Cooperative Communications

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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
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• Multiple antenna spatial diversity using only
  single antenna nodes
• Exploit two fundamental aspects of wireless
  channels
• Broadcast
• Multiple access

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Cooperative relays
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Whats 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

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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)

S2 transmits
S1 transmits
Tx
Conventional model
d receives
d receives
Rx
S1 tx
S2 tx
S2 repeats
S1 repeats
Tx
Cooperative source model
d, S2 rx
d,S1 rx
d rx
d rx
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
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

(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

(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
S1 bits

Inactive
S1
Rx S2 bits
S2 bits relay
S2 bits
S2
Inactive
S1bits relay
Rx S1 bits
N1 bits
N2 bits
N1 bits
N2 bits

• 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
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Analysis Pairwise Codeword Error Probability

• Slow fading

SNR in first frame
SNR in second frame
Diversity order 2

• Fast fading

Diversity order Hamming distance (Same for
non-cooperation case)
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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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Extension to Multiple Node Scenarios

• Non-orthogonal schemes
• Open-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

Laneman Wornell, IEEE Trans. on Inf. Theory,
2003
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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

Inter-cluster communications
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C
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Ochiai, Mitran, Poor Tarokh, IEEE Trans. Sig.
Proc. 2005
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Acquiring CSI in Cooperative Beamforming
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1. Broadcast data
2. Acquire CSI 3. Select relays

• Acquiring CSI requires extra energy and time

Madan, Mehta, Molisch, Zhang, To appear in IEEE
Trans. Wireless Commn., 2008
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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 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

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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 !

• General case

Mudumbai, Barriac Madhow, IEEE Trans. Wireless
Commn. 2007
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Receive Power Distribution
Phase uniformly distributed between -p/10, p/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

• Selection criteria
• Depends on SR and RD channels
• Criteria

• 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

Blestsas, Khisthi, Reed Lippman, IEEE JSAC,
2006 Luo et al, VTC 2005 Lin, Erkip
Stefanov, IEEE Trans. on Commn., 2006
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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 codes
• Order in which bits received doesnt matter
• 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
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Asynchronous Cooperation With Rateless Codes
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Broadcast
Best relay receives packet and starts
transmitting to destination
Second best relay also receives packet and starts
transmitting to destination
Time taken for best relay to decode packet
Molisch, Mehta, Yedidia, Zhang, IEEE Trans.
Wireless Commn, 2007
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Performance Transmission Energy Time
Mean transmission time and energy usage
Energy usage statistics
CDF (tx. time)
Mean tx. time
Mean tx. energy
Number of relays
Tx. time (normalized)
Performance primarily depends on inter-relay link
strength
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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

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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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Base Station Cooperation Is It Giant MIMO?
BS1
BS2
MS1
MS2

• No!

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Interference is fundamentally asynchronous

• Even with perfect timing-advance!

MS1
Zhang, Mehta, Molisch Zhang, IEEE Trans.
Wireless Commn. 2008
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Implications on Fundamental System Model
Changes the basic model!
Precoding at BS b for MS k
Should be
Channel from BS b to MS k
Was
Generalized zero forcing constraint is no longer
sufficient
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Asynchronous Interference-Aware Precoding

• Linear precoding design methods
• Sum rate maximization (CISVD)
• Non-trivial, non-convex
• Game theoretic approach in DSL Yu, Ginis,
  Cioffi 02
• Mean square error minimization (JWF)
• Zhang, Wu, Zhou, Wang 05
• Signal to leakage plus noise ratio criterion
  (JLS)
• Tarighat, Sadek, Sayed 05Dai, Mailaender,
  Poor 04

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Modeling Asynchronicity Helps
CISVD
JLS
Ave. spectral efficiency (bits/s/Hz)
JWF
2 cell, 2 UE set up
Transmit SNR per user dB

• Rate penalty for ignoring asynchronicity is
  significant

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Relay Cooperation System Model

• Linear precoding at relays

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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
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• 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

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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Progressive (Energy) Accumulative Routing
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• 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 r helps if and only if

• Source (s) and relay (r) transmit powers for
  maximal power savings

(Any eligible node can overhear source to
destination transmission)
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Progressive Accumulative Routing Protocol Design

• 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?

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Data Packet and Cooperation Packet Structures
u to v
Local CSI info
Energy accumulated thus far
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PAR Protocol
q
Sufficient conditions to be a useful relay
v
u
w
Ready to cooperate packet
w to u
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Simulations Gains from PAR
Box plot

• 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.

Total power consumed
Number of iterations
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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

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General Case Multiple Relays (Between Two Relays)

• Sufficient condition for inclusion Not conducive
  to distributed implementation

s
t
l
u
v
w

• Weaker condition

Add node between two relays (not after last relay)

• Only two nodes adjust transmit powers

Energy accumulated at last node (l)
(Parent relay)
(Last relay)
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Master-Slave Architecture for Phase
Synchronization
Mudumbai, Barriac Madhow, IEEE Trans. Wireless
Commn. 2007