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Power Control and Cross-Layer Design in Ad-Hoc and Sensor Networks

ECSE 6962

Di Wang11/07/2005

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Outline

OverviewDesign Principles for Power ControlPower Control ProtocolsUnintended ConsequencesControl-Theory Based ApproachConclusionReference

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Overview

Why is Power Control Important?Limited resources of energyAiming to bring better performances: Throughput, Delay,…

Why is Power Control a Cross-Layer Design Problem?Affect the physical layer: quality of the signalAffect the network layer: range of transmissionAffect the transport later: magnitude of the interference

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Overview

Multi-dimensional EffectMac Layer Performance: contention for the mediumTopology Control Problem: Connectivity of the networkEffect on several important metrics:

Energy ConsumptionThroughput CapacityEnd-to-End Delay

Impact on protocols in existenceCreate unidirectional linksAffect MAC/routing protocols:

Distributed Bellman Ford, RTS/CTS handshake in IEEE 802.11

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Design Principles For Power Control

To increase network capacity it is optimal to reduce the transmit power level

For transmit range r: The area of interference is proportional to r2

The relaying burden is proportional to 1/r, Then

The area consumed by a packet is proportional to r

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Reducing the transmit power level reduces the average contention at the MAC layer

For any given point in the domain:An average of cr2 transmitters within range;Traffic flowing through each node is proportional to 1/r, then

The net radio traffic in contention range is proportional to r

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The impact of power control on total energy consumption depends on the energy consumption pattern of the hardwareTerms:

PRxelec: the power consumed in the receiver electronics for processingPTxelec: the power consumed in the transmitter electronics for processingPTxRad(p): Power consumed by the power amplifier to transmit a packet at the power level pPIdle , PSleep

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The impact of power control on total energy consumption

If the energy consumed for transmission, PTxRad(p), Dominates:

Using low power level is broadly commensurate with energy efficient routing for commonly used inverse αth law path loss models, with α≥2

Energy efficient routing seeks to minimize :Can get the graph consisting of edges lying along some power optimal route between any pair of nodes

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Connections only with nearby nodes, and no intercections

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For α=2, can find an angle j < 90:

( ) ( ) ( )2ji2

jl2

li xxxxxx −<−+−

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When PSleep is much less than PIdle:

turning the radio off whenever possible becomes an important energy saving strategy

Estimates show that usually PIdle > 20PSleep

Power management protocols seeking to put nodes to sleep while maintaining the network connectivity

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When a common power level is used throughout the network:

There exists a critical transmission range rcrit, below which transmissions are sub-optimal with regards to energy consumptionGiven two nodes with distance d, the energy consumed for transmitting one packet:

Which can be minimized at:

( )αcrPPrd

TxelecRxelec ++

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The impact of power control on end-to-end delay

Power level and Traffic load jointly determine the end-to-end delay

Under high load a lower power gives lower delayUnder low load a higher power gives lower delay

A packet experiences:Propagation delay: neglectableProcessing delay: time taken in receiving, decoding and retransmitting, inversely proportional to range r; Queuing delay: can be shown it increases super-linearly with the power level p

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The impact of power control on end-to-end delay

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Power control can be regarded as a network layer problem

In fact it impacts multiple layersNumerous approaches attempt to solve it at MAC Layer

Adjust the transmit power level to make the SINR just enough for receiver to decode the packetOnly a local optimization

Network layer power control is capable of a global optimization

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Power Control Protocols

COMPOW ProtocolDesign Strategies

Choose a common power level;Set this power level to the lowest value which keeps the network connected;Keeps the energy consumption close to minimum, while restricting the lowest admissible power level to rcrit.

ImplementationRunning multiple proactive routing protocols at each power level, and find out the routing table with lowest p.

Appealing feature: Provides bidirectional links

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CLUSTERPOW ProtocolCOMPOW is not energy-efficient when there are outlying nodeDesign Strategies:

Select n different power levels to form a n-level hierarchical structure

ImplementationBuilding routing table for each power levelTransmitting packet at the smallest power level p such that the destination can be found on the p-routing table.

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CLUSTERPOW Protocol

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CLUSTERPOW Protocol

CLUSTERPOW is loop freeStill can be further improved

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Recursive Lookup Schemes

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Recursive Lookup Schemes

may not be loop-free

Solution: Tunnelled CLUSTERPOW

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Tunnelled CLUSTERPOW ProtocolWhen doing recursive lookup for an intermediate node, encapsulates the packet with the address of the node.

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MINPOW ProtocolDesign Objective: Provide a globally optimal solution with respect to total power consumptionImplementation:

Proactively sends “hello” at multiple transmit power levelsOnly the “hello” packets at the Pmax contain routing updatesFor each link, computes the power consumption per packet PTxtotal = PTxelec + PTxrad(p) at all power level and take the minimum as the link cost in the distance vector algorithm

Feature: a globally optimal solution for power consumption, but may not be the optimal solution for network capacity

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Simulation Results

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Simulation Results

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Simulation Results

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Unintended Consequences

Power Control can be addressed as

Multi-dimensional OptimizationUsually one objective is achieved at the expense of one another

Cross-Layer OptimizationShould not ignore the interactions between different layers

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Unintended Consequences

Example: the MINPOW Power Control ProtocolCompared with MHRP/802.11 solution (Min-Hop Routing)

MHRP/802.11: A->B and E->D can happen

concurrently

MINPOW: A has to resort to C to send

packets to BThen E->D cannot happen

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Control-Theory Based Approach Channel Model

It is simple to use the inverse αth law path loss model

It will be rather complicated when taking into account the time-variance of the channel gain

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Control-Theory Based Approach

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Feedback-based Power Control

Can Derive the closed loop system:

Time delay can be compensated for using the Smith predictor

Predict the power gain to improve the reactions so as to decrease the disturbulance

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Ts=0.015(solid)Ts=0.05(dashed)

With Smith Predictor (dark)Without Predictor (light)

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Conclusion

Power Control can be addressed as a cross-layer design problem, which involves a multi-dimensional optimization;Introduced the impact of power control on a variety of parameters and phenomenon, and then presented fundamental design principles;Introduced power control protocols achieving successful power saving, but sometimes at the expense of a reduction in the sense of other metrics;Put power control algorithms into a control theory context

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Reference

1. Kawadia, V.; Kumar, P.R.; Principles and protocols for power control inwireless ad hoc networks, Selected Areas in Communications, IEEE Journal on Volume 23, Issue 1, Jan. 2005 Page(s):76 – 88

2. Krunz, M.; Muqattash, A.; Sung-Ju Lee; Transmission power control in wireless ad hoc networks: challenges, solutions and open issuesNetwork, IEEE Volume 18, Issue 5, Sept.-Oct. 2004 Page(s):8 - 14

3. Fredrik Gunnarsson, Fredrik Gustafsson, Power control in Wireless Communications Networks – From a Control Theory Perspective

4. Cautionary Aspects of Cross Layer Design: Context, Architecture and Interactions, http://www.eas.asu.edu/~junshan/ICC/KumarICC.pdf

5. S. Narayanaswamy, V. Kawadia, R. S. Sreenivas, and P. R. Kumar, “Power control in ad-hoc networks: Theory, Architecture, Algorithm and implementation of the COMPOW protocol,” in European Wireless Conference, 2002.

http://www.eas.asu.edu/~junshan/ICC/KumarICC.pdf

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