1 Power Control and Cross-Layer Design in Ad-Hoc and Sensor Networks Di Wang 11/07/2005

2 Outline Overview Design Principles for Power Control Power Control Protocols Unintended Consequences Control-Theory Based Approach Conclusion Reference

3 Overview Why is Power Control Important? Limited resources of energy Aiming to bring better performances: Throughput, Delay,… Why is Power Control a Cross-Layer Design Problem? Affect the physical layer: quality of the signal Affect the network layer: range of transmission Affect the transport later: magnitude of the interference

4 Overview Multi-dimensional Effect Mac Layer Performance: contention for the medium Topology Control Problem: Connectivity of the network Effect on several important metrics: Energy Consumption Throughput Capacity End-to-End Delay Impact on protocols in existence Create unidirectional links Affect MAC/routing protocols: Distributed Bellman Ford, RTS/CTS handshake in IEEE

5 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 r 2 The relaying burden is proportional to 1/r, Then The area consumed by a packet is proportional to r

6 Design Principles For Power Control Reducing the transmit power level reduces the average contention at the MAC layer For any given point in the domain: An average of cr 2 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

7 Design Principles For Power Control The impact of power control on total energy consumption depends on the energy consumption pattern of the hardware Terms: P Rxelec : the power consumed in the receiver electronics for processing P Txelec : the power consumed in the transmitter electronics for processing P TxRad (p): Power consumed by the power amplifier to transmit a packet at the power level p P Idle, P Sleep

8 The impact of power control on total energy consumption If the energy consumed for transmission, P TxRad (p), Dominates: Using low power level is broadly commensurate with energy efficient routing for commonly used inverse α th 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

9 The impact of power control on total energy consumption Connections only with nearby nodes, and no intercections

10 The impact of power control on total energy consumption For α=2, can find an angle j < 90:

11 The impact of power control on total energy consumption When P Sleep is much less than P Idle : turning the radio off whenever possible becomes an important energy saving strategy Estimates show that usually P Idle > 20P Sleep Power management protocols seeking to put nodes to sleep while maintaining the network connectivity

12 The impact of power control on total energy consumption When a common power level is used throughout the network: There exists a critical transmission range r crit, below which transmissions are sub-optimal with regards to energy consumption Given two nodes with distance d, the energy consumed for transmitting one packet: Which can be minimized at:

13 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 delay Under low load a higher power gives lower delay A packet experiences: Propagation delay: neglectable Processing 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

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

15 Design Principles For Power Control Power control can be regarded as a network layer problem In fact it impacts multiple layers Numerous approaches attempt to solve it at MAC Layer Adjust the transmit power level to make the SINR just enough for receiver to decode the packet Only a local optimization Network layer power control is capable of a global optimization

16 Power Control Protocols COMPOW Protocol Design 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 r crit. Implementation Running multiple proactive routing protocols at each power level, and find out the routing table with lowest p. Appealing feature: Provides bidirectional links

17 Power Control Protocols CLUSTERPOW Protocol COMPOW is not energy-efficient when there are outlying node Design Strategies: Select n different power levels to form a n-level hierarchical structure Implementation Building routing table for each power level Transmitting packet at the smallest power level p such that the destination can be found on the p-routing table.

18 CLUSTERPOW Protocol

19 CLUSTERPOW Protocol CLUSTERPOW is loop free Still can be further improved

20 Power Control Protocols Recursive Lookup Schemes

21 Recursive Lookup Schemes may not be loop-free Solution: Tunnelled CLUSTERPOW

22 Power Control Protocols Tunnelled CLUSTERPOW Protocol When doing recursive lookup for an intermediate node, encapsulates the packet with the address of the node.

23 Power Control Protocols MINPOW Protocol Design Objective: Provide a globally optimal solution with respect to total power consumption Implementation: Proactively sends “hello” at multiple transmit power levels Only the “hello” packets at the P max contain routing updates For each link, computes the power consumption per packet P Txtotal = P Txelec + P Txrad (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

24 Power Control Protocols Simulation Results

25 Simulation Results

26 Simulation Results

27 Unintended Consequences Power Control can be addressed as Multi-dimensional Optimization Usually one objective is achieved at the expense of one another Cross-Layer Optimization Should not ignore the interactions between different layers

28 Unintended Consequences Example: the MINPOW Power Control Protocol Compared with MHRP/ 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 B Then E->D cannot happen

29 Control-Theory Based Approach Channel Model It is simple to use the inverse α th law path loss model It will be rather complicated when taking into account the time-variance of the channel gain

30 Control-Theory Based Approach

31 Control-Theory Based Approach 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

32 Control-Theory Based Approach T s =0.015(solid) T s =0.05(dashed) With Smith Predictor (dark) Without Predictor (light)

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34 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

35 Reference Kawadia, V.; Kumar, P.R.; Principles and protocols for power control in wireless ad hoc networks, Selected Areas in Communications, IEEE Journal on Volume 23, Issue 1, Jan Page(s):76 – 88 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 Page(s): Fredrik Gunnarsson, Fredrik Gustafsson, Power control in Wireless Communications Networks – From a Control Theory Perspective Cautionary Aspects of Cross Layer Design: Context, Architecture and Interactions,