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Chapter 10
Cooperation Link Level Retransmission in
Wireless Networks
M. Dianati, X. Shen, and K. Naik
Scope
• Link and MAC layer
for fading channels
Application
Layer
Presentation
Layer
Application
Layer
Application
adaptation and
interfacing
layers
Session Layer
TCP
Transport
Layer
Network Layer
IP
Data Link
Layer
Data Link
Layer
Physical Layer
Physical Layer
ISO OSI Model
The Internet
Model
• Two parts:
– Cooperative Scheduling
– Cooperative ARQ
Network and
unification
layers
Link
adaptation
layers
Introduction
Sample fading process
• Challenges in
wireless domain:
2
1
Envelope level (dB)
– Fading
– Interference
– Limited bandwidth
3
0
-1
-2
-3
-4
-5
-6
• Potentials:
– Again, fading
– Spatial diversity
0
10
20
30
40
50
60
Time (ms)
70
80
90
100
Introduction:
Stochastic model of flat fading process:
Power spectrum density
Complex envelope of
8
7
g (t )  g I (t )  jgQ (t )
6
m
(f)/( /2 f )
fading process:
p
I
4
S
 p
1

S g I g I ( f )   2f m 1  f / f m

0

S gQ gQ ( )  0
g gI
Power spectrum density:
5
| f | f m
3
otherwise
2
1
-1
-0.8
-0.6
0.4
0.2
0
-0.2
-0.4
Normalized Doppler frequency f/fm
0.6
Fading process is a non-white stochastic process with
relatively slow variations.
0.8
1
Introduction: Spatial diversity
•
•
Using independent transmission paths to increase:
–
Capacity
–
Reliability
–
Both
Examples:
–
Multiple antenna systems
–
Cooperative communications
–
Multiuser diversity
Cooperative ARQ: Motivations
•
ARQ: link level retransmission
–
•
•
Is de facto part of wireless link layer protocols
Cooperative ARQ uses:
–
Channel state info. (since fading is a non-white process)
–
Spatial diversity
To improve:
–
Throughput
–
Delay
Cooperative ARQ: Basic idea
• Let neighbor nodes assist the
retransmission trials
Neighbor
Transmission
X
Sender
Neighbor
Receiver
Cooperative ARQ: Basic idea
• Let neighbor nodes join
retransmission
Neighbor
Negative or positive ACK
NAK
Receiver
Sender
Neighbor
Cooperative ARQ: Basic idea
• Let neighbor nodes join
retransmission
Neighbor
Retransmission
Receiver
Sender
Neighbor
Cooperative ARQ: Basic idea
• Assuming that the physical layer can
handle multiple receptions, node
cooperation:
– Mitigates the impact of deep fading on
the primary path from the sender to the
receiver
– Improves the chance of successful
retransmission
Cooperative ARQ: System model
Coop. group 3
• Network model
Coop. group 1
Coop. group 2
Neighbor 1
• A single cooperation
group
Interim
channel
Relay
channel
Primary channel
Sender
Neighbor N
Cooperation group
Receiver
Cooperative ARQ: Basic scheme
•
Sender and receiver nodes
perform their normal
operations.
(a) Sender
Transmit the next
frame
NAK
Feedback
Retransmit the
current frame
ACK
(b) Receiver
Get next frame
from the physical
layer
Send ACK
Correct
Check the
frame
Erroneous
Send NAK
Cooperative ARQ: Basic scheme
•
Neighbor nodes:
1. Decode and store a copy of
each frame.
c) Neighbor
Listen to the next
frame
2. Drop the frame if ACK is
received.
Check the
frame
Erroneous
Correct
3. Transmit the frame in NAK
is received.
Listen to the
feedback
NAK
Transmit the
frame
•
Neighbors cooperate if
–
–
They will to cooperate
They have enough resources
ACK
Drop the frame
Cooperative ARQ: Analytical model
• Fading channel model
Received signal power
| (t)|
1-q
q
G
B
r
 (k)=G

 (k)=B
0
Time
 B |  (k ) | 
 (k )  
G |  (k ) | 
1-r
Cooperative ARQ: Analytical model
• Three steps:
– Model cooperation of a single node
– Combine multiple nodes into a super
node
– Obtain the protocol model
Cooperative ARQ:
Cooperation model of a single neighbor node
•
A tagged neighbor can
help if:
Neighbor i
Interim channel i
Relay channel i
1. It has correctly
received the previously
transmitted frame
Primary
channel
Sender
Receiver
(1-x)(1-b)
(1-x)(1-a)
(1-x)a
S0={GG}
S1={GB}
AND
(1-x)b
xa
x(1-a)
2. Its channel to the
receiver node is in good
condition.
xb
y(1-a)
x(1-b)
ya
y(1-b)
yb
(1-y)a
S2={BG}
S3={BB}
(1-y)b
(1-y)(1-a)
(1-y)(1-b)
Cooperative ARQ:
Cooperation model of multiple neighbor node
•
What if there are two
neighbor nodes?
–
(1-u1)(1-v2)
(1-u1)(1-u2)
Model as a single node
with a better cooperation
capability
(1-u1)u2
S0={C,C}
S1={C,NC}
(1-u1)v2
u1u2
u1(1-u2)
u1v2
u1(1-v2)
v1(1-u2)
v1u2
v1(1-v2)
v1 v2
(1-v1)u2
S2={NC,C}
S3={NC,NC}
(1-v1)v2
•
More than two neighbor
nodes:
–
(1-v1)(1-u2)
Iterative combination of all
neighbor nodes into a super
node
(1-v1)(1-v2)
Cooperative ARQ: The protocol model
•
The cooperation group is either in
Transmission state (T) or
Retransmission state (R).
X
1-X
T
R
1-Y
Y
O(k): Status of the protocol at discrete time k
O(k-1)
P(k)
N(k)
O(k)
P(k): Status of the primary channel
T
G
C
T
N(k): Status of the super node
T
G
NC
T
T
B
C
R
G: Good state
T
B
NC
R
B: Bad state
R
G
C
T
C: Cooperative state
R
G
NC
T
NC: Non Cooperative state
R
B
C
T
R
B
NC
R
Cooperative ARQ: The protocol model
S0
S1
X
S4
S5
S2
S6
S3
S7
Y
PS 2  PS3
PS0  PS1  PS 2  PS3
PS 4  PS5  PS6
PS 4  PS5  PS6  PS7
Cooperative ARQ: Application of the model
• Throughput:
 NCSW
Y

X Y
• Delay:
Definition of
delay: the total
time required to
transmit a single
packet from the
network layer
• Average delay:
Fragment 1
Fragment 2
Packet from upper layer
...
–
Fragment np
X Y
Dav 
Tf
Y
Cooperative ARQ: Application of the model
1
•
For a packet with np
fragments:
Snp,T
S(np-1),T
1-q
2np
q
1-q
...
2np - 2
q
r
Snp,R
S(np-1),R
2np -1
2np - 3
1-r
1-r
1-q
S(np-1),T
2
0
q
r
...
S(np-1),R
1
1-r
Di : The number of transitio ns from state i to the absorbing state
d i  E[ Di ]
 i2  E[ Di2 ]
•
Delay jitter:
 D  T f  22n  d 22n
p
p
S(np-1),T
1-q
r
Cooperative ARQ: Simulations
Parameters
Carrier freq.
2400 MHz
Maximum Doppler freq. shift
11 Hz
Frame duration
5 ms
Channel simulation
Sampling rate of fading channel
Jake’s model
8000 sample/s
Cooperative ARQ: Simulations

| (t)|
The definition of the normalized inverse fading margin
E[| (t)|]
•
0
Time
Normalized inverse fading margin:

L
E[|  (k ) |]
Cooperative ARQ:
Simulation results: Normalized throughput
N=2 (number of neighbor nodes)
1
0.9
0.8
Throughput
•
0.7
0.6
0.5
SW simulation
SW analytical
NCSW simulation (Lr=-5)
0.4
NCSW analytical (Lr=-5 dB)
NCSW simulation (Lr=-1 dB)
-5
NCSW analytical (Lr=-1 dB)
-4.5
-4
-3.5
-3
-2.5
Lp in dB
-2
-1.5
-1
-0.5
0
Cooperative ARQ:
Simulation results: Normalized throughput
Lp=-1 dB
0.85
0.8
Throughput
0.75
0.7
0.65
0.6
simulation (Lr=-5 dB)
analytical (Lr=-5 dB)
0.55
0.5
simulation (Lr=-1 dB)
analytical (Lr=-1 dB)
0
1
2
3
4
5
6
Number of neighbors
7
8
9
10
-1.5
-1
-0.5
0
0.75
Lp=-1 dB
N=2
0.74
0.73
Throughput
0.72
0.71
0.7
0.69
0.68
0.67
0.66
-5
Throughput vs. interim channel
Throughput vs. relay channel
-4.5
-4
-3.5
-3
-2.5
-2
Linterim/Lrelay in dB
Simulation results: Delay and Jitter
280
N=2
np=20
SW simulation
SW analytical
NCSW simulation
NCSW analytical
260
240
Delay (ms)
220
200
180
160
140
120
100
-5
-4.5
-4
-3.5
-3
-2.5
-2
Lp in dB
-1.5
-1
-0.5
0
-3
-2.5
-2
Lp in dB
-1.5
-1
-0.5
0
140
SW simulation
SW analytical
NCSW simulation
NCSW analytical
120
Jitter (ms)
100
80
60
40
20
0
-5
-4.5
-4
-3.5
Cooperative ARQ:
Summary and further direction
• Cooperation of few nodes can improve
performance of ARQ scheme significantly.
• Cooperative ARQ is backward compatible.
• There is not much signaling or
maintenance overhead.
• Further extensions:
–
Non-ideal feedback channels