A Power Controlled Multiple Access Protocol for Wireless

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A Power Controlled Multiple

Access Protocol for Wireless

Packet Networks

Jeffrey P. Monks, Vaduvur Bharghavan, and

Wen-mei W. Hwu University of IllinoisUrbana-Champaign, IL 61801


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Outline

INTRODUCTION

THE PROBLEM AND APPROACH TO THESOLUTION

THE PCMA PROTOCOL

PERFORMANCE OF PCMA CONCLUSION


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INTRODUCTION

A major issue in wireless networks is developingefficient medium access protocols that optimizespectral reuse

CSMA/CA fixed power controlled on/offcollision avoidance model

PCMA Power Controlled Multiple Access, usingvariable bounded power collision suppressionmodel

power:The rate of transfer or absorption of energy perunit time in a system


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INTRODUCTION (cont.)

PCMA

Does not require the presence of base stations to

manage transmission power (decentralized)Allows a greater number of simultaneous

transmissions (spectral reuse)

Improvements in aggregate channel utilization bymore than a factor of 2 compared to the IEEE

802.11 protocol standard


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THE PROBLEM AND APPROACH

TO THE SOLUTIONCSMA/CA in Wireless Networks

Time C A B D

RTS

CTS

DATA

ACK

A: sender

B: receiver

C: exposed station (within range

of sender, but not receiver)

D: hidden station (within range of

receiver, but not sender)

C hears RTS, defers

transmission

C doesnt hear CTS, resumes

transmission

D hears CTS, defers transmission

D hears ACK, resumes

transmission


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TO THE SOLUTION (cont.)

D C B A

Traditional CSMA/CA protocol : A could not send to B

If C reduced its transmission power such that it would be just enough for

D to capture its signal then other nodes in the region could also proceed

with their transmission


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TO THE SOLUTION (cont.)We can achieve PCMA by adhering to two key

principles

1.The power conserving principle: each station musttransmit at the minimum power level that is required to be

successfully heard by its intended receiver under current network

conditions

2.The cooperation principle: no station that commencesa new transmission must transmit loud enough to disrupt ongoing

transmissions

For these purposes every station must advertise its noise tolerance


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TO THE SOLUTION (cont.)Channel Propagation Models The amount of transmission power required for a node to

send a valid signal to its destination will depend on the gainbetween each source and destination.

Gain (Gij): The ratio of output current power to input current power

G is proportional to 1/d2 (inside the Fresnel zone), or 1/d4

(outside the Fresnel zone) Fresnel Zones are a series of concentric ellipsoids

surrounding the radio path
http://f/work/Fresnel.ppthttp://f/work/Fresnel.ppt


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TO THE SOLUTION (cont.) Channel Propagation Models (cont.)

A BData channel

Busy tone channel

In PCMA, we assume that:1. Data channel gains busy tone channel gains

2. GAB GBA

3. The channel gain is stationary for the duration of the control and

data packet transmissions


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TO THE SOLUTION (cont.) Channel Propagation Models (cont.)Three basic channel effects path loss, shadowing, multipath

path loss shadowing multipath

Assumption 1 N/A N/A N/A

Assumption 2 N/A N/A Yes

Assumption 3 Little Little Yes


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TO THE SOLUTION (cont.)Power ConstraintsPt_Max, Pt_Min : the maximum and minimum transmission powers for a

transmitter on the data channel, respectively

RX_Thresh, CS_Thresh : the minimum received signal power for receiving

a valid packet and for sensing a carrier, respectively

SIR_Tresh : minimum signal to interference ratio for which the receiver

can successfully receive a packet

Pti : the minimum power which a transmitterimust use to transmit apacket to a receiverj

Ek : the noise tolerance of a receiver k


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TO THE SOLUTION (cont.)Power Constraints (cont.) Pt_Min Pti Pt_Max

PtiGij RX_Thresh SIRj= PtiGij/ Pnj SIR_Thresh where Pnj=

+Nj, Njis the thermal noise

Ek

= (Prk

/ SIR_Thresh) Pnk

for all k, PtiEk/Gik

Pti mink{ Ek/Gik} = Pt_boundi

il

ljlGPt

KPr

k


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THE PCMA PROTOCOL

PCMA Protocol Overview request-power-to-send (RPTS) / acceptable-power-to-send

(APTS) handshake VS. RTS / CTS in IEEE 802.11 RPTS and APTS used to determine the minimum transmission power

that the sender must use

Noise tolerance advertisement is periodically pulsed in

busy tone channel VS. Carrier sense

The signal strength of the pulse indicates the noise tolerance


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THE PCMA PROTOCOL (cont.)

PCMA VS IEEE 802.11 (collision avoidance)

PCMA IEEE 802.11

sender Monitoring the busy

tone channel

Sensing the carrier

receiver Periodically pulsing

the busy tone

Sending a CTS

handshake RPTSAPTSDATAACK RTSCTSDATAACK

Power control Bounded power

model

On/of model


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THE PCMA PROTOCOL (cont.)

PCMA Protocol Steps

RPTS

APTS

DATA

Send Busy Tone pulses

ACK

i j kstep1

step2

step3

step4 step5

step6

step7

time


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PERFORMANCE OF PCMA

IPC , PCMA , and IEEE 802.11

Ideal power controlled protocol (IPC)

IPC is provided with perfect knowledge ofthe link gain between any two nodes, the

noise at any potential destination , and

the upper bound on a transmitters signalpower


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PERFORMANCE OF PCMA (cont.)

The source node is picked randomly

from the set of all nodes and the

destination is picked randomly from

the set of all nodes one hop away

Each data transmission between

source and destination will be referred

to as a flow

Flow rate refers to the number of

packets sent per second


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The performance of PCMA is demonstrated for differing number of busy tone pulses

sent per data transmission period (1, 4, 16, 64)

Sending busy tone

pulse for every 128

bytes


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The region is smaller than the transmission range. PCMA and 802.11 almost the same

throughput performance


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PCMA can sent packets simultaneously in both clusters by reducing its transmission

power


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Unfair phenomenon

if network load increases

1. the expected power for a source to reach its destination willincrease (increasing background noise)

2. the expected power bound decrease (increasing exposedreceivers)

source are more likely to backoff allowing a greater number ofshort range transmissions

unfair favoritism toward source-destination pairs sending overshorter distances


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The fraction of total packets received by destinations in five distance ranges

A perfect fair

protocol should

like this

source


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Packets

lost


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PERFORMANCE OF PCMA (cont.) Fixed power fair but more packets lost Unfixed power not fair but less packets lost

Fairness is improved

due to power boundincreasing, but less

throughput


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Mulitpath effect on the three assumption

X (dB) denotes the channel gain, X = -u with a

probability , u with probability , 0 withprobability ,


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CONCLUSION

PCMA allows for a greater number of

simultaneous senders than 802.11

PCMA can achieve more than a 2 timesimprovement in aggregate bandwidth compared to

802.11 for highly dense networks

PCMA is still a protocol design in progress, so

fairness properties and performance undermobility must be ongoing work

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A Power Controlled Multiple Access Protocol for Wireless

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