Channel Access Competition in Linear
Multihop Device-to-Device Networks
Vaggelis G. Douros
Stavros Toumpis
George C. Polyzos
IWCMC @ Nicosia,
07.08.2014
1
Motivation (1)

2
New communication paradigms will arise
Motivation (2)



3
Proximal communication-D2D scenarios
More devices…more interference
Our work: Channel access in such
scenarios  which device should
transmit/receive data and when
Problem Description (1)



4
1
2
3
4
5
6
1
2
3
4
5
6
Each node in this linear D2D network either
transmits to one of its neighbors or waits
Node 3 transmits successfully to node 4 IFF none of
the red transmissions take place
If node 3 decides to transmit to node 4, then none of
the green transmissions will succeed
Problem Description (2)




5
The problem: How can these
autonomous nodes avoid
collisions?
The (well-known) solution:
maximal scheduling…
is not enough/incentivecompatible 
We need to find equilibria!
1
2
3
1
2
3
1
2
3
Problem Description (3)





6
This is a special type of game called graphical game
Payoff depends on the strategy of nodes that are up
to 2 hops away
1-c: a successful transmission
-c: a failed transmission
0: a node waits
On the Nash Equilibria (1)




7
How can we find a Nash Equilibrium?
The (well-known) solution: Apply a best
response scheme…
will not converge 
A naive approach: A distributed iterative
randomized scheme, where nodes
exchange feedback in a 2-hop
neighborhood to decide upon their new
strategy
1
2
1
2
1
2
1
2
On the Nash Equilibria (2)



8
Each node i has |Di|
neighbors and |Di|+1
strategies. Each strategy
is chosen with prob.
1/(|Di|+1)
A successful
transmission is repeated
in the next round
Strategies that cannot be
chosen increase the
probability of Wait
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
This is a NE! 
On the Nash Equilibria (3)


9
1
2
3
4
5
1
2
3
4
5
By studying the structure of the NE, we can identify
strategy subvectors that are guaranteed to be part of
a NE
We propose a sophisticated scheme and show that it
converges monotonically at a NE
On the Nash Equilibria (4)
A sophisticated
approach: A successful
transmission is repeated
IFF it is guaranteed that
it will be part of a NE
vector
 Nodes exchange
messages in a 3-hop
neighborhood
 Is this faster than the
10 naive approach?

1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
1
2
3
4
5
This is a NE! 
Performance Evaluation (1)


11
The sophisticated outperforms the naive scheme
Even in big D2D networks, convergence at a NE is
very fast
Performance Evaluation (2)


12
Convergence at a NE for the sophisticated scheme
is ~ proportional to the logarithm of the number of
nodes of the topology
In <10 rounds, most nodes converge at a local NE
Take-home Messages

Channel access for selfish D2D networks can
lead to efficient NE with minimal cooperation
–

Studying the structure of the NE is very
useful towards the design of efficient
schemes
–
–
13
stronger notion than maximal scheduling
fast convergence
without spending much energy
 Ευχαριστώ! 
Vaggelis G. Douros
Mobile Multimedia Laboratory
Department of Informatics
School of Information Sciences and
Technology
Athens University of Economics and Business
[email protected]
http://www.aueb.gr/users/douros/
14
Acknowledgement (1)

Vaggelis G. Douros is supported by the
HERAKLEITOS II Programme which is cofinanced by the European Social Fund and
National Funds through the Greek Ministry of
Education.
This research has been co-financed by the European Union
(European Social Fund – ESF) and Greek national funds through
the Operational Program "Education and Lifelong Learning" of
the National Strategic Reference Framework (NSRF) - Research
Funding Program: Heracleitus II. Investing in knowledge
society through the European Social Fund.
15
Acknowledgement (2)

16
The research of Stavros Toumpis has been co-financed
by the European Union (European Social Fund ESF)
and Greek national funds through the Operational
Program “Education and Lifelong Learning” of the
National Strategic Reference Framework (NSRF)
Research Funding Program: THALES. Investing in
knowledge society through the European Social Fund.