Low Complexity Virtual Antenna Arrays Using Cooperative Relay Selection

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Low Complexity Virtual Antenna Arrays Using
Cooperative Relay Selection
Aggelos Bletsas, Ashish Khisti, and Moe Z.
Win Laboratory for Information and Decision
Systems (LIDS) Massachusetts Institute of
Technology moewin_at_mit.edu
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Outline

• Motivation
• System Model
• Protocols with Cooperative Relay Selection
• Zero-Feedback
• Single-Bit Feedback (single or multiple rounds)
• Concluding Remarks

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Motivation (1)

• Cooperative communications

• Node cooperation to improve the performance of
  wireless networks by coordination of terminals
  distributed in space.

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Motivation (2)

• Cooperation has been widely viewed as a
  distributed, multiple relay, transmission
  problem
• Using distributed Phased-array techniques
• or Using distributed Space-Time Coding

• Phased-array techniques require tracking and
  control of multiple carrier-phase differences
• Space-Time Coding for multiple antennas is an
  open area of research
• Both become less practical due to the distributed
  nature of the Relay Channel
• Both increase the complexity and cost of the
  transceiver.

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Motivation (3)

• Phased Array and Space-Time Coding Techniques
  increase the complexity and cost of the
  transceiver

Simplification of cooperative communication to
minimize the required hardware complexity and
cost Distributed single-relay selection Can we
achieve globally optimal cooperation simply by
single-relay transmission?
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System Model (1)

• Canonical case of half-duplex, narrow-band,
  dual-hop communication

Node A
Node B
Phase I
Phase II

• Received signal in a link A ? B

• Results has been extended to generalized fading
  models (e.g. Nakagami-m)A. Bletsas, A. Khisti,
  M. Z. Win, Unifying Cooperative Diversity,
  Routing and Feedback with Select and Forward
  Protocols, submitted to IEEE Transactions on
  Communications.

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System Model (2)

• Performance metric Diversity order-multiplexing
  gain tradeoff (DMT)

• DMT averages out relay topology(high SNR tool)
  
• DMT simplifies analysis

• Diversity order

reliability

• Multiplexing gain

achievable throughput (degrees of freedom)
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Protocols (1)

• Key idea 1 among a set of K possible relays,
  only one will be used
• Key idea 2 the selected, best relay will be
  chosen before source transmission (Proactive
  Relay selection)
• Key idea 3 the selected relay will be used only
  if needed (feedback availability)

• Which is the best relay to use? Select the
  relay that maximizes a function of the end-to-end
  channel conditions

• 2 Opportunistic functions min vs harmonic mean

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Protocols (2)

• Opportunistic Relay Selection based on channel
  conditions fading mitigation
• Proactive Relay Selection relays not used enter
  idle mode and total reception energy is minimized

• Those functions are simple and carefully chosen.

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Protocols (3)

• Distributed relay selection to find out the max
  element in a set, you dont need to know the
  individual value of all elements in the set.
• Distributed timer method has been proposed,
  analyzed and implemented in simple radios.
• Without requiring global CSI at each relay or at
  a central controller in the network.A. Bletsas,
  Intelligent Antenna Sharing in Cooperative
  Diversity Wireless Networks, Ph.D. Dissertation,
  Massachusetts Institute of Technology, September
  2005.

• Intuition to select the tallest student in a
  classroom, you dont need to measure each of
  them, but instead ask all of them to stand up and
  have the tallest observe the class and raise her
  hand.

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Protocols (4)

• Direct (non-cooperative) communication

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Discussion

• Proactive relay selection
• Simplify the receiver design and the overall
  network operation (which is equivalent to
  routing).
• May seem that selecting a single relay before the
  source transmission would degrade performance.

• Single relay transmission
• May seem that a single relay transmission would
  degrade performance.

Results show that there is no performance loss!
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Results (1)

• diversity order d on the number of cooperating
  nodes K1.
• feedback can improve rate r (from 0.5 -gt 1)
  without requiring simultaneous transmissions.
• Multiple rounds L of feedback further improve
  performance.

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Results (2)

• analysis includes both amplify-and-forward as
  well as decode-and-forward relays.
• recent results include generalized fading models
  as well as reactive protocols where relay is
  selected after source transmission.

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Concluding Remarks (1)

• Put forth simple opportunistic relay selection
  rules for decode-and-forward (DaF) or
  amplify-and-forward relays and provided DMT
  analysis.
• Studied the impact of feedback with multiple
  relays.
• Showed that single relay selection is equivalent
  to complex space-time coding, even though
  simpler.
• Proactive opportunistic relaying reduces the
  reception energy cost in the network.
• Energy-efficient routing

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Concluding Remarks (2)

• Our results reveal that relays in cooperative
  communications can be viewed not only as active
  re-transmitters but also as distributed sensors
  of the wireless channel.
• Cooperative relays can be useful even when they
  do not transmit, provided that they cooperatively
  listen.
• Cooperation benefits can be cultivated with
  simple radio implementation.

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Thank You!

• This research was supported, in part, by
• The Office of Naval Research Young
  Investigator Award N00014-03-1-0489,
• The National Science Foundation under Grant
  ANI-0335256,
• The Charles Stark Draper Laboratory Robust
  Distributed Sensor Networks Program

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Models and Protocols (4)

• Distributed relay selection

• Distributed timer method
• Without requiring global CSI at each relay or a
  central controller in the network