode and Forward
Case 2: Source forwards to destination instead of relay if SR channel is
poor
ˆrx x= ˆd rd dy h x w= +
( )( )
22 2
2 2
1 2 1log 1 2 ,2
1log 1 , else2
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 rdout DF
sd sr rd
P SNR R I RSNR
σ σσ σ σ
−+= < ≈
(Similar results apply for non-orthogonal scheme in which source transmits
to destination in both time slots, and relay repeats in second time slot)
Indian Institute of Science, Bangalore
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
Indian Institute of Science, Bangalore
Analysis: Pairwise Codeword Error Probability
• Slow fading
1 1 2 2
1 1 1( )
2 1 1d d
P dd SNR d SNR
= ÷ ÷+ +
( )1 1 2 2( ) 2 2d dP d Q d dγ γ= +
• Fast fading
1 2
1 2( ) 2 ( ) 2 ( )d dn n
P d Q n nη η
γ γ∈ ∈
= + ÷ ÷
∑ ∑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
Indian Institute of Science, Bangalore
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]
Indian Institute of Science, Bangalore
Cooperation in Ad Hoc Networks
• Basic 3 node scenario
• Multiple sources/relays case
Indian Institute of Science, Bangalore
Extension 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 1
1 Repeats 22 Tx 3 Repeats 2 …... N Repeats 2
1 Repeats 33 Tx 2 Repeats 3 …... N Repeats 3
1 Repeats NN Tx 3 Repeats N …... N-1 repeats N
time
freq
uen c
y
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
freq
uen c
y
Non-orthogonal scheme
Indian Institute of Science, Bangalore
C
22 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]
Indian Institute of Science, Bangalore
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 subset
selection by
destination
g1
g3
g2
h1
h2
h3
h5
s
r1
tr2
r3
r4
r5
4. Beamform data
α |g1|/(|g1|+|g3|)
α |g3|/(|g1|+|g3|)
x
Indian Institute of Science, Bangalore
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?
Indian Institute of Science, Bangalore
Average Energy Consumption: Including Cost of CSI
As a function of number of relays who decode message
Total energy consumed: Effect of relay selection rule
• Rule of thumb: Broadcast to reach 3-4 (best) relays, some of then beamform upon selection
Indian Institute of Science, Bangalore
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+ ejω| = 2 cos(ω/2)
– Even if ω = 300, amplitude = 1.93 (instead of 2) – Off by only 4% !
[Mudumbai, Barriac & Madhow, IEEE Trans. Wireless Commn. 2007]
• General case:
[ ] [ ]
22
1
2
1.
12. 2 ( 1) cos
i
Nj
R ii
R i
P g e
E P N EN
ω
ω
==
= + −
∑
Indian Institute of Science, Bangalore
Receive Power Distribution
Phase uniformly distributed between [-π/10, π/10]
[Mudumbai, Barriac & Madhow, IEEE Trans. Wireless Commn. 2007]
Indian Institute of Science, Bangalore
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
Indian Institute of Science, Bangalore
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 2
2 2
2 2
1. min ,
2.
i i i
i ii
i i
h g
h g
h g
µ
µ
=
=+
[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 µµ
Indian Institute of Science, Bangalore
Opportunistic Relay Selection and Cooperation Using Rateless 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 codes1. Order in which bits received doesn’t matter
2. If destination receives data streams from N nodes, it accumulates mutual information from all N nodes
[Shokrollahi, ISIT 2004; Mitzenmacher, ITW 2004; Luby, FOCS 2002; Palanki & Yedidia, ISIT 2004;
Erez, Trott & Wornell, CoRR 2007]
Indian Institute of Science, Bangalore
Asynchronous Cooperation With Rateless Codes
s
r1
dr2
r3
r4
s
r1
dr2
r3
r4
s
r1
dr2
r3
r4
Broadcast Best relay receives packet and starts transmitting to
destination
Second best relay also receives 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 max i i
Bt
h=
+
h1
h4
h2
h3
Indian Institute of Science, Bangalore
Performance: Transmission Energy & Time
Mean transmission time and energy usage
Energy usage statistics
Performance primarily depends on inter-relay link strength
Mea
n tx
.
ener
gy
Mean tx .
time
Number of
relays
CD
F (
tx.
time)
Tx. time (normalized)
Indian Institute of Science, Bangalore
Cooperation in Infrastructure-Based Networks
Indian Institute of Science, Bangalore
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]
Indian Institute of Science, Bangalore
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
Indian Institute of Science, Bangalore
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
Indian Institute of Science, Bangalore
Base Station Cooperation: Is It Giant MIMO?
No!
BS1 BS2
MS1
MS2
1H 2HSuper BS
MS1
MS2
12 , HH
Indian Institute of Science, Bangalore
Interference is fundamentally asynchronous
• Even with perfect timing-advance!
(1)2H
(1)1H
(2)1H
(2)2H
(1)1τ
(2)2τ(1)
2τ
(2)1τ
BS1 BS2
MS2
MS10 0
(1) (1)2 1τ τ−
(2) (2)2 1τ τ−
[Zhang, Mehta, Molisch & Zhang, IEEE Trans. Wireless Commn. 2008]
Indian Institute of Science, Bangalore
Implications on Fundamental System Model
( ) ( ) ( ) ( ) ( )
1 1 1
( ) ( ) ( )B K B
b b b b bk k k k k k jk k
b j b
m m m= = =
= + + ∑ ∑ ∑y H T s H T i n
( ) ( ) ( ) ( )
1 1 1
( ) ( ) ( ) ( )B K B
b b b bk 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
Indian Institute of Science, Bangalore
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]
Indian Institute of Science, Bangalore
Modeling Asynchronicity Helps
-5 0 5 10 15 200
2
4
6
8
10
12
Transmit SNR per User(dB)
Ave
rag
e S
pe
ctru
m E
ffici
en
cy P
er
Use
r(b
ps/
HZ
)
JWFJWF: Ignoring async. intf.JLSJLS: Ignoring async. intf.CISVDCISVD: Ignoring async. intf.
• Rate penalty for ignoring asynchronicity is significant
JWF
JLS
CISVD
Transmit SNR per user [dB]
Ave
. spe
ctra
l eff i
cien
cy
(bits
/s/H
z)
2 cell, 2 UE set up
Indian Institute of Science, Bangalore
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
= +
Received signals
BS-MS channel
Linear precoding
Information symbols
AWGN
• Linear precoding at relays
Indian Institute of Science, Bangalore
Asymmetric Relaying Arises Naturally
• Optimal asymmetric linear precoder is unknown!
• Can reduce the dimensionality of the optimization problem considerably
Indian Institute of Science, Bangalore
Cross Layer Aspects of Cooperation
Indian Institute of Science, Bangalore
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
Indian Institute of Science, Bangalore
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]
Indian Institute of Science, Bangalore
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]
Indian Institute of Science, Bangalore
Progressive (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]
Indian Institute of Science, Bangalore
1st Relay Addition: Necessary & Sufficient Conditions
• A node r helps 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 t
hrt > hst
(Relay doesn’t help)
hsr > hst
(Relay doesn’t help)
hst < min{hsr,hrt} (Relay saves power)
Indian Institute of Science, Bangalore
Progressive Accumulative Routing: Protocol Designs
r
t
s
r
t
q
s
r
t
q
s t
u v
l
w
• Update routes without tearing them 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?
Indian Institute of Science, Bangalore
s t
u v
l
w
s t u v whwt hwv
MSrc
MDest
RSrc RDest RelayID
GainD 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 1wt ut
uw uw uv
h h
h h h
>
+ <
Sufficient conditions to be a useful relay
Energy accumulated thus far
Indian Institute of Science, Bangalore
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
Tot
al p
o wer
con
s um
ed
Indian Institute of Science, Bangalore
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]
Indian Institute of Science, Bangalore
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
Indian Institute of Science, Bangalore
General Case: Multiple Relays (Between Two Relays)
• Sufficient condition for inclusion: Not conducive to distributed implementation
• Only two nodes adjust transmit powers
1 1and lt wt
uw uv wvuv uw lt ut
h hh h h
h h h h
−> − > ÷ −
1 1 1
uw wv uvh h h+ <• Weaker condition:
uuw
Ph
γ= 1 ( ) ut ut wtl
uw uv uw wv
h h hP A l
h h h h
γ = − + − − ÷
Energy accumulated
at last node (l)
s t
u v
l
w
Add node between two relays
(not after last relay)
(Parent
relay)
(Last
relay)
Indian Institute of Science, Bangalore
Master-Slave Architecture for Phase Synchronization
[Mudumbai, Barriac & Madhow, IEEE Trans. Wireless Commn.
2007]