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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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
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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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 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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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 transmitterimust use to transmit apacket to a receiverj
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, 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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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 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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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 boundincreasing, 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 , u with probability , 0 withprobability ,
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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 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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