Design Challenges for Energy- Constrained Ad Hoc Wireless Network

1
Design Challenges for Energy- Constrained Ad
Hoc Wireless Network

• 2004. 4. 16
• ???

2
Agenda

• 1. Introduction
• 2. Application
• 3. Cross layer design
• 4. Link design issue
• 5. Medium Access Control Design Issues
• 6. Network Design Issues
• 7. conclusion

3
Introduction
ltCellular system architecturegt

• Divided into cell
• Mobile terminal communicate directly with a base
  station
• There is no peer to peer communication between
  mobiles
• All communication is via base station through
  single-hop routing

4
Introduction

• Each base station is connected by a high-speed
  link (typically wired) to a mobile switching
  center (MSC) that in turn is connected to PSTN.
• Base station/MSC/PSTN infrastructure and
  centralized control
• Most WLAN have similar centralized single hop
  architecture.
• mobile-centralized access point backbone
  Internet, access point performs all networking
  and control functions for mobile nodes.

5
Introduction
ltAd Hoc network structuregt

• Ad Hoc wireless network has peer to peer
  communication, distributed networking, control
  functions among all nodes, and multi-hop routing
• Line width SINR (connectivity)

6
Introduction

• ltsingle hop routinggt
• Node send pkt directly to final destination.
• Excessive interference to surrounding nodes.
• Low rate, high probability of bit error,
  excessive delay
• ltmulti hop routinggt
• Routing via forwarding by intermediate nodes
• Increase capacity of ad hoc wireless networks
  (but achieving these gains through decentralized
  routing remains elusive)

7
Introduction

• ltEnergy constrainedgt
• This is not inherent to all ad hoc wireless
  network. (may be attached to large energy source)
• However many interesting ad hoc network nodes
  will be powered by batteries with limited
  lifetime.
• Interest are devices that cannot be recharged,
  sensors that are imbedded in walls or dropped
  into remote region.

8
Introduction

• All levels of protocol stack minimize power
  requirements
• Sleep modes for nodes conserve standby energy.
• Hardware and operating system design in the node
  can also conserve energy.

9
Introduction

• ltmobilitygt
• Link layer how fast link characteristics change,
  whether or not link connectivity is stable.
• MAC layer how scheduling algorithms perform.
• Network layer performance of different routing
  protocols.

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-- Application -- 1. data network

• Ad Hoc can support data exchange between laptops,
  palmtops, PDA .
• WLAN provide 10100Mb/s like wired LAN for low
  mobility and stationary users
• 802.11b provide data rates on this order. But
  many user, less rate
• Commercial LAN,Not based on Ad Hoc

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-- Application -- 1. data networks

• Wireless MAN
• multi hop routing
• Cover large area
• High data rate
• Link quality of each hop is changed
• Distributed collection of Laptop computer
• canonical example of Ad Hoc wireless data
  network.
• Highly limited in battery power, laptop acting as
  a router for other laptops

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-- Application --2.Home Networks

• Support communication between PC, laptop, PDA,
  cordless phone, consumer electronics,
  entertainment system anywhere in around home.
• All without getting up from bed.
• Sense people and movement, adjust light and
  heating.
• Alert police or fire department.
• Internet for remote control, software upgrade,
  schedule maintenance.

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-- Application --2.Home Networks

• Design challenges
• 1. support QoS
• Data rates delay constraint is quite stringent
  for home entertainment systems
• 2. need standardization
• All home network device follow same
  standardization
• 3. power constraints

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-- Application --2.Home Networks

• ltApproachgt
• Use existing WLAN standard such as 802.11.
• Limitation of 802.11
• Use star architecture all device talk directly to
  single access point. Eliminate benefit of multi
  hop routing
• Limited in data rate (802.11b110Mbps,
  802.11a1070Mbps for pkt data not media streaming

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-- Application --2.Home Networks

• Natural choice peer to peer ad hoc network
• Eliminates overhead of centralized node.
• Need only communicate with their nearest
  neighbors
• Most home networking applications involve
  stationary or low-mobility nodes, so protocols
  need not support high mobility.
• But provide high quality

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-- Application --3. Device Networks

• Primarily intended to replace in convenient
  cabled connections with wireless connections.
• Many of these devices have limited battery life,
  rechargeable.
• Main topology Bluetooth
• Short range communications.
• 10m-1mW, 100m- 100mW
• 2.4GHz (used without any licensing)
• Energy constraints
• 8 Bluetooth devices form star topology with one
  node acting as a master and other nodes acting as
  slaves. Master node is responsible for
  synchronization and scheduling transmissions of
  slave nodes

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-- Application --4. sensor networks

• Have enormous potential for both consumer and
  military applications.
• Enemy targets will be small, mobile.
• Used to identify and track targets.
• Electromagnetic, optical, chemical, biological
  sensors.
• Optical sensor provide navigation, routing
  vehicles around obstacles while guiding them into
  position for defense or attack.

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-- Application --4. sensor networks

• Examples of sensor networks for home environment
  
• electricity, gas, water meters can be read
  remotely through wireless connections.
• Smoke detector
• Detect spread of gas leaks or toxic fumes.

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-- Application --4. sensor networks

• Use at the sites of large accidents
• (collapse of a building)
• Sensor deployed at the site of an accident used
  to track heat, natural gas, toxic substances
• Used to detect and locate trapped survivors
• Low cost, low power sensor being inserted into
  concrete before it is poured
• Must be robust, self-configuring, long lifetime

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Cross-Layer design

• Requirements at each layer
• (1)link layer
• Multiple antenna, coding, power control
• (2) MAC layer
• Power control, scheduling
• (3) network layer
• Energy-constrained, delay constrained routing
• (4) application layer
• Application adaptation

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Cross-Layer design

• Current ad hoc wireless network protocol design
  is based on this approach.
• Designed and operated independently.

22
Cross-Layer design

• Inflexibility of this paradigm result in poor
  performance for ad hoc networks especially when
  energy is constraint or application need high
  bandwidth or stringent delay.
• So, cross layer design support adaptivity and
  optimization across multiple layers

23
Cross-Layer design

• Example 1
• If link connectivity (link SINR) is weak, this
  connectivity information is relayed to higher
  level.
• Network layer rerouting
• Applicationreduce rate compression

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Cross-Layer design

• Example 2
• Consider again weak link.
• Link connectivity can be measured accurately an
  quickly at the link layer.
• Link layer increasing transmit power or error
  correction coding
• However, if weak link is caused by something
  difficult to correct at the link layer, (mobile
  in tunnel) it is better for higher layer respond.
  Delaying pkt transmission until mobile leaves
  tunnel

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Cross-Layer design

• Example3
• If nodes are highly mobile
• Link characteristics and network topology will
  change rapidly.
• Network layer might change routing strategy from
  uni cast to broad cast in direction of intended
  user
• summary
• Integrated approach to adaptive networking each
  layer of protocol stack responding to local
  variations and information from other layer

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--Link Design Issue--

• Wireless channels are difficult communication
  medium.
• Low capacity per unit bandwidth, random
  amplitude, phase fluctuation.
• Goal of link layer design is to achieve rates
  close to fundamental capacity limits using little
  energy.

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--Link Design Issue--1. Fundamental Capacity
Limits

• Fundamental capacity of channel maximum
  data rate that can be transmitted over channel
  with small probability of error.
• (1) Shannon in 1948
• Capacity of an additive white Gaussian noise
  (AWGN)
• CBlog2(1SNR) b/s

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--Link Design Issue--1. Fundamental Capacity
Limits

• (2) recent work
• Capacity of single multiuser fading channels,
  capacity of diversity channels, capacity of
  channels with multiple antennas.
• (3) new area of research
• Channel capacity under hard transmit energy
  constraint

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--Link Design Issue--2. coding

• Channel coding can reduce power
• Engineers pursued use of very complex codes to
  minimize power.
• ?convolutional code
• ?Trellis code
• ?Turbo code

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--Link Design Issue--3. Multiple antenna

• Multiple antennas at the transmitter and receiver
  improve performance and reduce required power.
• Multiple antenna system use diversity, beam
  steering, multiple input multiple output(MIMO)
  techniques.

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--Link Design Issue--3. Multiple antenna

• ? diversity combining

• Mitigate flat fading,combine multiple
  independently fading copies of signal
• Reducing impact of flat fading, diversity
  combining can lead to power savings

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--Link Design Issue--3. Multiple antenna

• ? Beam steering
• Creates effective antenna pattern at the receiver
  with high gain in direction of desired signal and
  low gain in all other directions.
• Combining arrays of antennas with signal
  processing in both space and time
• Improve energy efficiency since transmitter power
  is focused in the direction of its intended
  receiver

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--MAC design Issues--

• Medium access control protocol how
  different users share available spectrum
• 2 components to this spectrum allocation
• How to divide spectrum into different channels
• How to assign these different channels to
  different users.

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--MAC design Issues--1. channelization

• 1. Frequency division
• Bandwidth is divided into nonoverlapping channels
• Each active user is assigned different channel
• Frequency division is the simplest channelization
  technique to implement, but it is rather
  inflexible
• It is difficult to allocate multiple channels on
  demand to a single user, since this requires
  simultaneous demodulation of multiple channels in
  different frequency bands.

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--MAC design Issues--1. channelization

• 2. Time division
• Time is divided into orthogonal time slots.
• One difficulty
• Need for synchronization among all nodes
  transmitting to the same receiver.
• Different nodes have different propagation delays
  to the receiver

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--MAC design Issues--1. channelization

• 3. Code division
• Time and bandwidth are used simultaneously by
  users.
• Orthogonal or semi-orthogonal code.
• There is hard limit on how many users can
  simultaneously occupy system. (orthogonal code)
• Semi-orthogonal code, number of users is
  interference-limited (no hard limit)

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--MAC design Issues--1. channelization

• Semi-orthogonal cod require power control to
  compensate for near-far problem
• Near-far problem
• If one user is very close to receiver, another
  user is very far away.
• If both users transmit ant the same power level,
  interference from close user will swamp signal
  from far user.
• Power control is used that received signal powers
  from all users are same

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--MAC design Issues--2. Random Access

• Most data users do not require continuous
  transmission
• Most systems have many more total users than
  channel.
• Assign channels to active users.

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--MAC design Issues--2. Random Access

• (1) pure Aloha
• Users transmit no channels whenever they have
  data to send
• (2) slotted Aloha
• Only begin transmitting at the start of a time
  slot.

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--MAC design Issues--2. Random Access

• (3) CSMA
• Hidden terminal
• Node 3, 5 wish to transmit to node 4.
• Node 5 starts its transmission
• Node 3 is too far away to detect this
  transmission
• Exposed terminal
• Node 2 wish to transmit to node 1, node 3 to node
  4
• Node 2 sensing and detect node 3s transmit,
  assume ch is busy
• Node 2 will not transmit even though no
  collision

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--MAC design Issues--2. Random Access

• Technique to avoid hidden and exposed terminals
• (1)Four-way handshaking (in 802.11 WLAN)
• (2)Busy tone transmission
• Listening for busy tone no separate control
  channel
• Not real busy tone, bit is set in field on
  control channel.

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--MAC design Issues--3. Scheduling

• If users have long strings of pkt or continuous
  stream data, random access works poorly
  (collision)
• Channel must be assigned to users in more
  systematic fashion by transmission scheduling
• scheduling access protocol is unavailable at
  startup. (use Aloha)

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--MAC design Issues--3. Scheduling

• Systematic approach to initialization that
  combines benefits of random access for burst data
  with scheduling for continuous data PRMA
• PRMA assumes slotted system with both burst,
  continuous users.
• After one user capture channel, she has dedicated
  channel for remainder of her transmission.
• Subsequent transmission are not corrupted by
  channel.

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--MAC design Issues--3. Scheduling

• Data users with short transmission benefit from
  random access protocol assigned to unused slots,
  users with continuous transmission get scheduled
  periodic transmissions after capturing an initial
  time slot

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--Network Design Issue--Routing

• Multihop routing protocol in ad hoc network is
  significant design challenge
• Most work in multihop routing protocols falls
  into 3 main categories flooding, proactive
  routing, reactive routing

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--Network Design Issue--Routing

• (1) flooding
• Broadcast all nodes within receiving range.
• Advantage
• Highly robust to changing network topologies and
  requires little routing overhead.
• Disadvantage
• Multiple copies of same pkt traverse through
  network, wasting bandwidth and battery power
• Just small network

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--Network Design Issue--Routing

• (2) proactive routing
• ?centralized route
• Information about channel conditions and network
  topology are determined by each nod and forwarded
  to a centralized location that computes routing
  tables for all nodes in network.
• ? distributed route
• Most common routing used in ad hoc
• Nodes send their connectivity information to
  neighboring nodes.
• Routes are computed from this local info.

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--Network Design Issue--Routing

• (3) reactive routing
• Routes are created only at initiation of source
  node that has traffic to send to a given
  destination.
• Eliminate overhead of maintaining routing tables
  for routes not currently use.
• Source node initiate a route discovery process
  when it has data to send.
• Route are maintained until source has no more
  data.

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--Network Design Issue--Routing

• ltEnergy constraints in routing protocolgt
• Exchange of routing information between nodes
  entails energy cost.
• Routing protocol under energy constraints must
  somehow balance delay constraints, battery
  lifetime, routing efficiency.
• 102 compare energy consumption of different
  well-known routing protocols.
• Their result indicate that reactive routing is
  more energy efficient.
• 102 J.-C. Cano and P. Manzoni, Evaluating the
  Energyconsumption Reduction in a MANET by
  Dynamically
• Switching-off Network Interfaces, Proc. IEEE
  Symp. Comp. Commun., 2001.

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conclusion

• Recent advances in ad hoc wireless network.
• There is still work improving link, mac, network,
  app protocols, interactions across these layers.
• Biggest design challenges in ad hoc networks are
  lack of centralized control, limited node
  capability.