A Distributed Relay-Assignment Algorithm A Distributed Relay-Assignment Algorithm for Cooperative Communications in for Cooperative Communications in

Wireless NetworksWireless Networks

ICC 2006

Ahmed K. Sadek, Zhu Han, and K. J. Ray LiuDepartment of Electrical and Computer Engineering,

and Institute for Systems ResearchUniversity of Maryland, College Park

Presented by Sookhyun Yang

2/14

Nearest Neighbor Protocol (NNP) Scheme• Is the relay assignment scheme at the cooperative networking

– Selects the nearest neighbor from the source as the relay node

– Considers only the uplink (toward the BS/AP)

• Neighbor discovery– Each relay sends out “Hello” message

– Each source node can know its distance to the BS/AP using TOA (Time of Arrival)

SourceBS/AP

Nearest neighbor relay

Relay

Relay

Direct transmission

Relay

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Source’s Location Distribution Model

• Derive PDF for the source location distribution when the distance between the user and the BS/AP is – The user’s angle is uniformly distributed between, [0, 360o)

Source

BS/AP: Cell radius

: the distance from the source to BS/AP

4/14

Received Signal Model

Transmitted power

Transmitted data with unit power (x=1)

Channel fading

Depends on Antenna’s design (constant)

Distance from source to

destination

Path loss exponent

( are assumed to be the same for all nodes)

noise

• Considered wireless link characteristics for received signal model– Random Rayleigh fading channel between two nodes -

– Propagation path loss – path loss exponent

– Additive white Gaussian noise

– No mutual interference because the nodes use the orthogonal channel• Each node has a single-element antenna and half-duplex mode

Voltage of received

power

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Metric: Outage• Is the event that the received SNR falls below a certain

threshold– It the received SNR is higher than the threshold, the receiver is

assumed to be able to decode the received message with negligible probability of error

– If the outage occurs, the packet is considered lost

• Outage probability

• For comparing the bandwidth efficiency, “outage probability” is computed for both the direct transmission and the nearest neighbor based cooperative transmission

threshold

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Average outage probability over the cell

Outage Probability for the Direct Transmission

The probability that the source is at the distance

Outage probability for the direct transmission when the source locates with the distance from BS/AP

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Outage Probability for NNP-based Cooperative Transmission

BS/APSource

Nearest Neighbor Relay

rsl rld

rsd

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Outage Probability for NNP-based Cooperative Transmission

Outage probability for NNP-based cooperative transmission

Outage probability for the direct transmission

Outage probability for the transmission through relay

Average outage probability over the cell

PDF of the nearest neighbor relay’s location?

depends on

Outage probability PDF of source location

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BS/APSource

Nearest Neighbor Relay

Outage Probability for NNP-based Cooperative Transmission

• The probability that the nearest neighbor is at distance from the source is equivalent to “the probability that the shaded area is empty”

=

Average outage probability for the nearest neighbor relay over the cell

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Approximated Outage Probability Formula for NNP-based Cooperative Transmission

• Because the formula in the previous slide can only be calculated numerically, they derive an approximated expression

• Assumption for the approximation– 1. The outage probability at the nearest neighbor relay is very low

– 2. Consider the worst case of the nearest neighbor selection, “a” or “b” when

Approximation

=

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Simulation Setup• Is modeled as a random Rayleigh fading channel

– Similarly configures with the indoor WLAN

– Cell radius is taken between 10m and 100m

– AWGN (Additive white Gaussian noise): variance = -70dBm

– Path loss exponent = 2.6

– The number of users in the cell attached to the AP = 10

– SNR threshold = 20dB

• For comparing the bandwidth efficiency (low outage probability), the average transmitted power is kept equal in the direct transmission and the cooperative transmission– The transmitted power does not affect the outage probability?

• Plotted the theoretical outage performance and the simulation results for both direct transmission and cooperative transmission– Which simulator?

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Average Outage Probability (y-axis) vs. Cell Radius (x-axis)

Direct transmission

Cooperative transmission

The cell coverage will increase in the cooperative transmission!

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Average Outage Probability (y-axis) vs. Transmission Power (x-axis)

Direct transmission

Cooperative transmissionThe cooperative

transmission is energy efficient

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Discussion• They did not show that the nearest neighbor can have the

best performance comparing with the other neighbors– What about the neighbor which locates in the middle between the

source and the BS/AP?

• They fixed the transmitted power as the same in both cases?– They considered the orthogonal channels for the cooperative

transmission, so the direct transmission will use less transmitted power than the cooperative transmission

Q&AQ&A

16/14

(1)

sdtr rPP

s

d

rP

tP

sdrTransmitted powerReceived power

ArP

P sdt

r 2

4

Path loss exponent

sdtr rPKPDepends on Antenna design

s

d

rP

tP

sdrFor transmitting the data, the antenna focuses on the specific direction (antenna directivity)

Distance

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(1)

sdtr rPKP rsd Py The relationship between Voltage and Power

sdtsd rPKy

sdsdsdtsd nxhrPKy

Multi-path fading

Noise

Voltage

Transmitted data per unit power(Usually x is set as “1”)

From the previous slide

Go to the next slide

18/14

sdsdsdtsd nxhrPKy

0

22

)(N

hrPK

n

hrPK

P

PrSNR sdsdt

sd

sdsdt

noise

signalsd

signalsdsdtsd PhrPKy

(2)

Compute the voltage of signal without noise, where x=1

2

sdsdtsignal hrPKP

?

From the previous slide

Go to the next slide

Voltage

Power

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(3)From the previous slide

0

2

)(N

hrPKrSNR sdsdtsd

)()())(( 02

0

2

sdt

sdsdsdt

sd rPK

NhP

N

hrPKPrSNRP

x

h

x

hexFexf

sdsd

1)(,)( 22

)()())(( 0022

sdth

sdtsdsd rPK

NF

rPK

NhPrSNRP

sd

Use

t

sd

sdt PK

rN

rPK

N

ee

00

11

(h^2 is exponential distribution)