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Title: Low Complexity Virtual Antenna Arrays Using Cooperative Relay Selection


1
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
2
Outline
  • Motivation
  • System Model
  • Protocols with Cooperative Relay Selection
  • Zero-Feedback
  • Single-Bit Feedback (single or multiple rounds)
  • Concluding Remarks

3
Motivation (1)
  • Cooperative communications
  • Node cooperation to improve the performance of
    wireless networks by coordination of terminals
    distributed in space.

4
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.

5
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?
6
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.

7
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)
8
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

9
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.

10
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.

11
Protocols (4)
  • Direct (non-cooperative) communication

12
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!
13
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.

14
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.

15
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

16
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.

17
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

18
Models and Protocols (4)
  • Distributed relay selection
  • Distributed timer method
  • Without requiring global CSI at each relay or a
    central controller in the network
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