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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 Illinois Urban

a-Champaign, IL 61801

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Outline

INTRODUCTION THE PROBLEM AND APPROACH TO THE

SOLUTION THE PCMA PROTOCOL PERFORMANCE OF PCMA CONCLUSION

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INTRODUCTION

A major issue in wireless networks is developing efficient medium access protocols that optimize spectral reuse

CSMA/CA – fixed power controlled “on/off” collision avoidance model

PCMA – Power Controlled Multiple Access, using “variable bounded power” collision suppression model

power: The rate of transfer or absorption of energy per unit 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 by

more 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

RTSCTS

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 doesn’t hear CTS, resumes transmission

D hears CTS, defers transmission

D hears ACK, resumes transmission

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

principles1.The power conserving principle: each station must

transmit 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 commences a new transmission must transmit loud enough to disrupt ongoing transmissions

For these purposes every station must advertise its noise tolerance

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THE PROBLEM AND APPROACH 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 gain between 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

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THE PROBLEM AND APPROACH 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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THE PROBLEM AND APPROACH 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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THE PROBLEM AND APPROACH 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 transmitter i must use to transmit a packet to a receiver j

Ek : the “noise tolerance” of a receiver k

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THE PROBLEM AND APPROACH TO THE SOLUTION (cont.)Power Constraints (cont.) Pt_Min ≦ Pti ≦ Pt_Max PtiGij ≧ RX_Thresh SIRj = PtiGij / Pnj ≧ SIR_Thresh where Pnj =

+Nj , Nj is the thermal noise Ek = (Prk / SIR_Thresh) – Pnk

for all k, Pti ≦ Ek/Gik

∴Pti ≦ mink{ Ek/Gik } = Pt_boundi

il

ljlGPt

KPrk

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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 RPTS→APTS→DATA→ACK

RTS→CTS→DATA→ACK

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 of

the link gain between any two nodes, the noise at any potential destination , and the upper bound on a transmitter’s signal power

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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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PERFORMANCE OF PCMA (cont.)

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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PERFORMANCE OF PCMA (cont.)

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

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PERFORMANCE OF PCMA (cont.)

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

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PERFORMANCE OF PCMA (cont.)

Unfair phenomenonif network load increases 1. the expected power for a source to reach its destination will inc

rease (increasing background noise) 2. the expected power bound decrease (increasing exposed recei

vers) source are more likely to backoff allowing a greater number of

short range transmissions unfair favoritism toward source-destination pairs sending over

shorter distances

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PERFORMANCE OF PCMA (cont.)

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

A perfect fair protocol should

like this

source

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PERFORMANCE OF PCMA (cont.)

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 bound increasing, but less

throughput

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PERFORMANCE OF PCMA (cont.)

Mulitpath effect on the three assumption

X (dB) denotes the channel gain, X = -u with a probability ¼, u with probability ¼, 0 with probability ½,

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PERFORMANCE OF PCMA (cont.)

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PERFORMANCE OF PCMA (cont.)

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CONCLUSION

PCMA allows for a greater number of simultaneous senders than 802.11

PCMA can achieve more than a 2 times improvement in aggregate bandwidth compared to 802.11 for highly dense networks

PCMA is still a protocol design in progress, so fairness properties and performance under mobility must be ongoing work