Chapter 10Cooperation Link Level

Retransmission in Wireless Networks

M. Dianati, X. Shen, and K. Naik

ScopeApplication

Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Application Layer

TCP

IP

Data Link Layer

Physical Layer

ISO OSI ModelThe Internet

Model

Link adaptation

layers

Network and unification

layers

Application adaptation and

interfacing layers

• Link and MAC layer for fading channels

• Two parts:

– Cooperative Scheduling

– Cooperative ARQ

Introduction• Challenges in

wireless domain:

– Fading– Interference– Limited bandwidth

• Potentials:

– Again, fading– Spatial

diversity

0 10 20 30 40 50 60 70 80 90 100-6

-5

-4

-3

-2

-1

0

1

2

3

Time (ms)

Env

elop

e le

vel (

dB)

Sample fading process

Introduction:Stochastic model of flat fading process:

)()()( tjgtgtg QI

Complex envelope of

fading process:

0)(

0

||/1

1

2)(

QQ

II

gg

m

mm

p

gg

S

otherwise

ffffffS

Power spectrum density:

Power spectrum density

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 11

2

3

4

5

6

7

8

Sg Ig I(f

)/(

p/2

f m)

Normalized Doppler frequency f/fm

Fading process is a non-white stochastic process with relatively slow variations.

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

Sender

Receiver

Neighbor

Neighbor

• Let neighbor nodes assist the retransmission trials

Transmission

X

Cooperative ARQ: Basic idea

Sender

Receiver

Neighbor

Neighbor

• Let neighbor nodes join retransmission

NAK

Negative or positive ACK

Sender

Receiver

Neighbor

Neighbor

Cooperative ARQ: Basic idea

• Let neighbor nodes join retransmission

Retransmission

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

Coop. group 2

Coop. group 1

Sender ReceiverPrimary channel

Cooperation group

Neighbor 1

Neighbor N

Interim channel

Relay channel

• Network model

• A single cooperation group

Cooperative ARQ: Basic scheme• Sender and receiver nodes

perform their normal operations.

Transmit the nextframe

FeedbackRetransmit thecurrent frame

NAK

ACK

Get next framefrom the physical

layer

Check theframe

Send NAKErroneousCorrect

(a) Sender

(b) Receiver

Send ACK

Cooperative ARQ: Basic scheme• Neighbor nodes:

1. Decode and store a copy of each frame.

2. Drop the frame if ACK is received.

3. Transmit the frame in NAK is received.

• Neighbors cooperate if– They will to cooperate– They have enough resources

Listen to the next frame

Check the frame Erroneous

c) Neighbor

Listen to the feedback

Correct

ACK

NAK

Drop the frame

Transmit the frame

Cooperative ARQ: Analytical model

• Fading channel model

G B1-q 1-r

r

q

|)(|

|)(|)(

kG

kBk

Received signal power

Time0

|(t)|

(k)=G

(k)=B

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:

1. It has correctly received the previously transmitted frame

AND

2. Its channel to the receiver node is in good condition.

SenderReceiver

Primary channel

Neighbor i

Interim channel iRelay channel i

S0={GG}

S3={BB}S2={BG}

S1={GB}

(1-x)(1-a)

(1-x)a

xa

x(1-a)

(1-x)(1-b)

xb

(1-x)b

x(1-b) y(1-b)

(1-y)(1-b)

(1-y)b

yb

(1-y)(1-a)

(1-y)a

y(1-a)

ya

Cooperative ARQ: Cooperation model of multiple neighbor node

• What if there are two neighbor nodes?

– Model as a single node with a better cooperation capability

S0={C,C}

S3={NC,NC}S2={NC,C}

S1={C,NC}

(1-u1)(1-u2)

(1-u1)u2

(1-u1)(1-v2)

(1-u1)v2

u1(1-u2) v1(1-u2)

u1u2

(1-v1)(1-u2)

u1v2

u1(1-v2) v1(1-v2)

(1-v1)u2

(1-v1)v2

v1v2

(1-v1)(1-v2)

v1u2

• More than two neighbor nodes:

– Iterative combination of all neighbor nodes into a super node

Cooperative ARQ: The protocol model

• The cooperation group is either in Transmission state (T) or Retransmission state (R).

O(k-1) P(k) N(k) O(k)

T G C T

T G NC T

T B C R

T B NC R

R G C T

R G NC T

R B C T

R B NC R

O(k): Status of the protocol at discrete time k

P(k): Status of the primary channel

N(k): Status of the super node

G: Good state

B: Bad state

C: Cooperative state

NC: Non Cooperative state

T R1-X 1-Y

Y

X

Cooperative ARQ: The protocol model

7654

654

3210

32

SSSS

SSS

SSSS

SS

PPPP

PPPY

PPPP

PPX

S0

S3

S2

S1

S7

S6

S5

S4

Cooperative ARQ: Application of the model

• Throughput: YX

YNCSW

• Delay:

– Definition of delay: the total time required to transmit a single packet from the network layer

• Average delay:

Packet from upper layer

Fragment 1

Fragment 2

Fragment np

...

fav TY

YXD

Cooperative ARQ: Application of the model

• For a packet with np fragments:

Snp,T

2np

Snp,R

2np -1

q

1-r

S(np-1),T

2np - 2

S(np-1),R

2np - 3

q

1-r

S(np-1),T

2

S(np-1),R

1

q

1-r

S(np-1),T

0

1-q 1-q 1-q 1-q

r r r

1

. . .

. . .

• Delay jitter:

][

][

state absorbing the to state from ns transitioofnumber The :

22ii

ii

i

DE

DEd

iD

22

22 pp nnfD dT

Cooperative ARQ: Simulations

Parameters

Carrier freq. 2400 MHz

Maximum Doppler freq. shift 11 Hz

Frame duration 5 ms

Channel simulation Jake’s model

Sampling rate of fading channel 8000 sample/s

Cooperative ARQ: Simulations• The definition of the normalized inverse fading margin

Time

E[| (

t)|]

| (

t)|

0

Normalized inverse fading margin: |])([| kE

L

Cooperative ARQ: Simulation results: Normalized throughput

• N=2 (number of neighbor nodes)

-5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0

0.4

0.5

0.6

0.7

0.8

0.9

1

Lp in dB

Thr

ough

put

SW simulation

SW analyticalNCSW simulation (L

r=-5)

NCSW analytical (Lr=-5 dB)

NCSW simulation (Lr=-1 dB)

NCSW analytical (Lr=-1 dB)

Cooperative ARQ:Simulation results: Normalized throughput

Lp=-1 dB

0 1 2 3 4 5 6 7 8 9 100.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

Number of neighbors

Thr

ough

put

simulation (Lr=-5 dB)

analytical (Lr=-5 dB)

simulation (Lr=-1 dB)

analytical (Lr=-1 dB)

-5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 00.66

0.67

0.68

0.69

0.7

0.71

0.72

0.73

0.74

0.75

Linterim

/Lrelay

in dB

Thr

ough

put

Throughput vs. interim channel

Throughput vs. relay channel

Lp=-1 dBN=2

Simulation results: Delay and Jitter

N=2 np=20

-5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0100

120

140

160

180

200

220

240

260

280

Lp in dB

Del

ay (

ms)

SW simulation

SW analytical

NCSW simulation

NCSW analytical

-5 -4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 00

20

40

60

80

100

120

140

Lp in dB

Jitt

er (

ms)

SW simulation

SW analytical

NCSW simulation

NCSW analytical

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