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Topology Control for Effective Interference Cancellation in Multi-User MIMO Networks
E. Gelal, K. Pelechrinis, T.S. Kim, I. Broustis Srikanth V. Krishnamurthy, B. Rao
IEEE INFOCOM 2010
Problem Motivation & Contributions
• MIMO communications are becoming prevalent Multiple antenna elements robust links
• 802.11n utilizes MIMO PHY CSMA/CA no exploitation of MIMO capabilities At most one transmission each time instance
• How can we realize multi-user MIMO communications?
• Precoding techniques can be used Accurate channel estimation, feedback from receiver.
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Successive Interference Cancellation
Problem Motivation & Contributions
• We design MUSIC (Multi-User MIMO with Successive Interference Cancellation) Uses SIC for enabling Multi-user MIMO communications
• Centralized and distributed approaches• Evaluation on a variety of settings
Our approach scales and the decoding error probability is bounded
MUSIC outperforms DoF approaches.
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Roadmap
• Problem motivation & Contributions• Background• SIC
Problem formulation
• Our approach• Evaluations• Conclusions
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Background
• Multi-user MIMO Precoding techniques
Tx sends pilot signals Rx receives pilot signals channel coefficients estimation Rx feedbacks channel coefficients to Tx Tx assigns weights at the antennas
Successive Interference Cancellation (SIC) Receiver iteratively extracts high interfering signals SINR requirement should be satisfied for every interferer.
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Background
• Selective diversity at Tx
Feedback from Rx to Tx for the best transmission element
One element used for subsequent transmission Feedback is required less often than with precoding
• Degrees of Freedom = k #antenna elements = k k simultaneous transmissions are possible
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Roadmap
• Problem motivation & Contributions• Background• SIC
Problem formulation
• Our approach• Evaluations• Conclusions
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SIC
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Node 1
Node 2
Node 3
Node 4
SIC
SIC tries to remove first the stronger interferers and then decode the weaker intended signal.
Models
• Selection diversity and SIC• Two kinds of interferers
Strong: signal strength higher than the intendedWeak: signal strength weaker than the intended
• Path loss and multipath
htr follows Rayleigh distribution, α is the path exponent, P the transmission power
9€
Ptr =P | htr |2
dtrα
Dealing with Weak Interferers
• Maximum weak interference tolerated on link (u,v):
• We want to assure that:
• Assuming all interferers at the same distance as of the strongest one Aggregate weak interference follows Erlang distribution with parameters n: number of intreferers σ: variance of the Rayleigh distributed variable h
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€
Puvγ
−N
€
Pr{ Pzv >Puvγ
−N} < δz≠u
∑
Dealing with Strong Interferers
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dBmStrongest interferer P1 P1/(N+P2+P3+….+Pk) > γ
Second strongest interferer P2
…Intended signal ((k-1) strongest) Pk-1
k stronger interferer (weak) Pk
P2/(N+P3+P4+….+Pk) > γ
Pk-1/(N+Pk) > γ SUCCESFUL DECODING !!
Compact rule: Iteratively for correct decoding on link (y,z), there must be at most one interferer u, with the following interfering power:
€
Puz ≥ Pyz(1+ γ )r,r ∈ {1,2,3,...,k −1}
Problem Formulation
• Interference Graph, Directed, edge and vertex weighted V’ : set of links, with weight the mean value of the received
signal strength E’ : set of directed edges among the links/vertices, with
weight the mean value of interference among the links connected.
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€
G'= (V ',E ',wV ',wE ' )
u v
x y
a(x,y) b(u,v)
Pxy
Puv
Pxv
Puy
Problem Formulation
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Time Slot 1 V1
’ linksTime Slot 2 V2
’ linksTime Slot m Vm
’ links
€
∪
€
∪
NP - Hard
Roadmap
• Problem motivation & Contributions• Background• SIC
Problem formulation
• Our approach• Evaluations• Conclusions
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C-MUSIC
• The centralized algorithm is iterative.• Global knowledge of the topology• Main steps
Priority to links not scheduled Include links that do not require SIC for
decodingAdd links that can be decoded with SIC Try to pack more links among those already
scheduled15
C-MUSIC
• Two interfering links cannot belong to the same sub-topology if:
The weak interferer causes more interference than the weak interference budget
The strong interference cannot be removed
The two links have the same transmitter (selection diversity)
A node is the transmitter for one of the links and a receiver for the other.
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D-MUSIC
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Transmitter Receiver
Overhearing Nodes
Roadmap
• Problem motivation & Contributions• Background• SIC
Problem formulation
• Our approach• Evaluations• Conclusions
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Simulation Set Up
• OPNET simulations• Traffic load: 10-30 pkt/sec, 1500 bytes packets• Path loss (α=4) and Rayleigh fading• Simulations with different
Node density, SINR requirement, number of antenna elements
• Metrics of interest: Number of time slots, average decoding success probability,
throughput
• Comparison with: Optimal (small topologies), DoF based topology control
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Evaluation results
• MUSIC is efficient in terms of number of time slots formed
• Density does not significantly decrease the probability of successful decoding
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Optimal C-MUSIC D-MUSIC
7.83 9.18 9.64
Evaluation results
• DoF based link activation cannot effectively exploit the benefits of multi-user MIMO DoF-based link activation leads to more decoding errors
MUSIC provides better throughput as compared with DoF
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Roadmap
• Problem motivation & Contributions• Background• SIC
Problem formulation
• Our approach: C-MUSIC• Evaluations• Conclusions
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Conclusions
• Identify the conditions for SIC to allow multi-packet reception in multi-user MIMO settings.
• Design a framework for exploiting SIC• Demonstrate through simulations the
applicability of our approach
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